Scipedia: Documents published in 2020

Estabilidad de la capa de hidroxizincato cálcico formada en las armaduras galvanizadas ante un posterior aumento del valor del pH

María Jesús Samper — Fri, 15 May 2020 11:24:32 +0200

In previous works on galvanized reinforcements in contact with Ca- containing highly alkaline media, the authors have reported the existence of a threshold pH of 13,3 ± 0,1, below which the Zn in contact with such a medium is passivated by formation of a continuous layer of calcium hydroxyzincate, but at pH values above this it corrodes continuously until it totally disappears. The investigation on the stability of the calcium hydroxyzincate layer after an increase of the pH to very high alkaline values, is the aim of the present paper. It has been establised that if the calcium hydroxyzincate is perfectly developed, a later increase of the pH does not affect the stability.

Corrosión de armaduras y su inspección en hormigones de cemento aluminoso

María Jesús Samper — Fri, 15 May 2020 10:55:16 +0200

Until recently has not become evident the magnitude of the problem that the corrosion of the reinforcement supposes for concrete technology. That is why, when aluminous cement preliminary degradations appeared it was not foreseen the dimension of future consequences..Now, the long term difficulty that aluminous cement presents is, in addition to the loss in mechanical strength, the possible corrosion of reinforcement. In present work are described aspects related to the processes which may originate the corrosion and given are some practical indications on how evaluate it.

Tendencias actuales en la investigación sobre corrosión de armaduras

María Jesús Samper — Fri, 15 May 2020 10:30:49 +0200

During the last 20 years, the corrosion of concrete rebars is increasily being considered a major problem due the high costs spent by some Governments on repair and of civil concrete structures. In the present paper, a brief description on the causes of this type of corrosion is offered together with some references to the costs of repairing of bridge decks published by USA researchs. Finally, the main lines of research which, in opinion of the authors have priority, are commented.

Report and database on the results of the fire performance experiments

Julio García-Espinosa — Fri, 15 May 2020 10:02:02 +0200

Today, fibre reinforced polymer (FRP) materials are extensively used for building lightweight hull structures of vessels with length up to about 50 metres, whereas in longer vessels their use is limited to secondary structures and components. In the European FIBRESHIP research project, innovative FRP materials are evaluated, new design and production procedures and guidelines are elaborated, and new validated software analysis tools are developed. As a result of the project, a comprehensive set of methods will be compiled, enabling the building of the complete hull and superstructure of over 50-metre-long ships in FRP materials. The results enhance significantly the use of FRP materials in shipbuilding and strengthen the competitiveness of the European shipbuilding industry on the world market.

In Task 2.4 of the FIBRESHIP project, an extensive experimental campaign was performed in two phases to characterize the fire performance of FRP materials and solutions.

For the first phase, seven commercially available resins or resin systems were selected for examination of fire performance. Laminates with glass fibre reinforcement and cured resins without reinforcement were produced for cone calorimeter tests and thermogravimetric analyses, respectively. From these seven candidates, two materials were down-selected on the basis of the mechanical performance, manufacturability and impact (including cost, claimed fire retardancy, worker health impact and recyclability).

The two material solutions chosen to continue to the second phase were LEO vinylester resin system and SR1125 epoxy resin system. The fire tests of the first phase showed that an intumescent coating on the surface of these laminates is essential for providing adequate fire performance.

In the second phase, a more comprehensive evaluation of thermal and fire properties was performed by carrying out more cone calorimeter tests, as well as dynamic mechanical thermal analysis, microscale combustion calorimetry, differential scanning calorimetry and transient plane source tests. Simultaneously, data for pyrolysis modelling, thermomechanical modelling and fire simulation was produced.

In cone calorimeter test at the irradiance of 50 kW/m2, the times to ignition of coated LEO and SR1125 were 75 and 52 seconds on the average, respectively. The maximum heat release was 261 kW/m2 for SR1125, but only 69 kW/m2 for LEO indicating good reaction-to-fire performance. The total heat release and the total smoke production were ca. 40 MJ/m2 and 9 m2, respectively, for both systems.

Cone calorimeter tests of coated laminate specimens were run also at the irradiance levels of 25 and 35 kW/m2 and for specimens representing different production batches. The results were not consistent in all cases. In addition, DSC tests revealed changes of the glass transition temperature when the specimens were re-heated, referring to incomplete curing in the manufacturing process. These observations highlight the importance of repeatable and well- controlled manufacturing process. The whole process must be carefully instructed, monitored and reported. The laminates and coatings must be of uniform quality to ensure the fire performance claimed on the basis of fire tests performed. Precise specifications and quality control play a key role in securing the fire safety of materials and products.

Thermogravimetric analyses and micro-scale combustion calorimetry showed that the mass loss of cured resins typically starts slightly above 300 °C both in inert (N2) and oxidative (air) atmosphere. The reactions in these atmospheres differ, the oxidative atmosphere revealing reactions such as char oxidation. At about 300 °C, however, a structure made of these FRP materials starts to produce combustible gases and contribute to fire.

Dynamic mechanical thermal analysis showed that the glass transition temperature is ca. 111 °C for LEO, and ca. 95 °C for SR1125. In general, the glass transition temperatures of FRP materials are typically about 100 °C. At this temperature, the material softens and loses its loadbearing capacity.

Structures made of FRP materials have a tendency to heat up locally, due to their relatively low thermal conductivity. In the case of a local fire, combustible gas production and heat release are the main concerns in terms of fire safety. If the fire threatens a large structure, like in the case of a compartment fire, the main problem is the softening of the material and the loss of the loadbearing capacity.

Análisis de las condiciones de fisuración en las estructuras de hormigón armado y su relación con la probabilidad de corrosión de las armaduras

María Jesús Samper — Fri, 15 May 2020 09:51:09 +0200

Cracks appears in reinforced concrete structures as something inherent to the material they are composed. Causes of diverse kind advise to control this cracking, considering it can not be prevented. The durability of reinforcements is, till now, the reason that has determined, in most structures, the study and the control of this cracking. In this report, the information contained in certain standards in force (CP-110; ACI-318-83; B.A.E.L. 80; MODEL CODE AND EH-82) about the characteristics the concrete must have and about the cracking in pieces of reinforced concrete under pliability attempts, are analysed and resumed, trying to judge later its connection with the corrosion phenomenon in reinforcements

FIBRESHIP Presentation delivered to academics, students and researchers at the "Composites at Bernal Seminar Series", Bernal Institute, University of Limerick, April 2019

Anthony Comer — Thu, 14 May 2020 22:51:02 +0200

Presentation given to Academics, students and researchers at the "Composites at Bernal Seminar Series", Bernal Institute, University of Limerick

Testing and modelling chloride penetration into concrete

María Jesús Samper — Thu, 14 May 2020 17:57:07 +0200

The design of concrete structures is usually made by means of a performance approach based on strength tests carried out following testing conditions agreed by convention. A similar approach should also be followed in the case of durability design regarding reinforcement corrosion. This type of consideration enables the establishment of verification of durability in categories. Three categories have been identified: (I) it would be the prescriptive method already given in present codes based on “deemed-to-satisfy” rules, (II) this is based in the testing through “durability indicators” that where still not quantification of the time to reach a limit state is explicit but the minimum life time is assured through performance based tests, (III) it is the category in which the calculation of aggressive ingress through models having explicit the time is made including probabilistic treatments. In the paper in addition to the comment on the most common methods of calculation of the chloride ingress and their limitations it is illustrated how for level II and III the measurement of the electrical resistivity results a very suitable proposal due to this parameter can be measured in a non-destructive test and serves to quality control. It is not only a performance test (level II) but it can be included in models of both the initiation and propagation periods. For the time to corrosion onset, the electrical resistivity represents the porosity and its connectivity and therefore can be used to model transport processes. It also results very suitable for measuring concrete aging and then accounting for the reduction of the diffusion coefficient with time. Concerning the propagation period, the electrical resistivity is an indication of the moisture content of concrete and therefore, it has a certain relationship with the corrosion cement. A model is proposed in which the resistivity is introduced in the square root of time law. Finally the paper comments on the tests methods used for the measurement of the diffusion coefficient and of resistivity

Linking fresh and durability properties of paste to SCC mortar

María Jesús Samper — Thu, 14 May 2020 17:33:19 +0200

In the last years many approaches to design SCC have been developed, but it remains a very complex process since it is necessary to manipulate several variables and understand their effects on concrete behaviour (fresh and hardened state). The prediction of concrete or mortar behaviour based on paste properties will be a significant contribution to simplify SCC design. With this purpose, two statistical experimental designs were carried out, one at paste level and the other at mortar level, to mathematically model the influence of mixture parameters on fresh and durability properties. The derived numerical models were used to define an area, labelled by self-compacting zone at paste level (SCZ), where fresh properties of the paste enable the design of SCC mortar. Furthermore, in order to extend this link to durability properties, the effect of including aggregate in cement paste was evaluated by means of the electrical resistivity test.

The Eurocode 2 and the assessment of concrete structures with corroded reinforcement

María Jesús Samper — Thu, 14 May 2020 16:43:27 +0200

The reinforcement corrosion is one of the major problems with relevant economic impact in the life cycle of concrete structures and that is why it is necessary to optimize the repair strategies in corroded structures. However, almost no international codes are available for structural evaluation and some criteria will be included in the revision of Eurocode 2 to evaluate existing deteriorated structures. A summary of Contecvet project funded EC is presented in this paper as well as some more recent developments. A first step in the evaluation of existing structures is to identify if the aggressive has reached the reinforcement and how much has affected to the structural performance. As soon as the corrosion rate has been calculated from the data taken in the structure, it is possible to estimate the rate of the structural deterioration process through the developed models and the remaining service life. Some models and expressions are summarized to estimate the residual bearing capacity of concrete structures with corroded reinforcement to be considered when drafting the new Eurocode 2.

Proposed amendment of concrete durability criteria in Eurocode 2

María Jesús Samper — Thu, 14 May 2020 16:23:30 +0200

The CEN/TC 250/SC2 and TC104/SC1 Committees have established a Joint Working Group, JWG TC250/TC104, in order to review all the durability aspects related to durability in Eurocode 2 “concrete structures” and EN 206-1 “concrete” standard. This JWG has studied the possibility of classifying concrete, not only by mechanical strength, but also by durability. This JWG has proposed that this classification be based on a direct test of carbonation or chloride resistance, which would oblige to make these types of tests routine in the future. As an alternative, the Spanish Mirror group has proposed to make this classification first by concrete mix proportioning as it is currently made and, in order to advance to a performance-based approach, by an indirect test (Durability Indicator). In present paper, the Spanish proposal is described and compared to that presented in the JWG. Advantages and disadvantages of both proposals are discussed and two Tables are presented that summarize what has been submitted by the AENOR CTN-140/SC2 for the revision of Chapter 4 of the Eurocode 2

Modelling the concrete‐real environment interaction to predict service life

María Jesús Samper — Thu, 14 May 2020 16:08:47 +0200

Recent durability studies on concrete structure

María Jesús Samper — Thu, 14 May 2020 15:23:40 +0200

The durability of concrete has attracted significant attention over the past several decades and is still a research hotspot until now. This paper reviews and discusses recent research activities on the durability of concrete, including: 1) major durability problems such as alkali aggregate reaction, sulfate attack, steel corrosion and freeze–thaw; 2) durability of concrete in marine environment; and 3) coupling effects of mechanical load and environmental factors on durability of concrete. Moreover, the consideration of durability in concrete structure design (DuraCrete and performance-based specifications) is also briefly reviewed.

Influence of Crack Width on Long Term Degradation of Concrete Structures

María Jesús Samper — Thu, 14 May 2020 15:12:56 +0200

The study of the influence of concrete crack width on corrosion of steel reinforcement due to exposure to aggressive environments is very important for durability design. Premature failure of structures may eventually occur during their service life. Since cracks in the concrete cover are inevitable due to the stress structures are subjected to, penetration of aggressive substances will occur more rapidly in cracked areas than in areas without cracking. This study addresses the corrosion effects on the reinforcement in cracked areas. Reinforced beams 2 m long and cracked at half-length with widths up to 1 mm were prepared 20 years ago, exposed to carbonation and chlorides and kept outdoors sheltered from rain until present. After 20 years, the carbonation depth in the faces of the cracks, and the reinforcement at the crack region have been examined. The results indicated that flexural cracks in high quality carbonated concrete present only a small additional risk of corrosion which, however, is significant in the presence of chlorides

Multiple linear regression model for the assessment of bond strength in corroded and non-corroded steel bars in structural concrete

María Jesús Samper — Thu, 14 May 2020 14:31:39 +0200

With the growth and ageing of the stock of existing structures, structural assessment and retrofitting are fast acquiring a significant role in the construction industry. The benefits of upgrading existing reinforced concrete (RC) structures or extending their service life and of ensuring greater durability in designs for de novo construction have led to a need to include deterioration as a factor in structural safety models. Bond between reinforcing steel and concrete is of cardinal importance in this respect. The present paper proposes a unified formulation for assessing bond strength in corroded and non-corroded steel bars, and an associated model to accommodate the effect of transverse pressure where appropriate. The formulation is the result of applying multiple linear regression analysis to a database built from the findings of over 650 bond tests on corroded and non-corroded reinforcing steel reported in the literature. The data collected include a wide range of variables affecting bond strength, such as bar diameter, concrete compressive strength, concrete cover, anchorage length, confinement ratio and cross-sectional loss. A number of statistical criteria are used to compare the proposed formulation to the other bond strength assessment models, including the fib Model Code 2010 proposal for corroded steel bars. Further to the statistical tests conducted, the model proposed can be usefully applied to assess the structural safety of corroded RC members.

Estimating corrosion attack in reinforced concrete by means of crack opening

María Jesús Samper — Thu, 14 May 2020 14:11:07 +0200

The corrosion of reinforcement in concrete is the most common degradation phenomenon of reinforced concrete structures. Reinforced concrete elements subjected to corrosion generally crack due to the expansive nature of oxides. One very important task is estimating the corrosion level using a non‐destructive method in order to establish both the actual safety of the structure and a priority intervention plan.

Many researchers have studied the relationship between the corrosion phenomenon and the corresponding crack openings and their evolution; several statistical analyses, based on test data from experimental campaigns under a wide range of test conditions, are available.

The present work attempts to contribute to finding a relationship between the crack opening and the amount of corrosion induced in the reinforcing bars. The result of the analysis is that only a reduced number of tests can be used to establish an empirical model based on a reliable set of test data. A simple relationship between crack opening and corrosion penetration is not recommended, due to the different parameters that are able to influence this correlation. Therefore, two fundamental parameters, the ratio of the concrete cover to the rebar diameter and the concrete strength, have also been considered. The considerations made regarding these parameter test results have been rearranged and the result is a formulation that shows reduced scatter

Indicator of carbonation front in concrete as substitute to phenolphthalein

María Jesús Samper — Thu, 14 May 2020 13:52:47 +0200

A method to substitute the recognized toxic phenolphthalein in mortars and concretes is proposed. It comprises putting a portland cement sample in contact with an innocuous solution, curcumin-based, which acts as an indicator yielding a red color in areas where there is alkaline reserve (pH ~ 12) and yellow in carbonate zones (pH ≤ 9).

Solutions containing curcumin extracted from commercial turmeric powder and from the rhizomes of the turmeric plant were prepared by using ethanol as a solvent. Subsequently it has been experienced with solutions in which curcumin has been used with a purity of over 95%.

The dissolution of curcumin can be used with the same reliability as phenolphthalein in the carbonation process with the advantage that it is safe and presents no health risks.

Review of the Current State of Knowledge on the Effects of Radiation on Concrete

María Jesús Samper — Thu, 14 May 2020 13:44:50 +0200

A review of the current state of knowledge on the effects of radiation on concrete in nuclear power production applications is presented. Emphasis is placed on the effects of radiation damage, as reflected by changes in engineering properties of concrete, in the evaluation of the long-term operation and for plant life or aging management of nuclear power plants (NPPs) in Japan, Spain, and the United States. National issues and concerns are described for Japan and the United States followed by a discussion of the fundamental understanding of the effects of radiation on concrete. Specifically, the effects of temperature, moisture content, and irradiation on ordinary Portland cement paste and the role of temperature and neutron energy spectra on radiation-induced volumetric expansion (RIVE) of aggregate-forming minerals are described. This is followed by a discussion of the bounding conditions for extended operation; the significance of accelerated irradiation conditions; the role of temperature and creep; and how these issues are being incorporated into numerical and meso-scale models. From these insights on radiation damage, analyses of these effects on concrete structures are reviewed, and the current status of work in Japan and the United States is described. Also discussed is the recent formation of a new international scientific and technical organization, the International Committee on Irradiated Concrete, to provide a forum for timely information exchanges among organizations pursuing the identification, quantification, and modeling of the effects of radiation on concrete in commercial nuclear applications. The paper concludes with a discussion of research gaps, including (1) interpreting test-reactor data, (2) evaluating service-irradiated concrete for aging management and to inform radiation damage models with the Zorita NPP (Spain) serving as the first comprehensive test case, (3) irradiated-assisted alkali-silica reactions, and (4) RIVE under constrained conditions.

Substantial global carbon uptake by cement carbonation

María Jesús Samper — Thu, 14 May 2020 13:26:23 +0200

Calcination of carbonate rocks during the manufacture of cement produced 5% of global CO2 emissions from all industrial process and fossil-fuel combustion in 20131,2. Considerable attention has been paid to quantifying these industrial process emissions from cement production2,3, but the natural reversal of the process—carbonation—has received little attention in carbon cycle studies. Here, we use new and existing data on cement materials during cement service life, demolition, and secondary use of concrete waste to estimate regional and global CO2 uptake between 1930 and 2013 using an analytical model describing carbonation chemistry. We find that carbonation of cement materials over their life cycle represents a large and growing net sink of CO2, increasing from 0.10 GtC yr−1 in 1998 to 0.25 GtC yr−1 in 2013. In total, we estimate that a cumulative amount of 4.5 GtC has been sequestered in carbonating cement materials from 1930 to 2013, offsetting 43% of the CO2 emissions from production of cement over the same period, not including emissions associated with fossil use during cement production. We conclude that carbonation of cement products represents a substantial carbon sink that is not currently considered in emissions inventories.

FIBRESHIP project: Engineering, production and life cycle management for the complete construction of large length fibrebased ships.

Cristobal Garcia Pariente — Thu, 14 May 2020 12:29:02 +0200

Fibre-Reinforced Polymer (FRP) materials are widely used in the construction of small-length vessels due to their light weight and high strength to weight ratio. However, the use of FRP materials in vessels above 50 m length is only allowed to secondary structural elements of the vessel. Hence, it is necessary to promote the creation of new regulatory frameworks to permit the construction of large-length vessels using composite laminates in all parts of the vessel structure to enable the development of this interesting technology for the reduction of weight. FIBRESHIP proposes to create a new market focused on the construction of large-length vessels based uniquely on lightweight composites. The results of this project are attracting considerable interest within the shipping industry as the extensive use of FRP materials in large-length vessels induces an important reduction of the weight with respect to the conventional steel ships. This significant weight reduction decreases the vessel bunkering consumption, increases the payload cargo capacity, and avoids the corrosion phenomena in the vessel among other identified benefits.

First steps of the FIBRESHIP project: Engineering, production and life cycle management for the complete construction of large length fibre-based ships.

Cristobal Garcia Pariente — Thu, 14 May 2020 11:58:02 +0200

The main objective of the FIBRESHIP project is to enable the building of the complete hull and superstructure of large-length seagoing and inland ships in FRP materials by overcoming few technical challenges. In order to achieve this objective, the project will develop, qualify and audit innovative FRP materials for marine applications, elaborate new design and production guidelines and procedures, generate efficient production and inspection methodologies, and develop new validated software analysis tools. Clear performance indicators will be designed and applied in the evaluation of the different solutions developed for three targeted vessels categories. Finally, the different technologies generated in FIBRESHIP will be first validated and then demonstrated by using advanced simulation techniques and experimental testing on real-scale structures. This paper is dealing with the presentation of the project scope and progress of the FIBRESHIP project over the first six months.

FIBRESHIP, the greatest challenge of using composite materials for the construction of large-length ships

Cristobal Garcia Pariente — Wed, 13 May 2020 19:05:02 +0200

FIBRESHIP (H2020, Grant 723360) is an ambitious innovation project funded by the European Commission, which is participated by 18 European partners led by the Spanish company TSI. The main objective of FIBRESHIP is to overcome the great technical challenge of replacing the metallic materials currently used for the construction of large-lenght vessels for composite materials.

To carry out this task, this project is addressed from different points of view: i) shipping market and bussiness analysis of composite vessels, ii) analysis and numerical modeling of the materials selected, iii) development of the structural design using a finite element analysis software, and, iv) analysis of the construction and assembly procedures, structural health monitoring techniques, and ship life cycle.

The study and implementation of this technology for the construction of large-length composite vessels is carried out through the design of three types of vessels over 50 meters, which are considered as the most promising in the market: a container ship of 245 meters length, a ROPAX of 185 meters and a special service oceanographic vessel of 80 meters. In addition, specific numerical models are being developed and validated through experimental testing campaigns. Eventually, different structural health monitoring strategies and production models adapted to this technology.

Materials for large length fibre-based ships. Characterization, selection, and numerical analysis

Joel Jurado Granados — Wed, 13 May 2020 18:51:02 +0200

Fourth dissemination action for university students for increasing the interest in the use of fibre-reinforced polymers for large-length vessels design and shipbuilding in the future naval architects and marine engineers. This presentation was held at the School of Naval Architecture and Ocean Engineering (ETSINO) of the Technical University of Madrid on the 1st of October of 2019. In this presentation, the process of materials selection for marine applicactions and the numerical models for characterizing and predicting the fatigue and failure of such materials were introduced.

Numerical tools for the design of fibre-based ships of large lengths

Joel Jurado Granados — Wed, 13 May 2020 18:36:03 +0200

First dissemination action for university students at the School of Naval and Ocean Engineering (ETSINO) of Cartagena University (UPCT) with the intention of increasing the interest of the students and future engineers and naval architects in the use of composite materials for the design and shipbuilding of large-length vessels. In this presentation, the numerical models developed for predicting the fatigue and failure of composite materials were showed. Besides, the new numerical tools to design and anayze fiber-based ships of large length were introduced, specifically a coupled seakeeping-FEA tool, a new graphical user interface (GUI) for materials definition, hull girder analysis and collapse assesemt.

Future trends in research on reinforcement corrosion

María Jesús Samper — Wed, 13 May 2020 17:51:14 +0200

The concept of service life in the science in reinforcement corrosion has evolved from a basic description of phenomena to a more detailed engineering perspective in which quantification of processes is of vital importance. This chapter treats the following subjects: (1) causes of reinforcement corrosion – carbonation, chloride ingress and stress corrosion cracking, (2) corrosion onset and chloride threshold, (3) corrosion propagation period, (4) modelling of service life, (5) additional preventive methods, (6) repair materials and techniques and (7) corrosion measurement techniques. The paper does not develop the known aspects of these different subjects, but rather identifies those aspects still needing further research work. In this exercise, the modelling of processes and their incorporation into structural and engineering calculations is identified as the most important subject needing to be improved and further developed in the near future.

Corrosion induced cracking: Effect of different corrosion rates on crack width evolution

María Jesús Samper — Wed, 13 May 2020 17:24:33 +0200

Reinforcement corrosion leads to several damage types that influence the structure service lifetime, among which can be mentioned the cracking of concrete cover due to the increase in volume of the oxides with respect to the original volume of the parent steel. The relation between crack width and corrosion level has been studied by some authors while its analytical or numerical modelling is attracting increasing interest. In the modelling attempts, it has not been found the consideration of the corrosion rate effect, which, on the other hand was experimentally demonstrated more than 20 years ago. In the present paper the impact of different corrosion rates on the crack width development of reinforced concrete (RC) was studied. Identical RC prisms were subjected to accelerated corrosion in galvanostatic conditions with distinct electrical currents each, in order to monitor the superficial crack width growth of every specimen. Crack width growth with steel attack penetration is analysed as a two stage bilinear process where the crack width grows faster in the first stage. An empirical relation is found between the ratio crack width/steel attack and the corrosion rate applied for the first stage of cracking. With the new-found relation, a formula relating crack width evolution through time with the corrosion rate is presented. This study shows that it is crucial to take into account the strong influence of the corrosion rate on the crack width/steel attack ratio variation in order to successfully predict corrosion induced cracking

Contactless safety evaluation of damaged structures through energetic criteria

María Jesús Samper — Wed, 13 May 2020 17:03:15 +0200

The reinforced concrete structures need to be monitored to ensure their structural integrity, but sometimes those measurements are very local, and the instrument is complex to locate physically in the structure and may interfere on it. Digital Image Correlation is a noncontact and nondestructive experimental technique capable to measure the displacement field in a big region of a structure with a great accuracy. This allows extracting valuable information from the fracture processes of reinforced concrete structures, critical for the evaluation of the structural integrity. The measurement of the energy dissipated by the structure is essential for the identification of the strength mechanisms that are failing in the structure and to identify a proper repair. Also, using fracture mechanics, other valuable information are extracted from the fracture processes of the reinforced concrete beam, such as the Modes I and II fracture energy released at each loading step, which is essential to evaluate the elastic energy that the structure can accumulate before collapse. The examples enable to anticipate the importance of Digital Image Correlation for future large scale studies of fracture in concrete and other materials related to construction.

Corrosion-induced brittle failure in reinforcing steel

María Jesús Samper — Wed, 13 May 2020 16:29:59 +0200

Steel rebar corrosion is the main cause of reinforced concrete structure deterioration and shortened service life. A number of corrosion mechanisms induce geometric flaws in rebar, reducing its mechanical strength. Interstitial hydrogen, which lowers steel toughness, may also prompt embrittlement.This paper discusses the effect of local, pitting and corrosion stress on steel rebar performance. The findings show that rebar bearing strength depends on corrosion morphology and weight loss, which would explain the scatter in experimental results when the latter is the sole parameter studied.

The effect of two types of modified Mg-Al hydrotalcites on reinforcement corrosion in cement mortar

María Jesús Samper — Wed, 13 May 2020 16:15:09 +0200

Two modified Mg-Al hydrotalcites (MHTs), (MHT-pAB and MHT-NO2) were incorporated into mortar (with different w/c ratios) in two different ways: (1) as one of the mixing components in bulk mortar; (2) as part of cement paste coating of the reinforcing steel. Accelerated chloride migration, cyclic wetting-drying and diffusion tests were performed to investigate their effect on reinforcement corrosion. The results indicated that MHTs could be promising alternatives for preventing chloride-induced corrosion when an appropriate dosage is adopted and applied in a proper way, particularly, replacing 5% mass of cement by MHT-pAB in bulk mortar or as a coating of reinforcing steel (MHT-pAB/MHT-NO2 to replace 20% mass of cement). The effect of MHT-pAB on time-to-corrosion initiation (TTC) of reinforcing steel was estimated using the DuraCrete model. It was found that the incorporation of 5% MHT-pAB in bulk mortar led to a more than double TTC relative to reference mortar without MHTs.

Reliability analysis of corrosion onset: initiation limit state

María Jesús Samper — Wed, 13 May 2020 15:35:46 +0200

The relatively premature failure of structures by steel corrosion has increased interest in incorporating to the standards calculation of the time to corrosion. These models in general identify the end of service life with the starting of bar corrosion, either by chlorides or by carbonation. Additionally, from the publications of the project Duracrete, funded by the EU, probabilistic treatments have been applied to steel depassivation which is associated with a serviceability limit state (SLS). Thus, fib Model Code (MC2010) recommends a probability of failure of 10% for calculation of the time to corrosion. However, this recommendation leads to uncertainties because, on the one hand, the chloride threshold is not a fixed value and the carbonation front affects first the external face of the bar, developing progressive perimetral damage and, on the other hand, the consideration of the limit state as an SLS, with so low a fixed failure probability, introduces some degree of contradiction with the classical definition of the SLS. This is because, while the SLS is defined as a failure to fulfil the design prescriptions, any structural design performance changes just at the moment of steel depassivation. Additionally, no treatment on how to consider the damaged section or corrosion propagation model is usually given in the MC2010 or other standard and how to verify the classical SLS and ultimate limit state (ULS) is undefined. The present work introduces new perspectives on the subject from a probabilistic point of view by, (1) treating statistically the depassivation step, (2) by introducing a corrosion propagation model and quantifying the corrosion limit states and (3) by calculation of the depassivation probability which is dependent on the rate of the deterioration process itself. The whole exercise stresses the need to not consider a fixed value (10%) associated with a SLS as the probability of corrosion, but to consider it a limit state of “corrosion initiation” following ISO 2394. The value of the adequate probability of corrosion would be dependent on the consequences of the failure and on the rates of chloride ingress, of carbonation or of steel corrosion.

Fracto-surface mobility mechanism in high-strength steel wires

María Jesús Samper — Wed, 13 May 2020 14:31:18 +0200

Stress corrosion cracking, a mechanism governing steel deterioration, has yet to be fully understood. This article introduces a supplement to the surface mobility mechanism proposed by Galvele to include fracture mechanics principles. In the model proposed, anodic dissolution is regarded as the source of vacancies that would be driven by a stress gradient to the crack tip under non-stationary conditions. Denominated the fracto-surface mobility mechanism (FSMM), it explains material mechanical failure based on vacancy diffusion across the metal surface.

Design and evaluation of service life through concrete electrical resistivity

María Jesús Samper — Wed, 13 May 2020 13:54:39 +0200

This paper describes the use of concrete electrical resistivity as durability performance parameter and the complementary information that resistivity can provide like: setting period, mechanical strength and degree of curing. Also, it is explained how to design the concrete mix to obtain a target resistivity. Current codes have prescriptive requirements for the durability of concrete and reinforcement corrosion. However, modern trends specify the performance rather than the concrete characteristics. This performance approach demands to define a durability controlling parameter, such as the chloride diffusion coefficient, with its corresponding test and the model to predict the time to steel corrosion.

Brief historical notes

María Jesús Samper — Wed, 13 May 2020 13:32:30 +0200

Anniversaries are a good excuse to make some summaries of historical events. Very briefly, some advances made from the initial research in the speciality of reinforcement corrosion which started in the 60’s are summarized. The use of electrochemical techniques was a milestone which enabled, from the decade of the 1970, to study the effect of each variable with much more rigour. The studies on service life started in the decade of 1980, although they were not of general interest until the next decade. From 1990 RILEM Committees and the Iberoamerican Program on Corrosion of CYTED extended the knowledge so widely that in the XXI century the subject attracts much research interest.

Model Predictive Control applied to thermostatic controlled systems

Florencia Lazzari — Wed, 13 May 2020 12:40:02 +0200

A Model Predictive Control (MPC) was developed to optimize the performance of a residential heat pump. To test the MPC, the real system was replaced by a Modelica emulator of the entire house. The building’s thermal behavior model was developed as an Auto-Regressive linear model with eXogenous variables (ARX) and the optimization algorithm was based in a genetic algorithm. The proposed predictive model to characterize the building’s thermal behavior was developed successfully. And the MPC is already implemented and validated, but the total integration with the emulator of the system and the feedback loop has not been yet assembled. In this way, we are looking towards to end this task and complete the further analysis.

Apparent diffusion coefficient of oxygen and corrosion control of reinforcement rebar coated with primers

María Jesús Samper — Wed, 13 May 2020 12:18:40 +0200

The present work evaluates the influence of five different types of coatings applied on the rebar on apparent diffusion coefficient of oxygen (Dap (O2)) and on the corrosion intensity (Icorr) in comparison with a reference coating composed of a cement-sand mortar. Oxygen flow (J (O2)) until the rebar was measured by potenciostático method in the steady state and the corrosion intensity (Icorr) was monitored by the Polarization Resistance technique (Rp). Evaluations for porosity of coatings were made through magnifying glasses, optical microscopy and SEM. The primers that represent barrier protection systems were, under the conditions tested, less permeable to oxygen. The apparent diffusion coefficient values (Dap (O2)) ranged from 2.1 x 10-6 cm2/s to 4 x 10-9 cm2/s, causing variations in the corrosion intensity measurements (Icorr), due to the cathodic corrosion process control.

Corrigendum to “Threshold concentration of chlorides in concrete for stainless steel reinforcement: Classic austenitic and new duplex stainless steel”

María Jesús Samper — Wed, 13 May 2020 11:53:01 +0200

The authors regret that Equation 1: 30%w Ni not is correct. Should be changed to 30%w N, so that is; PREn = %w Cr + 3.3%w Mo + 30%w N-%w Mn.

Footnote in Fig. 5 is not correct. Should be changed to “Micrographs (4×) of EN 1.4362/2304 duplex rebar; left: after removal from specimen; right: after pickling (4×)”.

The authors would like to apologise for any inconvenience caused

Threshold concentration of chlorides in concrete for stainless steel reinforcement: Classic austenitic and new duplex stainless steel

María Jesús Samper — Wed, 13 May 2020 11:46:30 +0200

Stainless steel rebars is used to lengthen the service life of structures in aggressive media. Corrosion resistance was studied in five grades of mortar-embedded stainless steel: EN 1.4307, EN 1.4404, EN 1.4482, EN 1.4362 and EN 1.4462. A modified accelerated chloride attack was applied to prevent steel rebar polarisation and artificial re-passivation. The critical chloride concentration was determined. Based on corrosion rate, service life was calculated for a given exposure at two probability levels.

Updating Carbon Storage Capacity of Spanish Cements

María Jesús Samper — Wed, 13 May 2020 11:03:53 +0200

The fabrication of cement clinker releases CO₂ due to the calcination of the limestone used as raw material, which contributes to the greenhouse effect. The industry is involved in a process of reducing this amount liberated to the atmosphere by mainly lowering the amount of clinker in the cements. The cement-based materials, such as concrete and mortars, combine part of this CO₂ by a process called “carbonation”. Carbonation has been studied lately mainly due to the fact that it induces the corrosion of steel reinforcement when bringing the CO₂ front to the surface of the reinforcing bars. Thus, the “rate of carbonation” of the concrete cover is characterized by and linked to the length of service life of concrete structures. The studies on how much CO₂ is fixed by the hydrated phases are scarce and even less has been studied the influence of the type of cement. In present work, 15 cements were used to fabricate paste and concrete specimens withwater/cement (w/c) ratios of 0.6 and 0.45 which reproduce typical concretes for buildings and infrastructures. The amount of carbon dioxide uptake was measured through thermal gravimetry. The degree of carbonation, (DoC) is defined as the CO₂ fixed with respect to the total theoretical maximum and the carbon storage capacity (CSC) as the carbonation uptake by a concrete element, a family or the whole inventory of a region or country. The results in the pastes where analyzed with respect to the uptake by concretes and indicated that: (a) the humidity of the pores is a critical parameter that favours the carbonation reaction as higher is the humidity (within the normal atmospheric values), (b) all types of cement uptake CO₂ in function of the CaO of the clinker except the binders having slags, which can uptake additional CO₂ giving aDoC near or above 100%. The CSC of Spain has been updated with respect to a previous publication resulting in proportions of 10.8–11.2% of the calcination emissions, through considering a ratio of “surface exposed/volume of the element” of 3 as an average of the whole Spanish asset of building and infrastructures.

Advances in Characterization of Gas Transport in Concrete: Determination of Oxygen Diffusion Coefficient from Permeability Coefficient and Porosity

María Jesús Samper — Wed, 13 May 2020 10:35:30 +0200

One of the most relevant mechanisms that influence the life service of concrete (carbonation, compactness, alkaline protection) is gas diffusion through its mass. However, determination of gas diffusion coefficient in concrete is not a simple task. There is not a general standard which determines the test procedure. Other approaches or alternative parameters could be used in order to obtain the gas diffusion coefficient, including the use of permeability coefficient. Both parameters are dependent on material porosity and moisture content. The literature already proposes generic correlations for these parameters, but these correlations do not support the direct derivation of the gas diffusion coefficient. This paper presents the results of research carried out to analyse these generic correlations and to propose specific expressions that support the derivation of a value for the oxygen diffusion coefficient, based on the porosity and permeability coefficients. The research was centred on a experimentation process to obtain these parameters. A diffusion chamber was designed and built for use with two types of concrete mix, two distinct concrete curing processes and three separate values for humidity. Each test case sought to derive values for oxygen diffusion and permeability coefficients.

Propagation of reinforcement corrosion: principles, testing and modelling

María Jesús Samper — Wed, 13 May 2020 10:15:58 +0200

Reinforcement corrosion is the risk most frequently cited to justify concrete durability research. The number of studies specifically devoted to corrosion propagation, once the object of most specialised papers, has declined substantially in recent years, whilst the number addressing initiation, particularly where induced by chlorides, has risen sharply. This article briefly describes the characteristics of steel corrosion in concrete that need to be stressed to dispel certain misconceptions, such as the belief that the corrosion zone is a pure anode. That is in fact seldom the case and as the zone is also affected by microcells, galvanic corrosion accounts for only a fraction of the corrosion rate. The role of oxygen in initiating corrosion, the scant amount required and why corrosion can progress in its absence are also discussed. Another feature addressed is the dependence of the chloride threshold on medium pH and the buffering capacity of the cement, since corrosion begins with acidification. Those general notions are followed by a review of the techniques for measuring corrosion, in particular polarisation resistance, which has proved to be imperative for establishing the processes involved. The inability to ascertain the area affected when an electrical signal is applied to large-scale elements is described, along with the concomitant need to use a guard ring to confine the current or deploy the potential attenuation method. The reason that measurement with contactless inductive techniques is not yet possible (because the area affected cannot be determined) is discussed. The method for integrating corrosion rate over time to find cumulative corrosion, Pcorr, is explained, together with its use to formulate the mathematical expressions for the propagation period. The article concludes with three examples of how to use corrosion rate to assess cathodic protection, new low-clinker cements or determine the chloride threshold with an integral accelerated service life method

Concrete Structures: On-site electrochemical techniques and concrete resistivity to predict service life Part 1 - Corrosion measurement techniques

María Jesús Samper — Tue, 12 May 2020 17:21:14 +0200

Concrete protects the steel of the reinforcements in a similar manner than do the paints but with the particularity that it is a material chemically active and evolving for decades. Concrete provides a very alkaline electrolyte encapsulated in its pore network. The steel may remain passive indefinitely but chlorides and atmospheric carbon dioxide penetrate the pores and reaching the reinforcement promote corrosion, which impairs the concrete structural properties inducing the cover cracking and the loss in steel-concrete bond. Calculation of service life is becoming an important subject in engineering terms due to the increasing demand to produce buildings and infrastructures with service life beyond 100 years. In the paper focus will be devoted to two main areas: 1) the measurement techniques in concrete and permanent probes, including on site methods for large structures and contactless (inductive) methods 2) the illustration of the corrosion resistance control exerted by concrete resistivity which enables the calculation of the diffusion of ions through the concrete and the prediction of the corrosion rate in function of the climatic conditions

Concrete structures: on-site electrochemical techniques and concrete resistivity to predict service life Part 2: Service life of structural concrete, present and future challenges

María Jesús Samper — Tue, 12 May 2020 17:12:28 +0200

Numerical Simulation of Fatigue in Composites

Joel Jurado Granados — Tue, 12 May 2020 17:07:03 +0200

For the past several years, composite marine structures have been designed but without having a complete characterization the composite materials. The main reason is due to the large quantity of variables that affect the behavior of said materials. One of the tasks that has not been properly characterized is the behavior of composite structures under fatigue loads that appear in marine structures that are subjected to cyclic loads. Therefore, numerical tools that characterize fatigue performance are required in order to design more reliable structures. The formulation proposed in this work is based on the Serial/Parallel Rule of Mixtures [1] and a fatigue damage model [2]. The Serial/Parallel Rule of Mixtures can be understood as a constitutive law manager that provides the response of the composite from the constitutive performance of its constituents. Therefore, the constitutive laws chosen to represent the behavior of each constituent material have to fit with their real performance. Also, the fatigue damage model is based on the use of a reduction function which takes into account the cyclic degradation of the materials, both strength and stiffness degradation, in function of the number of cycles, maximum stress and stress amplitude. Current work presents a numerical tool developed to characterize fatigue in composites. The fatigue behavior of constituent materials is defined using mechanic parameters taken from literature. Afterwards, a reproduction of the tests will be done in order to validate the fatigue formulation proposed.

[1] Car, E., Oller, S., Oñate, E. "Estudio del comportamiento no lineal en materiales compuestos", Techincal Report 264,CIMNE, 1997.

[2] Oller, Salomon, O., Oñate, E. “A continuum mechanics model for mechanical fatigue analysis", Composite Materials Science, Vol 32, Issue 2, pp 175-195, 2005.

Porosity and electrical resistivity-based empirical calculation of the oxygen diffusion coefficient in concrete

María Jesús Samper — Tue, 12 May 2020 16:52:09 +0200

Concrete carbonation induced by gas transport has an adverse effect on the service life of the steel-reinforced material. Gas tightness is crucial where concrete is used to contain radioactive materials and of utmost importance where acting as a radon barrier. Since as a rule gas travel inside concrete is governed either by diffusion or permeability, the material’s performance in that respect is assessed by analysing those two parameters.

The gas diffusion coefficient in concrete is not readily found, however, due to the practical difficulties involved in preventing gas from leaking out of standard diffusion cells. An alternative approach is to measure permeability at high gas concentration and pressure. The existence of a relationship between the oxygen diffusion coefficient and permeability in OPC concrete was established by the authors in a previous paper.

Diffusion has also been shown to be related to porosity and, in solutions, to electrical resistivity. Little is known, however, about the relationship between gas diffusion and resistivity where the pores may be filled not with a liquid, but with air. Inasmuch as resistivity is a non-destructive technique and porosity a property that can be readily measured, these two parameters could be used to directly assess concrete durability, its performance in nuclear facilities and its suitability as a radon barrier.

This paper describes a study conducted to formulate equations from which to analytically determine the oxygen diffusion coefficient. These equations are derived from empirical measurements of oxygen diffusion coefficient, concrete porosity and electrical resistivity. The findings, which corroborate the existence of such relationships, were used to formulate an equation to calculate the diffusion coefficient directly from the experimental values of concrete porosity and resistivity.

Since porosity depends primarily on concrete batching, curing and moisture content, two OPC concrete mixes were prepared using different water/cement ratios and two curing conditions. The combination resulted in four types of concrete. Pre-conditioning at three values of relative humidity was subsequently deployed.

Numerical and experimental procedure for material calibration using the serial/parallel mixing theory, to analyze different composite failure modes

Joel Jurado Granados — Tue, 12 May 2020 16:06:02 +0200

This work proposes a calibration procedure to obtain the material parameters required by the Serial/Parallel Mixing Theory for the analysis of composites. A set of experimental tests are defined to obtain the main composite failure modes. Then, it is proposed to calculate the parameters required by the formulation using the experimental results. The procedure proposed is validated by comparing the numerical results, with those obtained from the experimental campaign. This comparison shows that the Serial/Parallel mixing theory is capable of representing the failure modes of the composite for different loading scenarios as well as the material toughness.

High performance reduced order modeling techniques based on optimal energy quadrature: application to geometrically non-linear multiscale inelastic material modeling

María Jesús Samper — Tue, 12 May 2020 15:42:47 +0200

A High-Performance Reduced-Order Model (HPROM) technique, previously presented by the authors in the context of hierarchical multiscale models for non linear-materials undergoing infinitesimal strains, is generalized to deal with large deformation elasto-plastic problems. The proposed HPROM technique uses a Proper Orthogonal Decomposition procedure to build a reduced basis of the primary kinematical variable of the micro-scale problem, defined in terms of the micro-deformation gradient fluctuations. Then a Galerkin-projection, onto this reduced basis, is utilized to reduce the dimensionality of the micro-force balance equation, the stress homogenization equation and the effective macro-constitutive tangent tensor equation. Finally, a reduced goal-oriented quadrature rule is introduced to compute the non-affine terms of these equations. Main importance in this paper is given to the numerical assessment of the developed HPROM technique. The numerical experiments are performed on a micro-cell simulating a randomly distributed set of elastic inclusions embedded into an elasto-plastic matrix. This micro-structure is representative of a typical ductile metallic alloy. The HPROM technique applied to this type of problem displays high computational speed-ups, increasing with the complexity of the finite element model. From these results, we conclude that the proposed HPROM technique is an effective computational tool for modeling, with very large speed-ups and acceptable accuracy levels with respect to the high-fidelity case, the multiscale behavior of heterogeneous materials subjected to large deformations involving two well-separated scales of length.

Implicit–explicit integration of gradient-enhanced damage models

María Jesús Samper — Tue, 12 May 2020 15:12:16 +0200

Quasi-brittle materials exhibit strain softening. Their modeling requires regularized constitutive formulations to avoid instabilities on the material level. A commonly used model is the implicit gradient-enhanced damage model. For complex geometries, it still shows structural instabilities when integrated with classical backward Euler schemes. An alternative is the implicit–explicit (IMPL-EX) integration scheme. It consists of the extrapolation of internal variables followed by an implicit calculation of the solution fields. The solution procedure for the nonlinear gradient-enhanced damage model is thus transformed into a sequence of problems that are algorithmically linear in every time step. Therefore, they require one single Newton–Raphson iteration per time step to converge. This provides both additional robustness and computational acceleration. The introduced extrapolation error is controlled by adaptive time-stepping schemes. This paper introduced and assessed two novel classes of error control schemes that provide further performance improvements. In a three-dimensional compression test for a mesoscale model of concrete, the presented scheme was about 40 times faster than an adaptive backward Euler time integration.

Preface: Computational multiscale modelling and design of new engineering materials

María Jesús Samper — Tue, 12 May 2020 14:32:49 +0200

Making use of symmetries in the three-dimensional elastic inverse homogenization problem

María Jesús Samper — Tue, 12 May 2020 14:14:53 +0200

The objective of this paper is the design of three-dimensional elastic metamaterials with periodic microarchitectures. The microarchitectures of these materials are attained by following an inverse design technique jointly with an homogenization-based topology optimization algorithm. In this context, we have particularly studied the connection between the symmetry of the material layout at the microscale of 3D periodic composites and the symmetry of the effective elastic properties.We have analyzed some possible Bravais lattices and space groups, which are typically associated with crystallography, to study the way in which the symmetries of these geometrical objects can be usefully used for the microarchitecture design of 3D elastic metamaterial. Following a previous work of the authors for two-dimensional problems, we suggest adopting the design domain of the topology optimization problem coincident with the Wigner-Seitz cells of specific Bravais lattices having the same point group to that of the target elasticity tensor. The numerical assessment described in this paper aims at the design of an extreme material. The solutions obtained with this procedure show that different composite microarchitectures emerge depending on the cell shape selection.

Effect of Environmental Conditioning on the Properties of Thermosetting and Thermoplastic-Matrix Composite Materials by Resin Infusion for Marine Applications (PREPRINT)

Anthony Comer — Tue, 12 May 2020 13:19:02 +0200

Glass-fibre reinforced polymer (GFRP) laminates were manufactured using Vacuum assisted Resin Transfer Moulding (VaRTM) with a range of thermosetting resins and a novel infusible thermoplastic resin as part of a comprehensive down-selection to identify suitable commercially available resin systems for the manufacture of marine vessels greater than 50 m in length. The effect of immersion in deionised water and in an organic liquid (diesel) on the interlaminar shear strength (ILSS) and glass transition temperature (Tg) was determined. The thermoplastic had the highest Tg of all materials tested and comparable ILSS properties to the epoxy. Immersion in water, however, caused larger reductions in ILSS properties of the thermoplastic compared to the other systems. SEM showed a transition from matrix-dominated failure in the dry condition to failure at the fibre-matrix interface in the wet and organic-wet specimens. The overall performance of the infusible thermoplastic is good when compared to well-established marine resin systems; however, the environmental performance could be improved if the thermoplastic resin is used in conjunction with a fibre sizing that is tailored for use with acrylic-based resin systems.

Material design of elastic structures using Voronoi cells

María Jesús Samper — Tue, 12 May 2020 12:04:16 +0200

New tools for the design of metamaterials with periodic microarchitectures are presented. Initially, a two‐scale material design approach is adopted. At the structure scale, the material effective properties and their spatial distribution are obtained through a Free Material Optimization technique. At the microstructure scale, the material microarchitecture is designed by appealing to a Topology Optimization Problem (TOP). The TOP is based on the topological derivative and the level set function. The new proposed tools are used to facilitate the search of the optimal microarchitecture configuration. They consist of the following: (i) a procedure to choose an adequate shape of the unit cell domain where the TOP is formulated and shapes of Voronoi cells associated with Bravais lattices are adopted and (ii) a procedure to choose an initial material distribution within the Voronoi cell being utilized as the initial configuration for the iterative TOP.

Reduced order modeling strategies for computational multiscale fracture

María Jesús Samper — Tue, 12 May 2020 10:45:58 +0200

The paper proposes some new computational strategies for affordably solving multiscale fracture problems through a FE2 approach. To take into account the mechanical effects induced by fracture at the microstructure level the Representative Volume Element (RVE), assumed constituted by an elastic matrix and inclusions, is endowed with a large set of cohesive softening bands providing a good representation of the possible microstructure crack paths. The RVE response is then homogenized in accordance with a model previously developed by the authors and upscaled to the macro-scale level as a continuum stress–strain constitutive equation, which is then used in a conventional framework of a finite element modeling of propagating fracture. For reduced order modeling (ROM) purposes, the RVE boundary value problem is first formulated in displacement fluctuations and used, via the Proper Orthogonal Decomposition (POD), to find a low-dimension space for solving the reduced problem. A domain separation strategy is proposed as a first technique for model order reduction: unconventionally, the low-dimension space is spanned by a basis in terms of fluctuating strains, as primitive kinematic variables, instead of the conventional formulation in terms of displacement fluctuations. The RVE spatial domain is then decomposed into a regular domain (made of the matrix and the inclusions) and a singular domain (constituted by cohesive bands), the required RVE boundary conditions are rephrased in terms of strains and imposed via Lagrange multipliers in the corresponding variational problem. Specific low-dimensional strain basis is then derived, independently for each domain, via the POD of the corresponding strain snapshots. Next step consists of developing a hyper-reduced model (HPROM). It is based on a second proposed technique, the Reduced Optimal Quadrature (ROQ) which, again unconventionally, is determined through optimization of the numerical integration of the primitive saddle-point problem arising from the RVE problem, rather than its derived variational equations, and substitutes the conventional Gauss quadrature. The ROQ utilizes a very reduced number of, optimally placed, sampling points, the corresponding weights and placements being evaluated through a greedy algorithm. The resulting low-dimensional and reduced-quadrature variational problem translates into very relevant savings on the computational cost and high computational speed-ups. Particular attention is additionally given to numerical tests and performance evaluations of the new hyper-reduced methodology, by “a-priori” and “a-posteriori” error assessments. Moreover, for the purposes of validation of the present techniques, a real structural problem exhibiting propagating fracture at two-scales is modeled on the basis of the strain injection-based multiscale approach previously developed by the authors. The performance of the proposed strategy, in terms of speed-up vs. error, is deeply analyzed and reported

On the numerical modeling of granular material flows via the Particle Finite Element Method (PFEM)

María Jesús Samper — Tue, 12 May 2020 09:38:47 +0200

The aim of this work is to describe a numerical framework for reliably and robustly simulating the different kinematic conditions exhibited by granular materials while spreading ---from a stagnant condition, when the material is at rest, to a transition to granular flow, and back to a deposit profile. The gist of the employed modeling approach was already presented by the authors in a recent work (Cante et al., 2014), but no proper description of the underlying numerical techniques was provided therein. The present paper focuses precisely on the detailed discussion of such numerical techniques, as well as on its rigorous validation with the experimental results obtained by Lajeunesse, et al. in Ref. ( Lajeunesse et al., 2004). The constitutive model is based on the concepts of large strains plasticity. The yield surface is defined in terms of the Drucker Prager yield function, endowed with a deviatoric plastic flow and the elastic part by a hypoelastic model. The plastic flow condition is assumed nearly incompressible, so a u - p mixed formulation, with a stabilization of the pressure term via the Polynomial Pressure Projection (PPP), is employed. The numerical scheme takes as starting point the Particle Finite Element Method (PFEM) in which the spatial domain is continuously redefined by a different nodal reconnection, generated by a Delaunay triangulation. In contrast to classical PFEM approximations ( Idelsohn et al., 2004), in which the free boundary is obtained by a geometrical technique (a-shape method), in this work the boundary is treated as a material surface, and the boundary nodes are removed or inserted by means of an error function. One of the novelties of this work is the use of the so-called Impl-Ex hybrid integration technique to enhance the spectral properties of the algorithmic tangent moduli and thus reduce the number of iterations and robustness of the accompanying Newton-Raphson solution algorithm (compared with fully implicit schemes respectively). The new set of numerical tools implemented in the PFEM algorithm – including new discretization techniques, the use of a projection of the variables between meshes, and the constraint of the free-surface instead using classic a-shape – allows us to eliminate the negative Jacobians present during large deformation problems, which is one of the drawbacks in the simulation of granular flows. Finally, numerical results are compared with the experiments developed in Ref. (Lajeunesse et al., 2004), where a granular mass, initially confined in a cylindrical container, is suddenly allowed to spread by the sudden removal of the container. The study is carried out using different geometries with varying initial aspect ratios. The excellent agreement between computed and experimental results convincingly demonstrates the reliability of the model to reproduce different kinematic conditions in transient and stationary regimes.

Use of machine learning techniques in bank credit risk analysis

Bruno Samways dos Santos — Tue, 12 May 2020 02:18:42 +0200

The purpose of this article is to compare the performance of a credit scoring model by applying different Machine Learning techniques for the classification of payers in bank financing of companies (5 432 historical records). Clients were considered “non-default” or “default” depending on their default index, thus, 4 238 were considered “non-default” and 1 194 “default”, including the information related to 10 variables (features) that composed the database. First, a random undersampling technique was applied to solve the unbalanced data problem. The variables were then coded in two ways: Code I (categorical variables) and Code II (binary or dummy variables). This was followed by the feature selection methods to detect the most important variables. Finally, we used three classifier algorithms of Machine Learning (ML), Bayesian Networks (BN), Decision Tree (DT) and Support Vector Machine (SVM) comparatively. All these techniques were implemented in WEKA (Waikato Environment for Knowledge Analysis) software. The best performance was 95.2% using balanced classes, with the attributes coded in a binary way and the SVM machine learning technique. So, in this way, it is possible to automatically classify (“non-default” or “default”) new instances making use of the proposed methodology with high performance.

Retention of Mechanical Properties After Water Immersion for Glass-Fibre Polymer Composite Laminates with Thermoset & Thermoplastic Infusible Resins

Anthony Comer — Mon, 11 May 2020 22:24:02 +0200

Glass-fibre reinforced polymer (GRP) composite materials are the most widely adopted amongst fibrereinforced polymer composites globally, with approximately 1 million tons produced annually in the EU alone. GRP’s find very wide use and application in a number of industrial sectors (e.g. land & waterborne transport1 , marine, construction) due to their excellent balance between good performance and low cost compared to fibre reinforced polymers utilising other commercially available fibres (e.g. carbon, aramid). Particularly in marine applications, durability of composites and their ability to exhibit unchanged performance and stability in a marine context and environment is a crucial factor in order to select the most appropriate combination of polymer matrix and reinforcement. Ideally, a composite would retain its mechanical and thermo-mechanical profile even when exposed to a marine environment for extended periods. In this work, we conducted an extensive comparative study of the water absorption behavior and retention of mechanical properties of a group of GRP composite laminates manufactured with a range of infusible thermosetting and thermoplastics resins. Sample preparation for water immersion studies was according to ASTM D5229. This study was part of a comprehensive down-selection of commercially available resins in terms of their suitability for shipbuilding applications, as part of the EU H2020 project FIBRESHIP2 . All laminates were manufactured by Vacuum-Assisted Resin Transfer Moulding (VARTM; the most relevant manufacturing technique in shipbuilding) with a range of state-of-the-art thermosetting resins (Urethane acrylate Crestapol 1210, Epoxy SR1125, Bio-epoxy Supersap CLR, Phenolic Cellobond J2027X) and a novel infusible acrylic thermoplastic resin (Acrylic Elium 150). The reinforcement of choice for each laminate was a unidirectional glass fabric of 996 gsm. A selection of relevant properties of the laminates with different resin systems is presented in this paper including fibre volume fraction, apparent interlaminar shear strength (dry and wet condition), flexural strength (dry and wet condition) and flexural modulus (dry and wet condition). For the wet condition, samples were immersed in distilled water for 28 days at 35 oC (wet state) in accordance with classification society guidelines. The quality of the laminates (void content, fibre-matrix adhesion) was examined by scanning electron microscopy on fracture surfaces. The effects of water absorption on the microstructure, mechanical, thermal & thermomechanical properties of the laminates were studied. The average water absorption percentage varied across all resins systems from 0.19 to 1.37% in the interlaminar-shear specimens, and from 0.25 to 1.59% in the flexure specimens. The phenolic laminate was the one absorbing most water in both cases but the mechanical properties were relatively unaffected. Fibre volume fraction was in the range 0.56 to 0.6 for all of the laminates. The majority of the tested GRP laminates showed good retention of their flexural properties and interlaminar shear strength under the testing conditions. The laminate that appeared to be most adversely affected was the infusible thermoplastic, showing a reduction in flexural strength and interlaminar shear strength of 17.3% and 37.5%, respectively (in comparison to the dry state values). However, the water absorption for the Elium 150 was not excessive, ranging from 0.40 to 0.42% for the ILSS and flexure samples, respectively.

References: 1 Summerscales J, Marine applications of advanced fibre reinforced composites, Woodhead Publishing, Cambridge, 2016 2 H2020 project FIBRESHIP, funded by the European Commission under GA 723360 (www.fibreship.eu)

Experimental Investigation on the effect of water ingress on the flexural and interlaminar properties of glass/vinylester composite for marine applications

Anthony Comer — Mon, 11 May 2020 21:44:02 +0200

Fibre-reinforced polymer (FRP) composite materials find increasing acceptance and application in a number of transport sectors due to their lightweight nature, which provides a significant advantage in terms of lower fuel consumption and greenhouse gas emissions, in line with relevant EU directives. Particularly in the marine industry, FRPs are currently dominating the manufacture of vessels up to 50 m in length, with liquid resin infusion (LRI) being the most frequently used manufacturing technique and vacuum-assisted resin transfer moulding (VARTM) in particular the most widely adopted LRI variant. The wide-scale adoption of FRPs into large marine structures is often hindered by the lack of guidelines available for qualification of these materials by classification societies, particularly in relation to fire safety. FIBRESHIP is a Horizon 2020 funded EU project that aims to further the use of FRPs in long-length ship construction by addressing this issue in addition to tackling numerous other challenges associated with manufacturing FRP composite ships. It is important to characterise fully the performance of new commercially available marine resin systems as a potential candidates for selection in composite ship construction. This needs to be done under a wide range of environmental conditions, as durability of composites and their ability to exhibit unchanged performance and stability in a marine context and environment is a crucial factor in their selection. Ideally, a composite would retain its mechanical and thermo-mechanical profile even when exposed to a marine environment for extended periods. During the service life of marine composites (typically 20-25 years), water uptake is inevitable. This may cause plasticization, swelling, matrix hydrolysis or debonding of fibres from the matrix. As a result, the mechanical and thermal properties degrade accordingly, and the service life is shortened. This work represents part of a selection process for materials for the construction of long-length ships from FRPs and focuses on a commercially available fireretardant composite system (SAERTEX LEO®). The aim of the study is, therefore, to compare the flexural (ISO 14125) and interlaminar shear (ISO 14130) properties of the SAERTEX LEO® composite system under dry conditions and under “wet” conditions where the specimens have been immersed in deionised water at 35°C for varying durations (28 days, two months, three months). The flexural and interlaminar properties will also be assessed after soaking for 28 days, two months and three months followed by a drying process to remove all ingressed water. This will give an indication of the reversibility of the effects of water ingress and highlight the point at which permanent alteration of the properties begins to occur. Unidirectional laminates are manufactured by VARTM using the Saertex LEO Glass/Vinyl ester system (the system includes a fire-retardant gel coat, however the gel coat was not applied for the purpose of obtaining the mechanical properties of the FRP component of the system only). Dynamic Mechanical Analysis (DMA) is performed to establish that the laminates have been fully cured and fracture mechanisms are examined using scanning electron microscopy.

Experimental Investigation on the effect of thickness on the flexural properties of glass/vinyl-ester composite laminates for marine applications

Anthony Comer — Mon, 11 May 2020 21:21:02 +0200

Fibre-reinforced polymer (FRP) composite materials find increasing acceptance and application in a

number of transport sectors (aviation, land & waterborne transport) due to their lightweight nature, which

provides a significant advantage in terms of lower fuel consumption and greenhouse gas emissions, in line

with relevant EU directives. Particularly in waterborne transport and shipbuilding, FRP composites are

currently dominating the manufacture of vessels up to 50 m in length, with liquid resin infusion (LRI) being

the most frequently used manufacturing technique and vacuum-assisted resin transfer moulding (VARTM)

in particular the most widely adopted LRI variant. The wide-scale adoption of FRP composites into large

marine structures is often hindered by the lack of guidelines available for qualification of these materials

by classification societies. FIBRESHIP is a Horizon 2020 funded EU project that aims to further the use of

FRP composites in long-length ship construction by addressing this issue in addition to tackling numerous

other challenges associated with manufacturing FRP composite ships. This work represents part of a

selection process for materials for the construction of long-length ships from FRP composites and focuses

on a commercially available fire-retardant composite system (SAERTEX LEO®). As part of the selection

procedure for these materials, material properties, such as the flexural strength and modulus, are obtained

using coupon-sized test-pieces and are subsequently used as the basis for numerical models for ship design.

However, the actual material that is used in the final ship structure is significantly thicker than the coupons

from which the original material properties were derived. Additionally, the scale of the manufacturing

process of laminates for the extraction of coupons is drastically different to that of the manufacturing

process of a ship’s hull. The aim of the study is, therefore, to compare the flexural properties obtained from

a thin monolithic laminate manufactured in a research laboratory (University of Limerick, Ireland) to the

flexural properties obtained from a thick monolithic laminate representative of the thickness of a ship hull

manufactured in a shipyard (iXBlue Division H2x, Marseille, France) using the same material under

investigation. This will give an indication of how representative the thin test coupons are of the material

manufactured by the shipyards at the thickness used in the final structure. Unidirectional laminates are

manufactured in both the research and shipyard facilities by VARTM using the Saertex LEO Glass/Vinyl

ester system (the system includes a fire-retardant gel coat, however the gel coat was not applied for the

purpose of obtaining the mechanical properties of the FRP component of the system only). Dynamic

Mechanical Analysis (DMA) is performed on specimens from the thin and thick laminates to establish that

the laminates have been fully cured. Three-point-bend tests in accordance with ISO 14125 are performed

on 0° and 90° specimens extracted from thin and thick laminates. Another set of 0° and 90° specimens

extracted from thin and thick laminates are tested according to Bureau Veritas guidelines (NR456) in order

to investigate the comparison between the properties obtained using both methods. Fracture mechanisms in

thick and thin specimens are examined using scanning electron microscopy.

Mechanical characterization of polymer composite materials for long length ships

Amit Kumar Haldar — Mon, 11 May 2020 12:20:04 +0200

In the marine industry, Fibre-reinforced polymers (FRP) are currently dominating the manufacture of vessels up to 50m in length, with liquid resin infusion (LRI) being the most frequently used manufacturing technique, of which vacuum-assisted liquid resin infusion is the most widely adopted LRI variant. However, current regulations restrict the use of composite materials in vessels over 50m in length. FIBRESHIP is a Horizon 2020 funded EU project that aims to further the use of FRPs in vessels over 50m in length by addressing the regulatory restrictions and the numerous other challenges associated with manufacturing long-length FRP composite ships. The mechanical performance of new commercially available composite material constituents as potential candidates for selection in composite ship construction is central to this work. This paper provides an overview of selected work performed as part of the FIBRESHIP project in terms of evaluating various mechanical properties of selected laminates under dry and wet conditions. The laminates were immersed in seawater at 35°C for durations of one to three months. Three-point bend and interlaminar shear strength tests were undertaken in order to investigate the change in the mechanical properties of composite laminates subject to immersion. Finally, tested specimens were observed using micro-computed tomography (μCT) to evaluate the failure morphology.

Continuum approach to computational multiscale modeling of propagating fracture

María Jesús Samper — Mon, 11 May 2020 11:14:48 +0200

A new approach to two-scale modeling of propagating fracture, based on computational homogenization (FE2), is presented. The specific features of the approach are: a) a continuum setting for representation of the fracture at both scales based on the Continuum Strong Discontinuity Approach (CSDA), and b) the use, for the considered non-smooth (discontinuous) problem, of the same computational homogenization framework than for classical smooth cases. As a key issue, the approach retrieves a characteristic length computed at the lower scale, which is exported to the upper one and used therein as a regularization parameter for a propagating strong discontinuity kinematics. This guarantees the correct transfer of fracture energy between scales and the proper dissipation at the upper scale. Representative simulations show that the resulting formulation provides consistent results, which are objective with respect to, both, size and bias of the upper-scale mesh, and with respect to the size of the lower-scale RVE/failure cell, as well as the capability to model propagating cracks at the upper scale, in combination with crack-path-field and strain injection techniques. The continuum character of the approach confers to the formulation a minimal invasive character, with respect to standard procedures for multi-scale computational homogenization

Continuum approach to computational multi-scale modeling of fracture

María Jesús Samper — Mon, 11 May 2020 10:57:02 +0200

This paper presents a FE2 multi-scale framework for numerical modeling of the structural failure of heterogeneous quasi-brittle materials. The model is assessed by application to cementitious materials. Using the Continuum Strong Discontinuity Approach (CSD), innovative numerical tools, such as strain injection and crack path field techniques, provide a robust, and mesh-size, mesh-bias and RVE-size objective, procedure to model crack onset and propagation at the macro-scale

High-performance model reduction techniques in computational multiscale homogenization

María Jesús Samper — Mon, 11 May 2020 09:57:00 +0200

A novel model-order reduction technique for the solution of the fine-scale equilibrium problem appearing in computational homogenization is presented. The reduced set of empirical shape functions is obtained using a partitioned version that accounts for the elastic/inelastic character of the solution - of the Proper Orthogonal Decomposition (POD). On the other hand, it is shown that the standard approach of replacing the nonaffine term by an interpolant constructed using only POD modes leads to ill-posed formulations. We demonstrate that this ill-posedness can be avoided by enriching the approximation space with the span of the gradient of the empirical shape functions. Furthermore, interpolation points are chosen guided, not only by accuracy requirements, but also by stability considerations. The approach is assessed in the homogenization of a highly complex porous metal material. Computed results show that computational complexity is independent of the size and geometrical complexity of the Representative Volume Element. The speedup factor is over three orders of magnitude - as compared with finite element analysis - whereas the maximum error in stresses is less than 10%. A novel model-order reduction technique for the solution of the fine-scale equilibrium problem appearing in computational homogenization is presented. The reduced set of empirical shape functions is obtained using a partitioned version that accounts for the elastic/inelastic character of the solution - of the Proper Orthogonal Decomposition (POD). On the other hand, it is shown that the standard approach of replacing the nonaffine term by an interpolant constructed using only POD modes leads to ill-posed formulations. We demonstrate that this ill-posedness can be avoided by enriching the approximation space with the span of the gradient of the empirical shape functions. Furthermore, interpolation points are chosen guided, not only by accuracy requirements, but also by stability considerations. The approach is assessed in the homogenization of a highly complex porous metal material. Computed results show that computational complexity is independent of the size and geometrical complexity of the Representative Volume Element. The speedup factor is over three orders of magnitude - as compared with finite element analysis - whereas the maximum error in stresses is less than 10%.

Relación entre el Trastorno por Déficit de Atención e Hiperactividad y síntomas afectivos en estudiantes universitarios de un programa de Medicina de Manizales Caldas

Sandra Cañón — Sun, 10 May 2020 21:14:02 +0200

Introduction: Previous studies have reported high comorbidity between ADHD and anxiety and depressive spectrum disorders, however, few studies have been carried out on the subject in young university students. The present study evaluated the relationship between ADHD and the presence of depressive and anxious symptoms in university students from a medicine program in Manizales.

Materials and methods: Observational, analytical, cross-sectional research. Sample: 362 university students from a medical program. Instruments used: CAGE questionnaire, Beck's depression and anxiety scale, DSM-5 checklist for ADHD.

Results: Average age of 20.32 years. Of the total population 3.3% of a history of ADHD, of which 1.9% of treatment received. 56.36% symptoms current inattentive symptoms, 4.4% impulsive and hyperactive symptoms and 4.4% combined subtype. The absence of depressive symptoms according to the Beck scale is associated with symptoms of absence of inactive symptoms (Pvalor = 0.000), impulsive and hyperactive symptoms (Pvalor = 0.000) and mixed symptoms according to DMS-5 (Pvalor = 0.000). As the severity of depressive symptoms increases, inattentive symptoms, impulsive and hyperactive symptoms, and DSM-5 mixed symptoms increase (Pvalor = 0.000). A significant association was found between the presence of anxious symptoms and inattentive, impulsive, hyperactive and mixed symptoms according to the DSM-5 (Pvalor = 0.000).

Conclusions: ADHD symptoms are associated with the presence of anxious symptoms and depressive symptoms in university students of a medical program

Virtual Laboratories as a Strategy for Teaching Improvement in Math Sciences and Engineering in Bolivia

Francisco J. Triveno Vargas — Sun, 10 May 2020 15:38:05 +0200

STEM education is a strategy based on four disciplines (science, technology, engineering and mathematics), integrated in an innovative interdisciplinary approach. Although, the concept of STEM education is more relevant today, the discussion of a teaching model with special attention in the four subjects aforementioned began in the early 2000s. Taking into account this context, the strategy presented in this paper has been disseminated in Bolivia's main universities for the last five years. A country that has not yet managed to associate basic disciplines such as calculus, matrix algebra, and/or differential equations to solve problems of an applicative nature, that is, to establish the link between theory and practice. To establish the connection, it is necessary to deduce differential equations associated with practical problems; solve these equations with numerical methods, appeal to the simulation concept to later introduce programming languages like ''Python/VPython'' to build virtual laboratories. The classical problem addressed for this purpose is the satellite of two degrees of freedom.

Mechanical evaluation of a fire retardant through-thickness reinforced sandwich structure for marine applications

Anthony Comer — Fri, 08 May 2020 22:59:02 +0200

One of the main restrictions in adopting polymer composite materials for primary and secondary structural applications in marine vessels over 50 meters in length is concerns regarding fire retardancy and also a lack of design guidelines in general. The aim of this study is to evaluate the edgewise compression strength and core-shear strength of a fire retardant sandwich structure reinforced with through thickness composite ‘bridges’ under ambient conditions. These properties are important for structural components subjected to in-plane loads such as bulkheads. Core shear strength of the through-thickness reinforced sandwich far exceeded non-reinforced sandwich. However, edgewise compression strength and stiffness of the reinforced case was found to be similar to the unreinforced case.

Crack-path field and strain-injection techniques in computational modeling of propagating material failure

María Jesús Samper — Fri, 08 May 2020 15:50:28 +0200

The work presents two new numerical techniques devised for modeling propagating material failure, i.e. cracks in fracture mechanics or slip-lines in soil mechanics. The first one is termed crack-path-field technique and is conceived for the identification of the path of those cracks, or slip-lines, represented by strain-localization based solutions of the material failure problem. The second one is termed strain-injection, and consists of a procedure to insert, during specific stages of the simulation and in selected areas of the domain of analysis, goal oriented specific strain fields via mixed finite element formulations. In the approach, a first injection, of elemental constant strain modes (CSM) in quadrilaterals, is used, in combination of the crack-path-field technique, for obtaining reliable information that anticipates the position of the crack-path. Based on this information, in a subsequent stage, a discontinuous displacement mode (DDM) is efficiently injected, ensuring the required continuity of the crack-path across sides of contiguous elements. Combination of both techniques results in an efficient and robust procedure based on the staggered resolution of the crack-path-field and the mechanical failure problems. It provides the classical advantages of the "intra-elemental" methods for capturing complex propagating displacement discontinuities in coarse meshes, as E-FEM or X-FEM methods, with the non-code-invasive character of the crack-path-field technique. Numerical representative simulations of a wide range of benchmarks, in terms of the type of material and the failure problem, show the broad applicability, accuracy and robustness of the proposed methodology. The finite element code used for the simulations is open-source and available at http://www.cimne.com/compdesmat/.

PFEM-based modeling of industrial granular flows

María Jesús Samper — Fri, 08 May 2020 15:44:07 +0200

The potential of numerical methods for the solution and optimization of industrial granular flows problems is widely accepted by the industries of this field, the challenge being to promote effectively their industrial practice. In this paper, we attempt to make an exploratory step in this regard by using a numerical model based on continuous mechanics and on the so-called Particle Finite Element Method (PFEM). This goal is achieved by focusing two specific industrial applications in mining industry and pellet manufacturing: silo discharge and calculation of power draw in tumbling mills. Both examples are representative of variations on the granular material mechanical response—varying from a stagnant configuration to a flow condition. The silo discharge is validated using the experimental data, collected on a full-scale flat bottomed cylindrical silo. The simulation is conducted with the aim of characterizing and understanding the correlation between flow patterns and pressures for concentric discharges. In the second example, the potential of PFEM as a numerical tool to track the positions of the particles inside the drum is analyzed. Pressures and wall pressures distribution are also studied. The power draw is also computed and validated against experiments in which the power is plotted in terms of the rotational speed of the drum.

Computational modeling of high-performance steel fiber reinforced concrete using a micromorphic approach

María Jesús Samper — Fri, 08 May 2020 15:35:41 +0200

A finite element methodology for simulating the failure of high performance fiber reinforced concrete composites (HPFRC), with arbitrarily oriented short fibers, is presented. The composite material model is based on a micromorphic approach. Using the framework provided by this theory, the body configuration space is described through two kinematical descriptors. At the structural level, the displacement field represents the standard kinematical descriptor. Additionally, a morphological kinematical descriptor, the micromorphic field, is introduced. It describes the fiber–matrix relative displacement, or slipping mechanism of the bond, observed at the mesoscale level. In the first part of this paper, we summarize the model formulation of the micromorphic approach presented in a previous work by the authors. In the second part, and as the main contribution of the paper, we address specific issues related to the numerical aspects involved in the computational implementation of the model. The developed numerical procedure is based on a mixed finite element technique. The number of dofs per node changes according with the number of fiber bundles simulated in the composite. Then, a specific solution scheme is proposed to solve the variable number of unknowns in the discrete model. The HPFRC composite model takes into account the important effects produced by concrete fracture. A procedure for simulating quasi-brittle fracture is introduced into the model and is described in the paper. The present numerical methodology is assessed by simulating a selected set of experimental tests which proves its viability and accuracy to capture a number of mechanical phenomenon interacting at the macro- and mesoscale and leading to failure of HPFRC composites.

A micromorphic model for steel fiber reinforced concrete

María Jesús Samper — Fri, 08 May 2020 15:29:41 +0200

A new formulation to model the mechanical behavior of high performance fiber reinforced cement composites with arbitrarily oriented short fibers is presented. The formulation can be considered as a two scale approach, in which the macroscopic model, at the structural level, takes into account the mesostructural phenomenon associated with the fiber-matrix interface bond/slip process. This phenomenon is contemplated by including, in the macroscopic description, a micromorphic field representing the relative fiber-cement displacement. Then, the theoretical framework, from which the governing equations of the problem are derived, can be assimilated to a specific case of the Material Multifield Theory. The balance equation derived for this model, connecting the micro stresses with the micromorphic forces, has a physical meaning related with the fiber-matrix bond slip mechanism. Differently to previous procedures in the literature, addressed to model fiber reinforced composites, where this equation has been added as an additional independent ingredient of the methodology, in the present approach it arises as a natural result derived from the multifield theory. Every component of the composite is defined with a specific free energy and constitutive relation. The mixture theory is adopted to define the overall free energy of the composite, which is assumed to be homogeneously constituted, in the sense that every infinitesimal volume is occupied by all the components in a proportion given by the corresponding volume fraction. The numerical model is assessed by means of a selected set of experiments that prove the viability of the present approach. A new formulation to model the mechanical behavior of high performance fiber reinforced cement composites with arbitrarily oriented short fibers is presented. The formulation can be considered as a two scale approach, in which the macroscopic model, at the structural level, takes into account the mesostructural phenomenon associated with the fiber–matrix interface bond/slip process. This phenomenon is contemplated by including, in the macroscopic description, a micromorphic field representing the relative fiber–cement displacement. Then, the theoretical framework, from which the governing equations of the problem are derived, can be assimilated to a specific case of the material multifield theory. The balance equation derived for this model, connecting the micro stresses with the micromorphic forces, has a physical meaning related with the fiber–matrix bond slip mechanism. Differently to previous procedures in the literature, addressed to model fiber reinforced composites, where this equation has been added as an additional independent ingredient of the methodology, in the present approach it arises as a natural result derived from the multifield theory. Every component of the composite is defined with a specific free energy and constitutive relation. The mixture theory is adopted to define the overall free energy of the composite, which is assumed to be homogeneously constituted, in the sense that every infinitesimal volume is occupied by all the components in a proportion given by the corresponding volume fraction. The numerical model is assessed by means of a selected set of experiments that prove the viability of the present approach

Strain localization, strong discontinuities and material fracture: matches and mismatches

María Jesús Samper — Fri, 08 May 2020 15:20:49 +0200

The work focuses on the connections of strain localization modeling of material failure and discrete fracture mechanics. It is an attempt to give an answer to the old question of whether the finite element solutions of material failure problems based on strain localization techniques, using standard continuum stress–strain constitutive models equipped with strain softening, have physical sense as solutions of de-cohesive fracture mechanics problems. Based on some well-established links of the Continuum Strong Discontinuity Approach (CSDA) to material failure and cohesive fracture mechanics, some objective indicators are proposed to assess the quality of strain localization results. These indicators are simply derived on the basis of the inelastic strain distribution provided by the strain localization solutions and the direction of propagation of the localization band. They can be computed without knowledge of the exact fracture mechanics solution of the problem, and used as error indicators in a large variety of material failure situations. The proposed indicators are assessed, by means of their application to evaluation of a number of strain localization solutions of benchmark problems. Issues as the influence of the mesh structure and alignment, type of constitutive model and considered finite element techniques are examined. On the light of the obtained results, classical flaws in strain localization solutions, as mesh bias dependence and stress-locking are reinterpreted

On the contact domain method: a comparison of penalty and Lagrange multiplier implementations

María Jesús Samper — Fri, 08 May 2020 14:55:14 +0200

This work focuses on the assessment of the relative performance of the so-called contact domain method, using either the Lagrange multiplier or the penalty strategies. The mathematical formulation of the contact domain method and the imposition of the contact constraints using a stabilized Lagrange multiplier method are taken from the seminal work (as cited later), whereas the penalty based implementation is firstly described here. Although both methods result into equivalent formulations, except for the difference in the constraint imposition strategy, in the Lagrange multiplier method the constraints are enforced using a stabilized formulation based on an interior penalty method, which results into a different estimation of the contact forces compared to the penalty method. Several numerical examples are solved to assess certain numerical intricacies of the two implementations. The results show that both methods perform similarly as one increases the value of the penalty parameter or decreases the value of the stabilization factor (in case of the Lagrange multiplier method). However there seems to exist a clear advantage in using the Lagrange multiplier based strategy in a few critical situations, where the penalty method fails to produce convincing results due to excessive penetration.

A finite strain, finite band method for modeling ductile fracture

María Jesús Samper — Fri, 08 May 2020 14:07:54 +0200

We present a finite deformation generalization of the finite thickness embedded discontinuity formulation presented in our previous paper [A.E. Huespe, A. Needleman, J. Oliver, P.J. Sánchez, A finite thickness band method for ductile fracture analysis, Int. J. Plasticity 25 (2009) 2349–2365]. In this framework the transition from a weak discontinuity to a strong discontinuity can occur using a single constitutive relation which is of importance in a range of applications, in particular ductile fracture, where localization typically precedes the creation of new free surface. An embedded weak discontinuity is introduced when the loss of ellipticity condition is met. The resulting localized deformation band is given a specified thickness which introduces a length scale thus providing a regularization of the post-localization response. The methodology is illustrated through several example problems emphasizing finite deformation effects including the development of a cup-cone failure in round bar tension

Finite element modelling of ejection cracks in powder metallurgy die compaction processes: case study

María Jesús Samper — Fri, 08 May 2020 13:56:09 +0200

To date, the modelling of crack formation during the ejection stage in powder metallurgy die compaction processes has fallen outside the scope of conventional finite element studies on this process. In this paper, we attempt to make an exploratory step in this regard by presenting a case study that exemplifies how crack simulations can be harnessed to solve real powder metallurgy manufacturing problems. The part subjected to the study is a multilevel adapter whose design to production process proved problematic to the manufacturer, to the point that simplifications in the geometry of the part were to be made. The goal here is, through finite element simulations, to clarify the reasons behind the difficulties in ejecting free crack compacts, to understand the connection of such difficulties with the geometry modifications introduced in the design and to make recommendations on the prevention of similar problems in other situations

A robust approach to model densification and crack formation in powder compaction processes

María Jesús Samper — Fri, 08 May 2020 13:07:00 +0200

This paper deals with the question of how to efficiently integrate a constitutive model that describes the densification of powders and the potential formation of cracks in Powder Metallurgy (P/M) cold compaction processes. The analyzed model is a large strain, elastoplastic model of the Drucker–Prager/Cap type, refined to cover also the prediction of crack formation, and featuring non-conventional elements such as a density-dependent Von Mises yield surface; a parabolic plastic potential function for the Drucker–Prager envelope; and a softening law whose softening modulus is dependent on the level of densification. The employed integration procedure is a non-conventional hybrid or IMPLicit–EXplicit (IMPL-EX) scheme, whose essence is to solve explicitly for some variables and implicitly for others, with the peculiarity of the ‘explicit’ variables being but extrapolated values of the same quantities computed, at previous time steps, by means of a fully implicit scheme. The return-mapping equations stemming from this implicit scheme are solved using an unconditionally convergent, fractional step method-based iterative procedure. The performance of the IMPL-EX integration algorithm is critically assessed in two different situations: the densification of a cylindrical specimen, and the fracture process in a diametral compression test. Results obtained show conclusively that the proposed hybrid integration strategy offers an efficient solution to the trade-off between robustness and computational time requirements

On the proper characterization of tooling motions and initial conditions in powder die compaction modeling

María Jesús Samper — Fri, 08 May 2020 12:59:51 +0200

The present paper is concerned with the finite element modeling of Powder Metallurgy (P/M) cold die compaction process. Rather than on material constitutive theories or on numerical algorithmic issues, attention is confined exclusively on an scarcely addressed issue in the P/M modeling literature: the proper characterization of the boundary (tooling motions) and initial conditions of the problem. A case study of the compaction of an axially symmetric multilevel adapter in an advanced CNC press machine is used to convey the relevance of the accurate representation of these input data in the quality of model predictions. It is shown that unawareness or deliberate simplification of apparently insignificant details in this respect may cause errors far overshadowing those introduced by deficiencies in either the constitutive model or in the corresponding algorithmic solution procedure. The discussion of this case study serves also to provide useful modeling guidelines; illustrate frequent difficulties, as the unavailability of some information when guessing starting conditions; and reveal subtle, yet relevant for modeling purposes, technical details of advanced CNC press machines.

Flow regime analyses during the filling stage in powder metallurgy processes: experimental study and numerical modelling

María Jesús Samper — Fri, 08 May 2020 12:02:26 +0200

Experimental and numerical studies of powder flow during the die filling stage in powder metallurgy cold compaction processes are presented. An experimental setting consisting of a horizontal pneumatically activated shoe, a vertical die and high-speed video system has been designed. The experiments show the existence of three flow regimes: continuous, transitory and discrete, which are identified in terms of the particle size, the morphology and the speed of the shoe. In the continuous regime the powder flows in a progressive manner but in the discrete one some perturbations appear as a consequence of a shear band formation that forms discrete avalanches. A numerical model, based on a ratedependent constitutive model, via a flow formulation, and in the framework of the particle finite element method (PFEM) is also proposed. For the purpose of this study, the use of the PFEM assumes that the powder can be modelled as a continuous medium. The model, provided with the corresponding characterisation of the parameters, is able to capture the two fundamental phenomena observed during the filling process: (1) the irreversibility of most of the deformation experienced by the material and (2) the quick dissipation of the potential gravitatory energy of the granular system through the inter-particle friction processes, modelled by the plastic dissipation associated with the material model. Experimental and numerical results have been compared in order to study the viability of the proposed model

Simulación numérica del proceso de fractura en concreto reforzado mediante la metodología de discontinuidades fuertes de continuo. Parte II: Aplicación a páneles sometidos a cortante

María Jesús Samper — Fri, 08 May 2020 11:31:03 +0200

En este trabajo se presentan los resultados de la simulación numérica del proceso de fractura en páneles de concreto reforzado sometidos a cortante, utilizando un modelo basado en la metodología de discontinuidades fuertes de continuo (CSDA) y la teoría de mezclas. La CSDA describe la localzación de la deformación y la formación de una discontinuidad asociada con la aparición de una fisura. En cambio, la teoría de mezclas representa el comportamiento de un material compuesto, constituido por una matriz de concreto simple y uno o dos paquetes de barras largas de acero de refuerzo. El comportamiento del concreto simple y el acero se representan mediante un modelo de daño bidimensional y un modelo de plasticidad unidimensional, respectivamente. El modelo se implementa en el método de los elementos finitos considerando estado plano de esfuerzos, deformaciones infinitesimales y cargas estáticas. Se simularon tres páneles reforzados en una o en dos direcciones, los cuales estaban y sometidos principalmente a fuerzas cortantes. Los resultados de la simulación numérica, como la respuesta estructural y el patrón de fisuración, fueron satisfactorios. // The numerical simulation results of the fracture process in reinforced concrete shear panels are presented in this work. The simulation used a model based on the continuum strong discontinuity approach (CSDA) and mixing theory. CSDA describes strain localization and formation of discontinuity associated with the appearance of a crack. On the other hand, mixing theory represents composite material behaviour which is formed by a simple concrete matrix and one or two bundles of long reinforcement bars. The behaviour of simple concrete and steel is represented by a two-dimensional damage model and one-dimensional plasticity model, respectively. The model has been implemented in the finite element method which considers plane stress, infinitesimal strain and static loads. Three panels are simulated, reinforced in one or two ways; they are mainly subjected to shear forces. The numerical simulation results as well as structural response and cracking patterns were satisfactory.

Simulación numérica del proceso de fractura en concreto reforzado mediante la metodología de discontinuidades fuertes de continuo. Parte I: formulación

María Jesús Samper — Fri, 08 May 2020 11:24:40 +0200

En general, las estructuras de concreto reforzado como vigas, columnas y muros están conformadas por entramados complejos de barras de acero embebidas en una matriz de concreto, las cuales exhiben múltiples fisuras ante la aplicación de cargas externas elevadas. Este artículo presenta la formulación de un modelo numérico cuyo objetivo es describir el proceso de fractura en elementos de concreto reforzado a partir de la fracción volumétrica del concreto y del acero. El modelo utiliza un campo enriquecido de la deformación para describir la formación y propagación de fisuras en un material compuesto, tal como lo establecen la metodología de discontinuidades fuertes de continuo y la teoría de mezclas. El material compuesto está constituido por una matriz de concreto y uno o dos paquetes de barras de acero ortogonales entre sí. El acero y el concreto se representan con modelos de plasticidad unidimensional y de daño escalar con tracción y compresión diferenciada, respectivamente. La acción pasador y los efectos del deslizamiento entre las barras y la matriz, se describen con modelos adicionales que relacionan el esfuerzo y la deformación de los materiales componentes. Finalmente, se concluye que el modelo propuesto se puede implementar con facilidad en el método de los elementos finitos, dado que permanecen muchas características del procedimiento numérico no lineal convencional. Asimismo, el modelo permite analizar el problema en la escala macroscópica, lo cual elude la construcción de mallas de elementos finitos de cada material componente y de sus efectos de interacción, reduciendo así el costo computacional. // Reinforced concrete structures generally refers to beams, columns and walls which are constituted by complex lattices of steel bars embedded in a concrete matrix, exhibiting multiple cracks due to high external loads. This paper presents the formulation of a numerical model aimed at describing the fracture process in reinforced concrete, from the volumetric ratio of concrete and steel. Crack formation and propagation in a composite material is described in the model by an enhanced strain field, such as that established in the continuum strong discontinuity approach and mixture theory. The composite material is constituted by a concrete matrix and one or two steel bar orthogonal packages. The steel and concrete are represented by a one-dimensional plasticity model and a scalar damage model having different tension and compression strength, respectively. The dowel action and the bond-slip effects between the bars and the matrix are described with additional models relating component material stress and strain. It is concluded that the proposed model can easily be implemented in the finite element method, due to several conventional nonlinear numerical process characteristics which remain. The model would also allow the problem to be analysed at macroscopic scale, thereby avoiding a finite element mesh having to be constructed for each component material and its interaction effects and reducing computational costs

Mesoscopic model to simulate the mechanical behavior of reinforced concrete members affected by corrosion

María Jesús Samper — Fri, 08 May 2020 11:07:01 +0200

In this contribution, a finite element methodology devised to simulate the structural deterioration of corroded reinforced concrete members is presented. The proposed numerical strategy has the ability to reproduce many of the well-known (undesirable) mechanical effects induced by corrosion processes in the embedded steel bars, as for example: expansion of the reinforcements due to the corrosion product accumulation, damage and cracking patterns distribution in the surrounding concrete, degradation of steel–concrete bond stress transfer, net area reduction in the reinforcements and, mainly, the influence of all these mentioned mechanisms on the structural load carrying capacity predictions.

At the numerical level, each component of the RC structure is represented by means of a suitable FE formulation. For the concrete, a cohesive model based on the Continuum Strong Discontinuity Approach (CSDA) is used. Steel bars are modeled by means of an elasto-plastic constitutive relation. The interface is simulated using contact-friction elements, with the friction degradation as a function of the degree of corrosion attack. Two different (and coupled) mesoscopic analyzes are considered in order to describe the main physical phenomena that govern the problem: (i) an analysis at the cross section level and (ii) an analysis at the structural member level.

The resultant mechanical model can be used to simulate generalized reinforcement corrosion. Experimental and previous numerical results, obtained from the available literature, are used to validate the proposed strategy

Numerical modeling of crack formation in powder forming processes

María Jesús Samper — Fri, 08 May 2020 10:56:08 +0200

This paper presents a constitutive model describing the mechanical behavior of metal powders during (uniaxial) cold die compaction processes, placing special emphasis on the modeling of cracks formed during the ejection stage. The constitutive relationships are derived within the general framework of rate-independent, isotropic, finite strain elastoplasticity. The yield condition is determined by three surfaces intersecting non-smoothly in stress space, namely, an elliptical cap and the classical Von Mises and Drucker–Prager yield surfaces. The distinct irreversible processes are described in terms of two internal variables: an internal hardening variable, associated with accumulated compressive (plastic) strains, and an internal softening variable, linked with accumulated (plastic) shear strains. Motivated by both numerical and physical reasons, a parabolic plastic potential function is introduced to characterize the plastic flow on the linear Drucker–Prager failure surface. A thermodynamically consistent calibration procedure is employed to relate the softening modulus to fracture energy values obtained experimentally on Distaloy AE powder specimens. The predictive capability of the constitutive model is checked by simulating three representative cases: a diametral compression test, the ejection of an over-densified thin cylindrical part and the compaction of an axially symmetric multilevel part in an advanced CNC press machine. These simulations demonstrate the ability of the model to detect evidence of macroscopic cracks, clarify and provide reasons for the formation of such cracks, and evaluate, at least qualitatively, the influence of variations in the input variables on their propagation through the green compact

A 3D Frictionless Contact Domain Method for Large Deformation Problems

María Jesús Samper — Fri, 08 May 2020 09:52:17 +0200

This work describes a three-dimensional contact domain method for large deformation frictionless contact problems. Theoretical basis and numerical aspects of this specific contact method are given in [Oliver, Hartmann, Cante, Weyler and Hernández (2009)] and [Hartmann, Oliver, Weyler, Cante and Hernández (2009)] for two-dimensional, large deformation frictional contact problems. In this method, in contrast to many other contact formulations, the necessary contact constraints are formulated on a so-called contact domain, which can be interpreted as a fictive intermediate region connecting the potential contact surfaces of the deformable bodies. This contact domain has the same dimension as the contacting bodies. It will be endowed with a displacement field, interpolated from the displacements at the contact surfaces and will be subdivided into a non-overlapping set of contact patches, where the contact constraints will be applied. For the enforcement of these contact constraints a stabilized Lagrange multiplier method is used, which allows the condensation of the introduced Lagrange multipliers, leading to a purely displacement driven problem.

On the numerical resolution of the discontinuous material bifurcation problem

María Jesús Samper — Thu, 07 May 2020 17:11:56 +0200

The work focuses on the numerical resolution of the discontinuous material bifurcation problem as a relevant ingredient in computational material failure mechanics. The problem consists of finding the conditions for the strain localization onset in terms of the so-called bifurcation time, localization directions and localization modes. A numerical algorithm, based on the iterative resolution of a coupled eigenvalue problem in terms of the localization tensor, is proposed for such purpose. The algorithm is shown to be always convergent to the exact solution for the symmetric case (major and minor symmetries of the tangent constitutive operator). In the unsymmetric case (only minor symmetries), the solution is no longer exact, although it is shown that using the symmetric part of the localization tensor in the proposed algorithms provides enough accurate solutions for most of cases. Numerical examples illustrate the benefits of the proposed methodology in terms of accuracy and savings in the computational cost associated with the problem.

A contact domain method for large deformation frictional contact problems. Part 2: Numerical aspects

María Jesús Samper — Thu, 07 May 2020 16:59:30 +0200

This second part of the work describes the numerical aspects of the developed contact domain method for large deformation frictional contact problems. The theoretical basis of this contact method is detailed in the first part of this work. Starting from this, the present contribution focuses on describing important algorithmic details that go along with the finite element implementation for two-dimensional problems. Important aspects are the construction of the contact domain mesh, via a constraint Delaunay triangulation, the linearization of the discretized contact contributions and some important technical aspects about the extrapolation procedure used for the predictive active set strategy. Finally a set of numerical examples is presented to demonstrate the performance of the developed contact strategy. Demanding static and dynamic contact problems in the context of large deformations, including frictional effects as well as self contact, show the wide applicability and the robustness of the proposed method

A finite thickness band method for ductile fracture analysis

María Jesús Samper — Thu, 07 May 2020 16:27:46 +0200

We present a finite element method with a finite thickness embedded weak discontinuity to analyze ductile fracture problems. The formulation is restricted to small geometry changes. The material response is characterized by a constitutive relation for a progressively cavitating elastic–plastic solid. As voids nucleate, grow and coalesce, the stiffness of the material degrades. An embedded weak discontinuity is introduced when the condition for loss of ellipticity is met. The resulting localized deformation band is given a specified thickness which introduces a length scale thus providing a regularization of the post-localization response. Also since the constitutive relation for a progressively cavitation solid is used inside the band in the post-localization regime, the traction-opening relation across the band depends on the stress triaxiality. The methodology is illustrated through several example problems including mode I crack growth and localization and failure in notched bars. Various finite element meshes and values of the thickness of the localization band are used in the calculations to illustrate the convergence with mesh refinement and the dependence on the value chosen for the localization band thickness

A contact domain method for large deformation frictional contact problems. Part 1: Theoretical basis

María Jesús Samper — Thu, 07 May 2020 16:17:04 +0200

In the first part of this work, the theoretical basis of a frictional contact domain method for two-dimensional large deformation problems is presented. Most of the existing contact formulations impose the contact constraints on the boundary of one of the contacting bodies, which necessitates the projection of certain quantities from one contacting surface onto the other. In this work, the contact constraints are formulated on a so-called contact domain, which has the same dimension as the contacting bodies. This contact domain can be interpreted as a fictive intermediate region connecting the potential contact surfaces of the deformable bodies. The introduced contact domain is subdivided into a non-overlapping set of patches and is endowed with a displacement field, interpolated from the displacements at the contact surfaces. This leads to a contact formulation that is based on dimensionless, strain-like measures for the normal and tangential gaps and that exactly passes the contact patch test. In addition, the contact constraints are enforced using a stabilized Lagrange multiplier formulation based on an interior penalty method (Nitsche method). This allows the condensation of the introduced Lagrange multipliers and leads to a purely displacement driven problem. An active set strategy, based on the concept of effective gaps as entities suitable for smooth extrapolation, is used for determining the active normal stick and slip patches of the contact domain.

A mixture theory based method for three-dimensional modeling of reinforced concrete members with embedded crack finite elements

María Jesús Samper — Thu, 07 May 2020 16:03:40 +0200

The paper presents a methodology to model three-dimensional reinforced concrete members by means of embedded discontinuity elements based on the Continuum Strong Discontinuous Approach (CSDA). Mixture theory concepts are used to model reinforced concrete as a 3D composite material constituted of concrete with long fibers (rebars) bundles oriented in different directions embedded in it. The effects of the rebars are modeled by phenomenological constitutive models devised to reproduce the axial non-linear behavior, as well as the bond-slip and dowel action. The paper presents the constitutive models assumed for the components and the compatibility conditions chosen to constitute the composite. Numerical analyses of existing experimental reinforced concrete members are presented, illustrating the applicability of the proposed methodology.

On some topics for the numerical simulation of ductile fracture

María Jesús Samper — Thu, 07 May 2020 15:57:29 +0200

In this work, we analyze some aspects of the macroscopic Gurson–Tvergaard–Needleman (GTN) constitutive model when it is addressed to solve ductile fracture problems by means of numerical simulations: (i) The analytical solutions of the material discontinuous bifurcation problem is performed. Closed and exact formulas are obtained. The so determined critical conditions and the developed strain localization mode are afterward studied and compared in crack growth problems. Even when this methodology of analysis is rather standard at the present, the conclusions drawn from this study differ significantly from that obtained with a similar analysis in quasi-brittle fracture problems. (ii) A new very robust numerical integration method for the GTN model, namely the Impl–Ex Method, is proposed. It is a low computational cost algorithm, equivalent to a linear problem per each integration step, with a reasonable precision for engineering purposes. Its accuracy and convergence rate is assessed by means of an error study applied to a ductile fracture test simulation. (iii) A detailed analysis of a plane strain ductile crack growth problem is performed in a material containing two size-scale of voids. In the analysis, particular attention is given to the mesh size dependence and to the coalescence of the larger void

An implicit/explicit integration scheme to increase computability of non-linear material and contact/friction problems

María Jesús Samper — Thu, 07 May 2020 15:45:42 +0200

An implicit/explicit integration scheme for non-linear constitutive models is presented. It aims at providing additional computability to those solid mechanics problems were robustness is an important issue, i.e. material failure models equipped with strain softening, soft materials, contact-friction models, etc., although it can also provide important advantages, in terms of computational cost, with respect to purely implicit integration schemes. The proposed scheme is presented based on general families of constitutive models (continuum damage and elasto-plasticity) and its properties, in terms of robustness and accuracy, are analytically derived and computationally assessed by means of numerical simulations. An adaptive time stepping algorithm, based on a priori control of the committed error and the application of the proposed scheme to contact/friction interfaces are also presented.

Three-dimensional analysis of reinforced concrete members via embedded discontinuity finite elements

María Jesús Samper — Thu, 07 May 2020 14:52:59 +0200

This paper presents a methodology to model three-dimensional reinforced concrete members by means of embedded discontinuity elements based on the Continuous Strong Discontinuous Approach (CSDA). Mixture theory concepts are used to model reinforced concrete as a 3D composite material constituted of concrete with long fiber bundles (rebars) oriented in different directions embedded in it. The effects of the rebars are provided by phenomenological constitutive models designed to reproduce the axial non-linear behavior, as well as bond-slip and dowel action. This paper is focused on the constitutive models assumed for the components and the compatibility conditions chosen to constitute the composite. Numerical analyses of existing experimental reinforced concrete members are presented, illustrating the applicability of the methodology.

Two-dimensional modelling of material failure in reinforced concrete by means of a continuum strong discontinuity approach

María Jesús Samper — Thu, 07 May 2020 14:25:37 +0200

The paper presents a new methodology to model material failure, in two-dimensional reinforced concrete members, using the Continuum Strong Discontinuity Approach (CSDA). The mixture theory is used as the methodological approach to model reinforced concrete as a composite material, constituted by a plain concrete matrix reinforced with two embedded orthogonal long fiber bundles (rebars). Matrix failure is modeled on the basis of a continuum damage model, equipped with strain softening, whereas the rebars effects are modeled by means of phenomenological constitutive models devised to reproduce the axial non-linear behavior, as well as the bond–slip and dowel effects. The proposed methodology extends the fundamental ingredients of the standard Strong Discontinuity Approach, and the embedded discontinuity finite element formulations, in homogeneous materials, to matrix/fiber composite materials, as reinforced concrete. The specific aspects of the material failure modeling for those composites are also addressed. A number of available experimental tests are reproduced in order to illustrate the feasibility of the proposed methodology

Vulnerability analysis of large concrete dams using the continuum strong discontinuity approach and neural networks

María Jesús Samper — Thu, 07 May 2020 14:13:50 +0200

Probabilistic analysis is an emerging field of structural engineering which is very significant in structures of great importance like dams, nuclear reactors etc. In this work a Neural Networks (NN) based Monte Carlo Simulation (MCS) procedure is proposed for the vulnerability analysis of large concrete dams, in conjunction with a non-linear finite element analysis for the prediction of the bearing capacity of the Dam using the Continuum Strong Discontinuity Approach. The use of NN was motivated by the approximate concepts inherent in vulnerability analysis and the time consuming repeated analyses required for MCS. The Rprop algorithm is implemented for training the NN utilizing available information generated from selected non-linear analyses. The trained NN is then used in the context of a MCS procedure to compute the peak load of the structure due to different sets of basic random variables leading to close prediction of the probability of failure. This way it is made possible to obtain rigorous estimates of the probability of failure and the fragility curves for the Scalere (Italy) dam for various predefined damage levels and various flood scenarios. The uncertain properties (modeled as random variables) considered, for both test examples, are the Young’s modulus, the Poisson’s ratio, the tensile strength and the specific fracture energy of the concrete.

Stabilized mixed finite elements with embedded discontinuities for shear bans modelling

María Jesús Samper — Thu, 07 May 2020 14:01:00 +0200

A stabilized mixed finite element with elemental embedded strong discontinuities for shear band modeling is presented. The discrete constitutive model, representing the cohesive forces acting across the shear band, is derived from a rate-independent J(2) plastic continuum material model with strain softening, by using a projection-type procedure determined by the Continuum-Strong Discontinuity Approach. The numerical examples emphasize the increase of the numerical solution accuracy obtained with the present strategy as compared with alternative procedures using linear triangles

Stability and robustness issues in numerical modeling of material failure with the strong discontinuity approach

María Jesús Samper — Thu, 07 May 2020 13:49:51 +0200

Robustness and stability of the Continuum Strong Discontinuity Approach (CSDA) to material failure are addressed. After identification of lack of symmetry of the finite element formulation and material softening in the constitutive model as possible causes of loss of robustness, two remedies are proposed: (1) the use of an specific symmetric version of the elementary enriched (E-FEM) finite element with embedded discontinuities and (2) a new implicit–explicit integration of the internal variable, in the constitutive model, which renders the tangent constitutive algorithmic operator positive definite and constant. The combination of both developments leads to finite element formulations with constant, in the time step, and non-singular tangent structural stiffness, allowing dramatic improvements in terms of robustness and computational costs. After assessing the convergence and stability properties of the new strategies, three-dimensional numerical simulations of failure problems illustrate the performance of the proposed procedures

A comparative study on finite elements for capturing strong discontinuities: E-FEM vs X-FEM

María Jesús Samper — Thu, 07 May 2020 13:14:03 +0200

A comparative study on finite elements for capturing strong discontinuities by means of elemental (E-FEM) or nodal enrichments (X-FEM) is presented. Based on the same constitutive model (continuum damage) and linear elements (triangles and tetrahedra) optimized implementations of both types of enrichments in the same non-linear code are tested for a representative set of 2D and 3D crack propagation examples. It is shown that both methods provide the same qualitative and quantitative results for enough refined meshes. For the performed tests, E-FEM exhibited, in general, a higher accuracy, mostly for coarse meshes, whereas, convergence rate with mesh refinement, which is super-linear, showed slightly higher for X-FEM. As for the computational costs for single crack modelling X-FEM showed, depending on the case, from 1.1 to about 2.5 times more expensive than E-FEM. For multiple cracks, the computational cost of E-FEM keeps constant, whereas the cost associated to X-FEM increases linearly with the number of modelled cracks

On the fracture models determined by the continuum-strong discontinuity approach

María Jesús Samper — Thu, 07 May 2020 13:08:00 +0200

The paper focuses on the Continuum Strong Discontinuity Approach (CSDA) to fracture mechanics, and the traction-separation cohesive laws induced from continuum dissipative models as their projections onto the failure interface. They are compared with the cohesive laws commonly used for the fracture simulation in quasi-brittle materials, typically concrete. Emphasis is placed in the analysis of the mechanical stress-strain states induced by the CSDA into the fracture process zone: first when the damage mechanism is initiated and, after, when the cohesive model determines the crack response. The influence of the material parameters, particularly the fracture energy and the initial continuum softening modulus, in the obtained phenomenological responses is also analyzed. Representative numerical solutions of fracture problems are finally presented

On numerical simulation of powder compaction process: powder transfer modelling and characterisation

María Jesús Samper — Thu, 07 May 2020 12:39:29 +0200

A numerical model for the powder transfer stage in powder metallurgy cold compaction processes, and the corresponding material characterisation procedure, are proposed. They have been designed on the basis of the following requirements: robust and consistent computational mechanics ingredients, reliability of the obtained results for practical processes in powder metallurgy and industrial viability in the sense that characterisation of any mixture doesn't require either much effort or much time of the enduser. The starting point is a previously developed numerical model for powder compaction, formulated in terms of the large plastic deformation theory, which requires calibration of four parameters controlling the evolution of the yield surface. This calibration, which had been successfully carried out in the past in a range of moderate to high densities, is now extended to very low densities in order to make numerical simulations able to deal with compaction processes involving relevant powder transfer stages. To circumvent the difficulties inherent to direct measurements of very low powder densities, a simple apparatus, which allows studying the powder motion in the chamber, has been designed to provide an indirect way of calibration. On this basis, a set of calibration experiments is proposed. The proposed methodology appears to be simple and industrially viable. Although in the work it is applied to the specific constitutive model used by the authors, it also appears available for other families of constitutive models for powder compaction. As a relevant result, this would allow the same constitutive model to be used, via only the appropriated material characterisation, for simulation of densification in powder transfer stages as well as in pure compaction stages

Theoretical and computational issues in modelling material failure in strong discontinuity scenarios

María Jesús Samper — Thu, 07 May 2020 11:25:00 +0200

The paper deals with several aspects related to numerical modelling of material failure in strong discontinuity settings: (a) the onset and development of local material failure in terms of continuum constitutive models equipped with strain softening. Closed forms formulas for the solutions of the discontinuous material bifurcation problem are given for a class of those models; (b) finite elements with embedded discontinuities: nodal and elemental enrichments families are formulated in the continuum strong discontinuity approach (CSDA); (c) instability treatment: a discrete viscous perturbation method at the failure surfaces is presented as a way to substantially improve the robustness of the numerical simulations

Continuum approach to material failure in strong discontinuity settings

María Jesús Samper — Thu, 07 May 2020 11:11:53 +0200

The paper focuses the numerical modelling of material failure in a strong discontinuity setting using a continuum format. Displacement discontinuities, like fractures, cracks, slip lines, etc., are modelled in a strong discontinuity approach, enriched by a transition from weak to strong discontinuities to get an appropriate representation of the fracture process zone. The introduction of the strong discontinuity kinematics automatically projects any standard dissipative constitutive model, equipped with strain softening, into a discrete traction–separation law that is fulfilled at the discontinuity interface. Numerical issues like a global discontinuity tracking algorithm via a heat conduction-like problem are also presented. Some representative numerical simulations illustrate the performance of the presented approach

Computational modeling of cracking of concrete in strong discontinuity settings

María Jesús Samper — Thu, 07 May 2020 11:03:10 +0200

The paper is devoted to present the Continuum Strong Discontinuity Approach (CSDA) and to examine its capabilities for modeling cracking of concrete. After introducing the main ingredients of the CSDA, an isotropic continuum damage model, which distinguishes tension and compression states, is used to implicitly induce a projected traction separation-law that rules the cracking phenomena. Criteria for onset and propagation of material failure and specific finite elements with embedded discontinuities are also briefly sketched. Finally, some representative numerical simulations of cracking, in plain and reinforced concrete specimens, using the CSDA are presented.

On the strong discontinuity approach in finite deformation settings

María Jesús Samper — Thu, 07 May 2020 10:55:31 +0200

Taking the strong discontinuity approach as a framework for modelling displacement discontinuities and strain localization phenomena, this work extends previous results in infinitesimal strain settings to finite deformation scenarios. By means of the strong discontinuity analysis, and taking isotropic damage models as target continuum (stress–strain) constitutive equation, projected discrete (tractions–displacement jumps) constitutive models are derived, together with the strong discontinuity conditions that restrict the stress states at the discontinuous regime. A variable bandwidth model, to automatically induce those strong discontinuity conditions, and a discontinuous bifurcation procedure, to determine the initiation and propagation of the discontinuity, are briefly sketched. The large strain counterpart of a non-symmetric finite element with embedded discontinuities, frequently considered in the strong discontinuity approach for infinitesimal strains, is then presented. Finally, some numerical experiments display the theoretical issues, and emphasize the role of the large strain kinematics in the obtained results

A study on finite elements for capturing strong discontinuities

María Jesús Samper — Thu, 07 May 2020 10:46:39 +0200

The work focuses on the presently existing families of finite elements with embedded discontinuities and explores the possibilities of obtaining symmetric statically consistent finite elements that alleviate the stress-locking problem. For this purpose, mixed (reduced integration) and assumed enhanced strain techniques are applied to the basic symmetric four-noded element. Numerical simulations show the effectiveness of the proposed measures.

From continuum mechanics to fracture mechanics: the strong discontinuity approach

María Jesús Samper — Thu, 07 May 2020 09:39:26 +0200

The paper deals with the strong discontinuity approach and shows the links with the decohesive fracture mechanics provided by that approach. On the basis of 1D continuum damage models it is shown that, by introducing some few ingredients like the strong discontinuity kinematics, discrete constitutive models (traction vs. displacement jumps) are automatically induced. For the general 2D–3D cases it is shown that the weak discontinuity concept is an additional ingredient, necessary in order to fulfill the strong discontinuity conditions, which allows to establish additional links with the fracture process zone concept. Also classical fracture mechanics properties as the fracture energy are related to the continuum model properties in a straightforward manner