Abstract

An experimental procedure has been developed to characterize the interlaminar resistance of composite materials, specifically prepregs of vinylester-epoxy nature, based on the determination of their critical energy by the realization of consecutive low energy impacts on test specimens of the studied material.The impact sequence was performed by means of a certified impactor, based on a pendulum system, and the system used to measure the integrity of the material after each impact consisted in the NDT optical pulsed thermographic technique. In the present study, a series of 8 specimens made of unidirectional glass fiber with different arrangements and specific treatments, both on the reinforcement and the resin matrix itself, were used to carry out its characterization, as it is an asymmetric fabric based on a surface of continuous fibers and the other surface with glass veil finish. The NDT technique used has enabled, on the one hand, to quickly and efficiently identify the breakage of the materials and to determine the critical energy, and on the other hand, it has made possible to precisely measure the area of the delaminated zones and to determine their localization in depth through the graphic representation by means of projection in RGB channels.The results obtained in the characterization using the proposed procedure have been validated with fracture energy tests in mode I and II, obtaining satisfactory levels of correlation between both procedures.

Abstract

In order to obtain Mode II fracture toughness of unidirectional fiber composite materials, the End-Loaded Split (ELS) has enjoyed great success because of its higher stability. As a result, crack growth initiation and propagation values are obtained, the latter being especially used in interlaminar progressive damage models. The propagation values must be obtained when the Failure Process Zone (FPZ) has completely developed. In delamination tests of unidirectional composites, such as those contemplated in ISO 15114, the dimension of the FPZ can be neglected against the rest of the dimensions of the specimen, whereby the self-similar crack growth is reached just after the initiation of the crack propagation. However, in adhesive joints, the dimension of the FPZ increases significantly due to the plasticity of the adhesives. And even it can be larger than the entire length reserved for crack propagation. When this occurs, only initiation values are obtained, always inferior to the values of propagation, being these values too conservative to reproduce the behaviour of the joint. In this paper we present a methodology to define the configuration of the ELS test and the specimen dimensions required to obtain propagation results in adhesive joints. The methodology defined is based on two criteria that take into account the length of the FPZ and the stability of the test. As a result, a domain of validity of the test is obtained, which, for a certain type of adhesive and specimen configuration, it indicated is there will be propagation or not.

Abstract

Nowadays, the use of composite materials in primary structures has considerably increased, in particular in what refers to primary structures and bonded joints. Thus, numerical tools and experimental tests have to be implemented in order to understand failure modes and critical loads in this kind of structures. Usually, bonded joints quality is tested based on the interlaminar fracture test DCB (G1c). Nevertheless, these tests have some limitations. The aim of the present study is to create a feasible and accurate numerical model which allows the quality of a bonded joint as function of the peeling load to be determined. The idea is then an actual alternative to offer in-situ test by means of a transportable and easy to use device. In this study, a research analyzing two tests able to identify the quality of a bonded joints, as an alternative to the interlaminar fracture test, based on a classic peeling test on carbon laminates, is presented. The numerical simulation is based on a Finite Element Analysis using the commercial software Abaqus®. Onset and propagation of adhesive damage is taken into account by means of a cohesive zone model. A virtual test of the peeling test is done and its behavior is compared with the experimental results from an extensive laboratory test campaign. Numerical results will allow us to verify the feasibility to compute the value of G1c from experimental results.

Abstract

Fracture toughness is a critical property of composites, especially in the case of materials manufactured by liquid composite moulding processes because of the brittleness of the thermoset polymeric matrix. The use of thermoplastic interleaving veils can improve the fracture behaviour affecting only slightly other mechanical properties. Also, the veils can affect the manufacturability of the composites. This work presents a study of the influence of commercial and homemade veils on the manufacturability process and mechanical properties of carbon fibre reinforced composites. Results obtained show a significant improvement of fracture toughness without affecting manufactubarility.

Abstract

In this work, the influence of the loading rate on the mode I interlaminar fracture toughness of carbon fiber laminates manufactured with prepregs of the second (IM7/8552) and third generation (IM7/M91) was investigated. The study has been carried out using to loading rates ranges, namely, quasi-static between 0.5 and 450mm/min, and dynamic between 100 and 2000mm/s. The latter tests were performed using a Gleeble 3800GTC hydraulic testing system. The dynamic tests were carried out using the WIF methodology (wedge insert fracture) by inserting at high speed a cylinder between the arms of the specimen. Two strain gauges were used to measure the maximum flexural deformation of the loading arms allowing the estimation of the opening force of the specimen to infer the dynamic fracture energy. The results showed that the IM7 / M91 laminate was sensitive to the speed of solicitation presenting a maximum value of the fracture energy for 500mm/s being the toughness more than twice than the one measured under quasi-static conditions.

Abstract

While fiber reinforced polymer (FRP) composites are widely utilized in structural components due to their favorable mechanical properties, delamination between reinforcing plies remains a major problem, weakening the composite structure and limiting more widespread applications of FRPs. Carbon nanotubes (CNTs) carry the promise of enhancing this poor out-of-plane mechanical performance, although their integration has been challenging. In this work, macroscopic CNT veils with controlled nano-meso structure were drawn from the gas-phase using a semi-industrial process and then integrated into woven carbon fiber/epoxy matrix composites utilizing a facile and scalable approach. Interlaminar fracture toughness (ILFT) of the resulting composites was determined in Mode-I (opening mode) test. Additionally, crack propagation and interlaminar toughening mechanisms were systematically analyzed by means of optical microscope, SEM, and Raman analysis. The results showed that mode I ILFT was improved as much as 60% when interleaving as-received fluffy CNT veils and also revealed that interlaminar crossing between CNT veil/CF interfaces is of paramount importance in toughening mechanism.

Abstract

In this study, the interlaminar fracture toughness of 3D printed continuous carbon fibre-reinforced polyamide was characterized under Mode I and II loadings. 3D printing of fracture test samples presents difficulties, mainly shrinkage and excessive warping. To minimize these effects, two solutions were developed. The results show that the Mode I interlaminar fracture behaviour of the printed samples is similar to that obtained by hot compression moulding. In mode II, however, propagation is not effective due to the type of microstructure of the 3D printed material. The hot pressing of the printed material reduces the fracture toughness under Mode I, but significantly improves the fracture toughness under Mode II. The causes of these behaviours are discussed in the paper.

Abstract

Fiber bridging is a mechanism that may significantly alter the fracture behavior of composite laminates, adhesively bonded laminates, welded laminates, and co-consolidated laminates. It is therefore quite important for the finite element to take that mechanism into consideration. Such models have been developed for thermosetting laminates; however, this is not the case for thermoplastic laminates and thermoplastic joints. In the present work, a numerical model based on the cohesive zone modelling (CZM) approach has been developed to simulate mixed-mode fracture of co-consolidated thermoplastic laminates by considering fiber bridging. A modified traction separation law of tri-linear form has been developed by superimposing the bi-linear behaviors of the matrix and fibers. Initially, the data from mode I (DCB) and mode II (ENF) fracture toughness tests were used to construct the R-curves of the joints in the opening and sliding directions. The aforementioned curves were embedded into the numerical models through a user-defined material subroutine developed in the LS-Dyna FE code, in order to extract the fiber bridging law directly from the simulation results. The model was used to simulate fracture of a Single-Lap-Shear (SLS) specimen in which a considerable amount of fiber bridging was observed on the fracture area. The numerical results show that the developed model presented improved accuracy in comparison to the CZM employing the bilinear traction-separation law.

Abstract

Debonding between plies, or delamination, is a critical failure mechanism for laminated composite materials and has been analysed in several scientific investigation. Usually in real applications delamination initiates and propagates between interface layers with different reinforcement orientation. However, most of the experimental works are carried out using unidirectional specimens for determining the interlaminar fracture toughness because in laboratory conditions it is difficult to propagate the crack between multidirectional interface layers without other failure mechanisms that invalidate the test. Among these failure mechanisms, crack plane migration and crack branching at several planes are the most common. In this communication it will be detailed and analyzed the experimental characterization of the interlaminar fracture toughness in multidirectional interfaces of 3D-printed composite materials is detailed, including the manufacturing process of the Double Cantilever Beam (DCB) specimens that warranty crack propagation under pure mode I loading and without plane migration. Finally, a quantitative comparison is carried out between multidirectional interlaminar fracture toughness and the unidirectional one and a fractography analysis is reported.