Abstract

One of the main problems associated with the manufacture of composite materials by liquid moulding (injection, infusion, etc.) is the generation of porosity. The optimization of the properties of the composites passes through a better understanding of the mechanisms of pore formation and transport. This pore formation is associated with a non-homogeneity of resin flow during the manufacturing process as a consequence of the double-scale porosity present in textile preforms. In the channels between bundles of fibers, the resin flows controlled by the pressure gradient and its viscosity. However, within the fiber bundles, the resin flows as a result of the capillary pressure. As a consequence, if the viscous and capillary forces are not balanced, non-homogeneous local flow occurs and the formation of pores, either between the fiber bundles, or internally to the bundles is produced.In this work a systematic study of the spontaneous wetting process of an E glass fiber bundle through wicking tests, using several fluids with different properties, viscosity, surface tension and contact angle is presented. The visualization of the front position of the resin flow at microscopic scale, as well as the formation of the menisci between fibers has been performed using X-ray tomography techniques using synchrotron radiation. The reconstructed volumes have made it possible to obtain detailed quantitative information on the geometry of the menisci which occur as a consequence of the surface tension operating at such scale. These characterization techniques constitute a fundamental tool for the optimization of the processing of composite materials buy liquid moulding.

Abstract

The state-of-the-art additive manufacturing technology of continuous carbon fibre reinforced polymers (CFRP) is based on fused filament fabrication (FFF), which has a great potential for reproducing robust and complex geometries. However, 3D printed CFRP present relatively poor mechanical properties in comparison to traditional composites. This work analyses post-processing temperature effects on the microstructure and on the interlaminar mechanical properties of the printed CFRP. Post-processed pieces at 150°C (exceeding matrix glass transition) showed a reduction of porosity by 80% and improved mechanical properties by 250% with respect to unprocessed pieces.

Abstract

We derive analytical relationships between the spatial covariance of the (natural) logarithm of hydraulic conductivity (K) and that of representative soil particle sizes and porosity. The latter quantities can be directly measured during routine sedimentological analyses of soil samples and provide a way of incorporating K estimates into groundwater flow models at a relatively modest experimental cost. Here we rely on widely used empirical formulations requiring measurements of representative particle diameters and, in some cases, of medium porosity. We derive exact formulations relating the spatial covariance of these quantities and K and present workable approximations on the basis of perturbation methods. Our formulations provide a direct link between key geostatistical descriptors of sedimentological and hydraulic parameters of heterogeneous aquifers which can be employed in classical estimation and simulation procedures. The approach and theoretical results are tested on an extensive data set comprising 411 particle size curves collected at 12 boreholes in a small-scale alluvial aquifer.

Abstract

Abstract

This study explores the behavior of blended mortars of low reactive bottom ashes (BA) and ordinary Portland cement (OPC) in an alkaline solution. Mortar mixtures incorporating OPC with different replacement levels (0% to 30% mass) were studied. Isothermal conduction calorimetric analysis was studied and 28-and 60-day mechanical strength values were found. SEM images was used to identifies porosity structures at 28-day. Water absorption was also investigated. The results revealed different behaviors to low (OPC 2.5 and OPC 5) and moderated (OPC 10 and OPC 30) OPC content. Increase in percentage of Portland accelerated hydration kinetics. There is a second peak formation for moderated OPC content, associated with C-A-S-H gel formation. The partial replacement of bottom ash by OPC tend to reduce the absorption. There is a slower initial water absorption to low OPC content. This behavior is due the higher unreacted BA content, that works as a filler. Otherwise, the increase of water absorption for OPC content is due to the coexistence of Portland cement hydrates and alkali activated reactions.

Abstract

This study aim at understanding the void formation, transport mechanisms and porosity evolution during OoA consolidation process. The laminates were conformed by stacking and compacting of prepregs by two different technologies: manual (Hand lay-up, HLU) and automatic (Automated fiber placement, AFP). Subsequently the laminates were cured by the vacuum bag only (VBO) technique in an industrial oven at atmospheric pressure. The porosity characterization was performed using ultrasound and x-ray tomography technologies at different stages of the cure cycle. Ultrasonic inspections showed that the porosity is heterogeneously distributed, where the higher volumetric fraction of pores is located in the center of the laminate. X-ray tomography provided three-dimensional (3D) information of samples of the laminates and it allowed a quantitative and qualitative study of the pores within the tows of fibers and the porosity located between the layers of prepregs during the curing process. The prepregs designed for out of autoclave are partially impregnated in such a way to provide air evacuation channels in the fiber tows. The porosity of these channels was efficiently extracted and a volumetric fraction of pores entrapped between layers were evacuated during the curing process. The behavior of the evacuation channels and the porosity evolution for both stacking technologies were compared. An analytical model of the evolution of a pore was also developed.

Abstract

The addition of carbon fibers is the main alternative used for the manufacturing of conductive cements since, at a relative low price, provides a good combination of structural and functional properties. There are many different applications for this type of system, such as the manufacturing of heating elements or anodes for the cathode protection methodology. Most of the studies in this area are focused to the improvement of the dispersion of carbon fibers in the cement matrix. This objective is validated by measures of resistivity. According to this objective, there is no a universal method to verify these measures, but many alternatives can be found on the literature: measures with two or four electrodes, direct or alternating current testing…, impeding comparative analyses. This research offers the impedance methodology, already used for the characterization of common cement pastes without any reinforcements, as an option for the analysis of conductive cements. In this study, different variables are included, such as the type and quantity of dispersants to create a homogeneous mixture. The adjustment of the obtained results, following an equivalent electric model for this kind of systems, allowed us to determine the values of conductivity of different mixtures. With the use of this technique, it is possible to identify the polarization phenomena associated to the carbon fibers, being of interest in order to validate the dispersion of the fibers.