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

Interleaving high fibre areal weight veils, typically used to improve the interlaminar fracture toughness and impact tolerance of composite laminates, impairs their tensile and compressive properties. We characterized the mechanical response of a thin-ply quasi-isotropic baseline laminate and laminates interleaved with two types of light (4 g/m2) co-polyamide veil: tension, compression, mode-I interlaminar fracture toughness, impact and compression after impact. X-ray tomography provided comprehensive understanding into the effect interleaving has on the microstructure, impact resistance and fracture process zone of double cantilever beam specimens.

The veil fibre diameter was the key-parameter controlling the tensile properties (the veil with thinner fibres avoided resin accumulation at the interfaces and left the baseline tensile properties unaffected). The adhesion between veil fibres and resin controlled mode-I interlaminar fracture toughness (the veil with a suspected higher adhesion deflected crack propagation to the surrounding 0° plies, which enhanced mode-I interlaminar fracture toughness by 43%). Both veils reduced pristine compressive strength by up to 9%; however, the veil with thinner fibres improved CAI strength by up to 28% for impact at 14 J.

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

Nowadays, manufacturing of composite parts using out-of-Autoclave (OoA) processes is gaining importance because it allows a reduction of initial investments and recurring costs. Among these technologies, the consolidation of out-of-autoclave prepregs using vacuum bag only technologies (VBO) is proven to provide high-quality composite structural parts without the need of autoclaves.  OoA prepregs are designed to be semi-impregnated creating engineered channels within the tows to evacuate internal air or volatiles generated during the cure cycle. The aim of this work is to understand and explain the void formation mechanisms, their spatial distribution and transport mechanisms during the consolidation of OoA prepregs using vacuum bag only methods. This work includes parallel studies carried out using X-ray computed tomography (XCT) to determine the most important factors for air removal process in laminated composites produced by hand lay-up (HLU) and automated fiber placement (AFP). A big challenge is the accurate monitoring of the evolution of porosity during composite curing, which has not been done so far and even less with a 3D visual technique. This combination of monitoring the dynamics of pore evolution by laboratory XCT and synchrotron tomography (SXCT) at different resolutions during a cure process will provide a profound understanding of the pore evacuation process which is strategic for the further development of OoA materials.