A study of the factors influencing air removal in out-of-autoclave processing of composites

Resumo

Nowadays, manufacturing of composite parts using out-of-Autoclave (OoA) processes is gaining importance because the reduction of investments and recurring cost. 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 to use 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 thesis is to understand and explain the void formation mechanisms, spatial distribution and transport mechanisms during the consolidation of OoA prepregs using vacuum bag only methods. The thesis will include 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). The ability of XCT to classify porosity sources, inside of the plies (intraply) and within adjacent plies (interplay), and its evolution during the cure cycle was demonstrated in this work. This project is focuses on the commercial M56 prepreg, a Hexcel product marketed for OoA curing of composite aircraft structures. M56 resin is designed for low void content following vacuum bag only conditions. In addition, new interleaved prepreg configurations containing thermoplastic particles and a veil system will be studied. The interleave technologies are intended for use in aircraft primary structures, where damage tolerance is mandatory. As it will be described in the thesis, the interleaved laminates (particles or veils) presented additional difficulties for air removal during curing, since they hamper the air removal mechanisms and might lead to large volume fraction of voids in the composite parts. A major point in HLU laminates is related with debulking methods. Debulking is known as preconsolidation of the fresh laminate by means of the application of vacuum cycles to enhance the adhesion between the plies. An incorrect debulking will produce an excess of air entrapped within the plies in the fresh state, interply porosity, that lead to an important number of voids after curing. In HLU laminates, most of the interply voids are adequately transported to the evacuation channels, but many of them remained arrested. To avoid the high void content detected in HLU, the best alternative is to use automated layup methods, and in particular, automated fiber placement (AFP). In this case, the heated rollers used to lay the fiber tows exert the required pressure to eliminate, or almost, air entrapped between the plies producing the highest quality after curing. However, laminates prepared by AFP containing interleaves by either particles or veils showed some inefficiencies in the voids extraction mechanisms as compared with standard one. A lot of the studies presented in this thesis were carried out using a sequential inspection procedure. To this end, panels were partially cured stopping the cycle at different points. Such semicured panels were then inspected to address the evolution of the void content in terms of contents and spatial distribution. However, these studies lack on information of the dynamics of the process and 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.