Void Entrapment into Air Pathways in Partially Impregnated Prepregs in the Out-of-Autoclave Process

摘要

Out-of-Autoclave (OOA) thermoset prepreg manufacturing of aerospace quality parts is performed under low pressures, which makes it more susceptible to void formation and growth as compared to high pressure autoclave processing. Thus, OOA prepregs are intentionally partially impregnated with resin, with the goal to distribute the resin such that a completely connected network of empty channels is formed in the initial material. This network serves as pathways for evacuation of gases entrapped in the laminate before consolidation and cure. This work investigates how mechanically entrapped air can be removed from partially impregnated OOA prepreg laminates before oven curing. First, a model of void evacuation time is derived to estimate the time necessary to apply vacuum to remove air from within the laminate before placing it in the oven. Next, a flow visualization technique is presented where the resin film of the partially impregnated OOA prepreg is pressed into the fabric, while recording the resin flow on the dry side. This quantifies the degree of resin impregnation with processing time. A relationship between degree of resin impregnation and gas permeability is presented which influences the evacuation time necessary to remove air from the system. A large panel (~1.2m long) was fabricated in which low and high evacuation times were employed based on the model physics. The void content was quantified along the length via sectioning/polishing/image analysis and shown to have a void content of ~1%. The results of this study should prove useful to develop optimal vacuum application times and temperature and pressure cycles for void reduction and removal during processing of prepregs.The entrapment of voids is one of the most significant challenges of composites processing; a small percentage of voids will degrade mechanical properties by 10-15% [1]. The aerospace industry is motivated to replace aluminum and titanium structures with composites because of the additional weight benefits; however, composite structures of less than 1% void content are difficult to achieve outside the autoclave processing method. The elevated atmospheric pressure inside the autoclave compacts prepreg laminates to high fiber volume fraction and reduces void size. The drawback is that the autoclave process has high capital costs, which become prohibitively exorbitant as the size of the structure becomes larger. To counter these cost challenges, the Out-of-Autoclave (OOA) process is becoming attractive as it can process under vacuum pressure only in an oven to produce low void content large composite structures. OOA prepregs differ from autoclave prepregs in that resin only partially impregnates the fiber architecture - leaving dry, unimpregnated cross-sections to serve as ‘air pathways’ for removing air from the laminate, which if not removed will be entrapped between plies during cure and increase the void content in the composite. Additionally, any entrapped gases have a chance to migrate into air pathways during the application of vacuum, where they can be evacuated from the laminate easily. Some examples of partially impregnated prepregs are schematically shown in Figure 1. The initial architecture of these prepregs will play an important role in the formation of air and gas evacuation pathways which will influence the final void content in the fabricated composites by OAA process.The resin configuration has been shown to play an important role in air evacuation from partially impregnated prepreg laminates. Prepregs with low initial resin impregnation have less resistance to gas flow [2], making them suitable for evacuating gas from large structures. When fabricating sandwich structures with OOA prepregs, air pathways through the thickness have been shown to be highly beneficial for extracting air from the core [3,4]. This constraint makes UD prepregs (with no through thickness pathways) difficult t