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 aerospacernquality parts is performed under low pressures, which makes it more susceptible tornvoid formation and growth as compared to high pressure autoclave processing.rnThus, OOA prepregs are intentionally partially impregnated with resin, with therngoal to distribute the resin such that a completely connected network of emptyrnchannels is formed in the initial material. This network serves as pathways forrnevacuation of gases entrapped in the laminate before consolidation and cure. Thisrnwork investigates how mechanically entrapped air can be removed from partiallyrnimpregnated OOA prepreg laminates before oven curing. First, a model of voidrnevacuation time is derived to estimate the time necessary to apply vacuum tornremove air from within the laminate before placing it in the oven. Next, a flowrnvisualization technique is presented where the resin film of the partiallyrnimpregnated OOA prepreg is pressed into the fabric, while recording the resin flowrnon the dry side. This quantifies the degree of resin impregnation with processingrntime. A relationship between degree of resin impregnation and gas permeability isrnpresented which influences the evacuation time necessary to remove air from thernsystem. A large panel (~1.2m long) was fabricated in which low and highrnevacuation times were employed based on the model physics. The void content wasrnquantified along the length via sectioning/polishing/image analysis and shown tornhave a void content of ~1%. The results of this study should prove useful to developrnoptimal vacuum application times and temperature and pressure cycles for voidrnreduction and removal during processing of prepregs.The entrapment of voids is one of the most significant challenges ofrncomposites processing; a small percentage of voids will degrade mechanicalrnproperties by 10-15% [1]. The aerospace industry is motivated to replace aluminumrnand titanium structures with composites because of the additional weight benefits;rnhowever, composite structures of less than 1% void content are difficult to achievernoutside the autoclave processing method. The elevated atmospheric pressure insidernthe autoclave compacts prepreg laminates to high fiber volume fraction and reducesrnvoid size. The drawback is that the autoclave process has high capital costs, whichrnbecome prohibitively exorbitant as the size of the structure becomes larger.rnTo counter these cost challenges, the Out-of-Autoclave (OOA) process isrnbecoming attractive as it can process under vacuum pressure only in an oven tornproduce low void content large composite structures. OOA prepregs differ fromrnautoclave prepregs in that resin only partially impregnates the fiber architecture -rnleaving dry, unimpregnated cross-sections to serve as ‘air pathways’ for removingrnair from the laminate, which if not removed will be entrapped between plies duringrncure and increase the void content in the composite. Additionally, any entrappedrngases have a chance to migrate into air pathways during the application of vacuum,rnwhere they can be evacuated from the laminate easily. Some examples of partiallyrnimpregnated prepregs are schematically shown in Figure 1. The initial architecturernof these prepregs will play an important role in the formation of air and gasrnevacuation pathways which will influence the final void content in the fabricatedrncomposites by OAA process.The resin configuration has been shown to play an important role in airrnevacuation from partially impregnated prepreg laminates. Prepregs with low initialrnresin impregnation have less resistance to gas flow [2], making them suitable forrnevacuating gas from large structures. When fabricating sandwich structures withrnOOA prepregs, air pathways through the thickness have been shown to be highlyrnbeneficial for extracting air from the core [3,4]. This constraint makes UD prepregsrn(with no through thickness pathways) difficult to implement in sandwich structuresrn[5].rnThis study focuses on in-plane air evacuation from partially impregnatedrnprepreg laminates. The goal is to demonstrate the importance of vacuum hold timernprior to oven curing. Required vacuum hold time becomes longer as the part lengthrnincreases and as the degree of resin impregnation increases. Pressure decay raternscales with the square of the part length and linearly with the prepreg’srnpermeability, but the degree of resin impregnation has a non-linear effect onrnpermeability, and hence, the pre-cure cycle vacuum hold time.