Title: UNIDIRECTIONAL PREPREG FORMAT DESIGN FOR ROBUST, EFFICIENT OUT-OF-AUTOCLAVE PROCESSING
Abstract: Composite structures can be rendered affordable by reducing manufacturing costs associated with equipment and part rejection. Over the past decade, out-of-autoclave/vacuum bag-only (OoA/VBO) processing has been proposed as a low-cost alternative to traditional autoclave cure of high-performance structures. However, to date, VBO methods remain insufficiently robust and efficient. The initial distribution of uncured resin within prepreg plies significantly influences part quality in carbon fiber reinforced polymer (CFRP) laminates fabricated using OoA/VBO processes. Generally, in-plane air evacuation paths formed by reducing the level of impregnation can expedite air removal and ultimately improve part quality relative to fully saturated prepreg plies. Recently, research has also shown that internal and surface defects can further be reduced by designing fabric-based prepreg with optimized through-thickness permeability, achieved through selective deposition of uncured resin on tow overlaps, with underlaps remaining dry. However, since creation of through-thickness evacuation paths relies on the weave pattern of the fabric, this method cannot be directly translated to unidirectional (UD) fiber systems. Here, we introduce UDSCpreg—a prepreg system developed in-house and comprised of a UD fiber bed partially impregnated with discontinuous strips of uncured resin. UDSCpreg is fabricated using a mask-and-press process to customize prepreg format by controlling strip geometry and degree of penetration into the fiber bed. Laminates comprised of UDSCpreg are cured under ideal as well as sub-optimal processing conditions, and the defect contents are compared against a commercial prepreg system (CYCOM 5320-1). Results confirm that the increased through-thickness permeability of UDSCpreg leads to improved part quality and resistance to sub-optimal cure conditions compared to commercial OoA prepregs. Additionally, evidence is offered that minor differences in the degree of uncured resin penetration and the topology of resin strips may influence part quality in UDSCpreg systems (especially the proportion and distribution of surface defects). These results corroborate the assertion that though thickness permeability is critical in the design of robust OoA/VBO prepreg formats and demonstrate that transverse pathways are achievable in UD fiber systems. Looking forward, the development of robust VBO prepreg formats will encourage the transition from autoclave cure of performance critical composites toward manufacturing methods with improved technical, cost, and environmental efficiency.
Authors: William T. Edwards & Steven R. Nutt
Conference: CAMX 2017 –Orlando