Development of Vacuum Enhanced Resin Infusion Technology (VERITy) Process for Manufacturing of Primary Aircraft Structures

Abstract

Liquid Composite Moulding (LCM) is the process in which resin is infused into dry fabric and formed in the mould cavity under vacuum/closed moulds. This process has been the workhorse of marine industries for many years primarily due to relatively low cost of production. The advent of new and improved resin systems and suitable reinforcements makes infusion process and its variants more feasible for demanding aerospace applications. Many variations of this process like the SCRIMP (Seemann Composite Resin Infusion Moulding Process), VARTM (Vacuum Assisted Resin Transfer Moulding) have made significant progress. More recently, VARTM process is being explored by many in the industry for manufacture of primary structures for aircrafts. CSIR-NAL too has developed a proprietary infusion process called Vacuum Enhanced Resin Infusion Technology (VERITy) for manufacturing of primary composite components. The major objectives this development were twofold; namely to enhance the effectiveness of infusion process by nearly equaling mechanical properties similar to prepreg parts with fibre volume fraction approaching 60% and making the process cost effective for manufacturing large sized and complex composite components. Industry standard building block approach as per CMH-17 was used for the development of the VERITy process. The activities spanned from manufacturing large number of laminates for evaluating mechanical properties to the development of a large test box for structural testing to demonstrate the processing technology developed. Proper design of experiments and necessary qualification tests were done at each level to understand the nuances of the process. The study included selection of a suitable epoxy resin system, reinforcement (carbon unidirectional fabric) and consumables for infusion like resin distribution medium. It also involved understanding cure kinetics, characterization of resin system and its flow characteristics under vacuum. The resin infusion window (RIW) and pressure application window (PAW) were established. The cure cycle of the resin system was established in order to achieve required fibre volume fraction. Extensive evaluation of mechanical properties was done at three environmental conditions namely room temperature-dry (RTD), elevated temperature-wet (ETW) and cold temperature dry (CTD). Joints, panels and a test box were designed and manufactured, demonstrating the scalability of process to large and complex structures.