Improving safety and durability of composite structures requires advancements in mechanical fastening methods. Conventional drilling used for bolted joints in carbon fiber reinforced polymer composites lead to secondary damage and defects that compromise the strength and integrity of composite structural components. Minimizing defects/damage requires slow material removal using sharp drill tools employing coolants, all the while maintaining low cutting forces. The abrasive nature of carbon fibers contributes to rapid tool wear and requires increased machining forces for cutting. Furthermore, there are currently no optimum drilling tools or processes for stack drilling composites with metals which is often needed in repair and assembly operations. Friction flow drilling developed for metals has been explored for composites and composites/metal stack drilling. The emergence of friction drill fasteners that offer the possibility of a one step process for drill, finish and installation of fasteners is enticing. The proposed project seeks to develop and evaluate a new hybrid friction drilling tool/process/fastener that combines aspects of abrasive machining and friction flow drilling to achieve high quality friction drilled holes or fastened connections in composite/metal stacks. The key objectives are to minimize damage and defects at the drilled hole site, maximize the machining rate, and minimize the tooling costs. The process will be evaluated for single laminate drilling and for stack drilling in layers of thermosetting and thermoplastic polymers and metals. During Phase I, the M4 Engineering and San Diego State University team will design hybrid friction drill tools optimized for stack drilling and perform physical experiments to quantify tool and process parameter effects on hole quality and drilling-induced damage around the drilled holes. Computational process models that can guide process optimization will be explored. In future project phases, the drilling tool and process will be extended to the design of a friction drill installed fastener system that will be optimized using modeling-guided testing.
Benefit: Composite materials provide lightweight and tailorable structural solutions which enable lower operating costs for a range of commercial transport applications including aircraft, helicopters, tanks, trucks, and cars. Composites also find application for civil structures including buildings, wind turbines, shipping containers, as well as for medical and sports equipment. Essential to utilizing such composite materials is being able to join composite parts together or to other metallic structural components in a robust manner as designed or as part of a repair operation. Successful multiphase program execution will lead to an optimized friction drilling process for one or more Navy composite systems of interest, as well as a testing/modeling methodology to extend to it to other composite and composite/metallic material systems. This fastener process will find application for primary and secondary structures, as well as structural repair. The developed friction drill tool and process will significantly increase hole drill speeds, while retaining high geometrical tolerance and low secondary damage in composites and composite-metal stacks. Improved hole drilling and fastening methods can lead to lighter structures and longer fatigue life in composites. The development of direct friction drill fastener installation can help speed repair and assembly operations. As such, this technology will help to enable increased composite structure utilization throughout industry for primary and secondary structures, as well as for general structural repair.
Keywords: fiber reinforced polymer composites, fiber reinforced polymer composites, Composite Drilling, rapid assembly, Hybrid Drill Tool, Composite/Metal Stack Drilling, Friction drilling, Friction Drill Fasteners
