Friction Drilling Fasteners for Composite Structures

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.