Friction Drilling Fasteners for Composite Structures

The NAVY seeks a robust and rapid innovative process for joining thermoset composite structures and repair of damaged composite parts without destroying the adherends. The current practical joining technology used in the thermoset composites includes (a) mechanical fastening and (b) adhesive bonding. Holes drilled in the composite associated with the bolted connection involve stress concentrations and fiber breakage, reducing the overall strength of the structure. Adhesive bonding using mainly epoxy adhesive is labor intensive and requires extensive surface preparation and long curing times. A mixture of bonded and bolted configurations has been found in most repaired composite structures. Given the process-driven initial bond quality and its property evolution under a service/environmental condition, the added strength and stiffness from bonding to a repaired structure has usually been ignored and only the bolted effect has been included during the design and certification of the repaired structure. To fill in the current technology gap and demonstrate a novel joining technology for connecting thermoplastic to thermoset and thermoset to thermoset composite parts, Global Engineering and Materials, Inc. and collaborators propose to develop a hybrid joining technology based on the combination of ultrasonic welding and friction drill fastening. To establish the relation between the process parameters and the thermomechanical response, high-fidelity simulation models will be developed to ensure the process-induced temperature field is sufficient enough to promote the interface bonding without damaging the composite adherends. A temperature-dependent cohesive model will be applied to ensure the minimum level of delamination damage after the installation of the friction drilling fasteners. In Phase I, the research team will fabricate bonded and hybrid connected composite coupons of thermoset and thermoplastic materials and compare their tensile and flexural performance with the corresponding coupons fabricated from the current best practice.