As the use of fiber-reinforced polymer composites in mass-produced structural components in all domains of industry has grown, appropriate advancements in joining systems are necessary. Bolted joints are a common method used due to the simplicity of the process. Drilling holes in composites for fastening results in delamination, creation of locations for the onset of failure, and can reduce load-carrying capacities. Hybrid fastening techniques and other approaches for joining composite materials have been developed as a way to address problems related to conventional bolted joints. One such technique is the hybrid fastening system in which a structural adhesive insert is placed in the bolt-hole clearance. This approach has been shown to eliminate bolt-adherend slip, reduce delamination, and increase load-bearing capacities. Nevertheless, the extant work on such a hybrid fastening system is limited. In this work, experimental characterizations of hybrid fastening systems comprised of glass fiber reinforced polymer (GFRP) composite substrates fastened using a fully threaded grade 5 steel bolts in 1/2" (12.5 mm.) diameter with varying bolt hole clearance and three structural insert materials were performed. The GFRP substrates were manufactured using the vacuum-assisted resin transfer molding (VARTM) process. The preload was maintained at 75% of the bolt yield strength for all joints. The objective of this work was to characterize/quantify the effect of: a) the adhesive insert and b) the bolt-hole clearances on the efficiency of the hybrid fastening system. For the first parameter, namely the effect of the adhesive insert, four configurations, namely the hybrid fastening system with three different structural insert materials and one control/conventional joint without any structural insert were studied. Adhesive insert materials used were PRO-SET epoxy resin, DEVCON epoxy carrying aluminum particles, and polyurethane. All resulting joints were cured at room temperature for 48h prior to testing. For the second parameter, namely the effect of bolt clearance, four different bolt-hole clearances: close fit (0.5mm), normal fit (1.0 mm.), loose fit (1.5 mm.), and extra loose fit (2.0 mm.) were studied for each of the insert materials along with the control/conventional joint without any structural insert. The resulting joints were tested in a tensile-shear configuration at a rate of 5 mm./min. Hybrid joints were found to have 7 to 9 times higher load carrying capacities relative to slip-loads for conventional joints. All hybrid fastening systems showed no bolt-adherend slip along with delayed onset of delamination relative to conventional joints. Most of the joints with close fit (0.5mm) clearance were found to experience a catastrophic failure which resulted in bolt shearing failure. All other joints experienced progressive delamination failure without any bolt-shear failures. Further, the results indicate that the failure mechanism changed with the changes of bolt-hole clearances. The larger the clearances, the more the bolt tilts/rotates and experiences a combination of bending and shear. For small clearances, such as close-fit, shear dominates and bolt-shear occurs leading to bolt fracture. The combination of slightly larger clearance along with a structural adhesive insert allows tailoring the bolt- joint performance, leading to 7-9 times better performance than conventional joints with similar clearances. Future work should focus on quantifying the stress-concentrations and its reductions due to the addition of structural inserts. Overall, this work is novel and the first to report on the effect on clearance and varying adhesive insert materials for hybrid mechanical joints.
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