Experimental Study on Hybrid Fastening System With Varying Bolt Diameter and Dissimilar Substrates

Event Date/Time: 
December 1, 2017 - 10:00am
Event Location: 
1538 Engineering
Speaker: 
Andrew Valentine
Master's Thesis Defense

Abstract

Hybrid fastening system that uses an adhesive insert as a structural element has been shown to eliminate bolt-adherend-slip, reduce delamination, and produce peak loads that are 200% to 500% greater than slip-loads in conventional mechanically fastened joints, while still allowing versatility in joint design. These improvements were experimentally obtained for similar substrates with a constant diameter bolt/hole. The efficiency of hybrid fastening systems with varying bolt diameter and dissimilar substrates have not been studied or fully characterized.

In this work, Aluminum (6061) and glass fiber/SC15 epoxy (GFRP) substrates were joined together with bolts/holes of varying diameters, namely ¼" (6.25 mm.), ½" (12.50 mm.) and ¾" (18.75 mm.). The preload was maintained at 75% of the bolt yield strength for all joints. SC-15 adhesive was used as the structural insert along with biaxial carbon sleeve insert, and cured at room temperature for 24h. The resulting joints were tested in a tensile-shear configuration at a rate of 5 mm./min. The effect of adhesive, sleeve, and bolt diameter on the efficiency of the hybrid fastening system were compared with conventional fastened (no insert) joints. Experimental observations revealed that the failure mechanisms changed with bolt diameter. At small diameters, bolt properties governed the failure whereas at larger diameters it was controlled by the substrates. Hybrid joints were found to have 8 to 17 times higher load carrying capacities relative to slip-loads for conventional joints. While this work creates the benchmark for hybrid fastening in multi-material applications, further statistically significant experimental testing and studies on proper adhesive fill are needed to fully exploit the benefits offered by this hybrid fastening system.