Testing of components is a usual method to evaluate structures, joining methods, and materials prior to full scale testing. Ambient temperature dynamic crush testing was performed on steel and composite sub-component front frame rails to compare the energy absorption and evaluate crush behavior. The sub-component composite frame rails were fabricated from two parts, an upper and lower, and bonded using three adhesives: Ashland polyurethane, Lord epoxy, and 3M epoxy. Prior to the dynamic test of the rails, single lap shear coupon tests were performed at ambient temperature and elevated temperature, 135°C, to evaluate relative bond strength of these adhesives. Testing was performed at elevated temperature because adhesives used for structural bonding in automotive, specifically under-hood, applications can be subjected to elevated temperatures. All three adhesives tested showed reduced bond strength at elevated temperatures. At room temperature, the Ashland urethane and Lord epoxy adhesives were observed to have comparable higher bond strength with the composite-to-composite lap shear coupons compared to the 3M epoxy. However, the crush failure mode for the composite tubes was confined to the substrate and the mean crush load was independent of the adhesive used for fabrication.
Progressive crushing of the rail specimens was observed for all specimens tested. The amount of energy absorbed and crush mode for each rail design depended on its structural and material characteristics. The steel specimen absorbed energy by localized buckling in an accordion crush mode. The composite specimens absorbed energy by fracturing the composite matrix, delamination, fracture of the reinforcement fibers, and friction between the fracture and crushing surfaces. The crushing process of the steel rail was initiated by fabricated corrugations in the rail comers at the front, or impact, end of the rail. The composite rail crush event was initiated with an aluminum plug trigger designed to cause the composite rail to split at the comers with fracture of the composite matrix and delamination of the composite plies. Glass fibers were observed to fracture primarily at the tube corners. Fiber fracture elsewhere was infrequent. Close examination of the bonded joint fracture surface showed extensive fiber tear-out indicating that the failure was in the composite, not the adhesive. Mean crush load for the steel rail was 60% higher than the average mean load for the composite rails. The peak load for the steel rail was 71% higher than the average peak load for the composite rails. Specific energy absorption (SEA) of the steel rail was calculated to be 6.34 kJ/kgm compared with an average of 10.5 kJ/kgm for the composite rails.