Mechanisms of damage formation in transversely impacted glass-epoxy bonded lap joints

Adhesively bonded glass-epoxy lap joints are transversely impacted in order to determine the modes of damage resulting from out-of-plane impacts to the overlap region and to identify mechanisms by which damage formation occurs. Impacts over an energy range of 10-50 J are produced using a low-velocity, high-mass drop weight tower. Woven glass-epoxy joints bonded together with an epoxy adhesive film are tested. Localized debonding is observed for lower energy impacts in the region surrounding the impact point. These debonds, attributed to the transverse shear stress developed within the adhesive, are typically circular and concentric to the impact point, and can exist in the absence of debonds growing from the joint free edges (overlap boundaries). Debonds growing from the free edges initiate at the backside of the specimen (i.e., away from the impact side) where tensile peel stress in combination with shear stress causes failure of the adhesive at this location. Finite element analysis is used to determine the stresses that develop in the adhesive during impact, and to corroborate experimentally based conclusions on the mechanisms of damage formation. For bonded joint impacts, it is possible to experimentally establish damage threat thresholds at which impact damage initiates. Such thresholds are important for joint design, and also when making decisions related to the maintenance and repair of composite bonded joints. This is an important safety feature of bonded composite structures since partial joint-width debonding can occur at moderate impact energy levels, and the resulting damage is difficult (perhaps impossible) to detect by visual inspection.