Adhesively bonded lap joints from pultruded GFRP profiles. Part I: stress-strain analysis and failure modes

Quasi-static axial tension experiments were performed in a laboratory environment on epoxy bonded, balanced double-lap and single-lap joints from pultruded GFRP flat sections. Full-scale specimens were used to avoid size effects. Parameters investigated were the overlap length (from 50 to 200 mm), the adhesive layer thickness (from 1 to 3 mm) and the adherend thickness (from 3 to 12 mm). On nine of the double lap joints 40 strain gages were installed to measure the axial strain profiles along the overlaps. The measured axial strain profiles in the joints correlated well with numerical results obtained from a 2D finite element analysis. Failure initiation and progression were observed with a high-speed camera. Failure was initiated by the combination of local through-thickness tensile (peeling) and shear stresses at two different locations: in the adhesive fillet and in the outer fiber-mat layers of the adherends below the joint edges. Failure propagation always occurred in the outer fiber-mat layers of the adherends. The through-thickness strength of the adhesive-adherend interface was considerably higher than the through-thickness strength of the interfaces between the fiber-mats. The experimental investigation showed that the ultimate failure load converged to a constant value with increasing overlap length. The adhesive layer thickness had an insignificant influence on the ultimate failure loads and the stress-strain distributions along the overlaps.