Investigation of the Effects of Geometry on the Efficiency of the Double Lap Joint of Unidirectional Fiberglass Composites

The fracture toughness in shear mode (Mode II) has been studied for double lap adhesive joints where geometry of the joint is the major parameter. A unidirectional fiberglass - epoxy system was selected for experimental tests and theoretical analysis. Both rectangular and elliptical planform geometries were investigated. In order to evaluate the results, the adhesion area of the two geometries was purposely made identical. Through computer analysis, the effects on the stress concentration factor as a function of geometry was investigated. Then, using testing and fracture mechanics techniques, calibration factors for Mode II loading of the two geometries were obtained. The analysis of the results was performed by modeling the damage growth at the crack tip as a self similar crack extension through compliance matching procedure. The crack growth resistance at instability and the corresponding critical strain energy release rate are independent of initial crack length in the range of crack length investigated. The critical strain energy release rate in Mode II is roughly six times higher for the elliptical geometry than for the rectangular