Parametric study of size, curvature and free edge effects on the predicted strength of bonded composite joints

This paper presents the effects of size, curvature and free edges of laboratory lap joints on the debond fracture behaviour of joints that more realistically represent fuselage skin structures than conventional flat, narrow specimens. Finite Element Analysis is used in conjunction with Cohesive Zone Modelling (CZM) to predict the strength of selected joint features. The modelling approach was verified by simple single lap joint geometry. Four realistic joint features were then modelled by this validated modelling approach. The results show that moderate curvature has negligible effect on the peak load. There is a significant difference in the load vs displacement response of flat lab coupon joints with free edges and realistic curved joints with constrained edges. Further detail design features were investigated in this study, including (i) the joint runout and (ii) the presence of initial damage (thumbnail delamination). The modelling results show that the joggle configuration has an effect on the distribution of interlaminar stresses that affect the damage initiation and propagation. Fracture behaviour from different initial crack geometries associated with wider specimens has been simulated. From a design standpoint, an expansion of modelling capability is suggested to reduce the number of component tests in the traditional test pyramid.