Out-of-plane coupling effects on stress intensity factors at interface cracks in composite lap joints

The existence of off-axis plies in adhesively bonded lap joints leads to anisotropic stress-strain relationships where the in-plane and out-of-plane components of stress and deformation (in a plane perpendicular to the width of the joint) are coupled. In this paper, the anisotropic elasticity solution near a bimaterial crack tip is formulated using an eigenfunction expansion of the stresses and displacements. This elasticity solution is subsequently used in conjunction with finite elements in a global/local method in order to determine mode I, mode II, and mode III stress intensity factors. Numerical results for [(±Θ)₂]_s layups indicated that an out-of-plane mode III stress intensity factor, K_III, arises when off-axis plies are present even though the loading is in-plane. Depending on the layup angle and crack location, K_III can be as large as 43% of K_I and 55% K_II. It was also found that, in addition to producing a mode III intensity factor, out-of-plane coupling affects the magnitude of the in-plane stress intensity factors, K_I and K_II. Comparison of the 3D results with a 2D solution indicated that, depending on the layup angle, the 2D solution can either overpredict or underpredict the values of K_I and K_II by as much as 35%. Finally, the use of an effective stress intensity factor with thermoplastic or thermoset composites indicated that the 2D solution can overestimate failure loads by as much as 36% and corroborated the experimental observation that failure in composite bonded joints generally occurs as a first-ply failure of the adherends.