Interlaminar Fracture Processes in Resin Matrix Composites Under Static and Fatigue Loading

Both static and fatigue test results on a double-cracked-lap-shear specimen are presented under tensile and compressive loading. The specimen is made of AS4/3501-6 graphite/ epoxy material with (′45,0,90) quasi-isotropic balanced symmetric layups in the lap and strap. In order to suppress the Mode I contribution in this specimen which exhibits mixed mode behavior in tensile loading, a normal force is applied close to the delamination front. The response due to this force is estimated from a sublaminate theory which accounts for transverse shear strain and normal and axial displacements. The normal force application is effective only when the delamination growth is dictated by mixed mode behavior. On the basis of the limited experimental data generated thus far, it seems that the static and fatigue delamination growth thresholds are indistinguishable, though cyclic tensile loading induces larger delamination growth. Considerable improvement in fracture behavior was exhibited by Mode I suppression under both tensile and fatigue loading. This partially explains the effectiveness of stitching and wrapping free edges in practical structures.