A diagonal tension and failure study of stiffened stitched composite panels

Two panels are constructed of a skin and J stiffeners which are stitched to gether as dry preforms from a unidirectional carbon fabric material, which is then impregnated with epoxy resin and cured by the Resin Transfer Moldin g (RTM) technique. The intended purpose of the skin and skin to stiffener s titching is for delamination resistance after impact damage. An important s ide benefit shown by the data in this study is increased post-buckling stre ngth. A non-linear finite element model of the panel was constructed to det ermine the internal loads in the post-buckled state. A new failure criterio n, the Yeh-Stratton criterion, was compared to five other popular theories of failure and found to very reasonably predict the failure. The basis for calculating the failure load was first ply failure, failure considering the average laminate stresses, and cumulative damage ply-by-ply failure scheme . The Yeh-Stratton criterion was also examined in conjunction with a tradit ional semi-empirical diagonal tension calculation which has its basis in th e testing of many metallic panels and adapted for composites. The panel doe s not fail due to gross area stress, but to a stress concentration in the s kin at the edge of the stiffener, which occurs because the stiffener does n ot "pop off" the skin as in conventionally bonded stiffeners, but is held t o the panel with the stitching.