A Diagonal Tension and Failure Study of Stiffened Stitched Composite Panels

Two panels are constructed of a skin and J stiffeners which are stitched together as dry preforms from a unidirectional carbon fabric material, which is then impregnated with epoxy resin and cured by the Resin Transfer Molding (RTM) technique. The intended purpose of the skin and skin to stiffener stitching is for delamination resistance after impact damage. An important side benefit shown by the data in this study is increased post-buckling strength. A non-linear finite element model of the panel was constructed to determine the internal loads in the post-buckled state. A new failure criterion, 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 traditional semi-empirical diagonal tension calculation which has its basis in the testing of many metallic panels and adapted for composites. The panel does not fail due to gross area stress, but to a stress concentration in the skin at the edge of the stiffener, which occurs because the stiffener does not “pop off” the skin as in conventionally bonded stiffeners, but is held to the panel with the stitching.