Eight curved blade stringer-stiffened composite panels were tested under axial compression to obtain the "first" buckling and postbuckling behavior till collapse. Except for one panel, used as a reference panel, all of the panels had stringers without dropoff layers. Four panels contained either artificial damage or both artificial and impact-induced damage. Cyclic/repeated buckling was applied well in a relatively "deep" postbuckling region. It was demonstrated that neither repeated buckling, within the number of cycles applied in the present program, nor artificial damage and impact-induced damage, which were introduced into the panels, resulted in stiffness degradation of the panels. No premature failure of any of the tested panels was observed within their expected life cycle, i.e. exposure to a few hundred cycles deep in the postbuckling region, even in the presence of either type or a combination of the damage. All of the tested panels sustained repeated postbuckling loading till they were subjected to static loading aimed at determining their collapse loads. In spite of the present design, i.e. stiffeners with no dropoff plies aimed amongst others at providing a mechanism for initiating stiffener debonding, no skin–stringer separation was encountered till collapse of the panels. It was found that composite stringer-stiffened panels can be safely and repeatedly loaded in their deep postbuckling range with no degradation in their stiffness. Damage, due to either manufacturing or impact, which usually will result in rejection of a structural element, affected neither the load-carrying capacity nor the capability to withstand repeated loading in the relatively very deep postbuckling range within the present designed life cycle of the element. It was realized that manufacturing complexities and consequently costs can be reduced by employing a simplified design configuration where the use of a dropoff ply of the stringer base has been eliminated.
A new scheme for the optimum design of stiffened composite panels with geometric imperfections
