Residual Strength of Panels with Cracks Based on a Plastic Hinge

Abstract

The residual strength of a panel with a crack is dependent on the mode of failure. Generally, the panel will exhibit a brittle-fracture mode of failure (based on linear elastic fracture mechanics) or a mode of failure based on yielding. A common practice is to assume an elastic, perfectly plastic, material behavior, along with a net-section-yield criterion for panels that fail by yielding. However, the net-section-yield criterion is not appropriate for configurations such as panels with asymmetric geometry or loading, because equilibrium is violated. For asymmetric cases such as panels with edge cracks or off-center cracks, a plastic moment or hinge must exist, the details of which may be determined by satisfying equilibrium. The purpose of this study is to compare the experimental residual strengths of panels with edge cracks and off-center cracks with the residual strengths determined from the assumption of elastic, perfectly plastic, behavior and a plastic-moment (hinge) criterion. The material chosen for this study is 2024-T3 aluminum alloy. This material is commonly used for aircraft skin and it has a high fracture toughness, and so the mode of failure will less likely be brittle fracture. An elastic, perfectly plastic, material model was assumed, and expressions were developed for a plastic-hinge failure for panels with edge cracks as well as off-center cracks. A large number of panels with either edge cracks or off-center cracks were tested to failure, and the results were compared with the results obtained from the equations developed from the plastic-hinge assumption. The results from the plastic-hinge assumption compared favorably with the experimental results.