A Multiaxial Fatigue Damage Evaluation of As-Built Composite Structures with Ply Drops

This paper presents a high-fidelity fatigue damage modeling framework for composite structures with ply drops, incorporating several key advancements to capture localized fatigue behavior. The approach includes: (1) computation of local stress ratios at each fatigue cycle; (2) an R-ratio-dependent fatigue damage accumulation model; (3) implementation of a constant load diagram to construct S–N curves at arbitrary R-ratios; and (4) a cycle-jumping technique to account for the evolving rate of fatigue damage accumulation due to progressive stiffness redistribution. A combined experimental and numerical study was conducted on tapered composite beams subjected to mixed axial tension and vertical bending. A custom-designed fatigue test fixture was developed to capture displacement at the loading end, which was then used as a boundary condition in the fatigue life prediction model. To guide the selection of fatigue test peak loads, static failure analyses were first performed on representative tapered beams under constant axial tension and monotonic bending. Subsequent fatigue tests, conducted under constant axial tension and cyclic bending at two peak load levels, showed strong agreement between predicted and measured load–deflection responses, fatigue lives, and failure patterns, thereby demonstrating the accuracy and robustness of the developed framework.