Unveiling the role of peak load in the stability and growth of interlaminar fatigue damage in composite structures

Understanding the stability of delamination growth is essential for ensuring the reliability of composite structures under compressive fatigue loading. In post-buckling regimes, where crack propagation can become highly unstable and nonlinear, even small variations in the applied load may drastically alter damage evolution and residual strength. Despite its critical role, the stability of interlaminar crack growth under cyclic compression remains poorly understood, demanding refined numerical strategies to capture its complex behaviour. This work addresses this gap by investigating how the peak compressive load influences the stability and progression of delamination in composite structures. A dedicated numerical approach has been developed to study these effects. The methodology, named SMART LOOP, couples the Virtual Crack Closure Technique (VCCT) with the Paris Law-based damage accumulation model. Such method employs an adaptive timestep strategy to ensure precise load control and mesh-independent predictions. SMART LOOP transforms force-controlled fatigue simulations into a sequence of displacement-controlled static analyses, where the displacement input is automatically adjusted to reach the target load level. This produces delamination length results that are independent of element size, eliminating the need for mesh refinement strategies. A series of simulations has been carried out on an Omega-stiffened composite panel subjected to three different compressive peak loads (25.2, 27.1, and 29.1 kN). The numerical results have been compared with experimental data from the literature to enhance the understanding of the unstable delamination growth phenomenon. This provides additional insights into failure mechanisms and complements the experimental observations.