Fatigue delamination propagation in multi-directional woven composites: Mixed-mode deformation

Delamination is a critical failure mode in composite structures, often driven by complex cyclic loading in multi-directional (MD) laminates. This study aims to characterize the influence of mode mixity, cyclic displacement ratios (R_d), and mode coupling on delamination growth in dissimilar material interfaces. To achieve this, two MD material systems, wet-layup and prepreg with initial delaminations along 0°//±45° UD and woven plies and along 0°/90°//±45° woven plies, respectively, were evaluated using mixed-mode end-loaded split (MMELS) beam specimens. Fatigue tests were conducted at R_d ratios of 0.1, 0.5, and 0.75. The methodology employed the Virtual Crack Closure Technique (VCCT) within finite element analyses to determine energy release rates G_i and phase angles throughout propagation. The core contribution of this work is the application of a modified Hartman–Schijve master curve to account for displacement ratio effects and normalize growth data. Results indicate that while the da/dN vs. G_max slopes for prepreg specimens varied with R_d, the wet-layup slopes remained unaffected. Crucially, despite these differences, both material systems yielded similar master curves, demonstrating the robustness of the proposed normalization approach. Mode-mixity analysis confirmed steady-state conditions dominated by the K₁ component in both systems. This study provides a comprehensive framework for predicting fatigue delamination across diverse material interfaces, offering significant value for the design and safety assessment of complex composite architectures.