Experimental investigation & numerical modeling of the layup-specific damage behavior of composite bonded joints under mixed-mode loading

• Systematic analysis of layup dependent fracture in unidirectional thermoset prepreg within an aircraft-representative joint. • Decoupled effects of adherend bending stiffness and interface-ply fiber orientation via varying bending stiffness levels. • Mode-independent cohesive zone length provides convergent fracture loads and reliably predicts layup-specific damage progression. • Accurate crack-path prediction with comparatively coarse meshes, enabling significant savings in computational effort. This study investigates the influence of layup configuration on the failure behavior of composite bonded joints. Experimental results from quasi-static tests reveal that specimens with a 0°-oriented ply in contact with the adhesive consistently achieve higher fracture loads and exhibit simpler failure mechanisms compared to 45° configuration, where the crack propagates into the adherends and shows complex damage progression. Numerical simulations using cohesive zone modeling are conducted to predict this behavior. A method is applied to ensure sufficient resolution of the cohesive zone, and the influence of mesh size on predictive accuracy is assessed. The results show that at least nine cohesive elements are required within the cohesive zone for convergence in fracture load and accurate damage progression. The presented modeling approach accurately predicts both crack paths and fracture loads, particularly for complex failure scenarios. However, limitations remain for adhesive-layer-only failure due to sensitivity to surface pre-treatment and associated parameter uncertainty. The findings highlight the need for layup-sensitive modeling strategies and mesh resolution when analyzing bonded composite structures.