Failure behavior and damage model of composite sandwich panels under three-point bending fatigue load

Composite sandwich panels present a variety of damage modes due to their diverse components and variable structural parameters. This work conducted three-point bending static and fatigue tests on composite sandwich panels using specially designed fixtures to investigate their bending-shear characteristics and failure behaviors. The effects of core orientations, i.e., L-direction and W-direction, on the static strength, fatigue life, and fatigue crack initiation and propagation were compared. A stiffness-degradation-based fatigue damage model was constructed for the sandwich panels to describe the entire failure process from core cracking to core/plates debonding, achieving accurate modeling under the full load ratios. A differential evolution algorithm incorporating a penalty function was employed to optimize the model parameters under varying stresses. It was found that the designed fixture enables bi-directional fatigue loading and effectively avoids additional axial forces. Fatigue damage in sandwich panels resembles static but spans a wider area. While damage modes are similar in W-direction and L-direction, L-direction cores have higher stiffness and strength, and lower fatigue resistance in W-direction. Furthermore, the proposed fatigue damage model demonstrates excellent agreement with experimental results, achieving R² = 0.968.