Experimental research and modeling simulation of unidirectional thermoset prepreg considering non-constant bending stiffness

• Propose a method for calculating the shear angle in off-axis tension test. • The Bi-Linear Bi-Material (BLBM) model is used to model the non-constant bending stiffness of unidirectional (UD) prepreg in 0 ° and 90 ° direction respectively. • The elastic constitutive simulation model of UD prepreg is implemented by combining the nonlinear in-plane shear property, the BLBM model and fiber direction tracking model. • The out-of-plane bending deformation in the simulation is consistent with the experiment. During composite preforming, unidirectional (UD) thermoset prepregs are prone to out-of-plane wrinkling. To accurately predict such defects, this study investigates the mechanical properties of USN25000/7901 UD prepreg at 25 °C, focusing on bending stiffness. Free-cantilever bending tests of single-layer UD prepreg reveal strongly nonlinear bending moment–curvature ( M - κ ) responses in both the 0° and 90° directions, reflecting the material’s inherently non-constant bending stiffness, with the 90° bending stiffness much lower than that of 0°. The nonlinear M - κ responses are well captured by two independent Bi-Linear Bi-Material (BLBM) models. Simulations based on the BLBM model present that the predicted bending-moment accuracy improves by 42.39 % and 62.37 % in the 0° and 90° directions, respectively. The mean relative errors of the predicted free-cantilever deflection are 4.23 % and 3.39 % in the 0° and 90° directions, both superior to conventional models. The out-of-plane bending deformations from axial compression and 10° off-axis tensile simulations using the BLBM model also show better agreement with experiments. The normalized RMSE of the predicted 0° axial compression bending deformation remains within 5.5 %. These results demonstrate that the BLBM model offers a practical approach for predicting out-of-plane bending and wrinkling defects of UD prepregs during preforming.