Theoretical model of off-axis strength of CFRP composites considering the stress concentration features

Abstract

This study presents the development of an enhanced micromechanical failure model for predicting the strength of unidirectional (UD) carbon fiber reinforced polymer (CFRP) composites under off-axis loading. Existing micromechanical models often inadequately predict matrix stress by neglecting stress concentration effects, leading to discrepancies of up to 20–30% between theoretical and experimental results. To address this limitation, the proposed model integrates the Bridging Model to evaluate homogenized stresses in both the fiber and matrix, incorporates stress concentration factors to determine actual matrix stresses, and applies Tsai-Wu and maximum normal stress criteria to assess failure of the composite constituents. Off-axis tensile tests on 45° UD CFRP specimens show that the tensile strength predicted by the proposed method (98 MPa) closely matches experimental measurements (100 MPa), with a prediction error below 2%, compared to errors exceeding 15% in traditional approaches. . Furthermore, scanning electron microscopy (SEM) observations confirm that matrix crack initiation and propagation are the primary failure mechanisms, in agreement with the theoretical predictions. The novelty of this study lies in integrating stress concentration effects into a micromechanical framework, combining the Bridging Model with advanced failure criteria, and experimentally validating the improved predictive accuracy for off-axis CFRP composites.