Characterizing compressive failure mechanisms and their transitions in woven composites under on and off-axis loading

Abstract

The compressive failure of fiber-reinforced composites is complex, occurring via multiple mechanisms depending on the loading angle with the fiber directions. This work is aimed at the detailed characterization and understanding of this dependence in a 2 × 2 epoxy/carbon twill woven composite. Reported are the results of a variety of on-axis and off-axis compressive tests with loading angles ranging from 0°to 45°, carried out as per ASTM standards. The tests show that for angles from 0°to 10°, no appreciable change in compressive strength occurs. Failure in the load aligned tows is dominated by out of plane shear failure and out of plane fiber kinking. The behavior is very brittle, consisting of a sharp peak load immediately dropping to a non-zero residual stress plateau. Extensive micro-cracking in the transverse tow is also observed. Between 10°and 25°many transitions occur, including increased ductility, pre-peak non-linearity, increasing width of the out-of-plane fiber kink band, more gradual post-peak load decrease, and an increase in the residual stress. The lowest strength occurs at 45°where the behavior is completely ductile and failure occurs by in-plane fiber kink band formation. A correlation between the in-plane shear stress and kink band width appears to exist. The maximum stress based strength envelope theory captures this loading angle dependence of strength. The findings are crucial for designing multi-directional woven composite laminates subjected to compressive loading, and for the validation of computational models.