Strain-Rate-Dependent Yield Criteria for Progressive Failure Analysis of Composite Laminates Based on the Northwestern Failure Theory

The strain-rate-dependent failure of a fiber-reinforced toughened-matrix composite (IM7/8552) was experimentally characterized over the range of quasi-static (10⁻⁴ s⁻¹) to dynamic (10³ s⁻¹) strain rates by testing off-axis lamina and angle-ply laminate specimens. A progressive failure framework was proposed to describe the matrix-dominated transition from linear elastic to non-linear material behavior as determined from the experimentally measured stress-strain material response, and the Northwestern Failure Theory was adapted to provide a set of apparent yield criteria for predicting the matrix-dominated yielding of composites using the lamina-based transverse tension (F_2t^y), transverse compression (F_2c^y), and in-plane shear (F₆^y) yield strengths. The underlying theory was validated by determining the applicability of the new damage-mode-based yield criteria. Starting with the lamina, the proposed criteria accurately predicted the matrix-dominated yielding. Angle-ply laminates were then investigated to isolate the matrix-dominated laminate behavior based on fiber orientation, and the predictions were found to be in superior agreement with the experimental results compared to the classical failure theories. The results indicate the potential of using the Northwestern Yield and Failure Criteria to provide the predictive baseline for damage propagation from yield to ultimate lamina failure in composite laminates.