The Significance of the Critical Stress Ratio in the Formulation of Nonlinear Constant Life Diagrams for CFRP Laminate Life Prediction

The constant life diagram (CLD), a trusted method for life prediction, is generated based on static strength and constant amplitude fatigue data. This study examines the predictive accuracy of various nonlinear CLDs, including the innovative Anisomorphic CLDs, for a [+  45,− 45]_3S carbon fiber reinforced polymer composite laminate fabricated via autoclave curing with a 59% fiber volume. Initial calculations encompassed ultimate tensile strength (UTS) and ultimate compressive strength (UCS) using a constant crosshead speed of 1 mm/min. Subsequently, constant amplitude fatigue experiments were conducted for five stress ratios, encompassing the critical stress ratio (CSR) (χ = UCS/UTS), R = 0.1 and 0.5 (tension–tension), − 1 (tension–compression), and 5 (compression-compression). The predictive accuracy of three top-performing nonlinear CLDs, i.e., asymmetric Gerber, inclined Goodman, and the innovative Anisomorphic CLD, was assessed through necessary stress-life (S–N) curves. Notably, the novel Anisomorphic CLD, utilizing only static and CSR fatigue data, demonstrated optimal accuracy, positioning the maximum amplitude stress for any random fatigue life (N_f) on CSR. Its practical implementation undoubtedly streamlines the process, saving time, money, and effort required for extensive fatigue data generation.