An improved strain-life model based on the Walker equation to describe tensile and compressive mean stress effects

Abstract

Strain-life models that include Walker-like products can reproduce the higher mean stress effects in fatigue crack initiation observed in high-strength materials. These models are able to improve Smith-Watson-Topper’s strain-life predictions, at the cost of additional data-fitting exponents. This work reviews and compares several such models using 34 metallic alloy data sets from the literature that include non-zero mean stresses, totaling 1,355 uniaxial fatigue specimens. This comprehensive evaluation suggests that two different calibrations of Walker’s exponent are required to properly fit data measured under tensile and compressive mean stresses. Hence, an equivalent strain amplitude is proposed based on these two exponents, as well as on the ratio between the stress peak and amplitude. The new strain-life model resulting from this equivalent strain amplitude is able to describe uniaxial fatigue data under all stress ratios R < 1. Estimates for both exponents are provided as a function of the ultimate tensile strength of the material, which could be used in the absence of fatigue data under non-zero mean stresses, an asset for practical applications. A modified version of the proposed strain-life model, which considers mean stress effects only in the elastic damage term, is evaluated as well. Explicit equations are presented for the optimal calibration of all parameters from both models.