An Alternative Approach for Fracture Fatigue Entropy Estimation of Aluminum Alloys Subjected to Variable Amplitude Fatigue

Abstract Materials subjected to cyclic loading accumulate thermodynamic entropy until a critical value known as the fracture fatigue entropy (FFE) is reached, whereupon they experience final fracture. Research into FFE values commonly relies upon deformation entropy generation (DEG) theory to derive a model that is then validated with constant amplitude fatigue tests under low-cycle fatigue (LCF) conditions. Yet many engineering structures experience variable amplitude fatigue and/or high-cycle fatigue (HCF) conditions. In addition, the traditional DEG-based approach requires an adjustment to account for internal friction, which requires empirical relations with parameters whose values are known only through material-specific testing. This article presents an alternative approach to FFE estimation for aluminum alloys that accommodates variable stress amplitudes under either LCF or HCF conditions without the need to know the initial temperature rise in the test specimen or to account for internal friction. This alternative approach estimates FFE within the same bounds of variability as the FFE values provided by other researchers using the traditional DEG-based approach by utilizing an empirical correction factor (kload). Empirical fatigue test data validate the alternative approach for all test specimens except those with only two loading blocks in the LCF region, which produced results displaying the most variability.