Study of Load Sequence Effects in Combined Cyclic Fatigue Using CDM Approach

Abstract

In this study, Continuum Damage Mechanics (CDM) approach is used to model fatigue damage accumulation in spectra exhibiting combined cyclic fatigue (CCF) involving both low and high cycle fatigue (L/HCF) regimes. The CDM based results are compared and the traditional strain-life approach. The objective is to consider CCF type of variable amplitude spectra and determine conditions where the effects of load sequence are important in predicting fatigue life and situations and where the load sequence effects can be safely ignored. In the traditional strain or stress-life based approach, the fatigue damage under spectrum loading is calculated first by determining damage contributing strain cycles in the spectrum from a cycle counting technique and then calculating the damage for each of these strain cycles as a life fraction from the strain-life curve. Palmgren-Miner’s linear damage summation rule is finally employed to calculate total fatigue damage for the spectrum. However, the linear accumulation of damage may not hold under severe cases where there is large fluctuation in strain amplitude or when multiple modes of crack initiation mechanisms are encountered such as in HCF and CCF loading cases. The damage progression in these cases is inherently non-linear in nature when the actual micromechanical effects are taken into consideration. In the CDM based approach, the micromechanical causes of both high cycle and low cycle fatigue are modeled by considering damage as an additional internal variable at the meso scale and evolution of damage law is postulated based on thermodynamic principles. Based on actual micromechanical observations, the damage does not initiate until the accumulated plastic strain exceeds a threshold value. A crack is assumed to have initiated at the meso scale when the evolution of damage variable under cyclic loading reaches a critical value. These material threshold parameters, which are determined from critical experiments, are then incorporated in the damage evolution law. As the damage evolution in the CDM approach depends on the actual sequence of loading, the load sequence effects are directly modeled in this approach. CDM results for different variable amplitude loading cases involving HCF and CCF loadings are presented in the paper. Comparisons with results obtained from strain-life analysis are also provided to determine cases where load-sequence effects are important. The paper discusses the applicability and use of the CDM model to augment current strain-based models to determine reliable damage under CCF and VHCF type of loadings and also cases where load sequence effects are important.