Strain-controlled cumulative fatigue with mean strains and high-cycle and low-cycle interaction

This work reports on baseline fatigue and cumulative fatigue damage results of mild steel under cyclic strain control. All cumulative damage tests were conducted with a step loading in a decreasing order, from a high strain amplitude of 0.4 or 0.3% to a lower one of 0.2%. Different mean strain values of -0.2, 0, and 0.2% were applied in each strain amplitude level. Seven combinations of strain amplitude and mean strain were investigated. It was observed that cyclic softening in the second level was significantly stabilized by the presence of the first level, which accounts for approximately 20% of the total fatigue life. Mean stress relaxation in the second level depended upon the loading in the first level and was similar to that for single-level loading under certain strain amplitudes and mean strains. The damage summations in terms of cycle ratio (Miner's damage sum), both with and without mean strain correction, were calculated and discussed. For all of the cases investigated, the average damage summations were found to be in the range of 0.94 to 1.28. The degree of approximation and the deviation of each individual test from unity depends on whether mean strain correction was made. The competing effects of mean stresses induced and plastic deformation tended to overshadow the more classical loading sequence effects in terms of the damage summation. Fatigue damage accumulation has also been analyzed using Continuum Damage Mechanics modeling. The sources of the deviation of damage summation from unity and the nonlinear accumulation of damage are discussed.