STRAIN-CONTROLLED CUMULATIVE FATIGUE WITH MEAN STRAINS AND HIGH-CYCLEAND LOW-CYCLE INTERACTION

This work reports on baseline fatigue and cumulative fatigue damage re sults 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%. Differe nt mean strain values of -0.2, 0, and 0.2% were applied in each strain amplitude level. Seven combinations of strain amplitude and mean stra in were investigated. It was observed that cyclic softening in the sec ond level was significantly stabilized by the presence of the first le vel, which accounts for approximately 20% of the total fatigue life. M ean stress relaxation in the second level depended upon the loading in the first level and was similar to that for single-level loading unde r 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 case s 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 o f each individual test from unity depends on whether mean strain corre ction was made. The competing effects of mean stresses induced and pla stic deformation tended to overshadow the more classical loading seque nce effects in terms of the damage summation. Fatigue damage accumulat ion has also been analyzed using Continuum Damage Mechanics modeling. The sources of the deviation of damage summation from unity and the no nlinear accumulation of damage are discussed.