Experimental investigations on combined high and low cycle fatigue: Material-level specimen design and strain response characteristics

The paper designs a novel material-level specimen and its dedicated fixture suitable for applying Combined high- and low- Cycle Fatigue (CCF) loads. Unlike full-scale or simulation specimens, the CCF specimen eliminates geometrically induced stress gradients in the test region. Experimental data on CCF life and strain responses of ZSGH4169 alloy are acquired under different CCF loads. The Maximum Strain within Each (MSE) CCF cycle is demonstrated to be independent of the Low-Cycle Fatigue (LCF) loads and High-Cycle Fatigue (HCF) stress amplitudes, but exhibits a correlation with the Cycle Ratio of HCF/LCF (R_f). The growth law of MSE changes from linear to logarithmic as R_f decreases. Strain amplitudes in the dwell stage, observed unaffected by R_f, are quantified as a function of LCF nominal stresses and HCF stress amplitudes. However, under a defined CCF load, strain amplitudes in the dwell stage remain constant. Strain peaks in the dwell stage in a single CCF cycle decrease in a power function with increasing HCF cycles.