Experimental Study on the Coaxing Effect of Multi-Level Stresses with Different Sequences

The coaxing effect in fatigue refers to an increase in the fatigue strength of material imduced by prior cycling at a stress level below the fatigue limit. In this study, the coaxing effect of multi-level stresses in the fatigue process in the automotive drive shaft material 40Cr was investigated by applying three levels of stress amplitudes below the fatigue limit in different sequences prior to conducting the conventional constant amplitude fatigue test with standard round bar specimens. The existence of coaxing effect under multi-level stress with a load sequence effect was observed and confirmed by the results. The longest fatigue life is achieved through three levels of coaxing load with a step increase sequence, while a decrease sequence will result in the shortest fatigue life. The mechanisms of the coaxing effect which increased the fatigue life at the crack initiation and propagation stages were observed, through fracture morphology analysis, as the regulation of micropores, decreasing of pore density and the narrowed striation spacing. This finding is instrumental for extending the service life of mechanical components.