Abstract
The present study investigates the effect of tempering temperature, residual stresses and me-chanical induced stress gradients on the fatigue properties / resistance up to 109 cycles. Uni-axial tension-compression tests (50 Hz, 1 kHz and R = -1) were performed on specimens made of 42CrMo4 (AISI 4140) at room temperature. At first smooth specimens, which were tempered at six different temperatures to produce a wide range of ultimate tensile strength were tested to clarify the strength effect. With the decrease in tempering temperature the sensitivity of subsurface crack initiation at inner defects increases. The study indicated that for high-strength heat treatment conditions (Rm > 1400 MPa) the difference between the fatigue strength at 106 and 109 increases with increasing tensile strength. A functional relationship between these two fatigue strength was found and verified experimental. It seems that the stress intensity factor K which arises as a function of local loading conditions at inner stress-raisers depends on the yielding /hardening properties of the material around them. Based on K and ODA-size a lifetime prediction for crack initiation at inner defects was developed as a function of tempering temperature. In a second step residual stresses with different penetration depth were induced by macro- and micro-peening processes on smooth and notched (Kt = 1.23, 1.41 and 1.94) specimen. As a result of work-hardening and compressive residual stresses near the surface change the place of crack initiation from the surface of smooth and notched untreated specimen to subsurface for shot-peened treated specimen in the regime of high-stress amplitudes. The test results of notched/shot-peened specimen indicated that the depth-effect of residual stresses has a significant influence on the location of subsurface crack initiation. The stability of the residual stresses during cyclic loading in the VHCF regime was determined by interrupting the fatigue tests after a defined number of cycles.
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