Effect of natural and artificial defects on multiaxial fatigue of 4140 steel

Despite the prevalence of small defects and multiaxial stress conditions in load-bearing engineering components, their combined effect on fatigue strength is not usually considered in the literature. In this study, the high-cycle fatigue behavior of 4140 steel with its inherent non-metallic inclusions, and with an artificial surface microhole, is investigated under axial/torsional loading. The multiaxial fatigue tests were performed under in-phase and out-ot-phase loading, employing different ratios between the shear and axial stress amplitudes. Microscopy of failed specimens indicated a tensile-dominated crack formation mechanism in most of the tests. Analysis of the test data using a root area-based critical plane criterion showed that predictions of fatigue strength and crack direction can be made with accuracy sufficient for engineering calculations.