Effect of mean shear stress on the fatigue strength of notched components under multiaxial stress state

Abstract

The effect of a static and intermittent shear stress on the HCF strength of two quenched and tempered steel grades used to produce shafts in crane industry was studied on notched specimens (Kt=1.7 and 2.7) for being representative of critical areas. Three main load cases were considered: C1=rotative bending (RB), C2=RB and static torsion and C3=RB and mean torsion fluctuating in blocks to simulated start and stop cycles. In this last case the first investigated mean shear stress, τm, was equal to the material yield stress in pure shear, τy. Additional C3 variants were investigated too where τm was equal to 0.3τy and 0.7τy. It has been shown that τm has little effect on the rotating bending HCF strength at 3×106 cycles. For both steel grades and notch geometries studied, the results of the fatigue tests confirm that the influence of a static torsion on the rotating bending HCF strength is negligible even when the static torsion level is equivalent to the yield strength of the material in torsion. However, in intermittent service conditions (C3), it has been shown that torsion cycles can affect significantly the HCF strength in RB, depending on the notch acuity and torsion level. Elastic-plastic cyclic FEA has been done for the two specimen geometries to assess the stabilized stress-strain state at the notch root and then to compute the fatigue life by using the multiaxial HCF life models proposed by: Fatemi-Socie, Smith-Watson-Topper, and Wang-Brown. The Palmgreen-Miner hypothesis was used to cumulate damage mainly because of its simplicity for design purposes. According to our simulations and with the chosen cumulative damage rule, none of the tested fatigue life calculation methods give good results for all the load cases. The efficiency of the tested methods is very dependent on both the material and the load cases. However, the Smith-Watson-Topper approach gives the best results whereas the Fatemi-Socie models leads to the more conservative ones except in one load case