مقاومت به خستگی قطعات دارای شیار تحت شرايط تنش چند محوری
Abstract
1- Introduction
2- Experimental program and test results
3- Finite element analysis
4- Results of the finite element analysis
5- Fatigue life calculation methods
6- Results of the fatigue life simulations
7- Conclusion and prospects
References
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,bending=1.7 and 2.7) for being representative of critical areas. Three main loading configurations 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 finite element analysis has been done for the two specimen geometries to assess the stabilized stress-strain state at the notch root and then compute the fatigue life by using the multiaxial HCF 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 model leads to the more conservative ones except in one load configuration.
7- Conclusion and prospects
The effect of mean shear stress on the rotative bending (RB) fatigue strength at 3×106 cycles was studied on notched specimens (Kt=1.7 and 2.7) made of two quenched and tempered steels. As known in literature, it has been shown that a constant mean shear stress has a negligible influence on the HCF strength under RB for the two grades. But, in intermittent service conditions where repeated cycles of torsion are introduced, it has been shown that these cycles can affect significantly the RB HCF strength at 3×106 cycles, depending on the notch acuity and torsion level applied. In the case of Kt = 2.7, for both materials, the rotating bending strength reduction reached 30% for a torsion level equal to the yield strength in shear. This reduction becomes significant only for a mean torsion level over 30% of the yield strength in torsion. Although this effect was not observed in the case of Kt = 1.7, it seems relevant to consider in general that the damage induced by these repeated cycles is not negligible beyond a torsion level of 30% of the yield strength in shear. An elastic-plastic finite element analysis shows that the stabilized stress-strain field is not axi-symmetric all around the notch root because of plastic strains occurring during the first loading. This creates a non axi-symmetric field of residual stresses. According to our simulations and with the chosen cumulative damage rule (PalmgreenMiner), 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 or Wang Brown models lead to the more non conservative ones except in one load case.