This article deals with the effect of residual stresses on the fatigue behaviour of torsion bars exposed to cyclic torsional loading with different effective loading ratios, R. The residual compressive stresses on the surface were induced during technological processes by cold surface rolling and torsional overloading (presetting) into the plastic region due to the increase in the elastic linear range for torque. In the paper, we consider two different technological processes for introducing compressive residual stress on the surface of same material. We analysed the stress states affected by different residual and applied stress using the Drucker-Prager criterion in order to determine the actual stress state. Results show that the fatigue limit can be achieved if the maximum principal stresses (combined from residual and applied stresses) do not overcome the safe stress zone. As soon as the maximum principal stress reaches the edge of the safe zone, the number of cycles to failure rapidly reduces. Experimental results show that the effective loading ratio R_eff, and consequently the stress amplitude, varies through the cross section of the bar. This initiates the fatigue crack under the surface, in the highest amplitude stress zone, independent of the effective loading ratio R_eff. Consequently, increasing the compressive residual stresses on the surface by a second technological process has no significant effect on fatigue crack initiation in situ far from the surface. Increasing the plastic torsional prestress can shift the maximum stress amplitude far from the surface, but a significant volume of material should remain elastically loaded in order to ensure balance with compressive stresses from the surface of the solid bar section.

中文翻译：

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残余应力对扭杆疲劳行为的影响

本文探讨了残余应力对承受不同有效载荷比R的循环扭转载荷的扭杆疲劳行为的影响。在表面工艺过程中，表面冷轧和扭转超载（预设）在表面产生残余压应力）由于增加了弹性线性范围内的扭矩而进入塑料区域。在本文中，我们考虑在相同材料的表面上引入压缩残余应力的两种不同的工艺过程。我们使用Drucker-Prager准则分析了受不同残余应力和施加应力影响的应力状态，以确定实际应力状态。结果表明，如果最大主应力（结合残余应力和外加应力）未超过安全应力区域，则可以达到疲劳极限。一旦最大主应力到达安全区域的边缘，则失效循环的数量会迅速减少。实验结果表明有效载荷比R_eff，因此应力幅度在钢筋的横截面中变化。这会在最大振幅应力区域_内在表面下引发疲劳裂纹，而与有效载荷比R _eff无关。因此，通过第二工艺增加表面上的压缩残余应力对远离表面的原位疲劳裂纹萌发没有显着影响。增大塑料扭转预应力可使最大应力幅度远离表面，但应保持大量的弹性加载材料，以确保与实心杆截面表面的压缩应力保持平衡。