压痕对车轴钢疲劳极限的影响

Indentation effect on the fatigue limit of axle steel

利用硬度计在光滑沙漏状车轴钢疲劳试样上制造压痕,同时利用电火花在试样上加工缺陷,通过疲劳试验研究两种缺陷尺寸与试样疲劳极限之间的关系.将两类试样的测试结果和基于材料硬度、缺陷投影面积的Murakami模型计算结果进行对比.利用扫描电镜观察试样疲劳断口.结果表明,与计算结果相比较,压痕局部塑性变形导致的加工硬化和残余应力对试样的疲劳强度没有影响,裂纹依然从应力集中最大的压痕底部起裂.电火花缺陷表面粗糙度较大引起二次缺口效应,表面硬脆的重铸白层上还有微孔和微裂纹存在,导致此类试样疲劳强度低于模型计算结果,裂纹从电火花缺口底部多处萌生.

A hardness tester was used to make indentations on the hourglass fatigue specimens of axle steel,and notches were machined by electronic discharge machining（ EDM）.The fatigue limits of both indentation specimens and notched ones were examined and compared with the theoretical values according to the Murakami formula based on material hardness and defect projected area.The fracture surface was observed by scanning electron microscopy.It is found that local work hardening and residual stress caused by plastic deformation of indentations have no significant effect on the fatigue limits in comparison with the predicted values by the Murakami equation.Fatigue cracks originate from the bottom of indentations due to stress concentration effect.The fatigue limits of notched specimens are lower than the calculated values because of secondary notches caused by the high roughness of notch surfaces and the existence of microcracks and micropores within the re-cast layers.Cracks initiate from multiple sites on the bottom of electronic discharge machining notches.