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本文利用35NCD 16和35CD4型号高强度钢的ASTMT15型试样的试验研究了疲劳裂纹起裂的问题。试样具有底部半径为0.07—1毫米的机加工缺口。在本研究中,所使用的测量仪器可以测试扩展平面范围为几个平方毫米的表面裂纹。为了确定裂纹起裂阶段时间,我们力求用下述不同参数表征缺口底部金属的性能: (1)来自断裂力学概念的ΔK2~(1/P),它能集中地表达裂纹发生时间的数据。 (2)紐伯提出的公式中的缺口效应系数K_f。最有明显作用的参数是从所研究的材料低周疲劳性能的实验所确定的参数。相应于一个给定的裂纹起裂时间的标称应力数值而言已知相差近±10％,这相当于理论时间和实验数据之间的比值,对于给定的标称应力其比值变化范围为1～2。最后,我们研究了金相组织对裂纹起裂时间的影响,特别是残余奥氏体的存在对裂纹起裂时间的影响。
In this paper, the use of 35NCD 16 and 35CD4 high strength steel type ASTMT15 test specimens studied the problem of fatigue crack initiation. The specimen has a machined notch with a base radius of 0.07-1 mm. In this study, the measuring instruments used can test for surface cracks in the extended plane range of a few square millimeters. In order to determine the initiation time of the crack, we sought to characterize the properties of the metal at the bottom of the notch using the following different parameters: (1) ΔK2 ~ (1 / P) from the concept of fracture mechanics, which can collectively express the data of crack occurrence time. (2) The notch effect coefficient K_f in Newberg’s formula. The most significant parameters are the parameters determined from experiments on the low cycle fatigue properties of the materials studied. The known nominal stress values for a given crack initiation time are known to vary by approximately ± 10%, which is equivalent to the ratio between the theoretical time and the experimental data and for a given nominal stress the ratio varies by 1 ~ 2. Finally, we study the influence of microstructure on crack initiation time, especially the effect of retained austenite on crack initiation time.