Influence of Fracture Bhehvoir on the Rotating Bending Fatigue Properties of Sureface Hardened Steel

研究了表面硬化条件(表面硬度、芯硬度、有效渗层深度等)和缺口形状(应力集中系数)对旋转弯曲疲劳性能(疲劳极限、裂纹萌生强度、疲劳裂纹的萌生和扩展)的影响。对表面淬火钢的断裂模式进行了研究，分析了它们之间的相互关系。结果表明：1)疲劳极限直接影响疲劳极限E-C-D、岩心硬度和α-gt；，这些因素会改变断裂模式(表面源型断裂的内源型断裂)，从而影响疲劳极限；2)对于内源型断裂，E-C-D、岩心硬度或-lt；α-gt；会使疲劳极限提高，导致断裂模式由内源型向表源型转变。当内源型断裂转变为表面源型断裂时，疲劳极限成为最大值。其原因是裂纹萌生深度随应力强度的增大而加深，芯部硬度的增加使裂纹萌生强度增大，内部应力随α和gt；的增大而减小。&lt；α&gt；的增加。疲劳极限最大值后的E、C、D和芯部硬度使断裂模式为表面源型断裂，降低了疲劳极限。3)表面源型断裂时，疲劳强度随&lt；α&gt；E、C、D和芯部硬度的增加而降低。其原因是缺口根部的外加应力随着&lt；α&gt；的增大而增大，而缺口根部的残余压应力则随着E、C、D和岩心硬度的增大而减小。然而，当&lt；α&gt；超过一定值时，识别出非扩展裂纹，疲劳几乎是恒定的。

The influence of the surface hardening conditions (surface hardness,core harhness effective case depth (E C D).etc) and the notch shape (stress concentration factor <alpha> ) on the rotating bending fatigue properties (fatigue limit,crack initiation strength, initiation and propagation of fatigue crack. non-propagating crack, etc) and the fracture mode of surface hardened steel were investigated and those mutual relationshipe were analyzed. Thersults obtained are summarized as followes :1) The fatigue limit is directly influenced E C D, the core hardness and <alpha> ,and the fracture mode (inner origin type fracture of surface origin type fracture ) is changed by these factors,therefore the fatigue limit is influenced by the change of the fracture mode.2) In the case of the inner origin type fracture, the increase of E C D, the core hardness,or <alpha> makes the fatigue limit higher and induces the change of the fracture mode from the inner origin thpe to the surface origin type. When the inner origin type fracture changes to the surface origin type fracture, the fatigue limit becames the maximum value. The reason is that the fracture initiation becames deep with increasing E C D the increase of the core hardness makes the crack initiation strength higher, and the applied stress at the inner part dereases wity inreasing <alpha>. The increase of <alpha>. E C D and the core hardness after the maixmum value of the fatigue limit makes the fracture mode the surface origin type fracture and dereases the featigue limit.3) In the case of the surface origin type fracture, the fatigue decreased with increasing <alpha> E C D and the core hardness. The reason is that the applied stress at the notch root becomes higher wigh increasing <alpha> and the residual compressive stress at the notch root decreases with increasing E C D and the core hardness. However when <alpha> exceeded a certain value non-propagating cracks were recognized and the fatigue almost constant.