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、岩心硬度、有效壳体深度（E C D）等）和缺口形状（应力集中系数< α >）对旋转弯曲疲劳性能（疲劳极限、裂纹起裂强度、疲劳裂纹的起裂和扩展）的影响。研究了非扩展裂纹和表面淬火钢的断裂模式，并分析了它们之间的相互关系。结果表明：1)疲劳极限直接受E - C - D、岩心硬度和< α >的影响，断裂方式（内源型断裂或表面源型断裂）受这些因素的影响，因此疲劳极限受断裂方式变化的影响。2)内原点型断裂时，随着E - C - D、岩心硬度或< α >的增大，疲劳极限增大，导致断裂模式由内原点型向表面原点型转变。当内原点型断裂变为表面原点型断裂时，疲劳极限达到最大值。其原因是随着E - C - D的增大，起裂深度增大，岩心硬度的增大使起裂强度增大，而内部的外加应力随着< α >的增大而减小。< α >的增加。E、C、D和岩心硬度超过疲劳极限最大值后，使断裂方式为表面原点型断裂，使疲劳极限降低。3)表面原点型断裂时，随着< α - > E - C - D和岩心硬度的增加，疲劳程度降低。其原因是随着E - C - D和岩心硬度的增加，缺口根部的外加应力逐渐增大，而缺口根部的残余压应力逐渐减小。但当< α >超过一定值时，可识别出非扩展裂纹，且疲劳基本保持不变。

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.