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、心部硬度、有效硬化层深度（E C D）等）和缺口形状（应力集中系数<alpha>）对旋转弯曲疲劳性能（疲劳极限、裂纹萌生强度、疲劳裂纹萌生和扩展）的影响。非扩展裂纹等）和表面硬化钢的断裂模式，并分析了它们之间的相互关系。结果表明：1）E-C-D、心部硬度和心部硬度直接影响疲劳极限<alpha>，并改变了断裂方式（内源型断裂或表面源型断裂），从而影响疲劳极限; 2）对于内源型断裂，E-C-D、心部硬度和心部硬度的增加使<alpha>疲劳极限提高，导致断裂方式由内源型向表面源型转变。当内源型断裂转变为表面源型断裂时，疲劳极限达到最大值。其原因是随着E-C-D的增加，裂纹萌生深度变深，心部硬度的增加使裂纹萌生强度增加，而内部所受应力随E-C-D的增加而减小<alpha>。的增加<alpha>。当E-C-D和心部硬度达到疲劳极限最大值后，其断裂方式为表面起源型断裂，降低了疲劳极限; 3）在表面起源型断裂情况下，疲劳强度随<alpha>E-C-D和心部硬度的增加而降低。其原因是<alpha>随着E-C-D和心部硬度的增加，缺口根部的应力增大，而缺口根部的残余压应力减小。然而，当<alpha>超过一定值时，非扩展裂纹被识别，并且疲劳几乎恒定。

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.