Study on Statistical Giga-cycle Fatigue Property of High Strength Steels and Establishment of Reliability Assurance Engineering

主要研究结果如下：（1）几种高强度钢和表面硬化钢中常见的两步S-N曲线可以很好地解释为由表面诱导断裂和内部夹杂物诱导断裂两种不同断裂模式的S-N曲线组成的双重S-N曲线。(2)在长寿命区，上述双重S-N特性在旋转弯曲和轴向载荷下是共同的，但旋转弯曲时各S-N曲线的分离比轴向载荷时明显。(3)内部夹杂物诱发断裂的S-N曲线斜率小于表面夹杂物诱发断裂的S-N曲线斜率。这是由于在室内空气中和真空中疲劳裂纹扩展规律的不同造成的。(4)裂纹起始处夹杂周围细晶区前沿的应力强度因子范围对应于材料疲劳裂纹扩展门槛值ΔK<th>。(5)应力水平下的疲劳寿命分布可以很好地解释为由表面诱导断裂和内部夹杂物诱导断裂各自的寿命分量组成的Weibull分布的混合模式。统计疲劳性能可以通过威布尔参数的应力依赖性为各自的疲劳寿命分布进行定量分析。(6)轴承钢的疲劳寿命可分为裂纹萌生寿命、疲劳裂纹形成寿命和鱼眼形成寿命三部分。

Main results in this research are summarized as follows;(1) Two-step S-N curve often reported on several high strength steels and case-hardened steels can be well interpreted as the duplex S-N curves consisting of two different S-N curves for the respective fracture modes of surface induced fracture and interior inclusion induced fracture.(2) Above duplex S-N characteristics in the long life regime are common for the rotating bending and the axial loading but the separation of the respective S-N curves in the rotating bending is distinct more than that in the axial loading.(3) Slope of the S-N curve in the interior inclusion induced fracture is less than that in the surface induced fracture. This aspect can be attributed to the difference of the fatigue crack growth laws in the room air and in the vacuum.(4) Stress intensity factor range at the front of the fine granular area (FGA) around the interior inclusion at the crack origin is corresponding to the threshold value of the fatigue crack propagation of the material ΔK_<th>.(5) Fatigue life distribution at a stress level is well explained as the mixed mode of Weibull distribution consisting of the respective life components of the surface induced fracture and the interior inclusion induced fracture. Statistical fatigue properties can be quantitatively analyzed through the stress dependence of Weibull parameters for the respective fatigue life distribution.(6) Total fatigue life of the bearing steel can be separated into three components for the crack initiation life, the formation life of FGA and the formation life of the fish-eye.