Very‐high‐cycle‐fatigue (VHCF) strength properties are of interest to several technical applications assessed globally at different laboratories with long‐life fatigue testing capabilities. Also, VHCF failure mechanisms are a scientific topic with remaining open research questions. Herein, three automotive bar grade steels are studied with respect to VHCF strength and initiation mechanisms. A microalloyed ferritic–pearlitic steel (38MnSiV5, 870 MPa tensile strength), a quenched and tempered martensitic steel (50CrV4, 1410 MPa tensile strength), and a carburizing steel (16MnCr5, 1180 MPa core structure tensile strength) are studied to reveal characteristics regarding initiation and VHCF failure mechanisms. A 20 kHz ultrasonic fatigue testing instrument is used to obtain fatigue lives up to and above 10⁹ load cycles in uniaxial loading. Hour‐glass specimens, smooth or notched, are tested at R = −1 and R = 0.1. Fatigue strength and stress life (SN)‐diagram data are achieved, and crack initiation and growth mechanisms are studied using primarily field‐emission gun–scanning electron microscopy (FEG–SEM). Fatigue strengths are explained by a modified life‐dependent Murakami‐expression, the Haigh diagram, and notch sensitivity. Interior and surface crack initiations by surface defects, triple points, and inclusions are found. The fine granular area (FGA) to fish‐eye crack growth transition conditions are explored and schematic descriptions are given.

中文翻译：

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汽车钢的超高循环疲劳状态下的疲劳强度和断裂机理

超高循环疲劳强度（VHCF）强度特性是在具有长期疲劳测试能力的不同实验室中在全球范围内评估的几种技术应用的关注点。此外，VHCF失效机制是一个科学问题，尚待解决的开放研究问题。在本文中，针对VHCF强度和起爆机理研究了三种汽车棒材。研究了微合金铁素体-珠光体钢（38MnSiV5，抗拉强度为870 MPa），调质马氏体钢（50CrV4，抗拉强度为1410 MPa）和渗碳钢（16MnCr5，核心结构抗拉强度为1180 MPa），启动和VHCF失败机制。使用20 kHz超声波疲劳测试仪来获得10 ⁹以上的疲劳寿命单轴载荷下的载荷循环。在R  = -1和R  = 0.1的条件下测试光滑或有缺口的沙漏样品。获得了疲劳强度和应力寿命（SN）图数据，并主要使用场发射枪扫描电子显微镜（FEG-SEM）研究了裂纹的萌生和扩展机制。疲劳强度可以通过修正的依赖于生命的村上式表达，Haigh图和缺口敏感性来解释。发现由表面缺陷，三重点和夹杂物引起的内部和表面裂纹萌生。探索了细颗粒区域（FGA）到鱼眼裂纹扩展的过渡条件，并给出了示意图。