The fatigue life and fracture mechanism of friction stir spot welded tension–shear joints using 590‐MPa class steel as a base material under constant‐amplitude conditions were investigated with focus on welding dimension variations caused by tool wear. The fatigue limit of the friction stir spot welding (FSSW) joint used for this study is significantly low compared with the static tensile strength of the joint itself. It was clarified that the FSSW joint in this study exhibited two different failure morphologies regardless of the applied load level: base metal fracture and weld area fracture. Although the welding state changes due to the tool wear phenomenon that produce two types of fracture modes in relation to the welding rip diameter, they have no effect on the fatigue strength, regardless of the applied load.

中文翻译：

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搅拌摩擦点焊拉伸剪切接头钢的疲劳特性和断裂机理的阐明

研究了以恒定振幅条件下590MPa级钢为基础材料的搅拌摩擦点焊拉伸剪切接头的疲劳寿命和断裂机理，重点研究了工具磨损引起的焊接尺寸变化。与接头本身的静态拉伸强度相比，本研究中使用的搅拌摩擦点焊（FSSW）接头的疲劳极限明显较低。可以澄清的是，无论所施加的载荷水平如何，本研究中的FSSW接头均表现出两种不同的破坏形态：母材断裂和焊接区断裂。 尽管由于工具磨损现象而导致焊接状态发生变化，而这种磨损现象会产生两种与焊接裂口直径有关的断裂模式，但无论施加何种载荷，它们都不会对疲劳强度产生影响。