Fatigue Damage Of Spot Structure Under Two-level Loading

Resistance spot welding technology is widely used in various industry fields including automobile, aerospace, nuclear energy, electronics, light industry, household appliances, dueing to its operation simplicity, low cost and high efficiency. Spot structure is often under multiaxial stress and complex loading conditions, so its mechanical analysis is very difficult.In many fatigue studies, two level load is often used as the basic form of the complex loading conditions, so the fatigue studies of spot weld structure have very important practical significance.In this paper, a single spot welding specimen which is made of ST12low carbon steel plate is chosen researched. The spot welding specimens are set on MTS fatigue testing machine with constant amplitude and two-level loading test. And spot welding joints metallographic and Vickers hardness are tested. The effects of the load sequence and load levels on fatigue damage and life both under constant amplitude and two-level loading are analysed from theoretical and experimental aspects. The paper mainly studied the following aspects:1. The welding process test plan of spot weld specimens is designed reasonably. The best welding process is determined by spot welded specimen tensile-shear fatigue tests. Vickers hardness around the welding are measured. Through the macroscopical and microcosmic metallographic observation, the micro structure composition of different regions around weld joint and its formation are analysed.2. Constant amplitude cyclic loading is operated on the spot welded specimens. After analysis, obtained variation rule of fatigue life changing with stress amplitude, maximum load and load ratio. Crack form on the spot welded specimen and cause of formation are observed. 3. The spot welded specimens tests are carried out in three different loading levels and two stages loading (which refer to the low-high and high-low loading tests). Two level-loading load order and load level which are related to cumulative fatigue damage and fatigue life influence are analysed. Under two stages loading, variation of the total fatigue life, residual life and cumulative fatigue damage value with the first level load cycle number is found out.