运行条件对不锈钢点焊疲劳性能影响及开裂分析

以不锈钢车体底架电阻点焊接头为对象，研究了温度（-40℃、室温和70 ℃）、应力比（0.1、0.5）以及3.5%NaCl腐蚀介质对接头疲劳寿命的影响；借助电子背散射衍射（EBSD）技术、扫描电镜（SEM）、仪表球压痕系统(IBIS)和拉伸剪切试验，对晶粒取向、残余应力分布、疲劳断口形貌和力学性能进行分析。结果表明：低温条件下材料强度的提高可获得更好的疲劳性能，70 ℃条件下塑性性能的改善可使高周疲劳性能更优；应力比为0.1时的疲劳性能优于应力比为0.5时的疲劳性能；腐蚀介质和拉应力促使应力腐蚀开裂和裂纹尖端的阳极区快速溶解，加速裂纹的萌生和扩展，显著缩短了点焊接头的疲劳寿命；疲劳裂纹在界面应力集中尖角处形核，沿熔核软化区长大，逐渐向表面扩张，直至断裂失效。

Influences of Service Conditions on Fatigue Properties of Stainless Steel Spot Welding and Cracking Analysis

Joints of resistance spot welding on stainless steel body frame was used as research objects to find out the influences of temperature (-40 ℃, room temperature, 70 ℃), stress ratio(0.1，0.5) and 3.5% NaCl environmental medium on joint fatigue lifes. The electron back-scattered diffraction(EBSD) technology, scanning electron microscopy(SEM), instrumented ball indentation system(IBIS) and tensile-shear experiment were used to analyze the grain orientation, residual stress distribution, appearance of fatigue fractures and the mechanics properties. Results show that the fatigue property is better due to the increasing of material strength under low temperature conditions. And the high-cycle fatigue property is better due to the improvement of plastic property under 70 ℃. The fatigue property of stress ratio of 0.1 is better than that of 0.5. Corrosion medium and tensile stress may accelerate the strress corrosion, cracking and rapid dissolution of anode regions of crack top ends, accelerate the initiation and extension of the cracks，and may significantly reduce fatigue life of spot welding joints. The fatigue cracks are nucleated on the stress concentration interfaces, the cracks grow up along the softening zones and extend to the plate surfaces until fracture failure.