Study On Crack Propagation And Fatigue Life Of High Strength Steel Spot Welded Joint Under Residual Stress

High-strength steel,as a new generation of automotive steel,has excellent material properties.Resistance spot welding is the most commonly used connection technology in its body manufacturing.Spot welds play a role in helping various parts of the car body to transmit loads.However,they are prone to fracture when subjected to impact,collision,and fatigue loads.The residual stress from spot welding can affect crack propagation and reduce the structural safety and fatigue life.Therefore,when studying the crack propagation and fatigue life of high-strength steel spot welds,the factor of residual stress cannot be ignored.This article focuses on the spot welds in automobiles that are subjected to tensile-shear loads,and studies the mechanism of nugget formation and the distribution of residual stress during the welding process.Taking residual stress into consideration,the fracture process of spot welds under high strain rate loading is analyzed at both the microscopic and macroscopic levels.The influence of residual stress on crack propagation in car body welds when rapid fracture occurs is explored.By comparing the local stress intensity factors of spot welds with and without residual stress,the law of fatigue crack propagation under residual stress is derived.The expression of the equivalent stress intensity factor with respect to the ratio of crack depth is obtained using a polynomial extrapolation method,and a fatigue life prediction model of spot welds under residual stress is established.The fatigue life and crack propagation rate under the conditions of with and without residual stress are compared and analyzed,and the effect of the pre-stressing process to reduce residual stress on the fatigue life of spot welds is discussed.The main conclusions of this study are as follows:(1)The formation of the nugget is inseparable from the contact pressure and current density.It first appears at the contact surface of the two workpieces and grows both axially and radially,with the radial growth rate greater than the axial growth rate.During the welding process,the nugget region is mainly characterized by compressive stress,and the maximum compressive stress is also concentrated in this area.The residual tensile stress generated by spot welding mainly distributes in the nugget and the heat-affected zone.When the distance from the center of the nugget is greater than 2.2times the radius of the nugget,the residual stress appears as compressive stress.(2)At the macroscopic scale,residual stress reduces the load-bearing capacity of spot welded joints,shortens the time to crack initiation,and leads to premature failure of the component.The maximum shear force that the joint can withstand decreases by about 6.4%.Macroscopically,the fracture manifests as the initiation and coalescence of multiple intergranular cracks.When residual stress exists,the initiation location of intergranular cracks changes,and the initiation time is advanced.The number of cracking elements and the crack area increase significantly during the later stage of crack propagation,and residual stress causes the fracture area to increase by about60.8%.(3)Residual stress has almost no effect on the global stress intensity factor and the local stress intensity factor of an infinitesimal straight crack.When the depth of the straight crack is not zero,residual stress has the most significant effect on the opening stress intensity factor,and the degree of its influence increases with the increase of the ratio of crack depth.When the ratio of crack depth is 1,the increase in k_I is about 226%.(4)Under the action of residual stress,the overall trend of fatigue crack propagation remains unchanged,but the propagation rate significantly increases.The number of cycles that the crack undergoes to propagate to the same depth decreases significantly,and the fatigue life decreases by about 64.7%.Pre-stretching can significantly reduce residual tensile stress,improve fatigue life,and the improvement effect is positively correlated with the pre-stretching load.The optimal pre-stretching load is about 50%of the failure load of the joint.