Study On Characteristic And Crack Propagation Of Unequal-thickness Resistance Spot Weld Between DP590 And DC01

With the continuing improving requirement of automotive safety technology,energy saving and environmental protection legislations,it is a growing popularity in the designing and manufacturing of car body skeleton with the application of new High Specific Strength Materials represented by advanced high strength steel.In the modern vehicle body designing,the modern car body is commonly characterized by the hybrid welding of unequal thicknesses sheets of advanced high strength steel and ordinary low-cost low carbon steel.The mechanical performances of base metal materials are quite different and unequal sheet thicknesses and other factors lead the dissimilar steel and unequal thicknesses resistance spot weld joints between advance high strength steel and low carbon steel more complex than the traditional welding joints between the same material and thickness in microstructure and mechanical properties.The existing crack extends fatigue life prediction method is based on the equivalent stress intensity factor which is simplified by the joint surface crack.This method doesn`t consider the effect of propagation and extension of three dimensional crack,thus it does not meet the requirements of accurate prediction of the fatigue life of spot welding joint.In this paper,the unequal thicknesses of welding joints welded with DP590/DC01 are studied,and the influences on the stress intensity factor of three dimensional kinked crack and equivalent stress intensity factor is presented.Bases on the equivalent stress intensity factor,a fatigue life prediction method of spot weld is presented and corresponding experiments are carried out to verify the prediction method.Firstly,the microstructure characteristics,influencing factors and mechanical properties of weld joint are studied by experiments,and the crack initiation position of dissimilar steel and unequal thicknesses weld joint is determined by finite element analysis and experimental results.The fatigue crack initiation and propagation characteristics of spot welded joints are investigated by fatigue tests and fracture surface analysis of fatigue failure joints.And then,the influences of crack shape,crack depth and propagation angle on the stress intensity factor of the kinked crack are studied by using the finite element method.The equivalent stress intensity factor of three dimensional kinked crack and the fatigue life prediction method of welded joint are proposed.This fatigue life prediction method is based on the piecewise integral algorithm of equivalent stress intensity factor of three dimensional kinked crack.Finally,the rationality of the fatigue life prediction method is verified via the comparison of the experimental data.The main research results and conclusions of this paper are summarized as follows.?1?Characteristics and effect factors investigation of dissimilar steel and unequal thicknesses resistance spot weld jointThe increasing of welding current and welding time is helpful to improve the maximum tensile force of spot welding joint.The holding time is an important factor affects the crystallization structure growth and transformation in weld fusion zone and increasing of holding time is helpful to improve the micro hardness of the weld nugget,and improve resistance shear failure capability of welding joint.Meanwhile,when the holding time is exceeded than that of 15cycles,the continuing increasing of holding time does not significantly increase the welding nugget hardness,but reduces the welding efficiency.The deviated distance from the designed poison is another factor affects the mechanical properties of welding joint.The distance from weld center to the edge of base metal greater than 1.6 times the radius of weld joint can ensure to achieve more than 90%of the maximum tensile load of the weld joint at the ideal design position.?2?Failure characteristics study of dissimilar steel and unequal thicknesses resistance spot welding jointThe fatigue failure of spot welding joint is essentially the process of crack initiation at heat affected zone and expansion pierce through the sheet thickness and leading the fatigue facture of the weld joint.With typical welded joints fatigue test,the fatigue fracture morphology analysis and finite element analysis show that the heat affect zone of DP590/DC01 weld joint where the position with different material is the weak point,and it is the fatigue initiation in the fatigue process.The crack first penetrates the DC01 base metal along the plate thickness,and then expands along the circumference of the weld joint,causing the weld joint to pull out of the DC01 base metal,and an arc shaped crack surface is formed via the crack propagation path.?3?The study of stress intensity factors for dissimilar steel and unequal thicknesses weld jointThe investigation results via three dimensional finite element method show that the crack depth,crack angle and the shape of cracks affect the stress intensity factor of kinked cracks.The stress intensity factor of the main crack in the weld joint shows periodic distribution characteristics in the circumferential direction of the weld joint,and the stress intensity factors K<sub>I</sub> and K<sub>II</sub> are obviously larger than K<sub>III</sub>.The stress intensity factor K,which extends in the width direction of the weld joint,is 2 times equivalent to the stress intensity factor along the plate thickness direction.The length of the crack depth of the plate thickness reaches 30%,the stress intensity factor of kinked crack is 2 times equivalent to the main crack stress intensity factor,when the cracks extend to the thickness of the 50%depth,the stress intensity factor of kinked crack is 4times equivalent to the main crack stress intensity factor.With the crack growth,the stress intensity factor at the crack tip continues to increase obviously.Therefore,the effect of crack length in the crack propagation can`t be neglected in the calculation and application of the crack stress intensity factor.?4?Fatigue life prediction investigation of dissimilar steel and unequal thicknesses spot welding joint based on crack propagationThe equivalent stress intensity factor of three dimensional kinked crack of spot weld is proposed in this paper.It is combined with the investigation of crack propagation process of dissimilar steel and unequal thicknesses weld joint,and considered the crack propagation process,such as the effects of crack shape,crack depth and crack angle etc.According to the Paris law,a fatigue life prediction method based on equivalent stress intensity factor of kinked crack is proposed for dissimilar steel and unequal thicknesses welding joint,and the experimental results show the feasibility of the fatigue prediction method.