Nonlinear Cumulative Fatigue Damage And Life Prediction Of Coach Peel Type Spot Weldments

Resistance spot welding,as one of the important technologies for connecting metal sheets,has been gradually applied in the fields of aviation,aerospace,automobile,electronics and railway vehicles because of its advantages of high efficiency,low cost,easy operation and easy automation.However,spot welding components usually work under multi-axial stress and complex loads,so it is difficult to analyze their mechanical properties and life expectancy.At present,researchers often use two or more stages of loading as the basic form of spot welding fatigue study under variable amplitude conditions.However,due to the high load carrying capacity,tension-cutting spot welding has attracted more researchers' attention.And because of the simple linear Miner theory,Easy to calculate,usually widely used in spot welding luffing loading research.However,due to the low carrying capacity and complicated stress of the T-shaped tearing tensile specimens,the fatigue damage is usually easy to occur in the structural members.Therefore,the study of such geometric forms of spot welding fatigue can not be neglected.At present,most of the studies on the T-tearing tensile spot welding specimen fatigue data less,and to accurately describe the physical properties of nonlinear cumulative damage less.Therefore,in this paper,spot-welded specimens made of ST12 steel subjected to tearing tensile test are studied,and the cumulative damage and fatigue life prediction under linear and non-linear conditions are studied,and the influence of load level is considered.The main content of the thesis is divided into the following sections:First,the tensile test T-point tensile splint static tensile test to determine the mechanical properties.Through the static test results,the spot welding constant amplitude fatigue test with different load levels was designed and the relationship between the maximum load,load amplitude and load ratio and fatigue life was analyzed.The effects of different load levels on the crack initiation and fracture path of T-spot welding specimens were observed.Secondly,T-type tensile splitting tensile welding under different load levels of two low-high,high-low luffing load test,the use of linear Miner theory analysis of the experimental data to observe the load sequence and load levels between the two levels On the cumulative fatigue damage,and the difference between the two levels of load introduced Miner theory to predict the fatigue life under the second load.Four typical nonlinear fatigue cumulative damage theories are used to predict the fatigue life of the second-stage spot welding under two-stage loading.The influence of the load level between the two stages was taken as the influence factor to introduce Chaboche nonlinear model into the second-stage spot welding life prediction.The indices in the nonlinear cumulative damage model were fitted by experimental data and correlated with the load levels between the two levels to observe the effect of different load levels at two levels on the index.Finally,predict the fatigue life of spot-welded specimens under constant amplitude by finite element software.The first step is to use the solid element to do the geometric mesh modeling.Based on the relationship between the stress concentration factor and the maximum principal stress and stress amplitude,the maximum principal stress and fatigue life prediction equation is established,and this equation is used to predict the fatigue life of spot welding.After introducing the principle of equivalent structure stress method,different from the notched stress-strain method,nominal stress method,fracture mechanics and other life prediction methods for mesh-sensitive characteristics,this paper verifies the grid-insensitive characteristics of the two spot welding types,The fatigue life of spot welded joints is evaluated and compared.