Study Of Fatigue Behavior And Life Prediction Of Dual Phase Steel Spot Welds

It's very urgent for automotive manufacturers to improve their competitiveness by reducing fuel consumption to cope with the problem of energy shortage and environmental pollution. Auto light weight project has been the main focus of automotive technical research field. Traditional low carbon steel and high strength steel is now gradually replaced by dual phase steel (DP). DP steel's tensile strength reaches 600Mpa or higher, which makes it possible to decrease the auto-body weight while sustaining vehicle's strength and stiffness. Resistance spot welding is still the dominant joining method for auto-body spot-welded structures. There's severe stress concentration around these spot welds which could lead to fatigue crack. Most of the fatigue cracks in the auto-body appear at the spot welds. Spot weld fatigue means the crack initiation and propagation process in the spot weld microstructure. It's obvious that microstructure variation of DP spot weld changes the process of crack initiation and propagation, fatigue strength of DP spot weld is changed. Therefore traditional low carbon steel spot weld experimental fatigue data can't be applied for estimation of DP spot weld. Spot weld in autobody is under multi-axial variable load, which leads to the failure of fatigue prediction of S-N method. Based on above background, fatigue performance and life prediction of DP spot weld was studied in this dissertation. With the experiment results microstructure variation of DP spot weld and effect on fatigue performance was studied, and its influence on crack initiation and propagation was analyzed. Finite element analysis was applied to analyze the stress and strain layout at the spot weld, and based on the result fatigue crack initiation location was determined. With experimental strain measuring device the relationship between real-time strain and crack initiation and propagation stage was established. Crack depth and fatigue life was correlated by real-time strain. With all those experimental data and theoretical analysis model to predict fatigue life of DP weld was established based on the crack propagation path. Based on this fatigue life prediction model a fatigue life prediction system for auto-body structure's DP spot weld was established, which could estimate fatigue life of spot weld in the structure accurately under variable multi-axial load spectrum. To fulfill the above research objectives, the following five aspects of efforts are performed: (1) Microstructure features and fatigue performance analysis of DP spot weldSpot welding system using servo gun and fatigue test platform were built. Geometrical factors'influence on fatigue life of DP spot weld, including specimen geometry, nugget size, width, sheet thickness, were all studied. Considering interfacial fracture of DP spot weld its influence on fatigue performance was also discussed.Fatigue experiments were done to investigate the microstructure variation of DP spot weld. Fatigue behavior of DP and low carbon steel spot weld was compared. Microstructure variation's effect on DP spot weld was carefully analyzed.(2) Experimental analysis of crack initiation and propagation processCrack initiation location was also explained and verified by results from theoretical and finite element analysis of stress and strain layout of DP spot weld. Crack initiation location of DP spot weld was verified by sectioning the spot weld. Strain measuring and cross-sectioning was used to analyze the different stage of DP spot weld fatigue life. After real-time monitoring of strain at the different stage, it's correlated with crack depth of cross-sectioning specimens. And Mathematical relationship between fatigue life and crack depth was established. Ratio of different stages to the whole fatigue life of DP spot weld was determined.(3) Study of stress intensity factor of spot weldBased on the exact fatigue crack propagation route, two different stress intensity factors (SIF) were defined: global SIF at interface of spot weld and local SIF at the tip of propagating crack. Mathematical formulas were established to catch the global SIF for different geometrical types of spot weld, and then were verified by three-dimension finite element analysis (FEA). How the global SIF change with different peripheral angle was studied by FEA. FEA spot weld model with defined crack depth was established by sub-modeling technique. With this model variation of SIF at the crack tip was qualitatively calculated, which explained fatigue crack propagation characteristic of DP spot weld.(4) Fatigue life prediction model for DP spot weld based on crack propagation path Crack tip equivalent stress intensity factor was determined as the feature parameter correlating DP spot weld fatigue life based on crack propagation path. It's proven by experimental data that this parameter can effectively correlate DP spot weld of different geometry, and reduce the scattering of fatigue life data. Model for estimating fatigue life of DP spot weld was established based on Paris equation, which showed good agreement with new verifying fatigue life data.(5) Building and application of system for predicting fatigue life of DP spot weld of auto-body structuresA new system for predicting fatigue life of DP spot weld on auto-body structure was built based on the estimation model. It solved the problem of difficulty in predicting spot weld fatigue life under variable load range. It was applied to predict DP spot weld fatigue life of damper bracket, where spot weld fatigue normally appeared first in the whole vehicle body-in-white. Damage of different load range was taken into account by damage linear accumulation law. Fatigue result of DP spot weld under vehicle load spectrum was achieved with this new system, which could be used for indicating dangerous location of structure and compared with field test result of vehicle. Spot weld's design and optimization can be realized based on these analytical results.In this study, by absorbing and referring the exist achievements, study of fatigue behavior and life prediction of DP spot weld was carried out. DP spot weld fatigue behavior was presented and a new life prediction model was established. A new engineering system for estimating fatigue life of spot weld of structure in auto-body was built and applied to typical fatigue structure. The works not only provide tools for predicting fatigue life of DP spot weld, but also give suggestions on how to make spot weld design.