Study On The Failure Criterion And Modeling Methods Of High Strength Steel Spot Welding Joints

The spot welding has been the primary joining method used in the construction of the automotive body structure made of steel since 1940 s.With the increase of family car ownership in China,the car has become the main way to travel.The first part in the vehicle body to failure is the spot weld structure when a car crash happened,which will undermine the load capacity of the structure.So the security of the spot welding is significant to the automotive industry.Based on complex loading condition,there are fewer failure criterions related to the strength of base metal,thickness,nugget size and strain rate applied in vehicle simulation.So there are significant theoretical meaning and engineering value to do a research on the failure strength of the spot weld in complicated loading cases.This thesis proposes the static and dynamic failure criterion of the spot weld after doing stress analysis in HAZ,and the failure parameters has been gotten by the spot weld static and dynamic test.The plastic properties of the nugget materials is gotten through an inverse FE modeling combining the particle swarm optimization algorithm and the standard Vickers indentation test and the result is used as an input in the simulation.The paper's main research work and conclusions are as follows:This thesis do an analysis to the stress distribution in the region of the spot welding caused by the complex loading and propose a spot welding static failure criterion related to the strength of base metal,thickness and nugget size The static failure parameters are acquired through lap-shear and U-shape tests and the static failure criterion is verified by coach-peel and torsional tests.The influence of strain rate on the carrying capacity of the spot weld has been studied and a strain rate related failure criterion is purposed.The dynamic failure parameters which gotten from the lap-shear and cross tension tests are used in the simulation of the drop weight impact testThe plastic parameters in the spot welding region do a great influence to the precise of the simulation,so a new inverse FE modeling methodology to extract the plastic properties of the materials combining the particle swarm optimization algorithm and the standard Vickers indentation test was presented to solve the problems that some materials like nugget are too small to conduct the uniaxial tensile test to get the plastic properties parameters.In this thesis,the power law material model was used.The plastic property of the metal material was gotten by minimizing the difference between the force-displacement carve getting from the standard Vickers indentation test and the curve getting from the finite element analysis.To prove the validity of the spot welding failure criterion,the criterion has been compiled to generate a new solver in LS-DYNA.Geometric models of spot weld static test and dynamic test were modeled in UG and the file then was sent to Hypermesh to mesh?set the boundary condition and loading.Finally a k file was generated.The HEX element is selected to simulate the spot welding and the k file was submitted to the new solver to calculate.The comparison result of simulation and the experimental data show that the spot welding failure criterion proposed in the thesis can do a precise prediction to the strength of the spot welding.