Study On The Modelling And Failure Behaviors Of High Strength Steel Spot-Welding Joints

Spot weld is one of the major connection joints in automotive industry and its failureis an important issue for vehicle crash safety. As the improvement of vehicle lightweight,high strength steel has been applied in vehicle body and more and more attention has beenfocused on the investigation of spot weld of high strength steel. To save the cost and time,finite element simulation is widely used to optimize the structure design. As the vehiclesafety regulations are becoming stricter, it is very important to build a precise finiteelement model of spot weld of high strength steel.The specimen design of shear and notched tension tests were discussed in detail. Thefixtures of punch tests were also improved. To further research the material properties andfailure parameters of welded zone, a new design of small spot-weld test specimens andtheir test fixtures were provided in the thesis. This test method can simplify themanufacture process of specimen and test procedure. Moreover, due to the complicatedstructure in spot weld zone, the repeatability of spot weld tests usually cannot be wellguaranteed. A simple welding fixture was provided to improve the consistency of weldlocation in the specimen.Besides, a new identification method of high strength steel’s hardening curve wasprovided in this thesis, with which one can get the whole optimized hardening curve byadjusting only one parameter. The mesh size effect of failure prediction was discussed indetail using the finite element model of uniaxial tension. At the same time, parameters oftwo failure criterion：shear-modified Gurson model and Modified Mohr-Coulomb model(MMC) were identified with four different types of material tests results. With themeasurement of nugget in cross-tension specimens after test and hardness curve of spot-weld, the dimension of spot weld zones: Base Metal (BM), nugget and Heat AffectedZone (HAZ) was determined. The material properties of nugget and HAZ were alsocalibrated with the small spot weld specimen tests.In the thesis, simplified model and detailed model were evaluated. In simplifiedmodel, the material properties of HAZ and nugget were replaced by BM and Rigid,respectively. And two prevailing failure criterion, shear-modified Gurson model andModified Mohr-Coulomb model, were selected to predict pull-out failure of spot weld.The results shown that both models can predict failure of spot weld accurately in both lap-shear and cross-tension tests. In the detailed model, the parameters of these threezones in spot weld were used to predict the lap shear tests. The results shown that bothpull-out failure and interfacial failure can be predicted by the fine finite element model.In the end, the simplified model was discussed and provided reference for further research.