Study On Failure Behaviors Of Two Thin-walled Metal Structures On Vehicle

With the development of lightweight technology, thin-walled metal structures are widely used in vehicle. Two common thin-walled metal structures are thin-walled tubes and spot-welded components. At the early age of vehicle designing, finite element simulation method is applied by the engineers in order to simulate the mechanical behavior of vehicle structure, to increase the vehicle designing efficiency and to decrease product development cost. A good understanding of material mechanical behavior is the prerequisite to accurate simulation, especially to the failure behavior.This research firstly studied the mechanical behavior of thin-walled metal tubes. In order to characterize the behavior of 304 Stainless steel tubular material, the test system in different stress state was established, including axial tension, hoop tension, notched tension, punch test and shear test. The ring hoop tension test(RHTT) method was analyzed with the simplified mechanical model. The impact of friction and specimen position was investigated and the test method was verified with isotropic material. The hoop stress strain curve of 304 Stainless steel tube was measured and ―S-shape‖ was detected in the large plastic deformation phase. The impact of friction changing and martensitic transformation was analyzed with various experiments. The anisotropic property of tube material hardening curve in axial and hoop direction was detected, and the impact of manufacturing technology and martensitic transformation was revealed.The finite element model for each test was established with the fitted hardening curves of axial tension and hoop tension using the SHS(Swift-Hockett-Sherby) model. The Hill’48 anisotropic yield function was optimized and calibrated with Matlab and all the tests was simulated. Based on the simulation results, the history of stress strain evolvement on the fracture initiation point was detected and the MMC(Modified Mohr-Coulumb) model was reached. With the calibrated MMC model, the FE simulation can predict the failure behavior of tube material in some tests, while in some other tests the prediction was not very accurate. The reason for this difference was studied. Because the material has different martensitic transformation behavior in different tests, the hardening property in different tests could be different and cannot be simply described with current material yield function, thus the future research topic was put forward.The second part of this research was the study of the strain-rate effect of spot-weld failure behavior using finite element simulation. The simulation model for spot-weld coupon test was built and the material property for each area was analyzed. The impact of geometry on failure modes was studied. The fracture locus of base metal was optimized and the fracture development in low loading speed was accurately simulated. It was revealed that the phenomenon of fracture position moving from base metal to near fusion area along with loading velocity increasing could be accurately simulated, if the strain-rate effect of base metal hardening behavior was defined stronger than that of fusion area.