One of the most critical and important fracture mechanisms in a FMVSS207/210/225[1] test is the pull-thru of bolts from the body structure or spot weld separation. There are no analytically proven methods of making a judgment of pull-thru occurring except through evaluation of the plastic strain or through the thickness strain value around projection welds on Weld nut/stud bolt or spot welds. Therefore it is essential to have accurate criteria to evaluate the pull-thru. During elastic deformation, the sheet steel deforms while the quasi-static force is being applied and then returns to its original shape when the force is released. But when the force causes a stress that exceeds the yield strength, the sheet steel will permanently elongate with each additional unit of force applied, and it will not return to its original shape and size. The permanent change in shape of a part resulting from the application of a load exceeding the yield strength of the material is referred as plastic deformation or plastic strain which can be an indicator of failure plastic strain in a sheet steel.
This paper describes the method of establishing the criteria for failure plastic strain on frequently used materials to predict the pull-thru phenomenon during FMVSS207/210/225 test, and also demonstrates a sample test set-up and the corresponding model. A total of 51 sample tests, including mild steel, high strength(HS) and dual phase(DP) materials, and the corresponding analyses using LS-DYNA3D [2] have been conducted. It is possible to estimate accurate correlations between the analysis and the test, which can then be used to evaluate failure plastic strain. Eight parameters including mesh pattern and size, thickness, material, clamping force have been investigated.
Finally, failure plastic strain on the individual material has been established and proposed for users' application, and it enables the evaluation of FMVSS207/210 performance. The method was further extended to develop Strain-based spot weld and modeling technique.