| Owning to the advantages such as reducing body weight, enhancing vehicle safety, cutting down the fabricating cost and etc, multi-gauge high strength steel laser tailor-welded blanks has become the mainstream technique to fabricate lightweight vehicle.Introducing a weld into a blank alters its homogeneity and continuity, the traditional forming limit diagram (FLD) theory is not applicable to the multi-gauge high strength steel laser tailor-welded blanks. In order to evaluate its formability correctly and effectively, the FLD theory of multi-gauge high strength steel laser tailor-welded blanks needs to be established urgently.Based on the generalization and investigation on the failure mechanism of multi-gauge high strength steel laser tailor-welded blanks, the corresponding FLD theoretical model and research method are established. The Nakazima test is adopted to obtain its FLD. Finally the FLD established in the Nakazima test is applicated into the finite element analysis (FEA) of high strength steel laser tailor-welded blanks foming process as the failure criteria.In the second chapter, the failure type, failure cause of multi-gauge steel laser tailor-welded blank are overviewed. Based on FEA, the failure mechanism of multi-gauge high strength steel laser tailor-welded blanks is investigated in terms of its deformation characteristic, failure type, strain distribution and strain path alteration under the strain states of uniaxial tension, plane strain and biaxial tension respectively. Consequently, the FLD theoretical model and research method which are applicable for multi-gauge high strength steel laser tailor-welded blanks are established.In the third chapter, the Nakazima test is performed to generate the limit strain of multi-gauge high strength steel laser tailor-welded blanks under different strain states. The ARGUS strain measuring system is adopted to measure the limit strain, and the FLD of laser tailor-welded blanks is plotted subsequently. By investigating the deformation characteristic, failure type and strain distribution of different specimens, the FLD theoretical model established in the second chapter is verified.In the forth chapter, the FLD established in the Nakazima test is applicated into the FEA of multi-gauge high strength steel laser tailor-welded blanks forming process as the failure criteria. During the FEA of Nakazima test, the FLD of laser tailor-welded blanks is introduced into Dynaform post-processor as the failure criteria. Good correlation between the simulation and experimental results is indicated, which also shows the validity, reliability and practibility of the FLD research method for multi-gauge high strength steel laser tailor-welded blanks. |
