Theoretical And Experimental Investigation Of Forming Limit Stress Diagram In Sheet Metal Forming Analysis

Sheet metal forming is one of the most important metal shaping processes . It is extensively used throughout the major automotive, aeronautical and electrical appliance manufacturing industries and thus forms a vital part of the manufacturing industries of modern industries nations. In sheet metal forming analysis, it is indispensable to study forming limit and forming limit predict . The forming limit diagram (FLD), is a useful diagnostic tool for trouble shooting in sheet metal forming operations and is also an assessment method for determining the formability of different sheet materials. But FLD represented in strain coordinate system is strain path dependent and is therefore unable to give an accurate result in complicated deformation analysis, especially in analysis of multi-step deformation.This study is devoted to checking and verifying if the forming limit stress diagram (FLSD) could serve as a better criterion for limit analysis. This dissertation focuses on principal problem of FLSD in limit analysis application, correlative applied fundamental researches have been carried out.The main work and the results obtained in this dissertation are as follows:1. The shortcoming of FLD and the future of FLSD is discussed in sheet metal forming limit analysis complicated strain path are analyzed systemically, it is put forwarded that scheme using the combination of theoretical analysis with experiments to solve application of FLSD.2. The classic theory of plasticity interrelated with sheet metal forming was summarized, specially described different yield criterion and instable theory and its applied scope has been discussed. According to Swift instable theory , limit strain predicted derivation be carried out.3. Based on analysis of plasticity, according to"Sheet Metal Forming Property and Experimental Method", the forming limit experiments are performed with five kind of materials (SPCEN,DC01,DC03,08AL,LF21M)and three thick metal sheet of s=0.8mm, 1.2mm, 2.1mm. testing plans include different strain path such as proportion loading, uniaxial pre-strain, equ-biaxial pre-strain and plane pre-strain. FLDs in different strain path been established by testing and characteristic are analyzed and compared.4. The transformation relation from FLD to FLSD been derived in diverse yield criterion, FLSDs in different strain path have been built on the basis of data FLD and characteristic are analyzed and compared. At the same time, it be implemented to establish FLSD by FEM.5. The instable predict model be deduced according to principle of Continuum Damage Mechanics, then testing study by regarding the change of sheet metal,s density as the damage variable was carried out, on sheet metal of SPCEN and LF21M. FLDs are established by predict model, then the from FLD transform to FLSD by transformation relation and characteristic are analyzed and compared with FLSD established by testing. The result indicates that FLSD by means of CDM predict model is evident independent of strain path.6. By analysis relation between limit strain, limit stress with deformation parameters and materials properties parameters,integrating the advantages of the fuzzy set and the artificial neural network theory,a FLSD predict model sheet metal deformation has been established according to the experimental results. Meanwhile,the prediction model be checked. The comparison of the prediction results with the experimental of the test samples shows that the prediction model is stable and reliable.7. It is developed that the finite element analysis program based on the stress forming limit criterion applicable to the sheet metal forming analysis, which selects the commercial finite element code DYNAFORM as its development environment. Also it analyzes the finite element numerical simulation results of two step deep drawing parts by the developed program module and realizes the prediction of sheet metal forming limit adopting the FLSD as criterion.8. It is carried out that stamping test of two step drawing part for checking in Nanjing carbuilder, this project develops the program module of using FLSD as criterion for sheet metal forming, uses which to conduct analysis on actual sheet metal deep drawing parts and verdicts that the simulation result is in better coincidence with the result obtained from practical production, so it is feasible to use FLSD as criterion for sheet metal formability analysis.The research results in this dissertation show that that forming limit stress diagram is independent of strain paths. And it gives a good new criterion for multi-step sheet metal forming in finite element numerical simulation analytic.