Theoretical And Experimental Study On Prediction Of Plastic Forming Fracture Of 7075 High Strength Aluminum Sheet

High-strength aluminum alloy sheet has the advantages of low density,high strength and good impact performance,and is increasingly used in the aerospace field and automobile manufacturing field.However,during the forming process of the high-strength aluminum sheet,cracks are prone to occur,which leads to the generation of scrap parts.Therefore,how to accurately and effectively obtain the forming limit curve that reflects the deformation law of high-strength aluminum plates has been a key issue in the field of sheet forming research.Based on this,this work takes AA7075-T6 sheet as the research object,and studies the applicability of ductile fracture criterion and M-K model to AA7075-T6 sheet.Based on the fracture prediction error analysis and punch-stretch test,the Lou-Huh criterion and M-K model are used to evaluate the fracture prediction accuracy and applicable performance of AA7075-T6 sheet,which provided a reference for the selection of theoretical prediction models for high-strength aluminum sheet.In this paper,AA7075-T6 sheet is used.Ten tensile specimens with different stress states are designed.The material parameters and force curve of the sheet material are obtained.The experimental and simulation benchmarking methods were used to extract the stress triaxiality,Lode parameters,and equivalent plastic strain history data,which laid the foundation for solving the ductile fracture criterion.The Lou-Huh ductile fracture criterion was used to study the fracture of aluminum alloy sheet.The fracture prediction accuracy of different ductile fracture criterion parameters(integral equations method,analytical equations method,and surface fitting method)is discussed,and the optimization method(Method of BP neural network combined with genetic algorithm)is used to optimize the parameters of ductile fracture criterion.Fracture prediction error analysis is used to evaluate the fracture prediction accuracy of different fracture parameter calculation methods and optimization method,and verified by punch-stretch test.The results show that the optimal parameters obtained by the integral equations method and the analytical equations method are close to the global optimal solution obtained by the BP+GA method,and the fracture prediction result of the surface fitting method is the worst.The theoretical forming limit curve of the BP+GA method is very close to the measured limit strain point in the uniaxial tensile stress state to plane strain state region,and closely adheres to the lower limit of the measured data point.However,the lack of test specimens in the region of plane strain state to biaxial-balance tensile stress state results in a large gap between the theoretical FLC and experimental data,which reflects that the calculation of the Lou-Huh criterion parameters is highly sensitive to the stress state of the test specimen.Based on the M-K model,the forming limit curve of AA7075-T6 sheet is drawn,and the prediction accuracy of the M-K model and the Lou-Huh criterion is evaluated by the fracture prediction error analysis and punch-stretch test.The results show that the MK model has a higher prediction error of the fracture strain of the AA7075-T6 sheet specimen;the theoretical FLC based on M-K model and the test principal strain also show a large deviation.The mean value of Lou-Huh criterion for the AA7075-T6 sheet specimen fracture strain prediction error is small,and the forming limit curve is basically the lower contour of the test data,and the prediction result is safe.For a solution-treatment 7075 aluminum sheet with more pronounced ductile fracture characteristics,the fracture strain prediction performance of M-K model is enhanced.However,the experimental verification results show that M-K model cannot effectively predict the fracture of plane strain state.The Lou-Huh criterion has a small error in predicting the fracture strain of the solution-treatment 7075 aluminum sheet,and the theoretical FLC can effectively predict the fracture of the material.The research in this paper can provide a reference for the selection of theoretical prediction models for forming limit of high-strength aluminum sheets,and the calculation of ductile fracture criterion parameters.