Theory Research On Anisotropic Yield Criterion And Failure Model For The Plastic Deformation Of Light Weight And High Strength Sheet Metal

Light weight and high strength sheets,such as ultra-high strength steel,aluminum alloy,titanium alloy and so on,have been widely used in automobile industry and aerospace industry due to their low density,high specific strength.As these materials have characteristics of obvious anisotropy,low formability and serious springback,the numerical simulation technique is usually utilized to explore the design space of optimum forming process.Hence,precisely describing anisotropic yielding,hardening and forming limit of these advanced light weight and high strength sheets will be significant to realize high-precision metal forming simulation.At present,although some yield criteria under associated flow rule（AFR）can describe most material‘s tensile yield stress and Lankford coefficients,these yield criteria are not suitable to describe the characteristic of tension-compression asymmetry.With the development of plastic theory,there are still some issues that need to be addressed in the theory of non-AFR,that is,to take both accuracy and efficiency into account.How to accurately reflect the influences of strain path and stress state on the forming limit and damage evolution of light weight and high strength sheets are also the hotspots of plastic deformation theory.For light weight and high strength sheets,the critical basic mechanics and algorithm in the field of plastic processing,including the yield characteristics,the theory of non-AFR,forming limit,ductile fracture criterion and so on,have been studied deeply and systematically with the funding by National Natural Science Foundation of China（U1737210,51675332）,National Key Research and Development Project（2017YFB0304400）and Program of Shanghai Excellent Academic Research Leadership（19XD1401900）.The main research contents are as follows:Under AFR framework,a generalized yield criterion for describing both asymmetry and Lankford coefficients of FCC,BCC and HCP structured materials is developed.Through transforming existing stress invariants-based yield criteria from I-J₂-J₃ framework into η-ξ-（?） framework and analyzing the construction forms of these yield criteria,the essential rules for constructing stress invariants-based yield criterion are extracted.The conditions for the convexity of the proposed yield criterion are obtained by using the method of order principal minor determinant.The proposed isotropic yield criterion is extended to anisotropic yield criterion by introducing the fourth-order linear transformation tensor.This construction method effectively increases the flexibility and accuracy of yield function.The ability of the constructed yield criterion for describing materials‘tension-compression asymmetry and Lankford coefficients under AFR framework has been verified by applying to AA2008-T4 and AA2090-T3 and comparing with Yld2000-2d and Yoon2014 yield criteria.The proposed yield criterion is also utilized to describe the evolution of yield surface for a zirconium plate with various levels of through-thickness compressive pre-strains.It is found that the proposed yield criterion can capture the asymmetry and the local deformation characteristic of the yield surfaces.The differences of full method and simple method for non-AFR are analyzed.A restriction for calculating the parameters in plastic potential function for the simple method is clarified.An effective approach is introduced to improve the accuracy of the simple method aiming at the limitation of the simple method.Through comparing the fully implicit return-mapping schemes of full method and simple method,it‘s found that there are fewer equations in simple method and the key linearization equations are simpler than full method.Hence,the simulation time in simple method would be less than full method.In order to better understand the application of simple method,the fracture strains predicted by different non-AFR models are compared.The result shows that if the relative value of yield function and plastic potential function in the non-AFR along different tensile direction is less than 5%,the simple method can be adopted.A new efficient and robust algorithm for Marciniak-Kuczynski（M-K）model to calculate the forming limit diagrams is developed.Since the convergence of Newton-Raphson（N-R）method in the framework of M-K model is not promised,the relationship between the internal stress variables in the groove of sheet metal is investigated.The number of unknown variables in N-R method reduces to two from three.Based on the relationship between the internal stress variables in the groove of sheet metal,an increment method for M-K model is developed,which can unconditionally guarantee its convergence.The N-R method is changed as modified N-R method by considering this relationship.To improve the computational efficiency,a modified increment method is established by combining modified N-R method and increment method.Through applying these different methods to AA6111-T3,it‘s found that the modified increment method is recognized as the best method by considering from computation efficiency and convergence.An effective perturbation approach is developed to forecast anisotropic sheet metals‘forming limits.The phenomenon of localized necking is treated as a result of perturbation growth integrating throughout deformation history.The strain localization will appear if the perturbation accumulates to a critical value,and the resulting plastic strain outside the necking is the limit strain.The effectiveness and accuracy of the perturbation approach under linear and nonlinear strain paths are verified by applying it to AA5754-O.The influences of different hardening laws and yield criteria on forming limit curve（FLC）evaluated by the perturbation approach are systematically studied.The influences of pre-strains on the Forming Limit Diagram（FLD）and equivalent plastic strain（EPS）based（epFLD）are studied with bilinear strain paths.Investigations demonstrate that if the strain path changes before the EPS at the onset of deformation instability of plane strain deformation,the predicted forming limits will be the same as those predicted by original epFLD.However,the predicted forming limits will deviate from those predicted by original epFLD if the strain path changes after the EPS.The amount of deviations can be significant depending on the level of pre-strain and strain path.To consider the influence of normal stress on the forming limit strains,the perturbation approach is extended with normal stress.The conventional FLD established based upon plane loading would be inapplicable for the hydroforming processes due to the high level of normal stress.To solve this problem,the M-K model and the perturbation approach are extended to 3D stress state.To guarantee the convergence of solving process in M-K model with normal stress,the modified increment method is implemented into it.The influences of through-thickness normal stress on the predicted FLD and epFLD under proportional and non-proportional loadings are investigated.Besides,the influences of hardening law and yield criterion on the M-K model and the perturbation approach with normal stress are studied.The results show that the formability can be improved with the through-thickness normal stress,no matter for perturbation approach or M-K model.But the M-K model combined with normal stress can‘t be applied to strain rate sensitivity material.The sensitivity of FLD to normal stress is related to the hardening law and the range of strain path in FLD reduces with the increasing normal stress,which is related to the yield criterion.As the limitations in prediction of equivalent plastic strain at fracture（EPSF）in the wider scope of stress triaxiality for the existing uncoupled ductile fracture criteria（DFCs）,a new DFC is developed by analyzing voids nucleation,voids growth and voids coalescence.Through applying the proposed DFC to AA6061-T6 and AA2024-T351,the accuracy of this DFC is verified.To predict the onset of fracture under different strain rates and temperature,a generalized strain rate-and temperature-dependent DFC is formulated by combining Johnson-Cook fracture model and Zener-Hollomon parameter into Hu 2017DFC,which is called extended Hu DFC.For DP590,the loading path of the Lode parameter,stress triaxiality,strain rate and temperature to fracture are determined through studying the agreement between the force-displacement curves predicted by finite element simulation based on the extended Hu DFC and the corresponding experimental results.Through comparing the EPSFs predicted by extended Hu DFC,modified Hosford-Coulomb DFC and the corresponding experimental EPSFs,the accuracy and effectiveness of extended Hu DFC are validated.The influences of strain rate and temperature on EPSF predicted by extended Hu DFC have been explained.