Research On Fracture And Springback In Sheet Metal Forming Based On Mesoscopic Damage Mechanics

Stamping plays a very important role in manufacturing industry. It is widely used in aviation, aerospace, automobile and other areas of industry. Fracture, springback and wrinkling are three main defects occurred in sheet metal forming because of some factors, such as forming condition, process parameters and material selection. These defects increase the reject ratio and try and error. Numerical simulation technology is a valid means to solve the above problems. But the prediction precision of the fracture and springback defects is still unreliable. Predicting the two defects accurately and rapidly is very important to the product quality control, cost reduction, product development cycle shortening and competitive power improving.Constitutive model is a crucial factor affecting the reliability of numerical simulation. Owing to internal micro defects existing in actual metallic materials, the material mechanical property degrades gradually during the forming process. But up to present, which is seldom considered in numerical simulation analysis. Therefore, to improve the numerical simulation accuracy, introducing the damage factor into the constitutive relationship is very essential. In this paper, on the basis of GTN mesotropic damage theory, the damage model considering the plastic anisotropy and Bauschinger effect is established suitable for the sheet metal forming. The numerical realization of the damage model is improved. The model is applied to analyze forming process and springback. The ductile instability and fracture criteria are proposed. The main work in this paper is as follows.Hill'48-GTN and Barlat'89-GTN mesotropic damage models are proposed and established based on the initial GTN model. The macroscopic equivalent stresses denoted by Hill'48 and Barlat'89 yield criteria take the place of the original von Mises equivalent stress to reflect the plastic anisotropy. The constitutive equations and numerical realization equations are derived. The results of numerical example show that the Hill'48-GTN and Barlat'89-GTN models can describe both the damage evolution and plastic anisotropy in sheet metal forming.Because the original method is inefficient and converge difficultly when dealing with sheet metal forming, especially fracture problem. The constitutive numerical realization method is improved. Implicit back Euler constitutive integration algorithm is introduced into the explicit dynamic solver. The computational efficiency of the algorithm is improved and the accuracy is ensured. The corresponding GTN models user-defined subroutines VUMAT are developed based on the commercial software ABAQUS. The damage parameter, void volume fraction is displayed clearly in the sheet metal forming simulation. The reliability of the above damage models is validated through the circular cup drawing experiment.The precision of three main flow stress models: Hollomon, Swift and Voce equations are studied and compared. To comprehending the mechanism of ductile damage intuitively and thoroughly, the fracture sections of cracked specimens are analyzed using SEM. Combining the experimental tensile test and finite element simulation, the inverse method is carried out to identify the GTN damage parameters. No matter which method is used, the parameters of GTN model are variational. The effect of parameters variation on damage behavior is analyzed based on orthogonal experimental design and FEM numerical simulation.Several hardening laws are proposed based on GTN damage model. The back stress is considered into the model to reflect the Bauschinger effect. Springback analysis are simulated using the explicit-implicit algorithm using the above GTN models. The effects of different material hardening rules on springback are studied and compared. The effect of damage on the springback is also investigated.From the engineering point of view, the ductile instability criterion and ductile fracture criterion are proposed based on the damage variable, void volume fraction in GTN model. Because the void volume fraction can take account of the material forming history, these criteria can deal with the forming limit of worse ductile sheet, for example, the alumimum alloy, and the sheet forming limit under the non-proportional loading strain path. The forming limits of steel and AA5052-O cup drawing processes are analyzed. The simulation results agree with the experimental data well.