A Unified Damage Approach for Predicting Forming Limit Diagrams

Abstract

Plastic deformation in sheet metal consists of four distinct phases, namely, uniform deformation, diffuse necking, localized necking, and final rupture. The last three phases are commonly known as nonuniform deformation. A proper forming limit diagram (FLD) should include all three phases of the nonuniform deformation. This paper presents the development of a unified approach to the prediction of FLD to include all three phases of nonuniform deformation. The conventional method for predicting FLD is based on localized necking and adopts two fundamentally different approaches. Under biaxial loading, the Hill’s plasticity method is often chosen when α(=ε2/ε1) <0. On the other hand, the M-K method is typically used for the prediction of localized necking when α > 0 or when the biaxial stretching of sheet metal is significant. The M-K method, however, suffers from the arbitrary selection of the imperfection size, thus resulting in inconsistent predictions. The unified approach takes into account the effects of micro-cracks/voids on the FLD. All real-life materials contain varying sizes and degrees of micro-cracks/voids which can be characterized by the theory of damage mechanics. The theory is extended to include orthotropic damage, which is often observed in extensive plastic deformation during sheet metal forming. The orthotropic FLD model is based on an anisotropic damage model proposed recently by Chow and Wang (1993). Coupling the incremental theory of plasticity with damage, the new model can be used to predict not only the forming limit diagram but also the fracture limit diagram under proportional or nonproportional loading. In view of the two distinct physical phenomena governing the cases when α(=ε2/ε1) < or α > 0, a set of instability criteria is proposed to characterize all three phases of nonuniform deformation. The orthotropic damage model has been employed to predict the FLD of VDIF steel (Chow et al, 1996) and excellent agreement between the predicted and measured results has been achieved as shown in Fig. 1. The damage model is extended in this paper to examine its applicability and validity for another important engineering material, namely aluminum alloy 6111-T4.

Advances in anisotropy and formability

“…The effects of texture and damage evolution on the limit strains have been studied by Hu et al [144]. Chow et al have developed a ductile damage model and implemented it into the MK theory both for linear [78] and complex load paths [79,80]. An anisotropic model of Gurson type have been used by Huang et al [145] for the computation of the FLCs.…”

Failure analysis of anisotropic sheet-metals using a non-local plastic damage model

“…Once the damage model has been calibrated, it is used to predict the insurgence of tearing in drawing tests with flat-headed punch (Marciniack's tests) and compared with our experimental measurements of strains with the Digital Image Correlation method (D.I.C.). The formulation of the damage and coalescence allows to predict the Forming Limit Diagram (FLD) in a unified approach where necking and ductile rupture can be both detected depending of the strain path; this fact has been already shown by Chow et al [8] but with a different damage model and formulation.…”

Numerical and experimental investigations of forming limit diagrams in metal sheets

“…The new orthotropic damage model [20] for localized necking is employed to predict the FLD. The as-received materials are assumed to suffer no initial damage prior to load application.…”

“…Generally it is very difficult to determine the optimum parameters for satisfying the design specification without any problem during the forming process. Now, many researches have been being carried out with the FEA and the optimization techniques [12][13][14][15][16][17][18][19][20][21][22][23][24].…”