Naomichi Mori

New Method to Evaluate and Compensate 3D‐Springback

A new method for evaluating and compensating springback is presented. It is based on prediction of the change in curvature after springback by the mechanics of the elastic recovery. Starting from the nodal coordinates and the stress and strain distributions predicted by the FEM simulation at the beginning of springback, it first calculates the curvature and bending moments on the stamped sheet surface. Then, by modifying bending moment and curvature of sheet surface as well as process conditions it predicts an ideal sheet state at the beginning of springback. While predicting the ideal sheet state, it solves optimization problems to minimize springback defects with the aid of shape defect evaluation software named “NXT Defect Evaluator”. The springback process from this ideal state is expected to generate the desired shape. Therefore, the ideal sheet shape is helpful when designing compensational tools and renewing the FEM model.

Evaluation of Forming Limit by the 3 Dimensional Local Bifurcation Theory

A theoretical prediction and evaluation method for the sheet metal formability is developed on the basis of the three‐dimensional local bifurcation theory previously proposed by authors. The forming limit diagram represented on the plane defined by the ratio of stress component to work‐hardening rate is perfectly independent of plastic strain history. The upper and the lower limit of the sheet formability are indicated by the 3D critical line and the Storen‐Rice’s critical line on this plane, respectively. In order to verify the above mentioned behavior of the proposed forming limit diagram, the experimental research is also conducted. From the standpoint of the mechanical instability theory, a new concept called instability factor is introduced. It represents a degree of acceleration by current stress for developing the local bifurcation mode toward a fracture. The instability factor provides a method to evaluate a forming allowance which is useful to appropriate identification for a forming limit and to...

Plastic anisotropic constitutive equation based on stress-rate dependency related with non-associated flow rule for bifurcation analysis

In metal forming, progress in material models is required to construct a general and reliable fracture prediction framework because of the increased use of advanced materials and growing demand for higher prediction accuracy. In this study, a fracture prediction framework based on bifurcation theory is constructed. A novel material model based on the stress-rate dependence related to a non-associated flow rule is presented. This model is based on a non-associated flow rule with an arbitrary higher-order yield function and a plastic potential function for any anisotropic material. This formulation is combined with the stress-rate-dependent plastic constitutive equation, which is known as the Ito–Goya rule, to construct a generalized plastic constitutive model in which non-normality and non-associativity are reasonably included. Then, by adopting three-dimensional bifurcation theory, which is referred to the 3D theory, a new theoretical framework for fracture prediction based on the initiation of a shear band is constructed. Using virtual material data, a numerical simulation is carried out to produce a fracture limit diagram, which is used to investigate the characteristics of the proposed methodology.