Deformation Analysis of Shearing Process Using Results of Notched Round Bar Tension Test

Abstract

Deformation analysis of shearing process is carried out by using finite element method (FEM). Gurson-Tvergaard-Needlman (GTN) model is applied as a fracture criterion because ductile fracture behavior is a dominant factor in shearing process. Ductile fracture involves micro void nucleation, growth and coalescence of voids. To predict void nucleation in FEM analysis, a void nucleation critical strain model is proposed. Furthermore a notched round bar tension test using image analysis is proposed for deciding material properties relating ductile fracture. Gradual changes in the necking shape and tensile load are measured by using the test. From the results, strain and stress history at the necking section of tensile specimen and relationship between stress triaxiality and microvoid nucleation are obtained. Additionally, obtained damage parameters are introduced to the deformation analysis of shearing process and the results are compared with the experimental result. (doi:10.2320/matertrans.MF200903) Shearing process is used to cut off material by using punch, die and blank holder. This process is widely applied to make the billet in forging and electronic terminal parts and so on. Sheared surface has smooth area (sheared surface) which is made by metal flow in punch-die clearance, rough area (fracture surface) which is appeared by crack initiation and propagation at the tip of punch and die, burr and shear drop. These shape factors affect precision and quality of products, thus some researchers investigated the influence of process conditions on the shape of sheared surface. 1,2) Over the past few years, several studies have been performed on deforma- tion analysis of shearing process using finite element method (FEM), because the technology of CAE (Computer Aided Engineering) and computer performance has been pro- gressed. 3-5) On the shearing process, separation of material is completed by ductile crack connection which is generated at the tip of punch and die. Because sheared part shape is affected by the ductile fracture behavior, crack generation and propagation, optimum fracture criterion has to be used in FE analysis of shearing process. Ductile fracture involves micro void nucleation, growth and coalescence of voids. Micro voids appears on the inclusion-matrix boundary, grain boundary and in grain. Next, they grow with large plastic deformation of matrix material, and coalesce into macroscopic crack. In shearing process, sheared surface shape is affected this ductile crack behavior, and it has to be simulated in FE analysis using optimum ductile fracture criterion. In this study, Gurson- Tvergaard-Needlman model (GTN model) is applied to analysis of shearing process as a fracture criterion because this model express above micro voids behavior. There are some researches in which GTN model is applied to FE analysis of shearing process and the results show that GTN model is available for shearing analysis. 6-8) There are several material properties in GTN model and void nucleation critical strain is one of them. This parameter means that void nucleation is occurring when material strain reaches void nucleation critical strain. In the past studies, void nucleation critical strain is treated as a constant material value. However, it is known that stress state affects the nucleation and growth of micro voids. 9) In this study, new void nucleation critical strain model is proposed. In this model, void nucleation critical strain is expressed by a function of stress triaxiality and it is necessary to obtain the relationship between the stress triaxiality and void nucleation strain from some kind of material tests. For the purpose, notched round bar tension test using image analysis was developed and carried out. In this test, the specimen which has a circumferential notch was tensile and its notch curvature radius change was captured by CCD camera from the start of the test until material fracture continuously. It is possible to obtain a history of stress triaxiality and equivalent strain in notch center of specimen using the image analysis result of the notch shape change. From the results, material properties relating ductile fracture in the void nucleation critical strain model and GTN model were determined and introduced FE code we designed. Deformation analysis of shearing process is performed, and its accuracy is evaluated comparing experimental result. In this consideration, reli- ability and availability of proposed models are also inves- tigated.