J.H. Yoon

A three-dimensional rigid-plastic finite element analysis of bevel gear forging by using a remeshing technique

Abstract A three-dimensional remeshing scheme implemented by using a modular concept is proposed for the finite element analysis of a complicated forging process. In order to show the effectiveness of the scheme, forging of a bevel gear is simulated by using several basic modules in the general rigid-plastic finite element code (Yoon and Yang, Int. J. Mech. Sci.30, 887, 1988; Yang et al., Int. J. Mech. Sci.31, 145, 1989) developed for cold forging. Criteria for remeshing as well as a scheme for the mapping of state variables are proposed for three-dimensional remeshing. The computational results are compared with experimental data in order to check the validity of the simulation. The computational results show that the computation can be effectively carried out by using the proposed remeshing scheme and that it can be extended to other more complicated product geometry.

Finite element analysis of steady-state three-dimensional extrusion of sections through curved dies

Abstract A finite element analysis is made for steady-state three-dimensional extrusion of non-circular sections such as elliptic and clover-leaf sections through curved dies. In treating the incompressibility, the penalty constraint method, based on the rigid-plastic material model, is used. The initial guess for the initial velocity field is based on that of general upper-bound solutions for extrusion of arbitrarily shaped sections from round billets. Finite element computations are carried out for extrusion of elliptic and clover-leaf sections, but the present analysis can be further applied to general non-circular sections. The work-hardening effect is considered by integrating the effective strain rate along each stream line through interpolation by the least squares method. Experiments are carried out at room temperature and the experimental results are compared with numerical results in flow pattern and strain distribution. It is thus shown that the proposed numerical method is effective for detailed and reliable analysis of steady-state three-dimensional extrusion.

Rigid-plastic finite element analysis of three-dimensional forging by considering friction on continuous curved dies with initial guess generation

Abstract A three-dimensional finite element analysis of forging of blocks with continuous curved dies is carried out on the basis of a rigid-plastic material model. A method of imposing velocity boundary conditions on a three-dimensionally curved surface is proposed using a skew boundary condition. The frictional boundary condition is considered on a continuous curved surface. The initial velocity field is generated by introducing the assumption of linear viscous material. Forging of square blocks with hemispherical punches of various radii is simulated. Experiments are carried out to check the validity of the formulation. The comparison between computation and experiment shows good agreements in forging load as well as in deformed configuration.

Finite element analysis of large deformation by automatic renoding as a weak remeshing technique

Abstract An automatic renoding scheme is proposed as a new concept of “weak” remeshing for the finite element analysis of large deformation. In order to show the effectiveness of the scheme the following axisymmetric and three-dimensional backward extrusion processes are simulated; (i) backward extrusion of a cylindrical workpiece by a cylindrical punch with a rounded corner, and (ii) backward extrusion of a square block by a cylindrical punch with a rounded corner. Criteria for the renoding are automatically checked and the renoding scheme is carried out fully automatically. Experiments were carried out to check the validity of the proposed scheme for the three-dimensional case. Comparisons between the computed results and the experiments show that the proposed scheme can be effectively applied to metal forming processes of large deformation in connection with full remeshing.

Investigation of Elongation at Fracture in a High Speed Sheet Metal Forming Process

This paper investigates the dynamic elongation at fracture of conventional steels, advanced high strength steels and nonferrous metals, such as aluminium and magnesium alloys. Dynamic tensile tests were carried out using a high speed material testing machine at various strain rates ranging from 0.001/s to 200/s. The results show that the elongation at fracture of sheet metals does not simply decrease with the increase of the strain rate. The elongation of SPCC, SPRC450R, TRIP600 and AZ31 decreases when the tests are carried out under the quasi-static state at the strain rate of 0.1/s, but increases again when the tests are carried out at the strain rate of 0.1/s up to the strain rate of 200/s. Furthermore, DP600 and AA7003-T7 show the tendency that the tensile elongation increases as the strain rate increases. This tendency is related to the microstructure and forming history of the sheet metal. It is concluded that localized strain rate hardening in the necking region induces the enlargement of the necking region and thus the increased elongation. This phenomenon is worth being considered to predict the fracture of sheet metal products in high speed sheet metal forming.

Treatment of contact traction at the die-workpiece interface for the elastic analysis of die deformation

Abstract A method to calculate the contact traction on the die-workpiece interface is proposed. A systematic method for performing the elastic finite element analysis of the die deformation by using the information obtained from the finite element analysis of the workpiece deformation is proposed. The proposed method can be applied to both the two-dimensional and three-dimensional analyses of die deformation independently of the friction modelling and the constitutive equations. A rigid-plastic finite element analysis of workpiece deformation is carried out for the upsetting of a cylindrical workpiece. In order to check the validity of the present method for both flat and curved contact surfaces, the elastic finite element analysis of die deformation is then performed for flat square block forging as well as for a bevel gear forging (Yoon and Yang, Int. J. Mech. Sci. 32 , 277–291, 1990) [8]. From the computation it is shown that the error due to the transfer of data from the workpiece mesh to the die mesh does not influence appreciably the elastic deformation of the die. The proposed method can be applied to arbitrarily curved die geometries in three-dimensional metal forming problems.

Finite Element Analysis of Polycrystalline Deformation with the Rate‐dependent Crystal Plasticity

Constitutive models for the crystal plasticity have the common objective which relates the behavior of microscopic single crystals in the crystallographic texture to the macroscopic continuum response. This paper presents the texture analysis of polycrystalline materials using the rate‐dependent single crystal plasticity to develop a multi‐scale description of the mechanism at the grain and aggregate levels. The texture analysis requires a numerical algorithm for integrating the constitutive equations. The implicit deformation gradient approach is employed to update the stresses and texture orientations as an integration algorithm. It considers elastic or plastic deformation gradient as the primary unknown variables and constructs the residual of the elastic and plastic velocity gradients as the governing equations. This algorithm is shown to be an efficient and robust algorithm in rather large time steps. The texture analysis of the asymmetric rolling process is also presented to show investigation of th...

Efficiency enhancement in sheet metal forming analysis with a mesh regularization method

Abstract This paper newly proposes a mesh regularization method for the enhancement of the efficiency in sheet metal forming analysis. The regularization method searches for distorted elements with appropriate searching criteria and constructs patches including the elements to be modified. Each patch is then extended to a three-dimensional surface in order to obtain the information of the continuous coordinates. In constructing the surface enclosing each patch, NURBS (non-uniform rational B-spline) surface is employed to describe a three-dimensional free surface. On the basis of the constructed surface, each node is properly arranged to form unit elements as close as to a square. The state variables calculated from its original mesh geometry are mapped into the new mesh geometry for the next stage or incremental step of a forming analysis. The analysis results with the proposed method are compared to the results from the direct forming analysis without mesh regularization in order to confirm the validity of the method.

Finite Element Analysis of the Hydro-Mechanical Punching Process

This paper investigates the characteristics of a hydro-mechanical punching process. The hydro-mechanical punching process is divided into two stages: the first stage is the mechanical half piercing in which an upper punch goes down before the initial crack is occurred; the second stage is the hydro punching in which a lower punch goes up until the final fracture is occurred. Ductile fracture criteria such as the Cockcroft et al., Brozzo et al. and Oyane et al. are adopted to predict the fracture of a sheet material. The index value of ductile fracture criteria is calculated with a user material subroutine, VUMAT in the ABAQUS Explicit. The hydrostatic pressure retards the initiation of a crack in the upper region of the blank and induces another crack in the lower region of the blank during the punching process. The final fracture zone is placed at the middle surface of the blank to the thickness direction. The result demonstrates that the hydro-mechanical punching process makes a finer shearing surface than the conventional one as hydrostatic pressure increases.