Elena Lyamina

An Approach to Prediction of Evolution of Material Properties in the Vicinity of Frictional Interfaces in Metal Forming

The quality of surface of the product of metal forming processes depends on physical processes in the vicinity of frictional interfaces between the material and tool. It is well known that material properties in a narrow layer in the vicinity of such interfaces are usually quite different from the properties in the bulk. It is therefore necessary to develop a special approach to account for this feature of the distribution of material properties. A possible approach is proposed in the present paper. It is based on the concept of strain rate intensity factor.

Ductile Fracture in Metal Forming: A Review of Selected Issues

The paper reviews several theoretical and experimental methods for the assessment of ductile fracture criteria and for their application to the fracture prediction in metal forming processes. In particular, distinguished features of two widely used ductile fracture criteria are demonstrated in the case of free surface fracture. Conventional empirical ductile fracture criteria are not compatible with behaviour of plastic solutions in the vicinity of maximum friction surfaces. An approach to overcome this difficulty is discussed. Finally, a theoretical/experimental method to reveal a possible effect of geometric singularities on the applicability of ductile fracture criteria is reviewed.

Application of the Strain Rate Intensity Factor to Modeling Material Behavior in the Vicinity of Frictional Interfaces

This paper concerns with a novel approach to predicting the formation of a narrow layer of intensive plastic deformation in the vicinity of frictional interfaces. Theoretical solutions based on several conventional rigid plastic models are singular near maximum friction surfaces. In particular, the equivalent strain rate approaches infinity near such surfaces. This is in qualitative agreement with experimental observations that material properties in the vicinity of frictional interfaces are often quite different from the properties in the bulk. The new theory relates the strain rate intensity factor, which is the coefficient of the main singular term in a series expansion of the equivalent strain rate in the vicinity of maximum friction surfaces, and the thickness of the layer of intensive plastic deformation. Moreover, new constitutive equations involving the strain rate intensity factor are proposed for some parameters which characterize the structure of material. The process of plane strain extrusion is considered in some detail as an illustrative example. The strain rate intensity factor is determined from an approximate solution. Using this solution and experimental data the thickness of the layer of intensive plastic deformation is found. A non-local ductile fracture criterion is adopted to predict the initiation of ductile fracture in the layer. A numerical method for determining the strain rate intensity factor in the case of plane strain flow of rigid perfectly plastic material is discussed.

Design of an Annular Disc Subject to Thermomechanical Loading

Two solutions to design a thin annular disc of variable thickness subject to thermomechanical loading are proposed. It is assumed that the thickness of the disc is everywhere sufficiently small for the stresses to be averaged through the thickness. The state of stress is plane. The initiation of plastic yielding is controlled by Mises yield criterion. The design criterion for one of the solutions proposed requires that the distribution of stresses is uniform over the entire disc. In this case there is a relation between optimal values of the loading parameters at the final stage. The specific shape of the disc corresponds to each pair of such parameters. The other solution is obtained under the additional requirement that the distribution of strains is uniform. This solution exists for the disc of constant thickness at specific values of the loading parameters.

Simulation of Aluminium Alloy 5A06 Warm/Hot Hydromechanical Sheet Deep Drawing

The flow behavior and ductility of the Aluminium alloy 5A06 was investigated in the tensile test at the elevated temperatures from 20oC to 300oC and strain rates range of 0.0006s-1 0.06s-1. The influences of strain, strain rate and temperature to elongation strain at fracture , percentage elongation , tensile strength , strain hardening exponent and strain rate sensitive exponent were researched on the basis of experimental stress-strain curves, showing 5A06 is a temperature sensitive alloy. Warm/hot hydromechanical processing of cylindrical cup deep drawing was simulated. It is demonstrated that the differential temperature mode with cool punch and heated die can enhance the forming limit significantly compared with the constant temperature mode of all tools in the same temperature.

An accurate upper bound solution for plane strain extrusion through a wedge-shaped die

An upper bound method for the process of plane strain extrusion through a wedge-shaped die is derived. A technique for constructing a kinematically admissible velocity field satisfying the exact asymptotic singular behavior of real velocity fields in the vicinity of maximum friction surfaces (the friction stress at sliding is equal to the shear yield stress on such surfaces) is described. Two specific upper bound solutions are found using the method derived. The solutions are compared to an accurate slip-line solution and it is shown that the accuracy of the new method is very high.

Distribution of Stresses and Residual Stresses in FGM Discs Subject to Internal Pressure

A semi-analytic solution for the elastic/plastic distribution of stress and residual stress in a thin annular disc subject to pressure over its inner radius is presented. It is assumed that the disc is made of functionally graded material. Numerical methods are only necessary to solve simple transcendental equations. An illustrative example is presented.

Incorporation of a Fracture Criterion in Ideal Flow Design

The theory of sheet and bulk ideal plastic flows is used for the preliminary design of metal forming processes. The present paper develops an approach to incorporate the Cockroft and Latham ductile fracture criterion in this design method for stationary bulk flows. In particular, it is demonstrated that the initiation of ductile fracture can be predicted without having the ideal flow solution for stress and strain in the plastic zone (it is only necessary to know that the solution exists). Using the approach proposed the initiation of ductile fracture in axisymmetric drawing is predicted.

A semi-analytic method for elastic/plastic shrink-fit analysis and design:

A semi-analytic method for elastic/plastic shrink-fit analysis and design is developed. In contrast to many available semi-analytic methods, it is assumed that the outer disc obeys the von Mises yield criterion and its associated flow rule. The inner component of the assembly is purely elastic. The complete solution consists of three principal steps. First, the elastic/plastic solution in the outer disc is outlined. The only output of this solution required for the next step is the circumferential strain at the inner radius. It is shown that this strain can be found without having the strain distribution in the plastic region of the disc. This significantly simplifies the design of shrink fits. Moreover, only two parameters related to the outer disc (Poisson’s ratio and the dimensionless inner radius) are involved in numerical part of the elastic/plastic solution in the outer disc. Second, the found circumferential strain at the inner radius is used in conjunction with an analytic solution in the inner component of the assembly to match the two solutions. Any conventional design criterion can be adopted at this stage to determine optimal conditions. The complete solution involves several independent parameters. However, this does not cause any difficulty for design since these parameters are involved in analytic expressions. The final step is only necessary if the distribution of strains in the plastic region of the outer disc should be found.

Effect of a geometric singularity on a surface on ductile fracture

An experimental technique is proposed and applied to reveal a possible effect of a geometric surface singularity on ductile fracture during metal forming. The conditions of ductile fracture at the points of geometric singularity are shown not to be predicted by a standard workability diagram.