P. Hartley

Fracture prediction in plastic deformation processes

This paper describes the use of the finite-element technique to predict fracture initiation in a range of simple metalforming operations. These cover typical processes and enable deformation and fracture initiation to be examined under several different loading conditions. Three types of metalforming operation are considered, simple upsetting, axisymmetric extrusion, and strip compression and tension

Elastic-plastic finite-element modelling of cold rolling of strip

Abstract The rolling of copper strip has been simulated using an elastic-plastic finite-element method (EPFEM) for plane-strain deformation. The use of this program, which includes both material and geometric non-linearities, permits the identification of elastic and plastic deformation in rolling, and also the effects of unloading. The rolling model is based on the physically realistic concept that the strip is drawn into the roll gap by the action of interface friction. The results clearly show the maximum pressure associated with the neutral point and are in good quantitative agreement with earlier work. In addition, the stresses and deformation outside the nominal contact are presented.

A finite-element simulation of profile ring rolling using a hybrid mesh model

Abstract A hybrid mesh technique is used in conjunction with an elastic-plastic finite-element method to model the ring rolling process incorporating a full ring mesh analysis. The hybrid mesh technique consists of a material mesh and a computational mesh system. The computational mesh is used in the actual finite-element computation and derives its geometry from the material mesh system where data such as updated nodal positions and field variables are stored. Special tool modelling techniques are also used to model the process effectively. Both rectangular and V-section profiles are simulated taking into account the use of axial rolls. Lateral spread and the variation of field variables are observed during the simulation.

Finite-element modelling of deformation and spread in slab rolling

Abstract A simulation of metal flow in slab rolling is presented. The simulation is based on a three-dimensional elastic-plastic finite-element method, which allows a detailed analysis of flow, including spread in the width of the workpiece. Comparisons are made with previously published theoretical analyses and with experimental observations. The results presented here are found to be in close agreement with the experimental work over a wide range of workpiece width height ratios and reductions.

A study on the hot-deformation behavior and dynamic recrystallization of Al–5 wt.%Mg alloy

A numerical analysis was performed to predict flow curves and dynamic recrystallization behaviors of Al-5wt%Mg alloy on the basis of results of hot compression tests. The hot compression tests were carred out in the ranges of 350 ~ 500 and 5 ~ 3 /sec to obtain the Zener-Hollomon parameter Z. The modelling equation for flow stress was a function of strain, strain rate, temperature. The influence of these variables was quantifield using the Zener-Hollomon parameter. In the modelling equation, the effects of strain hardening and dynamic recrystallization were taken into consideration. Therefore, the modelling stress-strain curves of Al-5wt%Mg alloy were in good agreement with experimental results. Finally, the dynamic recrystallization kinetics were illustrated through the inspection of microstructure after deformation.

Simulation of the cold rolling of strip using an elastic-plastic finite element technique

Abstract Strip rolling is simulated using an elastic-plastic finite element technique which includes the extent of both elastic and plastic deformation outside the nominal contact deformation zone. Solutions for non-steady and steady-state rolling are obtained. The stress and strain distributions within the workpiece, the velocity fields (absolute and relative to the roll surface), and the normal pressure and shear stress distributions along the arc of contact are calculated. Analyses are conducted with different levels of friction, material properties, workpiece dimensions and reductions. The results are compared with previous theoretical and experimental work, with which good agreement is found. The usefulness of numerical analyses for the investigation of parameters relevant to industrial rolling practice is demonstrated.

Friction modelling and experimental observations in hot ring compression tests

Abstract Few studies have been undertaken to understand friction in hot metal forming, especially when addressing the critical issue of whether changing magnitudes of strain rate and temperature at the interface influence the level of friction. This paper describes an experimental investigation into friction under hot forming conditions using the ring compression test. The experiments shows how variations in temperature at the interface affected the frictional behaviour. Additional observations were made with regard to the suitability of using the ring compression test for hot metal forming, especially their ability to reproduce process conditions in hot rolling. Finite-element simulations of the ring compression test were also completed under similar temperatures as in the experiments. The correlation between the experimental measurements and the results of the process modelling is presented in the paper.

Three-dimensional finite-element modelling of ring rolling

Abstract In this paper, a three dimensional finite-element model for ring rolling modelling is described. An elastic-plastic finite-element formulation based upon the updated Lagrangian approach is employed to analyse the process and a special hybrid mesh model has been developed which significantly increase the accuracy and computational efficiency of the simulation. In addition to the friction-layer and the boundary surface techniques available in the finite-element program, some special methods are used to model the various tools involved in the process, namely the main roll, the mandrel and the guide rolls. As an example of this approach, a three-dimensional analysis of a steel ring rolling operation is carried out. The deformation, strain and force parameters are analysed. Some flow defects such as fish-tailing are also well predicted.

Observations on fracture in axi-symmetric and three-dimensional cold upsetting of brass

Abstract This paper describes the appearance of fractures in cold forming of brass during axi-symmetric collar upsetting tests and also in the upsetting of hexagonal shaped bars. The latter has been introduced to examine fracture under non-axi-symmetric conditions. The collar tests produced typical ductile fractures but the hexagonal bars displayed fractures similar to brittle fractures even though they were preceded by large plastic strain. Each experimental test was also modelled using the finite-element method to examine local stress/strain conditions at fracture. The simulations of the collar tests showed that fractures appeared in the presence of a circumferential tensile stress together with either a tensile or compressive axial stress. The compressive axial stress was present when fracture occurred at a much lower level of deformation than when the axial stress became tensile. The simulations of the hexagon bar upsetting revealed a maximum plastic strain coincident with the most likely site of fracture initiation. This mixture of differing stress and strain states at fracture leads to the conclusion that a single fracture criterion is unlikely to satisfy all the conditions that lead to fracture.

Use of a mean-normal technique for efficient and numerically stable large-strain elastic-plastic finite-element solutions

Abstract Large-strain elastic-plastic finite-element analyses require the use of finite-sized increments of deformation. Such solutions may prove to be unstable, even when the correct definitions of stress and strain increment are used. It has previously been suggested that the instability may be avoided by modifying the stiffness formulation by means of the mean-normal technique. The present comparison of finite-element results for the compression of a block of aluminium up to 50% reduction in height shows that the instability may also be avoided, with greater computational efficiency, by using the mean-normal method to calculate the stress changes at the end of each increment, even if the finite-element analysis contains a simple tangent-modulus solution of the stiffness equations. When the mean-normal method is used to calculate stress increments in strain-hardening materials, an iterative procedure must be adopted. The present analysis simplifies this procedure, reducing the computational effort, and allowing the technique to be incorporated easily into existing finite-element treatments.