The Journal of Strain Analysis for Engineering Design

The finite element method (FEM) is used to investigate the normal indentation problem of a deformable indenter in contact with a strain hardening substrate coated with an elastic—perfectly plastic layer. In order to assess the influence of the deformable indenter, the solution is compared with results of a rigid sphere indenting the same system. The comparison shows that, when the indenter is assumed to be rigid, the contact pressures are significantly higher than those for the deformable indenter during elastic deformation. When plasticity is more pronounced, the peak pressures at just inside the contact edge are also higher for the rigid indenter. Similarly, the results assuming a rigid indenter give a lower value of maximum radial tensile stress along the coating surface when compared with those using a deformable indenter, which is responsible for the cracking of the film. In order to examine the effects of strain hardening, the above solutions are compared with a system having an elastic—perfectly plastic substrate. Comparison between the two sets of results shows that the strain hardening medium develops a smaller contact area and higher central and peak contact pressures inside the contact edge during plastic deformation. It also shows that the use of a strain hardening substrate alleviates the maximum radial tensile stress just outside the contact edge.

The main purpose of this paper is to assess the influence of a deformable indenter by comparing the finite element results for a deformable indenter with those of a rigid indenter. Elastic—plastic solutions for the case of a stiff hard layer rigidly bonded to a softer substrate are given and the effect of the indenters (i.e. rigid; deformable, E = 600 GPa; deformable, E = 1000 GPa) on the contact pressures and stresses and strains are critically examined. The results showed that, when the indenter is assumed to be rigid, the contact pressures in the elastic regime are higher than those given by the deformable indenters. However, as deformation proceeds and plasticity becomes dominant, the differences between the three sets of results reduce. The rigid indenter results were also shown to give the lowest peak radial tensile stress which is responsible for cracking of the film normal to the surface. Finally, comparison of the von Mises stress contours revealed that varying the indenter properties affects the point at which yielding commences. Results showed that the rigid indenter caused the coating and substrate to yield prematurely, i.e. at a lower load than the deformable indenters.

Deformation and fracture are examined for a cyclic loading condition in which a 24 h period of forward creep in IN 597 at 850°C is reversed (a) by a similar period of creep and (b) by fast monotonic strain-limited plasticity with and without a subsequent 1 h period of relaxation. When ratchetting is evident, it is shown that a life fraction rule based upon the Monkman—Grant law describes the observed behaviour and that, for a given creep stress, the accumulated creep strain—time plot correlates with a simple creep curve. At lower-magnitude creep dwell stresses a cyclic equilibrium state is achieved. The absence of ratchetting during equilibrium results in a net increase in rupture life and fracture strain when compared with those for simple creep under the same stress. Time-limited cyclic behaviour is compared with further cyclic tests in which the reversal occurs at fixed limits of total strain. These did not show an equilibrium condition but a progressive cyclic softening in which the creep time and strain successively diminished.

Predictions to individual creep curves are made from the mechanical equation of state and the time hardening law. However, empiricism is preferred to describe the softening behaviour observed for the alloy more precisely. A theoretical assessment is made of the effect of imposing a period of relaxation at peak reversed strain upon cyclic creep deformation and life.

A finite difference technique for processing thermoelastic data for biaxial problems is presented. The method enables thermoelastic data to be smoothed at interior points and boundary information to be calculated from measured non-boundary values by an iterative procedure based on the compatibility equation. The technique has been successfully applied to quantify stress concentrations and to perform stress separation.

The quenching process of aluminium alloys is modelled using the finite element method. The study of residual stress field induced by quenching is divided into two: the thermal and mechanical aspects. In the thermal problem, the general heat conduction equation is solved and the temperature field during quenching is calculated. In the mechanical problem, the calculated temperature field and mechanical properties are used to predict the residual stress field.

In this paper, the two different boundary conditions used in the thermal problem are examined. The first is surface convection using the appropriate heat transfer coefficient. The second is the temperature variation measured at the surface of the part. These boundary conditions are compared, and the advantages and the drawbacks of each are shown.

The influence of different quenching parameters on the level of residual stress is studied. To validate the quenching modelling, the incremental hole drilling and neutron diffraction methods are used to measure the residual stress field in the studied parts. The hole drilling technique has been adapted to measure the residual stress through a larger thickness of the part. The aim of this paper is the combination of numerical and experimental techniques for the investigation of the through-thickness residual stress field.

A coupled finite element and boundary element method is introduced for the analysis of two-dimensional elastic contact problems without friction. The Lagrange multiplier method is used to apply the contact constraints. A computer program, which can analyse axisymmetric, plane strain and plane stress problems, has been developed and used to demonstrate the accuracy of the method. The program is applied to a sphere in contact with a flat surface, a rigid punch pressed on to an elastic foundation and an elastic cylindrical punch in contact with an elastic plate. In all cases good agreement is obtained with analytical solutions for stresses near the contact region.

Closed-form expressions for the contact pressure between a blunt wedge and an elastically dissimilar half-plane are presented. It is demonstrated that for practical values of the material mismatch parameter, β, and coefficient of friction, f, the results do not differ significantly from the elastically similar problem.