Post-yield characterisation of metals with significant pile-up through spherical indentations

Finite element simulations of spherical indentations accounting for frictional contact provide validated load–indentation output for assessing and improving existing methods used to determine the stress–strain curve of materials with significant pile-up. The importance of friction to the proper assessment of the pile-up effect is established. Weaknesses in current characterisation relations and procedures are also identified. Existing correction formulae accounting for pile-up are modified so that the contact area radius is more accurately determined. This modification is implemented in the context of a characterisation process that relies on analysing unloading portions of load–indentation curves. Post-yield material behaviour predictions from such analysis are found to be in very good agreement with the initial finite element material input

Fatigue crack shielding in plain bearings under large scale yielding

Multi-layered bearing systems used in the automotive industry show shielding and antishielding effects that reduce or amplify the crack driving force under large-scale yielding conditions. Using finite element analysis, it is shown that shielding in such systems results in path deflection and bifurcation despite the absence of mixed-mode loading. As the crack approaches a stiff layer, the tangential strains measured around a blunted crack tip model show a maximum corresponding to the direction of crack propagation. The distribution of such strains indicates the effect of shielding and the likelihood of the tip to deflect or bifurcate.The suitability of bi-layer and tri-layer bearing architectures is assessed through crack path and respective crack driving force prediction

A boundary element model for nonlinear viscoelasticity

The boundary element methodology is applied to the analysis of non-linear viscoelastic solids. Theadopted non-linear model uses the same relaxation moduli as the respective linear relations but with a time shiftdepending on the volumetric strain. Nonlinearity introduces an irreducible domain integral into the originalintegral equation derived for linear viscoelastic solids. This necessitates the evaluation of domain strains, whichrelies on proper differentiation of an integral with a strong kernel singularity. A time domain formulation isimplemented through a numerical integration algorithm. The effectiveness of the developed numerical tool isdemonstrated through the analysis of a plate with a central crack. The results are compared with respectivepredictions by the finite element metho

Comparison of fatigue performance of HVOF spray coated and conventional roll bonded aluminium bearing alloys

A comparative study on fatigue resistance of thin aluminium bearing linings (supported by harder backing steel layers) produced by high velocity oxyfuel (HVOF) spray coating and conventional roll bonding (RB) processes has shown that the former is superior to the latter with similar lining composition (Al-20%Sn-1%Cu-0.25%Mn) when compared on the basis of oscillating lining surface strains under a three point bend test condition. In terms of the integrity of the multi-layered bearing systems under oscillating stresses, the HVOF lining appeared to show worse fatigue resistance due to a poor/brittle bond between the lining and the backing steel layer resulting in the detachment of the lining from the backing layer. The newly developed RB alloy with reduced Sn content (Al-6.5%Sn-2.5%Si-1%Ni-1%Cu-0.25%Mn) and scattered intermetallics showed higher fatigue resistance than the HVOF and previous RB systems. This was linked to delayed initiation of short cracks leading to a longer overall lifetime compared to all other systems

Three-dimensional crack growth in multi-layered architectures for automotive plain bearings

The assessment of shielding and anti-shielding effects on crack growth in multi-layered architectures is achieved using elasto-plastic finite element models. Initially, the evolution of straight, deflected and bifurcated cracks is studied through two-dimensional models with blunted crack tips using the maximum tangential strain as a criterion for deflection and the crack opening displacement as the crack driving force. The methodology is extended to three dimensions assuming only elastic material behavior; some initial analysis results on the effects of shielding along the crack front are presented and discusse

Elasto-plastic finite element studies of fatigue crack shielding in multi-layered systems

Multi-layered systems used in automotive plain bearings show complex patterns of crack growth under service conditions. The studied architecture consists of a steel backing (1.8 mm), a compliant pure-aluminum interlayer (0.04 mm) and a lining of medium-strength aluminum alloy (0.38 mm). Previous experimental studies [1] with flat strip specimens of identical architecture under three point bending indicated similar crack propagation patterns despite the absence of mixed mode loadin

Application of optical measurement techniques to high strain rate deformations in composite materials

Traditionally high strain rate material characterisations have been conducted using strain gauges and/or cross head displacements in servo-hydraulic test machines, and force transducers in split Hopkinson bar experiments. Non-contact full-field techniques for experimental stress/strain analysis have been available for many years and used extensively for structural analysis under static or quasi static loading. These techniques have the advantage that they are non-contact and high resolution, so damage initiation can be captured within the field of view and the material behaviour is not modified by the sensor. In the paper, one such technique known as Digital Image Correlation (DIC) is used to assess the material behaviour by using high-speed digital cameras to capture images from material subject to high strain rate events. The high strain rate loading is achieved using an Instron VHS high speed tensile test machine that allows the applied strain rates to vary from 12.5 s–1 to 125 s–1. Although the strain rates that can be achieved are low in comparison to those achieved with the Hopkinson bar, the test machine provides better optical access and opportunities for illumination of the specimen necessary for the DIC. In the paper, a review of the literature associated with high strain rate testing using servo-hydraulic machines is first provided. Then, an experimental study of the high strain rate behaviour of the both composite material and the resin alone is described. The results from both the DIC and strain gauges are compared and discussed