Papers by Author: Xu Song

Abstract: The aim of the study presented here was to evaluate the residual stresses present in a bar of aluminium alloy 2124-T1 matrix composite (MMC) reinforced with 25vol% particulate silicon carbide (SiCp) using X-ray diffraction and 3D profilometry (curvature measurement using Mitutoyo/Renishaw coordinate measurement machine) and comparing these results with numerical models of residual strain and stress profiles obtained by a simple inelastic bending model and Finite Element Analysis (FEA). The residual strain distribution was introduced into the test piece by plastic deformation in the 4-point bending configuration. At the first stage of this study the elasticplastic behaviour of the MMC was characterized under static and cyclic loading to obtain the material parameters, hardening proprieties and cyclic hysteresis loops. Subsequently, synchrotron Xray diffraction and CMM curvature measurements were performed to deduce the residual stress profile in the central section of the bar. The experimental data obtained from these measurements were used in the inelastic bending and FEA simulations. The specimens were then subjected to incremental slitting using EDM (electric discharge machining) with continuous back and front face strain gauge monitoring. The X-ray diffraction and incremental slitting results were then analysed using direct and inverse eigenstrain methods. Residual stresses plots obtained by different methods show good agreement with each other.

Abstract: Most models based on continuum mechanics do not account for inhomogeneities at the micro-scale. This can be achieved by considering a representative volume of material and using (poly)crystal elasto-plastic deformation theory to model the effects of grain morphology and crystallographic orientation. In this way, the relationship between the macroscopic stress state and the stress state at the grain level can be investigated in detail. In addition, this approach enables the determination of the inhomogeneous fields of plastic strain, the identification of regions of localised plasticity (persistent slip bands), grain level shakedown, and the prediction of fatigue crack initiation using energy dissipation at the micro-scale. Elastic anisotropy is known to promote earlier onset of yielding, and to increase the magnitude of intergranular residual stresses. The effect of hardening behaviour of different slip systems on intergranular residual stresses is more subtle, as discussed in the text. The present study focuses on the analysis average intergranular residual strains and stresses that arise within the polycrystal aggregate following the application of single or cyclic external loading. These residual strains can also be evaluated experimentally using diffraction of penetrating radiation, e.g. neutrons or high energy X-rays, allowing comparisons with the model predictions to be made.

Abstract: Nickel base superalloys are the primary class of materials used in the manufacture of high temperature components for gas turbine aeroengines, including combustion casings and liners, guide vane and turbine blades and discs, etc. These components are subjected to complex cyclic loading induced by the combination of mechanical loading, changing temperatures and thermal gradients, inducing plastic deformation and creep, that ultimately may lead to crack initiation and propagation. The purpose of the present paper is to provide a necessarily brief overview of recent modeling activities in this field, including polycrystalline crystal plasticity modeling for the study of crack initiation, coupled non-local damage-plasticity modeling for crack initiation and propagation studies, and the incorporation of time and environment dependent processes (creep and oxidation) in the predictive modeling of fatigue crack growth rates in nickel base superalloys.