Showing papers by "Alexander M. Korsunsky published in 2011"

Improvement of fatigue properties by shot peening for Mg–10Gd–3Y alloys under different conditions

Abstract: In the present study we investigated the influence of shot peening on the high cycle fatigue (HCF) performance of the Mg–10Gd–3Y magnesium alloys in four different conditions referred to as-cast, cast-T6, as-extruded and extruded-T5, respectively. The results show that shot peening can cause different degree of enhancement of fatigue performance for Mg–10Gd–3Y alloys depending on the Almen peening intensity applied; and that the Almen intensity could always be found that conferred the optimum improvement. The effect of shot peening was quantified, and for the as-extruded and extruded-T5 alloys it was found to be superior to that for the as-cast and cast-T6 alloys. The peened extruded-T5 Mg–10Gd–3Y alloy showed the highest fatigue strength at 10 7 cycles of 240 MPa. The results of the analyses established a connection between the grain size, ductility and precipitates within the studied alloys. Microstructure affected the magnitude of the surface roughness induced by shot peening and also the maximum compressive residual stress and its relaxation during fatigue, and then determine the beneficial effect of shot peening.

Analysis of strain error sources in micro-beam Laue diffraction

Abstract: Micro-beam Laue diffraction is an experimental method that allows the measurement of local lattice orientation and elastic strain within individual grains of engineering alloys, ceramics, and other polycrystalline materials. Unlike other analytical techniques, e.g. based on electron microscopy, it is not limited to surface characterisation or thin sections, but rather allows non-destructive measurements in the material bulk. This is of particular importance for in situ loading experiments where the mechanical response of a material volume (rather than just surface) is studied and it is vital that no perturbation/disturbance is introduced by the measurement technique. Whilst the technique allows lattice orientation to be determined to a high level of precision, accurate measurement of elastic strains and estimating the errors involved is a significant challenge. We propose a simulation-based approach to assess the elastic strain errors that arise from geometrical perturbations of the experimental setup. Using an empirical combination rule, the contributions of different geometrical uncertainties to the overall experimental strain error are estimated. This approach was applied to the micro-beam Laue diffraction setup at beamline BM32 at the European Synchrotron Radiation Facility (ESRF). Using a highly perfect germanium single crystal, the mechanical stability of the instrument was determined and hence the expected strain errors predicted. Comparison with the actual strain errors found in a silicon four-point beam bending test showed good agreement. The simulation-based error analysis approach makes it possible to understand the origins of the experimental strain errors and thus allows a directed improvement of the experimental geometry to maximise the benefit in terms of strain accuracy.

Effect of microstructures and texture development on tensile properties of Mg-10Gd-3Y alloy

Abstract: In this study, influence of microstructures and texture development induced by hot-extrusion and/or heat treatments as T4, T5 and T6 on tensile properties of the Mg–10Gd–3Y magnesium alloys was investigated. The results show that obvious changes have taken place in microstructure, texture and tensile properties of the studied alloy after hot extrusion and/or heat treatments. The as-extruded and three aging-treated alloys exhibit an E D / / 1 0 1 ¯ 0 fiber texture. While the fiber texture almost completely disappeared after T4 and T6 heat treatments. Aging treatments especially peak-aging can significantly improve the tensile strength and hardness of as-extruded alloy but at the cost of decreasing elongation. In contrast, T4 and T6 heat treatments are detrimental to the tensile properties and hardness of as-extruded alloy.

Inverse Eigenstrain Analysis of the Effect of Non-uniform Sample Shape on the Residual Stress Due to Shot Peening

Abstract: This paper deals with the analysis of elastic strain and eigenstrain in non-uniformly shaped shot-peened 17-4PH stainless steel samples. Based on residual strain measurements by synchrotron X-ray diffraction, the finite element (FE) models are established for the inverse problem of eigenstrain analysis in slice conical sample. The eigenstrains obtained in the slice are then implemented into the FE model of the solid conical sample. It is found that the dependence of elastic strain distributions on the peening intensity and sample shape/thickness could be elucidated via the understanding of underlying permanent strain, or eigenstrain. The effect of the peening process is therefore best described in terms of the induced eigenstrain. The proposed framework is useful for the predictive modelling of residual stresses in non-uniformly shaped shot-peened materials, in that it allows efficient reconstruction of complete residual stress states. In addition, it provides an excellent basis for developing predictive tools for in service performance and design optimisation.

Fully Two-Dimensional Discrete Inverse Eigenstrain Analysis of Residual Stresses in a Railway Rail Head

Abstract: The aim of the present study was to introduce a new algorithm for reconstructing the eigenstrain fields in engineering components. A 2D discrete inverse eigenstrain study of residual stresses was carried out on a worn railhead sample. Its residual elastic strain distribution was obtained by neutron diffraction measurement in Stress-Spec, FRMII and used as the input for eigenstrain reconstruction. A new eigenstrain base function-tent was introduced to capture the fully two-dimensional variation of eigenstrain distribution. An automated sequential tent generation scheme was programed in ABAQUS ™ with its preprocessor to load the experimental data and postprocessor to carry out the optimization to obtain the eigenstrain coefficients. The reconstructed eigenstrain field incurs residual stress distribution in the railhead simulation, which showed good agreement with the experimental data.

A synchrotron tomographic energy-dispersive diffraction imaging study of the aerospace alloy Ti 6246

Abstract: A titanium alloy sample (#6246) containing a linear friction weld has been imaged nondestructively using tomographic energy-dispersive diffraction imaging (TEDDI). The diffraction patterns measured at each point of the TEDDI image permitted identification of the material and phases present (±5%). The image also showed the preferred orientation and size–strain distribution present within the sample without the need for any further sample preparation. The preferred orientation was observed in clusters with average dimensions very similar to the experimental spatial resolution (400 µm). The length scales and preferred orientation distributions were consistent with orientation imaging microscopy measurements made by Szczepanski, Jha, Larsen & Jones [Metall. Mater. Trans. A (2008), 39, 2841–2851] where the microstructure development was linked to the grain growth of the parent material. The use of a high-energy X-ray distribution (30–80 keV) in the incident beam reduced systematic errors due to the source profile, sample and air absorption. The TEDDI data from each voxel were reduced to an angle-dispersive form and Rietveld refined to a mean χ2 of 1.4. The mean lattice parameter error (δd/d) ranged from ∼10−4 for the highly crystalline regions to ∼10−3 for regions of very strong preferred orientation and internal strain. The March–Dollase preferred orientation errors refined to an average value of ±2%. A 100% correlation between observed fluorescence and diffraction peak broadening was observed, providing further evidence for vicinal strain broadening.

Investigation of the structure of human dental tissue at multiple length scales using high energy synchrotron X‐ray SAXS/WAXS

Abstract: High energy (>50keV) synchrotron X‐ray scattering experiments were carried out on beamline I12 JEEP at the Diamond Light Source (DLS, Oxford, UK) Although a complete human tooth could be studied, in the present study attention was focused on coupons from the region of the Dentin‐Enamel Junction (DEJ) Simultaneous high energy SAXS/WAXS measurements were carried out Quantitative analysis of the results allows multiple length scale characterization of the nano‐crystalline structure of dental tissues SAXS patterns analysis provide insight into the mean thickness and orientation of hydroxyapatite particles, while WAXS (XRD) patterns allow the determination of the crystallographic unit cell parameters of the hydroxyapatite phase It was found that the average particle thickness determined from SAXS interpretation varies as a function of position in the vicinity of the DEJ Most mineral particles are randomly orientated within dentin, although preferred orientation emerges and becomes stronger on approach to

Adaptive calibration of a nonlocal coupled damage plasticity model for aluminium alloy AA6082 T0

Abstract: Continuum Damage Mechanics (CDM) accounts for material degradation (softening and ultimately failure) by modifying the load-bearing properties of the material (stiffness and strength) through a special state variable referred to as damage. Damage is typically represented by a scalar or a higher dimension object (such as vector or tensor) with values between zero for virgin material and unity for the material that lost all its bearing capacity. Considered in this way, damage becomes an additional field quantity that needs to be considered along with strain and stress, and can be computed either incrementally, or as a certain function of a suitable physical parameter such as inelastic strain. The advantage of enriching the formulation of a continuum deformation problem with a damage parameter is that it allows considering the material post-critical behaviour, i.e. its response under deformations exceeding those when the maximum load-bearing capacity is reached. Typically, this post-critical behaviour is associated with strain localisation, initiation, growth and interaction of discontinuities, and final fracture. Within the CDM framework, cracks are represented by diffuse regions of material damaged so that it lost all its strength in at least one direction. Computationally, modelling the post-critical (softening) behaviour of material represents a challenge in terms of the numerical stability of algorithms. Nonlocal description of damage appears to offer a rational route towards stable modelling. Nonlocal averaging of the plastic strain for the evaluation of damage also renders CDM models independent of the mesh size and orientation, and helps overcome numerical instabilities. The formulation that emerges can be referred to as coupled nonlocal damage-plasticity modelling [1, 2]. An important challenge remains, however, in developing this general approach into a flexible and material-specific modelling tool. This concerns the need to calibrate a large number of material parameters that emerge in this formulation. In order to address this challenge, recently we developed an approach for the calibration of CDM models of ductile materials that we propose to refer to as adaptive calibration. The calibration of the damage function is accomplished by matching the model prediction to the experimental data obtained from a single tensile test with multiple gauge length extensometry [3] used to capture strain localisation and size effects. We describe the application and validation of this approach to the damage function parameter calibration for the aluminium alloy AA 6082 T0. Excellent agreement with experimental measurements is obtained.

Synchrotron X-Ray diffraction analysis of cyclic deformation behaviour of thin gold films

Abstract: Innovative micro-optical sensors involve metallic thin films deposited on polymer substrates. In the present study, we investigate in detail the state of macroscopic and microscopic stress that exists within thin gold films on polycarbonate substrate after deposition, and the evolution of these stresses during reversed in situ tensile testing and as a consequence of fatigue cycling.

Probing mesoscopic lattice misorientation by strain gradient crystal plasticity modelling and micro-beam Laue diffraction experiments

Abstract: A lengthscale and rate-dependent strain gradient crystal plasticity framework was employed to simulate the inter- and intra-granular deformation at the meso- to microscopic level in a large-grained polycrystalline Ni foil. The sample was characterised by micro-beam Laue diffraction experiments. Post-processing of the model was carried out both to forward predict the Laue diffraction patterns on the detector and to perform inverse analysis of the local lattice misorientation in the crystal. The anisotropic broadening of the Laue spots ('streaking') was correctly captured by the simulation and further analysed to reveal the local lattice arrangement.