Papers by Jonathan Belnoue
In the present study we consider an adaptation of Nguyen’s model [G.D. Nguyen, A thermodynamic ap... more In the present study we consider an adaptation of Nguyen’s model [G.D. Nguyen, A thermodynamic approach to constitutive modelling of concrete using damage mechanics and plasticity theory, D.Phil. Thesis, Dept. of Engng. Sci., University of Oxford, 2005] for brittle materials to the case of ductile rupture. We focus our attention on the formulation of a consistent tangent stiffness matrix which is essential for the efficiency and convergence of the implementation within a finite element framework. We demonstrate the capability of the model to generate mesh-independent results and to follow the rupture process very close to failure. Rupture analysis of thin plates of different geometry is presented (symmetric double notch-edged specimen, asymmetric double notch-edged specimen, double notch-edged specimen containing a hole). Further modifications are outlined required to simulate crack growth and paths under the conditions of creep and fatigue.
Monotonic and cyclic loading of polycrystals causes a complex evolution of the dislocation struct... more Monotonic and cyclic loading of polycrystals causes a complex evolution of the dislocation structure and internal stresses. These phenomena were studied in sheet samples of commercially pure (CP) Ni in heat-treated (large-grained) states. Various microscopy tools were used, namely, scanning electron microscopy (SEM) with electron back-scattered diffraction (EBSD) to image the surface grain structure; Focused Ion Beam (FIB) with channelling contrast to visualise the through-thickness grain arrangement; and synchrotron scanning transmission X-ray microscopy (STXM) to obtain absorption-contrast images. In order to investigate the internal defects and lattice distortion caused by them, synchrotron X-ray diffraction was used in a variety of modes. Reciprocal space mapping (RSM) was used to quantify the amount of lattice re-orientation (rotation) due to plastic deformation. Micro-beam Laue diffraction was used to obtain 2D images containing multiple reflections that undergo “streaking” due to plastic deformation. The combination of reciprocal space mapping and Laue micro-diffraction provided improved insight into the deformation processes within individual grains during plastic deformation. The results are interpreted and discussed in conjunction with dislocation dynamics and finite element modelling of plastic deformation by crystal slip.
Shot peening is an important surface impact treatment widely used in industry to improve the perf... more Shot peening is an important surface impact treatment widely used in industry to improve the performance of metallic parts subjected to fatigue loading, contact (fretting) fatigue, stress corrosion and other damage mechanisms by inducing beneficial compressive residual stresses in the surface region. Residual stresses induced by shot-peening are not limited to the material surface regions, but may extend to depths of up to 1mm into steel, depending on the shot-peening intensity and the material properties. The conditioning process is quantified using the ALMEN calibration method where a flat reference plate is simultaneously shot blasted in parallel to the sample being treated. This verification procedure however does not take into account differing material geometries, surface properties and residual stresses existing in practical specimens due to their manufacture. Direct quantification of the extent of the beneficial compressive residual stresses induced through the shot-peening process has become essential due to the occurrence of premature fatigue failures through cracking. We report a quantitative assessment of the efficiency of shot-peening on conical samples treated at three different shot-peening intensities, compared to a water quenced (without shot peening conditioning), as well as a control sample (as reference).
Investigations were performed at the ID31 instrument on 2mm thick slices wire EDM cut from the respective bulk conically shaped 17-4PH stainless steel samples to facilitate transmission measurements at contant pathlength through the sample thickness as function of depth below the surface. Results of the in-plane and surface normal components of residual strain indicate significant differences between line scans taken at three locations from the tip, reflecting an underlying material thickness dependence. An exploratory line scan taken at 3mm from the tip in the bulk of a conical sample indicates substantially larger compressive strains existing at the near surface region. The latter indicates that significant relaxation of the residual strains had occurred due to the sectioning of the slices.
Non-destructive, quantitative and precise determination of internal strain distributions within s... more Non-destructive, quantitative and precise determination of internal strain distributions within structural materials and components can be accomplished by only a few experimental techniques, amongst which diffraction of penetrating radiation (X-ray and neutron beams) plays a central role. However, due to the various limitations of these methods, the 2-D and 3-D mapping of internal strains within bulk cross-sections at a spatial resolution of 0.1 mm or better has long remained a challenge. The principle of “strain tomography” proposed by the present authors (an instance of broader “rich” tomography methodology) makes use of the reconstruction tomography technique widely employed in imaging applications. In the present paper we report successful practical implementation of this principle to the study of bending strains in sintered polycrystalline zirconia used in the fabrication of multi-unit fixed prosthetic dentures. The results demonstrate the power of the technique, and pave the way to wider application of this approach to cross-sectional strain mapping in complex-shaped components under various loading conditions.
When subjected to plastic deformation, grains within ductile FCC polycrystals fragment into ‘soft... more When subjected to plastic deformation, grains within ductile FCC polycrystals fragment into ‘soft’, low dislocation density cells separated by ‘hard’, dislocation-rich walls. Using a narrow-bandwidth, sub-micrometer X-ray beam, we have mapped the deformation structure inside a single grain within a deformed Ni polycrystal. Dislocation multiplication and entanglement was found to vary depending on the physical dimensions of the grain. The method we use overcomes current limitations in classical X-ray topography allowing topographic images to be formed from small, highly-deformed grains
A titanium alloy sample (#6246) containing a linear friction weld has been imaged nondestructivel... more 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 [chi]2 of 1.4. The mean lattice parameter error ([delta]d/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.
In this study, influence of microstructures and texture development induced by hot-extrusion and/... more 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 / / fenced(1 0 over(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.
Nickel base superalloys are the primary class of materials used in the manufacture of high temper... more 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.
This paper presents the results of eigenstrain analysis in non-uniformly shaped shot-peened 17-4P... more This paper presents the results of eigenstrain analysis in non-uniformly shaped shot-peened 17-4PH stainless steel samples. The finite element models are established for inverse eigenstrain analysis of slices and bulk conical samples. It is shown that the elastic strain distributions and relief are directly related to peening intensity and sample shape/thickness via the underlying permanent strain, or eigenstrain. Thus, the effect of the peening treatment is best described in terms of the induced eigenstrain. The proposed framework for predictive modelling of residual stresses in non-uniformly shaped shot-peened materials allows efficient reconstruction of complete residual stress state, and provides an excellent basis for developing predictive tools for in service performance and design optimization.
Accumulation of residual stresses in rails during service can contribute to crack initiation and ... more Accumulation of residual stresses in rails during service can contribute to crack initiation and fracture and may result in serious accidents. It is therefore necessary and important to quantify the residual stresses that evolve under repeated rolling contact between wheel and rail. In the present study, triaxial residual strain measurements were performed in a worn British railway rail using neutron diffraction. Localized stress is observed close to the region of contact, showing the asymmetry and complexity of distributions that arise from the non-uniform plastic deformation. Contact-induced plasticity is revealed by the broadening (increase in the full width at half-maximum (FWHM)) of the diffraction peaks.
The overload retardation effect on fatigue crack growth rate (FCGR) in titanium alloy Ti–6Al–4V i... more The overload retardation effect on fatigue crack growth rate (FCGR) in titanium alloy Ti–6Al–4V is studied. Synchrotron X-ray diffraction strain mapping of near-crack tip regions of pre-cracked fatigued samples is used to determine the effective stress intensity factors experienced by the crack tip. The effective stress intensity factor values are computed by finding the best match between the experimental strain maps and linear elastic fracture mechanics (LEFM) predictions. The dependence of the effective stress intensity factor, K, on the applied load is plotted, and an interpretation of the overload retardation effect is proposed. The present approach permits to reconcile the traditional LEFM fatigue crack propagation prediction and the experimental measurement of strain fields.
Procedia Engineering, Jan 1, 2009
In the present study we consider an adaptation of Nguyen’s model [G.D. Nguyen, A thermodynamic ap... more In the present study we consider an adaptation of Nguyen’s model [G.D. Nguyen, A thermodynamic approach to constitutive modelling of concrete using damage mechanics and plasticity theory, D.Phil. Thesis, Dept. of Engng. Sci., University of Oxford, 2005] for brittle materials to the case of ductile rupture. We focus our attention on the formulation of a consistent tangent stiffness matrix which is essential for the efficiency and convergence of the implementation within a finite element framework. We demonstrate the capability of the model to generate mesh-independent results and to follow the rupture process very close to failure. Rupture analysis of thin plates of different geometry is presented (symmetric double notch-edged specimen, asymmetric double notch-edged specimen, double notch-edged specimen containing a hole). Further modifications are outlined required to simulate crack growth and paths under the conditions of creep and fatigue.
International Journal of …, Jan 1, 2009
In this paper we present an overview of experimental and modelling studies of fatigue crack growt... more In this paper we present an overview of experimental and modelling studies of fatigue crack growth rates in aerospace titanium alloy Ti–6Al–4V. We review work done on the subject since the 1980s to the present day, identifying test programmes and procedures and their results, as well as predictive approaches developed over this period. We then present the results of some of our recent experiments and simulations. Fatigue crack growth rates (FCGRs) under constant applied load were evaluated as a function of crack length, and the effect of overload (retardation) was considered. Crack opening was measured during cycling using digital image correlation, and residual stress intensity factor was determined using synchrotron X-ray diffraction mapping. Modelling techniques used for the prediction of FCGRs are then reviewed, and an approach based on the analysis of energy dissipation at the crack tip is proposed. Finally, directions for further research are identified.
International Journal of …, Jan 1, 2007
This paper presents a new 1-D non-local damage-plasticity deformation model for ductile materials... more This paper presents a new 1-D non-local damage-plasticity deformation model for ductile materials. It uses the thermodynamic framework described in Houlsby and Puzrin (2000) and holds, nevertheless, some similarities with Lemaitre’s (1971) approach. A 1D finite element (FE) model of a bar fixed at one end and loaded in tension at the other end is introduced. This simple model demonstrates how the approach can be implemented within the finite element framework, and that it is capable of capturing both the pre-peak hardening and post-peak softening (generally responsible for models instability) due to damage-induced stiffness and strength reduction characteristic of ductile materials. It is also shown that the approach has further advantages of achieving some degree of mesh independence, and of being able to capture deformation size effects. Finally, it is illustrated how the model permits the calculation of essential work of rupture (EWR), i.e. the specific energy per unit cross-sectional area that is needed to cause tensile failure of a specimen.
Engineering Fracture …, Jan 1, 2010
A nonlocal coupled damage-plasticity model is presented for predicting crack growth within plates... more A nonlocal coupled damage-plasticity model is presented for predicting crack growth within plates made from a ductile metallic alloy. The damage law is explicitly defined in terms of the accumulated nonlocal plastic strain ensuring efficient numerical implementation. On the basis of this model, a UMAT for the finite element (FE) package ABAQUS (implicit) is developed. Thin plates of different geometries were modelled and analysed using this model, and the numerical stability of the model has been verified. The model shows the ability to capture the post-peak softening behaviour, shows mesh independence, and requires calibration of parameters against experimental data. The suitability of the model for crack path prediction is discussed
Procedia Engineering, Jan 1, 2009
Following the publication of several seminal studies, discrete dislocation dynamics has become we... more Following the publication of several seminal studies, discrete dislocation dynamics has become well-established as a means of analysing the response of ductile crystals and polycrystals to mechanical loading. Developments undertaken by different authors have followed two principal directions: (i) the use of simple 2D formulations that do not seek to capture correctly the details of slip geometry, but allow some insight to be developed into the trends and relationships, and (ii) large scale 3D simulations seeking to represent correctly the geometry of dislocation segments, and their spatial distribution and interaction. The former is computationally inexpensive and fast, but fails to capture the effects of grain orientation. The latter is associated with large overheads in terms of the computational effort. The purpose of the present study is to propose and develop an intermediate level approach, whereby the geometry of the crystal slip is captured to a greater degree, while computational difficulty is kept to a minimum. The results are analysed in terms of the dependence of yield stress and cyclic hardening on the crystal orientation and dislocation interaction with each other and with the grain boundaries.
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Papers by Jonathan Belnoue
Investigations were performed at the ID31 instrument on 2mm thick slices wire EDM cut from the respective bulk conically shaped 17-4PH stainless steel samples to facilitate transmission measurements at contant pathlength through the sample thickness as function of depth below the surface. Results of the in-plane and surface normal components of residual strain indicate significant differences between line scans taken at three locations from the tip, reflecting an underlying material thickness dependence. An exploratory line scan taken at 3mm from the tip in the bulk of a conical sample indicates substantially larger compressive strains existing at the near surface region. The latter indicates that significant relaxation of the residual strains had occurred due to the sectioning of the slices.
Investigations were performed at the ID31 instrument on 2mm thick slices wire EDM cut from the respective bulk conically shaped 17-4PH stainless steel samples to facilitate transmission measurements at contant pathlength through the sample thickness as function of depth below the surface. Results of the in-plane and surface normal components of residual strain indicate significant differences between line scans taken at three locations from the tip, reflecting an underlying material thickness dependence. An exploratory line scan taken at 3mm from the tip in the bulk of a conical sample indicates substantially larger compressive strains existing at the near surface region. The latter indicates that significant relaxation of the residual strains had occurred due to the sectioning of the slices.