618 research outputs found
Reconstruction of Residual Stress in a Welded Plate Using the Variational Eigenstrain Approach
We present the formulation for finding the distribution of eigenstrains, i.e.
the sources of residual stress, from a set of measurements of residual elastic
strain (e.g. by diffraction), or residual stress, or stress redistribution, or
distortion. The variational formulation employed seeks to achieve the best
agreement between the model prediction and some measured parameters in the
sense of a minimum of a functional given by a sum over the entire set of
measurements. The advantage of this approach lies in its flexibility: different
sets of measurements and information about different components of the
stress-strain state can be incorporated. We demonstrate the power of the
technique by analysing experimental data for welds in thin sheet of a nickel
superalloy aerospace material. Very good agreement can be achieved between the
prediction and the measurement results without the necessity of using iterative
solution. In practice complete characterisation of residual stress states is
often very difficult, due to limitations of facility access, measurement time
or specimen dimensions. Implications of the new technique for experimental
analysis are all the more significant, since it allows the reconstruction of
the entire stress state from incomplete sets of data.Comment: 16 pages, 17 figure
Numerical simulation and experimental validation of texture in extruded wires of a bcc metal
We present a comparison between a viscoplastic crystal plasticity finite element simulation of the extrusion process applied to a bcc polycrystal and the experimental evaluation of the preferred orientation (texture) in a tungsten wire by monochromatic synchrotron X-ray diffraction with an area detector. We perform a numerical simulation of sample texture evolution during large extrusion deformation with the elongation factor up to the value of fifty. By matching the predicted Orientation Distribution Functions (ODF) and the pole figures generated on the basis of the simulations to the experimental observations, the extrusion strain experienced by the sample during processing can be estimated
Synchrotron XRD study of residual stress in a shot peened Al/SiCp composite
In the present study, residual strain profiles in shot peened specimens of 2124-T4 aluminium alloy matrix composite reinforced
with 17vol% particulate silicon carbide (SiCp) were measured by means of synchrotron-based diffraction using monochromatic,
high energy X-ray beams. The stress state was considered in relation with the microstructural and morphological modifications
induced in the material by shot peening. Strain-induced changes in the lattice parameters were deduced from diffraction
measurements made by two detectors mounted in the horizontal and vertical diffraction planes, providing information on lattice
strains in two nearly mutually perpendicular in-plane directions. On the basis of these data, residual strain and stress profiles
through the specimen thickness were reconstructed for both phases (silicon carbide and aluminium alloy). Microstructural
characterization was performed by means of optical and scanning electron microscopy (SEM), and particle distribution and
hardness modification were identified. The effect of shot peening on the reinforcement and matrix stress states was evaluated.
The findings are discussed in the context of process optimization for fatigue resistance improvement in aluminium alloy-based
MMCs
Eigenstrain boundary layer modelling of the yttria-partially stabilised zirconia–porcelain interface in dental prostheses
The exceptional strength and appealing aesthetics of porcelain veneered yttria partially stabilised zirconia (YPSZ) dental prostheses, has led to the widespread adoption of these materials. However, near-interface chipping of the porcelain remains the primary failure mode. Advanced experimental techniques have recently revealed significant variations in residual stress and YPSZ phase distribution at the YPSZ–porcelain interface. Therefore, in order to improve existing understanding and effectively optimise the production of these devices, an enhanced model of the YPSZ coping that includes these newly discovered phenomena is presented in this study.
Macroscale stresses are shown to arise through the uneven temperatures within the coping during the sintering process and the coefficient of thermal expansion mismatch with the porcelain during veneering. In contrast, microscale stresses are driven by the YPSZ phase transformation and the associated volumetric expansion. The eigenstrain approach proposed here was found to demonstrate a good match between the phase variation determined experimentally, and the corresponding residual stress distribution showed an effective comparison with the empirical measurements. The proposed technique is a straightforward but powerful method for simulating this dominant mechanical behaviour, with significant potential to combine the resulting expressions into existing models. These enhanced simulations are the only viable approach for the precise, reliable and systematic optimisation of prosthesis production parameters that are needed to significantly reduce prosthesis failure rates.</p
Analytical computation of the lattice rotations induced by 3D dislocation loops
This paper presents the derivation of expressions for the lattice rotations induced by a triangular dislocation loop in an isotropic, elastic medium, based on the classical displacement field solution for a triangular dislocation loop. Using the simple example of a triangular dislocation loop with one segment of edge character, one segment of screw character and a mixed character segment, a comparison of the 3D lattice rotation fields with those predicted for straight, infinitely long 2D dislocations is made. Agreement is excellent. As an illustration of the utility of the rotation solution, the lattice rotations induced by a Frank-Read Source are studied at different stages during its evolution. The dislocation segment positions were computed using the discrete dislocation dynamics code ParaDiS. Post-processing of the lattice rotation maps in terms of lattice orientation spread reveals preferential lattice misorientation or streaking which is consistent with the single active slip system in the simulation. Streaking is a feature frequently observed in micro-diffraction measurements. The availability of the lattice rotation solution makes it possible to evaluate the lattice rotations arising from any 3D distribution of dislocation segments. This allows the computation of predicted diffraction patterns from computed dislocation substructures for direct comparison with experimental measurements. It also makes the inclusion of lattice rotations into 3D dislocation dynamics codes possible. This effect has thus far been treated as small, but was shown to be important in 2D dislocation dynamics simulations
Multi-scale characterisation of the 3D microstructure of a thermally-shocked bulk metallic glass matrix composite
Bulk metallic glass matrix composites (BMGMCs) are a new class of metal alloys which have significantly increased ductility and impact toughness, resulting from the ductile crystalline phases distributed uniformly within the amorphous matrix. However, the 3D structures and their morphologies of such composite at nano and micrometre scale have never been reported before. We have used high density electric currents to thermally shock a Zr-Ti based BMGMC to different temperatures, and used X-ray microtomography, FIB-SEM nanotomography and neutron diffraction to reveal the morphologies, compositions, volume fractions and thermal stabilities of the nano and microstructures. Understanding of these is essential for optimizing the design of BMGMCs and developing viable manufacturing methods
Efficient Raman Sideband Generation in a Coherent Atomic Medium
We demonstrate the efficient generation of Raman sidebands in a medium
coherently prepared in a dark state by continuous-wave low-intensity laser
radiation. Our experiment is performed in sodium vapor excited in
configuration on the D line by two laser fields of resonant frequencies
and , and probed by a third field .
First-order sidebands for frequencies , and up to the
third-order sidebands for frequency are observed. The generation
starts at a power as low as 10 microwatt for each input field. Dependencies of
the intensities of both input and generated waves on the frequency difference
(), on the frequency and on the optical
density are investigated.Comment: 7 pages, 6 figure
Design Methodology for a Very High Frequency Resonant Boost Converter
This paper introduces a design methodology for a resonant boost converter topology that is suitable for operation at very high frequencies. The topology we examine features a low parts count and fast transient response, but suffers from higher device stresses compared to other topologies that use a larger number of passive components. A numerical design procedure is developed for this topology that does not rely on time-domain simulation sweeps across parameters. This allows the optimal converter design to be found for a particular main semiconductor switch. If an integrated power process is used where the designer has control over layout of the semiconductor switch, the optimal combination of converter design and semiconductor layout can be found. To validate the proposed converter topology and design approach, a 75-MHz prototype converter is designed and experimentally demonstrated. The performance of the prototype closely matches that predicted by the design procedure, and the converter achieves good efficiency over a wide input voltage range
The Principle of Strain Reconstruction Tomography: Determination of Quench Strain Distribution from Diffraction Measurements
Evaluation of residual elastic strain within the bulk of engineering
components or natural objects is a challenging task, since in general it
requires mapping a six-component tensor quantity in three dimensions. A further
challenge concerns the interpretation of finite resolution data in a way that
is commensurate and non-contradictory with respect to continuum deformation
models. A practical solution for this problem, if it is ever to be found, must
include efficient measurement interpretation and data reduction techniques. In
the present note we describe the principle of strain tomography by high energy
X-ray diffraction, i.e. of reconstruction of the higher dimensional
distribution of strain within an object from reduced dimension measurements;
and illustrate the application of this principle to a simple case of
reconstruction of an axisymmetric residual strain state induced in a
cylindrical sample by quenching. The underlying principle of the analysis
method presented in this paper can be readily generalised to more complex
situations.Comment: 10 pages, 6 figure
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