The Experts below are selected from a list of 3621 Experts worldwide ranked by ideXlab platform
Matthew P Miller - One of the best experts on this subject based on the ideXlab platform.
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analyzing shear band formation with high resolution x ray diffraction
Acta Materialia, 2018Co-Authors: Darren C Pagan, Mark Obstalecki, Junsang Park, Matthew P MillerAbstract:Abstract Localization of crystallographic slip into shear bands during uniaxial compression of a copper single crystal is studied using very far-field high-energy diffraction microscopy (vff-HEDM). Diffracted Intensity was collected in-situ as the crystal deformed using a unique mobile detector stage that provided access to multiple diffraction peaks with high-angular resolution. From the diffraction data, single crystal orientation pole figures (SCPFs) were generated and are used to track the evolution of the distribution of lattice orientation that develops as slip localizes. To aid the identification of ‘signatures’ of shear band formation and analyze the SCPF data, a model of slip-driven lattice reorientation within shear bands is introduced. Confidence is built in conclusions drawn from the SCPF data about the character of internal slip localization through comparisons with strain fields on the sample surface measured simultaneously using digital image correlation. From the diffraction data, we find that the active slip direction and slip plane are not directly aligned with the orientation of the shear bands that formed. In fact, by extracting the underlying slip system activity from the SCPF data, we show that intersecting shear bands measured on the surface of the sample arise from slip primarily on the same underlying single slip system. These new vff-HEDM results raise significant questions on the use of surface measurements for slip system activity estimation.
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understanding local deformation in metallic polycrystals using high energy x rays and finite elements
Current Opinion in Solid State & Materials Science, 2014Co-Authors: Matthew P Miller, Paul R DawsonAbstract:Abstract A methodology for understanding the stress and elastoplastic deformation responses within a loaded polycrystal is presented along with illustrative examples. High energy synchrotron X-rays are used to penetrate bulk metallic samples and produce Diffracted Intensity from each deforming crystal – revealing the evolving internal structure. A virtual representation of the microstructure is constructed using the finite element method (FEM) to simulate the evolution of the elastoplastic deformations, stress fields, and lattice orientations within the deforming crystals as the polycrystal is loaded. Simulations are compared directly to experimental diffraction data. In the case of powder experiments, lattice strain pole figures (SPFs) measured experimentally are compared to SPFs calculated by projecting X-rays through the finite element mesh. During in situ loading experiments, the stress states are found to differ from one crystal to the next and to vary from the stress being applied at the macroscale. A SPF/FEM-based methodology for quantifying residual stress fields within processed polycrystalline components is described. SPFs were measured at many points within a shrink-fit sample. Finite element discretizations of both the sample and orientation space of each diffraction volume were used to formulate an optimization for the distribution of the stress tensor within the sample. A different experiment, one in which the X-ray beam and the crystals are closer to the same size, is used to investigate the aggregate crystal by crystal. The Debye–Scherrer rings reduce to a set of spots associated with each crystal within the diffraction volume. This method is demonstrated by tracking deformation of four grains within a deforming BCC titanium aggregate loaded in situ within the elastic regime to determine the single crystal elastic moduli. Plastic deformation can also be investigated by monitoring the size and shape of individual diffraction spots. Each spot contains geometrically exact information regarding the internal structure of the crystal. Instead of reconstructing the crystal structure by inverting the diffraction data, virtual diffraction experiments are performed on the finite element mesh and the resulting simulated diffraction patterns are compared directly to the experimental results. Once the experimental/simulation methodology is validated, the approximation of the subgrain distribution of stress and lattice orientation from the finite element model can be used to construct theories for failure phenomena such as microcrack initiation. As opposed to other methods of discretizing a polycrystalline aggregate, the finite element framework enables a seamless transition to analyses associated with mechanical design.
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quantitative analysis of crystal scale deformation heterogeneity during cyclic plasticity using high energy x ray diffraction and finite element simulation
Acta Materialia, 2014Co-Authors: Mark Obstalecki, Paul R Dawson, Su Leen Wong, Matthew P MillerAbstract:Abstract Modern high-energy X-ray diffraction (HEXD) experiments coupled with a crystal-based finite-element model employing forward projection of virtual X-rays through each element is applied to study cyclic plasticity. An Okegawa mold copper specimen was cyclically deformed in situ at the Advanced Photon Source. The strain amplitudes of the cyclic experiments reached well into the plastic regime and diffraction images were generated at several points in the loading history using a HEXD methodology. Four grains within the bulk of a polycrystalline sample were tracked and interrogated with X-rays. Diffraction peak data were reduced to center of mass (COM) and full width at half maximum (FWHM) values in the detector coordinates 2 θ (radial) and η (azimuthal). The peaks evolved with cycles and changed significantly when the plastic strain amplitude was increased. Large changes in the peaks (especially the azimuthal FWHM values) were also observed during the course of one loading cycle; larger η FWHM values were seen at the compressive end of the cycles. This trend was reversed when the sample was initially loaded in compression. Diffracted Intensity distributions were also seen to change significantly from one grain to the next. Using a virtual diffractometer model, COM and FWHM values were computed from the modeling results by projecting virtual X-rays through the finite-element mesh and compared to the experimental data. The finite-element polycrystal model serves as the final step in the data reduction process, revealing significant spatial heterogeneity of orientation, stress and plastic strain rate distributions. Studying these distributions collectively will be necessary to fully understand the detailed elastic–plastic deformation behavior within each grain and to explore problems such as microcrack initiation hypotheses in polycrystalline materials.
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a framework for generating synthetic diffraction images from deforming polycrystals using crystal based finite element formulations
Computational Materials Science, 2013Co-Authors: Su Leen Wong, Matthew P Miller, Junsang Park, Paul R DawsonAbstract:A framework for generating synthetic diffraction images on X-ray detectors from individual grains within polycrystals under in situ loading is described. Crystal plasticity-based finite element simulations of three-dimensional (3D) polycrystalline aggregates undergoing deformation were utilized to mimic a far-field High Energy Diffraction Microscopy (HEDM) experiment. The smearing of the diffraction spots on a two-dimensional (2D) area detector was consistent between the experiment and simulation for a target grain within a polycrystalline sample of a Cu–Cr–Zr alloy. The influence of crystallographic neighborhood and grain shape on the Diffracted Intensity distributions of the diffraction spots, the stress distribution and the misorientation distribution within a grain is also investigated. Key features of the diffraction spots are examined, differentiating between changes with applied stress and changes due to lattice misorientation associated with plastic straining.
Paul R Dawson - One of the best experts on this subject based on the ideXlab platform.
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understanding local deformation in metallic polycrystals using high energy x rays and finite elements
Current Opinion in Solid State & Materials Science, 2014Co-Authors: Matthew P Miller, Paul R DawsonAbstract:Abstract A methodology for understanding the stress and elastoplastic deformation responses within a loaded polycrystal is presented along with illustrative examples. High energy synchrotron X-rays are used to penetrate bulk metallic samples and produce Diffracted Intensity from each deforming crystal – revealing the evolving internal structure. A virtual representation of the microstructure is constructed using the finite element method (FEM) to simulate the evolution of the elastoplastic deformations, stress fields, and lattice orientations within the deforming crystals as the polycrystal is loaded. Simulations are compared directly to experimental diffraction data. In the case of powder experiments, lattice strain pole figures (SPFs) measured experimentally are compared to SPFs calculated by projecting X-rays through the finite element mesh. During in situ loading experiments, the stress states are found to differ from one crystal to the next and to vary from the stress being applied at the macroscale. A SPF/FEM-based methodology for quantifying residual stress fields within processed polycrystalline components is described. SPFs were measured at many points within a shrink-fit sample. Finite element discretizations of both the sample and orientation space of each diffraction volume were used to formulate an optimization for the distribution of the stress tensor within the sample. A different experiment, one in which the X-ray beam and the crystals are closer to the same size, is used to investigate the aggregate crystal by crystal. The Debye–Scherrer rings reduce to a set of spots associated with each crystal within the diffraction volume. This method is demonstrated by tracking deformation of four grains within a deforming BCC titanium aggregate loaded in situ within the elastic regime to determine the single crystal elastic moduli. Plastic deformation can also be investigated by monitoring the size and shape of individual diffraction spots. Each spot contains geometrically exact information regarding the internal structure of the crystal. Instead of reconstructing the crystal structure by inverting the diffraction data, virtual diffraction experiments are performed on the finite element mesh and the resulting simulated diffraction patterns are compared directly to the experimental results. Once the experimental/simulation methodology is validated, the approximation of the subgrain distribution of stress and lattice orientation from the finite element model can be used to construct theories for failure phenomena such as microcrack initiation. As opposed to other methods of discretizing a polycrystalline aggregate, the finite element framework enables a seamless transition to analyses associated with mechanical design.
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quantitative analysis of crystal scale deformation heterogeneity during cyclic plasticity using high energy x ray diffraction and finite element simulation
Acta Materialia, 2014Co-Authors: Mark Obstalecki, Paul R Dawson, Su Leen Wong, Matthew P MillerAbstract:Abstract Modern high-energy X-ray diffraction (HEXD) experiments coupled with a crystal-based finite-element model employing forward projection of virtual X-rays through each element is applied to study cyclic plasticity. An Okegawa mold copper specimen was cyclically deformed in situ at the Advanced Photon Source. The strain amplitudes of the cyclic experiments reached well into the plastic regime and diffraction images were generated at several points in the loading history using a HEXD methodology. Four grains within the bulk of a polycrystalline sample were tracked and interrogated with X-rays. Diffraction peak data were reduced to center of mass (COM) and full width at half maximum (FWHM) values in the detector coordinates 2 θ (radial) and η (azimuthal). The peaks evolved with cycles and changed significantly when the plastic strain amplitude was increased. Large changes in the peaks (especially the azimuthal FWHM values) were also observed during the course of one loading cycle; larger η FWHM values were seen at the compressive end of the cycles. This trend was reversed when the sample was initially loaded in compression. Diffracted Intensity distributions were also seen to change significantly from one grain to the next. Using a virtual diffractometer model, COM and FWHM values were computed from the modeling results by projecting virtual X-rays through the finite-element mesh and compared to the experimental data. The finite-element polycrystal model serves as the final step in the data reduction process, revealing significant spatial heterogeneity of orientation, stress and plastic strain rate distributions. Studying these distributions collectively will be necessary to fully understand the detailed elastic–plastic deformation behavior within each grain and to explore problems such as microcrack initiation hypotheses in polycrystalline materials.
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a framework for generating synthetic diffraction images from deforming polycrystals using crystal based finite element formulations
Computational Materials Science, 2013Co-Authors: Su Leen Wong, Matthew P Miller, Junsang Park, Paul R DawsonAbstract:A framework for generating synthetic diffraction images on X-ray detectors from individual grains within polycrystals under in situ loading is described. Crystal plasticity-based finite element simulations of three-dimensional (3D) polycrystalline aggregates undergoing deformation were utilized to mimic a far-field High Energy Diffraction Microscopy (HEDM) experiment. The smearing of the diffraction spots on a two-dimensional (2D) area detector was consistent between the experiment and simulation for a target grain within a polycrystalline sample of a Cu–Cr–Zr alloy. The influence of crystallographic neighborhood and grain shape on the Diffracted Intensity distributions of the diffraction spots, the stress distribution and the misorientation distribution within a grain is also investigated. Key features of the diffraction spots are examined, differentiating between changes with applied stress and changes due to lattice misorientation associated with plastic straining.
Daniele Spiga - One of the best experts on this subject based on the ideXlab platform.
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mirrors for x ray telescopes fresnel diffraction based computation of point spread functions from metrology
Astronomy and Astrophysics, 2015Co-Authors: Lorenzo Raimondi, Daniele SpigaAbstract:Context. The imaging sharpness of an X-ray telescope is chiefly determined by the optical quality of its focusing optics, which in turn mostly depends on the shape accuracy and the surface finishing of the grazing-incidence X-ray mirrors that compose the optical modules. To ensure the imaging performance during the mirror manufacturing, a fundamental step is predicting the mirror point spread function (PSF) from the metrology of its surface. Traditionally, the PSF computation in X-rays is assumed to be different depending on whether the surface defects are classified as figure errors or roughness. This classical approach, however, requires setting a boundary between these two asymptotic regimes, which is not known a priori. Aims. The aim of this work is to overcome this limit by providing analytical formulae that are valid at any light wavelength, for computing the PSF of an X-ray mirror shell from the measured longitudinal profiles and the roughness power spectral density, without distinguishing spectral ranges with different treatments. Methods. The method we adopted is based on the Huygens-Fresnel principle for computing the Diffracted Intensity from measured or modeled profiles. In particular, we have simplified the computation of the surface integral to only one dimension, owing to the grazing incidence that reduces the influence of the azimuthal errors by orders of magnitude. The method can be extended to optical systems with an arbitrary number of reflections ‐ in particular the Wolter-I, which is frequently used in X-ray astronomy ‐ and can be used in both near- and far-field approximation. Finally, it accounts simultaneously for profile, roughness, and aperture diffraction. Results. We describe the formalism with which one can self-consistently compute the PSF of grazing-incidence mirrors, and we show some PSF simulations including the UV band, where the aperture diffraction dominates the PSF, and hard X-rays where the X-ray scattering has a major impact on the PSF degradation. The results are validated with ray-tracing simulations, or by comparison with the analytical computation of the half-energy width based on the known scattering theory, where these approaches are applicable. Finally, we validate this by comparing the simulated PSF of a real Wolter-I mirror shell with the measured PSF in hard X-rays.
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mirrors for x ray telescopes fresnel diffraction based computation of point spread functions from metrology
arXiv: Instrumentation and Methods for Astrophysics, 2014Co-Authors: Lorenzo Raimondi, Daniele SpigaAbstract:The imaging sharpness of an X-ray telescope is chiefly determined by the optical quality of its focusing optics, which in turn mostly depends on the shape accuracy and the surface finishing of the grazing-incidence X-ray mirrors that compose the optical modules. To ensure the imaging performance during the mirror manufacturing, a fundamental step is predicting the mirror point spread function (PSF) from the metrology of its surface. Traditionally, the PSF computation in X-rays is assumed to be different depending on whether the surface defects are classified as figure errors or roughness. [...] The aim of this work is to overcome this limit by providing analytical formulae that are valid at any light wavelength, for computing the PSF of an X-ray mirror shell from the measured longitudinal profiles and the roughness power spectral density (PSD), without distinguishing spectral ranges with different treatments. The method we adopted is based on the Huygens-Fresnel principle for computing the Diffracted Intensity from measured or modeled profiles. In particular, we have simplified the computation of the surface integral to only one dimension, owing to the grazing incidence that reduces the influence of the azimuthal errors by orders of magnitude. The method can be extended to optical systems with an arbitrary number of reflections - in particular the Wolter-I, which is frequently used in X-ray astronomy - and can be used in both near- and far-field approximation. Finally, it accounts simultaneously for profile, roughness, and aperture diffraction. We describe the formalism with which one can self-consistently compute the PSF of grazing-incidence mirrors, [...] Finally, we validate this by comparing the simulated PSF of a real Wolter-I mirror shell with the measured PSF in hard X-rays.
Lorenzo Raimondi - One of the best experts on this subject based on the ideXlab platform.
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mirrors for x ray telescopes fresnel diffraction based computation of point spread functions from metrology
Astronomy and Astrophysics, 2015Co-Authors: Lorenzo Raimondi, Daniele SpigaAbstract:Context. The imaging sharpness of an X-ray telescope is chiefly determined by the optical quality of its focusing optics, which in turn mostly depends on the shape accuracy and the surface finishing of the grazing-incidence X-ray mirrors that compose the optical modules. To ensure the imaging performance during the mirror manufacturing, a fundamental step is predicting the mirror point spread function (PSF) from the metrology of its surface. Traditionally, the PSF computation in X-rays is assumed to be different depending on whether the surface defects are classified as figure errors or roughness. This classical approach, however, requires setting a boundary between these two asymptotic regimes, which is not known a priori. Aims. The aim of this work is to overcome this limit by providing analytical formulae that are valid at any light wavelength, for computing the PSF of an X-ray mirror shell from the measured longitudinal profiles and the roughness power spectral density, without distinguishing spectral ranges with different treatments. Methods. The method we adopted is based on the Huygens-Fresnel principle for computing the Diffracted Intensity from measured or modeled profiles. In particular, we have simplified the computation of the surface integral to only one dimension, owing to the grazing incidence that reduces the influence of the azimuthal errors by orders of magnitude. The method can be extended to optical systems with an arbitrary number of reflections ‐ in particular the Wolter-I, which is frequently used in X-ray astronomy ‐ and can be used in both near- and far-field approximation. Finally, it accounts simultaneously for profile, roughness, and aperture diffraction. Results. We describe the formalism with which one can self-consistently compute the PSF of grazing-incidence mirrors, and we show some PSF simulations including the UV band, where the aperture diffraction dominates the PSF, and hard X-rays where the X-ray scattering has a major impact on the PSF degradation. The results are validated with ray-tracing simulations, or by comparison with the analytical computation of the half-energy width based on the known scattering theory, where these approaches are applicable. Finally, we validate this by comparing the simulated PSF of a real Wolter-I mirror shell with the measured PSF in hard X-rays.
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mirrors for x ray telescopes fresnel diffraction based computation of point spread functions from metrology
arXiv: Instrumentation and Methods for Astrophysics, 2014Co-Authors: Lorenzo Raimondi, Daniele SpigaAbstract:The imaging sharpness of an X-ray telescope is chiefly determined by the optical quality of its focusing optics, which in turn mostly depends on the shape accuracy and the surface finishing of the grazing-incidence X-ray mirrors that compose the optical modules. To ensure the imaging performance during the mirror manufacturing, a fundamental step is predicting the mirror point spread function (PSF) from the metrology of its surface. Traditionally, the PSF computation in X-rays is assumed to be different depending on whether the surface defects are classified as figure errors or roughness. [...] The aim of this work is to overcome this limit by providing analytical formulae that are valid at any light wavelength, for computing the PSF of an X-ray mirror shell from the measured longitudinal profiles and the roughness power spectral density (PSD), without distinguishing spectral ranges with different treatments. The method we adopted is based on the Huygens-Fresnel principle for computing the Diffracted Intensity from measured or modeled profiles. In particular, we have simplified the computation of the surface integral to only one dimension, owing to the grazing incidence that reduces the influence of the azimuthal errors by orders of magnitude. The method can be extended to optical systems with an arbitrary number of reflections - in particular the Wolter-I, which is frequently used in X-ray astronomy - and can be used in both near- and far-field approximation. Finally, it accounts simultaneously for profile, roughness, and aperture diffraction. We describe the formalism with which one can self-consistently compute the PSF of grazing-incidence mirrors, [...] Finally, we validate this by comparing the simulated PSF of a real Wolter-I mirror shell with the measured PSF in hard X-rays.
Su Leen Wong - One of the best experts on this subject based on the ideXlab platform.
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quantitative analysis of crystal scale deformation heterogeneity during cyclic plasticity using high energy x ray diffraction and finite element simulation
Acta Materialia, 2014Co-Authors: Mark Obstalecki, Paul R Dawson, Su Leen Wong, Matthew P MillerAbstract:Abstract Modern high-energy X-ray diffraction (HEXD) experiments coupled with a crystal-based finite-element model employing forward projection of virtual X-rays through each element is applied to study cyclic plasticity. An Okegawa mold copper specimen was cyclically deformed in situ at the Advanced Photon Source. The strain amplitudes of the cyclic experiments reached well into the plastic regime and diffraction images were generated at several points in the loading history using a HEXD methodology. Four grains within the bulk of a polycrystalline sample were tracked and interrogated with X-rays. Diffraction peak data were reduced to center of mass (COM) and full width at half maximum (FWHM) values in the detector coordinates 2 θ (radial) and η (azimuthal). The peaks evolved with cycles and changed significantly when the plastic strain amplitude was increased. Large changes in the peaks (especially the azimuthal FWHM values) were also observed during the course of one loading cycle; larger η FWHM values were seen at the compressive end of the cycles. This trend was reversed when the sample was initially loaded in compression. Diffracted Intensity distributions were also seen to change significantly from one grain to the next. Using a virtual diffractometer model, COM and FWHM values were computed from the modeling results by projecting virtual X-rays through the finite-element mesh and compared to the experimental data. The finite-element polycrystal model serves as the final step in the data reduction process, revealing significant spatial heterogeneity of orientation, stress and plastic strain rate distributions. Studying these distributions collectively will be necessary to fully understand the detailed elastic–plastic deformation behavior within each grain and to explore problems such as microcrack initiation hypotheses in polycrystalline materials.
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a framework for generating synthetic diffraction images from deforming polycrystals using crystal based finite element formulations
Computational Materials Science, 2013Co-Authors: Su Leen Wong, Matthew P Miller, Junsang Park, Paul R DawsonAbstract:A framework for generating synthetic diffraction images on X-ray detectors from individual grains within polycrystals under in situ loading is described. Crystal plasticity-based finite element simulations of three-dimensional (3D) polycrystalline aggregates undergoing deformation were utilized to mimic a far-field High Energy Diffraction Microscopy (HEDM) experiment. The smearing of the diffraction spots on a two-dimensional (2D) area detector was consistent between the experiment and simulation for a target grain within a polycrystalline sample of a Cu–Cr–Zr alloy. The influence of crystallographic neighborhood and grain shape on the Diffracted Intensity distributions of the diffraction spots, the stress distribution and the misorientation distribution within a grain is also investigated. Key features of the diffraction spots are examined, differentiating between changes with applied stress and changes due to lattice misorientation associated with plastic straining.