João Quinta da Fonseca

Image processing issues in digital strain mapping

We have developed high density image processing techniques for finding the surface strain of an untreated sample of material from a sequence of images taken during the application of force from a test rig. Not all motion detection algorithms have suitable functional characteristics for this task, as image sequences are characterised by both short- and long-range displacements, non-rigid deformations, as well as a low signal-to-noise ratio and methodological artifacts. We show how a probability-based motion detection algorithm can be used as a high confidence estimator of the strain tensor characterising the deformation of the material. An important issue discussed is how to minimise the number of image brightness differences that need to be calculated. We give results from two studies of materials under axial tension: a sample of aluminium alloy exhibiting a propagating plastic deformation, and a preparation of deer antler bone, a natural composite material.

Local Plastic Strain Measurement by EBSD

Work has been carried out recently, which demonstrates misorientation measurements recorded by using electron backscatter diffraction (EBSD) enables one to undertake local post mortem plastic strain quantification once the degree of misorientation is calibrated against plastic strain. The present paper builds on this work and investigates the possibility of determining strain in individual grains. Due to the anisotropy of crystalline grains, polycrystalline material deform inhomogeneously on a microstructural level. In this study, the local strain induced in a pure copper specimen during tensile loading measured using EBSD was compared to in-situ strain measurements using optical microscopy imaging in conjunction with image correlation technique. By applying an averaging procedure for improving the accuracy of the measured EBSD data, the distribution of the misorientation within grains was quantified, and, as one would expect, it tended to be highest near the grain boundaries.

Comparison between a near-field and a far-field indexing approach for characterization of a polycrystalline sample volume containing more than 1500 grains

A comparison of the performance of X-ray diffraction tomography, a near-field diffraction technique, and a far-field diffraction technique for indexing X-ray diffraction data of polycrystalline materials has been carried out by acquiring two sets of diffraction data from the same polycrystalline sample volume. Both approaches used in this study are variants of the three-dimensional X-ray diffraction (3DXRD) methodology, but they rely on different data-collection and analysis strategies. Previous attempts to assess the quality of 3DXRD indexing results from polycrystalline materials have been restricted to comparisons with two-dimensional electron backscatter diffraction cross sections containing a limited number of grains. In the current work, the relative performance of two frequently used polycrystalline-material indexing algorithms is assessed, comparing the indexing results obtained from a threedimensional sample volume containing more than 1500 grains. The currently achievable accuracy of three-dimensional grain maps produced with these algorithms has been assessed using a statistical analysis of the measurement of the size, position and orientation of the grains in the sample. The material used for this comparison was a polycrystalline commercially pure titanium grade 2 sample, which has a hexagonal close-packed crystal structure. The comparison of the two techniques shows good agreement for the measurements of the grain position, size and orientation. Cross-validation between the indexing results shows that about 99% of the sample volume has been indexed correctly by either of these indexing approaches. The remaining discrepancies have been analysed and the strengths and limitations of both approaches are discussed.

Grain Breakup During Elevated Temperature Deformation of an HCP Metal

A combination of mechanical testing, EBSD and crystal plasticity finite element modeling were used to investigate the influence of temperature on the fragmentation of grains in a zirconium alloy. The results demonstrate that grains of Zircaloy-4 fragment more as the temperature rises. This trend can be explained by an increasing difference between the CRSS values for 〈c+a〉 slip and 〈a〉 slip as temperature rises. This change in relative slip activities with temperature is supported by experimental observations of macroscopic anisotropy and in-grain misorientation axes calculated from EBSD data, as well as plasticity modeling. By tracking the microstructural evolution during deformation, it is shown that the two major texture components fragment to different degrees under the action of prismatic slip. Grains in the

Introducing Heterogeneity into Brittle Fracture Modeling of a 22NiMoCr37 Ferritic Steel Ring Forging

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Effect of strain paths and residual delta ferrite on the failure of cold rolled austenitic stainless steels, type 304L

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The Effect of Lattice Misfit on Deformation Mechanisms at High Temperature

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