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Instruments / Technical Area	Publication Details	Published
I12-JEEP: Joint Engineering, Environmental and Processing	The effect of anisotropic microstructure on the crack growth and fatigue overload behaviour of ultrafine-grained nickel W. Zhang , C. A. Simpson , T. Leitner , X. Zhang , R. Pippan , P. J. Withers Acta Materialia DOI: 10.1016/j.actamat.2019.11.024 Diamond Proposal Number(s): [16647] Show Abstract ▼ Abstract: Changes in crack growth rate associated with overload events during fatigue are poorly understood, especially for materials with anisotropic microstructures. Here overload fatigue tests are reported for compact tension samples cut in two different orientations from high pressure torsion disc samples. During growth the crack planes reoriented either slightly, or significantly, to align with the elongated grain structure leading to low, and high, levels of mixed mode fatigue loading respectively. In both cases the ultrafine grained microstructure led to macroscopically flat crack faces. The fatigue crack growth rate was around 3.4 times slower for the case with the high mode II component than for the low. A 100% overload was then introduced and synchrotron X-ray diffraction and digital image correlation (DIC) were applied in-situ to map the bulk crack-tip elastic strain field (plane strain) and surface displacement field (plane stress) respectively prior to, during and after overload. The high mode II case displayed a larger degree of retardation after overload. Residual stress and plasticity-induced crack closure were found to be the primary causes for the retardation as the crack grows into the overload plastic zone. Significant crack face contact was observed for the high mode II case along with significant levels of compressive stress transferred across the crack faces at minimum load. Compared with conventional (coarse) grain Ni, the ultrafine grained Ni is less retarded by overload, because of its relatively flatter crack path and higher yield stress and thus less plasticity and residual stress induced closure.	Nov 2019
I12-JEEP: Joint Engineering, Environmental and Processing	Validating 3D two-parameter fracture mechanics models for structural integrity assessments C. A. Simpson , S. Tonge , T. Connolley , C. Reinhard , T. J. Marrow , M. Mostafavi Theoretical And Applied Fracture Mechanics 102281 DOI: 10.1016/j.tafmec.2019.102281 Diamond Proposal Number(s): [12606] Show Abstract ▼ Abstract: In-situ fracture tests were carried out on the I12 beamline at the Diamond Light Source. Four Al-Ti metal-matrix composites (MMCs), with two crack lengths, were studied to assess for the impact of in-plane constraint. Synchrotron X-ray computed tomography and synchrotron X-ray diffraction were used to measure total strain and elastic strain respectively. In this work, the measured elastic strains in the samples are detailed as a function of applied load and compared against those predicted from a 3D elastic-plastic finite element model. The modelled strains increased asymptotically towards the tip of the electro discharge machined notch. The experimental results do not highlight the same response, which is due to a combination of blunting and low experimental spatial resolution. Far field experimental and measured strain fields converged, notably in the test piece containing a long notch ( a/W = 0.5) and higher levels of constraint.	Jun 2019
I12-JEEP: Joint Engineering, Environmental and Processing	Validating 3D two-parameter fracture mechanics for structural integrity assessments C. A. Simpson , S. Tonge , A. Cinar , C. Reinhard , T. J. Marrow , M. Mostafavi Procedia Structural Integrity 13, 965 - 970 DOI: 10.1016/j.prostr.2018.12.180 Diamond Proposal Number(s): [12606] Open Access Show Abstract ▼ Abstract: In-situ fracture tests were carried out on the I12 beamline at the Diamond Light Source. Four Al-Ti metal-matrix composites (MMCs), with varying combinations of thickness and crack length, were studied to assess for the impact of in-plane and out-of-plane constraint. Synchrotron X-ray computed tomography and synchrotron X-ray diffraction were used to measure total strain and elastic strain respectively. The total strain was calculated via digital volume correlation, with Ti particles within the MMC providing sufficient texture to track the internal displacement vectors in 3D. The total, elastic-plastic strain energy release rate, Jtotal was calculated from 2D slices extracted from the 3D displacement field, with Jtotal reaching a maximum value at the sample surface. It is, however, still unclear whether calculating Jtotal on a slice-by-slice basis provides an accurate representation of strain energy release rate across the crack front; techniques to evaluate the J-integral from the full 3D displacement field are being developed.	Dec 2018

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