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Abstract: The propagation rate of a fatigue crack in a nodular cast iron, loaded in cyclic tension, has been studied in situ by X-ray computed tomography and digital volume correlation. The semi-elliptical crack initiated from an asymmetric corner notch and evolved to a semi-circular shape, initially with a higher growth rate towards one edge of the notch before the propagation rate along the crack front became essentially independent of po-sition. The phase congruency of the displacement field was used to measure the crack shape. The three-dimensional stress intensity factors were calculated via a linear elastic finite element model that used the displacement fields around the crack front as the boundary conditions. Closure of the crack tip region was observed. The cyclic change in the local mode I opening of the crack tip determined the local fatigue crack propaga-tion rate along the crack front.

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Abstract: Directed energy deposition (DED) is a promising additive manufacturing technique for repair; however, DED is prone to surface waviness (humping) in thin-walled sections, which increases residual stresses and crack susceptibility, and lowers fatigue performance. Currently, the crack formation mechanism in DED is not well understood due to a lack of operando monitoring methods with high spatiotemporal resolution. Here, we use inline coherent imaging (ICI) to optically monitor surface topology and detect cracking in situ, coupled with synchrotron X-ray imaging for observing sub-surface crack healing and growth. For the first time, ICI was aligned off-axis (24° relative to laser), enabling integration into a DED machine with no alterations to the laser delivery optics. We achieved accurate registration laterally (<10 µm) and in depth (<3 µm) between ICI measurements and the laser beam position using a single-element MEMS scanner and a custom calibration plate. ICI surface topology is verified with corresponding radiographs (correlation >0.93), directly tracking surface roughness and waviness. We intentionally seed humping into thin-wall builds of nickel super-alloy CM247LC, locally inducing cracking in surface valleys. Crack openings as small as 7 µm were observed in situ using ICI, including sub-surface signal. By quantifying both humping and cracking, we demonstrate that ICI is a viable tool for in situ crack detection.

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Abstract: The crystallographic texture development during processing of dual-phase Ti alloys like Ti-6Al-4 V is of fundamental technological importance. However, measuring texture in both phases in these materials is a significant challenge because of the spatial inhomogeneity of the texture and low volume fraction of the minority β-phase at room temperature. Here we demonstrate how synchrotron X-ray diffraction can be used to overcome these difficulties and measure texture and texture variation in hot-rolled samples in a reproducible manner. The texture in hot-rolled Ti-64 was calculated from 2D synchrotron diffraction patterns obtained along different directions. The data was analysed using MAUD, which is based on Rietveld refinement of the diffracted intensities, and using a Fourier series based analysis method, that extracts intensities directly from the 2D diffraction patterns, and then uses the open-source software MTEX to fit an orientation distribution function (ODF). By comparing the results with faithful EBSD measurements, we show that the Fourier series method produces much more accurate texture measurements, especially for the minority β-phase. We also show that a minimum of 2, and preferably 3, different measurement orientations are needed to fully represent the texture. This implies that measurements of texture which rely on diffraction data from a single sample orientation, like in fast in-situ studies or spatially resolved measurements, can only provide qualitative information and must be interpreted with care.

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Abstract: Superelastic alloys based on Ti-Nb have potential in the aerospace sector for vibration damping applications, due to their wide mechanical hysteresis and tuneable properties. However, their uptake is currently limited by functional fatigue, whereby a degradation in properties is seen on cyclic loading. To understand the mechanisms that underpin this change in behaviour, a combination of ex situ and in situ testing was used to characterise the evolution of the mechanical response in a commercial Ti-Nb based alloy. It was found that the behaviour of these materials changes via a two-step mechanism, driven by the accumulation of transformation related defects and their associated stress fields. This understanding rationalises many discrepancies within the literature and highlights how the overall shape of the load response of these alloys is dominated by changes occurring only in specific regions of the material.

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Abstract: NASA, ESA and the UK are collaborating on a Mars Sample Return (MSR) mission which aims to retrieve drill cores of Martian rock for terrestrial analysis, starting with the Mars2020 rover which landed successfully in Jezero Crater in Feb. 2021. Up to 30 samples, inside sealed titanium sample tubes, are planned to be returned to Earth in later missions. Due to the potential for back-contamination of Earth from possible extant life on Mars, strict contamination control measures must be taken for the purposes of planetary protection, as well as to prevent contamination of the samples by Earth’s environment. These measures place restrictions on the way measurements can be performed on the samples until they have been sterilised or judged safe. As the first step of scientific analysis, all samples will undergo a set of measurements called Pre-Basic Characterisation. Pre-BC will include weighing, X-ray CT, and magnetic measurements. These data along with Basic Characterisation data will be used to decide experimental plans for multi instrument analyses on the Mars samples. X-ray Diffraction (XRD) is currently planned for a later stage of sample analysis after the sample tubes have been opened due to limitations with conventional commercial X-ray diffractometers. [1, 2] While a conventional X-ray tube cannot provide an appropriate X-ray beam, a synchrotron source is capable of much higher intensities and precise wavelength selectivity. Synchrotron facilities also allow more suitable diffraction geometries for the size and shape of sample expected from MSR. We have carried out experiments with the help of Diamond Light Source’s I12-JEEP beamline to test the feasibility of XRD analysis of samples in sealed Mars2020 sample tubes and suitable instrument parameters for XRD of these samples. Titanium tubes were prepared as analogues to Mars2020 sample tubes. Three different geological analogues were used in place of the Mars samples: an Icelandic basaltic sand, a calcareous mudstone from Watchet Bay, UK, and a Devonian Fine Grained Sandstone, UK. Two different methods for preventing unwanted diffraction signal from the sample tube walls have also been tested: subtracting the diffraction spectrum of an empty tube from the tube-with-sample spectrum, and using energy-dispersive X-ray diffraction to exclude tube wall signal. We show that quantitative XRD phase analysis can be successfully carried out on returned Mars samples in unopened sample tubes using a synchrotron X-ray source, and thus could be included in the Pre-BC phase of returned sample science. This would provide mineralogical data much earlier in the sample science process, improving decision-making around sample science, curation, and handling.