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Abstract: The damage mechanisms and load redistribution taking place under high temperature (350°C), high cycle fatigue (HCF) of TC17 titanium alloy/unidirectional SiC fibre composites have been investigated in situ using synchrotron X-ray computed tomography (CT) and X-ray diffraction (XRD) under two stress amplitudes. The three-dimensional morphology of the fatigue crack and fibre fractures has been mapped by CT. At low stress amplitude, stable growth occurs with matrix cracking deflecting by 50-100 µm in height as it bypasses the bridging fibres. At higher stress amplitude, loading to the peak stress led to a burst of fibre fractures giving rise to rapid crack growth. Many of the fibre fractures occurred 50-300 µm above/below the matrix crack plane during rapid growth, contrary to that in the stable growth stage, leading to extensive fibre pull-out on the fracture surface. The changes in fibre loading, interfacial stress, and the extent of fibre-matrix debonding in the vicinity of the crack have been mapped over the fatigue cycle and after the rapid growth by XRD. The fibre/matrix interfacial sliding extends up to 600 µm (in the stable-growth zone) or 700 µm (in the rapid-growth zone) either side of the crack plane. The direction of interfacial shear stress reverses over the loading cycle, with the maximum frictional sliding stress reaching ∼55 MPa in both regimes. In accordance with previous studies, it is possible that a degradation in fibre strength at elevated temperature is responsible for bursts of fibre fracture and rapid crack growth under higher stress amplitude.

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Abstract: Environment Induced Cracking (EIC) has been observed in three dimensions (3D) at high spatial and temporal resolution for AA5083 H131 alloy using in situ synchrotron X-ray CT. The initiation of EIC cracks occurred in humid air at ∼0.5σy and started from corrosion sites associated with intermetallic particle clusters. Time-resolved monitoring of the crack growth confirmed cracks grow below threshold KIEIC and are susceptible to arrest under fixed displacement conditions. Even when the cracks exceed threshold KIEIC and enter sustained propagation they exhibit very uneven growth rates. Failure occurs when the K of the largest crack exceeds the fracture toughness KIC.

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Abstract: The fatigue overload behaviours of coarse grain (~30 μm), ultrafine grain (360 nm) and nanocrystalline (~30 nm) Ni are compared under constant amplitude loading at R = 0.1 after a 100% overload. Synchrotron X-ray diffraction is applied to map the elastic crack-tip strain fields at the mid-thickness of a compact tension specimen of nanocrystalline Ni at various stages through the loading cycles, from which the variation in stress before, during and after overload is estimated. Digital image correlation is used to measure the crack length and the displacement fields at the specimen surfaces for both grain sizes, from which the fatigue crack growth (FCG) rate, crack opening displacement and stress intensity factor range are determined. The FCG for coarse grain Ni is most significantly retarded whereas the nanocrystalline Ni is least affected by the overload due to an increased yield stress and a more planar crack surface morphology. As a result, FCG retardation by plasticity, surface roughness and residual stress-induced crack closure are reduced.

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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.

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Abstract: Thick corrosion scales form within carbon steel oilfield pipelines in sweet (CO2 saturated) environments. The morphology and the extent of the resultant pseudo-protective nature of these scales has been seen to be dependent on multiple factors including solution pH, temperature, flow rate, and partial pressure of CO2 present. Different techniques (SEM, XRD, FIB, etc.) have been used in the past to characterise these corrosion scales. However, limitations in these techniques occur due to the fact that only small regions of the scale can be characterised in a feasible time. Further limitations may result in difficulty to relate local scale features with corrosion morphology on the evolving metal surface (i.e. localised corrosion) as the morphology of the substrate surface can only be characterised once the scale is removed. The aim of this work is to address this issue through the use of high resolution x-ray absorption tomography to characterise the internal morphology of the corrosion scales from ex situ specimens exposed to CO2 environments as a function of pH and temperature. X-65 pipeline steel, high purity 99.99 Fe and low purity 99.0 Fe pins were used in these experiments. Specimens were exposed to CO2saturated solutions using two different methods, notably open circuit potential (OCP) immersion for 7-12 days and electrochemical polarisation for two hours at 200 mV(vs OCP). After the scaling experiments, samples were characterised using XRD and SEM. X-ray tomography was subsequently performed at the University of Manchester X-ray Imaging Facilities to characterise the corrosion morphology. Transmission X-ray microscopy (TXM) was used in absorption mode and phase contrast mode to obtain three dimensional reconstructions of the specimens, where the different features of the scale and the internal corroded substrate were imaged and characterised simultaneously. Results indicate differences in the morphology of the corrosion scale depending on the method that was used to corrode the samples. As a consequence of the supersaturation in Fe2+, polarised samples present a thicker scale than the scales obtained by immersion at OCP conditions. The polarisation process also produces a less uniform corroded substrate than when the sample is immersed for 7 days at open circuit in the corroding environments used. In this work high resolution X-ray tomography has been proven to be a very powerful technique to study the scale morphology as well as the features of localised corrosion occurring under scale in the substrate.