Open Access

Show Abstract ▼

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.

Show Abstract ▼

Abstract: An understanding of the petrogenesis of magmatic systems can be achieved by studying their constituent parts (e.g. crystals) and their chemical trends. Magmas are often travelling from source to emplacement levels together with a growing population of crystals. As they grow and respond to their environment through time, crystals can record the processes of contamination, magma mingling, crystal exchange and magma recharge occurring en route. Detailed quantification of the micro-scale textural parameters of igneous rocks such as crystal shape, size and spatial pattern provides important geometrical information about the crystal population, which can be linked with macroscale magmatic processes. The complex zoning often observed in crystals can also be quantified and linked back to the textures to show how magma chemistry changed during crystal growth. Finally, it is described how these techniques are developing toward 3D chemical mapping of igneous rocks as well as to record 4D experiments.

Open Access

Show Abstract ▼

Abstract: X-ray computed tomography and serial block face scanning electron microscopy imaging techniques were used to produce 3D images with a resolution spanning three orders of magnitude from ∼7.7 μm to 7 nm for one typical Bowland Shale sample from Northern England, identified as the largest potential shale gas reservoir in the UK. These images were used to quantitatively assess the size, geometry and connectivity of pores and organic matter. The data revealed four types of porosity: intra-organic pores, organic interface pores, intra- and inter-mineral pores. Pore sizes are bimodal, with peaks at 0.2 μm and 0.04 μm corresponding to pores located at organic–mineral interfaces and within organic matter, respectively. These pore-size distributions were validated by nitrogen adsorption data. The multi-scale imaging of the four pore types shows that there is no connected visible porosity at these scales with equivalent diameter of 20 nm or larger in this sample. However, organic matter and clay minerals are connected and so the meso porosity (<20 nm) within these phases provides possible diffusion transport pathways for gas. This work confirms multi-scale 3D imaging as a powerful quantification method for shale reservoir characterisation allowing the representative volumes of pores, organic and mineral phases to be defined to model shale systems. The absence of connected porosity at scales greater than 20 nm indicates the likely importance of the organic matter network, and associated smaller-scale pores, in controlling hydrocarbon transport. . The application of these techniques to shale gas plays more widely should lead to a greater understanding of properties in the low permeability systems.

Show Abstract ▼

Abstract: Geology is inherently dynamic, and full understanding of any geological system can only be achieved by considering the processes by which change occurs. Analytical limitations mean understanding has largely developed from ex situ analyses of the products of geological change, rather than of the processes themselves. Most methods essentially utilise “snap shot” sampling: and from thin section petrography to high resolution crystal chemical stratigraphy and field volcanology, we capture an incomplete view of a spatially and temporally variable system. Even with detailed experimental work, we can usually only analyse samples before and after we perform an experiment, as routine analysis methods are destructive. Serial sectioning and quenched experiments stopped at different stages can give some insight into the third and fourth dimension, but the true scaling of the processes from the laboratory to the 4D (3D + time) geosphere is still poorly understood. Micro computed tomography (XMT) can visualise the internal structures and spatial associations within geological samples non-destructively. With image resolutions of between 200 microns and 50 nanometres, tomography has the ability to provide a detailed sample assessment in 3D, and quantification of mineral associations, porosity, grain orientations, fracture alignments and many other features. This allows better understanding of the role of the complex geometries and associations within the samples, but the challenge of capturing the processes that generate and modify these structures remains. To capture processes, recent work has focused on developing experimental capability for in situ experiments on geological materials. Data presented will showcase examples from recent experiments where high speed synchrotron x-ray tomography has been used to acquire each 3D image in under 2 seconds. We present a suite of studies that showcase how it is now possible to take quantification of many geological processed into 3D and 4D. This will include tracking the interactions between bubbles and crystals in a deforming magma, the dissolution of individual mineral grains from low grade ores, and quantification of three phase flow in sediments and soils. Our aim is to demonstrate how XMT can provide new insight into dynamic processes in all geoscience disciplines, and give you some insight into where 4D geoscience could take us next.