Paul Mummery

Full-field strain mapping by optical correlation of micrographs acquired during deformation

Optical correlation is an emerging strain‐mapping technique that allows full‐field surface strain mapping by comparing the images of the same region before, during and after deformation. The fundamental aspects of optical correlation are presented, with emphasis on the applicability of the technique to the analysis of micrographs obtained during in situ deformation studies. Without considering specific algorithms, this paper discusses important practical issues such as accuracy and spatial resolution and how these are affected by image quality and other experimental difficulties. The technique was used to analyse image sequences obtained during in situ deformation tensile tests on two very different materials: antler bone and ferritic steel. As the technique does not require patterns or coatings to be applied on the surface of interest, the strain maps obtained could be used to relate strain heterogeneity to the underlying microstructure.

Acoustic emission from particulate-reinforced metal matrix composites

Abstract A systematic study of the effect of microstructural parameters on the fracture behaviour of silicon carbide particle reinforced aluminium matrix composites has been carried out. Acoustic emissions have been monitored during tensile testing, giving the size and number of emmissions as a function of strain. This has been shown to be simply related to the rate of void nucleation at the reinforcing phase. Both particle fracture and particle/matrix decohesion mechanisms can be detected. Void nucleation was observed from the onset of plastic deformation and a linear relationship between damage initiation rate and strain was found. The rate of emission increased with reiforcing particle size and volume fraction but was independent of matrix alloy composition and heat treatment. These results show that the failure strain of particulate metal matrix composites is not controlled solely by the onset of void nucleation at the reinforcing phase. Local failure processes in the matrix are shown to promote void coalescence and dominate the ductility. However, suppression of void nucleation at the particles increases the ductility. It is suggested that a critical number of fractured particles is required before failure.

The influence of microstructure on the fracture behaviour of particulate metal matrix composites

Abstract Fracture processes have been studied in a range of model particle-reinforced metal matrix composite systems: commercially pure aluminium matrices reinforced with silicon carbide particles of two volume fractions and three particle sizes. The fracture micromechanisms have been identified by a combination of fractography and in situ experiments. A transition in fracture mechanisms from decohesion at the particle-matrix interface to particle cracking has been observed on increasing both the particle size and the volume fraction. Fracture nucleation criteria have been developed to explain these phenomena as a function of the microstructural parameters.

The Measurement of Residual Stress in Railway Rails by Diffraction and other Methods

Residual stresses have been measured in a new roller-straightened railway rail and a worn ex-service rail. Synchrotron {211} lattice strain measurements at ID11 (ESRF) were used to map in-plane components of the stress tensor acting in cross-sectional rail slices. Stress maps made using laboratory X-rays and the magnetic measurement system MAPS, although coarser in detail, show similar trends. The validity of the measured data was examined using a stress balance requirement. Whilst generally true (to ±15 MPa), stress balancing was worst (±50 MPa) in regions with significant plastic deformation, suggesting that the measured {211} lattice strain had become uncharacteristic of the bulk elastic strain. Attributable to plastic anisotropy, this is a well-established issue with diffraction-based stress determination. To complement the in-plane stress measurements, the contour method was used to map the longitudinal stress component in a similar new rail sample, this component being relieved in the slices. On the...

Nanoindentation of histological specimens: Mapping the elastic properties of soft tissues

Although alterations in the gross mechanical properties of dynamic and compliant tissues have a major impact on human health and morbidity, there are no well-established techniques to characterize the micromechanical properties of tissues such as blood vessels and lungs. We have used nanoindentation to spatially map the micromechanical properties of 5-mum-thick sections of ferret aorta and vena cava and to relate these mechanical properties to the histological distribution of fluorescent elastic fibers. To decouple the effect of the glass substrate on our analysis of the nanoindentation data, we have used the extended Oliver and Pharr method. The elastic modulus of the aorta decreased progressively from 35 MPa in the adventitial (outermost) layer to 8 MPa at the intimal (innermost) layer. In contrast, the vena cava was relatively stiff, with an elastic modulus >30 MPa in both the extracellular matrix-rich adventitial and intimal regions of the vessel. The central, highly cellularized, medial layer of the vena cava, however, had an invariant elastic modulus of ~20 MPa. In extracellular matrix-rich regions of the tissue, the elastic modulus, as determined by nanoindentation, was inversely correlated with elastic fiber density. Thus, we show it is possible to distinguish and spatially resolve differences in the micromechanical properties of large arteries and veins, which are related to the tissue microstructure.

X-ray tomography observation of crack propagation in nuclear graphite

Abstract X-ray microtomography has been used to investigate the mechanisms responsible for rising crack growth resistance with crack propagation (R curve behaviour) in polygranular nuclear graphite. Tomography can be used to observe changes in the crack shape with propagation, and a side grooved specimen has been developed to produce the planar straight fronted crack necessary for fracture toughness measurement. Crack bridging from frictional contact between the fracture surfaces is observed. A zone of reduced X-ray attenuation, attributed to microstructural damage, is also observed around the crack tip and in its wake. These are the first in situ observations of the mechanisms of the R curve behaviour in nuclear graphites.

Elastic strains in antler trabecular bone determined by synchrotron X-ray diffraction.

The microstructure and associated mechanical properties of antler trabecular bone have been studied using a variety of techniques. The local trabeculae properties, as well as the three-dimensional architecture were characterized using nanoindentation and X-ray microtomography, respectively. An elastic modulus of 10.9+/-1.1 GPa is reported for dry bone, compared with 5.4+/-0.9 GPa for fully hydrated bone. Trabeculae thickness and separation were found to be comparable to those of bovine trabecular bone. Uniaxial compression conducted in situ during X-ray microtomography showed that antler can undergo significant architectural rearrangement, dominated by trabeculae bending and buckling, due to its low mineral content. High-energy synchrotron X-ray diffraction was used to measure elastic strains in the apatite crystals of the trabeculae, also under in situ uniaxial compression. During elastic loading, strain was found to be accommodated largely by trabeculae aligned parallel to the loading direction. Prior to the macroscopic yield point, internal strains increased as trabeculae deformed by bending, and load was also found to be redistributed to trabeculae aligned non-parallel to the loading direction. Significant bending of trabecular walls resulted in tensile strains developing in trabeculae aligned along the loading direction.

Characterization of the three-dimensional structure of a metallic foam during compressive deformation

X‐ray microtomography has been employed to collect three‐dimensional images of aluminium closed‐cell foam, enabling the internal structure to be characterized in three dimensions. An experimental technique and image analysis approach has been developed, and is described, in terms of the labelling of cells and the extraction of quantitative data such as the cell volume and cell compression. An in situ compressive deformation experiment has been performed on a single sample in order to illustrate the approach. The effect of the three‐dimensional cellular structure on the mechanisms of deformation suggests not only the position of large cell volumes to be very important in the local concentration of stress, but also the distribution of cell volumes of immediate neighbours.

Biomechanics of Dromaeosaurid Dinosaur Claws: Application of X-Ray Microtomography, Nanoindentation, and Finite Element Analysis

Dromaeosaurid theropod dinosaurs, such as Velociraptor, possess strongly recurved, hypertrophied and hyperextensible ungual claws on the pes (digit II) and manus. The morphology of these unguals has been linked to the capture and despatching of prey. However, the mechanical properties or, more importantly, the mechanical potential of these structures have not been explored. Generation of a 3D finite element (FE) stress/strain contour map of a Velociraptor manual ungual has allowed us to evaluate quantitatively the mechanical behavior of a dromaeosaurid claw for the first time. An X‐ray microtomography scan allowed construction of an accurate 3D FE mesh. Analogue material from an extant avian theropod, the pedal digit and claw of an eagle owl (Bubo bubo), was analyzed to provide input data for the Velociraptor claw FE model (FEM). The resultant FEM confirms that dromaeosaurid claws were well‐adapted for climbing as they would have been resistant to forces acting in a single (longitudinal) plane, in this case due to gravity. However, the strength of the unguals was limited with respect to forces acting tangential to the long‐axis of the claw. The tip of the claw functioned as the puncturing and gripping element of the structure, whereas the expanded proximal portion transferred the load stress through the trabeculae and cortical bone. Enhanced climbing abilities of dromaeosaurid dinosaurs supports a scansorial phase in the evolution of flight. Anat Rec, 292:1397–1405, 2009.

X-Ray Microtomographic Studies Of Metal-Matrix Composites Using Laboratory X-Ray Sources

X‐ray microtomography (XMT) is a non‐destructive technique that allows the internal structure of a material to be imaged by the spatial distribution of its linear X‐ray absorption coefficients. This paper demonstrates the use of XMT to investigate: (1) the distribution of TiB2 reinforcement in composites formed by powder processing; (2) the local void volume fraction as a function of position in highly deformed regions of failed tensile specimens of SiC‐reinforced material allowing a valid damage parameter to be defined at high strains; (3) absorption coefficients measured at different energies simultaneously using a multichannel analyser which can sometimes be used to separate linear absorption changes due to (a) density variations and (b) compositional variations in individual voxels; and (4) the use of sequential sections to provide a three‐dimensional representation of the failed specimens.