Papers by Author: Axel Steuwer

Abstract: The spatial resolution of time of flight neutron transmission diffraction was recently improved by the extension of photon/electron counting technology to imaging of thermal and cold neutrons. The development of novel neutron sensitive microchannel plates enables neutron counting with spatial resolution of ~55 um and time-of-flight accuracy of ~1 us, with efficiency as high as 70% for cold and ~40% for thermal neutrons. The combination of such a high resolution detector with a pulsed collimated neuron beam provides the opportunity to obtain a 2-dimensional map of neutron transmission spectra in one measurement. The results of our neuron transmission measurements demonstrate that maps of strains integrated along the beam propagation direction can be obtained with ~100 microstrain accuracy and spatial resolution of ~100 um providing there are sufficient neutron events collected. In this paper we describe the capabilities of the MCP neutron counting detectors and present the experimental results of 2-dimensional strain maps within austenitic steel compact tension (CT) crack samples measured at the ENGIN-X beamline of the ISIS pulsed neutron source.

Abstract: This paper presents microstructural, hardness and residual strain information for solid-state welds in creep-resistant Cr-Mo steel, made using the new local damage repair technique offered by friction taper stud welding (FTSW). The technique is suitable for making single welds to repair, for example, localised creep damage but can also be extended to deal with planar defects through the use of overlapping welds. Neutron diffraction was used to measure residual strains at a number of positions along a series of 5 overlapping FTS welds.

Abstract: Residual strain profiles were measured by synchrotron X-ray radiation in Al2O3/Y-stabilized ZrO2 (YSZ) ceramic laminates. Different stacking sequences were employed, including alternating layers containing 5 and 40 vol.% YSZ. Residual strains were found to be fairly constant within each layer; although they change at the interface between layers with different compositions. Different behaviour is observed for the strains along the in-plane and normal directions.

Abstract: There have been many theoretical studies to predict the stress fields around the tip of a growing fatigue crack. However, until recently the highly-localized, small scale nature of the stresses has meant that direct measurement has not been possible. With the current generation of synchrotron X-ray sources, sub-millimetre sampling dimensions are now possible, and it has become possible to evaluate directly the stresses at the tip of a fatigue crack and to see how the stresses evolve as the result of an overload, for example. In this paper we present results of synchrotron X-ray diffraction analysis of the stress fields around a fatigue crack in aluminium alloy 5091 (Al-Mg-Li-C-O); this is a dispersion-strengthened alloy with a fine grain size, which makes it ideal for such experiments. Compact tension (CT) specimens were prepared with constant amplitude fatigue loading. The energy dispersive X-ray diffraction (EDXRD) technique was used for measuring strains around the crack tip along the mid thickness of the specimen under in-situ loading. The measurement was carried out at the ESRF (European Synchrotron Radiation Facility), Grenoble, France on the ID15A beam line. The experimental crack tip stresses have been compared with the analytical fracture mechanics solution.

Abstract: We have used synchrotron x-ray diffraction to study the crystal orientation in human dental enamel as a function of position within intact tooth sections. Keeping tooth sections intact has allowed us to construct 2D and 3D spatial distribution maps of the magnitude and orientation of texture in dental enamel. We have found that the enamel crystallites are most highly aligned at the expected occlusal points for a maxillary first premolar, and that the texture direction varies spatially in a three dimensional curling arrangement. Our results provide a model for texture in enamel which can aid researchers in developing dental composite materials for fillings and crowns with optimal characteristics for longevity, and will guide clinicians to the best method for drilling into enamel, in order to minimize weakening of remaining tooth structure, during dental restoration procedures.

Abstract: This paper presents some ideas on incorporating output from advanced synchrotron and neutron scanning strain methods in improved assessment of the influence of weld process parameters on residual strains at welds and on their fatigue performance. It very briefly outlines two different cases involving synchrotron diffraction strain scanning of friction stir welds in a strain hardened aluminium alloy and neutron diffraction strain scanning of MIG welds in high strength steel.

Abstract: In this paper we discuss certain aspects of residual stress measurements using energy-dispersive synchrotron X-ray diffraction using very high X-ray energies in the range up to 200keV. In particular, we focus on the strain resolution and its relation to the geometric contribution to the instrumental resolution. This energy range together with the brilliance of insertion devices allows measurements in bulk materials with penetration approaching those of neutrons, and the technique is demonstrated to have a high potential for residual stress determination. However, the use of high X-ray energies implies a relatively small diffraction angle and in turn a relatively elongated gauge volume, which favours the application of the technique to essentially 2D problems.

Abstract: FaME38 is a new facility at the ILL/ESRF in Grenoble with the aim of improving the accessibility and effectiveness of neutron and synchrotron strain measurements. In addition to providing basic materials engineering facilities, it enables users from both commercial and academic backgrounds to prepare and to evaluate experiments on-site. The real success and impact of a strain scanning experiment depends on the quality of the collected data and its practical use. FaME38 provides a knowledge base and tools which can increase the efficiency of the measurement. These tools include a VAMAS standard sample base-plate, a materials support laboratory and enhanced visualisation software. The VAMAS base-plate allows pre-configuration of the sample position and set-up, as well as so-called “hot-swapping” of samples with minimum time required for re-configuration of the instrument. The visualisation tools feature web-based simulation and, in particular, 3D visualisation of both the experimental environment as well as the data. The use of the support facility can significantly reduce the set-up time, thus increasing the time available for measurement. The visualisation naturally enhances the understanding of the data and ties in with existing engineering code such as CAD/FEA software. We demonstrate how the use of additional technology can improve the effectiveness and impact of strain mapping experiments at neutron and synchrotron facilities by disseminating the workflow of a typical experiment undertaken in the FaME38 framework. This approach is aimed at paving the way toward technology-oriented application of synchrotron and neutron strain scanning.