G. Mills

Neutron diffraction measurements of residual stress and plastic deformation in an aluminium alloy weld

Abstract The neutron diffraction technique for measuring residual stresses non-destructively in an aluminium alloy weld has been examined. Calibration experiments have been performed to determine the influence of prior plastic strain and surface effects. Comparisons have also been made with strain gauge methods using the destructive block removal and layering technique for measuring the residual stresses. It has been shown that the neutron diffraction method gives an accurate indication of the three dimensional stress distribution present in a weld and that it can also be employed to estimate the amount of prior plastic strain encountered.

Residual stresses in alumino-thermic welded rails

Abstract Neutron strain scanning has been used to map the residual stress field that is generated in a railway rail by a standard gap alumino-thermic weld made using routinely specified procedures. The longitudinal and vertical residual stress fields in the sections well away from the weld are characteristic of many unwelded rails, being generally tensile in the head and foot with balancing compression in the web. In the vicinity of the weld the residual stress patterns are very different. At the top and bottom surfaces of the rail the longitudinal residual stress field is strongly compressive, which is generally beneficial in that it would tend to inhibit the initiation and propagation of fatigue cracks from surface defects. Just at the surface the vertical residual stress attenuates to zero but internally, in the web region, both longitudinal and vertical components of the residual stress field are strongly tensile, which increases the susceptibility of that region to crack initiation and propagation from internal material defects. This pattern is consistent with practical operating experience, which is that most of the small proportion of alumino-thermic welds that do fail do so as a result of porosity or inclusions in the weld. It is found that the ‘boundaries’ of the ‘weld type’ residual stress fields do not coincide with the boundary of the weld, nor of the heat-affected zone, but correlate reasonably well with the positions of the extremities of the mould assembly and with the location of the steepest longitudinal temperature gradients.

Residual stress changes in railway rails

Abstract Railway rail manufacturing involves repeated hot-rolling followed by cooling during which differential contraction and phase changes occur in the rail. Subsequent roller straightening, heat treatments and the passage of trains combine to generate a complex residual stress pattern. Results are presented of internal residual stress distributions measured in a new and a used rail using the neutron strain scanning technique. The results are compared with conventional destructive strain gauge measurements and with theoretical predictions and are discussed in terms of their effects on possible rail failure fatigue mechanisms.

Neutron strain scanning of a small welded austenitic stainless steel plate

Abstract Neutron strain scanning has been used to determine the residual stress distribution over the area of the mid-thickness plane of a small austenitic steel piate across which a weld had been laid. The longitudinal and transverse stresses in the centre of the plate and weld are strongly tensile. Stresses normal to the plane of the plate are low everywhere except near the ends of the weld where tensile ‘hot-spots’ are observed. Near the edges of the plate the effects of boundary and balancing conditions are evident. Longitudinal and transverse stresses tend to zero along the edges of the plate to which they are perpendicular but become strongly compressive near the middle regions of the edges to which they are parallel. The results are compared with published data from other welds that were made and constrained differently. The accuracy of the technique is discussed and the results obtained by measuring with different reflections are compared. It is shown that neutron strain scanning is an acceptable, and unique, method for determining non-destructively the residual stress distribution throughout small austenitic steel components with sufficient accuracy to provide data for design purposes and to validate model calculations.

Residual Stress Measurements in Rails by Neutron Diffraction

The relatively new technique of neutron strain scanning has several unique advantages over most traditional methods of measuring internal strains. It is non-destructive and, in principle, measurements can be made and repeated at any point in any direction within a sample. In practice its range of its application is limited in thick section samples, such as long lengths of rail, by neutron beam attenuation. In such cases it is often necessary to reach a compromise between what is ideally preferred by engineers for use in stress analysis calculations and what it is practical or economic to measure using the technique. Examples are given of how neutron strain scanning may be applied, in a cost effective manner, to the problem of the measurement of residual stresses in railway rails. Results are presented of representative longitudinal, transverse and vertical stresses measured down the centre-lines of rail sections and of stress contours measured in rail heads. The data are of sufficient quality and quantity to be used to validate theoretical calculations and reveal details of the residual stress distributions in rails not obtainable by other methods.

Residual stresses in a steel strand

Abstract Steel “strands”, which comprise several wires wound together, are used in many engineering applications where both tensile strength and some flexibility are required. Civil engineers use strands in “cable stayed” bridges which are used for moderate spans. Bridges are subjected in service to both static and time-varying loads, such as vibration and resonance from traffic, wind and water. Fatigue in the strands, which is influenced by residual stresses generated during manufacture and construction, is a potential problem. Results are presented of a neutron investigation of the residual stress field in a seven-wire steel strand generated by the manufacturing process, which involves multi-pass cold-drawing, annealing, coiling and winding. The results show a strong characteristic radial variation of axial, hoop and radial residual stress components. A discussion is also included of the treatment of instrumental aberration which could otherwise severely affect the quality and interpretation measurements close to a surface.

Problems with Railway Rails

Neutron diffraction is now being applied to measure macrostrain distributions inside components of engineering interest. The technique has several unique advantages but there are practical constraints which limit its widespread adoption. Compromises are often necessary to bridge the gap between what is required by engineers for use in stress analysis calculations and what it is practical to measure using the technique, especially when large sized components are involved.