Philip J. Withers

Residual stress. Part 1 – Measurement techniques

Abstract Residual stress is that which remains in a body that is stationary and at equilibrium with its surroundings. It can be very detrimental to the performance of a material or the life of a component. Alternatively, beneficial residual stresses can be introduced deliberately. Residual stresses are more difficult to predict than the in-service stresses on which they superimpose. For this reason, it is important to have reliable methods for the measurement of these stresses and to understand the level of information they can provide. In this paper, which is the first part of a two part overview, the effect of residual stresses on fatigue lifetimes and structural integrity are first summarised, followed by the definition and measurement of residual stresses. Different types of stress are characterised according to the characteristic length scale over which they self-equilibrate. By comparing this length to the gauge volume of each technique, the capability of a range of techniques is assessed. In the second part of the overview, the different nature and origins of residual stress for various classes of material are examined.

Friction stir welding of aluminium alloys

Abstract The comprehensive body of knowledge that has built up with respect to the friction stir welding (FSW) of aluminium alloys since the technique was invented in 1991 is reviewed. The basic principles of FSW are described, including thermal history and metal flow, before discussing how process parameters affect the weld microstructure and the likelihood of entraining defects. After introducing the characteristic macroscopic features, the microstructural development and related distribution of hardness are reviewed in some detail for the two classes of wrought aluminium alloy (non-heat-treatable and heat-treatable). Finally, the range of mechanical properties that can be achieved is discussed, including consideration of residual stress, fracture, fatigue and corrosion. It is demonstrated that FSW of aluminium is becoming an increasingly mature technology with numerous commercial applications. In spite of this, much remains to be learned about the process and opportunities for further research and development are identified.

Microstructure, mechanical properties and residual stresses as a function of welding speed in aluminium AA5083 friction stir welds

Friction stir welding (FSW), like other friction welding techniques, has the advantage that many of the welding parameters, e.g. tool design, rotation speed and translation speed, can be controlled in a precise manner, thus controlling the energy input into the system. However, the effect of different welding speeds on the weld properties remains an area of uncertainty. In this paper, we report the results of microstructural, mechanical property and residual stress investigations of four aluminium AA5083 friction stir welds produced under varying conditions. It was found that the weld properties were dominated by the thermal input rather than the mechanical deformation by the tool.

Residual stress. Part 2 – Nature and origins

Abstract Residual stress is that which remains in a body that is stationary and at equilibrium with its surroundings. It can be detrimental when it reduces the tolerance of the material to an externally applied force, as is the case with welded joints. On the other hand, it can be exploited to design materials or components which are resistant to damage, toughened glass being a good example. This paper, the second part of a two part overview, the first part having been devoted to measurement techniques, examines the nature and origins of residual stresses across a range of scales. This extends from the long range residual stress fields in engineering components and welded structures, through the interphase stresses present in composites and coatings, to the microscale interactions of phase transformations with local stresses.

The application of the eshelby method of internal stress determination to short fibre metal matrix composites

Abstract Eshelbys equivalent inclusion approach is used to provide a rigorous theoretical basis for the prediction of the mechanical properties of short fibre composites. The equivalent inclusion construction which is central to this method is described in detail. The elastic, thermoelastic and plastic behaviour of short fibre metal matrix composites is predicted, and, taking the Al/SiC system as an example, compared with experiment. Finally, it is shown that relaxation phenomena play an important role in the development of internal stresses, and that the energetics and the resultant stress redistribution between the two phases can be understood within the framework of the Eshelby model.

Residual stress and its role in failure

Our safety, comfort and peace of mind are heavily dependent upon our capability to prevent, predict or postpone the failure of components and structures on the basis of sound physical principles. While the external loadings acting on a material or component are clearly important, There are other contributory factors including unfavourable materials microstructure, pre-existing defects and residual stresses. Residual stresses can add to, or subtract from, the applied stresses and so when unexpected failure occurs it is often because residual stresses have combined critically with the applied stresses, or because together with the presence of undetected defects they have dangerously lowered the applied stress at which failure will occur. Consequently it is important that the origins of residual stress are understood, opportunities for removing harmful or introducing beneficial residual stresses recognized, their evolution in-service predicted, their influence on failure processes understood and safe structural integrity assessments made, so as to either remove the part prior to failure, or to take corrective action to extend life. This paper reviews the progress in these aspects in the light of the basic failure mechanisms.

X-ray nanotomography

Almost every area of science has been revolutionized by our ability to collect two-dimensional images of increasingly fine detail, ranging from radio signals of far-off solar systems, to high-resolution electron microscopy images. With the advent of digital image capture, tomography – the art of reconstructing a sliceable virtual three-dimensional copy of the object from two-dimensional images – is becoming increasingly popular across a range of imaging modalities and length scales. One area attracting a lot of attention is the area of submicron X-ray tomography, popularly dubbed X-ray nanotomography.

Methods for obtaining the strain-free lattice parameter when using diffraction to determine residual stress

The determination of residual stress by diffraction depends on the correct measurement of the strain-free lattice spacing d(hkl)(0), or alternatively the enforcement of some assumption about the state of strain or stress within the body. It often represents the largest uncertainty in residual stress measurements since there are many ways in which the strain-free lattice spacing can vary in ways that are unrelated to stress. Since reducing this uncertainty is critical to improving the reliability of stress measurements, this aspect needs to be addressed, but it is often inadequately considered by experimenters. Many different practical strategies for the determining of d(hkl)(0) or d(ref) have been developed, some well known, others less so. These are brought together here and are critically reviewed. In practice, the best method will vary depending on the particular application under consideration. Consequently, situations for which each method are appropriate are identified with reference to practical examples.

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.

Welding residual stresses in ferritic power plant steels

Abstract Many of the degradation mechanisms relevant to power plant components can be exacerbated by stresses that reside within the material. Good design or structural integrity assessments require therefore, an accounting of residual stresses, which often are introduced during welding. To do this it is necessary to characterise the stresses, but this may not be possible in thick components using non-destructive methods. These difficulties, and a paucity of relevant engineering data, have led to an increasing emphasis on the development and validation of suitable modelling tools. Advances are prominent in the estimation of welding residual stresses in austenitic stainless steels. The progress has been less convincing in the case of ferritic alloys, largely due to the complexities associated with the solid state phase transformations that occur in multipass welding. We review here the metallurgical issues that arise in ferritic steel welds, relate these to the difficulties in calculating residual stresses, and highlight some stimulating areas for future research.