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.

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Residual stress. Part 2 – Nature and origins

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

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Residual stress and its role in failure

A technique is described for the measurement of the residual stress tensor averaged over a specified volume within a component. The method involves measurement of small changes in lattice spacing using high resolution neutron diffraction. The stress is inferred from these measurements of the strain, and the theory of the relationship between the two quantities is described, including the effects of crystalline anisotropy. The various types of high resolution neutron diffractometer suitable for the work are described. Experimental results validating the method are given for a simple bent bar of mild steel of known strain, a plastically strained mild steel bar, and a mild steel tube of known torsional strain. Examples of the method in practical use are given by a cracked fatigue test specimen, a double-V test weld and a weld joining a tube to a plate. A more detailed example is the anisotropic response of a polycrystalline sample under elastic and plastic strain; this is illustrated by measurements...

Neutron diffraction methods for the study of residual stress fields

Metal additive manufacturing (AM) has garnered tremendous research and industrial interest in recent years; in the field, powder bed fusion (PBF) processing is the most common technique, with selective laser melting (SLM) dominating the landscape followed by electron beam melting (EBM). Through continued process improvements, these methods are now often capable of producing high strength parts with static strengths exceeding their conventionally manufactured counterparts. However, PBF processing also results in large and anisotropic residual stresses (RS) that can severely affect fatigue properties and result in geometric distortion. The dependence of RS formation on processing variables, material properties and part geometry has made it difficult to predict efficiently and has hindered widespread acceptance of AM techniques. Substantial investigations have been conducted with regards to RS in PBF processing, which have illuminated a number of important relationships, yet a review encompassing this information has not been available. In this review, we survey and assemble the knowledge existing in the literature regarding RS in PBF processes. A discussion of background mechanics for RS development in AM is provided along with methods of measurement, highlighting the anisotropic nature of the stress fields. We then review modeling efforts and in-process experimental measurements made to advance process understanding, followed by a thorough analysis and summary of the known relationships of both material properties and processing variables to resulting RS. The current state of knowledge and future research needs for the field are discussed.

An overview of residual stresses in metal powder bed fusion

Residual stress measurements by means of neutron diffraction

X-ray stress analysis represents a well-established method in the field of materials techno logy1–4. However, the determination of stresses by X-rays is restricted to surface layers less than 30 ym thick. This restriction can in principle be overcome by the use of neutrons instead of X-rays, for into most materials neutrons penetrate much deeper than X-rays, e. g. 7 mm of steel or 70 mm of aluminium are necessary to attenuate a neutron beam to half of its initial intensity, values which are by three orders of magnitude larger than the corresponding values for X-rays. Only a few materials absorb neutrons very strongly, such as cadmium, boron and some of the rare earths, but fortunately these are only of minor technological importance. In spite of this advantage neutrons have not yet been used for stress analysis because of the following drawbacks: 

1

In respect to brightness neutron sources are highly inferior to X-ray sources so that even at a modern high flux reactor the intensity of monochromatic neutron beams is much lower than that of an X-ray beam produced by a common X-ray tube. Moreover the spread in wavelength Δλ/λ of the neutron beam is comparatively large (≈ 1%) resulting in a poorer resolution of the diffractometer.




2

Neutron beam time is expensive which severely restricts the number of neutron diffraction experiments.




3

Neutron sources are found only at a few places in the world.

V. Jung

Papers

Residual stress measurements by means of neutron diffraction

Determination of Residual Stress Distributions in the Interior of Technical Parts by Means of Neutron Diffraction