http://doras.dcu.ie/17607/1/Review_of_residual_stresses-Final.pdf

Methods of measuring residual stresses in components

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

Many manufacturing processes can induce residual stresses in components. These residual stresses influence the mean stress during cyclic loading and so can influence the fatigue life. However, the initial residual stresses induced during manufacturing may not remain stable during the fatigue life. This paper provides a broad and extensive literature survey addressing the stability of surface and near-surface residual stress fields during fatigue, including redistribution and relaxation due to static mechanical load, repeated cyclic loads, thermal exposure and crack extension. The implications of the initial and evolving residual stress state for fatigue behaviour and life prediction are addressed, with special attention to fatigue crack growth. This survey is not a critical analysis; no detailed attempt is made to evaluate the relative merits of the different explanations and models proposed, to propose new explanations or models or to provide quantitative conclusions. Primary attention is given to the residual stresses resulting from four major classes of manufacturing operations: shot peening and related surface treatments, cold expansion of holes, welding and machining.

A literature survey on the stability and significance of residual stresses during fatigue

A large variety of transport properties have been observed at the interface between the insulating oxides SrTiO3 and LaAlO3 such as insulation, 2D interface metallicity, 3D bulk metallicity, magnetic scattering, and superconductivity. The relation between the structure and the properties of the SrTiO3/LaAlO3 interface can be explained in a meaningful way by taking into account the relative contribution of three structural aspects: oxygen vacancies, structural deformations (including cation disorder), and electronic interface reconstruction. The emerging phase diagram is much richer than for related bulk oxides due to the occurrence of interface electronic reconstruction. The observation of this interface phenomenon is a display of recent advances in thin film deposition and characterization techniques, and provides an extension to the range of exceptional electronic properties of complex oxides.

Structure–Property Relation of SrTiO3/LaAlO3 Interfaces

Residual stress measurements in a thick, dissimilar aluminum alloy friction stir weld

Cross-sectional mapping of residual stresses in a VPPA weld using the contour method

The contour method, a newly-invented sectioning technique for residual stress measurement, has the potential to measure the cross-sectional residual stress profile of a weld in a simple and time-efficient manner. In this paper we demonstrate the capability of the contour method to measure cross-sectional residual stress profiles, which are compared with neutron diffraction measurements and show excellent agreement. The results provide useful information for safetycritical design of welded components and optimization of welding parameters, and also illustrate the potential of the contour technique as a powerful tool for residual stress evaluation.

Residual Stress Mapping in Welds Using the Contour Method

In this paper, we present results of residual stresses around a cold-worked fastener hole in a thin, finite plate. An analytical model, with plane stress conditions, elastic nonlinear–plastic loading and unloading taken into account, is explicitly presented and has been developed from existing analytical models and methodologies. The solution also accounts for elastic deformation of the mandrel, and is applicable to different sizes of hole and mandrel. A parametric study of the strain-hardening exponent, plate size, Bauschinger parameter and expansion ratio was carried out in the context of the present solution. A sample study of a plate subjected to 4% hole-cold expansion was performed to compare this model with other analytical predictions, finite element simulation and X-ray diffraction experimental results. A brief review of related issues is given. This work is intended to present completely the closed-form equations able to generate residual stress profiles with ease around a cold-expanded hole in a thin, finite plate where boundary conditions are at issue.

Analysis of the Residual Stress around a Cold‐expanded Fastener Hole in a Finite Plate

Cold expansion is now a commonly accepted process for enhancing the fatigue endurance of structures containing fastener holes. However, precise knowledge of the residual stress profile around a cold-expanded fastener hole is clearly required if the fatigue life of such structures is to be accurately predicted. There have been many experimental studies of the residual stress distribution surrounding cold expanded holes. Hence, the determination of residual stress fields surrounding a split-sleeve cold-expanded hole in an En8 steel plate is ideal system on which to explore the effectiveness of a newly developed method of characterizing residual stress distribution, the Contour Method. In this paper, the two-dimensional residual hoop stress map obtained from the Contour Method is compared with the results form a three-dimensional finite element simulation. The Contour Method was found to be experimentally demanding but careful application of technique provided good agreement between the two methods showing that the Contour Method has the potential to be a powerful alternative to current residual stress measurement techniques.

Measurement of the residual stresses around a cold expanded hole in an en8 steel plate using the Contour Method

The contour method is a newly-invented destructive technique of residual stress measurement and has been receiving increasing interest. It is advisable to validate such new methods by comparison with other well-established techniques. For this purpose, contour measurements were performed on a MIG 2024-T351 aluminium alloy welded plate. The result has been compared with the results from neutron and synchrotron X-ray and shows a favourable agreement.

Ying Zhang

Papers

Cross-sectional mapping of residual stresses in a VPPA weld using the contour method

Residual Stress Mapping in Welds Using the Contour Method

Analysis of the Residual Stress around a Cold‐expanded Fastener Hole in a Finite Plate

Measurement of the residual stresses around a cold expanded hole in an en8 steel plate using the Contour Method

Validation of the Contour Method of Residual Stress Measurement in a MIG 2024 Weld by Neutron and Synchrotron X-ray Diffraction