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Octet-truss lattice materials have excellent potential for use as lightweight structures due to their high strength and stiffness, but low relative density. Octet-truss lattice specimens fabricated...

Mechanical constitutive models of metals can be difficult to validate without loss of generality. Creep-induced stress relaxation in stainless steels is one such example: stress triaxiality may play a role in the deformation rate but is challenging to measure experimentally. We aimed to determine whether the accuracy of constitutive laws for multiaxial deformation, particularly creep deformation, can be verified by measuring the evolution of the complete stress tensor at a point within a structure. Creep stress relaxation specimens of Type 316H stainless steel were exposed to 550 °C for different durations. We used time-of-flight neutron diffraction and finite element analysis to determine the complete stress tensors at points within the specimens, tracking their development as the residual stress field relaxed. Multiaxial stress relaxation of 316H occurs at 550 °C due to plasticity and creep, and can follow a non-proportional deformation path. In our measurements, over-determination of the elastic strain tensor using neutron diffraction was found to reduce stress uncertainty. A popular creep constitutive model for 316H, when used with finite element analysis, predicts slightly higher strain rates than were observed experimentally. Sequential neutron diffraction stress measurements can be used to validate material constitutive laws in situations involving multiaxial deformation. This could be used to substantiate models of plasticity, viscoelasticity and creep. The RCC-MR primary/secondary creep law for 316H stainless steel is conservative for cases involving a complex initial stress field.

The use of residual stress information in probabilistic fracture assessments is hindered by difficulties in the quantification of uncertainty. At the same time, it is often necessary to consider residual stress data derived via two or more independent methods in an assessment: typically from a model of the process which introduced the stress, and from a direct physical measurement. The uncertainty in single weld process models is difficult to quantify and is strongly dependent on the process being modelled, the material constitutive behaviour assumed, and so on. Likewise, most experimental techniques for measuring deep residual stresses on welded metallic components, including relaxation methods such as Deep Hole Drilling and diffraction-based methods, also have multiple physical sources of uncertainty associated with them. This makes the uncertainty associated with single measurements difficult to estimate reliably. We explore the use of inverse-variance weighting to combine such datasets through “characteristic” uncertainties derived from prior round robin studies, and we use data from the NeT TG4 residual stress measurement and modelling round robin to illustrate this approach. Although it requires some significant simplifications, it allows convenient synthesis of residual stress data while gaining more realistic uncertainty estimates than are typically available from single measurements. This is significant because straightforward yet robust uncertainty estimates will be key for enabling future structural integrity assessment methodologies.

Nickel Aluminium Bronze (NAB) continues to be used extensively in the marine industry and is most commonly seen in valves and propellers due to its strength and corrosion resistance. However, components used for marine applications often endure long exposure to air during storage periods prior to installation and NAB is vulnerable to pitting corrosion in air. To investigate the potential condition of stored NAB, this study measured the height, width and percentage coverage of corrosion pits observed in NAB samples after 4 months of air exposure at room temperature in a laboratory. Four different types of NAB samples were considered; off-the-shelf (sample 1a), polished (sample 1b), laser beam welded (sample 1c) and failed in service (samples 2a and 2b) and the microstructure of the samples was characterised using a mixture of microscopic techniques including surface roughness characterisation, SEM, EBSD, EDX and neutron diffraction. Examples of pitting corrosion were found on each sample type after 4 months of exposure to air. The results show significant correlation between surface roughness of the pre-corroded sample and pitting coverage, and this is thought to be due to the development of crevice corrosion during the 4 months of air exposure. It also highlights the inaccuracies in using Ra values alone to measure surface roughness in samples where localised corrosion pits are suspected due to high surface roughness or long periods of air exposure. The results of this study bring into question the pitting corrosion that would be found in components that are stored for longer period before being installed onto vessels. This has concerning safety implications as the initial surface characteristics of the metal may influence the residual load-bearing capacity of components.

A new test configuration called Holed-Cracked Square Plate (HCSP) is proposed to investigate mixed mode I/II fracture of brittle and quasi-brittle materials. This specimen is a square plate containing a central hole and two radial cracks initiated from the circumference of the internal hole. The finite element method was used to calculate mode I, mode II intensity factors and T-stress for various crack lengths and hole diameters. The numerical results show that a full range of fracture mode mixities from pure mode I to pure mode II can be realized by changing the orientation angle of two radial cracks in this specimen. The simple geometry and loading set up of this specimen make it a good substitute for other fracture tests.

We propose a method to predict the fatigue life of triangular lattices by using fatigue data from tests on single lattice struts. The method was validated using fatigue tests on 6 identical triangular lattice plates divided into three groups with different initial crack lengths. The predicted fatigue lives of lattices agree very well with the experimental results. A three-stage mechanism of lattice fatigue was observed in the tests, and the crack propagated in the direction 30° to the horizontal axis in the upper half of lattice plate in all three groups. The fatigue crack propagation rate was also studied and calculated by using the effective mechanical parameters from the homogenized structure. Experimental fatigue crack growth rate data were compared with predictions, showing that the proposed method can also predict the crack growth rate in triangular lattices very well.

In structural materials, ‘intrinsic’ toughening originates from plastic dissipation of strain energy at the tips of cracks. This depends on a material's microstructure and its stress–strain response. By introducing a spatially-varying distribution of prior strain-hardening into a material, we can modify the stress field which develops around a crack as it is loaded, producing an increased resistance to ductile tearing. We demonstrate this toughening effect using synchrotron X-ray diffraction and digital image correlation measurements of the crack tip region in a ductile ferritic steel. Localised strain-hardening also introduces a residual stress, but this is shown not to contribute significantly to the initiation of tearing in this material.

The strain history of a ductile material can affect its apparent initiation fracture toughness as well as its resistance to subsequent crack growth. Using neutron diffraction or synchrotron X-ray diffraction in conjunction with digital image correlation, it is possible to map both the total strain around the tip of a propagating crack, and the elastic strain component. Separating the elastic and irreversible strain components allows us to experimentally investigate the effects that residual stresses and prior strain-hardening have on crack propagation. This article presents the results of two experiments using these techniques. In the first experiment, it was shown that residual stresses in a 7xxx-series aluminium alloy affect not only the initiation of fracture but also fracture propagation. During crack propagation, residual stress relaxation occurs due to extension of the crack and plastic deformation in the region ahead of the crack tip. This affects the material’s apparent crack growth resistance in a non-linear manner and hence changes its fracture stability. In a second experiment, it was shown that the plastic zone ahead of a crack in a ductile material (in this case a ductile ferritic steel) can be modified by localised strain-hardening prior to fracture. Localised strain-hardening using an indentation tool was observed to cause small increases in the specimens’ peak load capacity and its energy absorption prior to the onset of unstable tearing. Using synchrotron diffraction, it was shown that is because hardening a region ahead of the crack tip causes stress redistribution to this region and away from the crack tip itself during fracture loading. These findings can inform how ductile and semi-ductile fracture is treated in structural integrity assessment procedures, particularly in the presence of residual stress. They also suggest that new localised peening treatments to improve the fracture resistance of structures in certain areas may be possible.

This study examines the influence of probabilistic treatment of transverse residual stress in a structural integrity assessment. Pipe girth weld measurement data gathered from numerous studies was statistically interpreted to provide realistic through-thickness stress distributions for use in fracture assessments. The database measurements are shown to follow a normal distribution which is capable of evaluating a probabilistic interpretation of weld residual stress within a fracture assessment. A comparison between deterministic upper bounds, provided by various standards, and probabilistic interpretation was undertaken. The presented results propose a 42% reduction in the estimated probability of failure of the case study using a probabilistic assumption of residual stress. Direct implications include a more realistic model for treatment in fracture assessments which can in turn improve acceptance criteria and avoid unnecessary weld repairs.

The fabrication of large components using a high deposition rate, near-net shape process like Wire + Arc Additive Manufacturing (WAAM) is a promising option for many industries, due to the potential for reduction in material wastage and shorter lead times in comparison to conventional methods. Specialist materials like nickel-base superalloys, which are typically used in high temperature and corrosive environments, are particularly attractive options due to their high raw material costs. Although nickel-base Alloy 718 seems well suited to the process due to its good weldability, process-induced defects can arise from unfavourable deposition conditions and elimination of these defects may not always be possible. In WAAM Alloy 718 deposited under such conditions, crack-like defects with planar morphology and hot cracking characteristics were observed. These defects were observable using conventional non-destructive testing techniques and displayed directionality relating to the deposition path. The fracture behaviour of WAAM Alloy 718 containing these defects was “semi-stable” – a mixture of fracture instability and stable crack extension. The apparent fracture toughness of WAAM Alloy 718 containing these defects was found to be anisotropic, which can be attributed to the interaction of the notched crack with pre-existing defects. WAAM Alloy 718 displayed an apparent fracture toughness comparable to that of wrought Alloy 718 when notched perpendicular to the defects; but only half that of wrought when notched parallel to the defects. Therefore, careful consideration of defect orientation and their effects on mechanical properties is important in assessing the fitness-for-service of WAAM Alloy 718.

Structural integrity assessment often requires the interaction of multiple closely-spaced cracks or flaws in a structure to be considered. Although many procedures for structural integrity assessment include rules for determining the significance of flaw interaction, and for re-characterising interacting flaws, these rules can be difficult to validate in a fracture mechanics framework. int_defects is an open-source MATLAB toolbox which uses the Abaqus finite element suite to perform large-scale parametric studies in cracked-body analysis. It is designed to allow developers of assessment codes to check the accuracy of simplified interaction criteria under a wide range of conditions, e.g. for different interacting flaw geometries, material models and loading cases. int_defects can help analysts perform parametric studies to determine linear elastic crack tip stress field parameters, elastic-plastic parameters and plastic limit loads for simple three-dimensional cracked bodies relevant to assessment codes. This article focusses on the validation of int_defects using existing fracture mechanics results. Through a set of validation examples, int_defects is shown to produce accurate results for a very wide range of cases in both linear and non-linear cracked-body analysis. Nevertheless, it is emphasised that users of this software should be conscious of the inherent limitations of LEFM and EPFM theory when applied to real fracture processes, and effects such as constraint loss should be considered when formulating interaction criteria.

Octet-truss lattice structures can be used for lightweight structural applications due to their high strength-to-density ratio. In this research, octet-truss lattice specimens were fabricated by stereolithography additive manufacturing with a photopolymer resin. The mechanical properties of this structure have been examined in three orthogonal orientations under the compressive load. Detailed comparison and description were carried out on deformation mechanisms and failure modes in different lattice orientations. Finite element models using both beam elements and three-dimensional solid elements were used to simulate the compressive response of this structure. Both the load reaction and collapse modes obtained in simulations were compared with test results. Our results indicate that three-dimensional continuum element models are required to accurately capture the behaviour of real trusses, taking into account the effects of finite-sized beams and joints.

int_defects is a free and open-source toolbox for MATLAB which uses the Abaqus finite element analysis suite to automate the analysis of interacting crack-like flaws in structures. It is designed to aid the development of flaw interaction criteria for structural integrity assessment procedures and to simplify sensitivity analysis of flaw proximity. It can also perform automatic 3D elastic, elastic-plastic or limit load analysis of elliptical embedded flaws and semi-elliptical surface flaws in plates and thick-walled pipes under a wide range of loading conditions. This technical note outlines the capabilities of int_defects and gives some examples of its use.

Residual stress in the welds that attach Control Rod Drive Mechanism nozzles into the upper head of a PWR reactor vessel can influence the vessel’s structural integrity and initiate Primary Water Stress Corrosion Cracking. PWSCC at Alloy 600 CRDM nozzles has caused primary coolant leakage in operating PWRs. We have used Deep Hole Drilling to characterise residual stresses in a PWR vessel head. Measurements of the internal cladding and nozzle attachment weld showed that although modest tensile stresses occur in the cladding, the attachment weld contains tensile residual stresses of yield magnitude. Despite the large dispersion of residual stress data for nozzle attachments of this type, all available data suggest that assuming a residual stress profile bounded by the weld material’s yield stress would be conservative for assessment purposes.

Residual stresses in materials affect their resistance to the initiation of fracture and to subsequent crack growth. Using full-field strain measurements and finite element analysis we demonstrate that the effect of residual stress on a material's crack growth resistance curve can be understood using elastic-plastic fracture mechanics. It is shown that Lei's modified J-integral formulation (Jmod) is a good predictor of the load vs crack extension behaviour of an elastic-plastic material containing residual stresses. Keywords: Residual stress, Fracture, Neutron diffraction, Digital image correlation, Finite element analysis

Crack-like surface flaws in pipes, pressure vessels and other structural components sometimes occur near to one another. When this happens, it is often necessary to take the mutual interaction of the flaws into account when performing fracture-mechanics-based fitness-for-service assessment. Integrity assessment procedures including ASME BPVC Section XI, BS 7910 and R6 include interaction criteria which are used during flaw characterisation to ensure that interaction is accounted for conservatively. This paper considers the interaction which occurs when flaws are loaded by a non-uniform through-wall distribution of stress, as may arise due to bending, thermal shock or welding/cladding residual stresses. Using parametric finite element analysis of a large number of crack pairs subjected to different distributions of stress, it is shown that the degree of flaw interaction can be enhanced under non-uniform loading. Therefore, care should be taken when performing integrity assessment using interaction criteria based on uniform tension loading only.

Dissimilar metal welds are complicated in nature because of the complex microstructure characteristics in the weld fusion zone. It is often necessary to know the phase distribution in a dissimilar metal weld especially at the interface such as fusion zone and heat affected zone to be able to predict the behaviour of the joint and its fitness for service. In this paper, a dissimilar metal weld made between ferritic/martensitic modified 9Cr-1Mo steel (P91) and austenitic AISI 316LN stainless steel using autogenous electron beam (EB) welding was analysed. The weld fusion zone has a local segregation of bcc and fcc phases. The EBSD technique was applied to determine the volume fractions of each of these phases in the weld fusion zone. This information was incorporated into the analysis of neutron diffraction data from the weld zone, and the macro-scale residual stresses were calculated from phase-specific stresses arising from the welding process. The results indicate that the overall macroscopic residual stress distribution in the weld centre is predominantly compressive in nature driven by the solid-state phase transformation of the weld pool during rapid cooling, with tensile peaks pushed adjacent to the heat affected zone (HAZ)/Parent boundaries on both sides of the fusion zone.

The introduction of cracks into loaded versus unloaded components has a significant effect on the apparent fracture toughness within finite element modelling. The history effects of crack introduction can be beneficial to defect assessment procedures that do not consider prior plasticity specifically from crack introduction. It is assumed that as strain energy is released due to plastic deformation during crack formation a reduction in the energy available for crack propagation under fracture conditions is experienced. This can be characterized by the formation of a plastic wake behind the crack tip and leads to significant increases in load at critical J and other crack growth parameters for modelling situations. However experimental evidence validating this apparent fracture toughness increase are needed. A beneficial increase in apparent fracture toughness can prolong the life of components that might be taken out of service prematurely if history effects are not considered. This paper discusses a series of experimental and modelling approaches that have been taken to assess the magnitude of the benefit in increase of apparent fracture toughness by the manipulation of crack introduction history effects. An initial parametric study of material properties on the effect of introducing cracks into loaded and unloaded components indicates that most benefit be derived from high hardness, high yield materials such as Aluminum 7000 series alloys. Further work has been carried out with experimental C(T) specimens of Aluminum Alloy 7475 T7351. Cracks were introduced by fatigue into the samples. One set of specimens was fatigued with a low mean load and the other with a high mean load, this was achieved by keeping a consistent ΔKI between specimens and changing the load ratio one set of specimens. Fracture test results indicate that the influence of prior plasticity on fracture initiation is much subtler in experimental trials than in the finite element model. Crack growth resistance curves and neutron diffraction results measuring the residual stress created ahead of the crack tip by this method are be discussed and contrasted with the parametric study and finite element modelling of the two different crack introduction scenarios.

Procedures for structural integrity assessment such as R6 and BS7910 often need to consider the possibility of closely-spaced structural defects interacting with one another. Typically, this comes in the form of a simplified interaction rule: if a criterion based on the size, shape and separation of the defects is satisfied, then they are judged to interact and cannot be analysed independently. When interaction is judged to occur, the defects may be re-characterised as a larger, enclosing defect. Rules for predicting defect interaction and then specifying the size of the re-characterised defect need to be conservative across a wide range of situations, so they are often informed by studies using parametric finite element analysis and laboratory testing. This article discusses the interaction of defects in inelastic materials, focussing on the determination of limit loads and the elastic-plastic crack driving force for co-planar crack-like defects. New methods for parametric modelling and data analysis are introduced, with examples demonstrating the use of inelastic modelling in formulation of interaction criteria.

BS 7910 “Guide to Methods for Assessing the Acceptability of Flaws in Metallic Structures” [1] is undergoing revision for the next edition. Updates are proposed for the Annex E flaw recharacterization guidance. The changes aim to make Annex E more applicable to real structures and to improve the synergy between the subcritical crack growth such as by fatigue and fracture of an embedded flaw. Several studies were done by WEE/37 fracture panel focusing on brittle and ductile material behaviour to support the proposed changes. These studies are summarised in the paper. The main modifications in the proposed updated Ann ex E are in: • Removal of use of Annex E for brittle fracture. • Updated guidance on recharacterized surface flaw length for ductile fracture. • Optional guidance on use of geometry-based criterion for embedded flaw break-through under ductile conditions. • Alignment of fatigue crack growth in Section 8 with the flaw break-through in Annex E.

Procedures for structural integrity assessment normally contain criteria to predict the significance of the interaction between neighbouring defects in a structure. Here, the elastic interaction between coplanar semi-elliptical surface cracks is examined in detail by considering a large number of dissimilar crack pairs with different depths and aspect ratios. Surface defect interaction criteria from several assessment procedures are critically assessed and found to be satisfactory for cracks loaded in uniform tension. The criterion used in the R6 Rev. 4 and BS 7910:2013 procedures is the least inherently conservative of those considered here. However, the amount by which interaction exacerbates the most severe crack front loading state can depend strongly on the distribution of stress applied to the cracks. This means that the loading mode should be taken into consideration when judging whether the interaction between surface defects is significant.

Since all residual stress measurement methods have inherent limitations, it is normally impractical to completely characterise a three-dimensional residual stress field by experimental means. This lack of complete information makes it difficult to incorporate measured residual stress data into the analysis of elastic–plastic fracture without resorting to simplified methods such as the Failure Assessment Diagram (FAD) approach. We propose a technique in which the complete residual stress field is reconstructed from measurements and used in finite element analysis of the fracture process. Residual elastic strains and stresses in three-point bend fracture specimens were measured using neutron diffraction and an iterative method was used to generate a self-consistent estimate of the complete residual stress field. This enabled calculation of the J contour integral for a specimen acted on by both residual stress and an externally-applied load, allowing the interaction between residual and applied stress to be observed in detail.

In this study, the effect of crack orientation on the fracture behaviour of two compact tension C(T) specimens extracted from a Wire + Arc Additively Manufactured (WAAM) wall made from Inconel (IN) 625 nickel-base superalloy was investigated. Both specimens had different levels of ductile tearing but their load vs. crack mouth opening displacement (CMOD) behaviour was relatively similar. The total-and-elastic strain distribution around a crack tip was measured in both specimens using Digital Image Correlation (DIC) and neutron diffraction respectively. The results show that the strain distribution and deformation around the crack tip are different in the two directions. In the specimen with crack orientation parallel to the build direction, banding was observed in both the total strain maps and the deformation pattern. Neutron diffraction measurements on this specimen also showed non-monotonic elastic strain evolution, suggesting the occurrence of intergranular load shedding mechanisms. These were not observed in the specimen with crack orientation perpendicular to the build direction. Electron Backscatter Diffraction (EBSD) maps show that the WAAM IN625 material is strongly textured with coarse columnar grains elongated in the build direction. The effect of microstructure has been correlated with the differences in strain distribution in the two specimens.

If a crack is introduced progressively into an elastic-plastic material containing a residual stress field, the incremental relaxation of the residual stress field causes the formation of a plastic wake along the crack boundaries. This leads to the reduction in the J parameter for a crack of a given size, compared to a crack with the same dimensions which has been introduced instantaneously, having the crack faces released simultaneously along the whole length, a 40% reduction is observed in the current analysis. This reduction in J is due to the dissipation of strain energy which is otherwise available for further crack extension, as in the instantaneously introduced crack. This is important for the current J-based fracture assessment common in the nuclear and petrochemical industries such as EDF Energy’s R6 and BS7910:2013 as they currently assume instantaneous insertion of cracks as this is inherently more conservative. Although many studies demonstrating this effect in FE are available, there is little experimental evidence for this phenomenon. Especially those including rigorous comparisons with specimens that have been ‘instantaneously’ cracked. This may be due to the difficulty inherent in manufacturing such a specimen as manufacturing processes rely on the incremental removal of material. The aim of this paper is to detail analysis of a novel method of crack introduction that aims to replicate the deformation behavior of an instantaneously introduced crack tip in a model that has had the nodes released in a progressive manner. This will allow specimens to be machined in a way that replicates ‘instantaneous’ cracking allowing for experimental techniques to be developed to display the difference between instantaneous and progressively introduced cracks.

Fitness for service assessment procedures rely on flaw interaction rules for assessment of multiple flaws in close proximity. Such rules are aimed at avoiding excessive amplification of the crack driving force that may result in a non-conservative fracture assessment. In BS7910, the 2013 edition [1] introduced a new flaw interaction rule for the co-planar flaws where the proximity of adjacent flaws is judged based on flaw height (i.e. s = 0.5*max(a1,a2) for surface flaws). The rule was introduced for flaws with aspect ratio of a/c < 1 for both flaws, while for other flaw shapes and combinations the earlier rule from the predecessor document PD6493:1993 [2] was retained. This paper summarises the recent work done by the authors and work from literature to examine the applicability of the s = 0.5*max(a1,a2) rule to flaws with aspect ratio a/c ≥ 1 and dissimilar flaw combinations. It is shown that the current BS7910 rule based on s = 0.5*max(a1,a2) produces a conservative flaw assessment with the use of BS7910 solutions for stress intensity factor and reference stress. An exception are cases of two deep surface flaws where the rule is proposed to change to: s ≤ max(a1, a2) for two surface flaws with a1/t & a2/t > 0.5

Fatigue fracture is one of the major degradation mechanisms in the low-alloy 4330 carbon steel pumps that are utilized in the hydraulic fracturing process operating under cyclic loading conditions. A weld cladding technology has been developed to improve the ability of these components to resist fatigue crack initiation by cladding them with a secondary material. This process introduces a residual stress profile into the component that can be potentially detrimental for fatigue performance. The cladding technology under examination is a low-alloy 4330 carbon steel substrate weld that is clad with the nickel-chromium–based superalloy IN625 and is investigated herein using several experimental residual stress measurement techniques. Understanding the magnitude and distribution of residual stress in weld clad components is of the utmost importance to accurately assess the performance of the component in service. This study summarizes the results of residual stress measurements that were determined using X-ray diffraction, i.e., hole drilling based on electronic speckle pattern interferometry, deep-hole drilling, and the contour method, to obtain the residual stress distributions from the surface of the weld clad, through the clad layer, and into the substrate material. The results of deep-hole drilling and the contour method show large-scale tensile residual stress in the clad layer and compressive residual stress in the majority of the substrate. However, the X-ray diffraction and hole drilling methods indicate the presence of short-scale compressive residual stress on the surface and near the surface of the clad layer. It was shown that these measurement techniques are complementary in assessing the residual stress profile throughout the entire component.

Closely-spaced cracks in structures can interact with each other; the presence of one crack can change the strain energy release rate at another crack nearby. Since this interaction is enhanced by the onset of plasticity, elastic analysis alone should not be used for judging whether interaction between cracks will have a significant effect on the integrity of a structure.

Elastic follow-up is a mechanical boundary condition lying between constant load and constant strain control. It exists in many engineering components operating at high temperature and can result in dramatically different creep stress relaxation and strain accumulation rates in a localized region of a component. We have performed creep tests under constant load, constant strain and elastic follow-up control on an aged (additional 22,000 h) 316H austenitic stainless steel after service in a nuclear power station for 65,000 h. Primary and secondary forward creep models with parameters derived from the constant-load data were able to describe constant-load creep adequately, but not able to predict stress relaxation and elastic follow-up. We show that this is because ageing has increased the constant-load creep strain rate significantly but has no effect on stress relaxation creep strain rate. Ageing promotes the formation of ferrite/chi phase at grain boundaries which are preferential sites for creep cavitation under load control. However, creep cavitation is less likely under constant strain and elastic follow-up control because a high creep strain rate, large creep strain and stress cannot coexist under these boundary conditions.

Residual and thermal stress fields in engineering components can act on cracks and structural flaws, promoting or inhibiting fracture. However, these stresses are limited in magnitude by the ability of materials to sustain them elastically. As a consequence, the stress intensity factor which can be applied to a given defect by a self-equilibrating stress field is also limited. We propose a simple weight function method for determining the maximum stress intensity factor which can occur for a given crack or defect in a one-dimensional self-equilibrating stress field, i.e. an upper bound for the residual stress contribution to KI. This can be used for analysing structures containing defects and subject to residual stress without any information about the actual stress field which exists in the structure being analysed. A number of examples are given, including long radial cracks and fully-circumferential cracks in thick-walled hollow cylinders containing self-equilibrating stresses.

AISI Type 304L austenitic stainless steels are extensively used in industries, and welding is an indispensable tool used for joining these materials. In a multi-pass weld, the development of residual stress to a large extent depends on the response of the weld metal, heat affected zone and parent material to complex thermo-mechanical cycles during welding. Earlier researchers on this area used either mechanical tensioning or heat treatment to modify the residual stress distribution in and around the weld. In this research, microstructural refinement with modification of residual stress state was attempted by using high pressure cold rolling followed by laser processing in 12 mm thick 304L austenitic stainless steels which is a novel technique. The hardening of the weld metal was evaluated after welding, post weld cold rolling, and post weld cold rolling followed by laser processing. The residual stress was determined non-destructively by using neutron diffraction. Residual stress analysis show that post weld cold rolling was effective in modifying the longitudinal residual stress distribution throughout the entire thickness. Post weld cold rolling followed by laser processing performed in this research was to induce recrystallization of the cold rolled grains. However, post weld cold rolling followed by laser processing showed minor grain refinement but was not effective as it reinstated the stress state.

A method for approximating an upper bound to the Mode I stress intensity factor for a crack in a two-dimensional object containing unknown thermal or residual stresses is described. It uses eigenstrain analysis and constrained maximisation to find the self-equilibrating stress distribution which imparts the maximum stress intensity after introduction of a crack. As an example we calculate limits on the stress intensity at an edge crack in a plane strain bar for thermal and residual stress loading. It is shown that uniform yield-magnitude stress perpendicular to the crack plane does not necessarily give a bounding estimate of stress intensity.

Residual stresses in welds pose a significant threat to the structural integrity of a component, especially in the presence of defects and are required to be accounted for in assessing component safety. Although the R6 assessment procedure suggests various approximate methods for incorporating these effects in defect assessment, most of them are overly conservative and not very cost-effective. A more reliable approach is to characterise the weld residual stresses around a defect and study how they interact with primary load. The current paper analyses the effects of weld residual stresses on the fracture of a dissimilar weld in the presence of defect. The weld is made between modified 9Cr–1Mo steel and 316LN stainless steel using autogenous electron beam welding. A C(T) specimen was extracted from the centre of the weld and a crack introduced in the fusion zone using electro-discharge machining. The residual stresses around the crack were measured on a grid of measurement points at mid-thickness of the C(T) specimen using neutron diffraction on the strain diffractometer SALSA at ILL, Grenoble. The measured residual stresses around the crack-tip were incorporated into a finite element model and the interaction of these with applied load was predicted under fracture.

Closely-spaced surface cracks in structures interact with each other when subjected to load. The degree of interaction depends strongly on the distribution of stress that is applied. In pressure boundary components, thermal shock, residual stress and global bending can all cause load distributions that are non-uniform through the wall thickness. A wide range of crack pairs subject to various non-uniform stress distributions have been modelled using finite element analysis. Cracks sometimes interact more strongly under non-uniform loading than when loaded in uniform tension. Consequently, interaction criteria developed by considering uniform tension may not be inherently conservative for all loading conditions. A simple weight function method for determining the interaction of twin cracks under an arbitrary through-wall stress is presented, and weight function coefficients for a wide range of crack sizes and aspect ratios are given.

Pressurised Thermal Shock (PTS) is one potential risk to the integrity of the reactor pressure vessel in a pressurised water reactor. It has been postulated that PTS could occur as a result of various initiating events such as loss-of-coolant accidents with subsequent re-pressurisation. Experimental studies of PTS are typically very difficult and expensive to perform because both a severe thermal shock and a primary load must be applied to the test specimen, while the specimen itself must be very large to imitate the behaviour of the RPV wall. We investigated the feasibility of using scaled-down PTS test specimens based on the spinning-cylinder concept. The use of scaled-down specimens could greatly reduce the difficulty and cost of experimental PTS testing. To explore this concept, we used a particularly well-characterised spinning-cylinder PTS test: the NESC-1 test which was performed in the late 1990s. A large parametric set of elastic-plastic finite element models was used determine a combination of specimen dimensions and test conditions that would very closely mimic the crack tip conditions which occurred during NESC-1. Specifically, the modelling demonstrated that it was indeed possible to replicate the KJ vs. temperature trajectory, and crack tip constraint, at a critical point on the crack tip line from which tearing initiated during the actual NESC-1 test. The reduced-size specimen must be carefully designed: it cannot be a simple linear scale-down due to the inherent non-linearity of both the thermal and mechanical processes which occur during PTS.

Pressurised thermal shock tests that simulate fracture conditions relevant to nuclear reactor pressure vessels are difficult to perform and require very large specimens. This study examines the feasibility of scaled-down specimens to allow easier testing. A series of finite element models was used to design scaled-down specimens that produce a crack-driving force, crack tip temperature trajectory and constraint conditions very similar to those which occurred in a previous large-scale spinning cylinder experiment known as NESC-1. It is shown that equivalent conditions can be achieved in much smaller specimens than NESC-1 using a practical set of testing parameters.

Detrimental residual stresses that occur in welded joints can be removed by rolling the weld seam. In this study we show that rolling could be applied to much thicker-section welds than has previously been attempted. A residual stress field introduced by localised rolling of an aluminium alloy specimen was studied to establish whether the plastic deformation caused by rolling would be sufficient to treat thick-section welds. It was modelled using finite element analysis and characterised using detailed neutron diffraction measurements. During rolling, plastic deformation occurs through the entire thickness of the specimen and strongly compressive residual stresses are created in the rolled area. Some features of the three-dimensional residual stress field, such as a region of tensile stress beyond the end of the rolled area, could be detrimental to structural integrity. It is recommended that these should be taken into account in the design of rolling-based weld treatment and surface treatment processes.

By finding stress states which are consistent both with any existing experimental measurements and with elasticity theory, residual stress fields can often be reconstructed from incomplete measurement data. We discuss such methods of residual stress reconstruction, their implementation using finite element analysis, and the measurement strategies which enable them. In general, reconstruction of residual stress fields must be formulated as an inverse problem, which can usually be solved using stress basis functions. However, prior knowledge of the form of the residual stress field and/or underlying eigenstrain distribution often allows the problem to be reduced such that inverse methods become unnecessary, greatly simplifying the analysis. Two examples of when residual stress field reconstruction can be simplified in this way are given.

A method is introduced by which the complete state of residual stress in an elastic body may be inferred from a limited set of experimental measurements. Two techniques for carrying out this reconstruction using finite element analysis are compared and it is shown that for exact reconstruction of the stress field via this method, the stress field must be measured over all eigenstrain-containing regions of the object. The effects of error and incompleteness in the measured part of the stress field on the subsequent analysis are investigated in a series of numerical experiments using synthetic measurement data based on the NeT TG1 round-robin weld specimen. It is hence shown that accurate residual stress field reconstruction is possible using measurement data of a quality achievable using current experimental techniques.

A method for predicting the fatigue life of triangular lattices is proposed in this paper by considering fatigue properties of single lattice struts. Fatigue tests of different sizes of lattice plates of aluminium alloy, and tests of single struts with different maximum fluctuating loads, have been conducted to validate this method. It is found that the struts in a triangular lattice break near to strut intersections, where stress and strain concentrations occur. Similar crack propagation paths were observed in different lattice plate specimens: the cracks grew at a 30° angle to the initial edge crack in the upper half of lattice plate. The mixed-mode fatigue crack propagation rate was also studied and expressed using an effective stress intensity factor. A size effect on the crack growth rate of triangular lattice plates was also observed: a fatigue crack will propagate slightly quicker in larger triangular plates than in smaller ones.

The post-welding rolling technique was used to modify the welding residual stress profile of butt joints of S355 structural steel. Two different residual stress measurement techniques were used: contour method and neutron diffraction. A good agreement between the results of both measurement techniques was found, illustrating the capability of the contour method to provide data otherwise available only using costly neutron and synchrotron radiation. The effect of post-weld roller tensioning on residual stress consisted in a substantial change in residual stress profile, with the large tensile stresses along the weld seam becoming compressive

Parts manufactured by Wire and Arc Additive Manufacture (WAAM) can have significant residual stress and distortion, as well as large grain sizes. To overcome these problems, each layer on a linearly deposited steel WAAM part was rolled with either a ‘profiled’ roller, which had a similar shape to the deposited layer, or a ‘slotted’ roller, in which a groove prevented lateral deformation. Both rollers reduced the distortion and surface roughness, but the slotted roller proved more effective – eliminating the distortion. The residual stresses in the rolled WAAM parts were measured and were lower than those in the unrolled control specimen – particularly adjacent to the baseplate. Rolling also induced additional grain refinement when the rolled material was reheated during the subsequent deposition pass. The application of rolling may be a key technology for enabling implementation of WAAM on large-scale structures.

Large residual stresses are an undesirable but inevitable side effect of fusion welding operations, and localised high pressure rolling of the weld seam is a proposed method for eliminating them. In this study, neutron diffraction has been used to map the residual stresses within low carbon steel weld seams treated with high pressure rolling. The effect on the residual stress distribution of using different roller types was determined, along with the influence of these different rollers on final weld seam geometry. Rolling was found to completely change the residual stress state in the weld, creating large compressive longitudinal residual stresses. It was effective for this purpose regardless of whether it was applied directly to the weld seam or to regions either side of it. The fatigue life of welded specimens was shown to be reduced by rolling; however, it is suggested that this is due to geometric and metallurgical effects.