M. R. Goldthorpe

Residual Stress and Constraint Effects on Fracture in the Transition Temperature Regime

This paper presents the results of a combined experimental and numerical study aimed at quantifying the influence of self-balancing residual stresses on the fracture toughness constraint benefit of a ferritic pressure vessel steel tested in the cleavage fracture regime. Tests were performed on standard and pre-compressed, high constraint, compact-tension (CT) and low constraint, single-edge-notched tension (SENT) specimens at a temperature close to the Master Curve reference temperature T0 . Pre-compression is undertaken prior to pre-cracking to establish a residual stress across the uncracked ligament, which is highly tensile at the pre-crack notch root and balanced by compressive stresses further ahead of the notch. The pre-crack is subsequently introduced into material ahead of the notch, within the tensile residual stress region, specimen by electro-discharge machining and fatigue. The tests demonstrate an influence of tensile residual stresses on the apparent fracture toughness properties for both CT and SENT specimens. The tests on low constraint specimens illustrate the constraint benefit on cleavage toughness for this material, and the influence of residual stresses in reducing this benefit. The paper shows how the observed behaviour can be quantified through using two parameter fracture mechanics. Here, the J-integral is determined by taking full account of the influence of preloading on the crack driving force. Both the elastic-T-stress and the elastic-plastic Q-stress are calculated and demonstrated as constraint indexing parameters. The results demonstrate a reduction in constraint benefit for cracks located within highly bending residual stress fields. Thus, when exploring any possible benefit in fracture toughness due to crack tip constraint, it is critical that the combined influence of the primary and secondary stresses on crack tip constraint be taken fully into account.© 2008 ASME

Assessment of Defects under Combined Primary and Residual Stresses

Residual stresses can provide a significant element of the crack driving force for defects in welded components. Structural integrity assessment methods are available, such as the R6 defect assessment procedure [1], which provide detailed guidance for the assessment of such defects under the combined influence of primary and residual stresses. However, in some circumstances these methods may be unduly conservative due, in part, to an over-estimation of the crack driving force due to the residual stress, KJ s. This over-estimation can lead to a pessimistic view of actual safety margins for welded components and premature replacement or repair strategies.

Residual Stress Effects on Ductile Tearing

Residual stresses are internal stresses generated during the fabrication and/or operation of engineering structures. Such stresses can provide the major element of the driving force for crack initiation and growth. Structural integrity assessment procedures, provide guidance for the assessment of defects located within regions of high residual stress. However, such guidance may be conservative where the defect develops progressively during service. This paper describes recent experimental and numerical work aimed at quantifying such conservatisms and providing improved guidance for undertaking more realistic analyses. The results demonstrate that pre-loaded compact-tension specimens provide a useful means for studying the behaviour of cracks within residual stress fields. The magnitude of calculated crack driving forces due to residual stresses is influenced by the approach used to introduce cracks into the stress field, with progressive cracks providing lower levels of crack driving force than instantaneously introduced cracks. The J R-curve associated with cracks under primary or combined primary + secondary loading can apparently be rationalized when the total crack driving force is calculated using methods that take proper account of the influence of prior plasticity on the J-integral. However, it is noted that due to differences in the form of the crack-tip stress and strain fields for static and growing cracks, such values of J may be path dependent and influenced by the magnitude of the growth increment.Copyright

Assessment of constraint effects on the local behaviour of a propagating crack under cyclic loading

During operation, reactor components experience a range of static and cyclic loading that have the potential to result in environmental-fatigue crack initiation and growth. Recent experimental work has indicated that the ASME XI fatigue ‘in air’ design curves are non-conservative for fatigue cracks propagating in primary water environments at fixed temperatures of relevance to the plant. The approach adopted to assess these tests has, to date, followed current best practice: in which global Linear Elastic Fracture Mechanics (LEFM) loading parameters are used to quantify crack growth rates.To help establish an improved understanding of these data, and to assist in their application to assess plant components, a local crack-tip finite element model has been developed. The model incorporates material constitutive behavior that simulates cyclic deformation of austenitic steel, can take account of plasticity-induced crack closure and can take into consideration cracks in structurally-representative geometries via the T-stress constraint parameter.The results of studies using the model suggest that highly compressive values of the T-stress constraint parameter tend to promote less severe reverse loading of the crack tip compared with high constraint geometries such as pre-cracked compact tension and bend test specimens. These findings indicate that rates of corrosion-fatigue in actual structural geometries might be different from those observed in pre-cracked test specimens.Copyright

Characterisation of the residual stress field and mechanical properties of a narrow-gap girth-welded stainless steel pipe and subsequent application to a numerical model

Girth-butt welds are used to join sections of stainless steel pipe in the primary circuit of Pressurised Water Reactors. The welding process creates residual stress fields across the weldment, which can contribute to the crack driving force when a defect is present. Assessment procedures account for such defects, enabling safety justifications to be made for continued operation of nuclear power plant. Such procedures require the size and nature of the residual stress field to be determined in order to make reliable structural integrity assessments. This paper describes the investigation of the residual stress field and fracture behaviour of a recently developed narrow-gap 304-stainless steel girth-butt weld in a primary circuit pipe. Two residual stress measurement techniques, Neutron Diffraction (ND) and incremental Deep Hole Drilling (iDHD), were used to measure the original residual stress field in the pipe weld. A second pipe weld specimen was used to fabricate tensile and fracture toughness specimens from which the mechanical properties of the weld material were determined. The residual stress and mechanical test data were used to develop numerical models of the pipe weld containing a postulated circumferential defect under an applied axial load. The numerical simulation results were applied within a failure assessment diagram, comparing different interaction parameters on the prediction of component failure load.Copyright

Mechanically-Based Calibration of Ductile Fracture Models

Cracks in high pressure plant are often associated with welds, where residual stresses can enhance the crack driving force due to primary loading acting alone. One approach for taking account of the influence of residual stresses on defect behaviour in ductile materials is to use ductile damage mechanics. This paper presents a mechanistic study aimed at using metallographic and fractographic observations of as-received and tested material as the basis for calibrating of the Gurson ductile damage mechanics with a view to applying the calibrated model to predict residual stress effects on the ductile tearing behaviour of a high strength, low toughness 2024-T351 aluminium alloy. Detailed examination of notched tensile test specimens has clarified that two populations of second phase particles are involved in the failure process. Large intergranular intermetallic precipitates between 5 and 10 μm in diameter fracture at low strains, creating isolated microscale voids that grow under increasing plastic strain. Subsequently, a much finer array of intragranular precipitates approximately 100 to 500 nm in diameter fail at high plastic strains, leading to the formation of an almost instantaneous sheet of nanoscale voids that cause the sudden final failure of the material. Results are presented from a series of finite element analyses of these tests incorporating Gurson parameters associated with either the microscale or the nanoscale damage. These analyses demonstrate that modelling the nanoscale damage provides far better agreement with the experimental data that simulating the microscale damage. The implications of this observation on the modelling of pre-cracked specimens are discussed.Copyright

Residual stress effects on cleavage fracture

It is recognised that the driving force for the initiation and propagation of defects in materials may, under some circumstances, include contributions from both externally applied loads such as internal pressure in pressure vessels and piping and secondary stresses such as weld residual stresses. For non stress-relieved welds, residual stresses can provide a significant proportion of the crack driving force. This paper describes the results obtained from an experimental programme aimed at extending the understanding of residual stress effects on cleavage fracture. The paper describes the preparation and testing of standard and preloaded compact-tension specimens of an A533B pressure vessel steel at its Master Curve reference temperature. Standard tests on compact-tension specimens provide fracture toughness data which are broadly consistent with the conventional three-parameter Weibull model, with Kmin = 20 MPa√m and an exponent of about 4. The preloaded compact-tension specimens included a high level of tensile residual stress at the crack location. Fracture toughness data obtained using the test standards from these specimens fall significantly below the standard specimen data, since the contribution from residual stresses is ignored. However, when due account is taken of the residual stress on the crack driving force using a correct definition of the J-integral, the distributions of fracture toughness data from both specimen types are found to overlay each other. The definition of J used in this paper allows residual stress effects on fracture to be accounted for in a single fracture parameter.Copyright

Load and Crack History Effects on Fracture

Assessments of the integrity of structures containing defects or cracks require estimates to be made of the elastic-plastic crack driving force (CDF) parameter J. This is the characterising parameter that controls the intensity of the fields of stress and strain close to the tip of a crack. Such estimates of J are inherently made in assessment procedures such as R6, Revision 4 [1]. Engineering components are typically subjected to load cycles, often with significant variations in magnitude. Normal operation cycles or overload (by a proof pressure test for example) may cause a re-distribution of weld residual stresses. A defect can be present at fabrication or develop during operation due to a sub-critical process such as fatigue or stress corrosion cracking. In these two cases, it is reasonable to suppose that the actual crack driving forces are different; since the development of a defect in a region of weld residual stress, in conjunction with additional primary loading, can cause significant non-proportional loading of the crack tip. The objective of the work described in this paper is to provide more accurate estimates of the crack driving force parameter for defects subjected to combined primary and secondary stresses, taking into account the effects of loading hisotory. The eventual aim is to reduce uncertainty in assessments of plant integrity, and to clarify advantage that can be taken from a reduction in crack driving forces due to weld residual stress resulting from overload, operational cycles and the progressive introduction of sub-critical defects. Finite element analyses and R6 calculations are undertaken and compared to examine the effects of inserting a crack at different times during the life of an engineering structure.Copyright

Further Analysis of Multiple Co-Planar Flaws Under Cleavage Failure Conditions

In assessing the integrity of structures, complex multiple flaws located in close proximity to each other are generally characterised as one, larger, single flaw. Guidance for the characterisation of multiple flaws is provided in codes such as R6 and BS 7910, which are routinely used in the UK and elsewhere in the structural integrity assessment of structures and components. Recent studies have shown that the current characterisation rules may be non-conservative under some circumstances. A combined experimental and analytical programme of work is underway within the UK in order to further investigate this potential non-conservatism for situations where the possibility of cleavage failure may have to be taken into account when assessing structures or components containing multiple flaws. Details of early stages of the analytical programme were reported at the 2006 ASME PVP Conference and comprised a number of finite element analyses to evaluate cleavage failure probability, via a master-curve based approach, for interacting twin flaws and the corresponding characterised single flaw under an applied tensile load. These analyses considered surface-breaking semi-elliptical flaws of the same depth but three values of aspect ratio and a range of separations. It was found that non-conservatism was indicated for flaws of high aspect ratio in contact. This paper describes further analytical work that has been undertaken to extend the results previously reported. The further work described has been centred on: • extending the three aspect ratios (a/c = 0.2, 0.44 and 0.8) for the twin surface flaws previously considered to a/c = 1. • extending the study to include an applied bending load. • extending the study on surface breaking flaws to include embedded flaws (for a/c = 0.8 and tensile loading only). The paper also includes results of the experimental work completed to-date.Copyright