Andrew H. Sherry

Methods for including constraint effects within the SINTAP procedures

Abstract To enable constraint-based approaches to be applied to the practical assessment of defective components, development work within SINTAP has addressed a number of issues: constraint parameters for surface defects, constraint parameters for secondary stresses, simplified weight function methods for calculation of T-stress and relevant fracture toughness data. This paper describes these developments and indicates how the methods have been included in the overall SINTAP procedures.

Preliminary Evaluation of Digital Image Correlation for In-situ Observation of Low Temperature Atmospheric-Induced Chloride Stress Corrosion Cracking in Austenitic Stainless Steels

Digital image correlation has been used to observe the growth of atmospheric-induced chloride stress corrosion cracking in type 304L stainless steel under controlled conditions of temperature, relative humidity and chloride-deposition density in a nondestructive manner. The technique is capable of detecting changes in crack dimensions that are difficult to discern via conventional optical microscopy, i.e. crack growth beneath salt layers and adherent corrosion product deposits, and measurement of crack opening displacements. Our results also demonstrate that suitable specimen design, combined with digital image correlation, will provide the means of comparing the growth behaviour of short atmospheric-induced chloride stress corrosion cracks with data obtained from conventional pre-cracked compact tension specimens as a function of mechanical “driving force”.

The creep fracture of a single-crystal superalloy

Abstract The creep fracture of the single-crystal superalloy SRR99 at 750°C was studied for crystals oriented close to the [001] and [011] directions. The results indicate that the fracture processes operating within both orientations are highly dependent on the deformation mechanisms which operate during creep deformation. In [001] orientations, homogeneous deformation leads to the formation of macroscopically “square” mode I cracks with sides parallel to 〈011〉 directions. These cracks propagate through the γ′ precipitates from microporosity. Final rupture in this orientation occurs by the interlinkage of these mode I cracks by fracture along four {111} planes when the resolved shear stress on the remaining material is equal to the critical resolves shear stress of the alloy. Conversely, failure in [011] orientations occurs in a fast and catastrophic manner, with a single crack growing along heterogeneous shear bands which lie along {111} type planes.

Influence of Pd and Ru additions on stress corrosion cracking of austenitic stainless steels

Abstract The influence of minor alloying platinum group metal additions on stress corrosion cracking (SCC) of austenitic stainless steels has been investigated in simulated pressurised water reactor environments. Crack propagation studies, employing tensile specimens precracked in acidified potassium tetrathionate and subsequently loaded in proving rings revealed that 1 wt-% ruthenium additions improved the resistance of 304 stainless steels to SCC. Analytical transmission electron microscopy was used to characterise crack tips and surface oxides after high temperature SCC tests. Enrichment of ruthenium and molybdenum within the dual oxide surface layers was observed in the ruthenium modified stainless steel, which is likely to have occurred during potassium tetrathionate pre-exposure and promoted intergranular SCC mitigation. Palladium additions (1 wt-%) showed no beneficial effects to SCC, and this was associated with the formation of second phases PdMn particles upon sensitisation, which decreased the availability of Pd.

Stress corrosion cracking of Ru doped 304 stainless steel in high temperature water

Abstract In pressurised water reactor primary water, there is generally growing concern that stress corrosion cracking (SCC) may occur in occluded locations where residual oxygen and other impurities may be trapped. For these critical components, the deployment of more SCC resistant materials is desirable. In this paper, the effect of 1 wt-%Ru additions on the SCC susceptibility of 304 austenitic stainless steels was investigated in high temperature water. Slow strain rate tensile tests were performed on standard and 1 wt-%Ru modified 304 stainless steels in both sensitised and cold worked conditions. Preliminary results showed that, although both ruthenium doped and standard 304 stainless steels exhibited intergranular SCC, the former was less susceptible as indicated by a greater strain to failure. The results obtained suggest an improved performance of the Ru doped 304 stainless steel towards SCC susceptibility in these environments.

Stress corrosion cracking of sensitized type 304 stainless steel in high-temperature water with anionic impurities contamination

This paper describes some results selected from a larger program that was aimed at understanding the stress corrosion cracking (SCC) initiation of Type 304 stainless steel (UNS S30400) in high-temperature deaerated water. Out of a large number of statically loaded samples, only a small minority of the tested samples underwent SCC. The occurrence of SCC indicates a synergism between sensitization, ionic impurities (mainly chloride and sulfate), and/or superficial defects and cold work. In fact, none of the nonsensitized materials initiated cracking (within the time scale of the tests), while only three sensitized samples underwent extensive SCC. The crack morphology of the fractured sample was predominantly inter-granular with some transgranular regions. Transmission electron microscopic samples containing crack tips were, in most respect, in line with the literature: a magnetite/spinel duplex layer on the crack surfaces, a Cr-rich oxide at the crack tip, and Ni enrichment at the metal/oxide interface and ...

Fracture toughness evaluation in C(T) specimens with reduced out-of-plane constraint

During fracture toughness testing of C(T) specimens, an important assumption is that the test specimen is highly constrained. This is ensured by testing a deeply cracked specimen, with in-plane and out-of-plane dimensions that are sufficient to guarantee an appropriate level of crack tip stress triaxiality. This condition guarantees that high-constraint fracture toughness values are derived, conservative for use in standard fracture mechanics assessments.In reality, many components have small out-of-plane dimensions (small thicknesses). This often causes a reduction in crack tip constraint of a sufficient amount to increase the effective fracture toughness of the components. However, there is currently limited understanding as to the magnitude of the benefits that could be claimed from out-of-plane constraint loss.Finite element and damage modelling of thin C(T) specimens under different loading conditions has been undertaken, looking at the effects of loss of out-of-plane constraint, to help validate the results of an on-going testing programme. When available, data from testing of thin C(T) specimens could allow the parameters of the damage model, based upon a ductile criterion, to be calibrated. Material resistance to fracture under different situations has been determined, leading to a correlation of toughness to the constraint condition for a nominal set of material parameters.Copyright

Residual Stress Driven Creep Cracking in Type 316 Stainless Steel

Specially designed AISI Type 316H austenitic stainless steel 25 mm thick compact tension specimens have been plastically deformed to produce significant tensile hydrostatic residual stresses at the notch root at mid-thickness. These specimens were thermally exposed at 550 °C for 4500 hours in order to study elevated temperature creep relaxation of residual stress and the development of reheat cracking creep damage. Residual strains within the specimens were measured using diffraction techniques before and after thermal exposure. A three-dimensional finite element model was developed both to predict the residual stress within the specimens before and after thermal exposure. No reheat cracking was found near surface, but due to the reduced creep ductility with increasing hydrostatic stress, significant creep cavitation was found mid-thickness. A previously developed creep damage model was applied to predict the onset of reheat cracking. Good correlation has been found between measurements and finite element predictions of strain and stress before and after thermal exposure. The extent of creep damage has also been assessed through destructive examination, providing validation for the creep damage prediction model.

Developments in Local Approach methodology with application to the analysis/re-analysis of the NESC-1 PTS benchmark experiment

Abstract Local Approach methods have received considerable attention in recent years as a complementary approach to structural integrity assessment. These approaches are based on the application of micro-mechanistic models of failure in which the stress, strain and ‘damage’ local to the crack-tip are related to the critical conditions required to initiate/propagate fracture. The models are calibrated in terms of material parameters that are deemed fully transferable and derived using a combination of reference test data and supporting stress analysis. Once calibrated, using small-scale test data, the models are assumed independent on geometry and loading configuration. For a given failure mechanism, the model parameters may be used in the assessment of a structure fabricated from the same material (for appropriate temperatures, loading rates, etc). The paper describes the work initially undertaken in relation to the NESC-1 (Network for Evaluating Steel Components) spinning cylinder test, in order to validate the application of Local Approach methods for the case of PTS loading. The predicted amount of pre-cleavage ductile tearing and the timing of the subsequent cleavage event are compared with the observed fracture behaviour of the defect. The paper then highlights several areas in which Local Approach methodology has been developed since the initial work on PTS. These include: • Calibration of the cleavage model across a range of temperatures and constraint states. • Treatment of 3D defects. • Reference to hydrostatic stress in cleavage fracture predictions. • Simplification and standardisation of analytical techniques for more routine use in integrity assessments. The paper concludes that results from large-scale structural experiments, such as the NESC-1 spinning cylinder test, will be of lasting value in validating developments in Local Approach methodology and other advanced methods of fracture assessment. This is particularly true in the authors’ current work that seeks to achieve an overall simplification in methodology, without sacrificing predictive accuracy.

TAGSI views on aspects of crack arrest philosophies for pressure vessels with thicknesses up to 100 mm

Abstract As part of its ongoing technical programme the UK nuclear industrys Technical Advisory Group on Structural Integrity (TAGSI) was asked to comment on three issues concerning use of crack arrest arguments in structural integrity assessments for pressure vessel applications. The paper summarises the content of the TAGSI discussions and conclusions.