P. John Bouchard

A Novel Cutting Strategy for Reducing Plasticity Induced Errors in Residual Stress Measurements Made With the Contour Method

The contour method (CM) has emerged as a valuable technique for the measurement of residual stresses (RS). The method involves cutting the sample in which residual stresses are to be measured, using wire electric discharge machining (EDM), and measuring the deformation that occurs on the newly created surface, which can be related to the residual stresses that existed beforehand. The contour method provides a full 2-D map of the stresses acting in a direction normal to the plane of the cut. It is ideally suited to measurements in power plant welds since, unlike diffraction-based techniques, it is not affected by microstructure gradients, and it is well suited to thick section components. However, as with other mechanical strain relief techniques, it is prone to errors arising from plasticity when residual stresses close to the yield strength of material are encountered. This paper describes contour method measurements in an AISI Type 316L austenitic steel three-pass. A novel cutting and restraint strategy is applied in an attempt to reduce plasticity errors following optimisation studies simulating the cutting process using the finite element (FE) method.

Advances in Residual Stress Modeling and Measurement for the Structural Integrity Assessment of Welded Thermal Power Plant

The safe operation of both thermal and nuclear power plant is increasingly dependent upon structural integrity assessment of pressure vessels and piping. Furthermore, structural failures most commonly occur at welds so the accurate design and remnant life assessment of welded plant is critical. The residual stress distribution assumed in defect assessments often has a deciding influence on the analysis outcome, and in the absence of accurate and reliable knowledge of the weld residual stresses, the design codes and procedures use assumptions that yield very conservative assessments that can severely limit the economic life of some plant. However, recent advances in both the modeling and measurement of residual stresses in welded structures and components open up the possibility of characterising weld residual stresses in operating plant using state-of–the–art fully validated Finite Element simulations. This paper describes research undertaken to predict residual stresses in stainless steel welds in order to provide validated reliable, accurate Structural Integrity assessment of nuclear power plant components

Measurements of Residual Stress in a Welded Compact Tension Specimen Using the Neutron Diffraction and Slitting Techniques

The residual stress field in a compact tension specimen blank extracted from a non-stress-relieved thick section butt weld has been measured using neutron diffraction and the slitting method. Significant triaxial residual stresses were found in the specimen that is normally assumed to be stress free. Moreover the level of stress was sufficient to make a significant contribution to the crack driving force in creep crack growth tests. The benefits of using more than one measurement technique in such investigations are demonstrated.

Optimising residual stress measurements and predictions in a welded benchmark specimen: a review of Phase two of the NeT Task Group 1 single bead on plate round robin

A single weld bead deposited on a flat plate is a deceptively simple problem that is in practice a challenge for both measurement and prediction of weld residual stresses. Task Group 1 of the NeT collaborative network has examined this problem in a two-phase programme extending from 2002 to 2008. Ten independent sets of residual stress measurements have been reported using diverse techniques, and over forty finite element simulations have been performed. This paper reviews Phase 2 of the Task Group 1 round robin. Here, the finite element simulations all made use of optimised thermal solutions, in which the global welding parameters, including efficiency, were fixed, and only the detailed heat source geometry was varied. These resulted in accurate far field welding temperature distributions, with significant variability only close to the weld bead itself. The subsequent mechanical analyses made use of kinematic, isotropic, and mixed isotropic-kinematic material constitutive models, and made a variety of assumptions about the introduction of weld filler material to the structure and the handling of high temperature inelastic strains. The large database of measurements allowed the derivation of statistical best estimates using a Bayesian “duff data” approach, and these best estimates were compared with the predictions to establish the most accurate material constitutive models. The most accurate predictions of residual stress were made using non-linear kinematic or mixed isotropic-kinematic constitutive models. The methods used to handle high-temperature inelastic strains influenced the predicted stresses only in regions where very high temperatures were predicted during welding. The results emphasise the importance and value of both well-characterised benchmark problems and international collaboration in the development of technologies to both measure and predict weld residual stresses.Copyright

Determination and mitigation of the uncertainty of neutron diffraction measurements of residual strain in large-grained polycrystalline material

For large-grained samples it is advantageous to perform pairs of neutron diffraction measurements at the same spatial location but rotated 180° around the geometric centre of the gauge volume as a means of minimizing the scatter coming from the random positioning of grains within the gauge volume.

Residual Stress Measurement of a Ferritic Bead on Plate Benchmark Test Specimen Using the Contour Method

This paper presents new measured residual stress data for a ferritic weld bead on plate benchmark specimen. The contour residual stress measurement method was applied because it could provide an informative map on the plane of interest capable of quantifying local variations in residual stress associated with phase transformations. The quality of the contour cut was optimised by carrying out initial cutting trials to tune the electro-discharge machining cutting parameters. The risk of plasticity during the cutting procedure was minimised by using an “embedded crack” contour cutting configuration, clamping with fitted bolts and a large diameter cutting wire in order to reduce the crack opening and stress concentration at the cut tip. The results of the contour method measurement are in reasonable overall agreement with published stresses measured by neutron diffraction. However, certain features in the results suggest that significant plasticity did occur during the contour cut despite the specific measures taken to minimise this effect.

Optimised Neural Network Prediction of Residual Stress Profiles for Structural Integrity Assessment of Pipe Girth Welds

Defect assessment procedures such as BS7910, R6, FITNET and API 579-1 provide simplified estimates or upper bound profiles that can be used to characterize residual stresses present in a weld. Some of the bounding through-thickness profiles used in these procedures are designed based on expert judgment and examination of residual stress measurements that exhibit wide scatter. As a consequence, structural integrity assessment of defects in welded components can be overly conservative by a large margin, and may lead to unnecessary and costly repair or inspection. This paper presents a novel approach based on artificial neural networks to predict residual stress profiles in austenitic stainless steel pipe girth welds. The network is trained using a set of baseline experimental residual stress data and then validated using previously unseen data. The committee of networks has been optimized using a Bayesian approach and the upper bound curve is determined from the histogram network of output distributions. The performance and suitability of the neural network approach is discussed by comparison with stress profiles recommended in the R6 procedure and followed by an assessment of whether the use of neural network bounding profiles can lead to non-conservative estimates of stress intensity factor in fracture assessments.Copyright

The Validation of Weld Residual Stresses for Use in Structural Integrity Assessment

The continued safe and reliable operation of plant invariably has to consider the assessment of defects in welded structural components. This requires some estimate of the residual stresses that have developed during the welding fabrication process. For as-welded structures these stresses can be of yield magnitude. Engineering critical assessment procedures such as R6, BS 7910, FITNET and API 579-1 provide simplified estimates, bounding profiles or advice on detailed analysis or measurement which can be applied to provide conservative estimates of the remaining life of plant. The use of finite element analysis (FEA) is being applied more frequently to predict residual stresses in welded components for assessment purposes. This calculation involves complex non-linear analyses with many assumptions. As a consequence, the accuracy and reliability of solutions is variable. In order to improve the consistency of weld modelling, and hence the accuracy and confidence in their use, a set of Guidelines covering the calculation of residual stresses have been developed. The residual stress calculations need to be validated before the results can be used in assessments and guidance on how to demonstrate the required standard of validation proof is provided with these Guidelines. The level of validation required, depends on the problem being solved and the sensitivity of the assessment to the presence of residual stress. For example a high level of validation may be required for assessments of safety critical plant. To support these calculations, measurements are required and a series of ‘Weld Residual Stress Benchmarks’, describing welded mock-ups which have been measured using various measurement techniques, are being collated which the users can then refer to when validating their finite element modelling techniques and thus provide a greater confidence in the predicted results.

Modelling and Measuring Residual Stresses in Pipe Girth Welds: Lessons From the Style Framework 7 Project

A number of girth-welded pipe mock-ups have been manufactured and investigated during the STYLE project, using a wide range of measurement techniques accompanied by extensive finite element simulation campaigns. This paper gives an overview of the work carried out and presents preliminary conclusions on the performance of finite element weld residual stress simulation techniques in the different mock-up designs.Copyright

Internal stress generation in austentic stainless steels during creep deformation

It has been well established in materials such as austenitic steels and aluminium alloys that plastic strain leads to generation of internal stresses. A number of intergranular factors such as the anisotropic stiffness and yield behaviour of a single crystal, orientation of grain families to the loading direction, and the constraint each grain places on its neighbours are responsible for creating these stresses. The presence of these accumulated internal stresses in power plant components is important because they interact with the applied external stress and play a critical role in the initiation and development of material degradation that may lead to eventual failure. This study focuses on measuring the generation of internal (intergranular) strains and stresses in austenitic stainless steels subjected to creep deformation. Creep processes increase the overall inelastic strain in a material and this correspondingly alters the internal strain state. A combination of in-situ and static neutron diffraction measurements was conducted to assess the internal strain generation through a material‟s creep life. These experiments have revealed similarities between internal strain generation during primary creep deformation and that during monotonic tensile deformation leading to the conclusion that common mechanisms may be responsible. Such studies are important in the current context when a number of power plants are being life extended. Components in high temperature service applications undergo a number of creep-fatigue cycles during their operation. It is vital to have accurate and robust life assessment procedures that can take account of long-term internal strain evolution effects to maintain economic but safe operation and avoid costly repairs or replacement.