Robert C. Wimpory

Measurement of residual stresses in T-plate weldments

Tensile welding residual stresses can, in combination with operating stresses, lead to premature failure of components by fatigue and/or fracture. It is therefore important that welding residual stresses are accounted for in design and assessment of engineering components and structures. In this work residual stress distributions, obtained from measurements on a number of ferritic steel T-plate weldments using the neutron diffraction technique and the deep-hole drilling method, are presented. It has been found that the residual stress distributions for three different plate sizes are of similar shape when distances are normalized by plate thickness. It has also been found that the conservatisms in residual stress profiles recommended in current fracture mechanics-based safety assessment procedures can be significant—of yield strength magnitude in certain cases. Based on the data presented here a new, less-conservative transverse residual stress upper bound distribution is proposed for the T-plate weldment geometry. The extent of the plastic zone developed during the welding process has also been estimated by use of Vickers hardness and neutron diffraction measurements. It has been found that the measured plastic zone sizes are considerably smaller than those predicted by existing methods. The implications of the use of the plastic zone size as an indicator of the residual stress distributions are discussed.

The New Materials Science Diffractometer STRESS-SPEC at FRM-II

In response to the development of new materials and the application of materials and components in new technologies the direct measurement, calculation and evaluation of textures and residual stresses has gained worldwide significance in recent years. Non-destructive analysis for phase specific residual stresses and textures is only possible by means of diffraction methods. In order to cater for the development of these analytical techniques the new Materials Science Diffractometer STRESS-SPEC at FRM-II is designed to be equally applied to texture and residual stress analyses by virtue of its flexible configuration. The system compromises a highly flexible monochromator setup using three different monochromators: Ge (511), bent silicon (400) and pyrolitic graphite (PG). This range of monochromators and the possibility to vary the take-off angles from 2θM = 35º to 110º allows wavelength adjustment such that measurements can be performed around a scattering angle of 2θS ~ 90º. This is important in order to optimise neutron flux and resolution, especially for stress analysis on components, since the gauge volume element in that case is cubic and large vertical divergences due to focusing monochromators do not affect the spatial resolution. The instrument is now available for routine operation and here we will present details of recent experiments and instrument performance.

Computational and Experimental Studies of High Temperature Crack Initiation in the Presence of Residual Stress

Residual stresses have been introduced into a notched compact tension specimen of a 347 weld material by mechanical compression. The required level of compressive load has previously been determined from finite-element studies. The residual stress in the vicinity of the notch root has been measured using neutron diffraction and the results compared with those obtained from finite-element analysis. The effect of stress redistribution due to creep has been examined and it is found that a significant reduction in stress is measured after 1000 h at 650°C. The implications of these results with regard to the development of damage in the specimen due to creep relaxation are examined.

The New and Upgraded Neutron Instruments for Materials Science at HMI - Current Activities in Cooperation with Industry

Recent progress in engineering includes the development of new materials and innovations in their processing and treatments. Material technologies, like the study of metals, alloys, ceramics and composites, especially non-destructive analyses of residual stresses profiles and textures, have gained an increasing importance. The dedicated residual stress diffractometers E3 and E7 at BENSC, HMI, Berlin are already equipped with new two-dimensional position sensitive detectors. An upgrade of the monochromator system is planned for 2006 which includes perfectly bent silicon crystals in order to optimise both intensity and angular resolution yielding a large gain of the diffractometer efficiency for strain measurements. A range of equipment for sample positioning is available, such as a closed Eulerian cradle for samples with weights of up to 5 kg, a second cradle for heavy samples (up to 50 kg) with the ability to tilt the samples up to 90° and a translation table carrying samples of up to 300 kg and 1000 mm in diameter. Gauge volumes can be adjusted by a new computer controlled variable slitsystem in a range from 1x1x1 mm³ up to several mm³. In-situ residual stress analysis can be performed within industrial components during mechanical or thermal loading (up to 2000 K). Rapid data visualization as well as evaluation is performed by the specially designed software. The powder diffraction pattern is calculated by summation over the scattering angle dependent Debye- Scherrer lines on the two-dimensional 400*400 mm² planar area detector. A large amount of beam time is exclusively used for industrial research. Among the components that were investigated are crankshafts, impellers, pistons, cylinder heads, turbine blades and welds. Both instruments are similarly designed, where E3 is set up for higher flux and therefore penetration depths and E7 is designed for higher angular resolution.

Stress Intensity Factors Due to Residual Stresses in T-Plate Welds

Residual stress distributions in ferritic steel T-plate weldments have been obtained using the neutron diffraction method. It is shown that the transverse residual stress distribution for two plates of different yield strength are of similar shape and magnitude when normalized appropriately and peak stresses are on the order of the material yield strength. The resultant linear elastic stress intensity factors for these stress distributions have been obtained using the finite element method. It has been shown that the use of the recommended residual stress distributions in UK structural integrity procedures leads to a conservative assessment. The stress intensity factors for the welded T-plate have been shown to be very similar to those obtained using a smooth edge cracked plate subjected to the same local stress field.

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

Specimen Geometry Effects on Creep Crack Initiation and Growth in Parent Materials and Weldments

High temperature crack growth in weldments is of great practical concern in high temperature plant components. Cracking typically occurs in the heat affected zone (HAZ) and often propagates into adjacent parent material (PM). Recently, the importance of constraint effects on creep crack growth behaviour has been recognised and creep crack growth testing on a range of specimen geometries has been performed. Experimental crack growth testing has been performed at 550 °C on a range of fracture specimens using sections taken from a non-stress-relieved 316 steel weldment. These specimens include the compact tension, C(T), middle tension, M(T) and circumferentially cracked bar, CCB, geometries. Results are presented from two long-term creep crack growth (CCG) tests performed on M(T) weldment specimens and these are compared with available data on C(T) and CCB weldment specimens together with both long and short term tests on parent material for a range of specimen geometries. The creep crack initiation (CCI) and growth (CCG) behaviour from these tests has been analysed in terms of the C* parameter. As high levels of residual stress exist in non-stress-relieved weldments, the residual stresses remaining in the weldment specimens have therefore been quantified using the neutron diffraction technique. Long-term (low-load) tests are required on PM specimen to observe specimen constraint effects in 316 steel at 550 °C. When interpreted in terms of the C* parameter the CCG behavior of PM and Weldment materials follow the same trendline on low constraint geometries. However, significant difference is observed in the CCG behavior of PM and weldments on the high constraint C(T) geometry. Long term tests on C(T) specimen weldments are required to confirm the results found.Copyright

Combined Neutron and X-Ray Diffraction Analysis for the Characterization of a Case Hardened Disc

The present work has been executed within the framework of the collaborative research center on Distortion Engineering (SFB 570) in order to evaluate the residual stress state of a disc after carburizing and quenching as well as to validate a simulation procedure. The combined use of X-ray and neutron diffraction analysis provided information about the residual stress states in the whole cross section. However, the stress free lattice spacing d0 for the neutron diffraction experiments is problematic and induces systematic uncertainties in the results and the application of a force balance condition to recalculate d0 might be a solution for improving the reliability of the results. Comparison of experimental results with simulation showed that an overall satisfying agreement is reached but discrepancies are still present.

Effect of residual stress on high temperature deformation in a weld stainless steel

This paper considers the measurement of residual stresses induced by mechanical loading in a weld Type 347 stainless steel. The work is based in part on an ongoing Round Robin collaborative effort by the Versailles Agreement on Materials and Standards, Technical Working Area 31, (VAMAS TWA 31) working on ‘Crack Growth of Components Containing Residual Stresses’. The specific objective of the work at Imperial College London and HMI, Berlin is to examine how residual stresses and prior straining and subsequent relaxation at high temperature contribute to creep crack initiation and growth for steels relevant to power plant applications. Tensile residual stresses have been introduced in the weld by pre-compression and neutron diffraction measurements have been carried out before and after stress relaxation at 650 oC. Significant relaxation of the residual stresses has been observed, in agreement with earlier work on a stainless steel. Preliminary results suggest that the strains local to the crack drop by over 60% after 1000 h relaxation at 650 oC for the weld steel. The results have been compared with finite element studies of elastic-plastic pre-compression and stress relaxation due to creep.

Residual Stress and Microstructural Variations in Thick Aluminium Alloy Forgings

The most critical stage in the heat treatment of high strength aluminium alloys is the rapid cooling necessary to form a supersaturated solid solution. During cold water quenching of thick sections, the thermal gradients are sufficient to cause inhomogeneous plastic deformation which in turn leads to the development of large residual stresses. Two 215 mm thick rectilinear forgings made from 7075 and 7010 were heat treated, and the through thickness residual stresses measured by neutron diffraction and deep hole drilling. The distribution of residual stresses was found to be similar for both alloys varying from highly triaxial and tensile in the core to a state of biaxial compression in the surface. The 7010 forging exhibited significantly larger tensile stresses in the core. 7075 is a much more quench sensitive alloy when compared to 7010. This results in loss of supersaturation by second phase precipitation during quenching in the core of the 7075 forging.