Mitchell D. Olson

Measured Biaxial Residual Stress Maps in a Stainless Steel Weld

Here, this paper describes a sequence of residual stress measurements made to determine a two-dimensional map of biaxial residual stress in a stainless steel weld. A long stainless steel (316L) plate with an eight-pass groove weld (308L filler) was used. The biaxial stress measurements follow a recently developed approach, comprising a combination of contour method and slitting measurements, with a computation to determine the effects of out-of-plane stress on a thin slice. The measured longitudinal stress is highly tensile in the weld- and heat-affected zone, with a maximum around 450 MPa, and compressive stress toward the transverse edges around ₋250 MPa. The total transverse stress has a banded profile in the weld with highly tensile stress at the bottom of the plate (y = 0) of 400 MPa, rapidly changing to compressive stress (at y = 5 mm) of ₋200 MPa, then tensile stress at the weld root (y = 17 mm) and in the weld around 200 MPa, followed by compressive stress at the top of the weld at around ₋150 MPa. Finally, the results of the biaxial map compare well with the results of neutron diffraction measurements and output from a computational weld simulation.

Characterisation of residual stresses in heat treated, high strength aluminium alloy extrusions

Residual stresses were measured in rectilinear aluminium bars quenched using an aqueous polyoxyethylene glycol (PAG) solution or cold-water. Residual stresses were measured with neutron diffraction and a superposition-based method using mechanical strain release measurements. Three orthogonal stress components were measured along two transverse lines using neutron diffraction. The longitudinal residual stresses were mapped over a transverse cross-section using the contour technique. A primary slice removal technique mapped three orthogonal residual stresses over a transverse cross-section in the PAG extrusion. Residual stresses were found to vary from biaxial compressive in the part boundaries to triaxial tensile in the interiors. There was close correlation between the neutron diffraction and mechanical strain release techniques. PAG quenching demonstrated lower residual stresses. This paper is part of a Themed Issue on Measurement, modelling and mitigation of residual stress.

Biaxial Residual Stress Mapping for a Dissimilar Metal Welded Nozzle

This paper describes a sequence of residual stress measurements made to determine a two-dimensional map of biaxial residual stress in a nozzle mockup having two welds, one a dissimilar metal (DM) weld and the other a stainless steel (SS) weld. The mockup is cylindrical, designed to represent a pressurizer surge nozzle of a nuclear pressurized water reactor (PWR), and was fabricated as part of a weld residual stress measurement and finite-element (FE) modeling round-robin exercise. The mockup has a nickel alloy DM weld joining an SS safe end to a low-alloy steel cylinder and stiffening ring, as well as an SS weld joining the safe end to a section of SS pipe. The biaxial mapping experiments follow an approach described earlier, in PVP2012-78885 and PVP2013-97246, and comprise a series of experimental steps and a computation to determine a two dimensional map of biaxial (axial and hoop) residual stress near the SS and DM welds. Specifically, the biaxial stresses are a combination of a contour measurement of hoop stress in the cylinder, slitting measurements of axial stress in thin slices removed from the cylinder wall, and a computation that determines the axial stress induced by measured hoop stress. At the DM weld, hoop stress is tensile near the OD (240 MPa) and compressive at the ID (−320 MPa), and axial stress is tensile near the OD (370 MPa) and compressive near the midthickness (−230 MPa) and ID (−250 MPa). At the SS weld, hoop stress is tensile near the OD (330 MPa) and compressive near the ID (−210 MPa), and axial stress is tensile at the OD (220 MPa) and compressive near midthickness (−225 MPa) and ID (−30 MPa). The measured stresses are found to be consistent with earlier work in similar configurations.

Further Comments on Validation Approaches for Weld Residual Stress Simulation

Tensile weld residual stress (WRS) in the presence of primary water has been identified as a main driver for stress corrosion cracking in dissimilar metal welds found in the cooling circuit of pressurized water reactors. Thus, WRS data are needed to support plant management decisions, and so are of interest to a broad range of stakeholders in pressure vessel technology. In an effort to validate predictive WRS models and quantify modeling uncertainty, the U.S. Nuclear Regulatory Commission (NRC) and the Electric Power Research Institute (EPRI) have cooperatively organized a research program on WRS. This paper is a follow-up to earlier work, which described a series of WRS data analysis methods that provide a range of figures of merit, from simple (e.g., root mean square difference from a benchmark) to complex (e.g., comparison of predicted crack growth behavior), that can be used to judge WRS data quality. The present work applies those methods to assess model outputs developed during the second NRC/EPRI international round robin study (Phase 2b) on WRS modeling. The results of exercising the data analysis methods are presented, and compared to results obtained from an analysis of data from the previous round robin (Phase 2a).Copyright

Residual Stress Mapping with Multiple Slitting Measurements

This paper describes the use of slitting to form a two-dimensional spatial map of one component of residual stress in the plane of a two-dimensional body. Slitting is a residual stress measurement technique that incrementally cuts a thin slit along a plane across a body, while measuring strain at a remote location as a function of slit depth. Data reduction, based on elastic deformation, provides the residual stress component normal to the plane as a function of position along the slit depth. While a single slitting measurement provides residual stress along a single plane, the new work postulates that multiple measurements on adjacent planes can form a two-dimensional spatial map of residual stress. The paper uses numerical simulations to estimate the quality of a slitting measurement, relative to the proximity of a given measurement plane to a free surface, whether that surface is the edge of the original part or a free surface created by a prior measurement. The results of the numerical simulation lead to a recommended measurement design for mapping residual stress. Finally, the numerical work and recommended measurement strategy are validated with physical experiments using thin aluminum slices containing residual stress induced by quenching. The physical experiments show that two-dimensional residual stress mapping with slitting has precision on the order of 10 MPa.

A Novel Approach for Biaxial Residual Stress Mapping Using the Contour and Slitting Methods

This paper describes a residual stress measurement approach that determines a two-dimensional map of biaxial residual stress. The biaxial measurement is a combination of contour method, slitting method measurements on a thin slices removed adjacent to the contour plane, and a computation to account for the effects of slice removal. The measurement approach uses only mechanical stress release methods, which is advantageous for some measurement articles. The measurement approach is verified with independent confirmation measurements. Biaxial mapping measurements are performed in a long aluminum bar (77.8 mm width, 51.2 mm thickness, and 304.8 mm length) that has residual stresses induced with quenching. The measured stresses are consistent with quench induced residual stress, having peak magnitude of 150 MPa and a distribution that is tensile toward the center of the bar and compressive around the boundary. The validating confirmation measurement results have good agreement with results from the biaxial mapping approach.

Contour Method Residual Stress Measurement Uncertainty in a Quenched Aluminum Bar and a Stainless Steel Welded Plate

This paper describes a newly developed uncertainty estimate for contour method residual stress measurements and presents results from two experiments where the uncertainty estimate was applied. The uncertainty estimate includes contributions from random error sources including the error arising from noise in displacement measurements and the smoothing of the displacement surfaces. The output is a two-dimensional, spatially varying uncertainty estimate such that every point on the cross-section where residual stress is determined has a corresponding uncertainty value. The current paper describes the use of the newly developed uncertainty estimate in a quenched aluminum bar with a cross section of 51 × 76 mm and a stainless steel weld plate with a cross-section of 25.4 × 152.4 mm, with a 6.35 mm deep groove, filled with a multi-pass weld. The estimated uncertainty in the quenched aluminum bar is approximately 5 MPa over the majority of the cross-section, with localized areas of higher uncertainty, up to 10 MPa. The estimated uncertainty in the welded stainless steel plate is approximately 22 MPa over the majority of the cross-section, with localized areas of higher uncertainty, over 50 MPa.

Numerical Analysis of Weld Residual Stress in a Pressurizer Surge Nozzle Full-Scale Mockup: The Effect of Hardening Constitutive Model and Interpass Temperature

In an effort to shed light on accuracy and reliability of finite element (FE) weld modeling outputs, the U.S. Nuclear Regulatory Commission (NRC) and the Electric Power Research Institute (EPRI) have been engaged in a program of cooperative research on weld residual stress (WRS) prediction. The current work presents numerical FE simulation of the WRS in a pressurizer surge nozzle full-scale mockup (Phase 2b), as a part of the broader NRC/EPRI program. Sequentially-coupled, thermo-mechanical FE analysis was performed, whereby the numerical solution from the thermal analysis was used as an input in the mechanical analysis. The thermal analysis made use of a dedicated weld modeling tool to accurately calibrate an ellipsoidal Gaussian volumetric heat source. The subsequent mechanical analysis utilized the isotropic and nonlinear kinematic hardening constitutive models to capture cyclic response of the material upon welding. The modeling results were then validated using a number of measurement techniques (deep hole drilling, contour method, slitting, and biaxial mapping). In addition, an effect of the interpass temperature (i.e. 24.5 °C, 150 °C, and 260 °C) on the final prediction of WRS is discussed.Copyright

Finite Element Modelling of Welded Austenitic Stainless Steel Plate With 8-Passes

The current paper presents a finite element analysis of an eight-pass groove weld in a 316L austenitic stainless steel plate. A dedicated welding heat source modelling tool was employed to produce volumetric body power density data for each weld pass, thus simulating weld-induced thermal loads. Thermocouple measurements and cross-weld macrographs taken from a weld specimen were used for heat source calibration. A mechanical finite element analysis was then conducted, using the calibrated thermal loads and a Lemaitre-Chaboche mixed work-hardening model. The predicted post-weld residual stresses were validated using contour method measurements: good agreement between measured and simulated residual stress fields was observed. A sensitivity analysis was also conducted to identify the boundary conditions that best represent a tack-welded I-beam support, which was present on the specimen back-face during the welding.Copyright

Simulation of Triaxial Residual Stress Mapping for a Hollow Cylinder

This paper describes a novel method to determine a two-dimensional map of the triaxial residual stress on a radial-axial plane of interest in a hollow cylindrical body. With the description in hand, we present a simulation to validate the steps of the method. The simulation subject is a welded cylindrical nozzle typical of a nuclear power pressurized water reactor pressurizer; in the weld region, the nozzle inner diameter is roughly 132mm (5.2in.) and the wall thickness is roughly 35mm (1.4in.). The pressure vessel side of the nozzle is carbon steel (with a thin stainless steel lining), the piping side is austenitic stainless steel, and between the two are weld and buttering deposits of nickel alloy. Weld residual stresses in such nozzles have important effects on crack growth rates in fatigue and stress corrosion cracking, therefore measurements of weld residual stress can help provide inputs for managing aging reactor fleets. Nuclear power plant welds often have large and complex geometry, which has made residual stress measurements difficult, and this work validates a new experimental technique for measurements on welded nozzles.