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Summary of Public Meeting and Webinar on the NRC/EPRI Weld Residual Stress Validation Program

December 18, 2014

Meeting Kick-Off The meeting convened on December 15, 2014 at 1:30 pm Eastern Standard Time in room 6-B1 at 21 Church Street, Rockville, MD. The meeting materials, including presentation slides, are attached to this document. The purpose of this meeting was to discuss the dataset obtained from an international finite element round robin study for the prediction of weld residual stress (WRS). Formal Presentations Michael Benson of the U.S. Nuclear Regulatory Commission (NRC) provided a presentation on NRC’s perspective on the Phase 2b round robin dataset. The NRC presentation provided a historical overview of the WRS Validation Program, as well as basic plots of the measurement and modeling data obtained from the round robin. Going forward, the NRC will be performing additional analysis of the data, including flaw growth calculations and statistical characterization. Michael Hill of the University of California, Davis provided a presentation on the Electric Power Research Institute’s (EPRI) perspective on the round robin dataset. The EPRI presentation provided additional analysis not discussed in the NRC talk, including proposing a preliminary benchmark (presentation materials used average of all modeling results, but a benchmark based on measurement data may be more suitable) and discussing potential outliers. Future work for EPRI includes developing WRS guidelines for the American Society of Mechanical Engineers Boiler and Pressure Vessel Code. Open Discussion The Open Discussion portion of the meeting provided an opportunity for the meeting attendees to confer on the challenges identified in the formal presentations moving forward in the research program. A number of technical issues were raised, including:

• Applying the room-temperature WRS profiles from the round robin to calculations related to high-temperature service.

• Including pre-stainless steel weld stress profiles in statistical analyses • Separating ANSYS and ABAQUS results to examine the effect of software tool. • Determining areas not specified well in the Phase 2b problem statement that may have

contributed to the observed scatter in the results (e.g., backweld geometry specification). • Accounting for other stress paths other than the dissimilar metal weld centerline. • Further examining the validity of supplied hardening law parameters. • Considering previously-proposed benchmarks and acceptance criteria for weld residual

stress finite element modeling (e.g., ± 10 ksi-in.0.5 on stress intensity factor benchmark). • Considering test case demonstration for acceptance of finite element modeling.

• Accounting for both measurement and modeling uncertainty when benchmarking finite element models against measurements

Action Items

1. Bud Brust, Engineering Mechanics Corporation of Columbus: interact with outside technical experts on hardening law parameters.

2. John Broussard, Dominion Engineering: set up a meeting on Wednesday of the next ASME Code Week for discussing acceptance criteria and incorporating lessons-learned from the round robin study into the developing third tier guidelines.

3. Zhili Feng, Oak Ridge National Laboratory: provide an update on the research program for developing an advanced hardening law for use in finite element calculations.

NRC Follow-Up Items

1. Investigate hiring statistics experts to assist with data analysis. 2. Participate in ASME Code process for residual stress guidance development. 3. Perform additional data analysis (e.g., flaw growth calculations).

Attachment A: NRC Slides

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U.S. NRC Perspective on Phase 2b Round Robin Study

Category 2 NRC Public Meeting and Webinar on the NRC/EPRI Weld Residual Stress Validation Program

December 15, 2014

Michael Benson, Joshua Kusnick, David Rudland RES/DE/CIB

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Feedback

Public Meeting

• Feedback forms available after the meeting

• For those attending in person:

– Fill out paper forms manually, or

– Scan the QR code to access the web form on a smart phone

• For those attending remotely:

– Web-based form

– http://feeback.nrc.gov/pmfs/feedback/form?meetingcode=20141833

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Purpose

Public Meeting

• Acknowledge the contributions of participants

• Publically present results of the latest round robin study for weld residual stress (WRS) prediction

• Discuss future plans for the EPRI/NRC WRS Validation Program

• Explore the challenges in applying the knowledge gained from this work

• Foster technical discussion on implications of the work

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Historical Overview

NRC/EPRI WRS Validation Program

• Overall Goal: Validation of WRS prediction for use in flaw initiation and growth assessment

• Four phases of research – Phase 1: measurement and modeling on scientific specimens

– Phase 2a: international round robin on pressurizer surge nozzle mockup

– Phase 3: measurement and modeling on nozzles from cancelled plant

– Phase 4: measurement and modeling of optimized weld overlay

• References – EPRI: MRP-316 and -317

– NRC: NUREG-2162

•Scientific Weld Specimens•Phase 1A: Restrained Plates (QTY 4)•Phase 1B: Small Cylinders (QTY 4)•Purpose: Develop FE models.

Phas

e 1 -

EPRI

•Fabricated Prototypic Nozzles•Type 8 Surge Nozzles (QTY 2)•Purpose: Prototypic scale under controlled conditions. Validate FE models.

Phas

e 2 -

NRC

•Plant Components•WNP-3 S&R PZR Nozzles (QTY 3)•Purpose: Validate FE models.

Phas

e 3 -

EPRI

•Plant Components•WNP-3 CL Nozzle (QTY 1)•RS Measurements funded by NRC•Purpose: Effect of overlay on ID.

Phas

e 4 -

EPRI

*

* WRS = weld residual stress

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Present Work

NRC/EPRI WRS Validation Program

• Goals

– Formulate guidance to reduce modeling uncertainty

– Establish acceptance criteria for WRS models

– Judge and account for uncertainty in WRS inputs for fitness-for-service evaluations

– Establish ASME Code guidelines

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Present Work

NRC/EPRI WRS Validation Program

• Phase 2b

– Second international round robin on a pressurizer surge line mockup

– 10 analysts from different organizations

– 4 hole drilling measurements, contour measurement, near-surface measurements

Pressurizer Surge Line Mockup

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Model Geometry

Finite Element Round Robin Study

CS Nozzle

SS Clad

Alloy 82

Butter

DMW

SS Safe End

SS Closure

Weld Alloy 82 Backweld

SS Pipe

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WRS Measurement Techniques

• Small diameter reference hole is drilled into part

• Diameter of reference hole, at multiple angles, is measured through the part thickness

• Stress is relieved by machining a larger hole (cylindrical core) around the reference hole

• Change in diameter of reference hole is measured and analyzed to calculate the original residual stresses present

• Measures bi-axial residual stresses in plane 90°to axis of hole

Hole Drilling

http://www.veqter.co.uk/residual-stress-measurement/deep-hole-drilling

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WRS Measurement Techniques

• Wire EDM used to cut a free surface in the specimen containing residual stresses

• Stresses released by the creation of a free surface cause distortions

• Coordinate measurement machine measures distortion

• Inverse displacements are applied to the surface of a finite element model to obtain the original residual stresses that were present in the component

• Produced a full field 2D stress map

Contour Method

http://www.lanl.gov/contour/principle.html

Cut

10 10

Experimental Setup

Phase 2b Measurements

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Raw Hole Drilling

Phase 2b Measurement Data

22°location 112°location

202°location 292°location

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Raw Contour – Axial Stress

Phase 2b Measurements

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Raw Contour – Hoop Stress

Phase 2b Measurements

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Processed Measurement Data

Phase 2b Measurements

Axial Residual Stress Hoop Residual Stress

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Modeling Package

Finite Element Round Robin Study

• Problem Statement

– Mockup geometry and fabrication details

– Information on measurements (no experimental data!)

– Modeling guidance based on lessons learned

– Material property files

– Submission guidelines

• Participant Questionnaire

– Establish point of contact

– Identify deviations from guidance

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Raw Modeling Data

Finite Element Round Robin Study

Axial Isotropic

Hoop Isotropic

Axial Kinematic

Hoop Kinematic

17 17

Processed Modeling Data

Finite Element Round Robin Study

Axial Isotropic

Hoop Isotropic

Axial Kinematic

Hoop Kinematic

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Future Work: Analysis and Deliverables

Technical Issues

• Flaw growth calculations (Q1 2015)

• Technical Letter Report (Q1 2015)

• Formal statistical analysis (Q1-Q2 2015)

• NUREG publication (Q4 2015)

• ASME Code guidance (Q4 2015)

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Discussion Topics

Technical Issues

• Uncertainty Characterization – What is the measurement uncertainty? Modeling uncertainty? – Are there outliers? If so, how do we identify and treat them? – Did we decrease uncertainty relative to Phase 2a? – How do we use the distribution in time to leakage?

• Model Validation – What is the proper method to compare the measurements and the models? – What are reasonable validation criteria?

• Modeling Guidance – What is the correct approach to hardening law? – How does this work affect development of ASME Code guidance?

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Thank You!

Questions?

Attachment B: EPRI Slides

NRC/EPRI Public MeetingDecember 15th, 2014

Rockville, MD

Michael R. Hill and Minh N. TranMech and Aero Engineering, UC Davis

Phase 2b Round Robin Study of Weld Residual Stress in

Pressurizer Surge Nozzle:An Initial Analysis

Analysis and Validation work supported byEPRI Materials Reliability Program

2

• Objective of this meeting– Describe Phase 2b round robin measurement and model outputs– Communicate validation analysis of model outputs

• Background on Phase 2b• Summary of submitted results

– Simulation outputs– Measurement data

• Validation: comparisons with benchmarks– Output from validation metrics

• RMS difference• Section forces (force and moment)• Stress intensity factors (circumferential and axial flaws)• Predicted crack growth behavior• Comments on validation metrics

• Assessments of outlying results• Summary and next steps

Outline of Presentation

Problem Statement Submitted Results Validation Outlying Results

4

• Mechanical strain relief measurement techniques:– Deep hole drilling: provides line stress– Contour method: provides stress field– Slitting: provides line stress

Problem Statement – Measurements

Deep Hole Drilling Setup Contour and Slitting Setup

Problem Statement Submitted Results Validation Outlying Results

PWR pressurizer surge nozzle mock-up

5

• 10 submissions were received, labeled A to J• Each included:

– Results from two hardening laws: isotropic and kinematic– Output stress field: axial and hoop– Output stress through thickness along DM weld centerline: axial and

hoop• Some variations:

– One participant used prior geometry (Phase 2a)– Two participants altered structural boundary conditions– One participant included 2D and 3D results using modified material

model– One participant included results at three different interpass

temperatures

Submitted Results – Simulation Outputs

Problem Statement Submitted Results Validation Outlying Results

6

• Center and Deep Hole Drilling:– Axial and hoop stress: 4 locations around pipe circumference, 90°

apart, in DM weld centerline region• Contour method:

– Axial stress: 5 locations around pipe circumference in DM weld centerline region

– Hoop stress: 5 locations along pipe length in DM weld centerline region

• Slitting: – Axial stress: DM weld centerline

Submitted Results – Measurement Data

Problem Statement Submitted Results Validation Outlying Results

7

• Submission A: selected 26 pt output file, 150 °C interpass– Similar to other modelers, can assess influence later

• Submission F: excluded; used Phase 2a geometry– Analyze this data later, as an outlier

• Submission H: selected 26 pt output file (not equally spaced)• Submission J: used results with provided material data• Normalized position through thickness with respect to

participant’s model thickness – All models have t within 3% of the mean

• Except excluded submission F

• Linearly interpolate to x/t = 0, 0.04, 0.08, …, 1.0– 26 points, evenly spaced through thickness, including ID and OD

• Compute mean of model outputs (Blue line)– Take mean at each x/t = 0, 0.04, 0.08, …, 1.0

Data Pre-process

Problem Statement Submitted Results Validation Outlying Results

13

• Validation, as defined by ASME V&V 10-2006:– The process of determining the degree to which a model is an

accurate representation of the real world from the perspective of the intended uses of the model

• Working from this definition, validation involves:– Comparison of model output with a benchmark– Benchmark should reflect the real world, perhaps drawn from

• Measurement data• Phenomenological correlation• Expert panel opinion• Exemplar model outputs

• Our objective today:– Propose approaches for comparison of WRS model outputs– Apply approaches to data from Phase 2a round robin– Discuss results

Model Validation

Problem Statement Submitted Results Validation Outlying Results

14

• EPRI MRP 287 (2010) – Primary Water Stress Corrosion Cracking (PWSCC) Flaw Evaluation Guidance– Suggestions on WRS validation (3 pages)

• Stress intensity factor, crack growth history• ASME V&V 10 (2006) – Guide for Verification and Validation in

Computational Solid Mechanics– Broad guidance for validation of computational solid mechanics (32 pages)– Nothing specifically dedicated to residual stress

• AWS A9.5 (2013) – Guide for Verification and Validation in Computational Weld Mechanics– Guidance on weld modeling practice– No detailed guidance on model validation (2 pages on conducting validation

measurements, 3 sentences on stress measurement)• R6 - Revision 4 (2013) – Assessment of the Integrity of Structures

Containing Defects– Substantial guidance on weld modeling

• Procedures for model parameter calibration using experimental data– Substantial guidance on WRS validation, including on experimental practice

• Differences between calibration and validation

Published Approaches to WRS Model Validation

Problem Statement Submitted Results Validation Outlying Results

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• Stress due to uniform section forces (average through wall)

• Stress due to bending section forces (linear through wall)

Stress due to Section Forces

Hoop

Hoop

22

• SIF from weight function (Wu and Carlsson):

Geometry: ri/ro = 0.8

• Total SIF, including applied loads:

where:

(ref MRP-115 for membrane, bending stress, and internal pressure)

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Total SIF: Complete Circ Flaw – Post-SS, Axial Stress

24

• SIF from weight function, deepest point A (Glinka):

Geometry: ri/ro = 0.8Aspect ratio: c/a = 2

• Total SIF:

where:

(ref MRP-115 for internal pressure)

25

Total SIF: Axial Semi-Ellip. Flaw – Post-SS, Hoop Stress

27

CG: Complete Circ Flaw – Post-SS, Axial Stress

31

High, deviation from problem statement:Assessment of Outlying Results

Back weld:3 extra beads

Mechanical BC:Axially fixed atbolt hole

Problem Statement Submitted Results Validation Outlying Results

32

Low, deviation from problem statement:• Mechanical BC: axially fixed the entire left side of stiffening plate• Back weld:

Assessment of Outlying Results

Back weld off center

37

• Described Phase 2b measurement data and model outputs• Described output from four validation metrics:

– RMS difference– Section forces (force and moment)– Stress intensity factors (circumferential and axial flaws)– Predicted crack growth behavior

• Mean of model outputs was used as benchmark– Some results similar if benchmark is mean of WRS measurements

• Two outlying model outputs examined– More apparent after application of SS weld

• Phase 2b and Phase 2a comparison– Phase 2b and Phase 2a show similar dispersion of model outputs

Summary

Problem Statement Submitted Results Validation Outlying Results

38

• Developing consensus– Confirm that model outputs are

• Interpreted correctly• Analyzed appropriately

– Are four figures of merit presented appropriate as validation metrics?• RMS difference• Section forces (force and moment)• Stress intensity factors (circumferential and axial flaws)• Predicted crack growth behavior

– Are there opportunities to validate model outputs relative to crack growth behavior?• Crack growth is the intended use of the model (see ASME V&V 10)• Measurement and model precision can be established• Measurement and model accuracy relative to crack growth still unknown

– How should model acceptance be determined?• Identify benchmark similar to intended application (an “accepted” reality)

– Within some limited space of materials and geometry• Demonstrate capability relative to a benchmark

Analysis Discussion

Problem Statement Submitted Results Validation Outlying Results

39

• Immediate need is to have verified one-dimensional stress vs thickness values to use in K calculation for crack growth analysis– Stresses are used to support relief requests for

inspection of Ni-based alloy welds and base metals• Address inspection coverage issues, inspection interval based on material

considerations• Calculate response of hypothetical flaw

– In service flaw evaluation is rare• Consensus based standards can help serve this immediate need• Three tiered approach currently in process with ASME Code

– Simple conservative stress vs thickness when time and/or conditions can support their use

– Prescribed stress vs thickness for common configurations• Must meet geometric and dimensional parameters• Uses FEA results, measurement data, consider uncertainty to develop profiles• Technical basis for prescribed stress vs thickness profiles in PVP2015 paper

– Discussion for FEA of configurations not considered for prescribed stresses• Possibly acceptance standards, point to other best practices published documents

– In process as Code Case

Model Acceptance Discussion

Problem Statement Submitted Results Validation Outlying Results

40

• Complete Analysis of Results and Model Validation – March• Publish Final Report for DMW Butt-weld WRS FEA Model

Validation Program – June• ASME Code Non-mandatory Appendix Inputs (as needed)• Potential Meetings (preliminary/tentative – for discussion)

– “WRS FEA Model Acceptance Requirements Workshops”• Technical approach for FEA model evaluation

- TBD, based on needs, approach, readiness and availability• Coordinated with ASME Code Weeks and PVP; Jan 25-30, Houston,

TX; Apr 26-May 1, Colorado Springs, CO; July 19-23, Boston, MA

• Summary of Anticipated MRP Final Report– Coordinated with Annual Materials R&D Tech Update – June 2-4,

Rockville, MD

Anticipated 2015 MRP Program Schedule

Problem Statement Submitted Results Validation Outlying Results

41

THANK YOU!

QUESTIONS?

Attachment C: Meeting Attendance

Attended In Person Name Organization Email Address Bud Brust Emc2 bbrust@emc-sq.com Andrew Cox Battelle coxa@battelle.org Lee Fredette Battelle fredette@battelle.org Minh Tran UC Davis mngtran@ucdavis.edu Michael R. Hill UC Davis mrhill@ucdavis.edu John Broussard DEI jbroussard@domeng.com Paul Crooker EPRI pcrooker@epri.com Bernie Rudell Exelon Generation bernie.rudell@exeloncorp.com Rob Tregoning NRC jobert.tregoning@nrc.gov Josh Kusnick NRC joshua.kusnick@nrc.gov Jay Collins NRC jay.collins@nrc.gov John Tsao NRC john.tsao@nrc.gov Joel Jenkins NRC joel.jenkins@nrc.gov Attended via Phone and Webinar Name Organization Email Address David Enos Sandia National Laboratories dgenos@sandia.gov Shawn Kleinsmith GE-Hitachi shawn.kleinsmith@ge.com Steve Marlette Westinghouse marletse@westinghouse.com Silvester Noronha Areva silvester.noronha@areva.com Francis Ku Structural Integrity fku@structint.edu Wilson Wong Structural Integrity wwong@strucint.com Steven Xu Kinectrics steven.xu@kinectrics.com