Macroscopic Residual Stress
Measurements by Neutron Diffraction
Thomas M. Holden, Northern Stress Technologies, Canada.
Neutron diffraction provides a measurement of stress at depth in engineering
components because of the high penetration of thermal neutrons through most
industrial materials. The limiting path length is about 50mm for nickel-based
alloys, 60mm for iron-based alloys but 250mm for aluminum alloys. Measurements
can be made at a reactor instrument with neutrons of a single wavelength by
recording the angle of diffraction, or at a spallation neutron source using
neutrons of all wavelengths by recording the time of arrival of the neutron
in a counter set at 90ş. The precision of measurements is sufficiently good
to give useful engineering information, but systematic errors from a number
of sources affect the accuracy and require a sound understanding of the
diffraction phenomenon. Measurement of a potentially tri-axial stress at
depth requires a knowledge of the spacing of the atomic lattice in the
absence of a stress field. The usual origin of a macroscopic stress is
inhomogeneous plastic deformation in the past history of the sample under
test. However, in addition to generating the stress field, inhomogeneous
plastic deformation always generates type-2 stresses on the spatial scale
of the grains, which vary for different diffraction lines. The latter bias
the diffraction measurements with a given diffraction line and must be
accounted for. Measurements of weld stresses are very common but require
separate measurements of lattice spacing since the chemical composition
and microstructure are frequently affected by welding. These chemistry a
nd type-2 effects are best determined by preparing small coupons from
the sample which destroys the stress field but leaves the chemistry and
stresses on smaller length scales unchanged. Components with large grain
sizes, 100µm and above, may also be prone to systematic errors which are
alleviated by improved statistical sampling of diffracting grains. An
international standard, ISO/TS 21432, for neutron diffraction measurements
of stress has been available since 2005 and draws attention to these
factors. The presentation will cover the concepts and difficulties involved
in such measurements and will be illustrated by practical examples drawn
from experience.
A Critical Analysis of Diffraction
Strain Measurements
Cev Noyan, Columbia University, New York.
Diffraction techniques for measuring internal strains in crystalline materials
are popular since they are non-contact, non-destructive and need little special
specimen preparation. Furthermore, data analysis is also deceptively simple and
will almost always yield a "result". Such a result, on the other hand, may or may
not be what is needed by the particular engineering program commissioning the
measurement. In this seminar the parameters that are measured by diffraction
techniques, and how they are related to the parameters defined by continuum
mechanics, will be reviewed. A few examples of "good" and "bad" results and
some simple tests that can be carried out to test the validity of diffraction
measurements will also be discussed.
Relaxation Methods for Measuring
Residual Stresses
Gary Schajer, University of British Columbia, Canada.
Relaxation methods, also called “destructive” methods, are commonly used to
evaluate residual stresses in a wide range of engineering components. While
seemingly less attractive than non-destructive methods because of the specimen
damage they cause, the relaxation methods are very frequently the preferred
choice because of their versatility and reliability. Many different methods
and variations of methods have been developed to suit various specimen
geometries and measurement objectives. Previously, only specimens with simple
geometries could be handled, but now the availability of sophisticated
computational tools and of high-precision machining and measurement processes
has greatly expanded the scope of the relaxation methods for residual stress
evaluation. This paper reviews several prominent relaxation methods, describes
recent advances, and indicates some promising directions for future developments.
Principles of the Deep Hole Drilling
Method for Through Thickness Residual Stress Measurement
David Smith, University of Bristol, UK.
Deep hole drilling (DHD) is one of suite of mechanical strain relaxation (MSR)
methods used to measure residual stresses deep within materials. As with all
MSR methods the DHD technique measures the distortion that arises during material
removal as a result of residual stress relaxation. The origins of the method
began over 40 years ago from the desire to measure stress in rocks as part of
mining operations. It is now widely used obtain the magnitude and distribution
in many thick section mechanical components, ranging from welded nuclear pipes to
large forged aluminium blocks. In this talk the developments made for its
application to engineered metallic components are detailed. The steps taken to
measure distortion and the analysis to transcribe the distortions to stresses are
described. Initially, the method for elastic relaxation of the residual stresses
was considered. Recently, it has been found that in some circumstances the
distortion is not solely related to elastic recovery of the residual stresses.
A new approach has been developed to ensure that the correct elastic distortions
are measured and incorporated into the analysis.
The Contour Method: Capabilities,
Limitations, and Recent Advances
Michael B. Prime, Los Alamos National Laboratory, NM.
The contour method can measure a 2-D map of residual stresses over a cross-section.
A part is cut in two using a precise and low-stress cutting technique such as
electric discharge machining. The contour of the resulting new surface, which will
not be flat if residual stresses are relaxed by the cutting, is then measured.
Finally, a conceptually simple finite element analysis determines the original
residual stresses from the measured contour. Only the stress component normal to
the cut is determined in the traditional implementation.
In this talk, the contour method for measuring residual stress is presented for an audience interested in practical application of the method. The principles and assumptions are presented. Next, details and applications are discussed. Error sources and other issues are discussed before discussing the strengths and weaknesses of the method. Some suggestions are given to improve the results and/or check for errors. Finally, some recent advances that permit the measurement of multiple stress components are presented.
Residual Stress Measurements that Correlate
Fatigue and Fracture Behavior
Michael R. Hill, University of California, Davis.
In recent years, advancements in residual stress measurements have improved our ability
to quantify the effects of residual stresses on structural performance. Typically,
residual stresses affect subcritical cracking due to fluctuating stress (fatigue) and
environmental exposure (environmentally assisted cracking) and may increase or decrease
the time required for cracks to nucleate and then the rate of crack growth; less
typically and more dramatically, residual stresses affect capability to sustain single
applications of service loading. Historically, except in special cases, available methods
have not allowed measurements of the spatial variation of residual stresses that could
enable reliable forecasts of component performance. Over the past few decades, a handful
of residual stress measurement methods have been put into practice with demonstrated
capability of enabling reliable performance forecasts. This paper provides a summary of
two of these residual stress measurement methods and describes their application to
correlate the fatigue and fracture behavior of residual stress bearing coupons.
Microscale Measurements of Residual
Stress
Philip Withers, Manchester University, UK.
Measurement of residual or applied stresses at the sub-micron or finer scale
poses substantial experimental and analytical challenges. Laboratory and
synchrotron micro-diffraction is well established as a means of mapping stress
laterally at the micron scale in crystalline materials and devices. On the
other hand, curvature methods have been used to depth profile over similar
length scales, but much larger areas laterally. Here, we present new micro-milling
techniques, based on mechanical-relaxation phenomena that are direct analogues
of their conventional macroscale counterparts. They allow mapping of the
in-plane stress tensor at micron scales laterally with sub-micron depth
resolution. They are applicable to both crystalline and amorphous materials.
Our microslotting and incremental micron-hole-milling methods use digital
image correlation (DIC) to record to nanometre precision the displacements that
develop around micron sized slits and holes respectively, as they are
progressively milled by a focused ion beam (FIB). We demonstrate these methods
by applying them to infer the residual stress profile in a Zr50Cu40Al10 bulk
metallic glass (BMG) with high spatial definition (200-300 nm). These methods
could find a wide range of applications across microelectronics,
micro-electronic machines, biomaterials, etc.
PWR Dissimilar Metal Butt-Weld
Residual Stress Finite-Element Model Validation Overview
Paul J. Crooker, Electric Power Research Institute and
Aladar A. Csontos, U.S. Nuclear Regulatory Commission
Permanent residual stresses in pressurized water reactor (PWR) components can
result from original fabrication processes, such as welding. If the resulting
residual stresses are of high magnitude, tensile, and applied to a susceptible
material, primary water stress corrosion cracking (PWSCC) can occur. This
combination of factors can exist in PWR primary coolant system components that
are fabricated from Alloy 600 and its welded forms Alloy 82/182. These components
include Reactor Pressure Vessel Head Control Rod Drive Mechanism Nozzles,
Bottom-mounted Instrumentation Nozzles, Steam Generators, and numerous dissimilar
metal (DM) J-welds and butt welds that seal vessel penetrations and join carbon
steel vessels to stainless steel pipes. Over the operating life of the existing PWR
fleet, numerous observations of PWSCC have occurred. Because of these events and a
similar history the BWR fleet, the nuclear power industry developed a PWR Alloy 600
management plan. As part of the required plan all DM Alloy 600 butt-welds must be
inspected by approved methods and schedules. In October 2006, during Alloy 600
inspections at a PWR generating station, significant circumferential indications were
identified in a pressurizer surge nozzle DM weld. Because of the size and location of
the indications, nine other PWRs that had planned to complete pressurizer inspections
in the Spring of 2008 were in jeopardy of being required to modify operations and
maintenance plans, inspect early or shutdown until the inspections or mitigation were
performed. Working cooperatively with the NRC, the industry developed and performed
advanced finite-element analyses to predict crack growth rates (CGR) in DM butt-welds
to determine if early inspections were required. The results of the analyses, documented
in EPRI report MRP-216, showed that there was sufficient safety margin to allow the nine
plants to continue operation and inspect as planned in 2008. However, the FEA results
also showed significant CGR sensitivity to welding residual stresses (WRS).
Because of CGR sensitivity to WRS and uncertainty in the WRS FEA calculations, the industry and NRC decided that further validation of WRS FEA models with PWR prototypic materials was needed. The validation study includes multiple prototypic mock-ups and residual stress measurements by multiple techniques. This presentation discusses WRS as a PWR materials degradation issue and the cooperative NRC-EPRI Materials Reliability Program (MRP) DM butt-weld FEA Model Validation Project.
Verification and Validation of Residual
Stress Analysis Methods for Airframe Structures
Pamela A. Kobryn and Rollie E. Dutton, Air Force Research Laboratory, and
Michael R. Hill, University of California, Davis
Over the last decade or so, the residual stress community has developed and refined
various analysis methods for quantifying and predicting residual stresses in complex
three-dimensional bodies. More recently, analysis methods for quantifying and
predicting the impact of residual stresses on fatigue and fracture behavior have
emerged. These analysis techniques have been invaluable in providing insight to
scientists and engineers working in the disciplines of manufacturing and
structural/mechanical engineering. However, their utility is severely limited by the
lack of verified and validated tools for quantitative engineering analysis. This
presentation will outline the verification and validation challenges associated with
residual stress analysis methods for application to the design, manufacture, and
lifecycle management of airframe structures. The goals of this presentation are
(a) to initiate discussion within the residual stress community regarding the
development of standardized verification and validation approaches for residual
stress analysis methods, and (b) to set the stage for a session on verification
and validation at the next Residual Stress Summit.
Modeling of Residual Stresses
and Machining Distortions of Aerospace Components
Shesh K Srivatsa, GE Aviation, Cincinnati, OH.
Aircraft engine and airframe structural components that are machined from
forgings represent a significant cost of both military and commercial
aircraft. Typical component applications are rotating disks in aircraft
engines and structural components in airframes. The buy-to-fly weight ratio,
which is the ratio of the forged material weight to the finished part
weight, is typically between 4 and 10 for such components. The excess
material is removed by various machining operations, which are a major
contributor to the cost of forged components. Machining distortions are
a problem with most forged components which are quenched rapidly in order
to generate the required mechanical properties. Distortion can be caused
by material bulk stresses resulting from heat-treating operations, or
from local near-surface machining-induced stresses. Typically additional
machining operations and setups are added in a time-consuming and costly
trial-and-error approach to minimize the effects of part distortion.
Manufacturing residual stresses can adversely impact the behavior of the
components during service. There is a need to understand and control the
effects of heat treating and machining on residual stresses and distortions.
The objective of this program is to establish a modeling method that
accurately predicts residual stress and distortion in forgings used in
aircraft engines and airframe structures. Prediction and validation of
machining distortions due to bulk and surface residual stresses will be
presented. This program is funded by the USAF Metals Affordability
Initiative (MAI).
An Integrated R&D Roadmap for
Residual Stress Management in Large Structural Forgings
Mark James, John Watton and Bob Bucci, Alcoa Technical Center, and
Dale Ball, Lockheed Martin Corp.
For more than a decade, the authors’ companies have been advancing their
respective visions for residual stress and machining distortion management
in large, three-dimensional components. During that time, substantial
progress has been made towards solving a myriad of complex technical challenges.
Recently, under the Metals Affordability Initiative, the Air Force provided
opportunity for stakeholder companies to join forces in an integrated planning
effort to outline a technology roadmap. The process affirmed that more than
ever, knowledge of residual stress influences on design/manufacturing processes
is essential to both assure conservatism and maximize performance. Furthermore,
expanded supply chain integration is necessary to realize the full range of
end-product benefits. This presentation summarizes the ongoing effort to
develop the roadmap and execute the vision.
Distortion Modeling of Airframe
Components
James Castle, Boeing Research and Technology.
Structural airframe design is driving to maximize structural unitization.
This enables appreciable cost, weight savings, and improved buy-to-fly in large
structural components. However, as parts become larger and more unitized the
demands imposed on the design and manufacturing process increases. While it
may be feasible to compensate for distortion during assembly of many small
parts, large unitized structure lacks the degrees of freedom to compensate
and is less forgiving of distortion. Distortion can be caused by material
bulk stresses resulting from processing operations and/or by local near-surface
machining induced stresses. Typically additional machining operations and
setups are added in a time-consuming and costly approach to minimize the
effects of part distortion. There is a need to understand the effects of
materials processing and machining on distortion and to predict, minimize,
and control these distortions. This presentation will review progress that
has been made on the machining modeling side to relate material and machining
process to distortion in airframe parts.
Bulk Residual Stress Modeling:
3D distortion, Contour Method and Neutron Diffraction Efforts to Validate Modeling Results
M.G. Glavicic, B. Ress and K. Ma, Rolls-Royce, Indianapolis, IN, R. Mitchell,
Rolls-Royce plc, Derby UK, W. Wu, Scientific Technologies, Columbus, OH,
S, Srivatsa, GE Aviation, Cincinnati, OH, J. Rolf and M. Preuss, Univerity of
Manchester, UK, and R. Ramanathan, Ladish Company, Cudahy, WI.
Bulk residual stresses that develop during the final cooling of a forging
pre-determines whether a component will be distortion free during machining or
inherently problematic to machine. A review of previous efforts to model and
validate bulk residual stresses will be presented. Topics to be covered will
include:
Computational Modeling and Optimization
of Bulk Residual Stress in Monolithic Aluminum Die Forgings
John D. Watton, Alcoa Technical Center, PA.
Towards Alcoa's goal of manufacturing forgings with consistently low residual
stress we have developed in-house and integrated commercial finite-element tools
to model the heat treatment quench induced residual stress process and secondly
the practice of cold work stress relief of monolithic aluminum (alloys AL7050
and AL7085) aerospace die forgings. We highlight Alcoa’s state-of-the-art
predictive capability with examples of its use to improve and optimize the
quench and cold work stress relief practice. In addition, Alcoa's modeling
tools are used to give guidance on the bulk residual stress contribution to
machining distortion. We also present validation work that compares measured
(using the slitting and contour method) and predicted residual stress for both
post quench and post cold work states. Finally, we discuss the limitations of
the computational modeling tools and our future direction.
Toward Understanding the Impact of
Bulk Residual Stress on the Life, Weight and Cost of Primary Aircraft Structure
Dale L. Ball and Bryan K. Tom, Lockheed Martin Aeronautics Co., and
Mark James and Bob Bucci, Alcoa Technical Center.
A variety of advanced material / structural concepts are being developed by,
or with the support of airframe manufacturers who are in pursuit of enhanced
performance at reduced cost. One of the more promising concepts, and one that
has already been adopted for a number of high profile applications, is the
unitization / integration of multiple, machined and subsequently assembled
parts, into a single monolithic component. When this is done with large
monolithic forgings, the presence of bulk residual stresses must be addressed
during design.
This presentation will describe methods for the explicit inclusion of bulk residual stresses in design analysis (specifically in fatigue life analysis). The life versus weight optimization that this inclusion enables will be described for several aircraft bulkheads. Finally, current results from ongoing weight and cost impact studies will be presented.
Advanced Software for Integrated
Probabilistic Damage Tolerance Analysis, Including Residual Stress Effects
M. P. Enright, R. C. McClung, Y.-D. Lee and W. Liang, Southwest Research Institute,
San Antonio, TX, and S. H. K. Fitch, Mustard Seed Software, Charlottesville, VA
New analysis methods and software tools are being developed for improved accuracy
and efficiency in performing damage tolerance analyses of critical aerospace components.
This presentation provides an overview of recent advances that automate and streamline
the process of fracture mechanics (FM) model development and life analysis, including
direct integration of finite element (FE) models that simulate the manufacturing process
and the service usage. Stress analysis results from the FE models are directly incorporated
into the life analysis through a powerful graphical user interface. Residual stresses can
be included in this analysis, including bulk residual stresses arising from forging and heat
treating as well as localized residual stresses from peening or other surface engineering
processes. The effect of these residual stresses on fatigue crack growth life and
component reliability can be automatically computed using algorithms that include advanced
weight function stress intensity factor solutions to accommodate arbitrary stress gradients
accurately. A novel scheme has been developed that automatically determines (without user
input) the orientation, size, and stress input for a FM model that will produce accurate
life results, given only a 2D model of the crack plane and an initial crack location, and
taking into account the actual component boundaries and stress fields. This capability is
then exercised to construct life contours that visualize the fatigue life response over an
entire component. The life calculation capabilities are combined with probabilistic
descriptions of key input variables and tailored probabilistic methods to calculate the
probability of fracture of the component. Long-range goals include performing this
reliability calculation in a similar automated fashion with minimal user intervention, as
well as optimizing the manufacturing and inspection plans through the integrated software
system in order to maximize the reliability.
Welding, Residual Stress, Distortion Control
Methods and Applications of Weld Modeling
Bud Brust, Engineering Mechanics Corporation of Columbus.
Computational weld modeling is challenging because many of the processes of
welding are highly nonlinear. Material melts and re-solidifies, very high
transient thermal gradients are experienced, non-linear temperature dependent
plastic straining and phase transformations can occur, among other sources of
nonlinearity. Moreover, for weld modeling to have practical advantages in
industrial production, computational solution times must be manageable since
an optimum weld design of large, complex fabrications requires numerous
separate analyses. Weld modeling technology is now advanced to where it can
have an important impact on numerous fabricated structures. These include
nuclear power plant components in commercial nuclear plants and nuclear ship
structures, including Aircraft Carriers and Submarines, NASA space vehicles,
and in industrial heavy fabrication. The benefits of weld modeling include
can be seen in the figure below and include:
Here we focus on weld residual stress and distortion control only. Methods for controlling both include weld sequencing, weld parameter definition, weld procedure, fixturing, pre-camber and pre-bending, thermal tensioning, heat sink welding, post weld heat treat, weld consumable, mechanical stress improvement (MSIP), weld overlay repairs, and combinations of these. Extensive full-scale experiments have validated the accuracy and predictive power of models. This presentation will provide a number of examples of both distortion and residual stress control. The examples will include several from the nuclear industry, several from the heavy manufacturing industries, several from the ship building industry, and one example of a NASA space flight example.
Simulation and Measurement of
Residual Strain Localization in a Laser Welded Titanium Ring
John O. Milewski, Ching-Fong Chen, William L. Stellwag, Jr.,
Thomas A. Sisneros and Donald W. Brown, Los Alamos National Laboratory, NM.
Elastic residual strains were measured in a laser welded commercially pure
titanium ring using a nondestructive neutron diffraction technique in order
to determine the usefulness and resolution of this method for the
characterization of small laser welds. These measurements were used to
validate calculations made using residual strain data obtained from simulation
of the residual stress in and near the weld region using SYSWELD ®. The
measured strains were in good agreement with the simulated results away from
the weld start - stop location. Simulation indicted a localization of
compressive and tensile strains within the welded ring longitudinal along the
weld start and stop location. Experimental measurements confirmed the presence
of these regions.
Weld Overlay Repairs
Pete Riccardella, Structural Integrity Associates.
Macroscopic Residual Stress Measurements
by Neutron Diffraction
Adrian T. DeWald, Hill Engineering, LLC, McClellan, CA.
Prediction of the fatigue and fracture performance of large, monolithic components
depends on knowledge of the bulk residual stresses that they contain. The contour
method is a new way to measure bulk residual stress fields that provides data
useful for forecasting fatigue and fracture performance, and it can be applied to
thick-section parts. Relying on simple assumptions and straightforward experimental
procedures, the method provides the two-dimensional spatial distribution of residual
stress normal to a plane of interest within the component. When the method is
applied at sections with high failure risk, the measured residual stress field may
be used directly with standard methods for predicting fatigue crack initiation and
growth. The presentation provides a summary of the experimental details of the
contour method and examples of its application with specific emphasis on aerospace
forgings and thick-section welds.
Application of Deep-Hole Drilling to
Dissimilar Metal Weld Components
Ed Kingston, VEQTER Ltd, UK.
The US Nuclear Regulatory Commission (US NRC) and the Electric Power Research
Institute (EPRI) are conducting a joint research program to validate the
through-wall weld residual stresses that develop at dissimilar metal butt-welds
from typical fabrication and failure mitigation processes. Surface and through-wall
residual stress measurements are being used to compare, improve and validate finite
element predictions of the residual stresses generated. Four phases of work are
being carried out focusing on basic laboratory mock-ups through to full-scale practical
components. This presentation focuses on through-wall Deep-Hole Drilling residual
stress measurements carried out by VEQTER Ltd for the program, showing results from
the lab mock-ups to the full-scale components and at different stages in manufacture
or mitigation.
Dissimilar Metal Weld Residual Stress
Mappings by Neutron and X-ray Diffraction and Incremental Hole Drilling Methods
Camden Hubbard, Josh Schmidlin, Matthew Klug, James Pineault,
Shane Van De Car, Zhili Feng, Fei Ren and Wei Zhang, Oak Ridge National Labs, TN.
The US Nuclear Regulatory Commission (NRC) and the Electric Power
Research Institute (EPRI) are conducting a weld residual stress validation
program aimed at both (1) refining computational procedures for residual
stress simulations in dissimilar metal welds, and (2) developing and
categorizing the uncertainties in the resulting residual stress predictions.
This program currently consists of four phases, with each phase increasing
in complexity from lab size specimens to component mock-ups. The US NRC and
EPRI are working cooperatively on this effort under a memorandum of
understanding, with this talk focusing on the characterization of residual
stresses in dissimilar Metal welds by by the incremental hole drilling
(IHD), X-ray diffraction (XRD) and neutron diffraction (ND) methods
conducted by Oak Ridge National Laboratory (ORNL). The three methods
provide stress maps at the surface (XRD), near surface (IHD) and through
thickness (ND).
Measurement of Residual Stresses
in Dissimilar Metal Welds Using Multiple Techniques
Don Brown, Los Alamos National Laboratory, NM.
With the recent push to continue licensing nuclear power reactors to increasing
ages, the structural integrity of welds, in particular dissimilar metal welds,
has come under increasing scrutiny. A major component of this is the residual
stresses present in the welds. Moreover, in such welds, which can have large
dimensions, large grains, strong chemistry variations, and other issues which
make the assumptions associated traditional residual stress measurements tenuous,
it is advisable to measure the residual stresses with multiple techniques based
on differing sets of assumptions. To this end, we have profiled the residual
stresses in a 16 mm thick model dissimilar metal plate weld using neutron
diffraction, the contour method, and hole-drilling. The three techniques are
both complementary, and provide a measure of self-consistency. For instance,
hole-drilling provides the most accurate results near the surface, where
neither the contour method nor neutron diffraction are well suited for
measurement. Further, neutron diffraction and the contour method both provide
bulk results, but are based on very different assumptions, and can be critically
compared. Finally, all of the results are compared with finite element models
of the weld.
Hole Drilling Evaluation of
Residual Stresses in Buried Pipelines using an Optical Interferometer with
Radial Sensitivity
Armando Albertazzi G. Jr. and Matias Vioti, Universidade Federal de
Santa Catarina, Mechanical Engineering Department, Brazil.
Pipelines used in the oil and gas industry are often exposed to complex stress
fields resulting from the combination of at least five components: stresses
generated by internal pressure, mounting stresses, thermal stresses, stresses
arising from the soil-pipe interaction and the pipe manufacturing residual
stresses. This paper discusses the use of the hole drilling method to assess
the state of combined stresses acting on a section and, from that, the
determination of the local bending and tensile loadings on the pipeline. The
combined stresses are measured in four points of each section by the hole
drilling method. The longitudinal stress components are computed and combined
in a linear model to estimate the local bending and tensile loadings on the
pipeline. This information is very useful for the pipeline owner in order to
assess its integrity due to soil-pipeline interaction induced by, for example,
after heavy rains. This approach was applied in the Bolivia-Brazil gas pipeline
in a sloping hill in five different sections to determine the axial tensile
forces and bending moments. The paper also presents a very practical and robust
residual stresses measurement device. It combines the hole drilling method and
a digital speckle pattern interferometer with in-plane radial sensitivity. An
appropriate configuration, using a special diffractive optical element, is used
to double illuminate an 8 mm diameter circular area, adjacent to the region
where the hole will be drilled. That configuration produces, at same time,
in-plane sensitivity in the radial direction and wavelength invariance, what
is very appropriate for in-field applications. Details of the device and its
performance are also discussed in the paper.
An On-Line Methodology for
Measuring Residual Stress and Producing Reliable Fatigue Life Assessments
S.W. Smith and J.A. Newman, NASA Langley, Hampton, VA,
M.A. James, R.L. Brazill, and R.W. Schultz, Alcoa Technical Center, PA,
J.K. Donald and A. Blair, Fracture Technology Associates, Bethlehem, PA, and
B.R. Seshadri, National Institute of Aerospace, Hampton, VA.
An on-line compliance-based method for the measurement of the component of
residual stress normal to a growing fatigue crack has been evaluated. Results
from this crack-compliance method for specimens containing a friction stir weld
are presented, and found to be in excellent agreement with residual stress data
obtained using the cut-compliance method. Variable stress-intensity factor
tests were designed to demonstrate that a simple superposition model, summing
the applied stress-intensity factor with the residual stress contribution, can
be used to determine the local crack-tip stress-intensity factor. Finite element
and J-integral analyses have been developed to predict weld-induced residual
stress using thermal expansion/contraction and an equivalent ΔT for the welding
process. An equivalent ΔT was established and applied to an analysis for each
specimen geometry tested to yield predicted residual stress distributions in very
good agreement with experimental results obtained using the crack and cut-compliance
methods.
Technical Issues in the EPRI/NRC
Weld Residual Stress Validation Program
John E. Broussard, III, Dominion Engineering, Inc., and
Howard J. Rathbun, U.S. Nuclear Regulatory Commission.
Pressurized water reactor (PWR) piping system dissimilar metal (DM) welds
are susceptible to primary water stress corrosion cracking (PWSCC) as an active
degradation mechanism. PWSCC is highly influenced by the state of stress within
the susceptible material, and tensile residual stresses in DM welds are an
established driving force for PWSCC. Hence, proper predictions or measurements
of such stresses are essential to accurate crack initiation and growth assessment.
Recent improvements in computational efficiency have facilitated advances in weld
residual stress predictions, but no universally accepted guidelines for these
procedures have been established. The assumptions and estimation techniques
employed vary from analyst to analyst, causing large variability in the predicted
residual stress profiles for a given weld. The Electric Power Research Institute
(EPRI) and the U.S. Nuclear Regulatory Commission (NRC), under a Memorandum of
Understanding (MOU), are cooperatively completing a weld residual stress
validation program aimed at both refining computational procedures for residual
stress simulations in DM welds and quantifying the uncertainties in the resulting
residual stress predictions. Technical challenges associated with the validation
effort include assumed geometry, constraints, thermal models (prescribed temperature,
cropped temperature and heat generation models), hardening laws and anneal modeling
techniques. Progress and remaining work in each of these areas will be discussed.
Summary and Next Steps in the MAI
Forging Residual Stress Program
Rollie E. Dutton, Air Force Research Laboratory.
The presentation will summarize some of the key aspects of
the MAI forging residual stress program and earlier discussions
taking place at the Summit. The presentation will also lay out
the planned, near term activities in the program.