Engineered
Residual Stresses in US Air Force Service
Michael Shepard. Air Force Research Laboratory, WPAFB, OH.
The introduction of engineered residual stresses into common aerospace design
allows the designer to add significant margin to new or existing hardware by
manipulating the residual stress states in critical features. Several processes,
including laser shock processing (LSP), low plasticity burnishing (LPB), hole
expansion techniques, and shot peening exist for this purpose. Best results are
obtained when the residual stresses are carefully optimized. Historically, this
optimization has been empirical, but new methods are emerging to minimize the
required iteration. As targeted applications become increasingly complex and
costly to develop these design tools are becoming critical. In this presentation
the major surface treatment processes will be briefly reviewed. Design methods
supporting the most advantageous use of engineered residual stresses will be
discussed. Progress toward the introduction of these processes and design
methods into Air Force service will be outlined.
GE
Aviation Experience in FOD Tolerance Improvement Using LSP
Paul Domas. GE Aviation, Cincinnati, OH.
This presentation briefly reviews the Laser Shock Peening (LSP) process and specific
application to aircraft engine gas turbine airfoil foreign object damage (FOD)
protection. Aggressive development and resource commitment has resulted in four
generations of Production laser systems and application on tens of thousands of
airfoils. Continued emphasis on quality control and cost reduction is noted.
The primary presentation purpose is to note key LSP lessons learned and similarity
to experience from other residual stress generating processes such as cold expansion
and shot peening. The role of fatigue testing, the potential occurrence of
undesirable side effects (and consequent process constraints) and avoidance of
“over-processing” are stressed. These observations are offered from an OEM Life
Management perspective as guidelines to be followed in garnering greater use of
Engineered Residual Stress.
Application
of Surface Treatments to Aerospace Components
John Schofield. Rolls-Royce, Derbyshire, UK.
Vision
for the Broad Application of Residual Stress - An Industry Perspective
Brad Cowles, Jay Littles, Sonia Martinez and Bob Morris. Pratt &
Whitney Aircraft, East Hartford, CT.
Foreign object damage (FOD) is most acute on the thin leading edges of titanium
airfoils that comprise the fan, low compressor and front stages of the high
compressor. Left undetected or uncorrected, FOD provides an ideal initiation
site for high cycle fatigue cracks to form and propagate within the airfoil.
In multi-engine military and commercial aircraft, this is not a safety issue
per se, but in single engine aircraft the results of an in-flight shutdown may
result in the total loss of the airframe or extensive damage. For these reasons,
FOD mitigation can have a substantial effect on maintenance costs, if the time
on wing could be extended, by reducing the number of unscheduled engine removals.
This presentation describes a vision for the application of deep surface
compressive residual stresses that can improve engine damage tolerance, average
time on wing (ATOW), and reduce engine maintenance costs. The perspective on the
use of an integrated design system that is required to reduce the time and cost
to implement these technologies as well as industry standards for advanced
surface treatments that could improve the speed and quality of original engine
manufacturers (OEM) applications will be discussed.
Shot
Peening: Some Background, Theory and Fundamentals
John Cammett. Cam-Met, Jacksonville, NC.
In principle, shot peening is a simple process traceable to early practices
of armorers and blacksmiths, yet, as in many cases, there is much more to it
than meets the eye. Modern shot peening traces its origins to the US automotive
industry in the 1930s wherein Dr J. O. Almen developed a peening technique to
enhance fatigue life and strength of automobile engine components. Enhancement
of fatigue resistance today remains as the primary purpose of the process,
now principally applied to aerospace structural and engine components and as well
still to automotive hardware. Enhanced fatigue resistance in metals stems from
compressive residual stresses induced in surface layers by plastic deformation
created by bombardment of surfaces with spherical media of various types. Shot
peening typically produces compressively stressed layers from a few thousandths
upwards to a few tens of thousandths of an inch in thickness depending upon media
size and intensity of bombardment. In addition to a brief anecdotal history
of peening, this presentation will involve the basics of what is important in
process performance, how residual stresses are created and how process parameters
affect resulting residual stresses.
Laser
Shock Processing Developments
Richard Tenaglia and David Lahrman. LSP Technologies, Dublin, OH.
Laser shock processing (also known as laser peening) is an innovative surface
enhancement technology used to increase tolerance of aircraft gas turbine
engine compressor and fan blades to foreign object damage (FOD) and to improve
high cycle fatigue (HCF) life. Laser peening has been implemented into
production operations for treatment of turbine engine blades for the B1-B
Lancer (F101 engine), the F-16 Falcon (F110 engine), and the F/A-22 Raptor
(F119 engine). Implementation of laser peening has provided considerable
savings in maintenance and inspection costs and boosted engine reliability,
crew safety, and mission readiness.
With technical achievements made during Air Force and Army ManTech programs, LSP Technologies, Inc. has been able to reduce the cost of laser peening by 50-75% and to increase processing throughput by six to nine times compared to earlier methods. LSP Technologies is now working with the Army and rotorcraft OEMs to develop laser peening for toughening propulsion system components for Army helicopters. An overview of laser peening and development efforts at LSP Technologies, Inc. is presented.
Low
Plasticity Burnishing in 2007
Paul S. Prevéy. Lambda Technologies, Cincinnati, OH.
Low plasticity burnishing (LPB) has been widely described in a variety of
applications to mitigate damage from foreign object damage (FOD), fretting,
corrosion pitting, corrosion fatigue, and stress corrosion cracking (SCC) in a
wide variety of aircraft and structural alloys. Comparison with laser shock
processing (LSP) has consistently shown that LPB can produce higher magnitude and
deeper compression in applications ranging from nuclear component welds to aircraft
structural features. LPB has now been in continuous production in a manufacturing
environment for over three (3) years, and is rapidly being introduced to a broad
array of aircraft engine and structural applications. The success of LPB is the
result of the development of a complete residual stress solution including design,
implementation, performance validation and continuous quality monitoring.
Lambda’s patented process for designing the required residual stress field, developing application specific tooling and CNC processing code, residual stress measurement and performance validation methods are described. The logistical benefits afforded by the ease of integration into existing manufacturing operations are presented with reference to a range of applications. The fatigue design methodology is described with application to an aircraft engine component. Examples of CNC processing control on both machine tools and multi-axis robots are described for a variety of LPB applications. Finally, the quality assurance system that provides statistical process control and 100% real time monitoring is presented including the benefits to insure that the required compression is achieved.
Laser Peening:
Hardware and Processes for Fatigue and Forming Production Applications
Jon Rankin. Metal Improvement Company, Columbia, MO.
Beneficial Effects of
Hole Cold Expansion Residual Stresses for Enhanced Fatigue and Damage Tolerance Life
Len Reid. Fatigue Technology, Seattle, WA.
Fatigue and damage tolerance life of metal structures subjected to cyclic
tensile stresses are greatly extended by practically inducing residual
compressive stresses around areas of high stress concentration such as
holes or penetrations in the structure. Aircraft structures in particular
benefit from a process that cold expands the hole using an expansion
mandrel pulled through an internally lubricated split sleeve, or by
radially expanding a pre-lubricated bushing in a hole, which locally
yields the surrounding material resulting in a zone of material with
beneficial residual stresses around the hole. These stresses are
typically compressive and effectively shield the hole from the applied
cyclic stresses. Furthermore they have been shown to reduce the stress
intensity factor for a crack growing from the hole, which will retard or
arrest the growth of small cracks. When used in either new construction
or repairs, application of the processes has been shown by test and
in-service experience to greatly extend the fatigue and damage tolerance
life of structures or components.
This paper will describe the methodology and include experimental test
results and service experience of the life enhancing benefits of the hole
cold expansion processes, including Split Sleeve Cold Expansion, expanded
bushings (the ForceMate process) and fastener systems. Examples of where
the technology is used in aircraft structures and other components such as
medical implants will be given along with supporting finite element
analysis of discrete applications and how these compare to practical
testing results will be shown. Recent methods to detect and measure
residual stresses around fastener holes will also be presented.
EDXRD
Capabilities for High-Resolution Mapping of Residual
Strain Fields and their Interplay with Applied Stress
Mark C. Croft1,2,
N. M. Jisrawi3,
Z. Zhong2,
V. Shukla,
R. Sadangi,
V. Holtz4,
K. Horvath1,
J. Skaritka2
K. Sadananda5, and
T. Tsakalakos3.
1: Rutgers University, Piscataway, NJ.
2: Brookhaven National Laboratory, NY.
3: Rutgers University, Piscataway, NJ.
4: Naval Research Laboratory, Washington, DC.
5: Technical Data Analysis, Falls Church, VA.
The technique of energy dispersive x-ray diffraction (EDXRD) for high resolution
mapping of the strain fields in engineering specimens will be discussed. This
techniques utilizes a high intensity/energy “white” beam synchrotron source and
probes the local material lattice parameter variations as the specimen is scanned
through a spatially fixed micro scattering (gauge) volume. Local strain variations
(in the interior of steel specimens up to 50 mm thick) over distances in the 10-50
µm range are routinely achievable. A very much larger spreading of the gauge
volume (GV) along at least one direction should be noted and techniques for
tailoring the GV for specific applications will be discussed. Strain gradients
induced by shot peening, sandblasting, laser peening, and welding processing will
be discussed. Recent high-resolution strain field mapping in the vicinity of
fatigue cracks, along with the response of these strain fields to in-situ loading
will be presented.
Measurement of
Residual Stresses in Full-Scale Welded Nuclear Components
David Smith. University of Bristol, Bristol, UK.
Knowledge of the origin, magnitude and distribution of residual stresses
generated during the manufacture of nuclear power plants is of vital importance
to their structural integrity assessment. The aim of the work described here was
to measure welding residual stresses in many different components prone to
stress-corrosion cracking in nuclear reactor pressure vessels. While there are
numerous residual stress measurement methods available, many are not amendable
to such large and complex components. The laboratory and on-site application of
the Deep-Hole Drilling (DHD) technique is described and applied to measure the
through-thickness residual stress distributions through five nickel alloy welds
representative of those found within nuclear reactor pressure vessels. The DHD
measurements have been compared with results from other measurement techniques
and finite element models, showing excellent agreements. Therefore the results
are important validations.
Role of Weld Residual
Stresses on Component Integrity Modeling of Pressurizer Nozzles
A. Csontos1,
D. Rudland2,
D. Shim2,
H. Xu2,
G. Wilkowski2,
F. Brust3 and
P. Scott3.
1: U.S. Nuclear Regulatory Commission,
2: Engineering Mechanics Corp.
3: Battelle Memorial Laboratory.
NRSF2 -
The Engineering Diffractometer at the HFIR
Cam Hubbard. ORNL, Oak Ridge, TN.
The second generation Neutron Residual Stress mapping Facility
(NRSF2) at HFIR has recently been commissioned and is available
to external users via the High Temperature Materials Laboratory
(HTML) User Program for studies of stress, texture and phase
mapping. The new facility replaces a facility that began operation in
1991 and has been shown to be nearly two orders of magnitude
more effective. The NRSF2 is located at the HB-2 beam at the High
Flux Isotope Reactor at ORNL. A doubly focusing monochromator
provides choice of six different wavelengths ranging from 1.45Å to
2.67 Å. The maximum flux is ~3x107 n/cm2/s. The sample
positioning system can handle samples up to 1000 lbs and four feet
in length. Slits, which define the gage or sampling volume, range in
size from 0.3 mm to 5 mm in width and to 20 mm in height. Seven
position sensitive detectors increase the fraction of the Debye cone
that is collected. Accessories include a uniaxial load frame, Huber
sample orienter, two different z-stages, furnaces and even a
superconducting magnet with induction heater insert. Advanced
software that provides calibration, real-time processing, and
experiment planning and rapid alignment further enhance the system.
Examples of studies by users will be highlighted.
Make
High Impact Discoveries with VULCAN
X.-L. Wang1, G. Q. Rennich1 and A. D. Stoica1,
T. M. Holden2,
P. K. Liaw3, H. Choo3 and
C. R. Hubbard4
1: Spallation Neutron Source, ORNL, Oak Ridge, TN.
2: Northern Stress Technology, Canada.
3: University of Tennessee, Knoxville, TN.
4: Metals and Ceramics Division, ORNL, Oak Ridge, TN.
The VULCAN diffractometer at the SNS is named after the Roman God of Fire and
Metalworking. As its name suggests, the instrument is designed for materials
science and engineering studies. Examples include high-speed and
high-resolution spatial mapping of residual stress distribution in components,
deformation behaviors under static and cyclic loading over a wide range of
temperatures, and time-dependent phase transformation phenomena.
The construction of VULCAN is supported by the Canada Foundation for Innovation.
In addition, the US National Science Foundation funded the sample environment
suite for VULCAN, which includes load-frames, furnaces, and electro-chemical
cells. The US Department of Energy, Office of Energy Efficiency and Renewable
Energy, provides additional funding for completing the instrument.
VULCAN is on schedule to be commissioned in 2008.
Analysis
of Hole Cold Expansion for Airframe Design and Service Life Extension
Dale Ball. Lockheed Martin Aeronautics Co., Fort Worth, TX.
The process of cold expanding holes in metallic components in order to extend
their fatigue life is now widely used throughout the aircraft industry.
Numerous experimental and analytical programs over the past three decades have
not only demonstrated how effective the process can be at extending fatigue
life, but have also shed considerable light on the mechanics of both the cold
expansion process and the growth of fatigue cracks in the resulting residual
stress fields. In spite of the progress that has been made, however, the
fatigue and fracture analysis community has yet to adopt a standard,
mechanics-based method for modeling the growth of fatigue cracks at cold worked
holes. In an effort to understand why this is so, this paper will review the
experimental and analytical techniques that have been employed over the past
several decades to the characterization of cold expansion induced residual
stress fields. The correlation (or lack thereof) between the two will be cited
in several cases. Methods for the inclusion of residual stresses in both
fatigue crack initiation and fatigue crack growth analyses will be described
in detail, and the ability of such analyses to simulate crack formation and
growth in laboratory experiments will be presented. Finally, results obtained
using current design analysis procedures for the inclusion of cold expansion
effects will be reviewed and contrasted with results based on explicit
representation of residual stresses in fatigue life calculations.
Modeling
of Residual Stress Improvements in FOD Tolerance
Adrian DeWald. Hill Engineering, McClellan, CA.
The development of effective surface treatments for new applications can be
difficult and time consuming. For cost and scheduling reasons it is often
advantageous to perform much of the iterative development work on prismatic test
coupons. Due to differences in geometry, however, the magnitude of the
compressive residual stress achieved in the test coupon can be different than
the magnitude of the compressive residual stress achieved in the part (using
the same processing steps). Furthermore, compressive residual stresses induced
by the surface treatment process must be equilibrated through the development
of tensile residual stresses elsewhere within the body. The location and
magnitude of the compensatory tensile residual stresses is difficult to predict
when moving from simple prismatic test coupons to complex 3D parts. Combined,
these two issues have resulted in poor performance of surface treated parts
during final check-out testing, causing significant portions of the development
process to be repeated.
This presentation provides an overview of the methodology and a summary of capabilities for a model that predicts the effects of residual stress inducing surface treatments on foreign object damage (FOD) tolerance. The proposed model gives an efficient means for predicting the residual stresses (compressive and tensile) in complex 3D parts, which is necessary for design. In addition, the model is capable of predicting the effects of changes in the processing parameters and coverage area, which provides an efficient means to iterate the process. Ultimately, the model is intended for use as a tool to help organizations adopt residual stress treatments with lower risk, lower cost, and reduced time to market.
Improving
Gear Performance by Surface Residual Compressive Stress
Lynn Ferguson, Andrew Freborg and Zhichao Li . Deformation Control Technology,
Cleveland, OH.
To meet a goal of at least a 25% improvement in helicopter gear tooth bending
fatigue life, several projects have been undertaken by DCT under US Army - Aviation
Advanced Technology Directorate (AATD) sponsorship to improve the compressive
residual surface stress state of carburized steel gears. Methods of increasing
the level of residual surface compression that have been evaluated include the
substitution of intensive quenching in place of conventional oil quenching during
quench hardening, conventional shot peening, and laser shockpeening. This paper
reports single tooth bending fatigue test results for carburized and quench
hardened Pyrowear 53 steel spur gears. The magnitudes and profiles of residual
surface stress developed by the mentioned methods are compared and related to the
bending fatigue data. Computer simulation of the processing methods is used to
help understand the differences in residual stress profiles. Also, initial models
of laser shockpeening and the potential increase in residual compression are shown.
Of significance is the importance of carrying the residual stress state from heat
treatment forward into the laser shockpeening models, and then to the final part
loading models for performance prediction.
Design,
Build, Monitor Near-Zero-Energy Homes
Jeff Christian. Buildings Technology Center, ORNL, Oak Ridge, TN.
This presentation describes 5 test houses with total energy costs as low as an
annual average of $0.42/day, compared to typical new construction of about $4/day
in the same East Tennessee environment. An opportunity is offered to pick
out either a one-story or two-story Zero-Energy Home, if you are ready to build.
Residual
Stress and Part Distortion in Machined Workpieces
Troy Marusich. Third Wave Systems, Minneapolis, MN.
Residual stresses in machined components arise both from primary processes in
the parent workpiece (rolling, forging, etc.) and from different plastic
deformation and thermal gradients induced by machining itself. There has been
significant research on predicting machining-induced residual stress, but
little work on the effects of both bulk and machining-induced stresses on part
accuracy. Third Wave Systems has formed a consortium of industries that are
interested in this problem, and is developing a comprehensive framework for
predicting part distortions.
Validation
of Etch Layer Removal Techniques Using Neutron and Synchrotron Radiation
Roger England1 and Thomas Watkins2.
1: Cummins, Inc., Charleston, SC.
2: ORNL, Oak Ridge, TN.
Planned
Upgrade for the BT8 Neutron Residual Stress Diffractometer
Thomas Gnaeupel-Herold. NIST Center for Neutron Research, Gaithersburg, MD.
The key elements of the proposed neutron diffractometer upgrade are discussed.
The design goal is to deliver both more neutrons onto the specimen as well as
counting more neutrons through better detectors. The first goal will be
accomplished by means of a composite monochromator consisting of up to three
individual monochromators, each delivering a different wavelength. As a result,
6 or more different specimen reflections can be measured simultaneously, thus
giving a performance comparable to neutron TOF instruments. The second goal
will be accomplished by a significant increase of detector coverage together
with improved neutron detection efficiency. A performance increase of at least
one order of magnitude is expected with the added benefit of obtaining stresses
less sensitive to elastic and plastic anisotropy. The simultaneous measurement
of multiple directions will allow the routine determination of the full stress
tensor.
Residual
Stress Measurement Using Piezospectroscopy
Michael Lance. ORNL, Oak Ridge, TN.
Piezospectroscopy is the measurement of stress in materials using characteristic
peak shifts of either vibrational or electronic spectra. The two most common
spectroscopic methods for stress measurement are Raman spectroscopy and
photo-stimulated luminescence spectroscopy (PSLS), which both employ the same
equipment for exciting and collecting spectra. Raman spectroscopy measures
vibrational energies which under load will slightly change due to the atomic
bonding in the material getting either weaker or stronger with load. The
resultant peak shift with stress can be calibrated and used to measure stress.
PSLS measures the emission of light caused by the excitation and subsequent
de-excitation of an electron of a dopant ion in a host lattice
(usually Cr3+ in Al2O3).
The energy of the emission will change under stress due to the crystal field
of the lattice altering the energy of the d- or f-shell electrons of the
dopant ion. Both techniques will be discussed with a focus on their
practical use for measuring residual stress in non-metallic materials such
as thermal barrier coatings, carbon nanotubes, silicon and alumina.
ASTM
Hole-Drilling Residual Stress Measurement Standard
Gary Schajer. University of British Columbia, Vancouver, Canada.
ASTM Standard Test Method E837 was first issued in 1981 and has had two major
revisions since then. This year, a third major revision is being undertaken to
extend the scope of the test method. Previously, the procedure was limited
to the case where the residual stresses are uniform through the thickness of the
test specimen. The proposed revision addresses the case where the residual stresses
can vary through the depth from the specimen surface.
ASTM
Committee E08 on Fatigue & Fracture – Investigations into Residual
Stress Effects on Fatigue and Fracture Test Methodologies
Steven R. Thompson. US Air Force Research Laboratory, WPAFB, OH.
Residual stresses are present in most all metallic materials to some degree or another.
Their presence were acknowledged by those who developed the fatigue and fracture
mechanics test methodologies currently in use today; however, there has always been
debate as to how to account for these stresses in test results, or even whether
they should be accounted for. Therefore, only cautionary notes were typically
inserted into the test standards. In recent years, there has been an increased
emphasis on quantifying the effect these stresses have on the properties resultant
from ASTM Committee E08’s test standards for fatigue and fracture. Committee E08
has initiated a research activity to determine the path forward for dealing with
residual stresses in test standards, whether they are manufacturing- or
intentionally-induced into the product form. This presentation will provide an
overview of this activity.
A
COncurrent approach for Manufacturing induced Part
distortion for Aerospace ComponenTs (COMPACT)
Richard Burguete. Airbus, Bristol, UK.
COMPACT is an European funded strategic research project to develop a better
understanding of how residual stress (RS) affects distortion in components
manufactured using aerospace aluminium alloys. Part distortion is a function
of RS and is caused by the complex relationships between material processing,
component design and manufacture. The aim of the project is to combine a
knowledge and understanding of materials processing, design and manufacturing
to predict and then mitigate the effect of RS so as to produce right first
time manufactured parts. In addition, a better understanding methods used to
correct distortion will be developed. Two further areas of research will enable
the research findings to be effectively applied to problem solving. These are
numerical simulation/modelling and knowledge integration. The knowledge
integration work will use this technology in order to assist or guide
cross-functional engineering teams in the decision making process. The work
will eventually enable costs savings to be made in manufacturing through the
minimisation of ‘scrap’, rework, concessions and the expensive re-processing
of aluminium. An outline of the project and some key results will be presented.