Residual stresses are of interest from an engineering perspective because they can have a significant influence on material performance. For example, fatigue initiation, fatigue crack growth rate, stress corrosion cracking, and fracture are all influenced by the presence of residual stress. Current design methods for aerospace structure typically assume that the material is residual stress-free.
Archives: Case Studies
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.
Forensic Determination of Residual Stresses from Fracture Surfaces
Residual stresses can be a main cause of fractures, but forensic failure analysis is difficult because the residual stresses are relaxed after fracture because of the new free surface. In this paper, a method is presented for a posteriori determination of the residual stresses by measuring the geometric mismatch between the mating fracture surfaces. Provided the fracture is not overly ductile, so that plasticity may be neglected, a simple, elastic calculation based on Bueckner’s principle gives the original residual stresses normal to the fracture plane. The method was demonstrated on a large 7000 series aluminum alloy forging that fractured during an attempt to cut a section into two pieces. Neutron diffraction measurements on another section of the same forging convincingly validated the residual stresses determined from the fracture surface mismatch. After accounting for closure, an analysis of the residual stress intensity factor based on the measured residual stress agreed with the material’s fracture toughness and fractographic evidence of the failure initiation site. The practicality of the fracture surface method to investigate various failures is discussed in light of the required assumptions.
Extending Life of Fighter Airframes
Fighter aircraft are developed at the cutting edge of technology. Aggressive engineering and reduced margins mean that early production aircraft can have performance shortfalls that are addressed by updates to manufacturing, materials, or software. Because it takes time to reveal shortfalls, there is a significant economic payoff in retrofits that bring the performance of early production aircraft in line with original performance objectives.
The F-22 Raptor is a tactical fighter aircraft developed for the United States Air Force (USAF) with a highly optimized titanium airframe. Full-scale structural testing of the F-22 airframe was carried out in parallel with initial production and identified early fatigue cracking in a number of structural details. A series of improvements were made in later airframes, and structural repairs and retrofits were deployed to improve performance of early production aircraft.
USAF F-22 Raptor tactical fighter aircraft
Hill Engineering had a role in repairing the F-22 wing-attach lugs that carry wing loads into the fuselage. The lug repairs were difficult because the lugs are an integral part of a single-piece, welded section of the titanium fuselage that includes attach points for the wing, engine, and horizontal tail. Working closely with Boeing, Lockheed Martin, and the USAF, Hill Engineering supported design, development, and certification of laser shock peening (LSP) treatments for structural repair. LSP creates a layer of compressive residual stress that improves resistance to fatigue crack initiation and slows fatigue crack growth, and initial tests showed that LSP could extend service life of the F-22.
Illustration of F-22 structure showing wing-attach lugs where laser shock peening was applied
Hill Engineering worked closely with Boeing during design and planning of the engineering program and performed residual stress measurements (slitting and contour methods), residual stress predictions (using ERS-Toolbox®), and assessments of fatigue crack growth to quantify service life improvements. The wing-attach repairs provided significant cost savings for the F-22 program as a result of increased airframe service life and reduced depot maintenance.
High-speed image of a plasma burst during laser shock peening on a test coupon
The F-22 wing-attach repair required Hill Engineering to further develop methods and tools to address challenging problems with structural materials. On-going efforts support complex, safety-critical applications in aircraft structures, petrochemical processing equipment, power generation systems, welded structures, and turbine engines.
Reference information:
Electron Beam Welding of F-22 Structures, R. Zenas, 41st Structures, Structural Dynamics, and Materials Conference, Atlanta, GA, Apr 2000.
Status of F/A-22 Full Scale Fatigue Test, S. Welsh, USAF ASIP Conference, Memphis, TN, Nov 2004.
F-22 Laser Shock Peening Depot Transition and Risk Reduction, K. MacGillivray, et al, USAF ASIP Conference, San Antonio, TX, Dec 2010.
Design and Analysis of Engineered Residual Stress Surface Treatments for Enhancement of Aircraft Structure, M. Hill, et al, USAF ASIP Conference, San Antonio, TX, Nov 2012.
Verification of Analytical Methodology to Minimize Inspection Burdens and to Utilize Full Benefits of Residual Stress Life Enhancement Technique, H. Cai, et al, USAF ASIP Conference, San Antonio, TX, Dec 2013.
Forging Residual Stress
Metallic materials inherit their mechanical properties through various processing steps that are optimized to provide useful spatial distributions of constituents, mechanical deformation, and orientation. Among forming methods for metals, the forging process typically provides parts with superior static strength and fatigue performance. For this reason, forgings are well-suited for use in key structural components requiring strength and durability.
In many aluminum alloys, quenching from high temperature enables high strength, but it also leaves significant levels of residual stress. Residual stress fields require specific attention, especially with respect to managing distortion of parts machined from forgings and in anticipating the effects of residual stress on part performance. When residual stress is not managed appropriately, excessive distortion drives part rejection in manufacturing, and residual stresses shorten service life by accelerating corrosion and fatigue cracking.
Hill Engineering works in the metallic materials supply chain to deliver residual stress technologies for measurement and modeling that benefit suppliers, system integrators, and end users across industry. Some of our most advanced work is in aircraft materials, where our measurements are being used to improve and validate advanced modeling aimed to revolutionize the design of forgings, with specific attention to residual stress fields.
Supported by the US Air Force, and working closely with the F-35 aircraft manufacturer and a key material supplier, Hill Engineering developed and applied residual stress measurement technology for very large aluminum forgings used to produce F-35 bulkheads, frames, and spars.
Cut-away of F-35 showing large single-piece bulkheads, frames, and spars made from forgings
Forging for a single piece bulkhead on F-35 variant with short takeoff and vertical landing capability
Contour method measurements confirmed what was predicted by modeling, that high residual stresses in quenched material are relieved to low levels by cold compression with specially designed dies. The measurement capabilities provided the F-35 team with data that supported their use of airframe materials.
By developing technology to serve the metallic materials supply chain, Hill Engineering delivers capabilities needed to solve challenging problems with structural materials. Contact us to see what we can do together.
Residual stress field in a quenched forging (top) and a cold-compressed forging (bottom) – model forecast (left) and measurement data (right) with tension in red and compression in blue
Reference information:
Forgings – What we Make, 2016, Alcoa Defense.
An Integrated R&D Roadmap for Residual Stress Management in Large Structural Forgings, M. James, et al, 4th Residual Stress Summit, Tahoe City, CA, Sep 2010.
The Impact of Forging Residual Stress on Fatigue in Aluminum, D. Ball, et al, AIAA SciTech, Kissimmee, FL, Jan 2015.
Development of the Contour Method
The contour method uses careful sectioning and precision inspection techniques to develop a two-dimensional map of residual stress on a plane through a part. The contour method was invented circa 2000 by Dr. Mike Prime of Los Alamos National Laboratory.
Hill Engineering staff were instrumental in commercialization of the contour method, working in close collaboration with Dr. Prime as early as 2000, and licensing the technology in 2006 from Los Alamos under US Patent 6,470,756 B1.
The first commercial applications of the contour method were related to the development of laser shock peening and improving manufacturing processes like welding and forging. Investments from the federal Small Business Innovative Research (SBIR) program have allowed us to demonstrate the usefulness of the contour method, and to enhance the quality, precision, and capability of the measurements we perform.
The contour method provides a unique capability to reveal the two-dimensional distribution of residual stress on a plane cut through a part, referred to as a residual stress map. After more than a decade of research and development, Hill Engineering is the industry leader in providing contour method stress mapping services for industry.
Further information on the contour method is provided elsewhere on our site, and other case studies illustrate its use.
Illustration of the contour method principle
Residual stress measured in a Ti-6Al-4V weld using the contour method superimposed on top of a photograph of the specimen