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.
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.
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.
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.
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.