Years of research have shown that high strength steels used in the main landing gear of aircraft are prone to stress corrosion cracking (SCC) and fatigue. Operators can spend millions each year on inspection and repair to ensure safety and remain compliant with airworthiness directives (AD). In certain cases, landing gear may even need to be completely replaced to solve the problem, adding a costly unscheduled maintenance cycle.
The combination of SCC with vibration during braking or “gear walk,” can be particularly problematic. On some popular models, landing gear is currently inspected every 450 landings, costing an estimated $2 million annually for the industry. Redesigned braking systems and constant sanding and repainting have done little to offset the issue.
Low Plasticity Burnishing
A solution to cracking in landing gear that is cost-effective, reliable and permanent is Low Plasticity Burnishing (LPB). Created by Lambda Technologies, LPB is a mechanical means to combat fatigue initiating damage, including FOD, corrosion pitting, fretting-induced cracking, and SCC. LPB induces a deep, stable layer of residual compression in the surface of a part, mitigating fatigue crack initiation and growth. Because SCC requires surface tension above a certain threshold level, LPB compression can eliminate SCC in high strength alloys.
Extensive work was done on LPB treated landing gear steels to determine the influence of many pre and post processing operations, like repeated shot peening, plating, stripping, and other MRO functions. Tests show that LPB eliminates SCC and the adverse effects of foreign object damage, restoring fatigue performance equivalent to, or sometimes even better than, brand new pieces. Existing fatigue damage up to 0.030 inch deep, easily detectible with conventional NDT, was fully arrested by LPB compression exceeding 0.050 inch deep. Treated pieces showed an overall 12X increase in fatigue life.
How does it work?
Tension in metallic components induces crack formation from stress concentrations formed by surface damage. Solutions involving changing the alloy or completely redesigning the component are expensive and time consuming. The LPB process eliminates the need to alter either the material or design. Producing a layer of high-magnitude compression deeper than the surface damage deters SCC and fatigue crack initiation, from undetectable micro cracking up to visible damage from FOD or corrosion.
Unlike traditional surface treatments like shot peening and deep rolling, LPB provides protection with very little cold work. This makes the compressive layer stable under either thermal exposure following plating or mechanical overload in service. The result is a mirror-like surface finish that is less chemically active than a highly cold worked surface.
Regulation and acceptance
LPB is FAA accepted for repair and alteration of both structural and engine aircraft components. Lambda has teamed with Delta TechOps to offer the process to the commercial fleet. Currently, Delta TechOps and Lambda are working to implement LPB as an A. D. Alternate Means of Compliance (AMOC) for landing gear that normally require expensive frequent inspections or replacement. The process is not a PMA replacement. All LPB treated parts remain 100 percent original OEM pieces.
Implementing solutions
The process mitigates cracking in a single maintenance cycle, and never requires reapplication. Basic CNC machines like mills, lathes, and robots are used making integration into production easy during either manufacture or by an MRO specializing in landing gear repair and overhaul. Technicians aren’t required to learn specialized foreign equipment or be certified in any special way. Robotic LPB systems can be used in a hangar environment, allowing landing gear components to be processed without removal from the aircraft and increasing time in service.
Summary
LPB can dramatically improve the damage tolerance of landing gear and mitigate failure from SCC. Parts can withstand larger loads with longer intervals between inspections, saving time and money on maintenance operations.
Doug Hornbach is the president and laboratory director for Lambda Technologies. He has been with the company since 1992. For more information please visit www.LambdaTechs.com.
