Commercial Technologies for Maintenance Activities

A variety of landing gear components are failing without having reached their desired service lives. The failures are due primarily to stress corrosion cracking (SCC) and fatigue. The objective of this project was to improve the fatigue life and SCC resistance through the application of laser peening technology to selected components. Team members on this project were the Aging Landing Gear Life Extension Program (ALGLE) at Ogden Air Logistics Center, the Lawrence Livermore National Laboratory (LLNL), the University of California (Davis), and the Metal Improvement Company. The project was funded and managed through the National Center for Manufacturing Sciences (NCMS) Commercial Technologies Maintenance Activities (CTMA) program.

The project focused upon the T-38 main landing gear aluminum side-brace trunnion, which had a history of SCC failures in the transition radius. A material change in the trunnion corrected the SCC failures, but fatigue failures increased at the same location. The trunnion transmits side loads from the main landing gear through a side brace to the aircraft frame. In addition, the trunnion also contains a linear extension-retraction actuator, which when extended or retracted, rotates the trunnion. The maximum stresses in the transition radius of the trunnion are from the extension and retraction loads the aircraft experiences in service.

Peening is used to improve material resistance to stress-induced damage such as fatigue and SCC. The peening process compresses the material just below the surface, resulting in a residual compressed load. When tensile stresses are applied to the material, the overall amount of stress is reduced since the material starts out under compression. This reduced stress thus leads to improvement in fatigue life and SCC. Typically, steel shot is used to peen the surface of the metal part, which compresses the top 0.25 mm of the component. However, lasers have seen increasing use for peening the surface of the part. A laser pulse creates a shock wave that drives a residual stress up to 2 mm deep into the metal part. The increased depth of peening helps prevent crack propagation, and imparts superior performance.

The concept of laser peening is not new, but has not been cost effective compared to conventional peening. LLNL has developed a more economically favorable system, which pulses at a rate 20 times faster than other available systems and can peen about one square meter of metal per hour. Thus laser peening can compete with shot peening for application on fatigue critical parts.

The project demonstrated that shot and laser peening significantly increased the fatigue life of 7049-T73 aluminum. These results are captured in the following table:

Surface Treatment	Relative Fatigue Life Improvement
Surface Treatment	40 ksi stress level	50 ksi stress level	60 ksi stress level
None	1	1	1
Steel shot peened	18	2.4	1.5
Laser peened	>21	4.8	2.9

For SCC resistance, both laser and shot peening significantly increased the resistance compared to the untreated material. Unfortunately, testing was discontinued before any difference was found between the two peening methods.

All results were conducted on test coupons. The next step is to conduct full-scale fatigue tests on laser peened T-38 main landing gear trunnions. Trunnions are now replaced every 6 years. The laser peened results indicate that treated trunnions should have their life extended 9–12 years before replacement.

An extensive cost analysis for the use of laser peening to eliminate fatigue failure on the T-38 trunnion is summarized in the following table:

T-38 Trunnion	Without Peening	Laser Peened Trunnions
Set-up costs		$75,000
Costs over life of weapon system	$8,720,818	$7,330,706 (max) $5,498,029 (min)
Total costs	$8,720,818	$7,405,706 (max) $5,573,029 (min)
Cost avoidance over life of weapon system		$3,147,788 (max) $1,315,112 (min)

Obviously, laser peening can be applied to other fatigue critical parts on Department of Defense (DoD) weapon systems, and similar results should be expected. Therefore, cost avoidance to the DoD with the implementation of this technology would be in the tens of millions of dollars.

Another encouraging benefit for laser peening is that its use will not alter form, fit, or function, and thus the technology could be implemented without the multiple coordination and approval layers that would otherwise be present with a new technology insertion.