N092-120 TITLE: Innovative, Low Cost Surface Treatment Method for Hydraulic Tube Fatigue Property Improvement

TECHNOLOGY AREAS: Materials/Processes

ACQUISITION PROGRAM: PMA-261, H-53 Heavy Lift Helicopter Program

OBJECTIVE: Develop and demonstrate an innovative surface treatment process to improve the fatigue performance and damage tolerance of aircraft hydraulic lines.

DESCRIPTION: Aircraft use thin-walled, high-strength tubing for hydraulic systems and, as wall thickness is reduced to save weight, the ability of the tubing to withstand flaws, induced either in manufacturing or in service, becomes critical. High performance platforms such as the H-53 will consider titanium alloys for tubing to save weight and provide high system pressure capability. Using higher operating pressures allows actuators to be smaller and hence lighter, saving weight. However, these thin walled tubes are very susceptible to outside diameter damage. In particular, where shallow marks even only 0.006" deep have been shown to cause rapid fatigue failure.

Novel solutions are being sought to enhance the fatigue life and damage tolerance by introducing deep residual compressive stresses on the outside and inside surface of hydraulic tubing. The novel process should be low cost and capable of being implemented on completed bent tubes, including welded or swaged end fittings. The process should not cause damage on the surface finishing of the tubes and the residual stresses must be adversely reduced after prolonged exposure to operating temperatures of 300 degrees F.

Goals include an increase in the flaw tolerance of the tubing thereby increasing aircraft reliability and availability, make tubing inspection easier due to the increase in the critical flaw size, and possibly allow even lighter tubing than the titanium tubing, specified in AMS 4945, to be employed. Since hydraulic systems are frequently flight critical, an increase in tubing reliability would increase the safety margin for personnel.

PHASE I: Design and demonstrate the feasibility of generating deep residual compressive stresses in titanium hydraulic tubing. Fatigue test a small number of processed tubes to demonstrate improved flaw tolerance. Measure tube residual stresses.

PHASE II: Finalize and optimize the process developed in Phase I, model the tube stress state, measure the induced residual stresses and demonstrate improved flaw tolerance with a statistically significant number of samples. Demonstrate full hydraulic tubes, including bends and end fittings surface treated and life tested.

PHASE III: Transition the technology into full scale development. Qualify and flight test the surface treated tubes. Transition the technology to program offices or primary contractors.

PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: If the technology proves to be successful in this application, it could be applied to other aircraft, including commercial jet hydraulic and engine fuel systems. The technology may also be suitable to other tubing applications where weight and cost and fatigue life are a concern, such as automotive diesel delivery systems.

REFERENCES:
1. Lin, Yaomin, et al., "Nanosecond pulsed laser micromachining for experimental fatigue life study of Ti-3.5Al-2.5V tubes" Optics and Lasers in Engineering,Volume 47, Issue 1, January 2009, pages 118-122.

2. Li, B., and Gupta, M. C., "Crack growth life of Ti-3Al-2.5V tubes under internal impulse pressure", Materials Science and Engineering, A431 (2006) 146-151.

KEYWORDS: surface treatment; hydraulic tubing; fatigue life; damage tolerance; titanium tubing; low cost process

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