Cavitation Peening of Hydraulic Tubes for Fatigue Improvement

Aircraft use thin-walled high-strength tubing for hydraulic systems. 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 aircraft use titanium alloys 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 even very shallow marks, 0.004” deep, have been shown to suffer substantial fatigue life reduction. The proposed solution is to make the tubing more tolerant of flaws by inducing deep residual compressive stresses using a new surface treatment process, cavitation peening. This project will develop the methods and tooling to adapt the cavitation peening technology so that it can be applied to tubing outside and inside diameters to generate high magnitude, deep and stable residual stresses. This low cost technology will add no weight to the aircraft while improving reliability and flaw tolerance. Phase I has demonstrated the benefits of the technology, improving the fatigue life of damaged tubing by a factor of 20.