Durability of Fe-Co thin films on Ti-6Al-4V
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MATERIALS damage prognosis has been proposed as a possible new approach for managing the health of aircraft structures, propulsion systems, as well as the entire system of military vehicles.[1] As highlighted in a recent symposium,[1] this revolutionary concept requires the integration of three key technologies: (1) real-time on-board interrogation of the material damage states during service, (2) physics-based models of the life-limiting processes in individual components and subsystems, and (3) automated reasoning to combine all available information on the past and present states of component subsystems and systems to make a robust prediction of the future capability of an aircraft. One of the key requirements of this revolutionary approach to vehicle health management is the use of on-board sensors[2,3] that are durable and maintain their functionality over a duration longer than the expected component life. A recent study demonstrated that real-time continuous health monitoring of critical engine components for fatigue damage or defects by on-board sensors can achieve significant improvement in component reliability with relatively low interrogation sensitivities.[3] In particular, the study[3] showed that a sensor with a crack detection limit as low as a 5-mm crack can significantly enhance the ability to detect and predict the onset of fracture and the fatigue life when the sensor monitors and measures the fatigue crack length continuously. In addition, magnetostrictive thin films have been shown to exhibit the required performance (resolution of a crack in the range of 2 to 3 mm in length) as thin film sensors for direct detection and monitoring of fatigue cracks at ambient temperatures.[3] Moreover, enhanced magnetostrictive sensor performance appears to be achievable through optimization of the thin film architecture and composition.[3] Comparatively very little is known about K.S. CHAN, Institute Scientist, S.J. HUDAK, Jr., Institute Scientist and Program Director, C.E. SMITH, Graduate Student, and G.M. LIGHT, Director of R&D, are with the Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78238. Contact e-mail: [email protected] B.R. LANNING, formerly with the Southwest Research Institute, is now Manager, Thin Films, ITN Energy Systems, 8130 Shaffer Parkway, Littleton, CO 80127. A. VEIT, formerly with the Southwest Research Institute, is now Quality Assurance Engineer, with the University of Iowa, 2401 Oakdale Blvd., Iowa City, IA 52242-5003. Manuscript submitted August 23, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS
the durability of magnetostrictive thin films under monotonic and cyclic loading conditions at ambient or elevated temperatures. For durability of thin films, the relevant material properties are the adhesion, interface toughness, and fatigue resistance. Studies of metallic thin films have revealed that the fatigue life of Cu and Ag thin films depends on film thickness.[4,5] In general, thin films are more fatigue-resistant than thicker films.[5] The stress amplitude at a constant fat
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