CAREER: Cyclic Plasticity and Fatigue Life of Small-Scale Metal Structures

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9876261VinciTwo objectives of this program on small-scale metallic structures are to understand plastic behavior related to cyclic loading and to identify mechanisms that determine fatigue lifetimes for low-and high-cycle regimes. Both cyclic plasticity and fatigue are evaluated in terms of sample state, particularly thickness and microstructure. The effects of testing conditions, including strain rate, ambient atmosphere and temperature, are evaluated. Emphasis is placed on identifying the key parameters that differentiate the behavior of freestanding metal thin films from that of bulk metals and that of thin metal films adhered to substrates. Unique apparatus under development fits into the specimen chamber of an Environmental Scanning Electron Microscope equipped with an Orientation Imaging Microscopy unit. This arrangement allows in-situ monitoring of surface morphology and microstructure during testing. Uniaxial tensile samples are used for simplicity of testing and analysis. Metal specimens are patterned at the Stanford Nanofabrication Facility using fabrication techniques developed in a preliminary study. Materials systems include aluminum (a FCC low melting point metal), iridium (a high melting point FCC metal), and molybdenum (a high melting point BCC metal). Improved understanding of thin film mechanical behavior will be transmitted to students through three paths: training of graduate and undergraduate research assistants, development of a specialized graduate mechanical behavior course, and integration of thin film mechanical behavior into the undergraduate curriculum.%%%Physics-based mechanical behavior models will allow dynamic modeling of device behavior and reliability for performance optimization. Technologies that will benefit include MEMS products such as high frequency RF switches and resonators for precision electronics. Relevant mesoscale applications include devices such as miniature aircraft and gas turbines for power generation. The target audience for this work includes university researchers, developers of design software, small-scale device manufacturers, and endusers of this technology.***