The present study examines the strain-rate sensitivity of four high strength, high-toughness alloys at strain rates ranging from 0.0002 s-1 to 200 s-1: Aermet 100, a modified 4340, modified HP9-4-20, and a recently developed Eglin AFB steel alloy, ES-1c. A refined dynamic servohydraulic method was used to perform tensile tests over this entire range. Each of these alloys exhibit only modest strain-rate sensitivity. Specifically, the strain-rate sensitivity exponent m, is found to be in the range of 0.004-0.007 depending on the alloy. This corresponds to a {approx}10% increase in the yield strength over the 7-orders of magnitude change in strain-rate. Interestingly, while three of the alloys showed a concominant {approx}3-10% drop in their ductility with increasing strain-rate, the ES1-c alloy actually exhibited a 25% increase in ductility with increasing strain-rate. Fractography suggests the possibility that at higher strain-rates ES-1c evolves towards a more ductile dimple fracture mode associated with microvoid coalescence.
Tensile and compressive stress-strain experiments on metals at strain rates in the range of 1-1000 1/s are relevant to many applications such as gravity-dropped munitions and airplane accidents. While conventional test methods cover strain rates up to {approx}10 s{sup -1} and split-Hopkinson and other techniques cover strain rates in excess of {approx}1000 s{sup -1}, there are no well defined techniques for the intermediate or ''Sub-Hopkinson'' strain-rate regime. The current work outlines many of the challenges in testing in the Sub-Hopkinson regime, and establishes methods for addressing these challenges. The resulting technique for obtaining intermediate rate stress-strain data is demonstrated in tension on a high-strength, high-toughness steel alloy (Hytuf) that could be a candidate alloy for earth penetrating munitions and in compression on a Au-Cu braze alloy.
Nano-electromechanical oscillators (NEMOs), capacitively-coupled radio frequency (RF) MEMS switches incorporating dissipative dielectrics, new processing technologies for tetrahedral amorphous carbon (ta-C) films, and scientific understanding of dissipation mechanisms in small mechanical structures were developed in this project. NEMOs are defined as mechanical oscillators with critical dimensions of 50 nm or less and resonance frequencies approaching 1 GHz. Target applications for these devices include simple, inexpensive clocks in electrical circuits, passive RF electrical filters, or platforms for sensor arrays. Ta-C NEMO arrays were used to demonstrate a novel optomechanical structure that shows remarkable sensitivity to small displacements (better than 160 fm/Hz {sup 1/2}) and suitability as an extremely sensitive accelerometer. The RF MEMS capacitively-coupled switches used ta-C as a dissipative dielectric. The devices showed a unipolar switching response to a unipolar stimulus, indicating the absence of significant dielectric charging, which has historically been the major reliability issue with these switches. This technology is promising for the development of reliable, low-power RF switches. An excimer laser annealing process was developed that permits full in-plane stress relaxation in ta-C films in air under ambient conditions, permitting the application of stress-reduced ta-C films in areas where low thermal budget is required, e.g. MEMS integration with pre-existing CMOS electronics. Studies of mechanical dissipation in micro- and nano-scale ta-C mechanical oscillators at room temperature revealed that mechanical losses are limited by dissipation associated with mechanical relaxation in a broad spectrum of defects with activation energies for mechanical relaxation ranging from 0.35 eV to over 0.55 eV. This work has established a foundation for the creation of devices based on nanomechanical structures, and outstanding critical research areas that need to be addressed for the successful application of these technologies have been identified.
Micromachines have the potential to significantly impact future weapon component designs as well as other defense, industrial, and consumer product applications. For both electroplated (LIGA) and surface micromachined (SMM) structural elements, the influence of processing on structure, and the resultant effects on material properties are not well understood. The behavior of dynamic interfaces in present as-fabricated microsystem materials is inadequate for most applications and the fundamental relationships between processing conditions and tribological behavior in these systems are not clearly defined. We intend to develop a basic understanding of deformation, fracture, and surface interactions responsible for friction and wear of microelectromechanical system (MEMS) materials. This will enable needed design flexibility for these devices, as well as strengthen our understanding of material behavior at the nanoscale. The goal of this project is to develop new capabilities for sub-microscale mechanical and tribological measurements, and to exercise these capabilities to investigate material behavior at this size scale.