Recently developed flexible mechanosensors based on inorganic silicon, organic semiconductors, carbon nanotubes, graphene platelets, pressure-sensitive rubber and self-powered devices are highly sensitive and can be applied to human skin. However, the development of a multifunctional sensor satisfying the requirements of ultrahigh mechanosensitivity, flexibility and durability remains a challenge. In nature, spiders sense extremely small variations in mechanical stress using crack-shaped slit organs near their leg joints. Here we demonstrate that sensors based on nanoscale crack junctions and inspired by the geometry of a spider's slit organ can attain ultrahigh sensitivity and serve multiple purposes. The sensors are sensitive to strain (with a gauge factor of over 2,000 in the 0-2 per cent strain range) and vibration (with the ability to detect amplitudes of approximately 10 nanometres). The device is reversible, reproducible, durable and mechanically flexible, and can thus be easily mounted on human skin as an electronic multipixel array. The ultrahigh mechanosensitivity is attributed to the disconnection-reconnection process undergone by the zip-like nanoscale crack junctions under strain or vibration. The proposed theoretical model is consistent with experimental data that we report here. We also demonstrate that sensors based on nanoscale crack junctions are applicable to highly selective speech pattern recognition and the detection of physiological signals. The nanoscale crack junction-based sensory system could be useful in diverse applications requiring ultrahigh displacement sensitivity.

Ultrasensitive mechanical crack-based sensor inspired by the spider sensory system

Field-assisted sintering technology/Spark plasma sintering is a low voltage, direct current (DC) pulsed current activated, pressure-assisted sintering, and synthesis technique, which has been widely applied for materials processing in the recent years. After a description of its working principles and historical background, mechanical, thermal, electrical effects in FAST/SPS are presented along with the role of atmosphere. A selection of successful materials development including refractory materials, nanocrystalline functional ceramics, graded, and non-equilibrium materials is then discussed. Finally, technological aspects (advanced tool concepts, temperature measurement, finite element simulations) are covered.

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Field-Assisted Sintering Technology/Spark Plasma Sintering: Mechanisms, Materials, and Technology Developments

Advances in physics

Review on superior strength and enhanced ductility of metallic nanomaterials

Bulk nanostructured (ns)/ultrafine-grained (UFG) metallic materials possess very high strength, making them attractive for high strength, lightweight and energy efficient applications. The most effective approach to produce bulk ns/UFG metallic materials is severe plastic deformation (SPD). In the last 30 years, significant research efforts have been made to explore SPD processing of materials, SPD-induced microstructural evolutions, and the resulting mechanical properties. There have been a few comprehensive reviews focusing mainly on SPD processing and the mechanical properties of the resulting materials. Yet no such a review on SPD-induced microstructural evolutions is available. This paper aims to provide a comprehensive review on important microstructural evolutions and major microstructural features induced by SPD processing in single-phase metallic materials with face-centered cubic structures, body-centered cubic structures, and hexagonal close-packed structures, as well as in multi-phase alloys. The corresponding deformation mechanisms and structural evolutions during SPD processing are discussed, including dislocation slip, deformation twinning, phase transformation, grain refinement, grain growth, and the evolution of dislocation density. A brief review on the mechanical properties of SPD-processed materials is also provided to correlate the structure with mechanical properties of SPD-processed materials, which is important for guiding structural design for optimum mechanical properties of materials.

Structural evolutions of metallic materials processed by severe plastic deformation

Plastic strain localization in metals: origins and consequences

Failure of metals II: Fatigue

The experimental and theoretical results of 50 years of research on the effects of electrical fields on deformation at high and low temperatures are reviewed. Application of currents to metals above a threshold in the range of about 3 × 102 to about 3 × 103 amp/cm2 (depending on material) increases the strain rate (i.e., decreases the stress). Above that threshold, the relationship between current density and strain rate is log-linear with a slope of about 3, independent of material. Many of the experiments were done in a “pulsed” mode to reduce Joule heating. The amount of Joule heating was measured and analytically eliminated from the results. Careful analysis of the data showed that the major effect of a field was to increase the frequency term . Reductions in the activation enthalpy and energy reductions due to the wind force were modest in terms of their effect on increasing the strain rate. The wind force on dislocations may be viewed as resulting from drift of the electron cloud; at low di...

The Effects of Electric Currents and Fields on Deformation in Metals, Ceramics, and Ionic Materials: An Interpretive Survey

Nickel-base superalloys are primarily used as components in jet engines and land-based turbines. While compositionally complex, they are microstructurally simple, consisting of small (501000nm diam...

https://royalsocietypublishing.org/doi/pdf/10.1098/rsta.2014.0128

Microstructural aspects of fatigue in Ni-base superalloys

The fatigue crack propagation (FCP) behavior of the nickel-base superalloy CMSX-2 in single-crystal form was investigated. Tests were conducted for two temperatures (25 and 700 °C), two orientations ([001][110] and [001][010]), and in two environments (laboratory air and ultra-high vacuum 10-7 torr). Following FCP testing, the fracture surfaces were examined using scanning electron microscopy (SEM). The FCP rates were found to be relatively independent of the temperature, environment, and orientation when correlated with the conventional mode I stress-intensity factor. Examination of the fracture sur-faces revealed two distinct types of fracture. One type was characterized by 111 fracture surfaces, which were inclined relative to both the loading and crack propagation directions. These features, al-though clearly a result of the fatigue process, resembled cleavage fractures along 111 planes. Such fea-tures were observed at 25 and 700 °C; they were the only features observed for the 25 °C tests. The second type had a macroscopically dull loading appearance, was microscopically rough, and grew normal to the loading axis. These features were observed on the specimens tested at 700 °C (in both air and vacuum) and appeared similar to conventional fatigue fractures. Although in this region the crack plane was mac-roscopically normal to the loading direction, it deviated microscopically to avoid shearing the y ’ precipi-tates. In view of the complex crack growth mechanisms, mixed fracture modes, and lack of any difference in FCP rates, it is hypothesized that the correlation between FCP rates and the stress-intensity parameter is probably coincidental. The implications for life prediction of higher temperature turbine components based on conventional fracture mechanics are significant and should be investigated further.

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Stephen D. Antolovich

Papers

Plastic strain localization in metals: origins and consequences

Failure of metals II: Fatigue

The Effects of Electric Currents and Fields on Deformation in Metals, Ceramics, and Ionic Materials: An Interpretive Survey

Microstructural aspects of fatigue in Ni-base superalloys

Fatigue crack propagation in single-crystal CMSX-2 at elevated temperature