Hysteresis and multistability are fundamental phenomena of driven nonlinear oscillators, which, however, restrict many applications such as mechanical energy harvesting. We introduce an electrical ...control mechanism to switch from the low to the high energy output branch of a nonlinear energy harvester by exploiting the strong interplay between its electrical and mechanical degrees of freedom. This method improves the energy conversion efficiency over a wide bandwidth in a frequency-amplitude-varying environment using only a small energy budget. The underlying effect is independent of the device scale and the transduction method and is explained using a modified Duffing oscillator model.
QCD critical point is a landmark region in the QCD phase diagram outlined by temperature as a function of baryon chemical potential. To the right of this second-order phase transition point, one ...expects first order quark–hadron phase transition boundary, towards the left a crossover region, top of it lies the quark–gluon plasma phase and below it the hadronic phase. Hence locating the QCD critical point through relativistic heavy-ion collision experiments is an active area of research. Cumulants of conserved quantities in strong interaction, such as net-baryon, net-charge, and net-strangeness, are suggested to be sensitive to the physics of QCD critical point and are therefore useful observables in the study of the phase transition between quark–gluon plasma and hadronic matter. We review the experimental status of the search for the QCD critical point via the measurements of cumulants of net-particle distributions in heavy-ion collisions. We discuss various experimental challenges and associated corrections in such fluctuation measurements. We also comment on the physics implications of the measurements by comparing them with theoretical calculations. This is followed by a discussion on future experiments and measurements related to high baryonic density QCD matter.
We describe a laser-launched micro-flyer apparatus designed for spall strength measurement. The launcher uses a single pulse from a pulsed laser that is stretched in time to nominally 20 nanoseconds ...using an optical ring cavity, while inexpensive multi-lens arrays are used to spatially homogenize the beam. The velocimetry technique that we developed for the experiment provides the required sub-nanosecond time resolution. We demonstrate the capability of the apparatus to interrogate the spall strength of AZ31B Mg alloy thin foils, a material system with potential applications as a lightweight protection material. Numerical simulations and fractography are very useful to determine the quality of the experimental data and help to interpret our results. The simulations and fractography analyses of the experiments suggest that the short shock pulse duration in the experiment causes incipient spallation. The short pulse also likely introduces stochasticity to the measured spall strength through limited activation of failure mechanisms within the samples. The shocked AZ31B Mg alloy has spall strengths that are greater than previously reported figures for fine grained Mg alloys, likely because the laser based system achieves higher strain rates than in prior work on this material.
The ratio of the yield strength to the density (specific strength) of magnesium and its alloys is relatively high, but cavitation-driven spall failure of these materials occurs at low threshold ...stresses and strains. The low failure threshold suggests a general susceptibility to catastrophic failure under tension at high rates of deformation. Higher material strength likely prevents such failure, but processing methods to achieve these strengths can introduce potential failure nucleation sites that can make failure more likely—the interplay between matrix strength and nucleation sites remains largely unknown. Here we examine the literature regarding experimental observation of spall failure of pure and alloyed magnesium with a particular emphasis on the trends between grain size and strain rate with respect to the spall failure strength. We then apply an analytical dynamic cavitation model to better understand the role of second phase particles in spall failure. We find that the conventional polycrystalline alloys with micron sized grains have microstructures containing second phase particles that act as failure nucleation sites, diminishing the potential to design against low failure threshold stresses. Additionally, nanocrystalline grained magnesium remains largely unexplored and may potentially offer an avenue for greater dynamic strength.
Pressure-shear plate impact experiments generate normal and transverse particle velocities during high strain rate deformations. Traditionally, freespace lenscoupled tabletop laser interferometry ...techniques are used together with diffraction gratings to interrogate the evolving velocity vector at the back face of the target plate. Recently, fiberoptic velocimetry (photon Doppler velocimetry or PDV) has become commonplace for measuring normal particle velocities above 200m/sec. In this work, we demonstrate transverse velocity detection using a modified PDV system where we subtract the measured normal velocity history from a concurrent velocity history measured at a canted angle to the target surface to obtain the transverse velocity component. This modified system is time-multiplexed to reduce the number of components, and uses an erbium doped fiber amplifier (EDFA) to boost the angled signal intensity while maintaining low noise. The system operates as a heterodyne interferometer, but features a frequency upshifted reference leg to improve data analysis at the particle velocities expected in the experiment. We demonstrate by direct comparison that this inexpensive and simple approach is as effective as traditional grating methods.
The linkage between the formal and informal credit markets in developing countries has largely been unexplored. This paper addresses one important aspect of the linkage by empirically investigating ...the impact of the intervention of microfinance programs on the moneylender interest rates in northern Bangladesh, and finds that moneylender interest rates increase with microfinance program coverage. Higher microfinance program coverage increases moneylender interest rates in the villages in which more loans are invested in productive economic activities than consumption. Borrowers resort to moneylenders for additional funds probably because of inadequate supply, unavailability of seasonal working capital from MFIs, and tight repayment schedules, which in turn increase demand for moneylender loans. PUBLICATION ABSTRACT
We examine the statistical distribution of critical nucleation pressures necessary to dynamically grow voids during the spall failure of an AZ31B magnesium alloy. The approach uses laser-driven ...micro-flyers to generate spall over times of the order of tens of nanoseconds, allowing us to focus on void nucleation processes rather than void coalescence processes. Our methodology combines quantitative postmortem characterization of void mediated failure with time-resolved interferometry of the failure event, and reveals the dynamics of the failure process. We infer the distribution of the underlying nucleation pressures and explore the associated strain rate dependence of spall strength in these alloys.
The high yield strength and low density, potential for further improvements in strength and ductility, and potential for use in defense applications of magnesium and its alloys have garnered serious ...interest in recent years. However, magnesium and its alloys have also shown a relatively low resistance to void-growth mediated spall failure when compared to other materials such as aluminum, copper, and titanium alloys. The lower spall strength indicates a propensity for failure under tension at the extremes. The common processing methods employed to manufacture these metals may introduce potential void nucleation sites at grain boundaries and second phase inclusions, which can make spall failure more likely. The relationship between the matrix strength and potential void nucleation sites remains largely unexplored in magnesium and its alloys. In this work, the literature regarding experimental observation of deformation mechanisms and spall failure processes active in single crystal and polycrystalline (pure and alloyed) magnesium is explored with a particular emphasis on examining deformation twinning and potential void nucleation sites through ex-situ (postmortem) microscopy. This concise note clearly exposes the need for more research to develop a better understanding of the underlying deformation mechanisms and failure processes active in single crystal and polycrystalline (pure and alloyed) Mg at the extremes.
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