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This investigation takes an analytical approach to the oscillations of a cantilevered plate immersed in a hypersonic flow with shock impingement. In particular, we derive a mathematical model keyed in on the fact that (1) the shock impingement point moves along the structure as it oscillates and (2) the local curvature of the structure changes the shock reflection angle and thus the compressible flow properties. For cantilever boundary conditions, large motion at the free end my render both effects significant. We show that the movement of the shock impingement point varies approximately with the cotangent of the oblique shock angle and this implicit relationship generates surprisingly strong nonlinear effects in the equation governing the first mode of vibration. A new, geometrically modified third-order Piston Theory is adapted to accurately capture structural curvature induced changes in flow properties as well as possible expansion wave interaction. Our model takes the form of a nonlinearly damped Duffing oscillator with quadratic and cubic nonlinearities stemming from the nonlinear aerodynamic generalized forces as well as geometric and inertial structural nonlinearities. Despite inviscid flow modeling, a perturbation solution to the governing equation shows good agreement with structural oscillations predicted by high-fidelity turbulent computational fluid dynamic simulations. [ABSTRACT FROM AUTHOR]

This paper examines the viability of completely passive control approach based on multiple timescale perturbation methods to elicit desired dynamic cancellation or suppression of nonlinear vibration characteristics. Without appeal to feedback, auxiliary oscillating systems, or nonlinear energy sinks, we demonstrate how inertial nonlinearity balancing can more simply realize distortion-free vibrational responses that are robust to very strong forcing amplitudes across resonant, super-harmonic, and sub-harmonic excitation frequencies as well as opportunities to passively suppress hysteresis, cusp-fold bifurcations, and higher harmonics. Of particular merit are the variegated results concerning critical design points for passive control of sub-harmonic resonances. To most simply capture the essence and broad relevance of the approach, we focus upon a variant of the Duffing oscillator that describes moderately large amplitude vibration of a cantilever beam, various link systems, and kinematically constrained particle motion. The results herein are anticipated to be the most relevant to NEMS/MEMS- based sensing research and technology as well as vibration-based mechanical energy harvesting or mechanical filtering. [ABSTRACT FROM AUTHOR]

We propose and experimentally validate a first-principles based model for the nonlinear piezoelectric response of an electroelastic energy harvester. The analysis herein highlights the importance of modeling inherent piezoelectric nonlinearities that are not limited to higher order elastic effects but also include nonlinear coupling to a power harvesting circuit. Furthermore, a nonlinear damping mechanism is shown to accurately restrict the amplitude and bandwidth of the frequency response. The linear piezoelectric modeling framework widely accepted for theoretical investigations is demonstrated to be a weak presumption for near-resonant excitation amplitudes as low as 0.5 g in a prefabricated bimorph whose oscillation amplitudes remain geometrically linear for the full range of experimental tests performed (never exceeding 0.25% of the cantilever overhang length). Nonlinear coefficients are identified via a nonlinear least-squares optimization algorithm that utilizes an approximate analytic solution obtained by the method of harmonic balance. For lead zirconate titanate (PZT-5H), we obtained a fourth order elastic tensor component of c1111p=-3.6673×1017 N/m2 and a fourth order electroelastic tensor value of e3111=1.7212×108 m/V. [ABSTRACT FROM AUTHOR]

The research findings reported in the 3D-Printed Actuators Special Issue show promising approaches for engineering biology-like actuation. The endurance of 3D-printed actuators, especially soft actuators, is a well-known engineering challenge. After decades of curiosity-driven innovation and engineering advancements of 3D-printed actuators, we continue to witness their endless impacts and ever-expanding opportunities in many applications that touch our lives. [Extracted from the article]

A review of past and recent developments in multiaxial excitation of linear and nonlinear structures is presented. The objective is to review some of the basic approaches used in the analytical and experimental methods for kinematic and dynamic analysis of flexible mechanical systems, and to identify future directions in this research area. In addition, comparison between uniaxial and multiaxial excitations and their impact on a structure's life-cycles is provided. The importance of understanding failure mechanisms in complex structures has led to the development of a vast range of theoretical, numerical, and experimental techniques to address complex dynamical effects. Therefore, it is imperative to identify the failure mechanisms of structures through experimental and virtual failure assessment based on correctly identified dynamic loads. For that reason, techniques for mapping the dynamic loads to fatigue were provided. Future research areas in structural dynamics due to multiaxial excitation are identified as (i) effect of dynamic couplings, (ii) modal interaction, (iii) modal identification and experimental methods for flexible structures, and (iv) computational models for large deformation in response to multiaxial excitation. [ABSTRACT FROM AUTHOR]

Nonlinear piezoelectric effects in flexural energy harvesters have recently been demonstrated for drive amplitudes well within the scope of anticipated vibration environments for power generation. In addition to strong softening effects, steady-state oscillations are highly damped as well. Nonlinear fluid damping was previously employed to successfully model drive dependent decreases in frequency response due to the high-velocity oscillations, but this article instead harmonizes with a body of literature concerning weakly excited piezoelectric actuators by modeling nonlinear damping with nonconservative piezoelectric constitutive relations. Thus, material damping is presumed dominant over losses due to fluid-structure interactions. Cantilevers consisted of lead zirconate titanate (PZT)-5A and PZT-5H are studied, and the addition of successively larger proof masses is shown to precipitate nonlinear resonances at much lower base excitation thresholds while increasing the influence of higher-order nonlinearities. Parameter identification results using a multiple scales perturbation solution suggest that empirical trends are primarily due to higher-order elastic and dissipation nonlinearities and that modeling linear electromechanical coupling is sufficient. This article concludes with the guidelines for which utilization of a nonlinear model is preferred. [ABSTRACT FROM PUBLISHER]

Absent in the human security literature is the role institutions play in determining levels of human security in developing nations. We attempt to determine if human security primarily is a phenomenon of economic development, institutional quality, geography, or some combination. Of these factors, we explore ‘deep determinants’: factors that are strongly correlated with increases or sustained levels of human security. We depart from the mainline literature by applying our model to a sample of former colonies. This narrow focus allows us to concentrate on those populations facing the greatest human security risks. While economists have investigated some elements of the general human security puzzle, thus far their research has produced statistically robust yet theoretically thin findings. Our model attempts to correct for these theoretical shortcomings through the inclusion of strictly focused political and social variables. We find that various deep determinants do indeed contribute to human security in surprising ways. [ABSTRACT FROM AUTHOR]

Although there has been much debate about whether democratization causes ethnic conflict, and many comparativists have argued about which kinds of political institutions are best for managing communal strife, little large- N work has addressed these issues. The authors apply a theory of ethnic conflict—the ethnic security dilemma—to derive predictions about the impact of democratization and political institutions on ethnic unrest. They then test these predictions by performing a series of pooled time-series analyses covering all ethnic groups in the Minorities at Risk data set from 1985 to 1998. The authors find that democratization, federalism, and presidentialism may not be as problematic as some argue and that proportional representation tends to reduce severe ethnic violence. They conclude by suggesting some directions for future research. [ABSTRACT FROM AUTHOR]

This letter investigates the nonlinear response of a bimorph energy harvester comprised of lead zirconate titanate (PZT-5A) laminates. For near resonant excitations, we demonstrate significant intrinsic nonlinear behavior despite geometrically linear motion. Fourth order elastic and electroelastic tensor values for PZT-5A are identified following methods recently published concerning a PZT-5H bimorph. A response trend indicative of a nonlinear dissipative mechanism is discussed as well as the inadequacy of linear modeling. The PZT-5A bimorph exhibits an increased softening frequency response in comparison to PZT-5H. The results contained herein are also applicable to electroelastic sensor and actuator technologies. [ABSTRACT FROM AUTHOR]

We model and experimentally validate a nonlinear energy harvester capable of bidirectional hysteresis. In particular, both hardening and softening response within the quadratic potential field of a power generating piezoelectric beam (with a permanent magnet end mass) is invoked by tuning nonlinear magnetic interactions. Not only is this technique shown to increase the bandwidth of the device but experimental results additionally verify the capability to outperform linear resonance. Engaging this nonlinear phenomenon is ideally suited to efficiently harvest energy from ambient excitations with slowly varying frequencies. [ABSTRACT FROM AUTHOR]

Integrating robotic collaborators into experimental practice may give rise to an exciting new paradigm. [ABSTRACT FROM AUTHOR]