scholarly journals Dynamic excitations of chiral magnetic textures

APL Materials ◽  
2020 ◽  
Vol 8 (10) ◽  
pp. 100903
Author(s):  
Martin Lonsky ◽  
Axel Hoffmann
Keyword(s):  
Author(s):  
Frank Fan Wang

It is a challenge to correlate different dynamic loads. Often, attempts are made to compare the peak acceleration of sine wave to the root mean square (RMS) acceleration of random vibration and shock. However, peak sine acceleration is the maximum acceleration at one frequency. Random RMS is the square root of the area under a spectral density curve. These are not equivalent. This paper is to discuss a mathematical method to compare different kinds of dynamic damage at the resonant point of the related electronic equipment. The electronic equipment will vibrate at its resonance point when there are dynamic excitations. The alternative excitation at the resonant frequency causes the most damage. This paper uses this theory to develop a method to correlate different dynamic load conditions for electronic equipment. The theory is that if one kind of dynamic load causes the same levels of damaging effects as the other, the levels of vibration can then be related.


Author(s):  
Bruce M. Douglas ◽  
Emmanuel A. Maragakis ◽  
Bhabananda Nath

This paper outlines a new method for performing base line corrections on accelerograms generated by the quick-release dynamic testing method. These accelerograms are produced when highway bridges are subjected to dynamic excitations by initially deforming the bridge structure and subsequently quick-releasing the loads causing the deformation. This base line correction method is developed in such a way that the step function character of the quick-release base line is preserved. It is this feature which allows the static deformations of the structure to be recovered by integrating the accelerograms. The reliability of the method is demonstrated by a series of laboratory measurements comparing the displacement time series obtained by double integration to independently measured displacement responses. These laboratory comparisons indicate that the offset displacements are accurate to within about 5% on the average. The method was applied on accelerograms obtained during a quick-release full-scale test of the Meloland Road Overcrossing, and some examples of this application are presented.


2006 ◽  
Vol 74 (3) ◽  
pp. 590-594 ◽  
Author(s):  
Amit Shaw ◽  
D Roy

An analysis of large deformations of flexible membrane structures within the tension field theory is considered. A modification of the finite element procedure by Roddeman et al. (Roddeman, D. G., Drukker, J., Oomens, C. W. J., Janssen, J. D., 1987, ASME J. Appl. Mech. 54, pp. 884–892) is proposed to study the wrinkling behavior of a membrane element. The state of stress in the element is determined through a modified deformation gradient corresponding to a fictive nonwrinkled surface. The new model uses a continuously modified deformation gradient to capture the location orientation of wrinkles more precisely. It is argued that the fictive nonwrinkled surface may be looked upon as an everywhere-taut surface in the limit as the minor (tensile) principal stresses over the wrinkled portions go to zero. Accordingly, the modified deformation gradient is thought of as the limit of a sequence of everywhere-differentiable tensors. Under dynamic excitations, the governing equations are weakly projected to arrive at a system of nonlinear ordinary differential equations that is solved using different integration schemes. It is concluded that implicit integrators work much better than explicit ones in the present context.


Author(s):  
Benjamin A. Goodpaster ◽  
Ryan L. Harne

Slender, lightweight structures are demanded to meet efficiency targets or to enhance vehicle system performance characteristics. Yet, when subjected to static stress for load-bearing purposes, the flexible structural members may buckle. Furthermore, additional dynamic excitations may activate adverse snap-through responses in such post-buckled components, which accelerates fatigue and failure. The severe nonlinearity associated with these phenomena challenges traditional forms of analysis and necessitates studious experimental methods for conclusive system characterization and model validation. This research builds upon state-of-the-art analytical and experimental strategies to examine the complex forced, dynamic behaviors of built-up structures that contain one or more post-buckled members. An analytical modeling and solution formulation is reviewed that is uniquely amenable to the study of multistable structures and permits experimentally-observable measures of impedance to be identified. Through theoretical and experimental studies, the efficacy of the impedance measures is evaluated towards their usefulness in identifying the onset of dynamic bifurcations in the multistable structural dynamics. For moderate amplitudes of input energy, the analysis is found to provide qualitatively accurate prediction of the drive point impedance changes observed prior to dynamic bifurcations from low to high amplitude of displacement.


Author(s):  
Claude Bohatier ◽  
Brahim Chetouane ◽  
Marc Vinches

The discrete element modelling allows the study of the mechanical behaviour of a collection of solids, submitted to dynamic excitations. The proposed definition of a stress tensor concerns a cell constituted of its kernel, and its neighbouring solids. We demonstrate that taking into account the inertial effects allows the symmetry of the stress tensor. The relative weight of the centrifugal effects remaining in the final expression of the stress tensor is evaluated in order to determine whether or not it has to be taken into account, depending on the application. The proposed definition allows the definition of the load level on different parts of the discontinuous structure. Applications to real masonry cases are presented.


Science ◽  
2019 ◽  
pp. eaat8687 ◽  
Author(s):  
Patrick Gallagher ◽  
Chan-Shan Yang ◽  
Tairu Lyu ◽  
Fanglin Tian ◽  
Rai Kou ◽  
...  

Graphene near charge neutrality is expected to behave like a quantum-critical, relativistic plasma—the “Dirac fluid”—in which massless electrons and holes rapidly collide at a rapid rate. We measure the frequency-dependent optical conductivity of clean, micron-scale graphene at electron temperatures between 77 and 300 K using on-chip terahertz spectroscopy. At charge neutrality, we observe the quantum-critical scattering rate characteristic of the Dirac fluid. At higher doping, we uncover two distinct current-carrying modes with zero and nonzero total momenta, a manifestation of relativistic hydrodynamics. Our work reveals the quantum criticality and unusual dynamic excitations near charge neutrality in graphene.


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