Non-dissipative Mechanisms Research Articles

Bioinspired Design of a Cartilage-like Lubricated Composite with Mechanical Robustness.

Natural articular cartilages show extraordinary tribological performance based on their penetrated surface lubricated biomacromolecules and good mechanical tolerance. Hydrogels are considered to be potential alternatives to cartilages due to their low surface friction and good biocompatibility, although the poor mechanical properties limited their applications. Inspired by the excellent mechanical properties and the remarkable surface lubrication mechanism of natural articular cartilages, one kind of cartilage-like composite material with a lubrication phase (Composite-LP) was developed by chemically grafting a thick hydrophilic polyelectrolyte brush layer onto the subsurface of a three-dimensional manufactured elastomer scaffold-hydrogel composite architecture. The Composite-LP exhibited good load-bearing capacities because of the nondissipation strategy and the stress dispersion mechanism resulting from the elastomer scaffold enhancement. In the presence of the top lubrication layer, the Composite-LP showed superior friction reduction functionality and wear resistance under a dynamic shearing process. This design concept of coupling the non-dissipative mechanism and interface lubrication provides a new avenue for developing cartilage-like hydrogels and soft robots.

Strength or Ductility Seismic Design? A case study in Sabah using the Eurocode 8 Malaysian National Annex

This paper shares the experience of a building project which piloted seismic design using Eurocode 8, with the Malaysian National Annex legislated in 2017. The 23-storey reinforced concrete building features geometric irregularity with podium and two towers, located in Kota Kinabalu, the capital city of Sabah, Malaysia. Sabah had experienced a magnitude-6 earthquake in 2015, where the epicentre Ranau was just 50 km away from Kota Kinabalu. Conventional Probabilistic Seismic Hazard Assessment has indicated that Kota Kinabalu has a peak ground acceleration (PGA) on rock of only 0.04g for a 475-year return period, which appears to be counter-intuitive for such a potentially destructive near-field magnitude 6 earthquake. Hence, four possible scenarios are postulated, i.e., PGA of 0.04g, 0.08g and 0.12g designed with ductility class low (DCL), assuming force-based non-dissipative mechanism and 0.12g designed with ductility class medium (DCM), engaging seismic detailing to form plastic hinges. This paper is written to give local engineering practitioners, who have either no prior experience in seismic design or have been using the basic equivalent force method, to develop a better sense of response spectrum seismic analysis, code-based design and choice of strength or ductility seismic design in low-to-moderate seismicity regions.

Non-Dissipative Structural Evolutions in Granular Materials

The structure of the contact network in granular assemblies can evolve due to either dissipative mechanisms such as sliding at contact points, or non-dissipative mechanisms through the phenomenon of contact gain and loss. Being associated with negligible deformations, non-dissipative mechanisms is actually active even in the small strain range of ~ 10−3 , especially in the case of densely packed assemblies. Hence, from a constitutive modelling point of view, it is crucial to be able to estimate such non-dissipative evolutions since both elastic and plastic properties of granular assemblies highly depend on contact network characteristics. The current study proposes an analytical scheme that allows us to estimate the non-dissipative contact gain/loss regime in terms of directional changes in the average contact force. The probability distribution of contact forces is used to compute the number of lost contact for each direction. Similarly, the number of newly formed contacts is estimated by considering the probability distribution of the gap between neighbouring particles. Based on the directional contact gain/loss computed, the changes in coordination number and fabric anisotropy can be found which, together with statistical treatments of Love-Weber stress expression, form a complete system of equations describing the evolution of other controlling microvariables. Finally, the results of the calculations have been compared with DEM simulations which verify the accuracy of the proposed scheme.

Nondissipative Saturation of the Magnetorotational Instability in Thin Disks

A new nondissipative mechanism is proposed for the saturation of the axisymmetric magnetorotational (MRI) instability in thin Keplerian disks that are subject to an axial magnetic field. That mechanism relies on the energy transfer from the MRI to stable magnetosonic waves. Such mode interaction is enabled due to the vertical stratification of the disk that results in the discretization of its MRI spectrum, as well as by applying the appropriate boundary conditions. A second order Duffing-like amplitude equation for the initially unstable MRI modes is derived. The solutions of that equation exhibit bursty nonlinear oscillations with a constant amplitude that signifies the saturation level of the MRI. Those results are verified by a direct numerical solution of the full nonlinear reduced set of thin disk magnetohydrodynamics equations.

Propagation of Shear Waves Generated by Acoustic Radiation Force in Nondissipative Inhomogeneous Media

We concentrate on the nondissipative mechanism induced shear wave in inhomogenous tissue. The shear wave equation of radiation force in inhomogeneous media is solved numerically with a finite-difference time-domain method. A rarely studied nondissipative mechanism of shear displacement due to a smooth medium inhomogeneity is evaluated. It is noted that unlike the dissipative effect, the nondissipative action on a localized inhomogeneity with its hardness parameter changing smoothly along the beam axis, compresses or stretches the focus area. The shear waves in nondissipative inhomogeneous media remain the property of sharp turn with 100% peak positive displacement and 64% peak negative displacement. This action is useful in discerning the water-like lesion.

Radiation force and shear motions in inhomogeneous media

An action of radiation force induced by ultrasonic beam in waterlike media such as biological tissues (where the shear modulus is small as compared to the bulk compressibility) is considered. A new, nondissipative mechanism of generation of shear displacement due to a smooth (nonreflecting) medium inhomogeneity is suggested, and the corresponding medium displacement is evaluated. It is shown that a linear primary acoustic field in nondissipative, isotropic elastic medium cannot excite a nonpotential radiation force and, hence, a shear motion, whereas even smooth inhomogeneity makes this effect possible. An example is considered showing that the generated displacement pulse can be significantly longer than the primary ultrasound pulse. It is noted that, unlike the dissipative effect, the nondissipative action on a localized inhomogeneity (such as a lesion in a tissue) changes its sign along the beam axis, thus stretching or compressing the focus area.

The Mechanisms of Damping of Geomagnetic Pulsations.

The physical mechanisms that might determine the damping rates and resonance widths of Pc3-5 pulsations are considered. To estimate the role of ionospheric dissipation, a numerical model, based on a calculation of the complex eigenfrequencies of Alfven field line oscillations within the approximation of a “thin” ionosphere and for a realistic ionosphere has been used. At middle latitudes the Joule dissipation in the ionosphere may account for the experimentally determined width of the resonance peak of the fundamental Alfven mode. The meridional spatial structure of higher harmonics is determined by nondissipative mechanisms. The calculations predict a rapid decrease of the Q factor of the magnetospheric Alfven resonator at L < 1.5. The effect is related to the violation of the thin ionosphere approximation at low latitudes. Experimental support for the calculations was found while analyzing the meridional distribution of the damping rates of Psc3 pulsations observed along the 210° magnetic meridian network.

Spherical nonlinear wave propagation in a dissipative stratified atmosphere

Nonlinear acoustic wave propagation is considered, for a signal which is initially sinusoidal at the input radius, subject to the effects of spherical spreading, exponential density stratification with altitude, and thermoviscous dissipation. A model equation is derived giving the signal variation along each straight line ray from centre of symmetry, and in the form of a Generalized Burgers Equation. This equation is studied in the small-dissipation limit. It is shown that along rays above the horizontal, shocks quickly form, but then rapidly thicken, the wave eventually dying in linear old age, with amplitude saturation. On rays below the horizontal shocks may or may not form, depending on the relation of the ray to an “initial shock curve” which is determined analytically. When shocks do form, they become increasingly thin with increasing distance, while outside them nonlinear effects become small and the main part of the wave evolves under linear non-dissipative mechanisms alone, displaying no trace of amplitude saturation.

Resonant Absorption of P-Modes by Sunspots

A substantial amount (≲50%) of solar acoustic (p-mode) absorption in sunspot regions has been reported by Braun, Duvall and LaBonte (1988). A number of suggestions have been advanced to account for the observed p-mode absorption in sunspots (see review by Davila in these proceedings). These include processes involving resonance absorption, radiation damping, and non-dissipative mechanisms. The main purpose of the present work is to demonstrate that any dissipative process like radiative, viscous, or resistive dissipation leads to the resonant absorption of acoustic waves incident on the sunspot tube and that the resultant heating rate can be shown to be consistent with the observed absorption of the p-mode power impinging on an isolated inhomogeneously structured sunspot.

Analysis of the Soft Mode Behavion in Li2Ge7O15in Terms of a Coupled-Mode Model

On the basis of a coupled-mode model, the coupling between a soft mode and another optical mode (called a partner mode) in Li 2 Ge 7 O 15 observed by means of the Raman scattering is analyzed. The observed Raman profiles are satisfactorily fitted to the adopted model. From the results of the fitting it has turned out that both real and imaginary parts of the coupling constant are almost constant over the examined temperature range and the coupling between two modes is mainly through the nondissipative mechanism. High pressure Raman scattering measurements were performed. It was found that there is some difference between the temperature and pressure dependences of the behaviors of the coupled system.