Smart dampers with on-demand controllable damping curves are key components for semi-active vibration control of structures. Smart dampers utilize friction or viscosity to dissipate mechanical energy by heat. The potential thermal problems are their major drawbacks. Novel colloidal dampers are recently developed with low-heat generation and high damping efficient, they are, however, passive and with no on-demand controllable damping capability. In this paper, we propose a smart colloidal damper by employing water-based ferrofluids in damping media. We find that the corresponding damping hysteresis loops can be affected by applied magnetic fields significantly and rapidly. We further retrieve the instant stiffness and damping coefficient of the smart colloidal dampers from the measured hysteresis loops. It is shown that the negative stiffness and the negative damping coefficient may occur during the operation of the smart colloidal dampers.
Dampers are the key devices for vibration control of structures. The mechanisms of current dampers are internal friction
or viscous flow to dissipate external mechanical energies by heat. The high-heat generation potentially causes thermal
problems to decrease the durability of dampers. Owing to the surface-tension dominated nanoflow on the porous
particles, colloidal dampers have been recently developed with low-heat generation and high damping efficiency. In this
paper, a new type of colloidal dampers are designed and fabricated. Its heat generation and hysteresis loops are tested. It
is found that the heat generation of the colloidal dampers is below 4% of that of hydraulic dampers with the same energy
dissipation capacity. Meanwhile, the hysteresis loops reveal that the colloidal dampers are highly nonlinear devices. We
introduce an efficient algorithm to retrieve its instant stiffness and damping coefficients from measured hysteresis loops
under cyclic excitations at different frequencies. The retrieved stiffness and damping coefficients are plotted against
damping forces or inner pressures. We find that, at low frequencies, the colloidal dampers exhibit the states with
negative stiffness and negative damping coefficients; nevertheless, at the frequencies above 6Hz, both the stiffness and
the damping coefficients are positive. Frequency is one of the key parameters dominating the damping mechanism of the
Research and development related to homeland security has emerged as one of the most challenging topics nationwide in the recent few years. Effective structural health monitoring, diagnosis, and prognosis are of great importance for the safety and reliability analysis for civil infrastructural systems. While the technologies of sensor-arrays embedded in host structures are widely employed for structural health monitoring, the key issue is how to set-up a physics-based model framework and its corresponding efficient algorithm to evaluate the quality of the host structures through the output of the sensors. It is a frontal interdisciplinary topic bridging the microstructural damage mechanics and signal estimation theory. By employing a multi-scale constitutive model of solids with damage, this paper conducts an exploratory research on the modeling and algorithm of estimating the mean value of crack density and the distribution of crack orientation of a cracked plate subjected to unidirectional tension. Simulation results reveal that the framework and algorithm provide a reasonable performance in recovering crack orientation.
This paper reviews the investigations of introducing magnetorheological elastomer (MRE)-based technologies to the design of smart electronic devices. Piezoelectric power actuators are required to operate at a resonant state in order to deliver maximum mechanical energy to loads. Owing to the field-dependent dynamic flexural rigidity of MRE-based structures, power actuators utilizing such structures exhibit the capability of compensating the change of the loads and keeping the resonant frequency at a fixed value. Four kinds of bender configurations for such smart actuators will be reviewed. They are: a cantilever suspended by an MRE patch at its free end, a single-layer MRE-based sandwich beam surface-bonded by piezoelectric patches, a multi-layer MRE-based sandwich beam surface-bonded by piezoelectric patches, and an inserts reinforced MRE-based sandwich beams surface-bonded by piezoelectric patches. Their driving capability and field-controllable capability are discussed in a detail. In addition, MRE-based structures are extended to propose linear time-variant systems for time-frequency signal processing. The system function is presented and the Wigner-Ville distribution is used to analyze the time-frequency distribution of the time-delayed response of the system. The system is proved to be a damped-vibration system with field-controllable resonant frequencies. Due to the field-controllable time-frequency pattern of the time-delayed response, the system can be used for data encryption and signal modulation.
This paper presents the research on stability analysis of carbon nanotubes (CNTs) via elastic continuum beam and shell models. The estimation of the flexural stiffness of a single-walled nanotube (SWNT) via elastic beam model is proposed based on the postulate analyzed and provided in the paper. The validation of the stiffness is conducted with the ab initio calculations of the vibration of a SWNT. Based on the stiffness proposed, the stability analysis of CNTs is further conducted and validated with the well-cited research results by Yakobson and his collaborators. In addition, more predictions of various buckling phenomena of carbon nanotubes by beam and shell models are provided and studied. In the end, the kink phenomenon in a SWNT under pure bending is discussed via the continuum model. Last but not least, the results on the kink of a SWNT under an initial bend is presented. It is hoped that this paper will pave the way toward a better understanding of continuum models’ application in the stability analysis of carbon nanotubes.
Magnetorheological elastomer (MRE) is a solid-state smart material with field-dependant dynamic shear modulus. But, its lower shear modulus, which is about 0.388MPa, does prevent its application. Sandwich configuration is an alternative to apply MRE in engineering since the outer thin skins will strengthen the bulk flexural stiffness and the transverse flexibility of the MRE core will affect the bulk flexural dynamic performance. In this paper, the field-dependant dynamic property of MRE-based sandwich beams, composed of conductive skins or non-conductive skins, is addressed theoretically through a high order model. By defining the maximum field-induced relative change of the bulk flexural dynamic stiffness as controllability index, structure designs to yield maximum controllability are presented through a non-dimensional analysis. The simulation on simply supported MRE-based sandwich beam indicates: (1) the anti-resonant frequencies and resonant frequencies of the sandwich beam change with applied magnetic fields up to 30%; (2) the bulk field-dependant flexural dynamic property is mainly depended on the field-dependant shear modulus of the MRE core; and, (3) there is an optimal combination of the thickness of the core and the thickness of the skins for maximum controllability; (4) around the optimal combination point, the controllability/mass ratio can be enhanced dramatically though decreasing the core thickness; (5) the normalized density of the skins affects the controllability slightly when the Young's modulus of the skins is low. This work indicates that sandwich structures can well utilize the controllable property of MRE to realize applicable stiffness changeable devices.
Magnetorheological Elastomer (MRE) is a new class of smart materials, whose modulus can be controlled by applied magnetic field. In this paper, we first show the field-dependent dynamic mechanical properties including shear and stretch of the MRE, cured by ourselves. By white light speckle method for deformation analysis, we present the dynamic deformation progress (the vector diagram of displacement or the whole-field quantitative displacement distribution, at various times) of the MRE and the elastomer-ferromagnetic composite (EFC) while the magnetic field turns on. The real-time deformation progress gives us a deep understanding to MRE and EFC.