Function Of Loading Frequency Research Articles

Design and experimental characterization of a bypass magnetorheological damper featuring variable stiffness and damping

Magnetorheological (MR) fluid dampers (MRFDs) with variable stiffness and variable damping capability (VSVD-MRFDs) have demonstrated excellent vibration mitigation performance. However, there are limited studies on the development of bypass VSVD-MRFDs which offer both higher dynamic range and output force, apart from simple maintenance and straightforward assembly. In this study, a novel large-capacity VSVD-MRFD with an annular-radial bypass MR valve, as opposed to the typical practice of implementing the valve within the traveling piston in the hydraulic cylinder of the MRFD, is proposed. The main contribution of the present work includes: (a) providing the conceptional design and experimental dynamic characterization of the proposed VSVD-MRFD; (b) investigating the feasibility of the proposed damper for realizing the VSVD characteristics under wide ranges of loading conditions. A test rig was, thus, designed to perform experimental characterization of the proposed VSVD-MRFD under wide ranges of mechanical loading and magnetic field conditions. A qualitative analysis including force-displacement, and force-velocity characteristics, together with a quantitative analysis including dynamic range, equivalent viscous and stiffness coefficients, were conducted as a function of loading frequency, displacement amplitude, and applied current. Results showed a maximum dynamic range and maximum output force of 4.5 and 7.8 kN, respectively. Also, the maximum relative increase in the equivalent viscous and stiffness coefficients were obtained, respectively, as 425% and 488%, when the applied current is increased from zero to 2 A. The results confirm the potential of the proposed VSVD-MRFD for applications in off-road suspension systems. The externally designed bypass MR valve permits a straightforward design modification for realizing wide scalability of damping force in different applications (e.g., off-road vehicle suspension systems).

Piezoelectric energy harvesting for powering a novel weigh-in-motion system

• Roadway piezoelectric energy harvester (PEH) developed to power weigh-in-motion system. • PEH was tested under sinusoidal and pulse forces in the laboratory. • An electromechanical model associates applied forces on PZT stacks to output voltage. • The developed PEH circuit includes a rectifier, a capacitor, and a flyback converter. • Traffic forces exceeding flyback input thresholds generate sufficient power for MCU. This paper describes the development and evaluation of a piezoelectric energy harvester (PEH) designed to power a novel system for weighing roadway vehicles in-motion. The PEH consists of four lead zirconate titanate piezoelectric (PZT) stacks connected in parallel. Each stack is made of six PZT elements of alternating polarity. The paper describes the electromechanical characterization of the PZT stacks, the design of the PEH electronic circuit and its power generation potential under realistic traffic conditions. An electromechanical model was fitted to laboratory data that allows relating PZT stacks voltage output to the applied forces, as a function of loading frequency (i.e., vehicle speed). The PEH circuitry consists of a rectifier, a flyback and a step-down converter to reduce voltage output to the 0–3.3 V range. The flyback input thresholds were 100 V and 300 mW. At highway speeds, these thresholds were exceeded for sinusoidal forces with amplitudes higher than 13 kN. Exceeding these thresholds generated over 200 mW of power, which is sufficient for supporting the low-power microprocessors considered. The amplitude of intermittent pulsating forces that can exceed these thresholds were estimated as a function of the rest period between pulses. For a rest period of 60 ms (i.e., typical rest period between tandem truck axles), the estimated amplitude was 33.4 kN. It was shown that this force is exceeded by roughly 35% of the typical truck tires.

Signalling molecule transport analysis in lacunar-canalicular system.

Mechanical loading-induced fluid flow in lacunar-canalicular space (LCS) of bone excites osteocyte cells to release signalling molecules which initiate osteo-activities. Theoretical models considered canaliculi as a uniform and symmetrical space/channel in bone. However, experimental studies reported that canalicular walls are irregular and curvy resulting in inhomogeneous fluid motion which may influence the molecular transport. Therefore, a new mathematical model of LCS with curvy canalicular walls is developed to characterize cantilever bending-induced canalicular flow behaviour in terms of pore-pressure, fluid velocity, and streamlines. The model also analyses the mobility of signalling molecules involved in bone mechanotransduction as a function of loading frequency and permeability of LCS. Inhomogeneous flow is observed at higher loading frequency which amplifies mechanotransduction; nevertheless, it also promotes trapping of signalling molecules. The effects of shape and size of signalling molecules on transport behaviour are also studied. Trivially, signalling molecules larger in size and weight move slower as compared to molecules small in size and weight which validates the findings of the present study. The outcomes will ultimately be useful in designing better biomechanical exercise in combination with pharmaceutical agents to improve the bone health.

Effect of loading frequency on deformations at the bone-implant interface.

This study considers the time-dependent behaviour of bone in the context of loosening of metal implants, which is one of the typical complications of joint replacement and fracture-fixation surgeries. We employed viscoelastic properties developed from our previous experimental studies for trabecular bone in a representative bone-implant construct, which was subjected to cyclic loading at varying loading frequencies. We found that the separation between the bone and the implant is a function of loading frequency and increases with number of loading cycles applied. Our analysis shows that at the start of cyclic loading, a higher frequency results in a lower displacement response of bone at the bone-implant interface; however, after the bone-implant system has been subjected to a large number of cycles (>500 cycles in this study), higher interfacial displacements are observed at higher loading frequencies. In other words, higher loading frequencies will not result in bone-implant separation if limited number of cycles are applied. In all cases, interfacial displacements increase as bone volume ratio decreases. This simple approach can be used to evaluate the mechanical environment in bone-implant systems due to cyclic loading which commonly used time-independent models that are unable to simulate. The approach can also be used to evaluate implant loosening due to cyclic loading.

Dynamic tensile properties of bovine periodontal ligament: A nonlinear viscoelastic model

As a support to the tooth, the mechanical response of the periodontal ligament (PDL) is complex. Like other connective tissues, the PDL exhibits non-linear and time-dependent behavior. The viscoelasticity of the PDL plays a significant role in low and high loading rates. Little information, however, is available on the short-term viscoelastic behavior of the PDL. Also, due to the highly non-linear stress–strain response, it was hypothesized that the dynamic viscoelastic properties of the PDL would be greatly dependent on the preload. Therefore, the present study was designed to explore the dynamic tensile properties of the bovine PDL as a function of loading frequency and preload. The in vitro dynamic tensile tests were performed over a wide range of frequencies (0.01–100Hz) with dynamic force amplitude of 1N and different preloads of 3, 5 and 10N. The generalized Maxwell model was utilized to describe the non-linear viscoelastic behavior of the PDL. The low loss factor of the bovine PDL, measured between 0.04 and 0.08, indicates low energy dissipation due to the high content of collagen fibers. Moreover, the influence of viscous components in the linear region of the stress–strain curve (10N preload) was lower than those of the toe region (3N preload). The data reported in this study could be used in developing accurate computational models of the PDL.

Loading frequency and microstructure interactions in intergranular fatigue crack growth in a disk Ni-based superalloy

The paper examines the role of the loading frequency on the dwell fatigue crack growth mechanism in the super-solvus nickel-based superalloy, ME3. This is accomplished by carrying out a set of crack growth experiments in air and vacuum at three temperatures; 650°C, 704°C and 760°C using a dwell loading cycle with hold time periods up to 7200s imposed at the maximum load level. Results of these tests show that the transitional transgranular/intergranular loading frequency is 0.1Hz, and are used to determine the apparent activation energy of the time-dependent crack growth process. Analysis of this energy in both air and vacuum showed that the intergranular cracking is governed by a mechanism involving grain boundary sliding. This mechanism is explained in terms of absorption of dissociated lattice dislocations into grain boundary dislocations. The gliding of these dislocations under shear loading is assumed to cause grain boundary sliding. A condition for this mechanism to occur, is that a critical minimum distance exists between slip bands impinging the affected grain boundary. This condition is examined by correlating the slip band spacing (SBS) and loading frequency using a model based on minimum strain energy accumulation within slip bands and that a unique configuration of number and spacing of bands exists for a given plastic strain. The model outcome expressed in terms of SBS as a function of loading frequency is supported by experimental measurements at both high and low loading frequencies. Results of the model show that a saturation of SBS, signifying a condition for intergranualr cracking, is reached at approximately 3μm which is shown to coincide with the transitional loading frequency of 0.1Hz.

Effect of Oxygen on the Degradation of Alloy 908 in Gaseous Environments

Abstract Alloy 908 (UNS N09908), a Ni-based, low-thermal expansion superalloy, has been developed as a structural material for high-field, Nb3Sn, superconducting magnets. Alloy 908 is characterized by high strength and toughness at cryogenic temperatures while, at the same time, exhibiting a thermal expansion coefficient close to that of Nb3Sn. While ideally suited for service at cryogenic temperatures, Alloy 908 is susceptible to embrittlement if exposed to an oxygen-containing environment in the temperature range from 350°C to 750°C. In this study oxygen-induced embrittlement (OIE) in Alloy 908 has been investigated. Constant ΔK fatigue crack growth testing was used as a tool to explore the effect of oxygen on embrittling behavior. Crack growth rate, da/dN, was evaluated as a function of loading frequency, oxygen partial pressure, ΔK, and load ratio over the temperature range from 350°C to 750°C. The effect of oxygen manifested itself as a change in crack growth rate and crack path. Transgranular (TG) a...

Effect of dynamic loading on the frictional response of bovine articular cartilage

The objective of this study was to test the hypotheses that (1) the steady-state friction coefficient of articular cartilage is significantly smaller under cyclical compressive loading than the equilibrium friction coefficient under static loading, and decreases as a function of loading frequency; (2) the steady-state cartilage interstitial fluid load support remains significantly greater than zero under cyclical compressive loading and increases as a function of loading frequency. Unconfined compression tests with sliding of bovine shoulder cartilage against glass in saline were carried out on fresh cylindrical plugs ( n=12), under three sinusoidal loading frequencies (0.05, 0.5 and 1 Hz) and under static loading; the time-dependent friction coefficient μ eff was measured. The interstitial fluid load support was also predicted theoretically. Under static loading μ eff increased from a minimum value ( μ min = 0.005 ± 0.003 ) to an equilibrium value ( μ eq = 0.153 ± 0.032 ). In cyclical compressive loading tests μ eff similarly rose from a minimum value ( μ min = 0.004 ± 0.002 , 0.003±0.001 and 0.003±0.001 at 0.05, 0.5 and 1 Hz) and reached a steady-state response oscillating between a lower-bound ( μ lb = 0.092 ± 0.016 , 0.083±0.019 and 0.084±0.020) and upper bound ( μ ub = 0.382 ± 0.057 , 0.358±0.059, and 0.298±0.061). For all frequencies it was found that μ ub > μ eq and μ lb < μ eq ( p < 0.05 ) . Under cyclical compressive loading the interstitial fluid load support was found to oscillate above and below the static loading response, with suction occurring over a portion of the loading cycle at steady-state conditions. All theoretical predictions and most experimental results demonstrated little sensitivity to loading frequency. On the basis of these results, both hypotheses were rejected. Cyclical compressive loading is not found to promote lower frictional coefficients or higher interstitial fluid load support than static loading.

Dynamic unconfined compression of articular cartilage under a cyclic compressive load

To study the effect of dynamic mechanical force on cartilage metabolism, many investigators have applied a cyclic compressive load to cartilage disc explants in vitro. The most frequently used in vitro testing protocol has been the cyclic unconfined compression of articular cartilage in a bath of culture medium. Cyclic compression has been achieved by applying either a prescribed cyclic displacement or a prescribed cyclic force on a loading platen placed on the top surface of a cylindrical cartilage disc. It was found that the separation of the loading platen from the tissue surface was likely when a prescribed cyclic displacement was applied at a high frequency. The purpose of the present study was to simulate mathematically the dynamic behavior of a cylindrical cartilage disc subjected to cyclic unconfined compression under a dynamic force boundary condition protocol, and to provide a parametric analysis of mechanical deformations within the extracellular matrix. The frequency-dependent dynamic characteristics of dilatation, hydrostatic pressure and interstitial fluid velocity were analyzed over a wide range of loading frequencies without the separation of the loading platen. The result predicted that a cyclic compressive force created an oscillating positive-negative hydrostatic pressure together with a forced circulation of interstitial fluid within the tissue matrix. It was also found that the load partitioning mechanism between the solid and fluid phases was a function of loading frequency. At a relatively high loading frequency, a localized dynamic zone was developed near the peripheral free surface of the cartilage disc, where a large dynamic pressure gradient exists, causing vigorous interstitial fluid flow.

The role of cartilage streaming potential, fluid flow and pressure in the stimulation of chondrocyte biosynthesis during dynamic compression

The effects of streaming potential, fluid flow and hydrostatic pressure on chondrocyte biosynthesis were studied by comparing the spatial profiles of these physical stimuli to the profiles of biosynthesis within cartilage disks subjected to dynamic unconfined compression. The radial streaming potential was measured using compression frequencies and disk sizes relevant to studies of physical modulation of cartilage metabolism; a general analytical solution to the unconfined compression of a poroelastic cylinder with impermeable, rigid, adhesive platens was derived using potential theory. The solution with adhesive platen boundary conditions, using measured values of cartilage material properties, predicted streaming potentials that were much closer to experimental results between 0.001 and 1 Hz than a solution using frictionless platen boundary conditions. The predicted radial profiles of streaming potential gradient and fluid velocity (but not hydrostatic pressure) were similar to the previously reported radial dependence of proteoglycan synthesis induced by dynamic unconfined compression. Changes in stiffness associated with reduction of disk diameter suggested that the relative contributions of collagen and proteoglycans to cartilage mechanical properties may be a function of loading frequency in unconfined compression; such anisotropies may explain the remaining discrepencies between measured stiffness and stiffness predicted by the present model.

Effect of hydrogen on the fatigue behavior of alloy 600 at cathodic potential

The purpose of this study is to investigate the fatigue crack growth behavior of mill annealed Alloy 600 in 0.1M NaCl solution at cathodic potential ({minus}1,300m V{sub SCE}) as a function of load frequency and load ratio (R = P{sub min}/P{sub max}), and to study the possible hydrogen-assisted cracking mechanism during fatigue testing. The hydrogen effects on Alloy 600 occur in air saturated NaCl solution at 25 C with specified load mode which differs from the operating conditions of PWR steam generators. The FCG tests in this study are for the basic research of fatigue behavior of Alloy 600 in aqueous environment. The enhancement of the FCG rates of Alloy 600 in 0.1M NaCl solution at 25 C and {minus}1,300m V{sub SCE} is due to hydrogen embrittlement, and the phenomenon is more significant as load frequency decreases and load ratio increases.

Environmental interactions in high-temperature fatigue crack growth of Ti-1100

The crack growth behavior of Ti-1100 is investigated for loading frequencies ranging from 30 to 0.0031 Hz at temperature levels extending from 23 °C to 650 °C in both air and vacuum environments. Two types of time-dependent damage mechanisms have been identified: oxidation and creep effects. It is concluded that the effect of oxidation on the crack growth acceleration is rapidly developed and only weakly dependent on total cycle time. Creep effects, on the other hand, are dominant at low frequencies in both air and vacuum and are loading rate dependent. The degree of contribution of each of these two damage modes during the steady state growth region has been phenomenologically determined by examining the frequency dependence on the exponent and coefficient parameters of the Paris-type crack growth equation. It is found that these parameters are largely determined by the extent of the viscoplastic response of the crack tip region below a specific, environment-sensitive transition loading frequency. Furthermore, the physical mechanisms involved in the environment-affected damage are identified with the nature of crack tip plastic work input as a function of loading frequency. The influence of frequency and environment on the anomalous appearance of pronounced stage I/stage II knee regions is also discussed with respect to closure levels and creep transient response.

液中で鋼の腐食疲労き裂進展に及ぼす荷重繰返し速度の影響 き裂先端で発生するキャビテーションの影響について

It was already reported that many bubbles were observed gushing from the tip of cracks in fatigue tests in liquid environments. These bubbles occur due to the cavitation accompaning with the opening and closing action of the crack. In this study, the influence of cavitation bubbles on the crack growth behavior was investigated at various load frequencies in synthetic seawater and in ion exchanged water. The following conclusions were drawn.(1) The ratio of crack growth rate in a liquid environment to that in laboratory air at a constant ΔKeff was obtained as a function of load frequency. This curve showed a constant in low frequency range (<0.2Hz) and in high frequency range (>5Hz), respectively, and a transition range was observed between these constant ranges.(2) It may be considered that the corrosion fatigue crack growth at high frequencies in liquid environments was reduced to the almost same value as in the laboratory air, because the cavitation bubbles were gushing at the tip of the crack.

Crack in a viscoelastic field of pure bending

Abstract The problem of a viscoelastic crack in a field of pure bending, recently considered[1], is re-examined using a different, in some ways more systematic, approach. Previous work is extended to allow demonstration of the fact that the crack may close smoothly. This is in contrast with the case of a purely elastic medium for which sudden closure always occurs. Also, a class of standard linear solids is defined which, under cyclic loading, close at only one end, at least in the early stages of loading history. This class is a function of loading frequency.