Increase In Dynamic Stiffness Research Articles

Variation characteristics of key dynamic measurement signals of anchor bolts with different anchorage qualities under pull-out loads

Pull-out load and the cement-sand ratio (CSR) can affect the non-destructive testing (NDT) results of anchor bolts. Therefore, in this article, NDT experiments were conducted on both fully and defectively grouted anchor bolts, and variation patterns of key dynamic testing signal parameters were analyzed. A longitudinal vibration model of defectively grouted anchor bolts considering dynamic and static damping was proposed, and simulated NDT of anchor bolts with varying qualities. The results indicated that grouting defects resulted in an increase in wave velocity, along with a decrease in the fundamental frequency and dynamic stiffness of anchor bolts. When grouting defects and pull-out load acted concurrently, the fundamental frequency, and dynamic stiffness of the defectively grouted anchor bolts were consistently smaller than those of fully grouted ones during the initial loading phase. With pull-out load increasing, wave velocity decreased first, then increased; fundamental frequency increased, followed by a decrease; dynamic stiffness rose. When the CSR of defectively grouted anchor bolts was reduced, wave velocity decreased, fundamental frequency increased slightly, and a substantial increase in dynamic stiffness was observed. Pull-out loads were more sensitive to anchor bolt key dynamic signals than defects and CSR. Simulated validation demonstrated the reliability of the proposed theory.

Relationship between morphometric and mechanical properties of superficial lumbosacral soft tissue layers in healthy young adults.

Introduction: It is commonly considered that myotonometry is a non-invasive method capable of quantifying linear elastic and viscoelastic properties of the myofascial tissue through the application of a weak mechanical impulse to the surface of the skin. However, before the impulse can reach the myofascial tissue, it must cross more superficial tissues such as the skin and subcutaneous tissue (ST). All these superficial tissues have different distributions and organizations of structural components. Therefore, the study aimed to examine the potential relationships between the mechanical and morphometric properties of various superficial soft tissues surrounding the lumbar multifidus muscle (LM). Methods: Myotonometric measurements of dynamic stiffness, logarithmic decrement, and creep, and ultrasonographic measurements of thickness and echogenicity of cutaneous, subcutaneous, perimuscular tissue, and LM were obtained from 50 healthy individuals in the resting prone position and during contralateral arm lift. Results: The most important findings were that in both the relaxed and contracted LM state, the dynamic stiffness strongly negatively (r = -0.69; p < 0.001 in relaxation, r = -0.83; p < 0.001 in contraction) and creep strongly positively (r = 0.79; p < 0.001 in relaxation, r = 0.85; p < 0.001 in contraction) correlated with the thicknesses of the ST. Similar but weaker correlations were noticed between both these measures and the perimuscular tissue thickness. Elasticity was uncorrelated to the thicknesses of the tissues. With LM contraction (change from the relaxed to contracted state), the relative increase in dynamic stiffness was correlated with the relative decrease in dermis (r = -0.51; p < 0.001) and ST (r = -0.47; p = 0.001) thickness, and with the relative increase in LM (r = 0.36; p = 0.010) thickness. Moreover, the relative decrease (thinning) in the ST thickness was correlated with the relative increase in logarithmic decrement (i.e., decrease in soft tissue elasticity, r = -0.37, p = 0.011). The mechanical properties of the soft tissues were not related to their echogenicity. Discussion: In conclusion, the thicker the subcutaneous and perimuscular layers, the lesser the stiffness and the greater the time-dependent deformation to the external force of the tissues surrounding the LM during its relaxation and isometric contraction. Moreover, the greater the thinning of the ST and the thickening of the LM during its contraction, the higher the increase in lumbosacral tissue stiffness and the decrease in elasticity. Therefore, one should consider the thickness of the ST before planning or analyzing the outcomes of myotonometric or other external biomechanical measurements to avoid drawing the wrong conclusions about the mechanical properties of the myofascial tissue.

Characteristic analysis under small-stroke and medium-high frequency of magneto-rheological damper with pressure controlled mechanism

Due to the high frequency hardening effect in applications of satellite and precision vibration isolation, it is an urgent need to analyze and improve the magnetorheological (MR) performance of MR damper under small-stroke and medium-high frequency excitations. In this paper, a compact MR damper with pressure controlled mechanism is proposed, analyzed and tested. Considering the inertial, pre-compression and compressibility of MR fluid, the dynamic physic model is established to analyze the output characteristics with pre-compressions under small-stroke and medium-high frequency excitations. And then, the critical model between the pre-yield region and post-yield region is obtained. The critical boundary between different excitations is derived and comparatively analyzed. Finally, the experiments with the proposed MR damper are carried out. Compared with theoretical results, the damping force and dynamic stiffness increase with the increase of the applied current and excitations. Besides these, the energy dissipated performance can be improved by the pre-compression. When the initial pressure increases to 5 MPa with the fixed frequency of 10 Hz, the energy dissipation is increased by 43.5%. So the pre-compression is an effective way to solve the high frequency hardening. Besides these, the derived critical model can predict the performance of MR damper at different excitations.

Dynamic stiffness modification by internal features in additive manufacturing

Dynamic stiffness, or the product of the modal stiffness and damping ratio, is an important consideration for the design of additively manufactured parts that will experience dynamic loading. This paper describes a demonstration component which was designed and manufactured in two configurations using a metal wire arc additive process. The first configuration was an open channel structure, while the second contained a dynamic absorber in the internal cavity. Frequency response measurements of the two components showed a significant magnitude reduction for the modified component at the original open channel structure's natural frequency and an overall increase in dynamic stiffness. Polymer damping material was then added to further increase the dynamic stiffness.

A Receptance Coupling Approach to Optimally Tune and Place Absorbers on Boring Bars for Chatter Suppression

A receptance coupling based method to optimally tune and place a tuned mass damper (TMD) on slender boring bars is presented. The boring bar and the TMD are treated as separate substructures. Substructural receptances are synthesized while optimizing TMD parameters as well as its location on the bar to target an increase in dynamic stiffness of the boring bar to improve chatter-free cutting conditions. Simulation based results with the proposed approach agree with classical analytical models reported in the literature. Methods presented are generalizable and can be extended to other slender cantilevered tools to increase their resistance against chatter vibrations.

Feasibility and Performance Studies of a Semi Active Journal Bearing

In this paper, details related to preliminary efforts made towards developing a novel semi-active journal bearing have been presented. The study which is predominantly experimental in nature uses a test rig designed and fabricated for the purpose of simulating a multi-lobed journal bearing and also to study the possibility of utilising smart materials for conversion of a circular bearing profile to a multi lobed bearing profile during operation. Preliminary experimental studies have indicated that while shape memory alloy (SMA) can be used as a medium to apply necessary forces at identified locations to obtain the desired profile shapes for a multi-lobe bearing configuration facilitating the development of semi active journal bearings, ovality ratios of the bearing profile under investigation ranging from 1.5 to 2 have been realised by suitable application of direct forces in opposing directions. The performance enhancement defined in terms of stiffness and damping of the newly configured bearing equivalent to two lobed bearing over the conventional bearing has been studied. The test rig is versatile in nature, with a facility to investigate varieties of semi active journal bearings. The study indicates an increase in dynamic stiffness and damping with increase in the ovality ratio of the semi-active journal bearings.

Cast Iron And Mineral Cast Applied For Machine Tool Bed - Dynamic Behavior Analysis

Abstract Cast iron and mineral cast are the materials most often used in the machine structural elements design (bodies, housings, machine tools beds etc.). The materials significantly differ in physical and mechanical properties. The ability to suppress vibration is one of the most important factors determining the dynamic properties of the machine and has a significant impact on the machining capabilities of a machine tool. Recent research and development trends show that there is a clear tendency to move away from the traditional iron casting to the mineral casting, due to better dynamic properties of the latter. However mineral cast as a structural material for the whole machine tools bed turns out to be insufficient due to its poor mechanical strength properties. The best solution should benefit from the advantages of the cast iron and mineral cast materials while minimizing their drawbacks. The paper presents numerical modal analysis of two lathe beds: the first one made of gray cast iron and the second one made of hybrid connection of cast iron and mineral cast. The analysis was conducted in order to determine the dynamic properties of two bodies of similar shapes made in the traditional (cast iron) and innovative hybrid (cast iron and mineral cast) technology. In addition, an analysis of the static structure rigidity of the two beds was performed. During the simulation studies it was found a significant increase in dynamic stiffness and static rigidity of the machine tool body made of hybrid connection of cast iron and mineral cast. The results of numerical simulations have confirmed the desirability of using hybrid construction because the dynamic properties of such a body are more advantageous in comparison with the conventional body made of cast iron.

Magnetorheological elastomer with stiffness-variable characteristics based on induced current applied to differential mount of vehicles

A differential mount with elastomers is installed to insulate vibration transmitted from the engine to the body through the propeller shaft. Since existing differential mounts adopt an elastomer of uniform stiffness, it is difficult to meet both the requirements for steering performance and driving comfort at the same time. In order to overcome this limitation, this study suggests a magnetorheological elastomer (MRE)-based stiffness-variable differential mount which allows the mount’s stiffness to vary reversibly or instantly. The stiffness-variable differential mount was designed with a new inner structure where a magnetic field can be induced in the MRE. Further, the geometry of the MRE was optimized by means of the response surface method to achieve a targeted level of stiffness. The variable performance of the stiffness-variable differential mount was evaluated with the dynamic stiffness when a current of 3 A (0.287 T) was applied. As a result, it was found that the average increase in dynamic stiffness was 4.41 kgf mm−1 over an excitation frequency range of 60–100 Hz, a critical point of variation for dynamic stiffness, and 3.60 kgf mm−1 over an excitation frequency range of less than 100 Hz.

Static and Dynamic Axial Response of Drilled Piers. I: Field Tests

A prototype pier load test program was performed to study the load-deformation and energy-dissipation characteristics of the response of drilled piers under static and dynamic axial loads. The field tests consisted of six fully instrumented drilled concrete piers 61–76 cm (24–30 in.) in diameter and 5.8–9.1 m (19–30 ft) in length. The piers were constructed on a stiff, sandy clay site adjacent to the University of California, Berkeley campus. A dynamic Fundex pile load test (PLT) was performed on each pier, followed by a static-compression or tension test and a second PLT. The field tests revealed that the stiffness and capacity of a soil-pier system depend significantly on the loading rate. For the type of piers and soil considered in the field test, the increase in dynamic stiffness versus static stiffness is approximately 20–40% at almost all displacement levels. The ultimate dynamic capacity increases approximately 30% compared with the static case. The multiple PLTs conducted on the same drilled pier also indicated that a pier may experience up to 50% stiffness and strength degradation when subjected to full load reversal. The test program showed that the PLT is a fast, innovative method to get useful site-specific information for seismic design of the pier foundation.

A Theoretical Study on the Influence of Frequency on the Dynamic Characteristics of Air Film

A general method is developed to deal with the compressibility of air film and to predict the linear dynamic stiffness and damping coefficients of air-lubricated bearings over the whole range of frequency. Numerical results reveal that both stiffness and damping coefficients are functions of not only the static parameters such as eccentricity and attitude angle, but also the frequency of disturbance. The coefficients are continuous and tend to certain values even when the frequency approaches zero or positive infinity. In general, the direct terms of the dynamic stiffness increase with the increase of the frequency, while the cross-coupling terms tend to an equal value when the frequency becomes infinite, and all the damping coefficients decrease in the higher frequency region and vanish when the frequency approaches positive infinity.

A Theoretical Study on the Influence of Frequency on the Dynamic Characteristics of Air Film

A general method is developed to deal with the compressibility of air film and to predict the linear dynamic stiffness and damping coefficients of air-lubricated bearings over the whole range of frequency. Numerical results reveal that both stiffness and damping coefficients are functions of not only the static parameters such as eccentricity and attitude angle, but also the frequency of disturbance. The coefficients are continuous and tend to certain values even when the frequency approaches zero or positive infinity. In general, the direct terms of the dynamic stiffness increase with the increase of the frequency, while the cross-coupling terms tend to an equal value when the frequency becomes infinite, and all the damping coefficients decrease in the higher frequency region and vanish when the frequency approaches positive infinity.

Dynamic stiffness enhancement of direct-driven machine tools using sliding mode control with disturbance recovery

This paper presents a disturbance adaptive discrete sliding mode controller for feed drives equipped with linear motors. The control law is expressed as a function of friction and cutting force disturbances which are estimated from the motor force and control states. The accurate prediction of disturbance forces is used to actively compensate low frequency machine tool structural modes which are within the bandwidth of the controller. The proposed control system is experimentally demonstrated on a high performance linear drive, which exhibited high bandwidth and significant increase in dynamic stiffness compared to classical cascaded control methods.

Comparison of the static and dynamic behaviour of carbon fibre composite golf club shafts

Golf shafts are characterized and sold by manufacturers on the basis of their static properties; however, their performance is controlled by their dynamic properties. This paper discusses the relationship between dynamic and static stiffness for a range of carbon fibre composite (CFC) shafts. In particular, the influence of ‘seams’ (specific orientations of the shaft with differing mechanical properties) on static and dynamic stiffness is considered. Vibrational frequency analysis (quasi-static testing) was used to locate the seams, and bend testing was used to determine the static stiffness of the low (seam) stiffness and high (90° to seam) stiffness positions. Dynamic stiffness values were determined by firing a golf ball from a gas cannon at a target attached to the end of the shaft and measuring the resulting deflection for different golf ball speeds. It was found that the dynamic stiffness values over a range of strain rates were similar to the static values for the sheet-laminated shafts indicating that there is no strain rate dependency for these shafts over the strain rates tested. However, for the filament wound shaft, a significant increase in dynamic stiffness, compared with the static value, was seen. This was related to the different CFC microstructure of this shaft. It was also found that the presence of seams resulted in statistically significant differences in static stiffness with respect to shaft orientation for a number of sheet-laminated shafts, whereas there was no difference in dynamic stiffness values.

New insight into deformation-dependent hydraulic permeability of gels and cartilage, and dynamic behavior of agarose gels in confined compression

Equilibrium, creep, and dynamic behaviors of agarose gels (2.0–14.8%) in confined compression were investigated in this study. The hydraulic permeabilities of gels were determined by curve-fitting creep data to the biphasic model (J. Biomech. Eng. 102 (1980) 73) and found to be similar in value to those published in the literature (AIChE J. 42 (1996) 1220). A new relationship between intrinsic permeability and volume fraction of water was found for agarose gel, capable of predicting deformation-dependent permeabilities of bovine articular cartilage and 2% agarose gel published in literature. This relationship is accurate for gels and cartilage over a wide range of permeabilities (four orders of magnitude variation). The dynamic stiffness of the gels increases with gel concentration and loading frequency (0.01–1.0 Hz). The increase in dynamic stiffness with loading frequency is less pronounced for gels with higher concentrations. The results of this study provide a new insight into deformation-dependent permeability behavior of agarose gel and cartilage, and are important for understanding biological responses of cells to interstitial fluid flow in gel or in cartilage under dynamic mechanical loading.

Measurement of the dynamic stiffness of recycled rubber based railway track mats according to the DB-TL 918.071 standard

Measurements of dynamic stiffness were performed on three samples of material (based on recycled rubber) used as a mat for railway track applications. Measurements were performed at three static pre-loads, 0.03, 0.06 and 0.1 MPa, in the frequency range 10–100 Hz. A steady increase in dynamic stiffness with increasing frequency was observed in the samples studied, ranging from 0.1 to 0.23 Nmm −3. At the same time, the measured dynamic stiffness shifted to higher absolute values with increasing static pre-load.

Deformational Behavior of a Unidirectional Graphite/Epoxy Composite Under Compressive Fatigue

Deformations of a unidirectional lamina, loaded in compressive-compressive fatigue in the fiber direction, perpendicular to the fibers, and in shear, were recorded continuously during fatigue tests. The deformations were separated into cyclic (AC) and creep (DC) components, and these values were used to define AC and DC moduli for each of the three modes of loading. The phase between stress and strain was monitored as well. The response to loading in the fiber direction was elastic, with negligible nonlinearity. Perpendicular to the fiber direction, the specimens crept slightly during the initial phase of the loading, and this was accompanied by a minor increase in dynamic stiffness. The shear loading caused large creep deformation, similar to creep observed in metals, and large change in phase, but the change in dynamic stiffness was minor. The results of creep are needed for modelling load transfer between layers in laminated composites, and the measurements of dynamic modulus have a potential for being a nondestructive test method for detecting fatigue life limit.

The characteristics and application of silicone gel. The fundamental study based on the experiment.

This paper describes the static and dynamic characteristics and the application of silicone gel. The dynamic characteristics are experimentally investigated to 200 Hz. The static relation between the draw ratios and the stress is expressed by the three-dimensional relation of the ideal rubber. The dynamic stiffness increase and the viscous damping coefficient decrease with an increase in the frequency. Their values increase as the draw ratios are decreased. The gel is applied to the spring-mass system and the beam system. As a result, the applications of the gel restrain the resonance gain of the system that doesn't have the gel. The restraining effect increases with a decrease in the draw ratios. Results of the analysis show that the damper of the gel restrains the gain of their systems and is superior to the rubber.

Dynamic elasticity in the initial phase of an isotonic twitch.

AbstractA method is described which allows a continuous determination of the elastic and viscous component of the dynamic stiffness of isolated frog muscle fibres in vibration experiments. Elastic and viscous stiffness increased gradually in the initial phase of preloaded isotonic twitches. The increase started already within the mechanical latent period. Within 25 per cent of the time to maximum shortening elastic stiffness increased by 100 per cent and viscous stiffness by 50 per cent. In slightly stretched fibres, at the length in the body, the increase in dynamic stiffness during tetanic contraction was identical with that found in a twitch. In highly stretched fibres dynamic stiffness in a tetanus exceeded that of a twitch, presumably due to the redistribution of load between the contractile substance and the sarcolemma. The increase in dynamic stiffness during a twitch immediately after a prolonged tetanus was markedly lower than in a twitch performed before the tetanus. The contributions to the dynamic stiffness from the parallel elastic, the series elastic and the contractile component were evaluated. Although the periodical deformations were essentially confined to the series elastic component, the dynamic elastic properties give information about the contractile substance as the series elastic component mainly represents the elasticity of the contractile material.