共查询到20条相似文献,搜索用时 93 毫秒
1.
The purpose of this investigation was to conduct a laboratory test program on how much induced vibrations on concrete during the period between initial set and final set affect the attainable strength of concrete. To achieve this purpose, a laboratory test program was conducted. The laboratory program consisted of casting 144 76?mm by 152?mm (3×6?in.) concrete cylinders and subjecting them to one of two levels of vibration for either 1 or 2?min at five different ages ranging in time from before, during, and after the setting period for the concrete. The levels of vibration correspond to typical frequencies of vibratory soil compactors and the peak particle velocity produced by the compactors. Both compression and splitting tensile tests were performed. The results of the laboratory study indicate that vibratory soil compaction should not be considered a significant hazard to foundation strength as long as the vibrations are within the limits in this study. 相似文献
2.
1. Responses of 73 fibers to dorso-ventral vibration were recorded in the saccular and utricular branchlets of Rana pipiens pipiens using a ventral approach. The saccular branchlet contained nearly exclusively vibration-sensitive fibers (33 out of 36) with best frequencies (BFs) between 10 and 70 Hz, whereas none of the 37 fibers encountered in the utricular branchlet responded to dorso-ventral vibrations. 2. Using a dorsal approach we recorded from the VIIIth nerve near its entry in the brainstem and analyzed responses to both sound and vibration stimuli for 65 fibers in R. pipiens pipiens and 25 fibers in Leptodactylus albilabris. The fibers were classified as amphibian papilla (AP), basilar papilla (BP), saccular or vestibular fibers based on their location in the nerve. Only AP and saccular fibers responded to vibrations. The AP-fibers responded to vibrations from 0.01 cm/s2 and to sound from 40 dB SPL by increasing their spike rate. Best frequencies (BFs) ranged from 60 to 900 Hz, and only fibers with BFs below 500 Hz responded to vibrations. The fibers had identical BF's for sound and vibration. The saccular fibers had BFs ranging from 10 to 80 Hz with 22 fibers having BFs at 40-50 Hz. The fibers responded to sound from 70 dB SPL and to vibrations from 0.01 cm/s2. 3. No differences in sensitivity, tuning or phase-locking were found between the two species, except that most BP-fibers in R. pipiens pipiens had BFs from 1.2 to 1.4 kHz, whereas those in L. albilabris had BFs from 2.0 to 2.2 kHz (matching the energy peak of L. albilabris' mating call). 4. The finding that the low-frequency amphibian papilla fibers are extremely sensitive to vibrations raises questions regarding their function in the behaving animal. They may be substrate vibration receptors, respond to sound-induced vibrations or bone-conducted sound. 相似文献
3.
RB Pollard 《Canadian Metallurgical Quarterly》1996,29(1):73-75
Points of subjective equality between continuous vibration and shock-type vibration (repeated vibration) were examined in hand-transmitted vibration to find a tendency of human response to shock-type vibration (repeated vibration with short duration). On time and off time of the repeated vibrations were changed from 10 ms to 5s. Each adjustment involved a 10 second exposure to the repeated vibration and subsequent 10 second exposure to the continuous vibration. Frequencies of the vibrations were 8, 16, 31.5 and 100 Hz. Subjective magnitude of the shock-type vibrations (repeated vibrations) decreased with increase of off-time and with decrease of on-time of the repeated vibrations. Results of this experiment were compared with calculated r.m.s. values, r.m.q. values and other quantity. R.m.s. values underestimated the repeated vibration and r.m.q. values overestimated the vibration compared with human responses. 相似文献
4.
5.
OBJECTIVE: The human body is often exposed to significant vibration stress in the workplace, at home, and during recreational activities. The current study was designed to evaluate whether low- to midfrequency vibrations present at the extraabdominal wall would be attenuated across this wall and what the levels of exposure would be once these vibrations reached the fetal head. STUDY DESIGN: Four pregnant sheep were instrumented with acceleration transducers to obtain acceleration levels at the extraabdominal and intraabdominal walls and at the fetal head. Sine-wave vibration stimulation was applied over a frequency range of 3 to 150 Hz at a constant acceleration level of 2.5 m/sec2 (root-mean-square). RESULTS: Vibration of the extraabdominal wall resulted in a frequency-dependent rise in vibration levels at the intraabdominal wall, from 4% to 140% of the input level. At the fetal head a broad peak in response was noted between 6 and 12 Hz, but the overall levels never exceeded 4% of the input level. CONCLUSION: Fetal exposure to localized vibratory stimulation of the maternal abdomen is maximal in the range of 6 to 12 Hz. 相似文献
6.
The authors treated white rats daily at one and the same time for a period of one hour with vibrations. One of the groups of animals were treated with vibrations and examined after that, but the other two groups were treated respectively for 45 and 90 days. Examination was carried out during vibration, immediately after that and three hours after the vibrations. They used the method of the turned bags of Wilson and Waisman, by means of which they checked the resorption of glucose through the intestinal wall in vitro, it was established that during continuous vibration action the values of the resorption glucose were statistically lower than those of the control group. There were statistically significant differences in the values of the resorbed glucose in the subgroups examined during the vibration immediately and there hours after that. The authors discuss the obtained results in the light of the current concepts for the action of vibrations as a stress factor and their influence on intestinal resorption. 相似文献
7.
Xizhong An Zhitao Xing Chengchang Jia 《Metallurgical and Materials Transactions A》2014,45(4):2171-2179
Physical experiments were carried out to study the cold compaction of copper powders under uniaxial compression using our self-designed equipment. Two kinds of copper powders with different particle sizes and distributions were considered. One-dimensional vibrations were utilized before compaction to systematically study the effect of parameters such as vibration frequency ω, amplitude A, and vibration intensity Г on the initial packing density. The macro-property and corresponding microstructures of compacts obtained from initial packings with and without vibrations were compared and analyzed. The results show that higher packing density can be obtained in the compaction of coarse powders with broad size distribution when other experimental conditions are fixed. For each powder, the evolution of packing density vs pressure takes on exponential correlation with high R 2 value. Much denser and more uniform compacts can be realized with the aid of vibration which can improve the particle rearrangement and result in the filling of macro pores formed in initial packing, and the characterization on the microstructure identifies that the particles inside the compact become polyhedrons with regular shape and uniformly distributed. 相似文献
8.
9.
《Acta Metallurgica Materialia》1992,40(10):2739-2751
Longitudinal steady-state sinusoidal vibration is introduced into the unidirectional dendritic solidification process of Al−3% Mg alloy. The first (470 Hz), second (1050 Hz), and third (1736 Hz) order resonant frequencies of solidification system is utilized to produce strong vibrational response out of less exciting energy. Solidification front temperature is raised by vibration under constant solidification conditions, whereas liquid temperature gradient ahead of solidification front and solidification rate are both reduced. Dendrite fragmentation is the most conspicuous variation of structural morphology, which is characterized by primary stalk disintegration and secondary arm detachment. The first order resonance results in a periodically fractured bamboo-like dendritic structure and most severe solute segregation. Contrary to its grain refinement effect, vibration expedites the coarsening of resultant structures. The alloy mechanical properties are markedly deteriorated under the first order resonance. But they are appreciably improved if the second and third order resonant vibrations are applied. 相似文献
10.
The long reach robots required for space applications necessitate active vibration control algorithms for a better pointing performance. The inherent flexibility of the long and flexible space arms can generate undesirable vibrations, making their end-point controls very difficult, although they provide a large reach volume and payload capabilities. This paper presents a novel technique to control the vibrations of N modes of a flexible link attached to a rigid robot employing an Impedance Control Technique using piezo-ceramic actuators and fiber-optic sensors. A piezo-ceramic actuator model has been developed by considering the moments∕forces generated by the piezo-ceramic actuator as nonconservative external forces. Successful simulation and experimental results have been obtained for vibration control of a cantilever beam which is attached to the end effector of the Titan-II robot arm. 相似文献
11.
A general asymptotic format is presented for the effect on the modal vibrations of a transverse damper close to the end of a cable. Complete locking of the damper leads to an increase of the natural frequencies, and it is demonstrated that the maximum attainable damping is a certain fraction of the relative frequency increase, depending on the type of damping device. The asymptotic format only includes a real and a complex nondimensional parameter, and it is demonstrated how these parameters can be determined from the frequency increase by locking and from an energy balance on the undamped natural vibration modes. It is shown how the asymptotic format can incorporate sag of the cable, and specific results are presented for viscous damping, the effect of stiffness and mass, fractional viscous damping, and a nonlinear viscous damper. The relation of the stiffness component to active and semiactive damping is discussed. 相似文献
12.
Continuous real-time estimating of compaction quality during the construction of a hot mix asphalt (HMA) pavement is addressed in this paper. The densification of asphalt pavements during construction usually is accomplished by using vibratory compactors. During compaction, the compactor and the asphalt mat form a coupled system whose dynamics are influenced by the changing stiffness of the mat. The measured vibrations of the compactor along with process parameters such as lift thickness, mix type, mix temperature, and compaction pressure can be used to predict the asphalt mat density. Contrary to existing techniques in the literature in which a model is developed to fit experimental data and to predict mat density, a neural network-based approach is adopted that is model-free and uses pattern-recognition techniques to estimate density. The neural network is designed to read the entire frequency spectrum of roller vibrations and to classify these vibrations into different levels. The intelligent asphalt compaction analyzer (IACA) is then trained to convert these vibration levels into a “number” indicative of the asphalt mat density at a given location. This two-step process eliminates the need for regression analysis and produces more accurate density measurements than those reported elsewhere in the literature. Compaction studies of HMA mixes on a stiff subgrade indicate that the changes in the vibration characteristics of the roller are attributable to an increased compaction of the HMA base. The results also show that, with the neural network working as a classifier, the IACA can estimate the density continuously, and in real time, with accuracy levels adequate for quality control in the field. 相似文献
13.
S. H. Ju 《Canadian Metallurgical Quarterly》2004,130(7):740-748
This paper explores the use of a three-dimensional finite-element analysis to model soil vibrations due to high-speed trains on bridges. Finite-element meshes include the bridge superstructure, bridge foundations, nearby building foundations, and piles. Wheel elements represented by appropriate mass, damping and stiffness factors were used to simulate a moving high-speed train. Along the mesh boundaries, absorbing boundary conditions were employed to avoid fictitious wave reflections. Isolation methods to reduce soil vibrations were investigated including construction of open and infilled trenches and soil improvement. Vibration isolation effects due to building foundations and piles were also studied. The finite-element results indicate that suitable axial stiffness between two simple beams can reduce vibration significantly, especially at a near-resonance condition. Operating with an appropriate train velocity to avoid resonance can be another way to reduce vibrations. Suitable mat foundations can significantly reduce soil horizontal vibration, but cannot isolate vertical vibration. Soil improvements near the bridge do not effectively attenuate low-frequency vibrations. Infilled and open trenches can isolate soil vertical vibration; however, their efficiency seems disproportionate to their cost. 相似文献
14.
Biswajit Basu 《Canadian Metallurgical Quarterly》2007,20(1):72-74
The purpose of this work is to study the nonlinear vibrations induced due to wheel contact. The contact force is nonlinear and is considered to be caused due to a nonpolynomial 3/2 type of spring (Hertzian) as is commonly assumed to model wheel contact. Approximate periodic free vibration solutions are derived using an analytical technique based on the averaging method and is compared numerically. The case of the nonlinear wheel with nonlinear damping (van der Pol) is also studied and the possibility of the existence of a limit cycle is explored. The existence of initial condition dependent free periodic vibrations or a limit cycle under certain circumstances may be important for passenger comfort and noise generation or for vehicle safety as it may cause wear and tear of the mechanical components of the vehicle. 相似文献
15.
J Zagórski R Jakubowski L Solecki A Sadlo W Kasperek 《Canadian Metallurgical Quarterly》1976,27(4):347-354
Investigations were carried out in a group of 20 males with a definite morpho-physiological type, at two selected ranges of acceleration values chosen according to ISO criteria in the range of 2-20 Hz. In the subjects exposed to vibration the values of vibration acceleration were measured in different points along the spine at the levels of S3, L3, Th7, C3 and at the vertex. The investigations demonstrated that the frequency is the parameter of crucial significance for the propagation of vibrations in the human organism. Vibration within the frequency range up to 12 Hz affects the whole human organism, while the vibrations above 12 Hz have only a local effect. 相似文献
16.
Takuya Tamura Yuuki Maehara Naoki Omura Kenji Miwa 《Metallurgical and Materials Transactions A》2008,39(8):1917-1921
The method for producing Mg-Cu-Y and Fe-Co-B-Si-Nb bulk metallic glasses using electromagnetic vibrations is effective in
forming the metallic glass phase. Disappearance or decrement of clusters by the electromagnetic vibrations applied to the
liquid state is considered to cause suppression of crystal nucleation, because the electromagnetic vibrations vibrate the
clusters vigorously in the melt. The purpose of this study was to investigate motion of the crystal particles by the electromagnetic
vibrations in Mg-Cu-Y bulk metallic glasses. The electromagnetic vibration force vibrated the crystal particles or the clusters
that become crystal nuclei in the melt, because the electric current for the electromagnetic vibrations concentrates in those.
Thus, the electromagnetic vibrations were found to select vibration particles from the melt. Moreover, it was considered that
composites for which second phases or other compounds are dispersed into the metallic glass phase or a nanostructure phase
can be produced by the electromagnetic vibration process.
This article is based on a presentation given in the symposium entitled “Bulk Metallic Glasses IV,” which occurred February
25–March 1, 2007 during the TMS Annual Meeting in Orlando, Florida under the auspices of the TMS/ASM Mechanical Behavior of
Materials Committee. 相似文献
17.
Glen Besterfield Sanjeev Nichani Autar K. Kaw Thomas Eason 《Canadian Metallurgical Quarterly》2003,8(4):204-211
To simulate a trunnion-hub-girder (THG) assembly for bascule bridges, two full-scale laboratory tests were conducted for quantifying stresses at previously observed failure locations and for identifying a favorable assembly procedure. One assembly procedure, AP#1, cools the trunnion for a shrink fit into the hub, followed by cooling of the trunnion-hub assembly to shrink fit it into the girder. Using AP#1, development of cracks on the hub was observed in one THG assembly, and, in yet another assembly, the trunnion got stuck in the hub before full insertion could take place. Large hoop stresses and low temperatures were observed at the trunnion-hub interface when the trunnion-hub assembly was cooled for insertion into the girder. Since fracture toughness of THG parts decreases with temperature, allowable crack lengths were small. In an alternative assembly procedure, AP#2, where the hub is shrink fitted into the girder first, followed by cooling the trunnion and shrink fitting it into the hub-girder assembly, the allowable crack length was determined to be double the allowable crack length of AP#1. Hence, for the given full-scale geometry and interference values, assembly procedure AP#2 was found to be better than AP#1. 相似文献
18.
This paper investigates the reduction of vibration due to the foundation slab both in horizontal and vertical directions. Similar reduction efficiency of the foundation slab was obtained among three-dimensional finite element analyses, field experiments, and the averaging scheme of the rigid slab theory. Then, two formulas fitted from numerical simulations were generated. Using those two simple formulas, one can estimate the reduction efficiency of horizontal and vertical vibrations due to foundation slabs. In summary, a suitable mat foundation can significantly reduce the horizontal vibration transformed from soil. The reduction efficiency depends on the size of the mat foundation over the soil wavelength. Enlarging the thickness of the foundation slab can decrease the vertical vibration transferred from soils. However, it is difficult to reduce vertical low-frequency vibrations for a reasonable slab thickness. 相似文献
19.
This paper proposes a multiresolution based wavelet controller for the control of linear time varying systems consisting of a time invariant component and a component with zero mean slowly time varying parameters. The real time discrete wavelet transform controller is based on a time interval from the initial until the current time and is updated at regular time steps. By casting a modified optimal control problem in a linear quadratic regulator (LQR) form constrained to a band of frequency in the wavelet domain, frequency band dependent control gain matrices are obtained. The weighting matrices are varied for different bands of frequencies depending on the emphasis to be placed on the response energy or the control effort in minimizing the cost functional, for the particular band of frequency leading to frequency dependent gains. The frequency dependent control gain matrices of the developed controller are applied to multiresolution analysis (MRA) based filtered time signals obtained until the current time. The use of MRA ensures perfect decomposition to obtain filtered time signals over the finite interval considered, with a fast numerical implementation for control application. The proposed controller developed using the Daubechies wavelet is shown to work effectively for the control of free and forced vibration (both under harmonic and random excitations) responses of linear time varying single-degree-of-freedom and multidegree-of-freedom systems. Even for the cases where the conventional LQR or addition of viscous damping fails to control the vibration response, the proposed controller effectively suppresses the instabilities in the linear time varying systems. 相似文献
20.
RW Shoenberger 《Canadian Metallurgical Quarterly》1982,53(5):454-457
In a previous series of experiments the subjective intensities of translational (Z-axis) and angular (roll, pitch, and yaw) vibrations were compared, using a psychophysical matching technique. To test the validity and generality of the matching results, an independent set of similar data was obtained in the present experiment, using the method of category production. Seated subjects set levels of translational vibrations, in the X-, Y-, and Z-axes, and angular vibrations, in roll and pitch, that they judged to be "uncomfortable" on a scale of vibration discomfort. Frequencies of 2.5, 3.15, 4.0, 5.0, 6.3, and 8.0 Hz were presented in each vibration direction. As frequency increased the mean acceleration judged to be uncomfortable increased for Y-axis and roll vibrations, decreased for Z-axis vibrations, and was essentially constant for X-axis and pitch vibrations. The Y- and Z-axis results correspond well to equal intensity contours in existing vibration exposure criteria, and the roll results show good agreement with data from the roll matching experiment. The X-axis and pitch results are similar to the results from the pitch matching experiment and indicate the importance of the backrest in determining the effects of X-axis translational vibrations and angular vibrations in pitch. 相似文献