Experiments on internal and combinational resonances of a three-story structure are investigated. The frequency ratio between the first and the second modal of the structure is set near 1:3 by modal design. The internal resonance behaviors are found when the excitation frequency is near that of the first or second modal frequency. Meanwhile, several types of combinational resonances are found in this structure. These results made a foundation for future modification of the nonlinear vibration equipment.
A new method to determine delay time is proposed, which is called nonlinear multiple autocorrelation function method. The method uses a high order multiple autocorrelation function, R(τ), to calculate the nonlinear correlation of system and estimates the optimal delay time by finding the first local minimum value of R(τ). The time complexity is low and the dependence on the length of data is not strong. The performance of the algorithm is tested by being applied to five noisy chaotic time series. The time series are generated by four chaotic maps adding different noise level Gauss white noise. The numerical results show that, the method is more appropriate and robust to noise.
Using three-dimensional human motion capture system and three-dimensional force plates, a series of experiments on dynamic properties of human-induced walking load at seven different walking frequencies was conducted. Seventy three test subjects had participated the experiments leading to 5 004 records of single footfall trace. The continuous time history of walking load is synthesized with single-step force by using its exact landing time. Based on the experiment measurements, the first five orders of dynamic load factors and phase angles from Fourier-series Model of both vertical and lateral walking load are given. By comparing with existing models from other countries, different characteristics of walking are noticed. The model proposed in this paper could be provided for structural design and analysis of long-span structures considering the problem of serviceability caused by human walking.
This paper took the metal thin-walled components as the investigation tool on the basis of different inducing grooves. The maximization of the Specific Energy Absorption (SEA) and the minimization of the maximum peak force ( ) were considered as the multiobjective functions, the radius of the inducing grooves|, distance between inducing grooves and free end, distance of inducing groovs were chosen as design variables. The optimization was based on the minimization of and the maximization of SEA. The objective functions were constructed based on the Radial Basis Function (RBF), the multiobjective optimization for the thin-walled with the different inducing grooves, which was presented by using the ideal point method, and considering the impact on the SEA and of the convex grooves, indentations, alternately convex grooves and indentations. After the optimization we can get the ideal indentations.
Firstly, the supercavitating vehicle was simulated by a cylindrical thin shell which was loaded with a axial dynamic load, and the dynamic stability differential equations and the unstable regions of structure were deduced. Secondly, the safety margin equations of dynamic buckling were given and linearized by limit step length iteration method when the randomness of axial load was taken into account, and step-by-step searching method was proposed in search of those effective safety margin equations. Finally, the reliability index of dynamic buckling was calculated with step-by-step equivalent plane method. Through numerical results, the influence of the change of load frequency, velocity and the coefficient of load rate to the dynamic buckling reliability were discussed. The calculate result give a theory foundation to how to choose the safety range of load frequency, velocity and the coefficient of load rate.
This paper investigates the vibration characteristics of the rotor system of permanent magnet synchronous motor with air-gap eccentricity. A dynamic model of the rotor system is built. According to the distribution of magnetic flux density generated by permanent magnet and armature current under load, Unbalanced Magnet Pull (UMP) is calculated with the Maxwell stress tensor and is substituted into the dynamic model. From several illustrating numerical examples, the effects of different eccentricities and loads on rotor vibration are discussed in detail. The results show that, the mass unbalance weakens the effect of the UMP while aggravates the vibration of the rotor, the magnitude and orientation of the initial eccentricity can also change the vibration characteristic of the rotor, and the vibration will be weakened when the orientation of initial eccentricity is close to the direction of the gravity. As the load changes, the corresponding vibration components will be excited with the same frequencies as those of the load.
A large number of structural, material, operational and environmental factors would have great influence on automotive brake squeal induced by friction, and the intrinsic influencing mechanism is a significant and challenging issue needs to be solved. Selecting a certain type of drum brake as the research object, a drum brake finite element model for frictional squeal is established using ABAQUS, a free-free modal test of individual brake components and the brake squeal bench test are performed to verify the model, and on the basis of which, the impact of Young’s modulus of shoes’ linings, brake pressure and friction coefficient on the squeal propensity is analyzed By introducing the contact pressure distribution properties between the drum and shoes, the intrinsic influencing mechanism of the factors above on squeal propensity is studied based on the complex eigenvalue theory. Results show that: (1) the squeal propensity of the drum brake decreases with the increase of Young’s Modulus of linings, while increases with the increase of friction coefficient and brake pressure; (2) the nonuniformity of contact pressure distribution is closely related to the squeal propensity and could make an unified explanation on the intrinsic influencing mechanism of the factors above; the squeal propensity decreases as the nonuniformity of contact pressure distribution increases, and the system tends to be more stable.
The absorption character in low frequency can be improved by adding cavum. The affect factor of absorption coefficient was studied, by three type of acoustic system model of stealth structure. Using the pulse tube system, the resonance peak and peak frequency of samples with cavum, different distance, shapes, and depth, were analyzed. The results show that the cavum enhance absorption in low frequency, and the first resonance peak is higher, peak frequency in lower in smaller cavum distance in air backing, and the result is opposite in water backing, and the first resonance peak is lower, peak frequency in lower in smaller cavum volume.
Based on the two-parameter Pasternak model of elastic foundation, the two-parameter model of viscoelastic foundation was derived by using fractional derivative. The equation of elastic and viscoelastic rectangular plate under the dynamic load on two-parameter viscoelastic foundation with the fractional Kelvin model was established. The equation of elastic and viscoelastic rectangular plate with four edges simply supported was solved by the Galerkin method and the segmented numerical method, the correctness of the solution was verified by the example of free vibration. The influences of the fractional order, viscosity parameter, horizontal shear parameter and modulus parameter on the displacement of the fractional Kelvin model with impact load were analyzed. The results show that the fractional derivative viscoelastic model may describe the mechanical behavior of different viscoelastic materials; the displacement response of rectangular plate appears different attenuation formation before and after the fractional order value of 0.5; the attenuation speed of the displacement response increases with the increasing of the viscosity parameter, horizontal shear parameter and modulus parameter.
Aiming at the two different installation modes of the screw nut at the condition that the screw is fixed at one end and only supported at another, this paper mainly presents a comparative study about their influences on the load distribution of the ball screw. The different load states of the screw at the two nut installation modes are firstly analyzed qualitatively. Then, a ball screw load distribution model considering the variations of actual contact angle is established. Using the model, the influence laws of the nut installation modes on the the screw’s load distribution, specifically, the normal load and actual contact angle of each steel ball, are quantitatively investigated. The numerical results show that the distributions of the normal load and contact angle both more balanced when the connecting flange of the nut is installed away from the screw’s fixed end. In addition the nut installation have no significant effect on the axial stiffness of ball screw.The conclusion of this study will provide a theoretical guide for reasonable installation or maintenance of ball screws, which may be significant for enhancing the performance of these functional components, as well as prolonging their life.
Generalized Differential Quadrature Rule(GDQR)was applied to investigate the stability of a complex elastic supported beam on an elastic foundation subjected to a partially tangential force. Based on the motion equations and boundary conditions of an elastic supported beam, the matrix eigenvalue equation consisted of the dynamic equations and boundary conditions was obtained after discreted by GDQR.After analyzing the corresponding eigenvalue equations,the effects of the elastic foundation modulus, shear coefficient, complex boundary conditions on critical load were discussed ,for a clamped-elastic supported beam,the influence of elastic foundation modulus and supported spring stiffness to the instability area was studied and some useful conclusions were obtained. It shows that GDQR is an effective way for solving the stability of such system.
This paper studies the validity of using impact-echo method to test the dynamic modulus of elasticity of concrete for standard prismatic specimens. It is found that the P-wave speed in a standard prism obtained from the impact-echo test well corresponds to the theoretical one-dimensional P-wave speed, and the dynamic elastic modulus obtained from the impact-echo method are essentially equal to those from the conventional transverse resonant frequency method. This paper adopts impact-echo method to detect the variation of P wave velocities of concrete specimen with the increase of freezing and thawing cycles, which is compared with the variation of fundamental vibration frequencies, thus to conduct parallel evaluation of the decline of relative dynamic elastic modulus using these two methods. The experimental results and theoretical analysis show that using impact echo method to detect the frost resistance of hydraulic concrete is feasible and can be used for durability assessment of hydraulic concrete.
A new vibration reduction control strategy is presented, which integrates modified negative input shaping (MNIS) technique with optimal control for active vibration control of the flexible systems such as flexible manipulator. An optimal state feedback controller is designed for the flexible manipulator maneuver and the vibration eliminating. To improve the performance of the vibration control and reduce the system response time duration, the designed modified negative input shaper as feed-forward controller which utilizes the frequency and damping ratio of the whole closed loop system under linear quadratic regulator (LQR) controller. Combining the feed-forward control and feedback control and utilizing the respective advantages, the performance of the control system can be improved. Both analytical and numerical results are presented to demonstrate the effectiveness for vibration suppression of the hybrid control strategy. It can reduce the time delay and thus increase the speed of the response of the system.
The shock load imposed on the equipments is multidirectional. At the same time, the isolators installed under equipments will appear multidirectional distortion. In this condition, the shock characteristics of isolators are distinct from the results in the unidirectional shocks in lab. The studies about the influence multidirectional shocks experts upon shock characteristics of isolators are little both home and abroad at present, and it is necessary for us to conduct groping works about this problem. Therefore, the shock characteristics along two directions of BE120 and EA120 isolators are tested using drop hammer impact test machine. The results show that the stiffness of isolators have dependencies on the main axle direction stiffness. While, the results can’t represent all isolators for the lack of sampling tests.