Low Frequency Structural Vibration Research Articles

Vibration Characteristics of Metamaterial Beams With Periodic Local Resonances

Vibration characteristics of metamaterial beams manufactured of assemblies of periodic cells with built-in local resonances are presented. Each cell consists of a base structure provided with cavities filled by a viscoelastic membrane that supports a small mass to form a source of local resonance. This class of metamaterial structures exhibits unique band gap behavior extending to very low-frequency ranges. A finite element model (FEM) is developed to predict the modal, frequency response, and band gap characteristics of different configurations of the metamaterial beams. The model is exercised to demonstrate the band gap and mechanical filtering capabilities of this class of metamaterial beams. The predictions of the FEM are validated experimentally when the beams are subjected to excitations ranging between 10 and 5000 Hz. It is observed that there is excellent agreement between the theoretical predictions and the experimental results for plain beams, beams with cavities, and beams with cavities provided with local resonant sources. The obtained results emphasize the potential of the metamaterial beams for providing significant vibration attenuation and exhibiting band gaps extending to low frequencies. Such characteristics indicate that metamaterial beams are more effective in attenuating and filtering low-frequency structural vibrations than plain periodic beams of similar size and weight.

Damage Identification of Steel Beams Using Local and Global Methods

Structural condition monitoring methods can be generally classified as local and global. While the global method needs only a small number of sensors to measure the low-frequency structural vibration properties, the acquired information is often not sufficiently sensitive to minor damages in a structure. Local methods, on the other hand, could be very sensitive to minor damages but their detection range is usually small. To overcome the drawbacks and take advantage of both methods, an integrated condition monitoring system has been recently developed for structural damage detection, which combines guided wave and structural vibration tests. This study aims at finding a viable damage identification method for steel structures by using this system. First, a spectral element modelling method is developed, which can simulate both wave propagation and structural vibration properties. Then the model is used in updating analysis to identify crack damage. Extensive numerical simulations and model updating works are conducted. The experimental and numerical results suggest that simply combining the objective functions cannot provide better structural damage identification. A two-stage damage identification scheme is more suitable for identifying damage in steel beams.

Fatigue Damage Monitoring and Remaning Life Assessment using Nonlinear Vibro-Modulation Technique

Assessment of structural deterioration due to in-service and environmental loads is an essential element in ensuring safety, operability and long life of various structures and structural components. We applied vibro-acoustic modulation technique to monitor material degradation at the micro/meso scale before the onset of the macro-scale fracture. The technique explores nonlinear acoustic interaction of high frequency ultrasound and low frequency structural vibration at the site of the incipient damage. It is shown that micro/meso scale degradation increases the material nonlinearity leading to modulation of the high frequency ultrasonic signal by low frequency vibration, quantified by the nonlinear acoustic Damage Index (DI). Numerous tests with hundreds of test coupons proved high sensitivity of the developed technique to micro/meso scale fatigue damage accumulation. The study further demonstrated that the remaining life of the fatigued material can be predicted using phenomenological damage accumulation rule expressed through measured Damage Index. Work supported by NAVAIR.

1315 非線形圧電インピーダンス変調法によるき裂進展の監視(J14-3 知的材料・構造システム(3) 計測・モニタリングII,ジョイントセッション,21世紀地球環境革命の機械工学:人・マイクロナノ・エネルギー・環境)

In this paper, a novel monitoring methodology to detect and characterize damages accompanied by changes in the micro/macroscopic condition of surface contact is proposed. The presented system consists of a PZT patch attached on the structural surface excited by a high frequency harmonic wave. When the structure is subjected to the operational or ambient load at low frequencies, it vibrates, and the inherent damages may introduce a nonlinear effect to the vibro-acoustic dynamics that induces an interaction between the low frequency structural vibration and the high frequency transmitted wave. This nonlinearity is observed as the modulation of the electric impedance of the PZT patch synchronous with the structural vibration. By investigating the relationship between the modulations and the structural vibration, the nonlinear characteristics of the damages can be specified. Experiments using a beam with low-cycle fatigue cracks are conducted for illustrative purpose.

206 非線形圧電インピーダンス変調法を用いた疲労き裂の検出(セッション5 センサ)

In this paper, a novel monitoring methodology to detect and characterize damages accompanied by changes in the micro/macroscopic condition of surface contact is proposed. The presented system consists of a PZT patch attached on the structural surface excited by a high frequency harmonic wave. When the structure is subjected to the operational or ambient load at low frequencies, it vibrates, and the inherent damages may introduce a nonlinear effect to the vibro-acoustic dynamics that induces an interaction between the low frequency structural vibration and the high frequency transmitted wave. This nonlinearity is observed as the modulation of the electric impedance of the PZT patch synchronous with the structural vibration. By investigating the relationship between the modulations and the structural vibration, the nonlinear characteristics of the damages can be specified. Experiments using a beam with low-cycle fatigue cracks are conducted for illustrative purpose.

Improvement of Dynamic Properties in Milling by Integrated Stepped Cutting

Die and mold machining usually necessitate employment of milling tools with long shanks. However, economic and technical design requirements often enforce low frequency structural vibration characteristics in work-pieces, especially in an area distanced from the tool holder. These structurally unstable designs pose a challenge for milling operations. The associated vibrations invariably reduce the quality of the work-piece produced and drastically increase average processing time and costs. This paper deals with cutting techniques used in face milling operations under semi-stable and unstable conditions. The strategy to use milling tools with integrated stepped cutting passes shows, that the use of smaller cutting ratios b/h, the chatter characteristics of the process could be significantly improved, thereby increasing the performance of the milling process.

A study of the vibro-acoustic modulation technique for the detection of cracks in metals

One implementation of the vibro-modulation technique involves monitoring the amplitude modulation of an ultrasonic vibration field transmitted through a cracked specimen undergoing an additional low frequency structural vibration. If the specimen is undamaged and appropriately supported, the two vibration fields do not interact. This phenomenon could be used as the basis for a nondestructive testing technique. In this paper, the sensitivity of the technique is investigated systematically on a set of mild steel beams with cracks of different sizes and shapes. A damage index was measured for each crack. The correlation obtained between the crack size and the strength of the modulation is fairly poor. The technique proved extremely sensitive to the initial state of opening and closing of the crack and to the setup due to the modulating effects of contacts between the specimens and the supports. A simple model is proposed which explains the main features observed and approximately predicts the level of sideband obtained experimentally.

Airfoil Performance at Low Reynolds Numbers in the Presence of Periodic Disturbances

The boundary-layer separation and wake structure of a NACA 0025 airfoil and the effect of external excitations in presence of structural vibrations on airfoil performance were studied experimentally. Wind tunnel experiments were carried out for three Reynolds numbers and three angles of attack, involving hot-wire measurements and complementary surface flow visualization. The results establish that external acoustic excitation at a particular frequency and appropriate amplitude suppresses or reduces the separation region and decreases the airfoil wake, i.e., produces an increase of the lift and∕or decrease of the drag. The acoustic excitation also alters characteristics of the vortical structures in the wake, decreasing the vortex length scale and coherency. Optimum excitation frequencies were found to correlate with the fundamental frequencies of the naturally amplified disturbances in the separated shear layer. The results suggest that acoustic waves play a dominant role in exciting the separated shear layer of the airfoil. Moreover, low-frequency structural vibrations are found to have a significant effect on airfoil performance, as they enhance the sound pressure levels within the test section.

Hybrid Control of Structures

Introduction of flexible structures, in a variety of application areas, has resulted in the need for low frequency structural vibration control. Due to structural complexities, spatial distribution, sensor and actuator requirements, new control system challenges have been identified. In any control application, robustness and performance appear as a design tradeoff. If a system is made very robust, the performance suffers and vise-versa. In addition, fixed controllers can be designed for robustness and adaptive controllers can be designed for performance. It is the goal of this paper to introduce a control scheme that primarily utilizes a fixed controller for disturbance rejection and uses an adaptive controller to handle small plant perturbations. Experimental results using fixed controllers and adaptive controllers are included and a combined fixed / adaptive control scheme is presented.

多点接続する柔軟付属物をもつ衛星ダイナミックスの導出

One of the promising technology to economize satellite communication system is the multi-beam system. The multi-beam satellite communication system requires large diameter satellite antenna. In order to satisfy several requirements, the large antenna learns to have low natural frequencies of about 1-2Hz or lower. The effects of the low frequency structural vibration on satellite attitude control have been discussed. In those effects the coupling coefficients, which shows the coupling between the rigid motion and the vibration, are important. But most of them are dealing with the flexible appendages which are connected to the satellite main body at one point. So those formulations of flexible satellite can be applied to a satellite with one-point connection. However, from the point of structural stability, it is desirable that the communication antenna reflector has multiple connection points to the main body. Using Lagrange multiplier, we revealed that the coupling coefficients are derived only by modal momentum and modal angular momentum regardless of the number of connecting points.

Soviet Physics—Acoustics

Reducing the vibration of smart structures based on adaptive active vibration control (AAVC) has been extensively studied in recent years, because it is a light weight and effective method for reducing low-frequency structural vibrations. The multi-channel AAVC methodology based on the filtered-X least mean square (FXLMS) algorithm is widely implemented in active control applications owing to its self-adjustment ability to adapt to dynamically varying structures. In this paper, a new multi-channel FXLMS with online secondary path modelling (SPM) is designed based on the auxiliary random noise technique. Through a defined indirect error signal, the proposed variable step-size (VSS) strategies can ensure that every online SPM filter and every active control filter have their own exclusive step size to update the coefficients. Moreover, the proposed auxiliary noise power scheduling (ANPS) strategy can ensure that the variation rules of auxiliary noise power applied to different secondary paths are different. A complete multi-channel AAVC real-time experimental system based on NI compact RIO is set up to conduct the experimental investigation. A series of AAVC control experiments on a piezoelectric smart cantilever plate with PZT sensors and actuators are conducted to demonstrate the validity and efficiency of the proposed method. The experimental results show that the vibration of the smart cantilever plate could be effectively attenuated with a high convergence rate. The proposed methodology has an important advantage in applications where active vibration control of piezoelectric smart structures is required.

Evaluation of B-52 structural response to random turbulence with stability augmentation systems.

A theoretical evaluation of the structural performance of benefits to be expected after installing a state-of-the-art stability augmentation system (SAS) on the B-52 is presented. The new system is expected to provide additional stability in the airplane rigid body motions and the low-frequency structural vibration modes. The mathematical model used in this study included rigid body motions, a large number of normal vibration modes, control surface rotations, and the SAS transfer functions. The dynamic response of the airplane to random atmospheric turbulence was determined using harmonic analysis techniques with provisions made to include the effects of nonlinearities due to SAS saturation. Quantitative estimates of the changes in airplane structural response indicate that fatigue-damage accumulation rates and peak-load amplitudes, due to atmospheric turbulence, can be significantly reduced without degrading the flutter margin.