André Preumont

Spectral methods for multiaxial random fatigue analysis of metallic structures

This paper presents computationally efficient frequency domain methods for estimating the high-cycle fatigue life of metallic structures subjected to a random multiaxial loading. The equivalent von Mises stress method proposed earlier by the senior author is first reviewed. It is then shown that the multiaxial rainflow method, initially formulated in the time domain, can be implemented in the frequency domain in a formally similar way. The consistency of the results are checked by comparison with a time domain method based on the critical plane. It is observed that frequency domain methods produce enormous computer savings and correlate fairly well with the time domain method in terms of localizing the critical areas in the structure. A frequency domain implementation of Crosslands failure criterion is also proposed; it is found in very good agreement and much faster than its time domain counterpart.

ACTIVE TENDON CONTROL OF CABLE-STAYED BRIDGES

This paper considers the active vibration control of cables and cable/structure systems with an active tendon controlling the axial displacement of the cable anchor point. It is demonstrated that a force feedback based on a collocated force sensor measuring the tension in the cable is feasible and that this control configuration can be associated with control laws with guaranteed stability properties. Experimental results are presented on a cable with small sag and on a cable/structure system. They show that the control algorithm can provide the structure with several percent of active damping and that the parametric resonance does not occur when the natural frequency of the structure is twice that of the cable.

Mixing Rules for the Piezoelectric Properties of Macro Fiber Composites

This article focuses on the modeling of structures equipped with Macro Fiber Composite (MFC) transducers. Based on the uniform field method under the plane stress assumption, we derive analytical mixing rules in order to evaluate equivalent properties for d31 and d33 MFC transducers. In particular, mixing rules are derived for the longitudinal and transverse piezoelectric coefficients of MFCs. These mixing rules are validated using finite element computations and experimental results available from the literature.

Spatial filters in structural control

This paper discusses the use of modal filters in structural control. Discrete piezoelectric array sensors are first discussed and their lack of roll-off due to spatial aliasing is pointed out. In the second part, a new porous distributed electrode concept is introduced, which allows the effective piezoelectric coefficient to be tailored in two dimensions.

Predicting Random High-Cycle Fatigue Life With Finite Elements

This paper extends the classical theory of random fatigue to multiaxial stress fields, using a quadratic criterion. An equivalent uniaxial random process is constructed by combining the power spectral densities of the normal and tangential stresses according to the von Mises criterion. This process, that we call the von Mises stress, is used to evaluate the damage with a uniaxial model of random fatigue. A finite element implementation is proposed. 17 refs.

Active damping of chatter in machine tools: demonstration with a “hardware-in-the-loop” simulator

Abstract The motivation of the work is twofold: (a) understand the physics behind regenerative chatter and the influence of structural damping and (b) demonstrate an active damping technique based on collocated actuator/sensor pairs. A numerical stability analysis is performed using the root locus method and it is shown that, along with the structural poles, eigenvalues due to the delay parameter may contribute to instability. Since experimental demonstration of chatter in real machines is difficult, an alternative way of demonstration via a mechatronic simulator is presented, using the ‘hardware-in-the-loop’ concept. The mathematical model of the regenerative cutting process in turning is simulated in a computer and this is interfaced to a beam, representing the structural dynamics of the machine, via a displacement sensor and force actuator. In this way, a hardware and a software loop are combined. In a second step, an additional control loop is added, consisting of an accelerometer sensor and a collocated inertial actuator. Numerical and experimental stability lobe diagrams are compared, with and without active damping.

ACTIVE DAMPING OF STRUCTURES WITH GUY CABLES

HE current design of the space station is largely based on trusses, but it is quite likely that the future large space structures (LSS) will use large trusses connected by tension cables, to increase their global stiffness, in a way similar to that used to stiffen the airplanes in the early days of aeronautics. This concept of tension trussstructure has already beenused for large mesh antennas (a 10-m deployable mesh antenna was e own by the Russians in 1979 ); it has the advantages of being deployable and easily recone gurable by changing the static tension in the cables and of lending itself naturally to shape control. 1;2 The active damping of LSS has long been recognized as a major issueforvariousreasonssuchastheinteractionofthee exiblemodes with the attitude control system, the pointing requirements of various instruments mounted on the station, or simply preservation of the microgravity environment. The active damping of truss structures has received considerable attention for the past 10 years or so, and effective solutions have been proposed 3;4 ; active struts including piezoelectric actuators have been developed, and control laws with guaranteed stability have been tested successfully. 5 The active damping of cable structures is more dife cult because cables and strings behave in a nonlinear manner and are prone to parametricexcitationwhenthefrequencyofthesupportingstructure is close to twice the natural frequency of the cable. Chen 6 showed thatthevibrationofastringcanbecontrolledby apositiveuseofthe parametric excitation resulting from the longitudinal motion of the support at afrequencyequal to twice the frequency of the transverse vibration of the string (stiffness control ). The damping of cable structures has also become a major issue in civil engineering because the ever-increasing span of the cable-stayed bridges 7 makes them more sensitive to wind and trafe c induced vibrations as well as to e utter instability. The distinctive feature here is the presence of some sag in the vertical plane, resulting from the gravity loads (typical value of the sag to length ratio is 0.5%). The active damping of cable-stayed bridges with an active

Responsive systems for active vibration control

Preface. An Introduction to Active Vibration Control A. Preumont. Active Sound Control S.J. Elliott. Active Vibro Acoustic Control S.J. Elliott, P. Gardonio. Actuators and Sensors in Structural Dynamics W. Gawronski. Robust Control of Vibrating Structures K.B. Lim. Finite Element Models for Piezoelectric Continua P. Gaudenzi. Latest Trends in the Development of Piezoelectric Multi-degree-of-freedom Actuators/Sensors R. Bansevicius. Damping Control in Systems Assembled by Semi-Active Joints R. Nitsche, L. Gaul. Piezoelectric Stack Actuator: FE-Modeling and Application for Vibration Isolation U. Stobener, L. Gaul. Active Vibration Control of a Car Body Based on Experimentally Evaluated Modal Parameters U. Stobener, L. Gaul. PVDF-Transducers for Structural Health Monitoring S. Hurlebaus, L. Gaul. Vibro-Isolation of Sensitive Equipment V. Ostasevicius. An Approach to Smart Structure Design using MEMS Technology P. Minotti. Power Amplifiers for Piezoelectric Actuators H. Janocha, et al. Index.

Active Tendon Control of Large Trusses

The use of tension cables to stiffen and control precision trusses as needed for future interferometric missions is investigated. A strategy for damping cable structures with active tendons is presented. Each tendon consists of a displacement actuator (piezoelectric in this case) collocated with a force sensor; the local control law consists of an integral force feedback, which has guaranteed stability if we assume perfect actuator and sensor dynamics. Then an approximate linear theory that allows one to predict the closed-loop poles of a cable structure with a root locus technique is developed; the methodology is applied numerically to a model of the Jet Propulsion Laboratory Micro-Precision Interferometer. Finally, a laboratory experiment on a guyed truss with three active tendons is described, and the experimental results are compared with the numerical predictions.

On the Peak Factor of Stationary Gaussian Processes

Abstract New simple approximate formulae are introduced for the average and the standard deviation of the peak factor of stationary Gaussian processes. The formulae take into account the bandwidth of the process and are based on the assumption that the extreme point process is Markovian. Also presented are simulation results for various spectral shapes (response of the linear oscillator to a white noise, bimodal spectrum, and ideal bandpass process). These suggest that none of the currently available bandwidth parameters can represent accurately the overall effect of the spectral shape on the peak factor. The approximate formulae, however, give reasonable estimates, if one excludes bimodal spectra with very light modal damping. The various definitions of the envelope process (essential in the Markov approximation) are reviewed and unified.