Structural-Damage Detection by Distributed Piezoelectric Transducers and Tuned Electric Circuits

A novel technique for damage detection of structures is introduced and discussed. It is based on purely electric measurements of the state variables of an electric network coupled to the main structure through a distributed set of piezoelectric patches. The constitutive parameters of this auxiliary network are optimized to increase the sensitivity of global measurements- as the frequency, response functions relative to selected electric degrees of freedom-with respect to a given class of variations in the structural-mechanical properties. Because the proposed method is based on purely electric input and output measurements, it allows for accurate results in the identification and localization of damages. Use of the electric frequency-response function to identify the mechanical damage leads to nonconvex optimization problems; therefore the proposed sensitivity-enhanced identification procedure becomes computationally efficient if an a priori knowledge about the damage is available.Comment: 18 page

Train-weight-in-motion identification measuring time-histories of rail strains

This paper deals with the identification of the weight of a train in motion, based on the measurement of the time-history of the response in terms of strains at the foot of the rail. The direct problem is initially addressed: the response of a rail modelled as a one-dimensional Euler-Bernoulli beam with constant properties, resting on a linear elastic foundation with viscous damping and subjected to a Dirac delta load travelling at constant speed is considered. For the model described, a closed-form expression of the solution can be obtained, which permits to investigate the sensitivity of the response to the main mechanical parameters. Analytical strains are compared to their experimental counterpart, showing their practical ability to describe the real phenomenon. As a second step, the inverse problem consisting in the identification of the loads for a given time-history of measured strains is addressed. The solution of the inverse problem is set up as a minimization problem whose objective function is based on the difference between experimental and model time-histories of strains. This inverse problem is nonlinear, and its solution can be pursued by the Newton method, which requires recursive application of a linearized expression for the evaluation of the optimal parameters. The Bayesian formulation enables to investigate identifiability of parameters and minimum number of measurements, and leads to conclude that the identification process must begin with an improving of the interpretative model. This model updating can be achieved by evaluating the model parameters, using the time-history of a train whose weight is known. After that, the actual identification of the loads can be performed. The procedure proposed is applied to experimental strains recorded at the foot of a rail on a stretch of line run by a locomotor moving at a low constant speed. The identified loads were in good agreement with the expected value, with errors smaller than 4%

Dynamic characterization of a system with degradation: A masonry wall

Characterization of the dynamic behavior of linear systems is exhaustively described with a single frequency response curve (frc). For nonlinear systems, which tend to depend on load amplitude, at least one frc for each excitation intensity is required to detect the main characteristics of the dynamic response. Nonlinear systems, more commonly dealt with in the literature, are invariant with respect to the deformation history and, thus, frcs obtained with increasing and decreasing driving frequency coincide, apart from the frequency range with coexistent solutions. This is not so for many real systems which suffer from their past, often exhibiting degradation of their mechanical properties. Here the focus is on the effects of damage on the dynamic signature of systems. The response of a masonry wall, representative of systems with a degrading restoring force, is analyzed under harmonic excitation. A refined finite element model is used to represent the typical degradation that occurs in masonry and its reliability is proved by comparing numerical results and experimental outcomes from shaking table tests. Particular attention is paid to the wall frcs, emphasizing the influence of the deformation history on the curves characteristics and their role in the dynamic characterization of a system with degradation

Modal curvature-based damage localization in weakly damaged continuous beams

Abstract Modal curvatures have been claimed to contain local information on damage and to be less sensitive to environmental variables than natural frequencies. However, simply using the difference between modal curvatures in the undamaged and damaged states can result into localization errors, due to the complex pattern that this quantity presents when considering broad damages or higher order modes. In this paper, we consider weakly damaged continuous beam and we exploit a perturbative solution of the beam equation of motion to obtain an analytical expression of the modal curvature variations in terms of damage distribution. The solution is then used to introduce a filtering technique of the modal curvature variation and to set up the inverse problem of damage localization based on modal curvatures only. Using numerical examples and experimental tests, we show that modal curvatures can be used for precise damage localization, once properly filtered

a hysteretic absorber to mitigate vibrations of rail noise barriers

Noise barriers for high-speed train lines are subject to strong vibrations due to fluid pressure generated by moving trains. The barriers, generally made of steel cantilever beams, suffer fatigue. For their safety, it is necessary to adopt more resistant solutions or reduce their vibration, as is applied in this study. Following the fundamental work by Den Hartog on viscoelastic tuned mass dampers, later contributions on vibration mitigation have shown a variety of phenomena exhibited by a primary structure connected to a nonlinear light attachment. Vibration reduction of the structure has been observed for selected characteristics of the attachment. Here, the use of a hysteretic absorber is exploited. The device, made of light mass on rubber elements, has the advantage of directly representing the elastic and damping elements. The Bouc-Wen model describes the absorber behavior and its parameters are determined by the experimental results. Due to the dependence of the nonlinear system response on the oscillation amplitude, an optimal tuning is adjusted. The system is calibrated to behave around the 1:1 internal resonance condition. Numerical and experimental results show that the absorber effectively reduces the amplitude and the number of vibration cycles

Post-critical response of an axially moving beam

In this paper the dynamic response of a simply supported traveling beam, subjected to a pointwise transversal load, is investigated. The motion is described by means of a high dimensional system of ordinary differential equations with linear gyroscopic part and cubic nonlinearities obtained through the Galerkin method. The system is studied in the super-critical speed range with emphasis on the stability and the global dynamics that exhibits special features after the first bifurcation. A sample case of a physical beam is developed and numerical results are presented concerning bifurcation analysis and stability, and direct simulations of global postcritical dynamics. In the supercritical speed range a regular motion around bifurcated equilibrium position becomes chaotic for particular values of frequency and force. The bifurcation diagram for varying force intensity is shown, it can be noticed that a chaotic motion occurs in a wide range of the forcing parameter, co-existiig with a 3T periodic solution in a limited window