Showing papers by "Alessandro Marzani published in 2011"

Graphical User Interface for Guided Acoustic Waves

Abstract: This paper presents GUIGUW v0.1, a graphical user interface (GUI) for the computation of stress-guided wave dispersive features. The software exploits semianalytical finite-element (SAFE) formulations for the calculation of wave-propagation characteristics. The interface allows for the selection of geometrical, mechanical, and frequency-related parameters for the computation. Isotropic and anisotropic materials with linear elastic and linear viscoelastic rheological behaviors can be considered, and any waveguide cross section can be modeled. For each existing wave, the dispersive results can be represented in terms of wave number, wavelength, phase velocity, group velocity (for undamped waveguides), energy velocity, and attenuation (for damped waveguides). By simply working with the GUI, original results for guided stress waves can be obtained.

A passive monitoring technique based on dispersion compensation to locate impacts in plate-like structures

Abstract: A method for impact location in plate-like structures is proposed. The approach is based on guided waves dispersion compensation. Procedures based on dispersion compensation are usually applied to active monitoring techniques, as they require the knowledge of the time of impact to effectively compensate the guided waves dispersive behaviour. Unfortunately, this knowledge is not given in passive monitoring techniques. Despite this limit, the proposed dispersion compensation procedure is useful as it removes in the group delay of the acquired signals the dependence on the travelled distance. By cross-correlating the signals related to the same event acquired by different sensors, the difference in travelled distances can be determined and used to locate the wave source via hyperbolic positioning. The results show that the developed tool could pave he way for a new class of procedures to locate impacts in waveguides.

Numerical and experimental study of guided waves for detection of defects in the rail head

Abstract: This paper gives insight to the ultrasonic wave propagation in arbitrary cross-section waveguides such as rails, with application to ultrasonic inspection. Due to the geometrical complexity of the rail cross-section, the analytical solution to the wave propagation in the rail is not feasible. A Semi Analytical Finite Element method is described as an alternative yet still robust approach to get the solution of the problem. The free-vibration solution and the forced solution to a laser excitation of the rail head are shown up to a frequency of 500 kHz. The effects of different loading patterns are discussed, and experimental results are provided. The analysis allows for the identification of certain wave modes potentially sensitive to specific types of rail head defects.

Cracks Interaction Effect on the Dynamic Stability of Beams under Conservative and Nonconservative Forces

Abstract: This study is an extension of the paper by E. Viola and A. Marzani [1] where the eigenfrequencies and critical loads of a single cracked beam subjected to conservative and nonconservative forceshave been investigated. Here the aim is to analyze the dynamic stability of T cross section beams withmultiple cracks. A doubly cracked Euler-Bernoulli beam subjected to triangularly distributed subtangential forces, which are the combination of axial and tangential forces, is considered. The governingequation of the system is derived via the extended Hamilton’s principle in which the kinetic energy, theelastic potential energy, the conservative work and the nonconservative work are taken into account. Thelocal flexibility matrix for a beam with T cross-section is used to model the cracked section. The resultsshow that for given boundary conditions cracked beams become unstable in the form of either flutter ordivergence depending on the crack parameters, the nonconservativeness of the applied load as well as theinteraction of the two cracks.

Warped-wigner-hough transformation of lamb waves for automatic defect detection

Abstract: To improve the defect detectability of Lamb wave inspection systems, application of non-linear signal processing was investigated. The approach is based on a Warped Frequency Transform (WFT) to compensate the dispersive behavior of ultrasonic guided waves, followed by a Wigner-Ville time-frequency analysis and the Hough Transform to further improve localization accuracy. As a result, an automatic detection procedure to locate defect-induced reflections was demonstrated and successfully tested by analyzing numerically simulated Lamb waves propagating in an aluminum plate. The proposed method is suitable for defect detection and can be easily implemented for real application to structural health monitoring.

A Defect Localization Procedure Based on Warped Lamb Waves

Abstract: Passive defect location procedures based on ultrasonic guided waves are widely used for structural health monitoring purposes of plate-like structures. Approaches based on the measured time-of-flight delay of propagating waves recorded at different locations are generally adopted. In these approaches, uncertainties are due to the fixed speed assumed for the incoming waves to convert their time delay in distances. These distances are next used to solve a triangulation scheme that leads to the defect location. In this paper, this inconvenient is avoided by processing the time transient measurements acquired at the different locations with a “Warped Frequency Transform” (WFT) that is capable to reveal the distance travelled by dispersive waves. In fact, by means of the WFT the recorded time waveform is converted into the incipient pulse at a distance from the origin which is proportional to the distance travelled by a mode within the signal, thus fully compensating its dispersive effect. Then, the processed time waveforms recorded from simple sensors can be used for locating defects by means of classical triangulation procedures.

Effect of an Internal Open Crack on Pipes Wave Propagation

Abstract: Stress guided waves in the sonic and ultrasonic regime are acknowledged as a powerful too lto inspect pipes in a non-invasive manner.A key point of the inspection procedure is related to the dispersive behavior of guided waves, that for agiven pipe is defined by the so-called dispersion curves. Such behavior, is generally predicted bymeans ofanalytical formulations. However, when the geometry of the pipe cross-section is not axially symmetric,such as in the presence of an open internal crack running along the pipe length, analytical formulations fail. Here, the computation of the guided waves properties for such a scenario is addressed via a SemiAnalytical Finite Element (SAFE) formulation in which the open crack is modeled at the mesh level.Different crack depths are considered and their effect on the waves dispersion curves are highlighted.The results could be of interest in pipeline inspection and monitoring.

Simulation of a strategy for the assessment of damage in human bones by using FEM and signal processing

Abstract: The aim of this work is to propose a non-invasive technique aimed at assessing the evolution of damage in elongated bones that could be caused, for instance, by metabolic diseases The technique exploits signals related to mechanical guided waves traveling along the bone to characterize its mechanical properties in a total non-destructive manner Such properties can be used as an indicator of the damage level present in the bone The preliminary results motivate future studies