Showing papers by "Laurence J. Jacobs published in 2001"

Time-frequency representations of Lamb waves.

Abstract: The objective of this study is to establish the effectiveness of four different time-frequency representations (TFRs)—the reassigned spectrogram, the reassigned scalogram, the smoothed Wigner–Ville distribution, and the Hilbert spectrum—by comparing their ability to resolve the dispersion relationships for Lamb waves generated and detected with optical techniques This paper illustrates the utility of using TFRs to quantitatively resolve changes in the frequency content of these nonstationary signals, as a function of time While each technique has certain strengths and weaknesses, the reassigned spectrogram appears to be the best choice to characterize multimode Lamb waves

Crack characterization using guided circumferential waves.

Abstract: This paper examines the propagation of guided circumferential waves in a hollow isotropic cylinder that contains a crack, with the goal of using these guided waves to both locate and size the crack. The crack is sized using a modified Auld's formula, which relates the crack's length to a reflected energy coefficient. The crack is then located by operating on the backscattered signal with a time-frequency digital signal processing (DSP) technique, and then comparing these results to those obtained if the cylinder is perfect. The guided circumferential waves are generated with a commercial finite element method (FEM) code. One objective of this work is to demonstrate the effectiveness of using sophisticated DSP techniques to describe the effect of scattering on dispersive waves, showing it is possible to characterize cracks systematically and accurately by quantifying this scattering effect. The results show that the need for high frequency signals to detect small cracks is significantly decreased by using these techniques.

Automated methodology to locate notches with Lamb waves

Abstract: This study develops an automated method capable of detecting notches in isotropic plates. Laser ultrasonic techniques are used to generate and detect Lamb waves in perfect and notched plates. These signals are first transformed into the time-frequency domain using a short time Fourier transform (STFT) and subsequently into the group velocity-frequency domain. Finally, the notch is located with an autocorrelation in the group velocity-frequency domain. A verification of the proposed methodology shows excellent agreement with the actual location of the notch.

Localization of a synthetic acoustic emission source on the surface of a fatigue specimen

Abstract: The finite geometry of a laboratory specimen influences a measured acoustic emission waveform because of reflections, transmission, and mode conversion at the interface and boundaries of the specimen, thus making it difficult to determine the location of an acoustic emission (AE) source. The objective of this investigation is to develop a model experiment to identifiy the exact source location on the surface using ``synthetic'' AE signals. The AE event is generated by a short local thermal expansion. This expansion is produced by the absorption of a short laser pulse which provides a noncontact and broad-band generation of elastic waves. The signals are detected by a noncontact, broad-band, and high-fidelity sensor: a laser interferometer. The triangulation with several detectors is replaced by a single probe laser interferometer located at different coordinates under reproducible conditions. The recorded signals are analyzed by wavelet transform in order to determine the arrival times of waves for several frequency levels. These arrival times are used to quantify the location of the AE source in the surface as well as the velocity of the most dominant feature, the Rayleigh wave, and the time lag between the instant of the AE and the recording of the signal. The accuracy of the method is demonstrated by comparing the identified source location with the exact one.

Characterization of adhesive bond properties with Lamb waves

Abstract: The objective of this research is to use analytical and computational models to develop a quantitative understanding of the propagation of guided Lamb waves in multi-layered, adhesive bonded components. A specific goal of this study is to develop a theoretical understanding of the experimental behavior observed in [1]. Key issues of this study include the effect of the adhesive bond layer, including its thickness, low stiffness (relative to the aluminum adherends) and viscoelastic behavior. The propagation of these guided waves are interpreted in terms of dispersion relationships, displacement profiles and attenuation curves (both as functions of frequency and wavenumber). The ultimate goal of this study is to determine the effectiveness and sensitivity of guided Lamb waves to determine the in situ properties of an adhesive bond.

Study of acoustic emission in plates using finite element method and laser-ultrasonics

Abstract: This paper studies the propagation characteristics of acoustic emission (AE) in plate structures and the modeling of AE sources using finite element (FE) method To validate the finite element results, propagation of ultrasonic pulses in a 2D plate is first compared with experimental measurements obtained using a laser-ultrasonic system Next the classical moment tensor model of AE sources, ie, force couples w and w/o moments, is simulated and the resulting AE waveforms are examined

One-Dimensional Pulse Propagation in a Nonlinear Elastic Media

Abstract: This paper considers the problem of wave propagation in a nonlinear elastic medium with a quadratic stress-strain relationship. The paper is limited to one-dimensional wave propagation. Under these conditions, the initial value problem is formulated into a hyperbolic system of conservation laws. The Riemann problem due to an initial step function excitation is considered first. Analytical solutions to the Riemann problem are obtained by solving the corresponding eigenvalue problem. In addition, a computer program is developed based on the high-resolution central scheme of Kurganov and Tadmor. The accuracy of this numerical procedure is verified by comparing the numerical results with the exact solutions. The second part of the paper considers several different types of initial excitations in order to determine special characteristics of the wave propagation due to material nonlinearity.

Nondestructive determination of unknown pile tip elevations using modal analysis

Abstract: A comprehensive experimental study and three corresponding numerical analyses were conducted to investigate the suitability of a modal analysis approach for identification of unknown pile embedment lengths. A small-scale pile facility containing partially embedded piles of differing lengths, cross section dimensions, and encasement attributes was constructed so that experimental pile response data could be gathered in a controlled laboratory environment. Impact tests were performed at a number of locations on each model pile, and the modal parameters for each were estimated from the resulting frequency response function data. Comparison of modal parameters estimated from model piles with similar cross section dimensions and different buried lengths showed essentially no variation in natural frequency as the buried length increased, in the frequency range that was practical to measure. Modal damping values showed a greater variation with pile embedment depth, but no discernable trends were apparent that wo...