Showing papers by "Laurence J. Jacobs published in 2000"

Characterization of adhesive bond properties using Lamb waves

Abstract: This research combines laser ultrasonic techniques with the two-dimensional Fourier transform (2D-FFT) to characterize adhesive bond properties. The experimental procedure consists of measuring a series of equally spaced, transient Lamb waves in specimens consisting of aluminum plates joined with an adhesive bond. The frequency spectrum (dispersion curves) for each specimen are obtained by operating on these transient waveforms with the 2D-FFT. This study quantifies the effect of bond stiffness on the dispersion curves of two different bonded specimens (a single aluminum plate with an adhesive transfer tape attached to one side, and two aluminum plates joined with the same adhesive tape) and four adhesive bond conditions (un-aged, and three different aging temperatures and times). The proposed procedure consists of first determining the frequency spectrum of the Lamb waves that propagate in each of the two bonded specimens (plus a single plate); these measurements provide the dispersion curves for each specimen in their un-aged state. Degradation causes changes in the stiffness of an adhesive bond, which causes changes in the dispersion curves of the aged specimens. Experimentally measured dispersion curves are used to quantitatively track changes in the bonded specimens, as a function of age. Finally, these experimental results are interpreted in terms of an analytical model that replaces the adhesive bond layer with a linear spring boundary condition.

Time-frequency representation of Lamb waves using the reassigned spectrogram

Abstract: This brief note reports on a study that applies the reassigned spectrogram (the reassigned energy density spectrum of the short-time Fourier transform [STFT]) to develop the dispersion curves for multimode Lamb waves propagating in an aluminum plate. The proposed procedure first uses the spectrogram to operate on a single, laser-generated and detected waveform to develop the dispersion relationship for this plate. Next, a reassignment procedure is used to refine the time-frequency resolution of the calculated dispersion curves. This reassignment operation clarifies the definition of the measured modes. This study demonstrates that the reassigned spectrogram is capable of distinguishing multiple, closely spaced Lamb modes in the ultrasonic frequency range.

Effect of Aggregate Size on Attenuation of Rayleigh Surface Waves in Cement-Based Materials

Abstract: This research uses laser ultrasonic techniques to study the effect of aggregate size on the attenuation of Rayleigh surface waves in cement-based materials. The random, multiphase, and heterogeneous nature of cement-based materials causes a high degree of material attenuation in the ultrasonic waves that propagate in these materials. Physically, these attenuation losses are due to a combination of absorption and the scattering losses due to material heterogeneity. Laser ultrasonics is an ideal methodology to measure attenuation in these materials because of its high fidelity, large frequency bandwidth, and absolute, noncontact nature. To investigate the effect of aggregate size on attenuation, this research uses a dual-probe, heterodyne interferometer to experimentally measure attenuation losses (as a function of frequency) in five different material systems (each with a different microstructure). These experimental results show that absorption, not scattering from the aggregate, is the dominant attenuation mechanism present in cement-based materials. As a result, aggregate size does not dominate attenuation.

A Civil Engineering Curriculum for the Future: The Georgia Tech Case

Abstract: This paper presents the results of an undergraduate civil engineering curriculum revision at the Georgia Institute of Technology. The process of revision included a variety of important constituencies that provided important input into the adopted curriculum. The curriculum provides emphasis on civil engineering systems, technical communications, sustainability, and computer-based analysis and design. In addition, the paper discusses efforts to encourage students to pursue a masters degree and the use of distance learning technologies as a platform for instruction.

Ultrasonic Monitoring of Material Degradation in FRP Composites

Abstract: This research uses laser ultrasonic techniques to monitor a (directly measurable) ultrasonic prop- erty—frequency-dependent Rayleigh wave velocity (material dispersion)—and then relates changes in this acoustic property to changes in the material's properties (such as stiffness) that characterize damage. The subject material system is a thick, glass-reinforced, vinylester (thermosetting) fiber-reinforced polymer (FRP) composite. Laser ultrasonics is an ideal methodology to monitor changes in the Rayleigh phase (or group) velocity of this material because of its high fidelity, broad bandwidth, point source/receiver, and noncontact nature. The ex- perimental procedure consists of measuring a series of transient elastic waveforms in a thick FRP specimen and then operating on these waveforms with the 2D fast Fourier transform to develop the dispersion relationship for that specimen. Material degradation (damage) is introduced into these specimens with environmental aging, and the dispersion curves are used to quantitatively track changes in material properties as a function of degradation.

Crack detection in thick annular components using ultrasonic guided waves

Abstract: This paper provides an overview of a study on circumferential guided waves in a thick annulus. Both steady state, time-harmonic waves and transient waves are considered. Several solution methods are reviewed and numerical solutions are presented for the propagation of ultrasonic circumferential waves in a thick, curved, two-dimensional annular waveguide. The modal content of the signal and the displacement profiles across the wall thickness are investigated. These studies provide valuable guidance in selecting optimal parameters for use in applications of the guided wave technique to the detection of cracks on the inner surface of annular components. Experimental results show that the technique can be used on parts with complex geometries (e. g. the pitch shaft of a helicopter) to detect cracks that would not be detectable by standard ultrasonic inspection.

Characterization of acoustic emission signals from fatigue fracture

Abstract: This paper discusses a comprehensive study that is developing a quantitative understanding of the acoustic emission (AE) signals that emanate from fatigue cracks. Two critical components of this study are the development of a transfer function that quantifies and removes geometric effects from a measured AE waveform and an experimental program that monitors and identifies AE signals that occur during the fatigue of cylindrical stainless steel specimens under torsion. Typical waveforms are collected during torsional fatigue and correlated with fracture mechanisms from different stages of testing. Three stages of fatigue are identified by AE waveform characterization and confirmed by microscopic replica observation. The other portion of this study demonstrates the effectiveness of using laser ultrasonic techniques to develop transfer functions to quantify and remove geometric effects from measured acoustic emission waveforms.

Experimental Lamb wave spectra of cracked plates

Abstract: This research combines laser ultrasonic techniques with the two-dimensional Fourier transform (2D-FFT) to investigate the effect of cracks on the dispersion of Lamb waves propagating in thin aluminum plates. The high fidelity and broad bandwidth of these optical techniques are critical elements to the success of this work. The experimental procedure consists of measuring a series of equally spaced, transient waves in aluminum plates containing notches; a crack is simulated with a 1 mm thick saw cut notch. The frequency spectrum (dispersion curves) for each plate is obtained by operating on these transient waveforms with the 2D-FFT; this procedure extracts steady-state behavior from a series of transient waveforms. This study quantifies the effect of notch depth (two notch depths are examined; one-fourth and one-half of plate thickness) on the dispersion curves of three different plate thicknesses (nominal thicknesses of 1, 1.5, and 3 mms). These dispersion curves show that a notch reduces the transmitted energy by an amount that is directly proportional to a notch’s depth. In addition, scattering by a notch causes definitive reductions in energy (evident in all modes) at certain frequency-wavenumber combinations, thus providing experimental evidence of the relationship between crack size and the scattered Lamb wavefield.

Application of the short time Fourier transform to interpret ultrasonic signals

Abstract: This research applies wavelet and short time Fourier transform (STFT) techniques to enhance the interpretation of experimentally measured transient ultrasonic waveforms. Specifically, this work applies these advanced signal-processing methodologies to two applications: (1.) detection of cracks in plate specimens; and (2.) the interpretation of acoustic emission (AE) signals. The first case examines experimentally measured (with laser ultrasonics) guided waves in both uncracked and cracked plates. This study investigates the sensitivity, accuracy, and robustness of both the wavelet transform and STFT to determine crack size and location. The second case examines AE signals that emanate from a fatigue crack. This work shows that wavelets enable the quantitative characterization of AE signals, enhancing the ability to determine both source type and location.

Scattering of circumferential waves in a cracked annulus

Abstract: This paper considers guided waves propagating in the circumferential direction of an annulus with a radial crack, with the objective of developing an ultrasonic technique that can detect and characterize these cracks. Specifically, the finite element method is used to simulate the propagation and scattering of guided circumferential waves in a cracked annulus. This method fosters a better understanding of the wave fields, so that a transducer configuration used in the field can be optimized for crack detection/characterization. Both a point source (simulating laser generated ultrasound) and a distributed source (simulating a PZT transducer) are modeled and compared to corresponding experimental results. Animations (snapshots at different instants in time) of the strain energy field in the annulus are given for various combinations of load profiles, incident angles, and incident frequencies. Results of this paper provide the necessary design guidelines for developing nondestructive ultrasonic techniques for the detection/characterization of radial cracks in cylindrical pressure vessels, gas/oil pipes, and shaft/bearing systems.

Quantitative acoustic emission from localized sources in material fatigue processes

Abstract: Fretting fatigue is the phenomenon where two contacting bodies undergoing a cyclic fatigue loading experience small amplitude oscillatory motion. Fretting fatigue is characterized by crack nucleation and the subsequent propagation of these cracks. The coupling of fatigue with fretting leads to the premature nucleation and acceleration of the early growth of fatigue cracks, resulting in a significant reduction in a structure’s service life. A better understanding of the mechanics of fretting fatigue is needed to prevent and reduce the severe consequences of such damage. This research uses quantitative acoustic emission (AE) techniques to study the fretting fatigue of PH 13-8 stainless steel under different loading conditions. Specifically, this work correlates AE signals to specific fretting characteristics such as frictional force history and frictional force-displacement hysteresis loops. These results indicate a close correlation between the various stages of fretting fatigue with the frequency of AE ev...