Durability of nanosized oxygen-barrier coatings on polymers

The paper presents the application of ultrasonic guided waves for fatigue crack detection in metallic structures. The study involves a simple fatigue test performed to introduce a crack into an aluminium plate. Lamb waves generated by a low-profile, surface-bonded piezoceramic transducer are sensed using a tri-axis, multi-position scanning laser vibrometer. The results demonstrate the potential of laser vibrometry for simple, rapid and robust detection of fatigue cracks in metallic structures. The method could be used in quality inspection and in-service maintenance of metallic structures in aerospace, civil and mechanical engineering industries.

https://iopscience.iop.org/article/10.1088/0957-0233/18/3/024/pdf

Fatigue crack detection in metallic structures with Lamb waves and 3D laser vibrometry

Guided ultrasonic waves have shown great potential for structural health monitoring. Various types of transducer can be used for actuating and sensing of these waves. This includes non-contact approaches such as optical/laser techniques. Classical laser methods usually involve high energy interferometers. The paper demonstrates that a commercial laser vibrometer, designed for vibration/modal analysis, can be used for crack detection in metallic structures. The study involves a simple fatigue test in order to initiate and grow a crack. Lamb waves generated by one bonded piezoceramic transducer were sensed using a multi-point scanning laser vibrometer. The results demonstrate the potential of laser vibrometry for simple, rapid and robust detection of fatigue cracks in metallic structures.

https://iopscience.iop.org/article/10.1088/0964-1726/14/6/031/pdf

Structural health monitoring using scanning laser vibrometry: II. Lamb waves for damage detection

Characterization of Electronic Components by Acoustic Microscopy G. Pfannschmidt. Interaction of Acoustic Waves with Solid Surfaces Y. Tsukahara, et al. Scanning Acoustic Microscopy with Phase Contrast W. Grill, et al. Ultrasonic Focusing with Time Reversal Mirrors M. Fink, C. Prada.

Advances in acoustic microscopy

A database of wideband acoustic emission (AE) modeled signals was used in Part 1 to examine the application of a wavelet transform (WT) to identify AE sources. The AE signals in the database were created by use of a validated three-dimensional finite element code. These signals represented the out-of-plane displacements from buried dipole sources in aluminum plates 4.7 mm thick and of small and large lateral dimensions. The surface displacement signals at three far-field distances were filtered with a 40 kHz high-pass filter prior to applying the WT. The WTs were calculated with AGU-Vallen Wavelet, a freeware software program. The effects of propagation distance, AE source type, and the depth of the AE source below the plate surface were examined. Specifically, a ratio of the WT magnitude (WT coefficient) from the fundamental anti-symmetric mode to that from the fundamental symmetric mode was studied for correlation with the AE source type. The WT magnitudes were those corresponding to a particular group velocity and signal frequency for each mode. For sources in the large plate located at the same depth, the ratios were able to distinguish different source types and exhibited only small changes with increasing propagation distance. But, when the variable of depth of the source was introduced, the ratios did not uniquely classify the AE source type. In the case of the small coupon plate specimen, reflections from the specimen edges distorted and complicated the WTs. Since the current coupon database excludes (except for one case) the parameter of changes in the distance of the source from the coupon sides, a full examination of these complications was not possible.

/pdf/a-wavelet-transform-applied-to-acoustic-emission-signals-2q51plqt34.pdf

A wavelet transform applied to acoustic emission signals: part 1: source identification #

Dispersion behavior of guided waves in hollow cylinders (cylindrical waves) was evaluated theoretically and experimentally. Observed dispersion behavior suggests an assignment, different from the traditional one, of longitudinal (L-), flexural (F-) and torsional (T-) modes which are consistent with Lamb waves and shear-horizontal (SH) mode waves. The L- and F-modes of the cylindrical waves have characteristics which are asymptotic to Lamb waves and to waves in a solid cylinder. Experimentally, wide-band cylindrical waves in aluminum pipes were generated using a laser-ultrasonic method. Wavelet transform of the cylindrical wave signals was utilized for time-frequency analysis in order to compare them with the theoretical dispersion curves. For the L(0, 1), F(1, 1), F(2, 1), L(0, 2), F(1, 2) and F(2, 2) modes of the cylindrical waves, which were efficiently excited, theoretical and experimental dispersion curves agree with each other.

http://iopscience.iop.org/article/10.1143/JJAP.40.364/pdf

Modal Analysis of Hollow Cylindrical Guided Waves and Applications

We present a novel method for generating 100 MHz band surface acoustic wave (SAW) by using a scanning interference fringe at the phase velocity of the SAW. The scanning interference fringe is obtained by intersecting two laser beams with different frequencies, and used as a thermoelastic source. The principle of this method is described, and experimentally demonstrated in the 110 MHz Rayleigh waves on an aluminum specimen generated by a long‐pulse (140 ns) Q‐switched Nd:YAG laser.

Excitation of high frequency surface acoustic waves by phase velocity scanning of a laser interference fringe

Propagation phenomena of wideband guided waves in a bended pipe.

We present a general theoretical formulation for the characteristics of surface acoustic waves (SAW) generated by the phase velocity scanning (PVS) method that employs a scanning single laser beam (SSB) or a scanning interference fringes (SIF). In the SSB approach, a broad band SAW pulse is generated and its amplitude is coherently enhanced when the laser scanning velocity V is equal to the phase velocity upsilon(R) of the SAW. The amplitude of the SAW follows a resonance curve represented by a sinc function of the scanning velocity V, but different spatial frequency components in the SSB significantly suppress the side lobes of the resonance curve. In the SIF approach, the scanning velocity upsilon(f) of the fringes is determined by the intersection angle and the frequency difference omega(a) of the laser beams. A narrow band tone burst of SAW with frequencies higher than 100 MHz can be excited. The SAW frequency omega depends upon a characteristic time t*, defined as a propagation time of the SAW across the laser beam spot. The SAW frequency omega is identical to the frequency difference omega(a) when the laser pulse width T is longer than the characteristic time t*. But, the SAW frequency omega is determined as a product k(f)upsilon(R) of the wave number of the SIF and the SAW velocity when the laser pulse width is shorter than the characteristic time. Precise frequency measurement provided by the amplitude enhancement effect and the narrow frequency bandwidth in the SIF approach make the PVS method particularly promising for the noncontact SAW velocity measurement.

Hideo Nishino

Papers

Modal Analysis of Hollow Cylindrical Guided Waves and Applications

Excitation of high frequency surface acoustic waves by phase velocity scanning of a laser interference fringe

Propagation phenomena of wideband guided waves in a bended pipe.

Analysis of excitation and coherent amplitude enhancement of surface acoustic waves by the phase velocity scanning method

Wavelet transform—Applications to AE signal analysis