Lamb waves can propagate over long distances which means that they are attractive for the quick, long range inspection of large structures, and they can also be useful for localized inspection, particularly in thin structures. This paper discusses the selection of the appropriate mode and frequency range for different inspection requirements and reviews the possible methods of excitation, response measurement and signal processing. It is usually desirable to transmit a single, non-dispersive mode, and excitation methods to achieve this are discussed. A variety of signal processing techniques from simple time domain to relatively complex two-dimensional Fourier analysis are available. Time domain processing can often be applied satisfactorily in low frequency-thickness regions where only two modes can propagate, but tends to be unreliable above the cut-off frequency of the a 1 mode. As an example of the design of a Lamb wave testing regime, a set of tests on a butt-welded steel plate with simulated weld defects of different depths is described. It is shown that by operating below the a 1 cut-off frequency with judicious selection of testing technique, the presence of defects can be detected reliably from changes in the shape of the received waveform.

Optimization of Lamb wave inspection techniques

The laser generation of ultrasound in solids is reviewed with particular emphasis on the application of this unique ultrasonic source. Three regimes for the generation of ultrasound in solids using lasers in the visible near infrared wavelength region exist: thermoelastic, plasma and constrained surface source regimes. The mechanism for ultrasonic generation in each of these regimes is given. Recent experimental investigations into laser-generated ultrasound are also described, including a description of a different mechanism for the generation of ultrasound in solids using a CO2 laser. Finally, the many applications of laser-generated ultrasound are reviewed. These applications range from nondestructive testing to the determination of candle flame parameters. Possible future applications are also outlined.

https://iopscience.iop.org/article/10.1088/0022-3727/26/3/001/pdf

Laser-generated ultrasound: its properties, mechanisms and multifarious applications

A single sided, air coupled ultrasonic NDT system based on the generation and reception of the a0 Lamb mode is described. 1–3 composite ultrasonic transducers are employed, transmitting and receiving transducers being oriented at the appropriate coincidence angle for the generation and detection of the mode; they are placed close together and scanned over the surface of the plate to produce a ‘C-scan’ image. Tests have been carried out on carbon fiber composite plates with delamination defects, the damaged areas being readily detected. The measurements have also been compared with numerical predictions based on a finite element model, good agreement being obtained.

Single Sided Inspection of Composite Materials Using Air Coupled Ultrasound

A novel laser-based ultrasonic technique for the inspection of thin plates and membranes is presented, in which a modulated continuous-wave laser source is used to excite narrow bandwidth Lamb waves. The dominant feature in the acoustic spectrum is a sharp resonance peak that occurs at the minimum frequency of the first-order symmetric Lamb mode, where the group velocity of the Lamb wave goes to zero while the phase velocity remains finite. Experimental results with the laser source and receiver on epicenter demonstrate that the zero-group velocity resonance generated with a low-power modulated excitation source can be detected using a Michelson interferometer coupled to a lock-in amplifier. This resonance peak is sensitive to the thickness and mechanical properties of plates and may be suitable, for example, for the measurement and mapping of nanoscale thickness variations.

/pdf/laser-based-ultrasonic-generation-and-detection-of-zero-1hxhxxakax.pdf

Laser-based ultrasonic generation and detection of zero-group velocity Lamb waves in thin plates

Lamb waves are guided ultrasonicwaves capable of propagating relatively long distances in thin plates and thin laminated structures, such as airframe skins, storage tanks, and pressure vessels. Their propagation properties in these media depend on the vibrational frequency as well as on the thickness and material properties of the structure. Structural flaws such as disbonds, corrosion, fatigue cracks, and voids represent changes in effective thickness and material properties, and therefore measurement of variations in Lamb wave propagation can be employed to assess the integrity of these structures. Lamb wave measurements can be made for a number of relative transducer positions (projections) and an image of the flawed region can be reconstructed tomographically. This paper presents a new technique in which two contact piezoelectric transducers are independently scanned along parallel lines in a fashion analogous to that commonly used in seismic crosshole tomography. These results are compared to those for parallel projection Lamb wavetomography data collected with an automated contact scanning apparatus. The advantages and drawbacks of these two methods in the development of automated tomographic Lamb wave scanners for quantitative mapping of thickness variation in platelike materials are discussed.

Parallel projection and crosshole lamb wave contact scanning tomography

Laser generation of narrow-band and directed ultrasound

Numerical inversion of the Hankel–Laplace transform has been performed for the case of ultrasonic displacements in an infinite, homogeneous, isotropic plate which is excited thermoelastically by a laser pulse. Values for the elastic moduli and the plate thickness may be extracted when the calculated displacements are compared directly to those obtained experimentally. Previous authors have demonstrated methods for determining the elastic modulus in thick plates; this letter shows that using a different method for the development of the theory allows similar modulus determinations to be made for thin as well as thick plates.

Quantitative theory for laser ultrasonic waves in a thin plate

A passively mode‐locked, flashlamp‐pumped Nd:YAG laser with a cavity length of 11.19 m has been developed to study the noncontact generation of narrow‐band ultrasound. The individual mode‐locked pulses acted as separate sources of ultrasound, producing a train of acoustic pulses with a repetition rate of about 13.4 MHz. The ultrasound was generated in an aluminum sample and remotely detected with a path stabilized Michelson interferometer. The energy in the multiple pulse acoustic signal was confined to a considerably reduced spectral range compared with that in a single pulse.

Generation of narrow‐band ultrasound with a long cavity mode‐locked Nd:YAG laser

Patterned illuminating sources and appropriate time modulation may be used to enhance certain features of laser-generated acoustic waves in time or frequency as a function of direction. Steerable, narrowband, “toneburst” ultrasonic signals have been generated from a single laser pulse which was spatially modulated by transmission through a lenticular array. In addition, narrowband toneburst ultrasonic waves have been generated from a mode-locked laser pulse train providing spectral narrowing of the laser-ultrasonic signal. Both temporal and spatial modulation of the laser pulse improve the signal-to-noise ratio for laser ultrasonics.

Modulated laser array sources for generation of narrowband and directed ultrasound

A novel dual-beam interferometer has been designed and constructed that enables two beams from a He–Ne laser to probe remotely the surface of a material. The separation of the two He–Ne beams is adjustable in the ∼15-to-∼40-mm range with a spatial resolution of 2 μm. Surface-acoustic-wave measurements have been performed with two different probe separations so that the travel time for the surface waves over a known distance can be determined accurately. With the aid of autocorrelation algorithms, the Rayleigh pulse velocity on 7075-T651 aluminum has been measured to be 2888 ± 4 m/s. The current precision of the system is limited mainly by the 10-ns sampling rate of the digital oscilloscope used. Rayleigh pulse interactions with a surface-breaking slot, machined to a nominal depth of 0.5 mm, have also been examined and the depth estimated ultrasonically to be 0.49 ± 0.02 mm. The system may also provide a technique for direct quantitative studies of surface-wave attenuation.

Andrew D. W. McKie

Papers

Laser generation of narrow-band and directed ultrasound

Quantitative theory for laser ultrasonic waves in a thin plate

Generation of narrow‐band ultrasound with a long cavity mode‐locked Nd:YAG laser

Modulated laser array sources for generation of narrowband and directed ultrasound

Dual-beam interferometer for the accurate determination of surface-wave velocity