Lamb wave

Interdigital Lamb Wave Transdusers for Rapid Monitoring of Plate Structures. 

PVDF film is readily commercially available in thicknesses up to 110 pm. If film of this thickness is bonded to a rigid substrate, the peak response is at a frequency of about 3.7 MHz which corresponds to the thickness of the layer being a quarter wavelength. This frequency is rather high for most Lamb wave testing since Fig 1 shows that even on a I mm thick... 

R. S.C. Monkhouse, P.D. Wilcox and P. Cawley, Flexible Interdigital PVDF transducer for the generation of lamb waves in structures. Ultrasonics (in press). 

Based upon a piezoelectric 1-3-composite material, air-bome ultrasonic probes for frequencies up to 2 MHz were developped. These probes are characterized by a bandwidth larger than 50 % as well as a signal-to-noise ratio higher than 100 dB. Applications are the thickness measurement of thin powder layers, the inspection of sandwich structures, the detection of surface near cracks in metals or ceramics by generation/reception of Rayleigh waves and the inspection of plates by Lamb waves. 

Generation and Detection of Lamb Waves for Plate Inspection... 

The use of Lamb waves offers the possibility of rapid long-range in-service inspection. Receiver and transmitter probes are positioned single sided - access is only required from one side of the specimen - in a pitch-catch-arrangement, the receiver being outside tbe field of the specular reflection. 

Castaings, M. and Cawley, P. The generation, propagation, and detection of Lamb waves in plates using air-coupled ultrasonic transducers... 

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Leak repair, fired heater, 10 162 Leaks and spills, secondary containment of, 24 311. See also Spills and leaks Leaky Lamb waves, propagating,... 

The difference between a specimen that is effectively a semi-infinite halfspace and one that has a boundary at a finite depth is illustrated in the V(z) curves of two glass specimens in Fig. 10.7. The dashed curve is V(z) for a microscope slide 2 mm thick. The solid curve is V(z) for a glass cover-slip 0.11 mm thick. In a thin plate such as a cover-slip Lamb waves can be excited (Auld 1973). At low frequencies the lowest-order symmetric mode is like a longitudinal wave, except that, because the plate is thin, the modulus governing the velocity is cn — c2n/cn (Table 6.1). If the frequency is increased until... 

Special conical lenses have been developed for Lamb wave imaging of layered structures because the waves are dispersive the frequency can be varied to tune the Lamb wave angle of the specimen to the angle for which the lens is designed (Atalar and Kbymen 1989). Such lenses could similarly be used for Sezawa wave imaging and measurement, and also for other dispersive waves. It may be that for layered structures measurements of V(f) at fixed defocus will become as prevalent as V(z) (Atalar et al. 1995). 

Atalar, A. and Koymen, H. (1989). A high efficiency Lamb wave lens for subsurface imaging. IEEE 1989 Ultrasonics Symposium, pp. 813-16. IEEE, New York. [56,218]... 

De Billy, M., Casakany, A., Adler, L., and Quentin, G. (1983). Excitation of backward ultrasonic leaky Rayleigh and Leaky Lamb waves. IEEE 1983 Ultrasonics Symposium, pp. 1112-15. IEEE, New York. [116]... 

In the devices discussed above, the acoustic wave propagates in a slab of piezoelectric material whose thickness is infinitely larger than the wavelength of the propagating wave (Table 4.2). When the thickness of the plate is reduced so that it becomes comparable to A, the whole plate becomes involved in the periodic motion and a symmetric and antisymmetric Lamb wave is created (White et al., 1987 Fig. 4.21). 

Fig. 4.21 Bulk Lamb waves in Acoustic plate-mode (Apm) and Flextural Plate Wave (FPW) oscillators...

Flextural Plate Wave (FWP) resonators belong to this family of devices. They are fabricated from a few microns thick rectangular membranes of Si3 N4 or ZnO that can be (but do not have to be) piezoelectric. The Lamb waves are excited piezoelec -trically (e.g., ZnO), either electrostatically or electromagnetically. The fundamental frequency is given by... 

In another approach, ultrasonic acoustic flexural plate waves (or Lamb waves) were used to generate fluid mixing motion of microspheres in water in a Si device. A 10-pm-thick piezoelectric ZnO film was deposited on the back side of a Si wafer (see Figure 3.48). This thickness is an odd-multiple of the acoustic half-... 

One finds in the simplest case of an elastically isotropic plate that there is an infinite set of waves that can exist. These are known as Lamb waves, after Sir Horace Lamb who published their first detailed description [60]. 

The characteristics of Lamb waves have been derived and explored by a number of authors [61,62,63,64]. Here we will focus on the aspects of the waves that are most relevant for sensing and actuating in devices made by micromachining techniques. These techniques include many of those customarily employed in making silicon integrated circuits. Examples are processes for depositing thin... 

Figure 339 Symmetric (S) and anti-symmetric (A) Lamb wave characteristics. Vertical axis Lamb-wave velocity normalized to transverse bulk-wave velocity. Horizontal axis Product k4 where kt = transverse wavenumber = 27r/At, where At is transverse wavelength and d is plate thickness. (Reprimed with permission. See Ref. [61]. 1967 Plenum Press.)...

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