Lamb waves technique

Rose, J.L. and J.J. Ditri, Pulse-echo and through transmission Lamb wave techniques for adhesive bond inspection. British Journal of NOT, 34(12), 1992. GogUo, L. and M. Rossetto, Ultrasonic testing of adhesive bonds of thin metal sheets. NDT E International, 32 323-331,1999. 

Rose JL, Ditri JJ. Pulse-echo and through transmission Lamb wave techniques for adhesive bond inspection. Br J NDT 1992 34(12) 591—4. 

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... 

Oblique ultrasonic waves sent to a composite at frequencies that excite plate wave modes induce the leaky lamb wave phenomenon. When the leaky Lamb wave is generated, the specular reflection is distorted. When the specular reflection and the leaky Lamb wave interfere, a phase cancellation occurs, and two components are generated with a phase between them. Because each type of defect has a unique response, this technique can be used to determine material eleastic constants and to estimate the volume content of resin as well as porosity content. Detection of transverse cracking and delamination in a 24-layer unidirectional graphite-epoxy laminate has also been reported [140], and oblique incidenee back-scattering techniques give accurate fiber orientation of the first composite layers [15],... 

Thompson et al. described a series of ultrasonic techniques used for in-.situ measurements of elastic constants on thick-walled submersible vessels [149]. The elastic constants can provide information about fabrication errors such as wavy fibers and fiber disbonds. Elastic constant measurements can be performed using Rayleigh or Lamb wave modes, or by using angle beam techniques, It was shown that the effect of the ani.sotropy increases... 

In addition, generation of acoustic waves in microstructures requires various processes and technologies, with compatibility as one of the main limitations in the development of smart sensors. Restrictions of manual handling and fab-ricatimi process limit minimum thickness of the devices to be about 100 microns. Adoption of silicon fabrication opens the possibihty for the integration of the acoustic devices and electronic driving circuitry, leading to the development of smart sensors. This technique involves localization of specific fabrication steps to common areas to minimize the total number of process steps in the fabrication process as well as to maintain SAW and Lamb wave device integrity and quality [26]. 

In the case where the waves have wavelengths shorter than the thickness of the structure, plate or Lamb waves have to be used and their group velocities considered. The 2D method to locate AE sources is usually applied when the accuracy of zonal technique is inadequate for the application. Applications of the 2D method in civil engineering are generally for monitoring large structures, like bridges (Kapphahn et al. 1993). 

Ultrasonic transducers can excite various wave modes, eg, guided Lamb waves that may be suitable for specific apphcations (143). Ultrasonic excitation is also used to detect the so-called acoustic nonlinearity associated with defects, eg, in PMC (144,145). The combination of recording guided ultrasonic waves with numerical back-calculation of wave propagation for detecting indications of defects in PMC pipes also looks promising (146). Surface-bonded fiber optics can detect ultrasonic waves via interferometric techniques (147). Beyond frequencies of 1 GHz, ultrasonics is called acoustic microscopy, for details, the reader is referred to Reference 148. This technique can be applied to polymers or PMC (149), specifically to polymeric microelectronics packaging (150). 

Other ultrasonic techniques which show promise are oblique incidence waves (Pilarski et al, 1990) and Lamb waves (Bork and Challis 1995). Rose and co-workers (Puthillath and Rose 2010 Rose et al. 1996 Rose and Ditri 1992 Rose et al. 1995) have used guided ultrasonic waves to detect defects in adhesively bonded joints. 

Experimental methods of measuring the Lamb shift can be broadly classified into two basic types. The first is the microwave resonance technique originally used by Lamb and Retherford, in which the 2s - 2p transition is observed directly in a microwave cavity. The prime factor limiting the accuracy is the precision with which the resonance line center can be located. Since the width r of the 2pi state is about one tenth of the Lamb shift, the line center must-be located to a precision of lOx ppm relative to F for a precision of x ppm n the Lamb shift. Line narrowing can be achieved by use of Ramsey s separated oscillatory fields technique, but at the expense of a lower signaj-to-noise ratio. Since the Lamb shift increases in proportion to Z, the f quency lies outside the micro-wave region for ions heavier than Li. A tunable laser can then be used in conjunction ith a fast ion beam as further discussed below. 

The Lamb dip is an important manifestation of the saturation of the gain which occurs in inhomogeneously-broadened transitions when the oscillation frequency coincides with the centre of the laser line. A similar phenomenon occurs if the laser frequency is tuned close to the centre of an absorption line of a sample of atoms or molecules interacting with the standing wave field of the laser. This saturated absorption has become an important new technique in atomic and molecular spectroscopy since it removes the limit on the attainable resolution which was formerly imposed by the Doppler broadening of absorption lines. 

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