Ultrasonic surface waves

In wide sectors of industry there is a growing need of inspection methods which go without liquid coupling media. The excitation of bulk and surface waves by means of air-coupled ultrasonic probes is therefore an attractive tool for NDE. This is tme e.g. for the rapid scanning of large composite structures in the aerospace industry [1]. In other cases, the use of liquid couplants is prohibitive like the thickness measurement of powder layers. 

The use of the surface ultrasonic waves seems to be convenient for these purposes. However, this method has not found wide practical application. Peculiarities of excitation, propagation and registration of surface waves created before these time great difficulties for their application in automatic systems of duality testing. It is connected with the fact that the surface waves are weakened by soil on the surface itself In addition, the methods of testing by the surface waves do not yield to automation due to the difficulties of creation of the acoustic contact. In particular, a flow of contact liquid out of the zone of an acoustic line, presence of immersion liquid, availability of chink interval leads to the adsorption and reflection of waves on tlie front meniscus of a contact layer. The liquid for the acoustic contact must be located only in the places of contact, otherwise the influence on the amplitude will be uncontrolled. This phenomenon distorts the results of testing procedure. 

Application of magnetic fluids in ultrasonic non-destructive testing [1-3] opens the real perspectives for automation of the testing methods, based on the surface waves. This report presents the results of investigations aimed at the creation of the transducer of the surface waves for the automated control. The basic attention is drawn to the analysis of the position of the front meniscus of the contact liquid when the surface waves excite through the slot gap. 

The dependencies described are sufficient for designing the different types of ultrasonic transducers for testing by surface waves. The constant transmission of acoustic energy is provided. 

Ultrasonic wave speed, Impact Echo and Spectral Analysis of Surface Waves... 

The common civil engineering seismic testing techniques work on the principles of ultrasonic through transmission (UPV), transient stress wave propagation and reflection (Impact Echo), Ultrasonic Pulse Echo (UPE) and Spectral Analysis of Surface Waves (SASW). 

Hydrogen embrittlement Processing and service Ferrous Magnetic particle method Ultrasonics method may be used by adopting surface wave technique... 

Laser ultrasonic transducers are truly non-contact devices which effectively avoid acoustic coupling problems (e.g. damping in the transducer and couplant reflection and transmission losses at the interface). Most laser ultrasonic devices have been used for excitation and detection of bulk elastic waves in point source or planar geometry, but also surface acoustic (Rayleigh or Brillouin) waves. Unlike the bulk wave regime, only one sample side is needed for excitation and detection when surface waves are used. This not only renders the measurements easier, but also avoids the need for an accurate knowledge and uniformity of the sample thickness. In addition, the excitation laser can be focused using cylindrical lenses in order to obtain an excitation line. 

Laser interferometry employs the principle of optical interference to recover the sought acoustic information from the light reflected from, or scattered by, a surface under ultrasonic vibration. Its non-contact nature makes laser probing a preferred alternative to contact methods in studying surface waves, their diffraction and damping by intrinsically rough surfaces. 

A back-scattered Rayleigh surface wave is the leaky ultrasonic energy returning to a transmitting transducer — along the opposite direction to the incident beam — from the... 

The mechanism of droplet formation from ultrasonic excitation of the liquid depends on the frequency and intensity applied [21]. At an intensity of 10 W/cm2, capillary surface waves are formed from cavitation at frequencies less than 100 kHz. Cavitation can occur at lower-power intensities when dissolved gases are present. Droplets are thrown outward by the rupture of the cavitation bubbles or the wave crests. For such conditions, the droplet diameter can be related to the capillary wavelength [21,22],... 

Other applications that were recently demonstrated with photorefractive polymers include homodyne detection of ultrasonic surface displacements using two-wave mixing [115]. With the development of new photorefractive polymers with response times in the millisecond range, numerous optical processing techniques can be performed at video rates. Image amplification and novelty filtering at video rates were demonstrated recently [116]. All-optical processing techniques compete with computational methods. Therefore, it is important that photorefractive polymers exhibit faster response times in the future. 

Ultrasonic Nebulizers. Ultrasound can be used to break up a liquid mass into smaller particles. In the ultrasonic nebulizer, an ultrasonic generator drives a piezoelectric crystal at a frequency between 200 kHz and 10 MHz. The surface of the liquid sample will breakdown into an aerosol when the longitudinal wave propagates from the surface of the crystal toward the liquid-air interface. The wavelength of the surface wave is given by the following equation ... 

The viscosity coefficients may also be determined by studying the reflexion of ultrasonic shear waves at a solid-nematic interface. The technique was developed by Martinoty and Candau. A thin film of a nematic liquid crystal is taken on the surface of a fused quartz rod with obliquely cut ends (fig. 3.7.1). A quartz crystal bonded to one of the ends generates a transverse wave. At the solid-nematic interface there is a transmitted wave, which is rapidly attenuated, and a reflected wave which is received at the other end by a second quartz crystal. The reflexion coefficient, obtained by measuring the amplitudes of reflexion with and without the nematic sample, directly yields the effective coefficient of viscosity. 

Takano H, Nakazawa M, Asai K, Itoh M. Performances of a new clinical nebulizer for drug administration based on the mesh-type ultrasonic atomization by elastic surface waves. J Aerosol Med 1999 12(2) 98. 

A major technical hurdle anticipated in tlie ultrasonic system development did not materialize when surface wave imaging was successfully applied to inspect cylindrical tensile rods (6-mm in diameter). A novel rotational stage was incorporated in the prototype specimen manipulator. Software for manipulator control, data acquisition, and image reconstruction was developed concurrently. The technical requirements included ... 

Resonetics Inc. of Nashua, NH has successfully machined simulated voids in silicon nitride specimens with an Excimer laser. Two longitudinally ground and lapped tensile specimens have been provided to them with plans for machining 100 each of 10,20, 50, and 100 mm voids. One of these IQIs was provided to Panametrics, Inc. for use in development of their surface wave technique for ultrasonic inspection of cylindrical specimens. 

Figure 6. Ultrasonic Inspection of a Cylindrical Tensile Specimen. Optical image of the tensile rod at low (top) and high (middle) magnification. Surface wave image (bottom) revealed voids. Note, that the optical and ultrasonic images are reversed.

For interference to be avoided, the duration of the tone burst must be less than the echo transit time. This means that flaws within a distance equal to one half of the tone burst duration of the surface cannot be resolved. In some applications (in torsion and flexure) the surface stresses are greater than the interior stresses, so surface flaws are of interest. Near-surface defects can be detected using a surface ultrasonic wave. One geometry for the generation of surface ultrasonic waves is shown in Figure 13.1. Surface waves reflect from surface-exposed defects in exactly... 

Figure 13.1 Schematic of ultrasonic setup for the generation and detection of surface waves. (From Roberts et al. )... 

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