Ultrasonic resonance methods

Ultrasonic properties Ultrasonic resonance method, Fokker Bond Tester A—>C 50,51... 

Delaminations reduce the time required for the reflected signal to arrive at the surface in pulse-echo setups (also shown in Fig. 2.6) which have both the transmitting and detecting transducers on the same side of the test sample. The same method can distinguish between flaws at different depths. A slightly different ultrasonic resonance method utilizes the fact that delaminations reduce the sample stiffness and hence produce a resonant frequency shift. 

Resonator ultrasonic relaxation method, 32 18 Respiratory chains, 45 351-354 aerobic growth, 45 354-357 anaerobic growth, 45 357-359 autotrophic growth, 45 359-362 complexes, proteins, 38 240-241 membrane-bound Fe—S enzymes, 38 302-303... 

One of the oldest and best known ultrasonic testing systems for NDT is the Fokker Bond Tester. This method uses a sweep frequency resonance method of ultrasonic inspection. Some degree of quantitative analysis is claimed with the Fokker Bond Tester in the aircraft industry. 

Chemical relaxation methods are widely used to study the kinetics of fast chemical reactions in solution. In particular, ultrasonic absorption techniques have been used to investigate fast exchange processes with relaxation times in the range 0.3 xs to 0.3 ns. The longer end of this time range has become accessible in recent years through the use of cylindrical resonator methods [1] which have lowered the frequency range covered by fictional pulse echo methods. 

Ultrasonic absorption measurements were carried out over the frequency range from 0 2 to 65 MHz. Three different techniques were used depending on the frequency ranges from 0 3 to 1.7 MHz, we used the plano-concave resonator method coupled with the optical detection techniques, from 2 to 7 MHz range we used the cylindrical resonator method and from 15 MHz to 65 MHz we used the pulse method. The details of the experimental system are described elsewhere. ... 

The indirect methods include ultrasonic Doppler, magnetic resonance imaging (MRI), and thermal anemometry, which measure the shear stress indirectly by correlating this parameter to flow rate. The ultrasonic Doppler method and MRI are noninvasive however, the spatial resolution near the wall is not as precise. Another indirect method is to correlate shear stress with convective cooling of a heated element based on the heat transfer principle. The advent of MEMS technology has elevated the significance of microfluidics in biomedical applications. 

Another variation of the normal beam ultrasonic technique is illustrated in Fig. 6 for a so-called resonance method. In this case, continuous wave ultrasonic energy travels from the ultrasonic transducer through the entire adhesive bond structure. When encountering a delamination, notice that the wave resonance set up is of a different wavelength than one where ultrasonic energy can nicely traverse the entire adhesive bond structure. There are many resonance techniques and instrumentation possibilities on the market that work very well for cohesive bonding problems and in some cases are even acceptable for adhesive defect situations. Standing waves are established and the frequency observed is equal to the wave velocity over twice the thickness, in this case either twice d or twice D. 

Figure 2.6 Cross sections of the cells used for ultrasonic absorption measurements by the fixed path resonance method (top) and variable path pulse method (bottom).

Before considering particular test methods, it is useful to survey the principles and terms used in dynamic testing. There are basically two classes of dynamic motion, free vibration in which the test piece is set into oscillation and the amplitude allowed to decay due to damping in the system, and forced vibration in which the oscillation is maintained by external means. These are illustrated in Figure 9.1 together with a subdivision of forced vibration in which the test piece is subjected to a series of half-cycles. The two classes could be sub-divided in a number of ways, for example forced vibration machines may operate at resonance or away from resonance. Wave propagation (e.g. ultrasonics) is a form of forced vibration method and rebound resilience is a simple unforced method consisting of one half-cycle. The most common type of free vibration apparatus is the torsion pendulum. 

It is not possible to discuss all the methods available for characterizing foods critically and systematically in a single volume. Methods pertaining to interfaces (food emulsions, foams, and dispersions), fluorescence, ultrasonics, nuclear magnetic resonance, electron spin resonance, Fourier-transform infrared and near infrared spectroscopy, small-angle neutron scattering, dielectrics, microscopy, rheology, sensors, antibodies, flavor and aroma analysis are included. 

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