Ultrasonic reflectance amplitude

Fig. 3. Ultrasonic reflectance amplitude from the interface between a piece of Plexiglas and sample of confectionary coating fat during cooling. As the sample crystallized, it became more acoustically similar to the Plexiglas and less sound was reflected.

As an ultrasonic wave encounters an interface between two materials, a portion of the energy is transmitted and a portion is reflected. The governing relationship for this reflection factor phenomenon for normal incidence is illustrated in Figure 10, where represents the incident ultrasonic pressure amplitude and corresponding reflected and transmit-... 

Several methods of displaying ultrasonic reflections are available, the most common being A and C-Scans. The simplest presentation is an A-Scan which shows the amplitude of the signal as a function of time (or distance, if a value for the velocity of sound in the medium is known), as shown in O Figs. 42.14 and O 42.15. In manual testing, an A-scan is obtained at each test point and is interpreted by the operator. 

Some of the problems often encountered during ultrasonic inspection of plane specimens are also found on cylindrical specimens. For example, problems associated with the directional characteristic of the ultrasonic transducer. Furthermore, the discontinuity influences the shape and propagation direction of a reflected pulse, causing wave mode transformation. In addition, the specimen influences the shape and amplitude of the reflected pulse by sound absorption. 

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. 

The impedance is practically important because it determines the proportion of an ultrasonic wave which is reflected from a boundary between materials. When a plane ultrasonic wave is incident on a plane interface between two materials of different acoustic impedance it is partly reflected and partly transmitted (Figure 3). The ratios of the amplitudes of the transmitted (At) and reflected (Ar) waves to that of the incident wave (Aj) are called the transmission (T) and reflection coefficients (R), respectively. 

A typical experimental configuration consists of a measurement cell which contains the sample, a pulse generator, an ultrasonic transducer and an oscilloscope (Figure 4). The pulse generator produces an electrical pulse of an appropriate frequency and amplitude. This pulse is converted into an ultrasonic pulse by the transducer. It then propagates through the sample until it reaches the far wall of the cell where it is reflected back to the transducer. The... 

Each echo has traveled a distance twice the cell length d further than the previous echo and so the velocity can be calculated by measuring the time delay t between successive echoes c = 2d/t. The cell length is determined accurately by calibration with a material of known ultrasonic velocity, e.g. distilled water 2d = cw.tw (where the subscripts refer to water). The attenuation coefficient is determined by measuring the amplitudes of successive echoes A = A0e-2cxd, and comparing them to the values determined for a calibration material. A number of sources of errors have to be taken into account if accurate measurements are to be made, e.g., diffraction and reflection (see below). 

The signal generator applies a continuous sine wave of suitable frequency and amplitude to the transducer. The transducer generates an ultrasonic sine wave which propagates into the sample and is reflected back and forth between the reflector plate and transducer. Standing waves are set up in the sample, and the amplitude of the signal received by the transducer... 

The US resonance phenomenon for the case of a plane parallel-type resonator consists of exoitation by one of the piezotransducers of the ultrasonic wave traveiiing in the direction of the second piezotransducer. Upon reflection by the second piezotransducer, the wave comes back and is reflected again. At the frequencies corresponding to a whole number of half wavelength between the piezotransducers, resonance occurs and increases the amplitude of the signal at the second piezotransducer. 

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. 

Since ultrasonic attenuation of samples is defined as the inverse of H(tn) defined by Eq. (5), frequency dependence of attenuation can be examined by comparing two power spectra obtained with and without the samples (Eq. (14)). However, there is a problem with reflection at the surface of a sample, and so, in the case of a solid sample, frequency dependence of attenuation is examined by comparing two power spectra obtained from samples of different thicknesses to overcome this problem. The signal amplitude A at frequency f after transmitting through the sample is expressed by Eq. (16) for a sample of tj thickness and by Eq. (17) for one of thickness [11] ... 

The importance of adequate calibration is paramount in any ultrasonic inspection and is generally caurried out both to monitor equipment stability and to enable defect echo amplitudes to be referred to those from known standard reflectors. Figure 6 depicts the calibration block specifically designed for this work. One surface was machined concave with the same radius of curvature.as the cone at the position of the outer weld. Two 3 mm diameter flat bottomed holes (FBH) and a 1.5 mm diameter side drilled hole (SDH) were provided at a depth equivalent to the cone plate thickness and spaced sufficiently far apart that reflections could be obtained from each one independently of the others. A second 1.5 mm SDH at 15 mm depth served two purposes, firstly as euti equivalent reflector to the SDH in the standard A2 block and secondly to provide a means, in conjunction with the other SDH, of checking the probe angle. One section of the block, V thick, simulated the cone plate itself and was used for recording backwall echo amplitudes for the focused and normal probes. 

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