Attenuation in actual structures the mechanisms

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Attenuation in Actual Structures The mechanisms discussed here may all contribute to the attenuation of acoustic emission signals in real structures. Under normal circumstances, the attenuation must be measured by tests on the actual structure. Figure 43 shows an example of the attenuation measured in a gas pressure vessel. 124 The vessel was cylindrical with hemispherical caps and had a total length of 12.2 m (40 ft) with an inside diameter of 1.2 m (4 ft). The wall thickness was 125 mm (5 in.) in the cylindrical section and 64 mm (2.5 in.) in the caps. The attenuation was measured by placing a mechanical random noise source at one position and an acoustic emission transducer at a second position on the outer surface of the tank. The plot in Fig. 43 shows the relative amplitude of the received acoustic signals at various frequencies as the separation between the noise generator and the receiving transducer was increased. The relative amplitudes are plotted on a logarithmic scale as attenuation in decibels equal to 20 10 ( A · A o –1 ), where A is the measured amplitude at separation distance r and A o is the amplitude at r = 0. Each of the vertical scales have been displaced to clearly show the variations with frequency. The measured data are shown by the circles in the figure; the lines are approximations of linear variations of the data. α η η = + k 1 2 E E j = ( ) 1 η y x t A e j k x t , ( ) = ( ) ω α α α dB = = 20 8 7 10 log . e y Ae e x j kx t = ( ) α ω 91 Fundamentals of Acoustic Emission Testing F IGURE 43. Attenuation of various frequencies as a function of distance traveled in a gas pressure vessel wall. 124 Relative amplitude (10 dB per division) 0 1.5 3.0 4.5 6.0 7.5 (5) (10) (15) (20) (25) Separation distance, m (ft) 100 kHz 150 kHz 200 kHz 300 kHz 400 kHz 540 kHz 600 kHz 700 kHz 850 kHz
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The plots clearly show an increased attenuation of the signal within the first 1.5 m (5 ft) of separation. Much of this is attributed to the geometrical spreading of the waves as they propagate outward from the source. As noted earlier, the amplitudes of the waves in this platelike structure would vary as r –0.5 if geometric attenuation were the only active mechanism. In this case, the amplitude at 1.5 m (5 ft) is 6.9 dB less than the amplitude at 0.3 m (1 ft) and the attenuation at 100 kHz is about 7 dB. Once the wave reaches the separation distance of 1.5 m (5 ft), the structure begins to act as a waveguide, effectively eliminating the geometric attenuation. For separations greater than 1.5 m (5 ft), the attenuation per unit length is reduced but still present. Most of this attenuation is probably caused by energy loss mechanisms with a smaller component caused by dispersion. The effects of frequency can clearly be seen on the attenuation plots of Fig. 43. These frequency effects are also shown in Fig. 44, a plot of the average attenuation per unit length from the straight line approximations in Fig. 43 at separations of 1.5 m (5 ft) and 7.5 m (25 ft). This plot also shows attenuation measurements made along the circumference of the vessel very close to the longitudinal attenuation measurements.
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  • Fall '19
  • Nondestructive testing, Acoustic Emission, Acoustic Emission Testing

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