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Measuring the Attenuation Coefficient of Ultrasonic Beams in Materials

Ultrasonic beams, similar to various types of energy, attenuate when passing through different media. The amount of ultrasonic beam attenuation per unit of length in different isotropic materials simply refers to the attenuation coefficient (α). This quantity is usually expressed in dB/in. (decibels per inch), dB/mm (decibels per millimeter), or even dB/m (decibels per meter). It is an important parameter in performing ultrasonic discontinuity size evaluation procedures in accordance with the AWS D1.1 standard (AWS, 2015). The distance gain size method also refers to this parameter when sizing a reflector inside a material using an ultrasonic beam (Krautkramer, 1990). The loss of ultrasonic energy when passing through materials depends on two main factors: first, the microstructure properties of the material; and second, on the characteristics of the ultrasonic beam involved. With regards to the first factor, the grain sizes, their orientations, the presence of any impurities at the grain boundaries, micro-porosity, and any history of heat treatment on that material can all contribute to the degree of attenuation of the ultrasonic beam. The second important factor is the characteristic of the ultrasonic beam, namely, the frequency and the beam profile (beam divergence) along its propagation path. This paper provides a simple procedure to separate these factors, and a practical way for measuring the attenuation coefficient (α) of the part under test.

References

AWS, AWS D1.1/1.1 M: 2015: Structural welding Code – Steel, American Welding Society, Miami, Florida, 2015. 

Krautkramer, J., and H. Krautkramer, Ultrasonic Testing of Materials, fourth edition, 1990, Springer-Verlag, p. 97. 

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