What are the physical processes of ultrasound in non-destructive testing?

Sound is a fascinating physical phenomenon that surrounds us constantly in everyday life, even if we are often unaware of its scientific nature. It is nothing more than mechanical vibrations that propagate through a medium—be it solid, liquid, or gas—and are perceived by our ears within a certain frequency range. 

But what happens when these vibrations exceed the range of human perception? In such cases, we enter the world of ultrasound. The same physical principles apply, but their implications and applications take on entirely new dimensions—particularly in the field of non-destructive material testing. To better understand how ultrasonic waves function in this context, it is important first to grasp the fundamental properties and behaviour of sound itself. 

With that basis established, we can then explore how ultrasonic waves differ, how they propagate through various media, and how they are utilized to detect internal flaws that would otherwise remain hidden to the human senses.

Basic Properties of Sound and Ultrasound

So far, we have only talked about the basic properties of sound, including those of audible sound. With ultrasound, the same physical processes take place as with audible sound, but the vibrations occur so quickly that they are no longer perceived by the human ear. Whereas the audibility limit is approximately at 20 kHz, ultrasonic testing uses frequencies between app. 0.5 MHz and 20 MHz (1 MHz = 1 000 000 oscillations per second). This property of ultrasound, i. e. that it can be neither seen, heard nor recognized in any other way by humans admittedly doesn’t simplify the understanding of its effects but it does have the advantage that, when testing, the sound intensity can be selected of such a high level that it would be unbearable if it was in the audible range.

Nature of Ultrasonic Oscillations

It’s now time to describe ultrasonic oscillations in greater detail. Any medium whether solid, fluid or gaseous can be made to oscillate. If the oscillations occur so rapidly that they lie above the human audibility limit they are referred to as being ultrasonic. It is actually the small particles of the medium (imagined as being elastically interconnected) which oscillate. The type of oscillation is determined by the elastic properties of the medium and by the pulse which causes the medium to oscillate. The sound wave propagates in the medium at a sound velocity c.

One can describe the sound oscillation as a movement of the particles in the material—for example with the particle deviation = f (z, t) as function of location and time or as a change of the sound pressure

p = f (z, t) with location and time. When, referring to sound pressure in materials testing, we mean the sound pressure which alternates with the oscillations. The sound pressure p is so important because the electric potential generated from a piezo-electric plate U = f (z, t) is directly proportional to the sound pressure of the impinging sound wave and vice versa.

Longitudinal and Transverse Wave Types

Fig. 14 shows the sound propagation in fluid, gases and solids. The particles oscillate in the direction of propagation of the wave. Such oscillations are called longitudinal or pressure waves. Closely packed particles mean smaller deviationthus high particle velocity and high sound pressure. If shear forces can be transferred in media (which is mostly the case in solids), the particles can also vibrate transverse to the direction of propagation of the wave and this is why they areknown as transverse or shear waves (fig. 15).

Wave Behaviour in Finite Media

But however, such ideal cases can only exist in infinite media. If, during a test, reflection occurs at the outer boundaries of the test specimen e.g. in plates, rods etc. complicated mixed wave formations develop plate waves, rod waves (fig. 16) and surface waves. The equations applicable to pure longitudinal and transverse waves do not apply to them. All these types of waves can be present at the same time in the test specimen, and they make the Interpretation of indications much more difficult.

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Fig. 14: Longitudinal wave (pressure wave)

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Fig. 15: Transverse wave (shear wave)

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Fig. 16a: Plate wave, unsymmetric

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Fig. 16b: Plate wave, symmetric

Important Physical Parameters in Ultrasonic Testing

The most important parameters of the physics of ultrasound for materials are:

The following relationships exist between the different parameters:

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The sound pressure then is proportional to the acoustic impedance Z, the frequency f and the particle deviation ξ.

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The acoustic power density J (intensity) is directly proportional to the acoustic impedance Z and the square of the frequency f and particle deviation ξ. The following relationships exist between the sound velocity c and the elastic properties:

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In summary

The transition from audible sound to ultrasound not only expands the frequency range beyond that detectable by humans but also enables completely new applications that are impossible with ordinary sound. 

Ultrasonic testing, based on the same universal physical laws, enables high resolution internal inspection of solid materials without damage or destruction.

While the intangible and invisible nature of ultrasound renders its study initially more theoretical, it is precisely this characteristic that enables intense sound energy to be harnessed in the industrial setting. 

Understanding the various modes of propagation of ultrasonic waves, their dependence on materials, and how mathematically they relate will allow one to interpret signals correctly and draw valid conclusions. In practice, it is typically the interplay between theory, measurement technique, and experience which is behind the success of ultrasonic techniques in material integrity.

Information on the basics of acoustics is given in:

[7]L. Bergmann: Der Ultraschall (Ultrasound), Hirzel-Verlag, Stuttgart (1954)