How is ultrasound used in non-destructive testing (NDT) of materials?

Sound is a phenomenon which does not cause any permanent change although its transient presence is very noticeable. This is why “sound” (with low energy) is so suited for non-destructive testing i. e. the aim is to obtain exact information on the condition of the specimen being tested. 

For this purpose, a method is required which produces a clear reaction within the specimen but does not change its condition. “Being passive” is, therefore, the most eminent feature of sound which is required for testing materials.

(In the special area of acoustic emission passive sound is used as well whereby the sound waves are generated inside the material by spontaneous changes in the condition of the material.)

The Propagation of Sound Wave

Sound waves are mechanical waves and thus require a medium which functions as a carrier. Each material whether solid, fluid or gaseous can be evaluated through the special effects on the sound waves. 

The entire ultrasonic test is based on how sound waves are influenced when they propagate within the medium being tested. The sound wave undergoes changes which can be measured and according to which the condition of the material can be evaluated. 

Indirect Material Evaluation

The evaluation of the properties of a material can then only follow indirectly. By means of models and empirical correlation-ships one can interpret certain changes in the sonic signal as a change in the structure of the material or the existence of an inclusion etc. 

The evaluation of the quality of a material always depends upon the reliability of the concept regarding the interpretation of the signal.

Principles of interference

The interference to the propagation of sound which is used for evaluating materials is always based on the same principles:

1.Interference due to an Interface

For example, due to the limiting faces of the test specimen or by macroscopic interfaces such as cracks and microscopic interfaces such as grain boundaries.

2.Interference due to absorption 

This is substantially a transformation of energy produced by internal friction. The evaluation in this case is done by the changed signal passing through the medium or by that signal reflected at an interface. These fundamental effects of interference have resulted in different ultrasonic testing methods, and they are:

3.Resonance method

Here use is made of the reflection between two parallel limiting faces of the test specimen (fig. 1).

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Fig.1: Resonance, if the wavelength λ is just 2t (resonance method)

4.Through-transmission method

Which, as in other languages, could have been better referred to as the “shadow method”. In this method the shadowing effect of a material interface (material dis- continuity) is used. Two opposing probes can be used (fig. 2) as can the “mirror-shadow-method” with the probes on one side of the test specimen (fig. 3).

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Fig.2: Shadow method (through transmission)

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Fig.3: Double disturbance of the sound wave before and after reflection on the backwall

5.Echo method

This method uses the signal which is reflected from a discontinuity in the material (fig. 4). Here the transmitter probe can be identical to the receiving probe, separate transmitter and receiver probes can also be used. The most important special group of echo methods and at the same time the most important ultrasonic testing method of all is the pulse- echo method. 

By using ultrasonic pulses not only the size of the reflection indication (echo amplitude) can be evaluated—the echo transit time can be evaluated too. Thus, one obtains data not only of the size of the reflector but also data regarding its position. 

If the position of the reflector is known (backwall) then, by using the transit time, the structure of the material can be evaluated. If the location of the reflector is not known but the proper- ties of the material are (attenuation, sound velocity) then e. g. wall thickness measurements can be carried out.

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Fig.4: Pulse-Echo Method

In Summary

This exploration of sound in non-destructive testing highlights its fundamental suitability due to its transient and low-energy nature, enabling material evaluation without permanent alteration. Leveraging the predictable propagation and interference of sound waves within a medium, ultrasonic testing indirectly reveals crucial information about a material's internal structure and integrity. 

Whether employing the resonance method, the through-transmission (shadow) method, or the versatile echo method, each technique strategically uses the interaction of sound waves with interfaces and material properties to effectively detect flaws, measure dimensions, and assess overall material quality.

Information on the problem of des- ignating a test method is given in:

[4]H. U. Richter: Verfahren der Ultra- schall-Materialprüfung (Ultrasonic testing methods), Materialprüfung 16 (1974) no 10,

308—310