Materials testing: how does ultrasonic testing work?

Ultrasonic testing uses sound to analyse distances, internal structures and the properties of materials and components. This may sound abstract at first, but we often encounter the principle in everyday life.

Anyone who has tapped a wall to find thin spots, dropped a stone into a shaft to estimate its depth, or used the time between lightning and thunder to work out how far away a storm is, is essentially using the same principle: sound and sound echoes.

What is sound?

All substances consist of atoms or molecules packed more or less closely together – more densely in solids than in liquids or gases. When these particles are set in motion, they transfer their momentum to neighbouring particles. Billiard balls provide a good illustration: when they collide, momentum is transferred.

In a material, a density fluctuation propagates in this way. The individual particles move only slightly around their equilibrium position and then oscillate back. The energy of the impulse, however, travels through the medium. A sound wave – a periodic density fluctuation in the material – has been created.

Echoes: when sound meets an obstacle

Sooner or later, a sound wave encounters an obstacle, such as the surface of a workpiece or a material defect. It is reflected there and returns as an echo, much like a billiard ball rebounding from the cushion.

This echo is detected by a receiver. In the human body, this is the eardrum; in ultrasonic testing, it is the sensor in the probe.

If the sound velocity in the material is known, the distance travelled can be determined from the signal’s transit time. Wall or layer thicknesses, for example, can be measured in this way. If the received signal differs from what a defect-free component would produce, this may indicate a material defect.

Why ultrasonic signals become weaker

Let us return to the billiard ball: as it rolls across the table, it slows down and eventually stops. The causes are energy loss at the cushion and friction from the table cloth and the air.

A sound wave behaves similarly in a material. It loses energy when reflected at walls and obstacles. Sound is also scattered within the material, for example by the microstructure, and partly absorbed – taken up by the material. In addition, sound spreads out spatially, distributing the available energy over increasingly large areas. The signal therefore weakens with increasing distance until it can no longer be detected.

Together, all these effects determine a system’s acoustic resistance, or acoustic impedance. In ultrasonic testing, high sound attenuation causes signals to become progressively smaller over long sound paths, making evaluation more demanding.

Ultrasonic testing technology

Ultrasound operates at frequencies outside the range of human hearing. Whereas people can hear vibrations between approximately 16 Hz and 16 kHz, typical ultrasonic testing frequencies range from 2 to 4 MHz – two to four million vibrations per second.

In the human body, transmitting sound through the vocal cords and receiving it through the eardrum are separate tasks: the vocal cords generate sound, while the eardrum detects it. Ultrasonic testing likewise uses probes with separate transmitting and receiving elements, as well as probes that perform both functions.

The central component is a piezoelectric crystal. When an alternating electrical voltage is applied, it starts vibrating at high frequency and generates sound waves. Coupling it with a couplant, such as water, gel or oil, enables sound to pass seamlessly from the probe into the workpiece.

When the echo reaches the crystal again, it causes the crystal to vibrate once more. This generates an electrical signal, which the instrument detects and displays as a measurement signal. The stronger the echo, the stronger the electrical signal.

The ultrasonic probe

The sensor used in ultrasonic testing is the probe. The sensor used in ultrasonic testing is the probe. Different designs, frequencies and sizes are used depending on the application. The simplest case is normal incidence using a straight-beam probe (SPK in German). In practice, however, component geometry does not always allow this testing direction.

Miniature probes with a transducer diameter of around 10 mm are often used for small components. High testing frequencies are generally chosen for very precise measurements, as they are associated with shorter wavelengths and therefore higher resolution.

Dual-element probes, with separate transmitting and receiving elements, are often used for very thin components or near-surface testing. Angle-beam probes (WPK in German) are used when sound needs to enter at an oblique angle, for example when testing welds.

Different wave types

Angle-beam probes usually work with a different type of wave from straight-beam or dual-element probes. The latter mainly use longitudinal waves, whereas angle-beam probes mainly use transverse waves. These are different ways in which sound waves propagate through a medium.

• Longitudinal waves propagate in the direction of particle vibration, occur in all media and are faster than transverse waves in the same material.
• In transverse waves, the particles vibrate perpendicular to the direction in which the sound wave propagates. These waves occur only in solids and are slower than longitudinal waves in the same material.

An illustrative comparison: longitudinal waves resemble people swaying together, while transverse waves resemble a Mexican wave in a stadium.

Examples of typical sound velocities are:

• Air: approximately 330 m/s (longitudinal waves)
• Water: approximately 1,480 m/s (longitudinal waves)
• Forged steel: approximately 5,920 m/s (longitudinal waves)
• Forged steel: approximately 3,255 m/s (transverse waves)

What do ultrasonic signals look like on the instrument?

An ultrasonic testing instrument displays signals in a similar way to an oscilloscope. The position of a signal can be used to determine the distance to a reflector, such as a wall or a material defect.

From the shape and height of the signal, an experienced inspector can often draw conclusions about the type of reflector and distinguish sidewall lack of fusion (an unfused weld interface) from a pore (a gas inclusion), a shrinkage cavity (a void caused by the cooling of castings) or dross (deposits in a casting that rise towards the surface).

Although conventional ultrasound cannot determine the shape and size of a defect with absolute precision, distances can often be measured to within a few tenths of a millimetre in practice.

What is evaluated with ultrasonic testing?

As with all non-destructive testing (NDT) methods, ultrasonic testing uses references for evaluation: an indication is assessed against known calibration values. For wall thickness measurements, the instrument is first calibrated on specimens of known thickness. Weld testing often uses reference blocks whose geometry and reference defects resemble those of the test object.

Two methods are frequently used to evaluate indications:

1. DAC (Distance Amplitude Correction): a reference curve is recorded showing the echo height of a reference defect at different sound paths, for example 20 mm, 40 mm, 60 mm, 80 mm and 100 mm. The test object is then examined, and the signals found are compared with the reference echoes, taking account of the depth (distance) of each reflector.
2. DGS (Distance-Gain-Size, known as AVG in German): in this method, the reference curve is calculated from physical models that describe how signals change with distance and size. Modern instruments perform most of this calculation automatically.

DGS is often used with standard probes, steel materials and simple geometries, whereas DAC is more universally applicable.

Ultrasonic testing: conclusion

Ultrasonic testing is an extremely versatile method of non-destructive materials testing. Among other things, it enables cracks to be detected, wall and layer thicknesses to be measured, and welds to be analysed without damaging the component.

A major advantage is its flexibility: an ultrasonic instrument and a selection of probes can perform numerous testing tasks. Modern methods such as phased array ultrasonic testing or TOFD (Time of Flight Diffraction) further extend its capabilities and improve both the accuracy and the information provided by the test.

W.S. Werkstoff Service – expertise in NDT

If you would like to expand your knowledge of ultrasonic testing or gain qualifications in non-destructive testing methods, the team at W.S. TrainingCenter will be happy to help with practical continuing education and training. Further information is available on our course and seminar pages.