Ultrasonic testing

Ultrasound testing is a non-destructive test method that can be used for detecting defects and irregularities in the material. This method uses acoustic waves with a frequency above the human audible range (> 16 kHz). These are called ultrasonic waves. There are various way of creating these ultrasonic waves.

Piezoelectric effect

Ultrasonic waves are most commonly created by using the piezoelectric effect. The properties of some materials are used so that they change their geometric dimensions when creating electric current.

Ultrasonic waves can be created by periodically exciting so-called piezoelectric material with ac voltage.

The so-called piezo element is the core of an ultrasonic head. During ultrasound testing, the ultrasound created in the test head has to be transferred to the test material. As air is unsuitable for this purpose, a coupling agent such as water, oil, or gel, as used in the medical sector, is used.

Another method for creating ultrasonic waves is the EMAT method, where ultrasound is created electro-dynamically directly in the test material, which means no coupling agent is required. The EMAT method is described in more detail at the bottom of this page.

Impulse-echo method

the impulse-echo method is particularly important for ultrasound testing. A short ultrasonic impulse is created and coupled into the test material, after which the test system waits for the echo if this impulse. The echo can be received with the same piezo element which has been used for creating the ultrasonic impulse. So-called SE test heads, which contain two piezo elements – receiver and sender – in one housing, can also be used.

When sounding in a vertical position, echoes of the ultrasonic waves are created by reflecting the waves on the back wall of a plate, tube, or rail. The so-called A-image shows the amplitude of the ultrasonic wave recorded by the receiver in a timeline. In the case described above, this is a send impulse and sequence of receding back wall echoes. This effect is created by the ultrasonic impulse ping-ponging between the back wall and surface of the plate or tube, thus being dampened in the process.

The position of the back wall echo can be used for determining the wall thickness of the tube or the thickness of the plate. Any defects contained in the material, such as lamination, enclosures, or precipitation, can result in an additional reflection of the ultrasound. The A-image then shows an additional echo which can be used for detecting the defects.

EMAT operating principle

1. The eddy currents are initially created with an excitation coil near a
suitable magnetic field.
2. Lorentz
forces are created in the process.
3. Periodic eddy currents create periodic
Lorentz forces.
4. The periodic Lorentz forces create
acoustic waves with the same frequency
(ultrasound).
 
5. The acoustic waves are reflected on the
back wall.
6. The back wall echoes induce a signal in the
receiver coil. The duration of the
back wall echo can be used for determining the wall thickness.