Couplant
A liquid or pasty medium inserted between the probe and the component to reduce impedance mismatches and enable sound transmission. Without this medium, the ultrasound would be reflected at the air boundary.
Physico-chemical mechanism of action
Couplants reduce the differences in acoustic impedance between the probe and the component. This allows more efficient sound transmission of ultrasound from the transmitter to the receiver, without the sound being reflected at the air boundary.
Metallurgical relevance & risks in welding
Without a couplant, a large portion of the ultrasound would be reflected at the air-component boundary, resulting in a strong loss of sound energy. This can lead to inaccurate or missing detection results when testing for cracks and defects.
Workshop practice & action instructions
Always use a suitable couplant that matches the specific component material. The application specifications can be found in the probe manufacturer's catalog. Make sure the medium is fully applied to the component surface and there are no air bubbles or air layers between the probe and the material.