Ultrasonic couplants are utilized in all contact testing applications to encourage the transmission of ultrasonic sound energy between the transducer and the test piece. Couplants will commonly be respectably viscouse non-toxic fluids, gels, paste or glues. Their utilization is essential since sound energy at the ultrasonic frequencies ordinarily used for nondestructive testing isn’t viably transmitted through the air. Aside from attenuation effects, air shows an extreme acoustic impedance mismatch regarding both transducer wear plates and typical test materials. Indeed, even a very slight air hole between the transducer and the test piece will prevent sound energy transmission and make regular testing impossible.
Various basic substances, for example, water, engine oil, grease, and even some products like hair gel can be utilized as ultrasonic couplants in numerous applications. Notwithstanding, for best outcomes in high-temperature and ordinary occurrence shear wave testing, it is important to utilize extraordinarily figured couplants. For testing in the atomic power plant, couplants whose halogen and sulfur substance are constrained and record are required.
Glycerin (Couplant B2)
Glycerin is a broadly useful couplant. The benefits of utilizing glycerin are that it is highly viscous and has a high acoustic impedance, making it the favoured couplant for rough surfaces and attenuating materials. Glycerin has an acoustic impedance of 2.42 × 10 gm-cm/sec (versus 1.61 for propylene glycol, around 1.5 for engine oil, and 1.48 for water). Glycerin’s acoustic impedance intently coordinates that of plastics, and it is nearer to that of metal than other basic coupling fluids, giving progressively productive sound transmission between the transducer and the test piece. In metal testing, glycerin normally gives a 3 to 6 dB increment in sign quality when contrasted with propylene glycol.
The drawback in using glycerin is that in the event that it is not removed from the test surface, it can advance rusting or corrosion in certain metals by engrossing and holding water from the air. While glycerin is frequently recommended for testing castings because of its great acoustic properties, care must be taken to wash it out after use. Note that this consumption impact does not happen in plastics, fibreglass, or composites, so glycerin might be uninhibitedly used on those materials. The greatest suggested temperature for glycerin is roughly 200 °F (90 °C).
Gel (Couplant D12)
Gel type couplants are used for rough surfaces, for example, sand-cast metals and fibreglass layups, where their high consistency, viscosity and moderately high acoustic impedance will boost sound coupling in a circumstance where the transducer can’t reach the test surface. Gel couplants are likewise typically used in weld part inspection where it is important to slide the transducer over a wide surface area. Here, the merit is just that gel can be quickly spread over a huge area and that it will maintain great coupling as the transducer is filtered along the weld. Gels are likewise helpful in assessing overhead surfaces or vertical dividers since they won’t trickle or run. Most gel couplants can be utilized on decently hot surfaces up to roughly 200 °F (90 °C).
High-Temperature Couplant (Couplant H)
Couplant H is a gel that can be utilized at high temperatures. Its producer expresses that it very well may be utilized at temperatures up to 750 °F (398 °C) under certain open conditions. Similarly, as with any high-temperature couplant, the client is in charge of discovering that it is reasonable for a given application.
Shear Wave Couplant – SWC-2
Normal incident shear wave transducers require couplants of exceptionally high thickness, since fluids, for example, oil or glycerin won’t support shear waves. Regularly, sticky tars have been utilized for typical frequency shear wave coupling. A limited quantity is to be set on the substance of the transducer or the test piece and after that squeezed into a slender layer. Note that for ideal coupling, it is significant that the couplant layer is thin and slim, a little part of the test wavelength. Following testing, buildup might be washed off with warm water. SWC might be utilized at temperatures up to roughly 100 °F (38 °C).