Unlocking Shear Wave Inspections: Choosing the Right Couplant For the Job

High-Temperature Ultrasonic Testing

You’ve calibrated your probe, dialed in the perfect frequency, and set up signal processing yet your readings are faint or nonexistent. The culprit? It might not be your equipment at all—it could be your couplant.

For ultrasonic testing (UT) in non-destructive testing (NDT), focus is usually on the probe, frequency, and signal processing. However, there is one other vital but often overlooked factor that can make or break the quality of your inspection—the couplant.

This is especially true for normal incidence shear wave inspections, where the rules of wave transmission are different than in standard shear wave inspections. Here, we will explore the key differences, identify the challenges, and recommend the couplant that is best suited for this special application.

What’s Different about Shear Waves?

Inspection becomes more challenging when dealing with shear waves at normal incidence. Unlike angle beam inspections, which generate shear waves through refraction, normal incidence shear wave inspections use shear wave transducers placed perpendicular to the surface, generating pure shear waves directly into the test piece.

These waves travel through the material and can be used to detect flaws such as cracks, delaminations, and other discontinuities that are oriented parallel to the inspection surface.

The problem? Unlike longitudinal waves, shear waves cannot travel through liquids. This means, your usual go-to couplants won’t transmit shear waves effectively at normal incidence. You’ll lose signal, or worse, never generate one at all.