Equipment 3 of the GWT training Series. It covers how the MsS probe generates a guided wave.
Start with the problem it solves.
Steel is magnetostrictive — A changing magnetic field causes mechanical deformation in steel. That has been known for a long time.
The difficulty is that structural steel is a poor magnetostrictive material. Relying on the pipes own property
gives you a weak wave, and for long-range inspection a weak wave is not enough.
The step that made the method practical was to stop relying on the structure.
Bond a strip of strong magnetostrictive material to the pipe, excite the strip, and let the strips deformation pass into the pipe wall. The transducer is now a material chosen for the job rather than whatever the structure happens to be made of. This is the same reasoning as conventional UT: nobody hopes the specimen is piezoelectric. They bring a strong piezoelectric element to it.
Now, here's the point I want to clarify, because many inspectors have been told otherwise: the strip must transfer its deformation into the pipe wall, and that transfer depends on proper mechanical coupling.
The first successful approach used a nickel strip bonded with epoxy. Epoxy provided effective coupling and a narrow dead zone, as described in the original patent. However, this was the starting point, not the only method.
Next came shear couplant — the same class of couplant used for shear wave UT — which transfers the strips deformation without bonding.
Today, most magnetostrictive sensor (MsS) field testing uses dry-coupling tools. No epoxy, no couplant, and no residue left on the pipe.
I still hear the claim that "MsS only works if you glue the strip to the pipe." That's an understandable assumption if your information comes from the original patent. However, the technology has evolved considerably since then. If you've been evaluating guided wave inspection options based on that assumption, it's worth taking another look.
What has remained constant across all three approaches is the underlying physical requirement: the strip's localized deformation must be transferred into the pipe wall to generate shear waves. Epoxy bonding, shear couplant, and dry coupling are three different ways to achieve the same essential function.
The module also covers torsional and longitudinal mode generation, iron-cobalt strip properties and handling, and the standard coil and adapter sizes used in the field.