Lorentz Force vs. Magnetostriction for Generation of Guided Waves

Ultrasound in EMAT guided waves can be generated using two methods:

  • Guided waves with Lorentz force
  • Guided waves with magnetostrictive technique

Guided Waves with Lorentz Force

As explained under the EMAT Technology section, for guided waves with Lorentz force, ultrasonic waves are induced into a test object with two interacting magnetic fields within the EMAT sensor. A relatively high frequency (RF) field generated by electrical coils interacts with a low frequency or static field generated by magnets to generate a Lorentz force in a manner similar to an electric motor.

This disturbance is transferred to the lattice of the material, producing an elastic wave. In a reciprocal process, the interaction of elastic waves in the presence of a magnetic field induces currents in the receiving EMAT coil circuit.

Ultrasonic Sound Generation with Lorentz Force
Figure 2: Ultrasonic Sound Generation with Lorentz Force

Techniques that use Lorentz forces (such as the MRUT-Lamb technique) require the sensors to be in close proximity to the part, but can handle some lift-off caused by coatings or contamination. Although it varies with frequency, MRUT-Lamb techniques can typically handle up to 3mm lift-off.

Guided Waves with Magnetostrictive Technique

Innerspec’s patented guided waves using magnetostriction method to produce ultrasonic waves requires the adherence of a magnetostrictive strip (made of FeCo) on the inspected part. This strip can be pressure-coupled or taped / glued to the inspected part.

With the magnetostrictive sensor, the inspector will swipe the magnetostrictive strip with a permanent magnet (in the sensor’s direction of inspection). This action generates a biased magnetic field on the strip. The interaction between the biased magnetic field and the dynamic field induced by the tangential current flow within the sensor’s coil creates magnetostrictive strains on the strip. In turn, this strain induces ultrasonic waves in the magnetostrictive strip, which is then transferred to the lattice of the inspected material.

Ultrasonic Sound Generation with Magnetostrictive Technique
Figure 3: Ultrasonic Sound Generation with Magnetostrictive Technique

Magnetostrictive techniques (such as the MRUT with Shear Horizontal wave modes technique) require that the strip is applied to the base material. The material can have paint or epoxy as long as it is hard and strongly adhered to the part. Once the strip is properly coated to the pipe, the thickness of coating is not relevant.