Introduction
EMAT or Electro Magnetic Acoustic Transducer is an Ultrasonic Testing (UT) technique that generates the sound in the part inspected instead of the transducer.
An EMAT induces ultrasonic waves into a test object with two interacting magnetic fields. 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.
For ferromagnetic conductors, magnetostriction produces additional stresses that enhance the signals to much higher levels than could be obtained by the Lorentz force alone. Various types of waves can be generated using different combinations of RF Coils and Magnets.
Because the sound is generated in the part inspected instead of the transducer, EMATs have the following advantages over more conventional piezoelectric transducers:
- Dry inspection. EMATs do not require couplant for transmitting sound, which makes them very well suited for inspection of hot parts, and integrated in automated environments.
- Impervious to surface conditions. EMATs can inspect through coatings and are not affected by pollutants, oxidation or roughness.
- Easier probe deployment. Not having wedges or couplant, Snell's law of refraction does not apply, and the angle of the probe does not affect the direction of propagation. This makes them easier to control and deploy, especially in automated environments.
- Ability to generate unique wave modes. EMATs are the only practical means for generating shear waves with horizontal polarization (SH waves), which do not travel through low-density couplants. The ability to easily produce Guided SH waves at 90' and lamb waves make EMAT ideal for generation of guided waves, used in the inspection of plates, tubes and rounds.
Wave Modes
EMAT is capable of generating all wave modes used in ultrasonic testing, including some modes that are very difficult or impractical with conventional piezoelectric transducers.
The table below provides a summary guide of the type of wave and technique available for different applications.
| Bulk/Guided | Beam Orientation | Wave Mode | Technique | Main Applications |
|---|---|---|---|---|
| Bulk | Normal | Longitudinal | Piezo EMAT |
- Thickness and Velocity Measurements
- Flaw Detection - Properties Measurement |
| Shear Horizontal | EMAT1 | |||
| Angled | Shear Vertical | Piezo EMAT |
- Flaw Detection | |
| Shear Horizontal | EMAT1 | - Flaw Detection, including austenitic materials | ||
| Guided | Surface | Rayleigh | Piezo EMAT2 |
- Flaw Detection (Surface) |
| Volumetric | Lamb | Piezo EMAT2 |
- Flaw (including Corrosion) Detection
- Velocity and Properties Measurements |
|
| Shear Horizontal | EMAT1 | - Flaw (including Corrosion) Detection
- Velocity and Properties Measurements |
||
|
1 Generation restricted to EMAT for practical purposes 2 Especially well suited for generation with EMAT | ||||
Quality Standards
As an ultrasonic technique EMAT can be used to meet ISO, AWS, API, MIL-STD and other international standards. Innerspec Technologies has already deployed systems designed to meet the following:
- API 5CT (ISO11960) and API 5L8 for tube and casing (OCTG)
- EN10160 for plate
- MIL-STD 2154 for ultrasonic inspection of wrought metals
- CGA C-20 for high-pressure cylinder
EMAT is also specifically referenced in the following ASTM guides:
- ASTM E1774-96 Standard Guide for Electromagnetic Acoustic Transducers (EMATs)
- ASTM E1816-96 Standard Practice for Ultrasonic Examinations Using Electromagnetic Acoustic Transducer (EMAT) Technology
- ASTM E1962-98 Standard Test Methods for Ultrasonic Surface Examinations Using Electromagnetic Acoustic Transducer (EMAT) Technology
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