EMAT 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.

1 Generation restricted to EMAT for practical purposes
2 Especially well-suited for generation with EMAT

Normal (zero degree) Beam

Characteristics

• Direction of Propagation: Perpendicular to the entry wall.
• Sensor Configuration: Pulse-echo (transmitter = receiver) or pitch-catch (transmitter ≠ receiver).
• Wave modes: Shear Horizontal and Longitudinal waves in frequencies ranging from 500KHz to 10MHz. Whereas EMAT can generate both Shear and Longitudinal waves at 0 degrees, Shear (Horizontal) waves are easier to generate.
• Materials Inspected: Ferromagnetic and non-ferromagnetic metals.

Applications

• Thickness Measurement
• Corrosion and erosion measurement.
• Flaw detection, such as, inclusions, de-laminations and disbond.
• Acoustic velocity measurement.
• Rolling direction recognition.
• Anisotropy and stress measurement.
• Nodularity measurement.
• Bolt-Load measurement.

Electro-Magnetic Acoustic Transducer (EMAT) Uniqueness

• Dry and non-contact. Practical working distance from the coil to the part (lift-off) is usually between 0-3mm. Greater lift-off can be achieved (up to 10mm in laboratory settings), depending on material, equipment and type of inspection. Ideal for automated and hot environments.
• Not affected by surface conditions (coatings, oil, oxide).
• Maintains readings even when the probe face is not parallel to the part. The only restriction in coil/sensor angle is derived from the loss of signal due to lift-off, so depending on the application, the coil/sensor can be angled as much as 30º from the part and still obtain good signals.
• Capable of generating Shear wave energy (Shear Horizontal). Shear waves have approximately half the velocity of Longitudinal waves providing better time resolution (especially important for defects next to walls). Shear waves are also is capable of detecting defects perfectly perpendicular to the direction of sound, and attenuate less than Longitudinal waves.
• Ability to select the direction of polarization when using Racetrack or Butterfly style coils (see RF Coil section).
• Because EMAT by definition cannot use a delay line (or water column), there is a dead zone of approximately 4µs (equivalent to around 6mm of material).
• This dead zone can be circumvented when parallel walls are present by relying on the 2nd bounce from the wall to perform the inspection.

Angled Beam (including Phased Array)

Characteristics

• Direction of Propagation: At an angle from the entry wall.
• Sensor Configuration: Pulse-echo or pitch-catch, including Phased Array.
• Wave modes: Shear Horizontal and Shear Vertical at angles from 10º to 80º in frequencies ranging from 500KHz to 10MHz.
• Materials Inspected: Ferromagnetic and non-ferromagnetic metals.

Applications

• Flaw detection.
• Corrosion and erosion measurement.
• Detection of hydrogen damage and pitting.
• Austenitic weld inspection in heavy walls (>0.5” or 13mm).
• Inspection of welds while welding (e.g. submerged arc welding).
• Volumetric flaw detection.

Electro-Magnetic Acoustic Transducer (EMAT) Uniqueness

• Dry and non-contact (up to 2.5mm lift-off depending on application and frequency). Ideal for automated and hot environments.
• Not affected by surface conditions (coatings, oil, oxide). Capable of inspecting on severely pitted surfaces.
• While Angled Beam Shear Vertical energy is easy to generate using refracting angles on piezoelectric transducers (PZT), Shear Horizontal Angled Beams do not travel through high-density couplants so they are difficult to generate and excluded from applications that require scanning of the probe.
• The polarity of the energy (vertical Vs horizontal) is important since shear waves do not mode convert when striking surfaces that are parallel to the direction of polarization thus Shear Horizontal waves are especially well suited for inspection of austenitic welds and other materials with dendritic grain structures.
• Inspection at temperatures of up to 1,382ºF (750ºC).

Guided Waves

Characteristics

• Direction of Propagation: Parallel to the entry wall and within the boundaries of the top and bottom walls. Limited to approximately 0.5” (13mm) plate thickness for internal flaw detection.
• Sensor Configuration: Pulse-echo or pitch-catch.
• Wave modes: Shear Horizontal at 90º, Lamb waves and Rayleigh waves in frequencies ranging from 50KHz to 10MHz.
• Materials Inspected: Ferromagnetic and non-ferromagnetic metals.

Applications

• Weld inspection in thin plates (
• Flaw detection in plates, tubes and rods.
• Corrosion and erosion measurement.
• Material properties characterization (e.g. acoustic velocity measurement).

Electro-Magnetic Acoustic Transducer (EMAT) Uniqueness

• Dry and non-contact (up to 2.5mm lift-off depending on frequency and type of application). Ideal for automated environments.
• Not affected by surface conditions (coatings, oil, oxide).
• Ability to normalize the signal for automatic and continuous self-calibration.
• Less sensitive to probe positioning. Especially well suited for automated weld inspection.
• Ability to concentrate the energy on the outside boundaries or center of the material to be more or less sensitive to surface or internal defects (e.g. to avoid or ignore root and crown in weld inspection).