EMAT Technology

Overview

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.

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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. EMAT does not require couplant for transmitting sound, which makes it very well suited for inspection of very hot and very cold parts, and integration in automated environments.
• Impervious to surface conditions. EMAT can inspect through coatings and are not affected by pollutants, oxidation, or roughness.
• Easier sensor deployment. Not having wedges or couplant, Snell’s law of refraction does not apply, and the angle of the sensor does not affect the direction of propagation. This makes EMAT transducers easier to control and deploy.
• Ability to generate SH modes. EMAT is the only practical means for generating shear waves with horizontal polarization (SH waves) without high mechanical pressure or low-density couplants that impede scanning of the part.
• Mode selectivity. The antenna-type construction of the EMAT coil combined with a multi-cycle excitation provides great specificity in the frequency domain, thus the ability to precisely select the wave mode of interest, which is of great importance for guided wave generation and interpretation.

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 (<0.5” or 13mm).
• 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).

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EMAT Applications

Electro-Magnetic Acoustic Transducer (EMAT) is an ultrasonic Non Destructive Testing (NDT) which works without contact or couplant. The sound is directly generated within the material adjacent to the transducer. This couplant-free feature makes Electro-Magnetic Acoustic Transducer uniqueness for several applications:

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).

EMAT FAQ

What is Ultrasonic Testing (UT)?

Ultrasonic Testing is the use of high-frequency sound waves to detect imperfections, take measurements or to locate changes in material properties of solid objects. Because of its capabilities and reliability, UT is one of the fastest growing Non Destructive Testing techniques. It is the method of choice for critical applications where full volumetric inspection is required, especially when there is only limited access to the object being inspected.

How is ultrasound generated with a piezoelectric transducer?

The piezoelectric crystal creates a mechanical disturbance (ultrasonic wave) when subjected to voltage. This ultrasonic wave is from the transducer into the object to perform the inspection. Because ultrasonic waves do not transmit easily through air due to differences in impedance, the transducer needs to be to the solid being inspected using a liquid between the transducer and the solid being inspected.

How is ultrasound generated with an EMAT?

An EMAT or Electro Magnetic Acoustic Transducer consists of a magnet and a coil of wire and relies on electro-magnetic acoustic interaction for elastic wave generation (ultrasound). Using Lorentz forces and magnetostriction, the EMAT and the metal test surface interact and generate an acoustic wave within the material. The material being inspected is its own transducer and there is no need to couple the EMAT with the material, which is one of the EMAT advantages. For more information see EMAT Technology.

What kind of waves and what frequencies can you generate with EMAT?

EMATs can easily generate any wave mode available with piezoelectric UT (Rayleigh, creeping, shear Vertical, longitudinal) plus others that are very difficult to generate with conventional methods (shear horizontal, Lamb). Also, EMATs can generate shear waves at any angle and sweep the material from 0º to 90º using the same transducer by simply modifying the frequency. In terms of frequencies available, Innerspec has deployed systems ranging from 50KHz to 12MHz.

What are the advantages of EMAT?

Because the sound is generated in the part inspected, EMAT test has several unique advantages:

• It doesn’t require couplant (dry inspection).
• It is not affected by surface conditions.
• It permits easier deployment of probes (especially important for automated inspections).
• It can generate unique wave modes (i.e. horizontally polarized shear energy and lamb waves) that are often difficult to reproduce with piezoelectric transducers. For more information see EMAT Technology.

What is horizontally polarized shear energy used for?

Because of the specific particle motion, horizontally polarized shear energy is the only practical means to ultrasonically inspect austenitic welds and materials with dendritic grain structures (e.g. stainless steel welds). Moreover, as a guided or plate wave, horizontal shear waves fill up the thickness of the material and provide complete, thru-thickness inspection in one pass of the transducer.

In summary, what is a good application for EMAT?

EMAT is especially well suited for real-world, industrial applications where speed and accuracy of inspection are paramount and the conditions of the material and process are not always ideal due to the EMAT test advantages we have seen before.

What materials can be inspected with EMAT?

EMAT works with any material that conducts electricity. Overall, EMAT works with almost all metals, however some metals are better suited to the technique than others.

Why aren’t EMATs used more frequently?

EMAT is a relatively new technique still unexplored by many potential users. EMAT transducers also require high power and specific electronic equipment that is not widely available. As industry discovers the advantages of EMAT tests its use will spread to an increasing number of applications.

Are there standards for EMAT inspection?

In terms of wave propagation and response to defects, the same standards that apply to conventional UT apply to EMAT. In addition to this, there are recognized ASTM standards on EMAT inspection. See EMAT Technology.

Is EMAT equipment expensive?

Because EMAT requires more power to generate the ultrasound, EMAT instruments are generally more expensive than piezoelectric UT instruments. However, by eliminating couplant delivery and disposal and being less affected by process and material conditions, they are significantly more cost effective in integrated inspection systems. In general, integrated EMAT systems are normally equivalent to or less expensive than piezoelectric UT systems.

Can you have fast inspection with EMAT?

Real-time inspection at production speeds is one of EMAT’s most interesting advantages. We currently have weld inspection systems operating at 3m/s, and some applications have reached inspection speeds of 26m/s.

Is EMAT safe?

Yes. There are no health risks associated with either the technology or the equipment.

Can EMAT transducers be used in phased array?

Yes, EMAT transducers can be used in phased array. Innerspec Technologies has developed several applications with phased array for inspection of welds and large volumes.

Are EMAT systems difficult to use?

We design our integrated to be used by line operators without any specific ultrasonic training. Our portable instrumentation is similar to conventional UT systems, with only some special features that are unique to EMAT.

Do you also sell EMAT sensors?

Yes. Innerspec Technologies sells sensors and has research capabilities to develop custom solutions for specific applications. However, it is important to note that most EMAT sensor designs require high-power instrumentation that is not widely available. For these cases, we can also provide specific instrumentation modules to meet individual requirements.

I have an application that is not on your list of products, how do I find out if you can do it?

Contact us with the details of the application. Once we receive this information, we will immediately let you know if it is a good application for EMAT. We might also ask you to send samples to perform a proof-of-principle or feasibility test. Unless specific equipment is required (e.g. custom sensors), Innerspec Technologies will perform this initial test completely free of charge. If the results of the test are positive, we will provide you with a complete quotation. Once we quote your application, the final system will have the same 100% performance guarantee as our existing products.

Do you sell equipment worldwide?

Most of our systems are available worldwide through our network of offices, and distributors although some restrictions apply. Contact us to inquire about availability in your country.

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