Electromagnetic acoustic transducer

An EMAT ultrasonic transducer (UT) shown with a conventional piezoelectric UT.

Electromagnetic Acoustic Transducer (EMAT) is a transducer for non-contact sound generation and reception using electromagnetic mechanisms. EMAT is an ultrasonic nondestructive testing (NDT) method which does not require contact or couplant, because the sound is directly generated within the material adjacent to the transducer. Due to this couplant-free feature, EMAT is particularly useful for automated inspection, and hot, cold, clean, or dry environments. EMAT is an ideal transducer to generate Shear Horizontal (SH) bulk wave mode, Surface Wave, Lamb waves and all sorts of other guided-wave modes in metallic and/or ferromagnetic materials.[1][2] As an emerging ultrasonic testing (UT) technique, EMAT can be used for thickness measurement, flaw detection, and material property characterization. After decades of research and development, EMAT has found its applications in many industries such as primary metal manufacturing and processing, automotive, railroad, pipeline, boiler and pressure vessel industries.[2]

Basic Components in EMAT Transducer

There are two basic components in an EMAT transducer. One is a magnet and the other is an electric coil. The magnet can be a permanent magnet or an electromagnet, which produces a static or a quasi-static magnetic field. In EMAT terminology, this field is called bias magnetic field. The electric coil is driven with an alternating current (AC) electric signal at ultrasonic frequency, typically in the range from 20 kHz to 10 MHz. Based on the application needs, the signal can be a continuous wave, a spike pulse, or a tone-burst signal. The electric coil with AC current also generates an AC magnetic field. When the test material is close to the EMAT, ultrasonic waves are generated in the test material through the interaction of the two magnetic fields.

Transduction Mechanism

There are two mechanisms to generate waves through magnetic field interaction. One is Lorentz force when the material is conductive. The other is magnetostriction when the material is ferromagnetic.

Lorentz Force

The AC current in the electric coil generates eddy current on the surface of the material. According to theory of electromagnetic induction, the distribution of the eddy current is only at a very thin layer of the material, called skin depth. This depth reduces with the increase of AC frequency, the material conductivity, and permeability. Typically for 1 MHz AC excitation, the skin depth is only a fraction of a millimeter for primary metals like steel, copper and aluminum. The eddy current in the magnetic field experiences Lorentz force. In a microscopic view, the Lorentz force is applied on the electrons in the eddy current. In a macroscopic view, the Lorentz force is applied on the surface region of the material due to interaction between electrons and atoms. The distribution of Lorentz force is controlled by the design of magnet, and design of the electric coil, the properties of the test material, relative position between the transducer and the test part, and the excitation signal for the transducer.

Magnetostriction

A ferromagnetic material will have a dimensional change when an external magnetic field is applied. This effect is called magnetostriction. The flux field of a magnet expands or collapses depends on the arrangement of ferromagnetic material having inducing voltage in a coil and the amount of change is affected by the magnitude and direction of the field.[3] The AC current in the electric coil induces an AC magnetic field and thus produces magnetostriction at ultrasonic frequency in the material. The disturbances caused by magnetostriction then propagate in the material as an ultrasound wave.

In polycrystalline material, the magnetostriction response is very complicated. It is affected by direction of the bias field, direction of the field from AC electric coil, the strength of bias field, and the amplitude of the AC current. In some cases, one or two peak response may be observed with the increase of bias field. In some cases, the response can be improved significantly with the change of relative direction between bias magnetic field and AC magnetic field. Quantitatively, the magnetostriction may be described in a similar mathematical format as piezoelectric constants.[3] Empirically, a lot of experience is needed to fully understand the magnetostriction phenomenon.

Magnetostriction effect has been used to generate both SH-type and Lamb type waves in steel products. Recently, due to the stronger magnetostriction effect in nickel than steel, magnetostriction sensors using nickel patches are also developed for nondestructive testing of steel products.

Comparison between EMAT and Piezoelectric Transducers

As an Ultrasonic Testing (UT) method, EMAT has all the advantages of UT compared to other NDT methods. Just like piezoelectric UT probes, EMAT probes can be used in pulse echo, pitch-catch, and through-transmission configurations. EMAT probes can also be assembled into phased array probes, delivering focusing and beam steering capabilities.[4]

Advantages

Compared to piezoelectric transducers, EMAT probes have the following advantages:

  1. No couplant is needed. Based on the transduction mechanism of EMAT, couplant is not required. This makes EMAT ideal for inspections at temperatures below the freezing point and above the evaporation point of liquid couplants. It also makes it convenient for situations where couplant handling would be impractical.
  2. EMAT is a non-contact method. Although proximity is preferred, a physical contact between the transducer and the specimen under test is not required.
  3. Dry Inspection. Since no couplant is needed, the EMAT inspection can be performed in a dry environment.
  4. Less sensitive to surface condition. With contact-based piezoelectric transducers, the test surface has to be machined smoothly to ensure coupling. Using EMAT, the requirements to surface smoothness are less stringent; the only requirement is to remove loose scale and the like.
  5. Easier for sensor deployment. Using piezoelectric transducer, the wave propagation angle in the test part is affected by Snell’s law. As a result, a small variation in sensor deployment may cause a significant change in the refracted angle.
  6. Easier to generate SH-type waves. Using piezoelectric transducers, SH wave is difficult to couple to the test part. EMAT provide a convenient means of generating SH bulk wave and SH guided waves.

Challenges and Disadvantages

The disadvantages of EMAT compared to piezoelectric UT can be summaried as follows:

  1. Low transduction efficiency. EMAT transducers typically produce raw signal of lower power than piezoelectric transducers. As a result, more sophisticated signal processing techniques are needed to isolate signal from noise.
  2. Limited to metallic or magnetic products. NDT of plastic and ceramic material is not suitable or at least not convenient using EMAT.
  3. Size constraints. Although there are EMAT transducers as small as a penny, commonly used transducers are large in size. Low-profile EMAT problems are still under research and development. Due to the size constraints, EMAT phased array is also difficult to be made from very small elements.
  4. Caution must be taken when handling magnets around steel products.

Applications of EMATs

EMAT has been used in a broad range of applications and has potential to be used in many other applications. A brief and incomplete list is as follows.

  1. Thickness measurement for various applications[5]
  2. Flaw detection in steel products
  3. Plate lamination defect inspection
  4. Bonded structure lamination detection[6][7]
  5. Laser weld inspection for automotive components
  6. Various weld inspection for coil join, tubes and pipes.[8]
  7. Pipeline in-service inspection.[9]
  8. Railroad and wheel inspection
  9. Austenitic weld inspection for power industry[4]
  10. Material characterization[10][11]

References

  1. ↑ R.B. Thompson, Physical Principles of Measurements with EMAT Transducers,Ultrasonic Measurement Methods, Physical Acoustics Vol XIX, Edited by R.N. Thurston and Allan D. Pierce, Academic Press, 1990
  2. 1 2 http://www.innerspec.com
  3. 1 2 Masahiko Hirao and Hirotsugu Ogi, EMATS For Science and Industry, Kluwer Academic Publishers, 2003
  4. 1 2 Gao, H., and B. Lopez, "Development of Single-Channel and Phased Array EMATs for Austenitic Weld Inspection", Materials Evaluation (ME), Vol. 68(7), 821-827,(2010).
  5. ↑ M Gori, S Giamboni, E D’Alessio, S Ghia and F Cernuschi, ‘EMAT transducers and thickness characterization on aged boiler tubes’, Ultrasonics 34 (1996) 339-342.
  6. ↑ S Dixon, C Edwards and S B Palmer, ‘The analysis of adhesive bonds using electromagnetic acoustic transducers’, Ultrasonics Vol. 32 No. 6, 1994.
  7. ↑ H. Gao, S. M. Ali, and B. Lopez, “ Efficient detection of delamination in multilayered structures using ultrasonic guided wave EMATs” in NDT&E International Vol. 43 June 2010, pp: 316-322.
  8. ↑ H. Gao, B. Lopez, S.M. Ali, J. Flora, and J. Monks (Innerspec Technologies), “Inline Testing of ERW Tubes Using Ultrasonic Guided Wave EMATs” in 16th US National Congress of Theoretical and Applied Mechanics (USNCTAM2010-384) , State College, PA, USA, June 27-July 2, 2010.
  9. ↑ M Hirao and H Ogi, ‘An SH-wave EMAT technique for gas pipeline inspection’, NDT&E International 32 (1999) 127-132
  10. ↑ H. Ogi, H. Ledbetter, S. Kim, and M. Hirao, "Contactless mode-selective resonance ultrasound spectroscopy: Electromagnetic acoustic resonance," Journal of the ASA, vol. 106, pp. 660-665, 1999.
  11. ↑ M. P. da Cunha and J. W. Jordan, "Improved longitudinal EMAT transducer for elastic constant extraction," in Proc. IEEE Inter. Freq. Contr. Symp, 2005, pp. 426-432.

Codes and Standards

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