词条 | Electromagnetic acoustic transducer |
释义 |
Basic componentsThere 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 mechanismThere 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 forceThe 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. MagnetostrictionA 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 with piezoelectric transducersAs 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] AdvantagesCompared to piezoelectric transducers, EMAT probes have the following advantages:
Challenges and disadvantagesThe disadvantages of EMAT compared to piezoelectric UT can be summaried as follows:
ApplicationsEMAT 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.
References1. ^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 Innerspec Technologies 3. ^1 Masahiko Hirao and Hirotsugu Ogi, EMATS For Science and Industry, Kluwer Academic Publishers, 2003 4. ^1 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. ^Stéphane Sainson, ‘Inspection en ligne des pipelines : principes et méthodes, Ed. Lavoisier 2007’ 11. ^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. 12. ^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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