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Now electromagnetic ultrasonic transducers (EMAT) have been developed. These can generate and receive SH - Waves [100, 107]. They don't need a couplant for transmitting waves in metals, just close contact with the surface. The following chapter briefly describes the different methods of generating electromagnetic SH - Waves, EMAT transducers and some of their characteristics.
The Lorentz force is caused by interaction between the electric current (current density J) which is inducted through the eddy current coil and the magnetic flux (B0). The direction and intensity of the force FL is determined by the vector equation:
Fig 5.1: Principle of electromagnetic acoustic generation with Lorentz-force.
Fig 5.2: Principle of electromagnetic acoustic generation with Magnetostriction
Fig 5.3: Different kind of Radio Frequency (RF) coils
Fig 5.4: EMAT - Probes for SH - Waves with Electromagnet
FL = J x B0 (5.1)
The Lorentz force, electric current and magnetic field lie at a 90° angle from each other, Abb. 5.1.
Physical distortion occurs in almost all ferromagnetic materials when a magnetic field is applied. This phenomenon is called 'magnetostriction' (in simple words: it is the magnetic equivalent of the piezoelectric effect).
The distortion appears generally parallel to the applied magnetic field. If an eddy current coil is mounted on the surface of a ferromagnetic material (steel), in addition to the eddy current field, dynamic magnetic fields are inducted generating the dynamic forces Fms. These forces lie parallel to the applied magnetic flux Bo, Fig. 5.2.
To achieve a sufficient sound beam it is generally necessary to use big apertures by means of big eddy current coils. Two concepts are available. A big aperture can be achieved by using a large scale meander coil or by an arrangement of many small coil segments, Fig 5.3. Such a transducer of multiple coil segments works like a phase controlled transducer array. The eddy current coils or the coil segments are operated by a frequency burst and alternating current. The burst length relies on the coil's size.
If the burst frequency and the sound speed of the generated wave mode is known, the wave length can be calculated. The distance s between two elements of the eddy current coil of the same current direction is called track wavelength.
What are the advantages and disadvantages of both concepts: non-segmented and segmented coils (phased controlled arrays)? Big non-segment coils generating narrow band signals mean the axial resolution is bad. Furthermore, EMATs of non-segmented coils distribute the same energy forward and backward.
EMAT - Probes with segmented coil, which work as a phase controlled array, provide ultrasound signals of wider bandwidth than non-segmented probes and deliver a forward to backward ratio of 30 - 40 dB. Such electromagnetic, phase controlled arrays are better suited for ultrasonic testing than EMAT Probes with non-segmented coils.
The U-shaped electromagnet provides a quasi static magnetic field which lies parallel to the material's surface. The magnet is operated by alternating current (e.g. 50 Hz) so it can be easily moved on the work piece. The eddy current coil provides multiple turns and its axis lies parallel to the magnetic field. The eddy current and the magnetic field propagate parallel to each other so that the magnetostrictive effect on ferromagnetic materials is dominant.
Such electromagnetic probes are built as non-segmented as well as segmented eddy current probes (principle of the phased control array), Fig. 5.4. The coils are driven by amplifiers which generating the burst signal. In case of segmented coils each coil is driven by a separate amplifier of the delayed burst signal.
EMAT probes are generally built as separate transmitters and receivers (eddy current coil), either both coils are interlocked or the coils are side by side in a V-shape.
With such a SH-Wave Probe it is possible to achieve a dynamic range of 45 dB on ferrite steel. Typical probe frequencies are in the range of 0,6 - 1,5 MHz and typical track wave lengths are in the range of 3 - 5 mm. Phase controlled arrays (probes with segmented eddy current coils) deliver a forward-backward radio of typically 30 dB for angles of incidence in the range of 30° - 60°.
The sound beam divergence of an EMAT probe depends on the angle of incidence. Generally it is true that a wider angle of incidence equals a wider beam. A meander probe (transmitter probe) with 12 elements and a track wavelength of 4 mm for 45° angle of incidence (f = 1,15 MHz) typically provides +-7 mm beam width (at 6 dB drop). The increase of the angle of incidence to 60° and the frequency change to 0,95 MHz increases the beam divergence to +10°. An increase in the number of array elements reduces the beam divergence. A phased controlled array probe, for instance with 6 segments, each containing 3 array elements with 4mm track wave length provides a beam divergence of +-3° for an angle of incidence of 45°. The increase of the angle of incidence to 60° increases the beam divergence to +-5,5°.
Fig 5.5: EMAT probe for SH-waves with Permanent Magnet
In the case of segmented (phased controlled array) probes, an arrangement of permanent magnets (segments) is used to generate the magnetic bias field, as shown at the right of Fig. 5.5; the eddy current is generated with the meander coil.
The propagation direction of the SH - wave, which is generated by the meander coil and a segment of single permanent magnets, is led directly into the material perpendicular to the plane of the permanent magnets.
If the meander coil is driven time delay controlled, the probe operates as a phased control array, and the angle of incidence of the SH - wave can be controlled between 30° and 90°.
Fig 5.6: EMAT- SE- Probe, layout of permanent magnet
Fig 5.7: Control of the angle of incidence with variation of the frequency (non-segmented EMAT- Probe)
The track wave length s of a SH - wave - probe with permanent magnets is determined by the distance of two permanent magnets of the same polarity lying side by side. Typical values of the track wave length s are 4 mm to 6 mm, and a frequency range of 0,8 MHz to 0,5 MHz. The probe bandwidth of 50 - 100% depends on the width of each element, and the forward-backward ratio is typically larger than 35 dB for separate transmitter-receiver probes. The sound beam divergence of a phased controlled array with six segments of 3 elements each and track wavelength of 5 mm is +-5,5°, for an angle of incidence of 60°.
These permanent probes were mainly developed for testing of non-ferrite materials. There they can be applied with no problems since they are easy to handle. On ferrite materials this probe can very difficult to move, because of its magnetic force. But with the help of special probe manipulators it is possible to use permanent magnetic probes on ferrite materials also.
So it is easy to calculate = c/f, where c is the sound velocity of the generated wave mode.
The angle of incidence () of an EMAT- probe can be determined in the case of a non-segmented probe by the operation frequency and the track coil wavelength (s) of the transducer , Fig. 5.7, accordingly the ratio:
sin = /s (5.2)
Fig. 5.8 shows the angle of incidence of a probe as a function of the transducer's frequency and the track wavelength for generation of longitudinal - and shear waves.
Fig. 5.9 demonstrates that the main lobe of a non-segmental probe can be controlled by frequency variation . Here the sound distribution of a transmitter at a halve cylinder of ferrite steel of 150 mm radius was measured.
Incidence angle of a probe as a function of the transducer's frequency and the track wavelength [mm].
Fig 5.9: Sound distribution of a non-segmented EMAT probe (track wavelength: 4 mm, 12 array elements)
The angle of incidence can change from 60° and 40° when the frequency varies between 950 kHz and 1,3 MHz. The level of side lobes is very low; that is based on an amplitude distribution of each coil element (variation of turns).
Fig 5.10: Control of incidence angle by variation of the delay time (segmented EMAT probe)
sin = c t/d (5.3)
The frequency of the phased array is determined by the track wavelength of each element. If the number of elements within a segment is small (1-3), it is possible to control a large range of glancing incidence angle (90° - 50°) by varying the delay time.
That is possible since the main lobe of a segment with, for instance, 3 elements is very wide, Fig. 5.11 . Within this wide main lobe of the single element it is possible to control the lobe of the complete array only by varying the delay time.
If a large number of array elements (> 3) are used for a segment, the frequency of the probe must be changed, if the delay time is changed, to achieve another angle of incidence.
Since the main lobe of the segments gets smaller when the number of array elements increase, for an unchanged frequency a loss of sensitivity outside the main lobe maximum is predictable.
Sound distribution of one segment of a segmented EMAT probe.
Fig 5.12: Sensitivity of an EMAT probe along the acoustic axis
5.3.2 Sound distribution perpendicular to the beam axis
The main lobe of an EMUS - TR - Probe with permanent magnets (four receiver and four transmitter elements), 17 mm receiver and transmitter width and a 18° V - position between transmitter and receiver is shown in Fig. 5.14. The 6 dB - width of the main lobe divergence is 15° .
Sound distribution of an EMAT - TR - Probe perpendicular to the beam axis
Fig. 5.15 Distributions of an EMAT - TR - Probe in front of the probe.
For EMAT - Probes with segmental RF coils (arrays) the angle of incidence is controlled by varying the delay time. Fig. 5.15 shows the distribution of a permanent magnetic probe with four transmitter and receiver segments (s = 4,4mm, 4 elements per segment).
The distribution was measured at an Aluminum - half cylinder with a 10 mm radius. The EMAT - Probe was designed for inspection of piping of 324 mm outer diameter. The distributions shown in Fig. 5.15 were performed by using the following delay times:
Angle of |
This article is the Chapter 5 of the Book: |
"Ultraschallprüfung von austenitischen Plattierungen, Mischnähten und austenitischen Schweißnähten" Eberhard Neumann et al Eberhard.Neumann@bam.de
See also online the Chapter 4: Ultrasonic Testing of Austenitic and Dissimilar Metal Welds
Expertverlag 1995 Kontakt & Studium; Bd. 377: Werkstoffe ISBN-3-8169-1078-5 Mit 340 Seiten, 204 Bildern und 210 Literaturstellen (c) Expert Verlag; D- 71272 Renningen Malmsheim. Homepage: www.expertverlag.de