Abstract

A new fabrication concept of EMAT-SH and SV Probes for a frequency range of 1 MHz to 2.5 MHz has been developed. Models of arrays with up to 6 transmitter and 6 receiver elements in a housing of 40 mm x 40 mm x 30 mm are available. Probe modifications for curved geometries of a radius as low as to 5.4 mm are possible. The following examples are discussed:
• Wall thickness measurement and flaw detection.
• Results of sound field measurements of SH-Wave Probes
• Flaw detection in an austenitic pipe weld using SH-Wave Probes
• Flaw detection in a non-ferrous pipe (10.8 mm x 0.8 mm)

Background

In recent years, EMAT techniques have been developed as an important supplement to the piezoelectric ultrasonic technique. The development of new solutions for inspection tasks by EMAT technique is usually much more difficult than with piezoelectric technique.

When an inspection task is performed using the piezoelectric technique, the process is usually the following:
A suitable standard ultrasonic instrument together with a large range of standard piezoelectric transducers is used to investigate factors like test frequency, focus, piezoelectric element size, incidence angle, wave mode, etc.. At last, there is a solution - a supplier can manufacture the best probe form.

This process is usually not possible when the EMAT technique is applied. To begin with, there are just a few commercial providers of EMAT probes and electronic equipment, and something like a standard EMAT probe doesn't exist anyway. The main problems are the high loss of signal energy when the gape of the object increases as well as the change in efficiency when transforming electromagnetic energy into ultrasound, which is affected by the object's material characteristics. That can make the investigation impossible if standard instead of customized probes are used.

Development of an EMAT probe family

For an effective usage of the EMAT technique for NDT tasks in our company we have trials done during the last 18 months to develop our own EMAT probe family. We had to fulfill the following requirements:
• To provide the most important wave modes such as SV-,SH-,Rayleigh/Lamb-Waves in a wide range of angles.
• Phased-Array-Technique, that means the operation of multiple transmitter and receiver elements as an array system.
• Test frequency from 1 MHz to 2.5 MHz
• Probe housing size comparable with conventional piezoelectric technique.
• Simple, cost efficient and highly reproducible manufacturing, especially for objects' geometry modified probes.
• Applicable to ferrous and non-ferrous objects.
• High quality standard due to manufacturing by modular-design principle by means of a minimum of easy-to-manufacture probe components.

Figure 1 Connection to pre-amplifier and transmitter unit Soldering plate with adjustable capacitor Perspex Support for permanent magnet and coil Permanent magnet and coil support

Figure 1 shows our probe's principal construction. An EMAT probe usually consists of the following single components:

• Housing (not displayed in the figure)
• Soldering plate with adjustable capacitor
• Perspex -support for coil and permanent magnet
• Permanent magnet
• Ferrite coil core with coil winding
• Ceramic protection layer (thickness depends on application)



Based on these components we currently manufacture 4 different types of standard probes for application to planar or less curved objects. Figure 2 shows the scope of applications as a function of frequency and incidence angle. All probes work in a frequency range of 1.0 MHz to 2.5 MHz. We manufacture our standard probes with up to 6 transmitter- and 6 receiver elements in a housing of 40 mm x 40 mm x 30 mm (without pre-amplifier), whereby the transmitter and the receiver transducer are always placed separately. The construction of the transmitter and the receiver transducer differs only by the coil wire thickness and the capacity value of the adjustable capacity.

Depending on the application we position the transmitter-receiver probe either side by side (e.g., for wall thickness measurement), parallel behind (e.g., for ADEPT) or side by side at a certain angle to the contact surface (e.g., depth focusing).
[UT Encyclopedia: ADEPT]

Figure 3: Inspection of wall thickness and 3 mm notch at near surface and far surface.

Modification of the EMAT probe for curved objects makes possible a special design of the perspex support; the probe matches exactly the object's geometry, today performed by high precise manufacturing techniques. The modification of the originally supplied plan permanent magnet on curved geometry is possible by simple grinding. Until now probes were modified for radiuses as small as 5.4 mm.

Results
Wall thickness Determination and Flaw Detection with SV-Wave-Probes

Figure 3 shows the possibility of using a SV-Probe with a track wave length of 3 mm, each consisting of 4 transmitter and receiver elements operating as an array. The active probe face for each transmitter and receiver is 10 mm x 33 mm. With a proper choice of the exciting frequency and the delay times; it is possible to perform a wall thickness measurement, a 45° flaw detection and a 90° surface inspection with Rayleigh waves, all with the same probe.

Fig 4: Wall thickness inspection by multiple reflections with 2.5 MHz of an 8 mm aluminum plate

Figure 5

Figure 4 illustrates the results of a wall thickness measurement of an 8 mm aluminum plate using the same SV-Probe. Much more than 50 backwall reflections are available in praxis. Because of limitations of the electronic testing device, it is only possible to perform a wall thickness measurement with more than 20 backwall reflections. This application uses an exciting frequency of 2.5 MHz.

The results of another application is shown in Figure 5. Dry contact ultrasonic testing is required for an 8mm thin aluminum plate in one of the plate's plane axis (in plate's length or in width), where only the 8 mm width edge face of the plate is available as a coupling surface for the EMAT probe. So we built a transmitter and receiver probe with an active probe face of total 8 mm x 20 mm. A test plate of 8 mm thickness and 320 mm width was inspected. The width was determined. The EMAT probe generates a linear polarized 2.5 MHz 0°- shear wave; its orientation is perpendicular to the plate thickness and perpendicular to the wave propagation.

In Figure 5 below you can see the rectified echo train of the first 23 echo reflections of the plate far-surface. (the 23rd echo is magnified at top right) We can predict that this EMAT probe can also measure a 7 m width plate with sufficient S/N ratio.

Results of Sound field measurements of SH-Wave-Probes

Figure 6 shows the sound field characteristics parallel to the plane of incidence of a SH-Wave-Probe with a track wavelength of 2 mm and 4 transmitter- and receiver elements. The active probe face for each transmitter and receiver is 10 mm x 30 mm. The nominal angle of incidence is adjusted to 60°, 70° and 90° by a suitable selection of exciting frequency and delay time.

	Fig 6: Sound field characteristics of a SH-Wave-Probe parallel to the plane of incidence	Fig 7: Sound field characteristics of a SH-Wave-Probe in perpendicular to the plane of incidence.
Incidence Angle 60° Frequency 1.73 MHz
Incidence Angle 70° Frequency 1.70 MHz
Incidence Angle 90° Frequency 1.158 MHz

Figure 7 shows the related sound field characteristics for nominal incidence angle perpendicular to the plane of incidence.

Flaw detection of an austenitic pipe weld using SH-Wave Probes

Figure 8 shows another application: flaw detection of an austenitic weld ( Boundary of material numbers 1.4550/1.4551/1.4550 ) in a pressure pipe of a control unit in a nuclear power plant. This particular pipe section has a diameter of 55 mm and a wall thickness of approx. 6 mm. Therefore, the applied probe must be built to match the pipe's diameter. The Transmitter-Receiver Probe is built with 6 array elements each and operates longitudinally to the pipe's axis (ADEPT technique). The active probe face for each transmitter and receiver is 10 mm x 40 mm.

Artificial defects are applied as notches of 1 mm depth, 5 mm length (longitudinal to the weld) and 0.4 mm width and lying in relation to the probe index in front of, beside, and behind the austenitic weld. The EMAT probe was applied with a 78° nominal angle and an exciting frequency of 1.58 MHz.

Figure 9 shows the A-scan results of the artificial defects compared to A-scans of an defect-free weld section. The prospective defect zone lies between 42 µs to 52 µs signal time of flight. The artificial flaw before the weld is detected with a 15 dB S/N ratio, the one central to the flaw has 14 dB and the one behind the weld has 13 dB.

Figure 10

Figure 10 illustrates another adapted EMAT probe application to a pipe radius. This time, it's an inspection of non-ferrous thin-wall piping ( Zr.- alloy ) with a diameter of approx. 10.8 mm and wall thickness of approx. 0.8 mm. The probe consists of 4 transmitter and 4 receiver elements in ADEPT arrangement and generates a 1.3 MHz -SH guided wave under a 70° incidence angle. The active probe face for each transmitter and receiver is 10 mm x 30 mm. The direction of polarization lies circumferential and perpendicular to the pipe axis. Bottom left in Figure 10 depicts the undisturbed reflection echo at the end of the investigated pipe. The probe was positioned approx. 170 mm from the pipe end.

At bottom right of the figure displays the measurement of a comparative pipe with an artificial defect lying 38 mm before the pipe end (approx. 80% wall-thinning over approx. 20% pipe circumference). The artificial defect is detectable with a better than 12 dB S/N ratio.

Figure 11

Flaw detection of a non-ferrous pipe (10.8 mm x 0.8 mm)

Figure 11 shows the application of an inspection of longitudinal defects in the above mentioned tube. The EMAT probe consists of one transmitter and receiver element each, positioned at a 180° angle to each other. The active probe face is 10 mm x 7.5 mm. A 1.0 MHz SH-wave is generated; the direction of polarization lies parallel to the pipe axis At bottom left of Figure 11 we see the RF-signal of an defect-free test pipe. More than 20 circumferential signals are clearly visible in the time of flight view. The bottom right shows the display of a pipe consisting of a 1 mm test drill. Beginning with the 4th cycle, no clear circumferential echo is visible. That means an evaluation is possible based on the number of circumferential cycled echoes, illustrated for the 1 mm test drill.

R.Meier
born 1947, studied Mechanical Engineering as well as Electrical Engineering. 20 years experience in automation and computerization of UT and ET examination. Special knowledge in the use of stochastic methods for signal improvement, Neural Networks for defect recognition, EMAT probes and EMAT applications and in the examination of small laserwelds using extremely focused piecoelectric probes. Working at Siemens in the NDT-field since 1978. Presently responsible for the development and exploitation of innovative NDE-products and -services.
Siemens AG - Power Generation Group - KWU NP
Freyeslebenstraße 1, D-91058 Erlangen
Phone: +49 9131 18-3012, Fax: +49 9131 18-2468
Email: rainer.meier@erl11.siemens.de
Homepage: http://w2.siemens.de/kwu/e/foa/n/products/s9.htm
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