Alternative Electromagnetic Techniques

A large number of other electromagnetic testing techniques exist which may prove to be viable alternatives to, or be complementary to, the eddy current method. Background to the following methods, which were ultimately deemed unsuitable for this project, can be found in Appendix D.

• Magnetic Flux Leakage

• Remote Field Sensing

• ACPD

• ACFM

• Meandering Winding Magnetometer (MWM).

An additional two methods, Pulsed Eddy Current and Barkhausen Noise, which are relevant to the second project on creep cavitation are explained below.

3.6.1. Pulsed Eddy Current

A technique that has become more feasible in recent years due to the advances in computing technology and sensor design is pulsed (or transient) eddy current. The concept of using a step function voltage to excite the probe has been around for many years. Where conventional eddy current inspection uses a continuous gated sinusoidal waveform of a specified frequency, pulsed eddy current uses a step*function voltage which contains a continuum of frequencies. The great benefit of this is that the electromagnetic response to the many different frequencies can be measured with a single pulse. Since depth of penetration is frequency dependent, so information from a range of penetration depths can be obtained simultaneously. Consequently, by

observing the data in the time*domain, we have a system analogous to ultrasonics where features nearest the probe are observed prior to those at greater depth.

Another similarity with ultrasonics produces what many consider to be the most useful feature of pulsed eddy current – its improved depth of penetration compared to conventional eddy current. By way of comparison, using a conventional eddy current system, the useful signal within the test material is approximately three times the standard depth of penetration, for a pulsed system it can be as much as ten times.

In pulsed eddy current testing it is beneficial to use a reference or nulling signal, similar to some differential coil arrangements in conventional eddy current testing, to improve data interpretation. This signal is representative of the ideal or undamaged state of the inspected component. By subtracting the reference signal only the variations that occur within the test are displayed.

Large amounts of information can be gathered from the acquired data, provided this can be accurately extracted. The method lacks the precision of the conventional eddy current technique and so has its applications in different areas which require relatively lower sensitivity, such as corrosion detection in thin plates.

The fields resulting from pulsed eddy currents can be detected by a receiving probe in a similar manner to a conventional transmit*receive configuration, or by a Hall*effect sensor, located either adjacent to the transmission coil or, for a through*transmission test, on the opposite face of the material being tested. Hall*effect sensors offer better sensitivity for deep defects, at the low frequency part of the spectrum, whereas coils are

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better for shallower defects where the higher frequencies are of most significance.

The pulse rise*time and duration determine the frequency content and could typically extend from a hundred hertz to several megahertz. The wave disperses as it penetrates the material and the pulse shape will change since the frequency and thus the wave speed, will vary with depth. Viewing the pulse data in the frequency*domain, further information can be gained from the variation of amplitude and/or phase with frequency.

By sampling different delay times within a pulse, specific sections of the spectrum can be evaluated. From the amplitude and phase properties measured parameters such as presence of defects, thickness variations and changes in lift*off can be evaluated for each frequency selected.

3.6.2. Barkhausen Noise

The Barkhausen Noise (or Micromagnetic) NDE technique is a material characterisation method applicable to ferromagnetic metals, which are made up of small magnetic regions called domains. These domains are separated by domain walls which are most easily magnetised in one particular crystallographic direction. Applying an alternating magnetic field to a ferromagnetic material will cause the domain walls to move back and forth as the domains either side of it shrink and grow respectively. This will cause a change in the overall magnetisation of the metal which can be detected by a conducting coil placed nearby. This detection is in the form of an electrical pulse, which will be created for each domain movement. Combining the pulses from all the domain movements gives a stepped signal resembling electrical noise, containing information about the magnetic characteristics of the metal under test. There is a second method of

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Barkhausen Noise testing, Acoustic Barkhausen Noise (ABN), which uses ultrasonic transducers to detect the domain movements rather than using a Hall effect (or similar) sensor to detect the change in magnetic field associated with the domain movement.

As with eddy current testing there is a skin*effect preventing deep penetration: typical application depths are 0.01mm to 1.5mm. Two main characteristics will affect the Barkhausen Noise signal; distribution of elastic stresses and the material microstructure.

Due to the phenomenon known as magnetoelastic interaction, compressive stresses in the material will cause a reduction in noise intensity while tensile stresses will cause an increase in noise intensity. Modifications to the metal by grinding, shot*peening, carburizing and induction hardening can all be detected by Barkhausen Noise. The MBN method will be discussed in greater detail in Chapter 7.

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