Eddy Current Testing Method Principles

The eddy current test method was briefly described in Chapter 2, here it is presented in more detail using the theory presented in Chapter 2. .

3.1.1. Application of Lenz's Law to Eddy Current Testing

The alternating current passed through the eddy current test coil produces a magnetic field that varies in magnitude and can induce a current in a conducting medium within its influence. Lenz’s law indicates that this induced current opposes the current/field that caused it and thus the opposing current manifests itself as a measurable impedance in the test coil. Since the phase and magnitude of this impedance will vary with test parameters and test material properties, it can be used to imply the latter. This is the principle of eddy current testing and is illustrated in Figure 3.1. The simplest eddy current test coil arrangement, using a single coil, is known as absolute, because the test measurement is the absolute coil impedance measured across the terminals of the coil.

Figure 3.1 – Formation of eddy currents from a coil’s magnetic field Primary magnetic

field from coil

Secondary magnetic field from eddy currents

Coil excited by A.C.source

Electrically conducting material Eddy currents in

closed loops

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3.1.2. Impedance of an Eddy Current Coil

A multiple*turn coil with an alternating current passing through it will have an impedance comprising resistive and reactive elements regardless of whether or not it is acting on a conducting medium. The resistive element corresponds to the resistance of the coil wire, which is frequency dependent. The effect of this frequency dependence is likely to be fairly insignificant in fine wire, where the skin depth, at all but the highest frequencies, is likely to exceed the wire thickness. The reactive part comprises two parts, capacitive and inductive. The winding of the coil, creates a capacitance between each element of each turn; the sum of these capacitances is the total capacitance of the coil. The resulting capacitance is generally small (10^*9*10^*12 Farads), enough to be neglected at all but the highest frequencies. The inductive element of the reactance is also caused by the interaction between neighbouring turns in the coil, this is 180º out of phase with the capacitive reactance. Each turn on the coil creates an alternating magnetic field which induces opposing currents in neighbouring turns as per Lenz’s law, this is known as self inductance. The resistive element of the coil and the net value of the two reactances form the real and imaginary components of the unloaded coil impedance.

When analysing eddy current data, the unloaded coil impedance is often not of interest and is used to normalise impedances measured in the loaded coil case. On an eddy current test instrument this is done using a null or balance operation with the coil in air.

Physically this alters an impedance of a reference part of an internal measurement circuit within the eddy current test instrument. Performing this operation with the coil in the loaded state will normalise all measurements to that particular loaded coil impedance.

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When the coil is placed so that a conducting material intercepts the coil’s magnetic field the impedance of the coil will change from its unloaded state. The coil’s magnetic field induces currents in the conducting material, which is usually analogous to a flat plate.

Unlike the interaction between coil turns where the currents flow along the coil wire, in the case where current is induced in a flat plate the currents flow in closed loops in the material. It is the swirling pattern of these currents like eddy currents in fluid flow that give eddy current testing its name. The magnitude and distribution of the eddy currents within the conducting material is dependent on the test frequency, the conductivity of the test material, the magnetic permeability of the test material, the local geometry of the test material and the coil parameters and current.

The induced eddy currents will have their own magnetic field which will oppose the magnetic field that created the eddy currents, thus it will be varying constantly. This magnetic field will induce a current in the eddy current test coil, which will manifest itself as an inductive reactance. Additionally, due to the conservation of energy, the act of generating eddy currents will cause a resistance in the test coil. The amount of magnetic flux coupling the coil to the test piece is termed the magnetic flux linkage.

Localised variations in the material under test (which affect the conductivity of the test material), the magnetic permeability of the test material or the local geometry of the test material, will all cause variations in the impedance measured across the eddy current coil. Thus by observing the variations in measured impedance an eddy current test can detect changes in material properties and geometric variations in the material, such as cracks and thickness variation, though the latter is dependent on the depth of penetration of the eddy currents, a concept which will be discussed later in this chapter.

3.1.3. Lift1off

Another important factor in the strength of the eddy currents generated in the test material is the distance between the coil and the test surface which is termed lift=off.

This factor affects the amount of flux linkage between coil and test material, so can also be affected by the incident angle between the coil and the test material. Lift*off causes an exponential reduction in flux linkage resulting in lower test sensitivity, which is usually undesirable. This effect can be beneficial as it can be used to measure thin non*

conducting coatings on conducting materials. By plotting the impedance at a range of known lift*off distances between the coil and test material, actual coating thicknesses on materials with the same electromagnetic properties can be estimated by measurement of the impedance with the coil in contact with the coating.

3.1.4. Coil Configurations

In eddy current testing the method of generating the eddy currents is the same for all configurations as it is for absolute coil testing. There are however, a number of variations on the configuration for the detection part of eddy current test. Rather than having a common coil for excitation and detection, the detecting coil can be a separate coil, this is known as transmit=receive or driver=pickup. In this case the receiving coil has no unloaded impedance, so all measurements of impedance are just attributed to the interaction with the material under test; in this case the interrogated area lies between the driver and pickup coil. Other variations on this theme use magnetic field sensors to analyse the variation in eddy current distribution. This can provide improved sensitivity but generally it is more difficult to infer information about material properties due to the lack of impedance data.

Another eddy current test configuration is known as differential. This uses two adjacent coils which are wound in opposite directions. The coils are connected in series and the impedance is measured across both coils together. Consequently, when the coils are above identical regions of conducting material their impedances cancel each other out.

This makes the pairing insensitive to gradual variations in impedance, which may not be desirable, but sensitive to sudden changes, like cracks. Because the unloaded coil impedances cancel each other out there is no normalising of the data. Differential coils are only effective if the coils are influenced by features of interest at different times, similarly they are only beneficial if structural features causing undesirable signals affect both coils equally at the same time. This is entirely dependent on the orientation of the coils relative to one another.

Common undesirable features that can be suppressed by differential coils are geometric effects. The most common is known as the edge=effect where as the eddy currents near the edge of a conducting material they are bound by the physical dimensions of the material and so are redistributed resulting in an impedance change at the coil terminals.

This often produces a large unwanted signal on the eddy current test instrument’s Complex Impedance Plane display, which is the subject of Chapter 5.