The name “absolute sensor” is derived from the fact that the absolute value of the voltage induced in the test coil is measured. The signal of a defect or a change in the properties of the test specimen is superimposed on the signal when the test specimen is free of defects or when the coil is empty. The test signal is therefore influenced not only by defects, but also by the material properties of the test piece and the environmental conditions. If, for example, the sensor heats up during operation, its electrical resistance changes and the absolute value of the induced voltage starts to “wander” (drift). This is the major disadvantage of these sensor systems. To suppress such and similar interferences, an additional compensation coil is often connected in between. This must be arranged in such a way that no interaction with the test specimen occurs but environmental influences are effectively suppressed.
If only the changes in the measured value caused by deviating sample properties are to be displayed, the absolute value must be compensated for this change. The simplest way to achieve this is through the so-called settlement procedure. Here, two absolute sensors, which are as identical as possible, are used, which are connected against each other (like two identical batteries, which are connected with their negative poles). The sum of the two partial voltages is zero if there are test specimens with identical properties in both coils. In this case, both coils provide equal measurement signals, which cancel each other out due to the electrical circuitry.
Such a coil system is called a difference system with arm’s length comparison. In it, the test object is compared with a sample piece of specified quality, the “test standard”. A display only occurs if the test object deviates in its properties from the test standard. A difference of 1% can fill the entire display scale only with suitable instrument sensitivity. In order to always obtain reproducible measurement results with a certain instrument setting, it is expedient to mark the sensors so that the same sensor is always used as a “reference sensor”.
The bandwidth is an elementary parameter in signal processing. It describes the width of the frequency spectrum, i.e. the frequency components contained in a signal.
The bandwidth is characterized by a lower and an upper cut-off frequency. The upper cut-off frequency is limited internally in the device by the damping behavior of the system. The lower bandwidth may well be zero. In this case, the bandwidth corresponds to the upper cutoff frequency. Usually, the 3 dB criterion is used, which corresponds to a drop of the signal amplitude to approx. 71 %.
Note: The maximum bandwidth is an important feature of eddy current test equipment and should not be confused with the available range of test frequency. It refers to the frequency spectrum of the demodulated eddy current signal and can be specifically limited to the application-specific requirements via filter settings (high-pass filter > lower cutoff frequency or low-pass filter > upper cutoff frequency). In this way, interfering signals can be weakened or eliminated, provided that their frequency spectrum differs significantly from that of the signals of interest (e.g. crack indications).
A bandpass filter is created by combining a lowpass filter and a highpass filter. This means that only a middle frequency segment containing only the signals of interest is allowed to pass unaffected. Only low-frequency and high-frequency interference signals are suppressed.
For bandpass filters, a lower and an upper cutoff frequency must be set (see bandwidth).
In eddy current testing, bandpass filters can be used, for example, to attenuate or eliminate conductivity variations, geometry variations, distance signals (lift-off), as well as high-frequency electromagnetic interference and electronic device noise.
It should be noted that the frequency spectrum of the interfering signals and the signals of interest depends on the current test speed as well as on the type and geometry of the sensor used:
1) The higher the test speed and the smaller the coil effective width, the higher the frequency of the signals (> “shorter pulses”).
2) For low test speeds and for relatively large coil dimensions, the frequencies of the signals become correspondingly smaller (> “longer pulses”).
The application of a bandpass filter is also referred to as “dynamic” testing, since the frequency spectrum of the demodulated signal contains only variable (dynamic) components (e.g. in rotor applications). This means that the signal point always returns to the coordinate origin, even if the sensor is not moved.
In a differential sensor system, the same coil arrangement is used as in the foreign comparison method, except that the coils are arranged so that one location on the test piece is compared with another location on the same test piece that is only a short distance away.
The test piece is compared with itself in this arrangement. Since it can be assumed that the alloy and the microstructure do not change or change only very insignificantly on the small distance between the two receiving coils, only the suddenly occurring defects or other material inhomogeneities will be displayed in this way.
This method is thus mainly used to detect localized material defects (e.g. cracks), while changes in the workpiece properties, which occur continuously over the entire length, are largely compensated for.
The disadvantage of this arrangement consists of a direction dependence of the sensor. While elongated flaws (cracks) running transversely to the two receiving coils are detected well (since only one of the two receiving coils is affected at any time), these flaws are no longer detected or only detected to a very limited extent as soon as they run longitudinally (the elongated flaws now cover both receiving coils simultaneously). This can be remedied by a multiple arrangement of receive coils (so-called multi-difference arrangements). However, this still has preferred directions, i.e. defects in certain orientations are still only detected to a limited extent.
Basically, two main groups of sensors are distinguished: flow sensors and touch sensors. In the case of through-flow sensors, a distinction is made between: External pass-through sensor, which encloses the test object and is guided through it (e.g. bar testing with comprehensive test coils) and internal pass-through sensor, which is enclosed by the test object, i.e. the sensor is guided through the test object (e.g. in internal tube testing). Continuous sensors always detect a complete circumferential section of the test object, outside or inside.
In principle, eddy current testing can be classified as a surface testing method. Due to the process, the induced eddy currents are concentrated on a more or less thin layer near the surface. The strongest eddy currents flow immediately at the surface. Therefore, the maximum test sensitivity can be achieved there.
The decrease in eddy current strength with increasing depth (distance from the surface) is caused by the shielding effect of flowing eddy currents (“skin effect”). As a measure of the depth-dependent decrease in eddy current strength, the so-called standard penetration depth is usually used in eddy current testing.
The standard penetration depth δ corresponds to the distance at which the eddy current strength has dropped to about 37% of the value at the test specimen surface (this corresponds to a decrease by a factor of 1 / e ~ 1 / 2.7). It is not a fixed quantity, but depends on the respective test conditions: the test frequency (f), the electrical conductivity (σ) and the relative permeability of the test object (µr) and can be calculated approximately with the following formula:
δ – standard penetration depth in mm
σ – electrical conductivity in MS / m
µr – relative permeability (unitless)
f – test frequency in Hz
Thus applies:
The greater the electrical conductivity or relative permeability or the higher the test frequency, the more the eddy currents are concentrated at the surface of the test object and the smaller the standard penetration depth becomes.
The relative eddy current strengths for selected integer multiples of the standard penetration depth δ are:
1δ: -> 36.8 %
2δ: -> 13.5 %
3δ: -> 5.1 %
5δ: -> 0.7%.
The 3δ depth is also called the “effective penetration depth”. Material changes or defects located at greater depths can generally no longer be reliably detected with sufficient sensitivity, since the eddy current strength has already dropped too much.
Test objects with a wall thickness greater than 5δ are considered “thick-walled”; further increasing wall thickness would not cause any further change in measured value at the eddy current coil.
Based on the standard penetration depth, the depth detection capability can thus be roughly estimated – taking into account the existing test conditions (material properties and test frequency).
When performing eddy current tests, a number of interfering or undesirable signals may appear in practice. These unwanted signals include, for example:
1) Conductivity variations, thermal drift, mechanical vibrations, geometry changes or the so-called lift-off signal, which usually occur over a longer period of time than a defined reference error (low-frequency signals).
2) electromagnetic interference or the electronic noise of the test instrument, which is usually present for a shorter period of time than a defined reference error (high-frequency signals).
In the worst case, all the above-mentioned disturbances occur simultaneously, i.e. they overlap in such a way that the signals of interest (e.g. crack indications) can no longer be reliably detected and thus evaluated.
However, certain frequency components in the demodulated signal can be weakened or eliminated by means of filtering.
In order to be able to specifically suppress the interfering signals, the following requirements must be met:
* First, the frequency spectrum of the signals of interest and that of the interfering signals to be suppressed must be known.
* Second, the frequency spectra of the signals of interest and the signals to be suppressed must differ significantly.
* In addition, care must be taken (when using a time-based filter) to maintain a constant test speed.
In this way, it is possible to avoid not only pseudo indications but also misinterpretations and consequently to significantly increase the reliability of test statements.
For filtering, the filter types high-pass filter, low-pass filter and band-pass filter are available.
High-pass filters are used to suppress interfering low-frequency signal components of the frequency spectrum, while leaving the frequencies above an upper cutoff frequency (i.e., the signals of interest) unaffected (see also bandwidth).
In eddy current testing, high-pass filters can be used, for example, to suppress conductivity or permeability variations, geometry variations, and especially distance signals (lift-off).
It should be noted that the frequency spectrum of the interfering signals and the signals of interest depends on the current test speed as well as on the type and geometry of the sensor used:
1) The higher the test speed and the smaller the coil effective width, the higher the frequency of the signals (-> “shorter pulses”).
2) For low test speeds and for relatively large coil dimensions, the frequencies of the signals become correspondingly smaller (-> “longer pulses”).
A particularly common unit of electrical conductivity in the USA is IACS (for International Annealed Copper Standard).
Here, the electrical conductivity σ is expressed as a percentage of the conductivity of electrolytically pure annealed copper (with 58 MS/m).
For the conversion of SI units to the IACS system applies:
σ – electrical conductivity
In eddy current testing, one uses a coil to:
1) generate (induce) eddy currents in the test object, and
2) to include the repercussions from the test object reflecting its properties.
In the simplest case, an eddy current sensor consists of only one coil, which then acts as both transmitter and receiver (parametric sensor). The principle of operation is that the test object imprints its properties on the coil – more precisely, on the coil impedance.
Impedance is the resistance to alternating current. For a coil, this is composed of two components:
1) the ohmic resistance R (This corresponds to the DC resistance of the coil wire.) and
2) the inductive reactance XL (This occurs because the coil wire is wound into turns. If an alternating current flows through the coil, the coil windings are in the sphere of influence of their own alternating magnetic field. Consequently, currents are induced in them which flow in the opposite direction to the coil current causing them and superimpose it. The resulting total current is therefore out of phase, i.e. time-delayed).
The ohmic resistance R is independent of the test frequency, but depends on the geometry and material of the conductor wire:
R – Ohmic resistance in Ω
l – length of the conductor in m
A – cross-sectional area of the conductor in mm2
ρ – specific electrical resistance in Ω mm2 / m
The inductive resistance XL becomes all the greater:
* the higher the frequency f and
* the greater the inductance L of the coil is
and can be calculated with the following formula:
XL – inductive reactance in Ω
f – frequency in Hertz (Hz = 1 / s)
L – inductance in Henry (H)
The coil inductance depends on the number of turns, the coil dimensions and the material filling the inside of the coil:
L – inductance in H (Henry, 1 H = V s / A)
μ – magnetic permeability inside the coil in V s / A m (with μ = μ0 * μrel)
n – number of turns
A – cross-sectional area in mm2
l – coil length in mm
The total resistance of the coil (impedance or apparent resistance) results from the vectorial addition of ohmic and inductive reactance. The value of the impedance is determined by:
Z – impedance in Ω (Ohm)
R – Ohmic resistance in Ω
XL – inductive reactance in Ω
The total voltage of the coil US is composed of the active voltage UR, which drops at the coil wire (real component) and the inductive reactive voltage UL (imaginary component due to induction). The magnitude of the coil voltage is calculated from the vectorial addition of both components:
US – Total voltage in V (volts)
UR – active voltage in V
UL – inductive reactive voltage in V
It is out of phase with the coil current I by the value ϕ.
The phase shift ϕ of a coil takes values between 0° and 90° and is calculated according to the following formula:
ϕ – phase shift in degrees (°)
R – Ohmic resistance in Ω
XL – inductive reactance in Ω
All the mentioned quantities are represented in the complex plane as follows:
* real quantities (R, UR and I) in the horizontal and
* imaginary sizes (XL and UL) in the vertical.
The phase shift ϕ is represented in the complex plane as the angle (clockwise) between the coil impedance Z and the ohmic resistance (effective resistance) R, or as the angle between the total voltage US and the current intensity I.
In order to make the range and possible applications of our sensors clear for our customers at first glance, we label our sensors in the following way in all presentations:
Electrical conductors are all materials that have mobile charge carriers (e.g. valence electrons in metals) that can conduct the electric current.
The electrical conductivity σ (sigma) is a material-specific parameter. It describes how well a material conducts electricity.
The reciprocal of the specific conductivity is the specific resistance ρ (rho). It expresses the resistance of a material to the flow of charge carriers.
To determine these material properties, the specimen geometry (length and cross-sectional area) and electrical quantities (voltage drop and current intensity or ohmic resistance) are linked:
σ – specific conductivity in S / m (Siemens / m, 1 m / Ω mm2 = 1 MS / m)
ρ – specific resistance in Ω mm2 / m
U – voltage drop in V (volts)
I – current intensity in A (amperes)
R – Ohmic resistance in Ω (Ohm)
l – length of the conductor in m
A – cross-sectional area of the conductor in mm2
In the Anglo-American area, the electrical conductivity is specified in the so-called IACS system.
The specific conductivity, and thus also the specific resistance, are temperature-dependent. In metals, the specific conductivity generally decreases with increasing temperature, since the increasing thermal motion of the atoms opposes the flow of the charge carriers with increased resistance.
Multiplexing (also called MUX or multiplex) is a method for serial transmission of multiple signals over just one signal line. In this process, all signals are transmitted seemingly simultaneously (i.e. at the same time); strictly speaking, however, they are interleaved with each other in time, as it were, “in tidbits”. The shared transmission channel is divided into time slices. Each signal is assigned such a time slice. For transmission, a multiplexer (MUX) switches one signal after the other onto the transmission link, each for the duration of a time slice. At the other end of the line, a demultiplexer (DEMUX) synchronously switches the transmitted signals to the associated receivers. A complete multiplex cycle consists of the sum of all time slices.
Parameter multiplexing in eddy current testing means that a sensor is operated sequentially with several test parameters. For this purpose, the system switches between the individual parameters in rapid succession, each of which is thus only effective for a very short duration. In this way, e.g. a multi-frequency test can be realized. Of course, other test parameters can also be switched over according to a defined time pattern, such as filters, thresholds or even various combinations of these.
The main advantages of parameter multiplexing in eddy current testing are significant time savings or reduction of hardware requirements compared to conventional methods. The mutual interference (crosstalk) between the test channels is also reduced compared to simultaneous testing.
The realization of a multi-frequency test offers further possibilities for signal evaluation compared to the single-frequency test. It thus provides a significant “more” of information and also leads to an increase in the reliability of test statements.
The alternating magnetic field generated by the eddy current coil propagates in the outer space of the coil and penetrates the volume of the electrically conductive test object. The maximum field strength and the maximum eddy current strength occur directly at the surface of the test object.
The eddy currents flowing directly at the surface are directed in the opposite direction to the coil current, i.e. 180° out of phase.
Due to the skin effect, on the one hand the eddy current strength is reduced in the depth direction. On the other hand, with increasing depth, an increasing phase shift of the eddy currents can be observed, i.e. an increasing time delay compared to the surface of the test object.
This phase shift of the eddy currents increases approximately linearly with depth. It can be calculated according to the following formula:
x – depth position (in mm)
δ – standard penetration depth (in mm)
β – phase shift of eddy currents (in °)
At the standard penetration depth δ, a phase shift of βδ = 57° occurs with respect to the eddy current profile at the surface. For two times the standard penetration depth, it is 114°, etc.
The phase shift of the flowing eddy currents provides important information about the test object, especially with regard to the nature of property changes and the depth of certain features.
It is specifically exploited in a special analysis procedure – phase evaluation – and is used, for example, in defect depth determination (preferably combined with multi-frequency testing).
When selecting the test frequency for eddy current testing, the specific requirements of the application and the sensor to be used must be taken into account. The frequency range recommended for the sensor should be taken from the manufacturer’s sensor data sheet.
The test frequency decisively determines the so-called penetration capacity, i.e. the distribution of the eddy current strength in the depth direction:
As the distance from the surface increases, the eddy current strength decreases significantly. The eddy currents are largely concentrated at the surface; they are shielded to a certain extent in the depth direction. This is also known as the skin effect. A measure of the depth-dependent drop in eddy current strength is the standard penetration depth.
The higher the test frequency, the greater the strength of the eddy currents generated at the test specimen surface (according to the law of induction). On the other hand, the eddy current strength decreases even faster in the depth direction with increasing test frequency (more pronounced skin or shielding effect). It should be mentioned in this context that the electrical conductivity and the relative permeability of the test object have the same influence on the distribution of the eddy current strength as the test frequency.
Therefore, both the test sensitivity and the interaction volume, i.e. the volume of the test object penetrated by eddy currents, can be specifically controlled with the frequency selection:
Here, high test frequencies generate strong eddy currents at the test specimen surface and provide excellent sensitivity for surface defects.
Low test frequencies, on the other hand, provide good sensitivity (detection capability) for subsurface defects (hidden defects) due to better penetration capability.
The choice of test frequency also affects the phase separation angle of defects of different depths. This is used, for example, when testing pipes with an internal flow sensor (also known as a “bobbin” probe) by means of phase evaluation:
At low test frequencies, the signal excursion directions (or signal phases) of different depth internal defects or different depth external defects hardly differ from each other.
As the test frequency increases, the separation angle of the errors of different depths increases. This corresponds to an improved resolving power in the depth direction. In this context, the electrical conductivity and the relative permeability of the test object have the same influence on the phase separation angle as the test frequency.
Segment sensors represent an intermediate stage of the two basic types of flow sensors and touch sensors. They do not completely enclose the test object, but usually cover a wide circumferential range between 90° and 180°. Their resolving power is roughly between that of a flow sensor and a touch sensor.
Sensor multiplexing in eddy current testing means that several sensors (or several test coils) are operated quasi-simultaneously with the same set of test parameters (test frequency, gain, phase and filter settings, thresholds, etc.). Strictly speaking, a multiplexer (MUX) is used to switch between the individual sensors in rapid succession. This means that each individual sensor is only operated for a short duration.
In this way, multiple sensors or sensor arrays can be operated very efficiently. For example, to scan a test surface two-dimensionally, it is only necessary to scan mechanically in one direction. The second dimension is covered by “virtual scanning” via multiplexer (electronic shifting in a fixed time grid).
Sensor multiplexing results in a considerable time saving compared to the single sensor method or a significant reduction in hardware expenditure compared to conventional testing technology (lower number of required test modules or test devices).
Compared to simultaneous multi-sensor testing, there is a reduction in mutual interference (crosstalk). The use of sensor arrays thus allows the inspection of even relatively large areas with high inspection sensitivity and spatial resolution in the shortest possible time.
The art of sensor development consists in the realization of a sensor setup that brings the required magnetic field (and thus the eddy current field) to the ‘test location’ in the workpiece with the required test frequency, in the optimal alignment and required strength, while always minimizing undesirable effects as much as possible. The basic principle is that the best test instrument can only obtain the information that the sensor system has also recorded. A ‘blind’ sensor does not allow sensitive testing. In addition to this basic sensitivity, the exact reproducibility of the sensors is a decisive factor.
A layer model can be used to illustrate the course of the eddy current strength in the depth direction.
Eddy currents are generated directly at the surface of the test object, which run in mirror image to the coil current and build up a magnetic field opposite to the coil field. According to Ohm’s law, effective losses occur in the test object, so that the opposing magnetic field is weaker than the exciting field of the coil. The superposition of both results in a weakened overall magnetic field; the magnetic field of the coil is practically strongly shielded in the depth direction.
This weakened magnetic field now generates lower eddy currents in the imaginary layer below, whose direction is reversed again. These eddy currents in turn act like a coil and induce eddy currents in the next layer down.
This process continues in the depth direction. In this way, with increasing distance from the surface, the excitation field penetrating the test object is more and more weakened and phase-shifted by the flow of eddy currents. The phase shift corresponds to the time delay resulting from the limited velocity of the flowing eddy currents.
The displacement of the magnetic field and the concentration of eddy currents on the outer surface of the test specimen are therefore referred to as the skin effect.
A measure of the decrease in eddy current density with increasing depth is the standard penetration depth. The time delay of the flowing eddy currents, which increases with the distance from the surface, leads to the depth-dependent phase shift of the eddy currents.
Low-pass filters are used to suppress interfering high-frequency signal components of the frequency spectrum, while leaving the frequencies below a lower cutoff frequency (i.e., the signals of interest) unaffected (see also bandwidth). This type of filter is also called a static filter because the frequency spectrum of the demodulated signal also contains a constant (static) component.
In eddy current testing, low-pass filters are used, for example, to suppress high-frequency electromagnetic interference, as well as electronic device noise.
It should be noted that the frequency spectrum of the interfering signals and the signals of interest depends on the current test speed as well as on the type and geometry of the sensor used:
1) The higher the test speed and the smaller the coil effective width, the higher the frequency of the signals (-> “shorter pulses”).
2) For low test speeds and for relatively large coil dimensions, the frequencies of the signals become correspondingly smaller (-> “longer pulses”).
The lower cut-off frequency of the low-pass filter is set correctly, i.e. high enough, when the higher frequency interfering signals are effectively suppressed, but the signals of interest are still displayed with maximum signal height.
The minimum lower cutoff frequency fTPmin for the low-pass filter can be approximated using the following formula:
fTPmin > vtest / Bw( where: vtest = test speed andBW = coil effective width).
Eddy current testing can be used to check objects made of electrically conductive materials for integrity, composition and quenched and tempered condition, or for geometric dimensions. Eddy current testing is based on the physics of electromagnetic fields.
A coil through which alternating current flows forms an alternating (primary) magnetic field in its environment. This generates currents on the surface of an electrically conductive test object. These so-called “eddy currents” flow parallel to the coil windings, but in the opposite direction to the coil current. Therefore, they generate a (secondary) alternating magnetic field which is directed in the opposite direction to the magnetic field of the coil. This eventually results in a weakening of the coil’s magnetic field. This can be measured as a change in the coil’s AC resistance (impedance).
On the basis of the variations of the coil impedance, the properties of the test object (if they influence the expression of eddy currents) can thus be incl. possible defects are detected and characterized. This requires certain analysis methods, e.g. magnitude or phase evaluation, waveform evaluation or harmonic analysis.
Using reference parts, which should have certain properties in terms of geometry (dimensions, shape), material characteristics (electrical conductivity, permeability, hardness) and material defects (defects), the eddy current tester must be set up (test frequency, gain, phase setting, filter settings, etc.) before starting the test.
To ensure reliable test results, care must be taken to maintain the test conditions during the test (e.g. constant sensor spacing and constant test speed). Any interference (e.g. mechanical vibrations, temperature fluctuations or electromagnetic interference fields) should also be excluded or minimized as far as practicable.
Another important prerequisite for successful eddy current testing is the selection or development of a suitable sensor, i.e. the number and arrangement of the coils used, the type of their electrical interconnection, their dimensions, number of turns, and, if necessary, a magnetic core or shielding.
Due to the “skin effect”, the strongest eddy currents are formed at the surface of the test object; their strength decreases rapidly with increasing distance from the surface. Therefore, eddy current testing is to be classified as a surface method, which in principle can be used for all electrically conductive materials.
Since the characteristics of the eddy currents are influenced by numerous properties of the test object, there are many fields of application for eddy current testing (e.g. testing for material defects, wall thickness determination, measurement of material characteristics for the purpose of sorting, measurement of layer thicknesses, etc.).
Compared to other non-destructive testing methods, the eddy current method is characterized by the following advantages:
– non-contact,
– no surface preparation and post-processing necessary,
– no coupling agent required,
– high test speeds possible (up to several m/s).
This makes it ideally suited for use in automatic testing systems.
A current-carrying electrical conductor is surrounded by a circular magnetic field (vortex field). If the straight conductor wire is now wound into a circular conductor loop, the vortex-like field lines overlap in such a way that they form a magnetic dipole (with north/south pole structure). The strength of the magnetic field generated can be increased by winding coils, such as those used as sensor elements in eddy current testing, with a larger number of turns. As the coil length increases, its magnetic field becomes more and more similar to that of a bar-shaped permanent magnet.
The magnetic field outside the coil penetrates the electrically conductive test object. Since an alternating current flows through the coil, circular currents are induced in the area near the surface of the test specimen, which are also known as eddy currents. These eddy currents run in the opposite direction to the coil current; in a sense, they can be seen as a mirror image of the coil current. The flowing eddy currents are in turn surrounded by a magnetic vortex field.
In a defect-free test specimen (homogeneous material), the eddy currents can propagate unhindered.
The magnetic field generated by the eddy current is also characterized by a dipole structure. This so-called secondary magnetic field is directed in the opposite direction to the primary magnetic field of the coil. The superposition of both magnetic fields leads to a resulting magnetic field which has a lower field strength compared to the primary magnetic field of the coil.
If local defects (e.g. cracks, corrosion scars, pores, non-metallic inclusions or similar) occur in the test object, the eddy currents can no longer flow unhindered. Such inhomogeneities represent an insurmountable obstacle, so to speak. The eddy currents have to escape laterally and/or in the depth direction and therefore experience weakening. Consequently, the surrounding magnetic vortex field is also weakened. The reduced magnetic opposing effects on the primary magnetic field of the coil lead to a change in the resulting magnetic field compared to the fault-free test specimen.
The strength of the magnetic field under the influence of an electrically conductive test object can be detected by suitable sensors (receiving coils), subsequently evaluated and displayed in a suitable manner. This allows conclusions to be drawn about the properties of the test specimen, e.g. with regard to geometry, dimensions, material characteristics and the presence of local defects.