The Eddy Current Inspection Method

Insight Vol 46 No 5 May 2004 279

ScopeThis article is only intended as a brief familiarisation with the eddy current inspection method. It is not intended as a complete theoretical course, and a number of aspects are intentionally simplified. The bibliography at the end of this series will document details several appropriate reference books which should be consulted if a more complete understanding is required. The text is based on various documents that have existed in Hocking NDT Ltd for many years but has been substantially revised for this article.

This feature will appear in four parts namely; Part 1. History and electrical theory Part 2. The impedance plane and probes Part 3. Instrumentation Part 4. Applications, practical testing and advanced concepts

Historical perspectiveEddy current testing has its origins with Michael Faraday’s discovery of electromagnetic induction in 1831. In 1879 Hughes recorded changes in the properties of a coil when placed in contact with metals of different conductivity and permeability, but it was not until the Second World War that these effects were put to practical use for testing materials. Much work was done in the 1950s and 60s, particularly in the aircraft and nuclear industries, and eddy current testing is now an accurate, widely used and well-understood inspection technique.

Basic electrical theoryWhen a voltage is applied to a circuit containing only resistive elements, current flows according to Ohm’s Law:

I = V/R or V = I.R

If a circuit consists of more than one element, the overall voltages, resistance and capacitance can be calculated by simple algebra, for example, with two resistors in series, as show in Figure 1, current (I) must be the same for both resistors, so:

V1 =I.R1, V2=I.R2,

Vtot = V1+V2 = I.R1+ I.R2 = I (R1+R2) = I.Rtot

So Rtot = R1+ R2.

Electromagnetic inductionIn 1824 Oersted discovered that current passing though a coil created a magnetic field capable of shifting a compass needle. Seven years later, Faraday and Henry discovered the opposite: that a moving magnetic field would induce a voltage in an electrical conductor.

The two effects can be shown in a simple transformer connected to a DC supply as shown in Figure 2.

The meter needle will deflect one way when current is applied, then back the other way when it is removed. A voltage is only induced when the magnetic field is changing.

Such a voltage is also induced in the first winding, and will tend to oppose the change in the applied voltage.

The induced voltage is proportional to the rate of change of current:

€

didt

A property of the coil called inductance (L) is defined, such

that:

€

Induced voltage = Ldidt

If an AC current flows through an inductor, the voltage across

the inductor will be at maximum when the rate of change of current is greatest. For a sinusoidal waveform, this is at the point where the actual current is zero. See Figure 3.

Thus the voltage applied to an inductor reaches its maximum value a quarter-cycle before the current does - the voltage is said to lead the current by 90 degrees.

The value of the voltage and current can be calculated from the formula:

V = I.XL

where XL is the inductive reactance, defined by the formula:XL = 2πf L

where f is the frequency in Hz.As we saw above, for series DC circuits calculation of total

resistance is simply a matter of adding the individual resistance values.

BACK TO BASICS

The eddy current inspection methodPart 1. History and electrical theory

J Hansen

John Hansen, Hocking NDT Ltd, Inspec House, 129-135 Camp Road,St Albans, Herts AL1 5HL. UK. Tel: 01727 795509; Fax: 01727 795409; E-mail: [email protected]; web: www.hocking.com

Figure 1. Circuit containing only resistive elements

Figure 2. Simple transformer


For an AC circuit it is not so simple, but the same basic principles apply: the current through both elements must be the same, and at any instant the total voltage across the circuit is the sum of the values across the elements (see Figure 4). However, the maximum voltage across the resistance coincides with zero voltage across the inductor and vice versa - see Figure 5.

We can represent this graphically using a vector diagram, as shown in Figure 6.

The impedance of the circuit is therefore given by the formula:

€

Z = XL2

+ R2( )

Total resistance in AC circuits - IMPEDANCEThe phase angle between voltage and current is given by:

€

Θ = sin−1 XLZ

TheorySimple coil above a metal surface

When an AC current flows in a coil in close proximity to a conducting surface (see Figure 7) the magnetic field of the coil will induce circulating (eddy) currents in that surface. The magnitude

and phase of the eddy currents will affect the loading on the coil and thus its impedance.

As an example, assume that there is a deep crack in the surface immediately underneath the coil (see Figure 8).

This will interrupt or reduce the eddy current flow, thus decreasing the loading on the coil and increasing its effective impedance.

This is the basis of eddy current testing. By monitoring the voltage across the coil (Figure 9) in such an arrangement we can detect changes in the material of interest.

Figure 3. Sinusoidal waveforms of voltage and current in an inductive circuit

Figure 4. Circuit containing resistive and inductive elements

Figure 5. Sinusoidal waveforms of voltage and current in an AC circuit containing resistive and inductive elements

Figure 6. Vector diagram

Figure 7. Induction of eddy currents

Figure 8. Eddy currents are affected by the presence of a crack

Figure 9. Circuit showing the basis of eddy current testing

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Note that cracks must interrupt the surface eddy current flow to be detected. Cracks lying parallel to the current path will not cause any significant interruption and may not be detected (see Figure 10).

Factors affecting eddy current response

A number of factors, apart from flaws, will affect the eddy current response from a probe. Successful assessment of flaws or any of these factors relies on holding the others constant, or somehow eliminating their effect on the results. It is this elimination of undesired response that forms the basis of much of the technology of eddy current inspection. The main factors are:

Material conductivity The conductivity of a material has a very direct effect on the eddy current flow: the greater the conductivity of a material, the greater the flow of eddy currents on the surface. Conductivity is often measured by an eddy current technique, and inferences can then be drawn about the different factors affecting conductivity, such as material composition, heat treatment, work hardening etc.

PermeabilityThis may be described as the ease with which a material can be magnetised. For non-ferrous metals such as copper, brass, aluminium etc., and for austenitic stainless steels the permeability is the same as that of ‘free space’, ie the relative permeability (μr) is one. For ferrous metals however the value of μr may be several hundred, and this has a very significant influence on the eddy current response, in addition it is not uncommon for the permeability to vary greatly within a metal part due to localised stresses, heating effects etc.

FrequencyAs we will discuss, eddy current response is greatly affected by the test frequency chosen, fortunately this is one property we can control.

GeometryIn a real part, for example one which is not flat or of infinite size, geometrical features such as curvature, edges, grooves etc. will

exist and will effect the eddy current response. Test techniques must recognise this, for example in testing an edge for cracks the probe will normally be moved along parallel to the edge so that small changes may be easily seen. Where the material thickness is less than the effective depth of penetration (see below) this will also effect the eddy current response.

Proximity/lift-off

The closer a probe coil is to the surface the greater will be the effect on that coil. This has two main effects:q The ‘lift-off’ signal as the probe is moved on and off the

surface.q A reduction in sensitivity as the coil to product spacing

increases.

Depth of penetration

The eddy current density, and thus the strength of the response from a flaw, is greatest on the surface of the metal being tested and declines with depth. It is mathematically convenient to define the ‘standard depth of penetration’ where the eddy current is 1/e (37%) of its surface value (see Figure 11).

The standard depth of penetration in mm is given by the formula:

€

δ = 50 ρ( f .μr )

Where:ρ is resistivity in mΩ.cm (ρ = 172.41/ material conductivity)f is frequency in Hzμr is the relative permeability of the material, for non ferrous = 1

from this it can be seen that depth of penetration: q Decreases with an increase in frequencyq Decreases with an increase in conductivityq Decreases with an increase in permeability – this can be very

significant – penetration into ferrous materials at practical frequencies is very small.The graph in Figure 12 shows the effect of frequency on

standard depth of penetration.

It is also common to talk about the ‘effective depth of penetration’ usually defined as three times the standard depth, where eddy current density has fallen to around 5% of its surface value. This is the depth at which there is considered to be no influence on the eddy current field.

Next month’s article in this series will deal with the impedance plane and probes.

Figure 10. Cracks must interrupt the surface eddy current flow to be detected

Figure 11. Standard depth of penetration

Figure 12. The effect of frequency on standard depth of penetration

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BACK TO BASICS

The eddy current inspection methodPart 2. The impedance plane and probes

J Hansen


The impedance planeEddy current responses of a single coil may be conveniently described by reference to the ‘impedance plane’. This is a graphical representation of the complex probe impedance where the abscissa (X value) represents the resistance and the ordinate (Y value) represents the inductive reactance – see Figure 13.

Note that, while the general form of the impedance plane remains the same, the details are unique for a particular probe and frequency.

The display of a typical CRT eddy current instrument represents a ‘window’ into the impedance plane, which can be rotated and ‘zoomed’ to suit the needs of the application.

For example, in the impedance plane diagram in Figure 13 a rotated detail of the ‘probe on aluminium’ area would appear as Figure 14.

This shows the display when moving over a series of simulated cracks of varying depths. Note that in the example shown, both the amplitude and the phase of response from the different-sized cracks varies.

Coil configurationsAppropriate coil selection is the most important part of solving an eddy current application; no instrument can achieve much if it does not get the right signals from the probe.

Coil designs can be split into three main groups: q Surface probes used mostly with the probe axis normal to the

surface. In addition to the basic ‘pancake’ coil, this includes pencil probes and special-purpose surface probes such as those used inside a fastener hole.

q Encircling coils that normally used for in-line inspection of round products. The product to be tested is inserted though a circular coil.

q ID probes are normally used for in-service inspection of heat exchangers. The probe is inserted into the tube. Normally, ID probes are wound with the coil axis along the centre of the tube.

Figure 13. Impedance plane diagram

Figure 14. Typical eddy current instrument and its display

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These categories are not exhaustive and there are obviously overlaps, for example between non-circumferential wound ID probes and internal surface probes.

Up to this point we have only discussed eddy current probes consisting of a single coil. These are commonly used in many applications and are commonly known as absolute probes because they give an ‘absolute’ value of the condition at the test point.

Absolute probes are very good for metal sorting and detection of cracks in many situations, however they are sensitive also to material variations, temperature changes etc. Absolute probes are usually circular and thus respond uniformly to defects regardless of orientation.

Another commonly used probe type is the ‘differential’ probe this has two sensing elements looking at different areas of the material being tested. The instrument responds to the difference between the eddy current conditions at the two points.

Differential probes are particularly good for detection of small defects, and are relatively unaffected by lift-off (although the sensitivity is reduced in just the same way), temperature changes and (assuming the instrument circuitry operates in a ‘balanced’ configuration) external interference. However, differential probes are directionally sensitive with a null occurring when both sensors pass over an identical area.

Figure 15 shows a typical response from a differential probe. Note the characteristic ‘figure of eight’ response, as first one

probe element, then the other, move over the defect. In general, the closer the element spacing, the wider the ‘loop’ in the signal, which is caused by the field from when element interfering with that of the other element.

Lift-off should be cancelled out assuming that the probe is perfectly balanced, but there will still be a ‘wobble’ response as the probe is moved and tilted slightly.

Reflection, or driver pick-up probes have a primary winding driven from the oscillator and one or more sensor windings connected to the measurement circuit. Depending on the configuration of the sensor windings, reflection probes may give a response equivalent to either an absolute or differential probe – see Figure 16.

Main advantages of reflection probes are: q Driver and pick-up coils can be separately optimised for their

intended purpose.q Wider frequency range than equivalent bridge connected

probes.q The larger driver coil gives a more even field, resulting in better

penetration and lift-off characteristics.All the coils in a probe need not necessarily be wound together

but may be separate elements for instance, such as:q in sliding probes (with separate transmit and receive elements)

which achieves wider coverage and hence more rapid scansq the Hocking FastScan probe, where a large drive coil is used

to produce a large field and four small coils are configured differentially to give a high signal-to-noise.

Typical coil connectionsModern eddy current equipment now uses two methods for coil connection, namely bridge or reflection.

Bridge

Also known somewhat confusingly as differential. See Figure 17.The two coils (differential or absolute plus balancing coil) form

the ‘legs’ of a bridge. When the bridge is balanced the measured voltage will be zero. Any change in the condition of either coil will result in an unbalanced bridge, the degree of imbalance corresponds to the change in coil impedance. When one coil is used as a sensor and the other acts as a balancing load (and may be positioned in the instrument or in the probe away from the test surface) this is an absolute probe in response but differentially connected.

Reflection

Also known as driver pick-up, transmit receive or transformer connected. See Figure 18.

Here the coils are connected with one coil acting as a drive (or transmitter) and one or two coils acting as pick-ups (or receivers).

As can be seen the essential elements are the same for a driver pick-up configuration as for a bridge, the necessary changes can be achieved by simple switching or probe connection changes. In fact, by connection of two resistors and providing the amplifier input is differential, then a reflection configuration can be made into a bridge connection.

Next month’s article in this series will deal with instrumentation and applications.

Figure 15. Differential probe

Figure 16. Reflection probe configurations

Figure 17. Bridge-type coil connection

Figure 18. Reflection-type coil connection

BACK TO BASICS

The eddy current inspection methodPart 3. Instrumentation and applications

J Hansen


Basic instrumentationThere are a number of basic groups of eddy current instrumentation.

These may be categorised as:q Meter display instrumentsq Portable impedance plane instrumentsq Tube testing instrumentsq Array Systemsq Automation systemsq Measuring equipment

Although it is easy to categorise instrumentation the boundaries are typically very fuzzy.

Meter display instruments

Simple easy-to-use equipment, operating at a restricted number of fixed frequencies – typically several hundred kHz – with a meter or bar-graph display. Only suitable for surface crack detection and simple sorting applications.

The controls are usually restricted to a gain control (a potentiometer) for sensitivity adjustment, coarse frequency selection switch, material selection switch and a balance control pushbutton.

The balance control normally gives some means of compensating for lift-off (for example phase rotation and/or fine frequency adjustment) so that the meter is most sensitive

to crack-like indications. An alarm threshold is usually included to provide audible and visual output that an indication has occurred.

The information presented to the operator is, by definition, extremely simple. The use of mechanical meters means that response to defects is very slow as the meter requires time to respond to rapid changes.

Typical frequencies used are:q 200 to 500 kHz for general purpose crack detectionq 2 MHz for detection of small (0.1 mm deep) cracks in

aluminiumq 6 MHz for crack detection in low conductivity materials

particularly titanium.This type of instrument has now been almost entirely superseded

by impedance plane instruments.

Portable impedance plane eddy current flaw detectors

The availability of low cost graphic displays and improvements in battery technology has meant the widespread availability of

cost-effective low weight battery-operated digital impedance plane instruments.

Such instruments give a real impedance plane display on a CRT (see Figure 20) or, now more commonly, electronic display (LCD, electroluminescent, etc). Generally these have comprehensive capabilities: wide frequency ranges from around a hundred Hertz to several mega-Hertz, extensive alarm facilities, general purpose units may have signal filtering. It is now common for instruments to offer dual frequency operation, allowing the combination of signals at two or more test frequencies (termed mixing) in order to reduce or eliminate specific interfering effects.

Low current consumption displays and higher energy density batteries have led to extremely portable units with long battery lives. At first battery operation for this type of instrument was by using nickel cadmium (NiCd) batteries. NiCd suffers from the memory effect, which is problematic in the typical inspection industry usage patterns. Modern Lithium Ion (Li-Ion) batteries have no memory effect and much greater charge for weight density. The charging is necessarily more complex but brings advantages in rapid charging and no disadvantage in top-up charging.

Microprocessor control has enabled easy use through menus and user-definable softkeys with internal storage of settings and test data. Connection of instruments to a PC gives the ability to; back-up settings and test results, copy information to other PC applications (for example word processor or database) and transfer of inspection procedures from one instrument to another.

These features are contained in an ergonomic case weighing under 1 kg including the battery.

Typical applications include surface inspection, but now the impedance plane display adds the ability to perform more complex inspections, such as weld inspection, whereas the frequency range, which can be as low as 10 Hz, enables corrosion and sub surface defect inspection to be performed.

At the top end of the range for this type of equipment, manufacturers add the capability to measure electrical conductivity on non-magnetic material and support for rotary drives. Rotary

Figure 19. Typical meter display instrument

Figure 20. Portable impedance plane eddy current flaw detector

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drives enable rapid and reliable inspection of holes (rotating at up to 3000 rpm). The main application for this equipment is in in-service inspection of aircraft. Such instruments provide the three major applications for eddy current in aircraft inspection all in one convenient package. A rotary system, apart from requiring the capability to drive the motor in the drive, also must have high and low pass signal filters that can be set extremely precisely to optimise the signal to noise ratio and thus enable the reliable detection of extremely small defects.

Tube testing instruments

These are used almost exclusively to test heat exchanger tubing with internal diameter probes. Typical equipment will test simultaneously at four frequencies using differential bobbin probes. As a differential test is only sensitive to short defects, then the test is repeated in absolute mode for all frequencies.

Figure 21 shows a typical response obtained in the differential mode. By testing at four suitably chosen frequencies, signals from ferrous support plates, ferrous inclusions and dents may be suppressed whilst defects may be characterised such as corrosion pitting and cracks. This method is widely used because of the speed of test (1 to 2 m/s), ability to automate (for instance in remote pressurised water reactor steam generator inspection) and lack of any other NDT technique that offers any significant advantages. Vast amounts of data are produced leading to problems with data handling and reliable analysis. Computerised analysis is the answer but not widely used at present.

Remote field inspection is a method developed for inspecting carbon steel tubes from the inner diameter. The probe is a transmit receive probe with the driver and receiver spaced approximately three diameters apart. Over this distance, the near field is attenuated and a field that takes a path through the tube thickness along the outer surface and then returns to the bore. Signal amplitude is related to defect volume and phase to depth (as in conventional eddy current inspection), however there is no discrimination between internal and external defects. As carbon, steel tubes typically become heavily corroded during use and still be within their design tolerances it is not practical to detect small defects due to the high background noise levels.

Array systems

An array system tests simultaneously with numerous identical eddy current probe elements, for practical reasons varying in size from 8 to 128 elements. The advantage is that instead of scanning the probe mechanically over a surface, multiple elements uniformly spaced scan one width of the surface giving near equal sensitivity over a surface profile. The elements are electronically scanned (multiplexed). One disadvantage is cost because the probe and its associated electronics are necessarily complex; if the component shape or size changes then a new probe is required and currently the element size means that the transverse resolution is very coarse when compared with a conventional mechanical scan. This means that inspection is usually carried out on high value components such as aircraft engine disk inspection.

Industrial automation systems

Eddy current inspection provides a simple way of inspecting components for surface-breaking defects. High surface scanning speed, typically two metres per second, coupled with non-contacting probes means that rotationally symmetrical components (such as bearings, bolts, ball joints, cylinder liners, gudgeon pins, clutch plates, wheel axles etc.) may be inspected during the manufacturing process.

These are intended for factory operation, often in automatic or semi-automatic inspection machines. Generally, they are similar in operation to impedance plane portables but usually have extensive input and output facilities such as relays and photocell inputs. Emphasis is not on low power consumption, portability and weight. The capability to mount the equipment in an industrial environment is important. The systems may be custom built for a specific purpose, in which case features not needed for the intended application are often omitted.

One example of a standard system is for aircraft wheel half inspection (see Figure 22). Aircraft wheels are subject to high stresses during landing and take-off, and are inspected at each tyre change or heavy landing. Wheel halves are coated to prevent corrosion and not having to remove this coating to inspect is a major cost saving. With an automated system, an inspection can take as little as 30 seconds to inspect over a path length of 250 m. The probe used is an absolute probe. Data presentation is in the form of a chart record showing a defect channel and a lift-off/signal coupling channel.

A bar-code scanner is used to automate test set-up selection and all data may be stored to a local hard disk. A chart recorder is included to provide a paper test result. Test reporting may also be paperless and the system attached to a network giving the ability to view test results remotely. This unit uses a standard hand held

Figure 21. ID probe and typical impedance plane traces from a calibration standard

Figure 22. Automated system for aircraft wheel inspection

Figure 23. AutoSigma conductivity meter

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eddy current impedance plane instrument that may be detached for use in manual inspection.

Eddy current systems may also be used for determining heat treatment status of metals using a comparative test.

Special-purpose measuring equipment

A number of instruments use the eddy current principle to provide actual numeric data, examples are:q Coating thickness metersq Conductivity meters (AutoSigma 3000). q AC field crack depth metersq AC field stress measurement

Generally these are designed to give a digital readout without demanding interpretation of an indication (see Figure 23). Conductivity meters find applications during metal manufacturing for quality control of non-ferrous alloys and heat treatment and in service for heat damage assessment.

Next month’s concluding article in this series will discuss applications, considerations in setting up an instrument and a bibliography.

Enquiry No 407-11

Enquiry No 407-12416 Insight Vol 46 No 7 July 2004

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BACK TO BASICS

The eddy current inspection methodPart 4. Applications, practical testing and advanced concepts

J Hansen


ApplicationsEddy current applications encompass a wide range of capabilities. Here is given a summary of some of these and a discussion about their practical requirements. If we refer to part one of this series we will find the standard equation for calculating the effective depth of penetration. This equation has parameters for resistivity, test frequency, relative magnetic permeability and depth of penetration. The change in any one of these parameters gives the basis for an eddy current. Figure 24 illustrates some of the applications possible.

Defect detectionSurface crack detection

This is normally carried out with pencil probes or ‘pancake’ type probes on ferrous or non-ferrous metals. Frequencies from 100 kHz to a few MHz are commonly used. Depending on surface condition it is usually possible to find cracks 0.1 mm or less in depth. Shielded probes, with their focused field, add the ability to test very close to edges or dissimilar materials such as ferrous fasteners in an aluminium structure.

Differential probes are sometimes used, particularly in automated applications, but care must be taken to ensure that the orientation of flaws is correct for detection.

Sub-surface crack/corrosion detection

This is primarily used in airframe inspection. By using a low frequency and a suitable probe, eddy currents can penetrate aluminium or similar structures to a depth of 10 mm or so, allowing the detection of second and third layer cracking, which is invisible from the surface, or thinning of any of the different layers making up the structure.

Test frequencies are generally in the range 100 Hz to 10 kHz. Probe size should also be two or more times wider than the depth of penetration required.

Material sortingNon-ferrous metal sorting

This is conductivity testing, and for dedicated applications a conductivity meter may be a better choice. From the impedance plane diagram one can observe that the indication from a conductivity change is essentially the same as from a crack, and both meter and impedance plane type crack detectors can be successfully used to sort similar metals using a suitable absolute probe. It should be remembered that:q widely different metals may be a similar conductivity;q the allowable values for similar alloys may overlap;q an alloy of one material can, in vastly different states of heat

treatment, have the same electrical conductivity; andq there is no direct relationship between conductivity and

hardness.However, once these caveats are understood then conductivity

measurement can be used as part of a quality control system. Suitable test frequencies used are in the range 10 kHz to 2 MHz, although account should be taken of the material thickness to ensure the depth of penetration is less than one third of the material thickness.

Ferrous metal sorting

Ferrous material may be sorted using eddy current impedance plane equipment. Unfortunately it is not possible to produce quantitative values due to the reading obtained being related to electrical conductivity, magnetic permeability and the depth of the change in material properties. Frequencies to use are 100 Hz to 10 kHz. The use of two or more frequencies gives additional information about the depth of the material properties such as in induction hardening.

Coating thickness assessment

In the simple case of a non-conductive coating (for example paint) on a conductive material, then the eddy current lift of signal can be used. The probe type to be used should have an absolute response with reflection spot face probes offering some advantage in temperature stability and frequency range. Higher frequencies are preferred (100 kHz and higher) and for non-ferrous materials it should be checked that the frequency is sufficiently high so as not to be influenced by material thickness (say 10 times that to make the wall thickness equal the effective depth of penetration).

To obtain quantitative readings, a calibration piece with several different thickness of coating in the range of interest is essential, and a calibration curve created.

Some instruments have an intrinsic function as part of conductivity measurement for obtaining direct readings.

For the more complex case where the coating is conductive, then the following needs to be taken into account. The two materials must have different conductivities and/or relative permeabilities and the top coating must be non-magnetic. Choose a frequency that will make the effective depth of penetration equal the nominal

Figure 24. Eddy current applications


wall thickness. If the surface coating has higher resistivity than the lower coating, then by using a frequency that is sufficiently low to penetrate the surface coating, results will be similar to that for non-conductive coatings.

Wall thickness assessment

This is possible in the same way as it is possible to determine non-ferrous conductive coating thickness and the same rules apply about choice of frequency.

Tube inspectionTubes may be inspected from the outer diameter (OD), usually at the time of manufacture and from the inner diameter (ID), usually for in-service inspection, particularly for heat exchanger inspection.

ID heat exchanger tube testing

Heat exchangers used for petrochemical or power generation applications may have many thousands of tubes, each up to 20 m long. Using a differential Internal Diameter (ID or ‘bobbin’) probe, these tubes can be tested at high speed (up to 1 m/s with computerised data analysis) and by using phase analysis, defects such as pitting can be assessed to an accuracy of about 5% of tube wall thickness. This allows accurate estimation of the remaining life of the tube, allowing operators to decide on appropriate action such as tube plugging, tube replacement or replacement of the complete heat exchanger.

The operating frequency is determined by the tube material and wall thickness, ranging from a few kHz for thick-walled copper tube, up to around 600 kHz for thin-walled titanium. Tubes up to around 50 mm diameter are commonly inspected with this technique. Inspection of ferrous or magnetic stainless steel tubes is not possible using standard eddy current inspection equipment.

Dual or multiple frequency inspections are commonly used for tubing inspection, in particular for suppression of unwanted responses due to tube support plates. By subtracting the result of a lower frequency test (which gives a proportionately greater response from the support) a mixed signal is produced showing little or no support plate indication, thus allowing the assessment of small defects in this area. Further frequencies may be mixed to reduce noise from the internal surface.

Remote field

Remote field is a branch of eddy current testing that has evolved over the last decade or so. By using specially designed equipment and ID probes it is possible to obtain indications of wall thickness changes on magnetic material.

In-line inspection of tubing

External eddy current encircling test coils are commonly used for inspecting high quality metal tubing of wall thicknesses less than 6 mm. When the tube is made of a magnetic material there are two main problems: q Because of the high permeability, there is little or no

penetration of the eddy current field into the tube at practical test frequencies.

q Variations in permeability (from many causes) cause eddy current responses which are orders of magnitude greater than those from defects.These problems can be overcome by magnetising the tube using

a strong DC field. This reduces the effective permeability to a low value, thus increasing the depth of penetration and masking the permeability variations, hence allowing effective testing.

Ferromagnetic tubing up to around 170 mm diameter are commonly tested using magnetic saturation and encircling coils.

Testing may be in-line during manufacture or off line on cut length tube.

When tubes are welded (usually by the ERW method) the weld area is the usual site of defects and as the weld position is well controlled, it is more efficient to inspect the weld area only by means of a sector (or saddle) probe.

Ferrous weld inspection

The geometry and heat-induced material variations around welds in steel would normally prevent inspection with a conventional eddy current probe, however a special purpose ‘WeldScan’ probe has been developed which allows inspection of welded steel structures for fatigue-induced cracking. The technique is particularly useful as it may be used in adverse conditions, or even underwater, and will operate through paint and other corrosion-prevention coatings. Cracks around 1 mm deep and 6 mm long can be found in typical welds both in the root area and the cap.

Dynamic hole inspection

Here, differential probes are used attached to high-speed rotary scanners with test speeds as high as 3000 rev/min then the inner bore of holes may be inspected rapidly and reliably with the eddy current technique. Probes may be as small as 1 mm diameter and test frequencies used follow the same rules as for surface defect detection. The use of high- and low-pass filters (so called band-pass filters) is essential to ensure optimum signal to noise. Target calibration notch is usually a 0.5 mm corner notch at 45º.

Practical testingAny practical eddy current test will require the following: q A suitable probe.q An instrument with the necessary capabilities.q A good idea of size, location and type of the flaws it is desired

to find.q A knowledge of the material conductivity and whether it is

magnetic or not.q A suitable test standard to set up the equipment and verify

correct operation.q A procedure or accept/reject criteria based on the above.q The necessary operator expertise to understand and interpret the

results.

Operating frequency

Selection of operating frequency is the primary eddy current test parameter under operator control. Frequency selection affects both the relative strength of response from different flaws and the phase relationship. Thus, selection of operating frequency is very important in obtaining good resolution of flaw signals in the presence of other variables which may affect the test.

Instrument set-up

While the precise details of setting up an instrument will vary depending on the type and application, the general procedure is usually the same. Once the application has been tested the required values for many test parameters will be known, at least approximately. 1. Connect up the appropriate probe and set any instrument

configuration parameters (mode of operation, display type etc.).

2. Set the frequency as required for the test.3. Set gain to an intermediate value, for example 40 dB. 4. Move the probe on/over the calibration test-piece and set phase

rotation as desired (for example lift-off or wobble horizontal on a phase plane display). It may help the stability of the readings to attenuate the horizontal (x axis) gain by 12 dB (1⁄4 of the vertical gain).

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5. Move over the defects and adjust gain (and horizontal/vertical gain ratio if fitted) to obtain the desired trace size/meter indication. It may be necessary to re-balance after changing gain.

6. Further optimise phase rotation as required by setting the dominant source of noise whilst scanning the probe in the horizontal axis.

7. Use filters etc. to further optimise signal-to-noise ratio (see below).

8. Set alarms etc. as required.9. Run over the calibration test-piece again and verify that all

flaws are clearly detected. 10. Perform the test, verifying correct operation at regular intervals

using the calibration test piece.

Use of filtering

Searching for defects in an eddy current test conventionally implies probe movement. So when indications are detected then, due to the probe size, these will vary with time in a way which is fairly consistent (assuming that the probe movement speed is reasonably constant). As a result of this speed and the probe size, defects have a characteristic frequency of response (probe width divided by probe speed).

For example, if an absolute probe with diameter 2 mm moves over a narrow crack at a speed of 1 m/s the resulting indication will last for approximately 2 ms. If the material composition, thickness or probe lift-off is also varying gradually, the indication from this will change much more slowly. Therefore, a high-pass filter set to a frequency around 100 Hz or so will pass the rapidly changing signal from the defect but not the slowly varying changes. Further rapidly varying signals such as electronic noise or noise caused by surface roughness may be reduced by low-pass filtering. It is good practice to ensure that the low-pass filter is set sufficiently low to

ensure the test signal displays the lowest amount of high-frequency noise but high enough to ensure that the smallest target defect is not attenuated by the filter (see Figure 25).

Advanced conceptsIn this section, a few of the advanced testing concepts are outlined.

Simultaneous use of multiple frequencies

Choice of frequency determines how well surface and subsurface defects may be detected. By using more than one frequency it is possible to achieve both good detection of surface defects and sub-surface defects. Further, generally the more information that is available from a test, the easier it is to categorise difficult-to-interpret defects (for example ferrous inclusions in non-ferrous material).

Mixing of signals from a test at two frequencies allows unwanted signals to be suppresses, for example support plate signals in heat exchanger inspection.

Simultaneous use of absolute and differential tests

Differential testing is excellent for finding small defects but can be poor at detecting very large defects.

By testing simultaneously in absolute and differential then it is possible to preserve good sensitivity to small defects (for example cracks, small pits) in the differential channel and large defects (for example corrosion, material property changes) in the absolute channel.

The risk in both simultaneous testing modes is that the data becomes more complex to analyse whilst not greatly improving the reliability of defect detection.

Spatial considerations in selecting probes

Probe geometry has an influence on the efficiency of an eddy current test.

Smaller probe elements will give better signals from smaller defects. Shielded probes will further improve this.Figure 25. Use of high- and low-pass filters

Figure 26. Illustration of mixing

A mix exploits the changes in phase separation and amplitude that occur when testing at different frequencies on an unwanted signal. By suitable manipulation of the phase and x/y gain it is possible to minimise the signal from the unwanted signal, after subtraction of the two frequencies whilst maintaining sensitivity to wanted signals.


Larger probes will allow the eddy current signal to penetrate more deeply (probe diameter should be typically two times higher or more than the depth of the material to be penetrated. Note this means that choice of frequency is not the only consideration in determining depth of signal penetration.

Noise sources and how to them minimise

There are numerous noise sources in eddy current testing but they may be summarised as follows:q Intrinsic electronic noise from the instrument electronicsq External electronic noise also known as electromagnetic

interferenceq Noise from the material, in that noise can be defined as any

signal that may obscure the signal that it is required to detect.A probe that is well matched to the instrument being used will

produce a better signal-to-noise ratio (NOTE: signal-to-noise ratio is the ratio of the wanted to unwanted component, usually expressed in dB). Further larger probes will be better at averaging out noise caused by surface roughness and other small variations.

By ensuring that the probe drive signal is as high as possible, then both forms of electronic noise may be minimised. The probe type and whether the instrument electronics become saturated will limit this.

External electronic noise is also influenced by the quality of the cables used and earthing of both the test-piece and the instrument.

Similarly, the intrinsic electronic noise may be further minimised by using as high an input (or pre-amplifier gain).

Filters may be used to ensure the signal is detected using the smallest possible bandwidth (NOTE: bandwidth is the numeric difference between the low- and high-pass filters).

The setting of the lift-off signal in the horizontal is one simple form of optimising the removal of unwanted material signals.

Using two frequencies or more to minimise unwanted material signals by mixing can further improve the situation but the electronic noise will generally decrease by 6 dB for each mix.

This concludes the series.

Bibliography

There are a number of excellent books available on eddy current testing:

Advanced Manual For: Eddy Current Test MethodCAN/CGSB-48.14-M86, Canadian General Standards BoardASNT Level III Study Guide: Eddy Current Testing MethodPublished in 1983, 72 pages, ASNT. ISBN: 0-931403-80-4Eddy Current Characterization of Materials and Structures Birnbaum & Free. Published June 1, 1981 ASTMISBN: 0803107528 Eddy Current Testing Theory and PracticeBy E Dane HarveyPublished in 1995, 76 pages. ISBN: 0-57117-007-3

Eddy Current Testing By Cecco, Van Drunnen and SharpPublished in 1987, 196 pages, Nicholas PublishingISBN: 0-87683-890-5Eddy Current Testing By Cecco, Van Drunnen and SharpPublished in 1987, 196 pages, Nicholas PublishingISBN: 0-87683-890-5Electrical and Magnetic Methods of Nondestructive Testing By Jack BlitzPublished in 1997, Chapman and Hall, 261 pages. ISBN 0-412-79150-1Mathematics Formulas and References for Nondestructive Testing – Eddy CurrentJ Mark Davis and Mike King. Published in 2001, 40 pages. ISBN: 1-884285-02-3Nondestructive Testing Handbook, 3rd Edition, Volume 5: Electromagnetic TestingSatish S Udpa, (technical editor) and Patrick O Moore, (editor)Published in 2004, 536 pages. ISBN: 1-57117-046-4 (book)1-57117-116-9 (CD-ROM)

Thanks

John Hansen acknowledges that a great deal of the original material in this series was due in part to the work of Joe Buckley whilst he worked at Hocking NDT. Thanks are also due to John Rudlin, John Calvert, Richard Lewis, Don Hocking, Nigel Thorpe and Caroline Akeroyd who contributed to the author’s understanding, sometimes without realising it!


The Eddy Current Inspection Method

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