Eddy Current Theory Introduction: Scope This document is only intended as a brief familiarisation course. It is not intended as a complete theoretical course in eddy current non-destructive testing, and a number of aspects are intentionally simplified. The bibliography at the end of the document details several appropriate reference books which should be consulted if a more complete understanding is required. Historical perspective Eddy 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 1950’s and 60’s, particularly in the aircraft and nuclear industries, and eddy current testing is now an accurate, widely used and well-understood inspection technique
Principles: Current flow in DC circuits When a voltage is applied to a circuit containing only resistive elements, current flows according to ohms 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:
Current (I) must be the same for both resistors, so: V1 =I.R1, V2=I.R2, Vtot = V1+V2 = I.R1+ !.R2 = I (R1+R2) = I.Rtot so Rtot = R1+ R2 Electromagnetic induction In 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 below:
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. A property of the coil called inductance (L) is defined, such that
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 wave form this is at the point where the actual current is zero.
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 = 2 pi f L where f is the frequency in Hz. Current flow in AC circuits - impedance
As we saw above, for series DC circuits calculation of total resistance is simply a matter of adding the individual resistance values. For an AC circuit it is not so simple, but the same basic principles apply: the current though 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. However, the maximum voltage across the resistance coincides with zero voltage across the inductor and vice versa.
We can represent this graphically using a vector diagram:
The impedance of the circuit is therefore given by the formula:
and the phase angle between voltage and current is given by:
Instrumentation: There are a number of basic groups of eddy current instrumentation. Special purpose equipment: Coating thickness meters, conductivity meters (e.g. AutoSigma 3000). Generally designed to give a digital readout without demanding interpretation of an indication. "Crack detectors" Fairly simple equipment, generally operates at a restricted number of frequencies typically several hundred kHz, meter or bar-graph display. Suitable for surface crack detection and simple sorting applications only. e.g. Locator and QuickCheck These normally have some means of compensating for lift-off (e.g. phase rotation and/or fine frequency adjustment) so that only crack-like indications give a reading on the meter or bargraph. An alarm threshold is usually included. Portable impedance plane eddy current flaw detectors Give a real impedance plane display on a CRT or other electronic display (LCD, Electroluminescent, etc.) Generally have fairly extensive capabilities: wide frequency ranges from around a hundred hertz to several megahertz, extensive alarm facilities, general purpose units may have rate filtering (see below) some instruments may be capable of multifrequency operation, allowing combination of results at two or more test frequencies in order to reduce or eliminate specific interfering effects, e.g. Phasec 2200 and Phasec D62. "Systems" eddy current units Intended for factory operation, often in automatic or semi-automatic inspection machines. Generally similar operation to impedance plane portables but usually have extensive input and output facilities such as relays and photocell inputs. May be custom built for a specific purpose, in which case features not needed for the intended application are often omitted, e.g. Phasec D62 system. Meter/CRT instruments Typical examples (simplified).
Eddy Current Crack Detector Locator UH is the automatic solution to the problems of detecting surface breaking cracks in metals. Portable, rugged and reliable it can be set up rapidly and is simple to operate. Three frequency settings cover the full range of metals and components, providing more accurate inspection at lower costs.
Specifications Frequency Ranges Switchable to 500kHz (or 200kHz), 2MHz and 6MHz. Lift-Off Push button automatic compensation. Zero Automatically occurs with lift-off compensation.
Universal application Three instruments in one. 500kHz (200kHz optional) for general purpose use. 2MHz for inspection of bolt holes, threads, splines and gears. 6MHz provides outstanding performance on edges, titanium, high nickel alloys, turbine blades and discs. Repeatable, simple test set-up Push button lift-off compensation achieves near perfect
Additional instantaneous push button or manual control for geometry or edge compensation. Sensitivity or Gain Ten turn control with digital 0-999 unit display. Alarm Threshold Calibrated 0-100 to correspond with meter display amplitude. Audio/Visual Alarm Internal or on Remote Control. May be continuous, pulsed or muted. LED on instrument front panel. Indication also on Remote Control bar graph display. Metals Selector Virtually all metals can be examined. Separate positions for Aluminium, Magnesium, Titanium, Austenitic Stainless Steel and Ferrous metals. Power Supply Built-in NiCad rechargeable cells. Alternatively may be connected through battery charger input to AC line voltage and used continuously. Battery Duration 8 to 10 hours per charge. Low Battery Warning Flashing LED on front panel and Remote Control. Instrument 'On' Indicator Steady LED on front panel. Battery Charger Built in. Re-charge Time Set for 10 hours as standard. Charger is safe for permanent connection. Recorder Output Pin connections for output to recorder remote unit or lift-off indicator. Lift-Off Indicator Optional plug-in LED indicates when probe has excessive lift-off from the metal surface. Voltage Selection Automatic detection of incoming line voltage between 100-130V and 200-250V. 40-60Hz. Size Instrument only: 290 x 167 x 80mm (11.3 x 6.6 x 3.2") L x W x D. Instrument in carrying case: 295 x 175 x 150mm (11.6 x 6.9 x 6") L x W x D. Weight Instrument only: 3.1kg (6.8lb)
correction over critical small spacings. Push button zeroing compensates for edge testing and geometry changes. Calibrated, lockable gain control for accurate, repeatable settings. Total portability Battery or AC operation. Built-in charger. Detachable case lid stores probes, cables, reference blocks, shoulder strap, power cord and optional remote unit. Extra ruggedness Steel/aluminium, rubber sealed instrument case fits in tough ABS carrying case. Shock absorbent mounting of instrument within carrying case. Probes for every application Select from a wide variety of standard and special probes in pencil, right angle, bolt hole and thread root configurations. Choose 500kHz, 2MHz or 6MHz in unshielded, semi or fully shielded designs, including 6MHz microfocus. Powerful accessories Remote Control/Display unit fits in the palm of your hand for work in crawl spaces. Has controls for lift-off and zeroing, plus bar graph display, audio/visual alarm and low battery indicator. 4 metre (160 inch cable). Electronic Reference Unit checks instrument standardisation. Verifies Locator UH response to reference flaws, including national standards. Ideal for 'fleet' users. Automatic line voltage detection/selection Instrument sets itself to correct voltage.
Instrument in carrying case: 4.1kg (9.0lb) Temperature Range Operating: -20؛C (-4؛F) to 52؛C (125؛F) Storage: -24.5؛C (-12؛F) to 60؛C (140؛F). Moisture Resistance Splash proof. Accessories Remote Control – a hand held unit which contains controls for lift-off compensation and meter zero; has visual and audio alarm, a bar graph display and low battery indicator with 4 metres (160 inches) attached cable and lift-off indicator. Electronic Reference Unit (ERU) – simulates response of artificial defects at 3 frequencies on three different metals – Aluminium Alloy, Titanium, Ferrous.
Typical application: Surface crack detection in aircraft parts using absolute probe. Controls Meter display indicates ‘crack severity’ - imbalance from zero point. Zero - balance internal circuitry Zero Offset - shift zero point, useful for sorting/material verification applications. Train - zero and set lift-off compensation circuitry. Frequency - choice of three operating frequencies: 500kHz for Aluminium and carbon steel, 2 MHz for small cracks in Al + Stainless & carbon steels, 6 MHz for low conductivity Alloys, Titanium etc. Metal type - optimises lift-off compensation circuitry and adjusts sensitivity to match response curves from particular metals. Alarm Level - sets meter indication at which the alarm will light/sound.
Advanced Eddy Current Instrument
Class leading package – small, light (less than 3kg), tough poly-carbonate enclosure, reliable in the harshest environment. Fully PC compatible. Comprehensive Eddy Current features – Dual frequency with mixing, range of rotating probe drives, Conductivity and Coating Thickness Measurement Intelligent signal processing – User selectable 'Auto' functions, including Automix, Auto Lift-Off, Auto Balance and Auto probe Matching Widest range of applications – Corrosion detection, fastener hole inspection, surface and subsurface crack detection Enhanced products for specific inspections – Connects to a wide range of systems, for in-situ fastener inspection, weld inspection and C-scan corrosion detection.
Specifications Dimensions 249 x 133 x 146mm Weight 2.7kg Operating Frequency Single frequency mode: 60Hz-6MHz Dual frequency mode: 100Hz-2MHz, F1/F2 ration 10:1-1:10 (multiplexed) Channels Single and Dual Frequency probes Rotating probes, 1 channel Gain Overall adjustment 0-90dB
Take the weight of inspection off your shoulders – with the advanced NDT capabilities of the Phasec 2200 you can concentrate fully on the tests themselves. The light compact package reduces operator fatigue and allows unhindered inspections all day long. This combination of power, features and portability will lighten your inspection load. Class Leading Package The innovative design of the Phasec 2200 puts a high technology Eddy Current instrument inside an ultralight, splashproof case that has the ruggedness to perform reliably in the harshest environments. It has the benefit of a large Electroluminescent display to reduce eyestrain, comprehensive battery management, extensive context-sensitive help screens and advanced user functions. The Phasec 2200 includes all the functions needed to enable you to achieve accurate, reliable and repeatable test results. It minimises operator fatigue by being both the lightest available unit of its kind, and by automating many repetitive operations.
Filters Low Pass and High Pass Alarms Box Gate, Circular/Sector Gate, Flashing LEDs, internal sounder and open collector output. Internal Data Storage Capacity for 50 traces and 100 settings. 14 character file descriptions. Conductivity Measurement Range 1%-110% IACS, 0.4-64 MS/m Requires probe ref: 47P001 Coating Thickness Measurement Needs probe as above. Inputs/Outputs Serial interface for PC communication or serial printer. Headphone output. 2 selectable Analog outputs for chart recorder or similar. Display Electroluminescent, 152mm diagonal. Power External power supply/charger: 120/240 VAC 50/60Hz Batteries 6 x 'D' size Alkaline or NiCad cells. Environment Operating temp. 0؛C to 40؛C Storage temp. -20؛C to +55؛C.
Comprehensive Eddy Current Features The dual channel, dual frequency, Phasec 2200 offers a full range of capabilities for the NDT operator: Wide choice of alarms – Box Gate, Circular/Sector gate, flashing LEDs, audible alarm and open collector output. Supports the range of Hocking rotating drives for dynamic inspection. Conductivity and non-ferrous metals plus coating thickness measurements in conjunction with a conductivity probe. Comprehensive reporting features: storage of data and traces within the instrument, and export to a PC. Choice of full alphanumeric names for these files.
Intelligent Signal Processing Digital techniques have vastly improved data manipulation, flaw discrimination, and automation of tasks, allowing more rapid and accurate inspection. Inspection results can be recorded and replayed. During playback parameters can be changed to find the optimum inspection setup. Digital filters allow high quality signal processing. Setting lift-off and optimising balance loads can be performed automatically. Mixing out unwanted signals, such as support plates during dual frequency inspection, is performed in seconds by the instrument – automatically. Full connectivity to available PC software packages for control and recording.
Aerospace Applications Corrosion The low frequency performance of the Phasec 2200 in both single and dual frequency mode makes it ideal for sub-surface inspection in thick multi-layer structures such as aircraft skins. For positional scanning the Phasec 2200 can use the AndScan scanning system. Fastener Hole Inspection The Inspection of fastener holes in aircraft and other structures benefits greatly from the Phasec's use of versatile filters, advanced display software and low noise. The result is an excellent signal to noise ratio allowing very small flaws to be detected. A complementary range of probe drives is available. These range from the ultralight MiniDrive for tight access areas, to the fully indexing incrementing drive, providing for a wide choice of dynamic inspections. FastScan The FastScan system has been designed specifically for detection of surface and sub-surface cracking under installed fasteners. Using dual frequency techniques the operator can get an indication of the depth and severity of flaws to a depth of about 10mm. This system can inspect under ferrous or non-ferrous fasteners and can identify top-surface cracking under the fastener head. Conductivity With the appropriate probe, the Phasec 2200 measures the conductivity of nonferrous metals over the full range of 1% to 110% IACS or 0.4-64MS/m. A readout of coating thickness to 1.25mm is also displayed.
Other Applications Surface Cracking The Phasec 2200 gives high sensitivity surface inspection using all standard absolute and differential probes. The built-in balance loads and wide probe matching ensure compatibility with other manufacturers' application-specific probes. Weld Inspection In conjunction with the Hocking WeldScan range of probes, the Phasec 2200 offers
an advanced system for checking the integrity of weld on steel structures, such as bridges, ships, oil rigs and steel framed buildings. It is designed to be insensitive to lift-off and can be used on structures with anti-corrosion coatings. Tube Inspection (non-ferrous) With ID probes the Phasec 2200 can perform a range of tube inspection tasks.
There are a number of basic groups of eddy current instrumentation. Special purpose equipment: Coating thickness meters, conductivity meters (e.g. AutoSigma 3000). Generally designed to give a digital readout without demanding interpretation of an indication. "Crack detectors" Fairly simple equipment, generally operates at a restricted number of frequencies typically several hundred kHz, meter or bar-graph display. Suitable for surface crack detection and simple sorting applications only. e.g. Locator and QuickCeck These normally have some means of compensating for lift-off (e.g. phase rotation and/or fine frequency adjustment) so that only crack-like indications give a reading on the meter or bargraph. An alarm threshold is usually included. Portable impedance plane eddy current flaw detectors Give a real impedance plane display on a CRT or other electronic display (LCD, Electroluminescent, etc.) Generally have fairly extensive capabilities: wide frequency ranges from around a hundred hertz to several megahertz, extensive alarm facilities, general purpose units may have rate filtering (see below) some instruments may be capable of multifrequency operation, allowing combination of results at two or more test frequencies in order to reduce or eliminate specific interfering effects, e.g. Phase 2200 and Phase D62. "Systems" eddy current units Intended for factory operation, often in automatic or semi-automatic inspection machines. Generally similar operation to impedance plane portables but usually have extensive input and output facilities such as relays and photocell inputs. May be custom built for a specific purpose, in which case features not needed for the intended application are often omitted, e.g. Phase D62 system. Meter/CRT instruments Typical examples (simplified).
Typical application: Surface crack detection in aircraft parts using absolute probe. Controls Meter display indicates ‘crack severity’ - imbalance from zero point. Zero - balance internal circuitry Zero Offset - shift zero point, useful for sorting/material verification applications. Train - zero and set lift-off compensation circuitry. Frequency - choice of three operating frequencies: 500kHz for Aluminium and carbon steel, 2 MHz for small cracks in Al + Stainless & carbon steels, 6 MHz for low conductivity Alloys, Titanium etc. Metal type - optimises lift-off compensation circuitry and adjusts sensitivity to match response curves from particular metals. Alarm Level - sets meter indication at which the alarm will light/sound.
Typical applications: Surface crack detection, weld inspection, bolthole inspection with rotating probe
drive, metal sorting, tube inspection etc. Controls: On, Help, Record, Freeze, Automatic Processing, Clear, Zoom, Display, Execute and Balance. Menu commands for: Sophisticated menu structure for adjusting all parameters relating to an inspection. 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
Testing: Simple coil above a metal surface When an AC current flows in a coil in close proximity to a conducting surface 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. 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 in such an arrangement we can detect changes in the material of interest.
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.
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. Permeability This 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’, i.e. the relative permeability (mr) is one. For ferrous metals however the value of mr 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. Frequency As we will discuss, eddy current response is greatly affected by the test frequency chosen, fortunately this is one property we can control. Geometry In 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: The "lift-off" signal as the probe is moved on and off the surface. 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.
The standard depth of penetration in mm is given by the formula:
where r is resistivity in mW.cm and f is frequency in Hz from this it can be seen that depth of penetration: Decreases with an increase in frequency Decreases with an increase in conductivity
Decreases with an increase in permeability – this can be very significant – penetration into ferrous materials at practical frequencies is very small.
The graph above 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 3% of its surface value. This is the depth at which there is considered to be no influence on the eddy current field. The impedance plane Eddy 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.
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 above impedance plane diagram a rotated detail of the "probe on aluminium" area would appear as below:
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 configurations Appropriate coil selection is the most important part of solving an eddy current application, no instrument can achieve much if it doesn’t get the right signals from the probe. Coil designs can be split into three main groups: 1. 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. 2. Encircling coils are normally used for in-line inspection of round products, The product to be tested is inserted though a circular coil. 3. 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. These categories are not exhaustive and there are obviously overlaps, for example between noncircumferential wound ID probes and internal surface probes. 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. 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.
The diagram 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. 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 response equivalent to either an absolute or differential probe.
Main advantages of reflection probes are: Driver and pick-up coils can be separately optimised for their intended purpose. Wider frequency range than equivalent bridge connected probes. The larger driver coil gives a more even field, resulting in better penetration and lift-off characteristics. Typical coil connections Bridge
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. Driver pick-up
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. Practical testing Any practical Eddy current test will require the following: A suitable probe. An instrument with the necessary capabilities. A good idea of size, location and type of the flaws it is desired to find. A suitable test standard to set up the equipment and verify correct operation A procedure or accept/reject criteria based on the above. The necessary operator expertise to understand and interpret the results.
Applications: Surface crack detection Normally carried out with pencil probes or ‘pancake’ type probes on ferrous or non-ferrous metals. Frequencies from 100kHz to a few MHz are commonly used. Depending on surface condition it is usually possible to find cracks as small as 0.1 mm deep. 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. Typical instruments used would be the Phase 2200 and Phase D62. Non-ferrous metal sorting This is essentially conductivity testing and for dedicated applications a conductivity meter may be a better choice. From the impedance plane diagram it will be seen 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. The Hocking AutoSiqma 3000 is an instrument that gives a direct reading of conductivity for non-ferrous metals. It should be remembered that widely different metals may be a similar conductivity and that the allowable values for similar alloys may overlap, so conductivity measurement should only be used as an indication that a metal is of correct composition or heattreatment. Sub-surface crack/corrosion detection 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 10mm 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. Typical instrument used would be the Phasec 2200. Heat exchanger tube testing Heat exchangers used for petrochemical or power generation applications may have many thousands of tubes, each up to 20m long. Using a differential ID ‘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 600kHz for thin-walled titanium. Tubes up to around 50mm 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. Typical instruments used would be the Phasec D62. In-line inspection of Steel tubing Almost all high quality steel tubing is eddy current inspected using encircling coils. When the tube is made of a magnetic material there are two main problems: Because of the high permeability there is little or no penetration of the eddy current field into the tube at practical test frequencies. Variations in permeability (from many causes) cause eddy current responses which are orders of magnitude greater than those from defects. These problems may be overcome by magnetically saturating the tube using a strong DC field. This reduces the effective permeability to a low value, allowing effective testing. Tubes up to around 170mm diameter are commonly tested using magnetic saturation and encircling coils. When tubes are welded this is usually where the problems occur, and so welded tubes are commonly tested in-line using sector coils which only test the weld zone. Typical instruments used would be the Phasec D62 system or QuickCheck. 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 "WeldScanWeldScan" 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 corrosionprevention coatings. Cracks around 1mm deep and 6mm long can be found in typical welds. Typical instrument used would be the MiniPhasec. 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 tried 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. 4. Move the probe on/over the calibration test piece and set phase rotation as desired (e.g. lift-off or wobble horizontal on a phase plane display). 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. 7. Use filters etc. to further optimise signal to noise ratio. 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.
Rate filtering Most eddy current tests involve movement. Therefore the indications obtained will vary with time in a way which is fairly consistent (assuming the movement is regular) and which can be interpreted in terms of the speed of probe movement over various discontinuities. For example, if an absolute probe with diameter 2mm moves over a narrow crack at a speed of 1 m/s the resulting indication will last for approximately 2 milliseconds. If the material composition or thickness is also varying gradually over a distance of say 50cm the indication from this will change much more slowly. Therefore a high-pass filter set to a frequency around 100Hz or so will pass the rapidly changing signal from the defect but not the slower indication from the material changes. The Passec D62 system has a comprehensive set of filters.
Bibliography: There are a number of excellent books available on eddy current testing. Many are available from ASNT or other national institutes. The following are particularly recommended to ‘fill in’ if a more complete understanding is required. Fundamentals of Eddy Current Testing Donald J. Hagemaier, ASNT, 1990 ISBN 0-931403-90-1 Advanced Manual For: Eddy Current Test Method CAN/CGSB-48.14-M86, Canadian General Standards Board NDT Handbook 2nd Edition Volume 4b, Electromagnetic Testing Ed. Paul McIntyre/Mike Mester, ASNT, 1986 ISBN 0-931403-01-04