1. Eddy Current Testing-An Introduction2014-NovemberMy ASNT Level III Pre-ExamPreparatory Self Study NotesCharlie Chong/ Fion Zhang

2. Charlie Chong/ Fion Zhang https://www.nde-ed.org/EducationResources/CommunityCollege/EddyCurrents/Graphics/Flash/DifferentialvsAbsoluteAnim.swf 3. Charlie Chong/ Fion Zhang 4. Fion Zhang2014/Novemberhttp://meilishouxihu.blog.163.com/Charlie Chong/ Fion ZhangShanghai 5. Shanghai Charlie Chong/ Fion Zhang 6. Shanghai Charlie Chong/ Fion Zhang 7. Greek letterCharlie Chong/ Fion Zhang 8. YouTube., Charlie Chong/ Fion Zhang 9. Charlie Chong/ Fion Zhanghttp://v.qq.com/cover/u/ujnamwpqg1xg8qm/y0012j6s11e.html 10. Offshore InstallationsCharlie Chong/ Fion Zhang 11. Offshore InstallationsCharlie Chong/ Fion Zhang 12. ET Expert at WorksCharlie Chong/ Fion Zhanghttp://ropax.co.uk/eddy-current-inspection.html 13. Expert at WorksCharlie Chong/ Fion Zhang 14. ET Expert at WorksCharlie Chong/ Fion Zhang 15. Contents1. Introduction2. The Physics3. Instrumentation4. Probes (Coils)5. Procedures Issues6. Applications7. Advanced Techniques8. QuizzesCharlie Chong/ Fion Zhang 16. 1.0 Introduction1.1 Basic Principles of Eddy Current InspectionEddy current inspection is one of several NDT methods that use the principalof electromagnetism as the basis for conducting examinations. Severalother methods such as Remote Field Testing (RFT), Flux Leakage andBarkhausen Noise also use this principle.Eddy currents are created through a process called electromagnetic induction.When alternating current is applied to the conductor, such as copper wire, amagnetic field develops in and around the conductor.This magnetic field expands as the alternating currentrises to maximum and collapses as the current is reducedto zero. If another electrical conductor is brought into theclose proximity to this changing magnetic field, currentwill be induced in this second conductor. Eddy currentsare induced electrical currents that flow in a circular path.They get their name from eddies that are formed when aliquid or gas flows in a circular path around obstacleswhen conditions are right.Charlie Chong/ Fion Zhang 17. Eddy current - Surface probeCharlie Chong/ Fion Zhanghttp://criterionndt.com/eddy-current-testing/eddy-current-theory 18. Eddy current - Encircling ProbeCharlie Chong/ Fion Zhanghttp://www.keytometals.com/page.aspx?ID=CheckArticle&site=kts&NM=197 19. Charlie Chong/ Fion Zhang https://www.nde-ed.org/EducationResources/CommunityCollege/EddyCurrents/Graphics/Flash/EddyCurrentAnim.swf 20. One of the major advantages of eddy current as an NDT tool is the variety ofinspections and measurements that can be performed. In the propercircumstances, eddy currents can be used for:1. Crack detection2. Material thickness measurements3. Coating thickness measurements4. Conductivity measurements for: Material identification Heat damage detection Case depth determination Heat treatment monitoringCharlie Chong/ Fion Zhang 21. Some of the advantages of eddy current inspection include: Sensitive to small cracks and other defects, Detects surface and near surface defects, Inspection gives immediate results, Equipment is very portable, Method can be used for much more than flaw detection, Minimum part preparation is required, Test probe does not need to contact the part, Inspects complex shapes and sizes of conductive materials,Charlie Chong/ Fion Zhang 22. Some of the limitations of eddy current inspection include: Only conductive materials can be inspected, Surface must be accessible to the probe, Skill and training required is more extensive than other techniques, Surface finish and roughness may interfere, Reference standards needed for setup, Depth of penetration is limited, Flaws such as delaminations that lie parallel to the probe coil winding andprobe scan direction are undetectable.Charlie Chong/ Fion Zhang 23. Flaws such as delaminations that lie parallel to the probe coil winding andprobe scan direction are undetectable.Charlie Chong/ Fion Zhang 24. Surface finish and roughnessmay interfereFlaws such as delaminations that lie parallel to the probe coil winding andprobe scan direction are undetectable.Charlie Chong/ Fion Zhang 25. Eddy current transducersCharlie Chong/ Fion Zhang 26. Eddy current circuitCharlie Chong/ Fion Zhang 27. Eddy current circuitCharlie Chong/ Fion Zhang 28. Flaws such as delaminations that lie parallel to the probe coil winding andprobe scan direction are undetectable.Charlie Chong/ Fion Zhang 29. 1.2 History of Eddy Current TestingEddy current testing has its origins with MichaelFaraday's discovery of electromagnetic inductionin 1831. Faraday was a chemist in Englandduring the early 1800's and is credited with thediscovery of electromagnetic induction,electromagnetic rotations, the magneto-opticaleffect, diamagnetism, and other phenomena. In1879, another scientist named Hughes recordedchanges in the properties of a coil when placed incontact with metals of different conductivity andpermeability. However, it was not until theSecond World War that these effects were put topractical use for testing materials. Much workwas done in the 1950's and 60's, particularly inthe aircraft and nuclear industries. Eddy currenttesting is now a widely used and well-understoodinspection technique.Charlie Chong/ Fion Zhang 30. 1.3 Present State of Eddy Current InspectionEddy current inspection is used in a variety of industries to find defects andmake measurements. One of the primary uses of eddy current testing is fordefect detection when the nature of the defect is well understood. In general,the technique is used to inspect a relatively small area and the probe designand test parameters must be established with a good understanding of theflaw that is to be detected. Since eddy currents tend to concentrate at thesurface of a material, they can only be used to detect surface and nearsurface defects.In thin materials such as tubing and sheet stock, eddy currents can be usedto measure the thickness of the material. This makes eddy current a usefultool for detecting corrosion damage and other damage that causes a thinningof the material. The technique is used to make corrosion thinningmeasurements on aircraft skins and in the walls of tubing used in assembliessuch as heat exchangers. Eddy current testing is also used to measure thethickness of paints and other coatings.Charlie Chong/ Fion Zhang 31. Eddy currents are also affected by the electrical conductivity and magneticpermeability of materials. Therefore, eddy current measurements can be usedto sort materials and to tell if a material has seen high temperatures or beenheat treated, which changes the conductivity of some materials.Eddy current equipment and probes can be purchased in a wide variety ofconfigurations. Eddyscopes and a conductivity tester come packaged in verysmall and battery operated units for easy portability. Computer basedsystems are also available that provide easy data manipulation features forthe laboratory. Signal processing software has also been developed for trendremoval, background subtraction, and noise reduction. Impedance analyzersare also sometimes used to allow improved quantitative eddy-currentmeasurements. Some laboratories have multidimensional scanningcapabilities that are used to produce images of the scan regions. A fewportable scanning systems also exist for special applications, such asscanning regions of aircraft fuselages.Charlie Chong/ Fion Zhang 32. 1.4 Research to Improve Eddy current measurementsA great deal of research continues to be done to improve eddy currentmeasurement techniques. A few of the these activities, which are beingconducted at Iowa State University, are described below.1.4.1 Photo-inductive Imaging (PI)A technique known as photo-inductive imaging (PI) was pioneered at CNDEand provides a powerful, high-resolution scanning and imaging tool.Microscopic resolution is available using standard-sized eddy-current sensors.Development of probes and instrumentation for photo-inductive (PI) imagingis based on the use of a medium-power (5 W nominal power) argon ion laser.This probe provides high resolution images and has been used to studycracks, welds, and diffusion bonds in metallic specimens. The PI technique isbeing studied as a way to image local stress variations in steel.Charlie Chong/ Fion Zhang 33. 1.4.2 Pulsed Eddy CurrentResearch is currently being conducted on the use of a technique calledpulsed eddy current (PEC) testing. This technique can be used for thedetection and quantification of corrosion and cracking in multi-layer aluminumaircraft structures. Pulsed eddy-current signals consist of a spectrum offrequencies meaning that, because of the skin effect, each pulse signalcontains information from a range of depths within a given test specimen. Inaddition, the pulse signals are very low-frequency rich which providesexcellent depth penetration. Unlike multi-frequency approaches, the pulse-signalslend themselves to convenient analysis.Measurements have been carried out both in the laboratory and in the field.Corrosion trials have demonstrated how material loss can be detected andquantified in multi-layer aluminum structures. More recently, studies carriedout on three and four layer structures show the ability to locate cracksemerging from fasteners. Pulsed eddy-current measurements have also beenapplied to ferromagnetic materials. Recent work has been involved withmeasuring the case depth in hardened steel samples.Charlie Chong/ Fion Zhang 34. Photo-inductive ImagingCharlie Chong/ Fion Zhanghttp://www.mdpi.com/1424-8220/13/12/16146/htm 35. Photoinductive ImagingCharlie Chong/ Fion Zhanghttp://www.mdpi.com/1424-8220/13/12/16146/htm 36. Pulsed Eddy current inspectionCharlie Chong/ Fion Zhanghttp://radio.rphf.spbstu.ru/a263/eddy.htm 37. Pulsed Eddy current inspectionCharlie Chong/ Fion Zhanghttp://www.ndt.net/article/ecndt02/251/251.htm 38. DiscussionTopic: What is Pulse Eddy CurrentCharlie Chong/ Fion Zhang 39. 2.0 The Physics2.1 Properties of ElectricitySince eddy current inspection makes use of electromagnetic induction, it isimportant to know about the scientific principles of electricity and magnetism.For a review of these principles, the Science of NDT materials on this Internetsite may be helpful. A review of the key parameters will be provided here.2.1.1 ElectricityIt is well known that one of the subatomic particles of an atom is the electron.Atoms can and usually do have a number of electrons circling its nucleus.The electrons carry a negative electrostatic charge and under certainconditions can move from atom to atom. The direction of movement betweenatoms is random unless a force causes the electrons to move in one direction.This directional movement of electrons due to some imbalance of force iswhat is known as electricity.Charlie Chong/ Fion Zhang 40. Electricity- Flow of ElectronCharlie Chong/ Fion Zhang 41. 2.1.2 AmperageThe flow of electrons is measured in units called amperes or amps for short.An amp is the amount of electrical current that exists when a number ofelectrons, having one coulomb of charge, move past a given point in onesecond. A coulomb is the charge carried by 6.25 x 1018 electrons or6,250,000,000,000,000,000 electrons.2.1.3 Electromotive ForceThe force that causes the electrons to move in an electrical circuit is calledthe electromotive force, or EMF. Sometimes it is convenient to think of EMFas electrical pressure. In other words, it is the force that makes electronsmove in a certain direction within a conductor. There are many sources ofEMF, the most common being batteries and electrical generators.Charlie Chong/ Fion Zhang 42. EMF- Electromotive forceCharlie Chong/ Fion Zhanghttp://www.askmrtan.com/physics/17currentofelectricity/image032.gif 43. 2.1.4 The VoltThe unit of measure for EMF is the volt. One volt is defined as theelectrostatic difference between two points when one joule of energy is usedto move one coulomb of charge from one point to the other. A joule is theamount of energy that is being consumed when one watt of power works forone second. This is also known as a watt-second. For our purposes, justaccept the fact that one joule of energy is a very, very small amount of energy.For example, a typical 60-watt light bulb consumes about 60 joules of energyeach second it is on.2.1.5 ResistanceResistance is the opposition of a body or substance to the flow of electricalcurrent through it, resulting in a change of electrical energy into heat, light, orother forms of energy. The amount of resistance depends on the type ofmaterial. Materials with low resistance are good conductors ofelectricity. Materials with high resistance are good insulators.Charlie Chong/ Fion Zhang 44. 2.2 Current Flow and Ohm's LawOhm's law is the most important, basic law of electricity. It defines therelationship between the three fundamental electrical quantities: current,voltage, and resistance. When a voltage is applied to a circuit containing onlyresistive elements (i.e. no coils), current flows according to Ohm's Law, whichis shown below.I = V / RWhere:I = Electrical Current (Amperes)V = Voltage (Voltage)R = Resistance (Ohms)Charlie Chong/ Fion Zhang 45. Ohm's law states that the electrical current (I) flowing in an circuit isproportional to the voltage (V) and inversely proportional to the resistance (R).Therefore, if the voltage is increased, the current will increase provided theresistance of the circuit does not change. Similarly, increasing the resistanceof the circuit will lower the current flow if the voltage is not changed. Theformula can be reorganized so that the relationship can easily be seen for allof the three variables.The Java applet below allows the user to vary each of these three parametersin Ohm's Law and see the effect on the other two parameters. Values may beinput into the dialog boxes, or the resistance and voltage may also be variedby moving the arrows in the applet. Current and voltage are shown as theywould be displayed on an oscilloscope with the X-axis being time and the Y-axisbeing the amplitude of the current or voltage. Ohm's Law is valid for bothdirect current (DC) and alternating current (AC). Note that in AC circuitsconsisting of purely resistive elements, the current and voltage are always inphase with each other.Charlie Chong/ Fion Zhanghttps://www.nde-ed.org/EducationResources/CommunityCollege/EddyCurrents/Physics/PopUps/applet1/applet1.htm 46. 2.3 Induction and Inductance2.3.1 InductionIn 1824, Oersted discovered that current passing though a coil created amagnetic field capable of shifting a compass needle. Seven years later,Faraday and Henry discovered just the opposite. They noticed that a movingmagnetic field would induce current in an electrical conductor. This process ofgenerating electrical current in a conductor by placing the conductor in achanging magnetic field is called electromagnetic induction or just induction. Itis called induction because the current is said to be induced in the conductorby the magnetic field.Faraday also noticed that the rate at which the magnetic field changed alsohad an effect on the amount of current or voltage that was induced. Faraday'sLaw for an uncoiled conductor states that the amount of induced voltage isproportional to the rate of change of flux lines cutting the conductor.Faraday's Law for a straight wire is shown below.Charlie Chong/ Fion Zhang 47. Where:VL = the induced voltage in voltsd/dt = the rate of change of magnetic flux in webers/secondCharlie Chong/ Fion Zhang 48. Charlie Chong/ Fion Zhang http://hyperphysics.phy-astr.gsu.edu/hbase/electric/farlaw.html 49. Faraday LawCharlie Chong/ Fion Zhang 50. Electromagnetic Induction and Faraday's Law www.youtube.com/embed/vwIdZjjd8fo https://www.youtube.com/watch?v=vwIdZjjd8foCharlie Chong/ Fion Zhang 51. 2.3.2 InductanceWhen induction occurs in an electrical circuit and affects the flow of electricityit is called inductance, L. Self-inductance, or simply inductance, is theproperty of a circuit whereby a change in current causes a change in voltagein the same circuit. When one circuit induces current flow in a second nearbycircuit, it is known as mutual-inductance. The image below shows an exampleof mutual-inductance.Charlie Chong/ Fion Zhang 52. When an AC current is flowing through a piece of wire in a circuit, anelectromagnetic field is produced that is constantly growing and shrinking andchanging direction due to the constantly changing current in the wire. Thischanging magnetic field will induce electrical current in another wire or circuitthat is brought close to the wire in the primary circuit. The current in thesecond wire will also be AC and in fact will look very similar to the currentflowing in the first wire. An electrical transformer uses inductance to changethe voltage of electricity into a more useful level. In nondestructive testing,inductance is used to generate eddy currents in the test piece.It should be noted that since it is the changing magnetic field that isresponsible for inductance, it is only present in AC circuits. High frequencyAC will result in greater inductive reactance since the magnetic field ischanging more rapidly. Self-inductance and mutual-inductance will bediscussed in more detail in the following pages.Keywords: Induction Inductance L, Self inductance, Mutual inductance Inductive reactanceCharlie Chong/ Fion Zhang 53. 2.4 Self-Inductance and Inductive ReactanceThe property of self-inductance is a particular form of electromagneticinduction. Self inductance is defined as the induction of a voltage in a current-carryingwire when the current in the wire itself is changing. In the case ofself-inductance, the magnetic field created by a changing current in the circuititself induces a voltage in the same circuit. Therefore, the voltage is self-induced.The term inductor is used to describe a circuit element possessing theproperty of inductance and a coil of wire is a very common inductor. In circuitdiagrams, a coil or wire is usually used to indicate an inductive component.Taking a closer look at a coil will help understand the reason that a voltage isinduced in a wire carrying a changing current.Charlie Chong/ Fion Zhang 54. The alternating current running through the coil creates a magnetic field inand around the coil that is increasing and decreasing as the current changes.The magnetic field forms concentric loops that surround the wire and join toform larger loops that surround the coil as shown in the image below. Whenthe current increases in one loop the expanding magnetic field will cut acrosssome or all of the neighboring loops of wire, inducing a voltage in these loops.This causes a voltage to be induced in the coil when the current is changing.Charlie Chong/ Fion Zhang 55. By studying this image of a coil, it can be seen that the number of turns in thecoil will have an effect on the amount of voltage that is induced into the circuit.Increasing the number of turns or the rate of change of magnetic fluxincreases the amount of induced voltage. Therefore, Faraday's Law must bemodified for a coil of wire and becomes the following.Charlie Chong/ Fion Zhang 56. Where:VL = induced voltage in voltsN = number of turns in the coild/dt = rate of change of magnetic flux in webers/secondCharlie Chong/ Fion Zhang 57. What is self inductance- Lec 20 | MIT 8.02 Electricity and Magnetism,Spring 2002 https://www.youtube.com/watch?v=UpO6t00bPb8Charlie Chong/ Fion Zhang 58. The equation simply states that the amount of induced voltage (VL) isproportional to the number of turns in the coil and the rate of change of themagnetic flux (d/dt). In other words, when the frequency of the flux isincreased or the number of turns in the coil is increased, the amount ofinduced voltage will also increase.In a circuit, it is much easier to measure current than it is to measuremagnetic flux, so the following equation can be used to determine theinduced voltage if the inductance and frequency of the current are known.This equation can also be reorganized to allow the inductance to becalculated when the amount of inducted voltage can be determined and thecurrent frequency is known.Where:VL = the induced voltage in voltsL = the value of inductance in Henriesdi/dt = the rate of change of current in amperes per secondCharlie Chong/ Fion Zhang 59. Lenz's LawSoon after Faraday proposed his law of induction, Heinrich Lenz developed arule for determining the direction of the induced current in a loop. Basically,Lenz's law states that an induced current has a direction such that itsmagnetic field opposes the change in magnetic field that induced the current.This means that the current induced in a conductor will oppose the change incurrent that is causing the flux to change. Lenz's law is important inunderstanding the property of inductive reactance, which is one of theproperties measured in eddy current testing.Charlie Chong/ Fion Zhang 60. Lenzs Law:Heinrich Friedrich Emil Lenz (12February 1804 10 February 1865)was a Russian physicist of BalticGerman ethnicity. He is most noted forformulating Lenz's law inelectrodynamics in 1833. The symbol L,conventionally representing inductance,is chosen in his honor. This was duringthe reigns of Alexander II the LiberatorCharlie Chong/ Fion Zhang1804-1865http://en.wikipedia.org/wiki/List_of_Russian_rulers 61. Inductive ReactanceThe reduction of current flow in a circuit due to induction is called inductivereactance. By taking a closer look at a coil of wire and applying Lenz's law, itcan be seen how inductance reduces the flow of current in the circuit. In theimage below, the direction of the primary current is shown in red, and themagnetic field generated by the current is shown in blue. The direction of themagnetic field can be determined by taking your right hand and pointing yourthumb in the direction of the current. Your fingers will then point in thedirection of the magnetic field. It can be seen that the magnetic field from oneloop of the wire will cut across the other loops in the coil and this will inducecurrent flow (shown in green) in the circuit. According to Lenz's law, theinduced current must flow in the opposite direction of the primary current. Theinduced current working against the primary current results in a reduction ofcurrent flow in the circuit.It should be noted that the inductive reactance will increase if the number ofwinds in the coil is increased since the magnetic field from one coil will havemore coils to interact with.Charlie Chong/ Fion Zhang 62. Inductive ReactanceCharlie Chong/ Fion ZhangDirection of the primary current isshown in red, and the magnetic fieldgenerated by the current is shown inblue. Induce current flow (shown ingreen) in the circuit. 63. Similarly to resistance, inductive reactance reduces the flow of current in acircuit. However, it is possible to distinguish between resistance and inductivereactance in a circuit by looking at the timing between the sine waves of thevoltage and current of the alternating current. In an AC circuit that containsonly resistive components, the voltage and the current will be in-phase,meaning that the peaks and valleys of their sine waves will occur at the sametime. When there is inductive reactance present in the circuit, the phase of thecurrent will be shifted so that its peaks and valleys do not occur at the sametime as those of the voltage. This will be discussed in more detail in thesection on circuits.Charlie Chong/ Fion Zhang 64. 2.5 Mutual InductanceMutual inductance is the Basis for Eddy Current Inspection.The magnetic flux through a circuit can be related to the current in that circuitand the currents in other nearby circuits, assuming that there are no nearbypermanent magnets. Consider the following two circuits.Charlie Chong/ Fion Zhang 65. The magnetic field produced by circuit 1 will intersect the wire in circuit 2 andcreate current flow. The induced current flow in circuit 2 will have its ownmagnetic field which will interact with the magnetic field of circuit 1. At somepoint P, the magnetic field consists of a part due to i1 and a part due to i2.These fields are proportional to the currents producing them.The coils in the circuits are labeled L1 and L2 and this term represents theself inductance of each of the coils. The values of L1 and L2 depend on thegeometrical arrangement of the circuit (i.e. number of turns in the coil) and theconductivity of the material. The constant M, called the mutual inductance ofthe two circuits, is dependent on the geometrical arrangement of both circuits.In particular, if the circuits are far apart, the magnetic flux through circuit 2due to the current i1 will be small and the mutual inductance will be small. L2and M are constants.Charlie Chong/ Fion Zhang 66. We can write the flux, B through circuit 2 as the sum of two parts.B2 = L2i2 + i1MAn equation similar to the one above can be written for the flux through circuit1.B1 = L1i1 + i2MThough it is certainly not obvious, it can be shown that the mutual inductanceis the same for both circuits. Therefore, it can be written as follows:M1,2 = M2,1Charlie Chong/ Fion Zhang 67. Eddy currentCharlie Chong/ Fion Zhang 68. Eddy currentCharlie Chong/ Fion Zhang 69. How is mutual induction used in eddy current inspection?In eddy current inspection, the eddy currents are generated in the testmaterial due to mutual induction. The test probe is basically a coil of wirethrough which alternating current is passed. Therefore, when the probe isconnected to an eddy-scope instrument, it is basically represented by circuit 1above. The second circuit can be any piece of conductive material.When alternating current is passed through the coil, a magnetic field isgenerated in and around the coil. When the probe is brought in closeproximity to a conductive material, such as aluminum, the probe's changingmagnetic field generates current flow in the material. The induced currentflows in closed loops in planes perpendicular to the magnetic flux. They arenamed eddy currents because they are thought to resemble the eddy currentsthat can be seen swirling in streams.Charlie Chong/ Fion Zhang 70. Eddy CurrentCharlie Chong/ Fion Zhang 71. Eddy CurrentCharlie Chong/ Fion Zhang 72. Eddy CurrentCharlie Chong/ Fion Zhang 73. Eddyscope- PC InterfaceCharlie Chong/ Fion Zhanghttp://www.mkckorea.com/catalog/Eddyscope/Eddyscope-2020.htm 74. Charlie Chong/ Fion Zhang 75. The eddy currents produce their own magnetic fields that interact with theprimary magnetic field of the coil. By measuring changes in the resistanceand inductive reactance of the coil, information can be gathered about the testmaterial. This information includes the electrical conductivity and magneticpermeability of the material, the amount of material cutting through the coilsmagnetic field, and the condition of the material (i.e. whether it containscracks or other defects.) The distance that the coil is from the conductivematerial is called liftoff, and this distance affects the mutual-inductance of thecircuits. Liftoff can be used to make measurements of the thickness ofnonconductive coatings, such as paint, that hold the probe a certain distancefrom the surface of the conductive material.Keywords: Electrical conductivity Magnetic permeability Lift-offCharlie Chong/ Fion Zhang 76. It should be noted that if a sample is ferromagnetic, the magnetic flux isconcentrated and strengthened despite opposing eddy current effects. Theincrease inductive reactance due to the magnetic permeability offerromagnetic materials makes it easy to distinguish these materials fromnonferromagnetic materials.Charlie Chong/ Fion Zhang 77. In the applet below, the probe and the sample are shown in cross-section.The boxes represent the cross-sectional area of a group of turns in the coil.The liftoff distance and the drive current of the probe can be varied to see theeffects of the shared magnetic field. The liftoff value can be set to 0.1 or lessand the current value can be varied from 0.01 to 1.0. The strength of themagnetic field is shown by the darkness of the lines.Charlie Chong/ Fion Zhanghttps://www.nde-ed.org/EducationResources/CommunityCollege/EddyCurrents/Physics/PopUps/applet5/applet5.htm 78. 2.6 Circuits and PhaseA circuit can be thought of as a closed path in which current flows through thecomponents that make up the circuit. The current (i) obeys Ohm's Law, whichis discussed on the page on current flow. The simple circuit below consists ofa voltage source (in this case an alternating current voltage source) and aresistor. The graph below the circuit diagram shows the value of the voltageand the current for this circuit over a period of time. This graph shows onecomplete cycle of an alternating current source. From the graph, it can beseen that as the voltage increases, the current does the same. The voltageand the current are said to be "in-phase" since their zero, peak, and valleypoints occur at the same time. They are also directly proportional to eachother.Charlie Chong/ Fion Zhang 79. In-Phase - Simple resistance circuitCharlie Chong/ Fion Zhang 80. In the circuit below, the resistive component has been replaced with aninductor. When inductance is introduced into a circuit, the voltage and thecurrent will be "out-of-phase," meaning that the voltage and current do notcross zero, or reach their peaks and valleys at the same time. When a circuithas an inductive component, the current (iL) will lag the voltage by onequarter of a cycle. One cycle is often referred to as 360o, so it can be said thatthe current lags the voltage by 90o.Charlie Chong/ Fion Zhang 81. This phase shift occurs because the inductive reactance changes withchanging current. Recall that it is the changing magnetic field caused by achanging current that produces inductive reactance. When the change incurrent is greatest, inductive reactance will be the greatest, and the voltageacross the inductor will be the highest.When the change in current is zero, the inductive reactance will be zero andthe voltage across the inductor will be zero. Be careful not to confuse theamount of current with the amount of change in the current. Consider thepoints where the current reaches it peak amplitude and changes direction inthe graph below (0o, 180o, and 360o). As the current is changing directions,there is a split second when the change in current is zero. Since the changein current is zero, no magnetic field is generated to produce the inductivereactance. When the inductive reactance is zero, the voltage across theinductor is zero.Charlie Chong/ Fion Zhang 82. The resistive and inductive components are of primary interest in eddy currenttesting since the test probe is basically a coil of wire, which will have bothresistance and inductive reactance. However, there is a small amount ofcapacitance in the circuits so a mention is appropriate. This simple circuitbelow consists of an alternating current voltage source and a capacitor.Capacitance in a circuit caused the current (ic) to lead the voltage by onequarter of a cycle (90o current lead ).Charlie Chong/ Fion ZhangKeyword: In capacitor circuit, the current ic isleading the voltage Vc by 90o. In inductor circuit, the current iL islagging the voltage VL by 90o. 83. Phase Shift Capacitor Circuit iC leading Vc by 90oCharlie Chong/ Fion Zhang 84. Phase Shift Inductor Circuit ii lagging Vi by 90oCharlie Chong/ Fion Zhang 85. When there is both resistance and inductive reactance (and/or capacitance)in a circuit, the combined opposition to current flow is known as impedance.Impedance will be discussed more on the next page.Keywords: Resistance Inductive reactance Capacitive reactanceCharlie Chong/ Fion Zhang 86. Capacitor & Inductor phase change https://www.youtube.com/watch?v=ykgmKOVkyW0Charlie Chong/ Fion Zhang 87. 2.7 ImpedanceElectrical Impedance (Z), is the total opposition that a circuit presents toalternating current. Impedance is measured in ohms and may includeresistance (R), inductive reactance (XL), and capacitive reactance (XC).However, the total impedance is not simply the algebraic sum of theresistance, inductive reactance, and capacitive reactance. Since the inductivereactance and capacitive reactance are 90o out of phase with the resistanceand, therefore, their maximum values occur at different times, vector additionmust be used to calculate impedance.Keywords: Vector additionCharlie Chong/ Fion Zhang 88. Current leadvoltage by 90oCurrent laggingvoltage by 90oCharlie Chong/ Fion Zhang 89. In the image below, a circuit diagram is shown that represents an eddycurrent inspection system. The eddy current probe is a coil of wire so itcontains resistance and inductive reactance when driven by alternatingcurrent. The capacitive reactance can be dropped as most eddy currentprobes have little capacitive reactance. The solid line in the graph belowshows the circuit's total current, which is affected by the total impedance ofthe circuit. The two dashed lines represent the portion of the current that isaffected by the resistance and the inductive reactance componentsindividually.It can be seen that the resistance and the inductive reactance lines are 90oout of phase, so when combined to produce the impedance line, the phaseshift is somewhere between zero and 90o. The phase shift is alwaysrelative to the resistance line since the resistance line is always in-phase withthe voltage. If more resistance than inductive reactance is present in thecircuit, the impedance line will move toward the resistance line and the phaseshift will decrease. If more inductive reactance is present in the circuit, theimpedance line will shift toward the inductive reactance line and the phaseshift will increase.Charlie Chong/ Fion Zhang 90. Current Phase Shift Inductance a vector sum of resistance & reactanceCharlie Chong/ Fion ZhangIf more resistance than inductivereactance is present in the circuit,the impedance line will movetoward the resistance line and thephase shift will decrease. If moreinductive reactance is present inthe circuit, the impedance line willshift toward the inductivereactance line and the phase shiftwill increase. 91. The relationship between impedanceand its individual components(resistance and inductive reactance) canbe represented using a vector as shownbelow. The amplitude of the resistancecomponent is shown by a vector alongthe x-axis and the amplitude of theinductive reactance is shown by a vectoralong the y-axis.The amplitude of the impedance isshown by a vector that stretches fromzero to a point that represents both theresistance value in the x-direction andthe inductive reactance in the y-direction.Eddy current instruments withimpedance plane displays presentinformation in this format.Charlie Chong/ Fion Zhang 92. Eddy Impedance plane responsesCharlie Chong/ Fion Zhang 93. Eddy current impedance plane displaysCharlie Chong/ Fion Zhanghttp://www.geocities.ws/raobpc/EC-Def.html 94. The impedance in a circuit with resistance and inductive reactance can becalculated using the following equation. If capacitive reactance was present inthe circuit, its value would be added to the inductance term before squaring.The phase angle of the circuit can also be calculated using sometrigonometry. The phase angle is equal to the ratio between the inductanceand the resistance in the circuit. With the probes and circuits used innondestructive testing, capacitance can usually be ignored so only inductivereactance needs to be accounted for in the calculation. The phase angle canbe calculated using the equation below. If capacitive reactance was presentin the circuit, its value would simply be subtracted from the inductivereactance term.Charlie Chong/ Fion Zhangor 95. The applet below can be used to see how the variables in the above equationare related on the vector diagram (or the impedance plane display). Valuescan be entered into the dialog boxes or the arrow head on the vector diagramcan be dragged to a point representing the desired values. Note that thecapacitive reactance term has been included in the applet but as mentionedbefore, in eddy current testing this value is small and can be ignored.Charlie Chong/ Fion Zhanghttps://www.nde-ed.org/EducationResources/CommunityCollege/EddyCurrents/Physics/PopUps/applet2_6/applet2_6.htm 96. Impedance and Ohm's LawIn previous pages, Ohm's Law was discussed for a purely resistive circuit.When there is (1) inductive reactance or (2) capacitive reactance also presentin the circuit, Ohm's Law must be written to include the total impedance in thecircuit. Therefore, Ohm's law becomes:I = V / Z (the usual R = Z)Ohm's law now simply states that the current (I), in amperes, is proportional tothe voltage (V), in volts, divided by the impedance (Z), in ohms.Charlie Chong/ Fion Zhang 97. The applet below can be used to see how the current and voltage of a circuitare affected by impedance. The applet allows the user to vary the inductance(L), resistance (R), voltage (V) and current (I). Voltage and current are shownas they would be displayed on an oscilloscope. Note that the resistanceand/or the inductive reactance values must be changed to change theimpedance in the circuit.Charlie Chong/ Fion Zhanghttps://www.nde-ed.org/EducationResources/CommunityCollege/EddyCurrents/Physics/PopUps/applet3/applet3.htm 98. Also note that when there is inductance in the circuit, the voltage and currentare out of phase. This is because the voltage across the inductor will be amaximum when the rate of change of the current is greatest. For a sinusoidalwave form like AC, this is at the point where the actual current is zero. Thusthe voltage applied to an inductor reaches its maximum value a quarter-cyclebefore the current does, and the voltage is said to lead the current by 90o.Charlie Chong/ Fion Zhang 99. Resistance, reactance and impedance 59:35! https://www.youtube.com/watch?v=FEERuJlwBxECharlie Chong/ Fion Zhang 100. Impedance www.youtube.com/embed/Pj4Rq1ZIeDI www.youtube.com/embed/FEERuJlwBxE www.youtube.com/watch?v=xyMH8wKK-Ag www.youtube.com/embed/y1ES6WrALzICharlie Chong/ Fion Zhang 101. 2.8 Depth of Penetration & Current DensityEddy currents are closed loops of induced current circulating in planesperpendicular to the magnetic flux. They normally travel parallel to the coil'swinding and flow is limited to the area of the inducing magnetic field. Eddycurrents concentrate near the surface adjacent to an excitation coil and theirstrength decreases with distance from the coil as shown in the image. Eddycurrent density decreases exponentially with depth. This phenomenon isknown as the skin effect.Charlie Chong/ Fion Zhang 102. Depth of Penetration & Current DensityCharlie Chong/ Fion Zhanghttp://www.suragus.com/en/company/eddy-current-testing-technology 103. The skin effect arises when the eddy currents flowing in the test object at anydepth produce magnetic fields which oppose the primary field, thus reducingthe net magnetic flux and causing a decrease in current flow as the depthincreases. Alternatively, eddy currents near the surface can be viewed asshielding the coil's magnetic field, thereby weakening the magnetic field atgreater depths and reducing induced currents.Charlie Chong/ Fion Zhang 104. Eddy current inspectionCharlie Chong/ Fion Zhanghttp://www.azom.com/article.aspx?ArticleID=8016 105. The depth that eddy currents penetrate into a material is affected by thefrequency of the excitation current and the electrical conductivity andmagnetic permeability of the specimen. The depth of penetration decreaseswith increasing frequency and increasing conductivity and magneticpermeability.The depth at which eddy current density has decreased to 1/e, or about 37%of the surface density, is called the standard depth of penetration (d).The word 'standard' denotes plane wave electromagnetic field excitationwithin the test sample (conditions which are rarely achieved in practice).Although eddy currents penetrate deeper than one standard depth ofpenetration, they decrease rapidly with depth. At two standard depths ofpenetration (2d), eddy current density has decreased to 1/e squared or 13.5%of the surface density. At three depths (3d), the eddy current density is downto only (1/e)3 5% of the surface density.Charlie Chong/ Fion Zhang 106. Eddy current inspection penetrationCharlie Chong/ Fion Zhanghttp://www.cnde.iastate.edu/faa-casr/engineers/Supporting%20Info/Supporting%20Info%20Pages/Eddy%20Pages/Eddy-uses.html 107. Eddy current inspectionCharlie Chong/ Fion Zhang 108. Since the sensitivity of an eddy current inspection depends on the eddycurrent density at the defect location, it is important to know the strength ofthe eddy currents at this location. When attempting to locate flaws, a frequency is often selected whichplaces the expected flaw depth within one standard depth of penetration(1/e). This helps to assure that the strength of the eddy currents will besufficient to produce a flaw indication. Alternately, when using eddy currents to measure the electrical conductivity of amaterial, the frequency is often set so that it produces three standarddepths (1/e)3 of penetration within the material. This helps to assure thatthe eddy currents will be so weak at the back side of the material thatchanges in the material thickness will not affect the eddy currentmeasurements.Charlie Chong/ Fion Zhang 109. Defect Detection / Electrical conductivity measurement1/e or 37% ofsurface density attargetCharlie Chong/ Fion Zhang(1/e)3 or 5% ofsurface density atmaterial interfaceDefect Detection Electrical conductivity measurement 110. The applet below illustrates how eddy current density changes in a semi-infiniteconductor. The applet can be used to calculate the standard depth ofpenetration. The equation for this calculation is:Where: = Standard Depth of Penetration (mm) = 3.14f = Test Frequency (Hz) = Magnetic Permeability (H/mm) = Electrical Conductivity (% IACS)Charlie Chong/ Fion Zhang 111. (Note: The applet has an input box for relative permeability since this is oftenthe more readily available value. The applet multiplies the relativepermeability of the material by the permeability of free space to get to H/mmunits.) The applet also indicates graphically the phase lag at one and twostandard depths of penetration. Phase lag will be discussed on the followingpage.Charlie Chong/ Fion Zhanghttps://www.nde-ed.org/EducationResources/CommunityCollege/EddyCurrents/Physics/PopUps/applet7/applet7.htm 112. 2.9 Phase LagPhase lag is a parameter of the eddy current signal that makes it possible toobtain information about the depth of a defect within a material.Phase lag is the shift in time between the eddy current response from adisruption on the surface and a disruption at some distance below the surface.The generation of eddy currents can be thought of as a time dependentprocess, meaning that the eddy currents below the surface take a little longerto form than those at the surface. Disruptions in the eddy currents away fromthe surface will produce more phase lag than disruptions near the surface.Both the signal voltage and current will have this phase shift or lag with depth,which is different from the phase angle discussed earlier. (With the phaseangle, the current shifted with respect to the voltage.)Charlie Chong/ Fion Zhang 113. Eddy current inspection- Types of ProbesCharlie Chong/ Fion Zhanghttp://fatheata.blogspot.com/2009/05/eddy-current-non-destructive-testing.html 114. Phase lag is an important parameter in eddy current testing because it makesit possible to estimate the depth of a defect, and with proper referencespecimens, determine the rough size of a defect. The signal produced by aflaw depends on both the amplitude and phase of the eddy currents beingdisrupted. A small surface defect and large internal defect can have a similareffect on the magnitude of impedance in a test coil. However, because of theincreasing phase lag with depth, there will be a characteristic difference in thetest coil impedance vector.Phase lag can be calculated with the following equation. The phase lag anglecalculated with this equation is useful for estimating the subsurface depth of adiscontinuity that is concentrated at a specific depth. Discontinuities, such asa crack that spans many depths, must be divided into sections along itslength and a weighted average determined for phase and amplitude at eachposition below the surface.Keywords:Phase lag Depth of defectAmplitude Depth, size (current density)Charlie Chong/ Fion Zhang 115. In RadianIn Degree=Phase Lag (Radian or Degrees)x =Distance Below Surface (in or mm)=Standard Depth of Penetration (in or mm)Charlie Chong/ Fion Zhang 116. At one standard depth of penetration, the phase lag is one radian or 57o. Thismeans that the eddy currents flowing at one standard depth of penetration (d)below the surface, lag the surface currents by 57o. At two standard depths ofpenetration (2d), they lag the surface currents by 114o. Therefore, bymeasuring the phase lag of a signal the depth of a defect can be estimated.On the impedance plane, the liftoff signal serves as the reference phasedirection.The angle between the liftoff and defect signals is about twice the phaselag calculated with the above equation.As mentioned above, discontinuities that have a significant dimension normalto the surface, will produce an angle that is based on the weighted average ofthe disruption to the eddy currents at the various depths along its length.Charlie Chong/ Fion Zhang 117. 3.0 Instruments3.1 Eddy Current InstrumentsEddy current instruments can be purchased in a large variety ofconfigurations. Both analog and digital instruments are available. Instrumentsare commonly classified by the type of display used to present the data. Thecommon display types are analog meter, digital readout, impedance planeand time versus signal amplitude. Some instruments are capable ofpresenting data in several display formats.The most basic eddy current testing instrument consists of an alternatingcurrent source, a coil of wire connected to this source, and a voltmeter tomeasure the voltage change across the coil. An ammeter could also be usedto measure the current change in the circuit instead of using the voltmeter.Charlie Chong/ Fion Zhang 118. While it might actually be possible to detect some types of defects with thistype of equipment, most eddy current instruments are a bit moresophisticated. In the following pages, a few of the more important aspects ofeddy current instrumentation will be discussed.Charlie Chong/ Fion Zhang 119. 3.2 Resonant CircuitsEddy current probes typically have a frequency or a range of frequencies thatthey are designed to operated. When the probe is operated outside of thisrange, problems with the data can occur. When a probe is operated at toohigh of a frequency, resonance can occurs in the circuit. In a parallel circuitwith resistance (R), inductance (XL) and capacitance (XC), as the frequencyincreases XL decreases and XC increase. Resonance occurs when XL andXC are equal but opposite in strength. At the resonant frequency, the totalimpedance of the circuit appears to come only from resistance since XL andXC cancel out.Every circuit containing capacitance and inductance has a resonantfrequency that is inversely proportional to the square root of the product of thecapacitance and inductance.Charlie Chong/ Fion Zhang 120. Resonant Circuits https://www.youtube.com/watch?v=hqhV50852jACharlie Chong/ Fion Zhang 121. Eddy current inspectionCharlie Chong/ Fion ZhangAt resonant frequency Xc and XLcancelled out each other. Thus thephase angle is zero, only theresistance component exist. Thecurrent is at it maximum. 122. Current leadvoltage by 90oCurrent laggingvoltage by 90oCharlie Chong/ Fion Zhang 123. Phase angle Charlie Chong/ Fion ZhangCurrent laggingvoltage by angle Voltage (lead current) 124. RCL Circuit Resonant frequencyCharlie Chong/ Fion Zhanghttp://hyperphysics.phy-astr.gsu.edu/hbase/electric/rlcser.html 125. Resonant Frequency https://www.youtube.com/watch?v=lWRTzmvk2lUCharlie Chong/ Fion Zhang 126. Resonance part of 59:35! https://www.youtube.com/watch?v=FEERuJlwBxECharlie Chong/ Fion ZhangA good explanationon resonantfrequency at thisportion of this lengthvideo. 127. More on Resonant FrequencyCharlie Chong/ Fion Zhanghttp://hyperphysics.phy-astr.gsu.edu/hbase/electric/serres.html 128. Resonant FrequencyCharlie Chong/ Fion Zhanghttp://www.electronics-tutorials.ws/accircuits/parallel-resonance.html 129. Resonant FrequencyCharlie Chong/ Fion Zhanghttp://www.electronics-tutorials.ws/accircuits/parallel-resonance.html 130. More on Resonant Frequency: Average power versus frequency for aseries RCL circuit. The width of each curve is measured between the twopoints where the power is half the maximum at the resonance frequency 0Charlie Chong/ Fion Zhanghttp://www.kshitij-school.com/Study-Material/Class-12/Physics/Alternating-current-circuits/Resonance-in-a-series-RLC-circuit.aspx 131. In eddy current probes and cables, it is commonly stated that capacitance isnegligible. However, even circuits not containing discreet components forresistance, capacitance, and inductance can still exhibit their effects. Whentwo conductors are placed side by side, there is always some capacitancebetween them. Thus, when many turns of wire are placed close together in acoil, a certain amount of stray capacitance is produced. Additionally, thecable used to interconnect pieces of electronic equipment or equipment toprobes, often has some capacitance, as well as, inductance. This straycapacitance is usually very small and in most cases has no significant effect.However, they are not negligible in sensitive circuits and at high frequenciesthey become quite important.The applet below represents an eddy current probe with a default resonantfrequency of about 1.0 kHz. An ideal probe might contain just the inductance,but a realistic probe has some resistance and some capacitance. The appletinitially shows a single cycle of the 1.0 kHz current passing through theinductor.Charlie Chong/ Fion Zhang 132. RCL circuit in parallelCharlie Chong/ Fion Zhanghttps://www.nde-ed.org/EducationResources/CommunityCollege/EddyCurrents/Instrumentation/Popups/applet1/applet1.htm 133. Exercise 1: Using your mouse, adjust the resistance by sliding the slide bar.Does the frequency change?Exercise 2: Note that changing the inductance and/or the capacitancechanges the resonant frequency of this resonant circuit. Can you find severalcombinations of capacitance and inductance that resonate at 1.0 kHz?Charlie Chong/ Fion Zhang 134. 3.3 BridgesThe bridge circuit shown in the applet below is known as the Maxwell-Wienbridge (often called the Maxwell bridge), and is used to measure unknowninductances in terms of calibrated resistance and capacitance. Calibration-gradeinductors are more difficult to manufacture than capacitors of similarprecision, and so the use of a simple "symmetrical" inductance bridge is notalways practical. Because the phase shifts of inductors and capacitors areexactly opposite each other, a capacitive impedance can balance out aninductive impedance if they are located in opposite legs of a bridge, as theyare here.Unlike this straight Wien bridge, the balance of the Maxwell-Wien bridge isindependent of the source frequency. In some cases, this bridge can be madeto balance in the presence of mixed frequencies from the AC voltage source,the limiting factor being the inductor's stability over a wide frequency range.Charlie Chong/ Fion Zhang 135. Maxwell-Wien bridgeCharlie Chong/ Fion Zhang 136. Maxwell-Wien bridgeCharlie Chong/ Fion Zhanghttps://www.nde-ed.org/EducationResources/CommunityCollege/EddyCurrents/Instrumentation/Popups/applet2/applet2.htm 137. Exercise: Using the equations within the applet, calculate appropriate valuesfor C and R2 for a set of probe values. Then, using your calculated values,balance the bridge. The oscilloscope trace representing current (brightestgreen) across the top and bottom of the bridge should be minimized (straightline).In the simplest implementation, the standard capacitor (C) and the resistor inparallel with it are made variable, and both must be adjusted to achievebalance. However, the bridge can be made to work if the capacitor is fixed(non-variable) and more than one resistor is made variable (at least theresistor in parallel with the capacitor, and one of the other two). However, inthe latter configuration it takes more trial-and-error adjustment to achievebalance as the different variable resistors interact in balancing magnitude andphase.Another advantage of using a Maxwell bridge to measure inductance ratherthan a symmetrical inductance bridge is the elimination of measurement errordue to the mutual inductance between the two inductors. Magnetic fieldscan be difficult to shield, and even a small amount of coupling between coilsin a bridge can introduce substantial errors in certain conditions. With nosecond inductor to react within the Maxwell bridge, this problem is eliminated.Charlie Chong/ Fion Zhang 138. A Maxwell bridge (in long form, a Maxwell-Wien bridge) is a type ofWheatstone bridge used to measure an unknown inductance (usually of lowQ value) in terms of calibrated resistance and capacitance. It is a real productbridge.It uses the principle that the positive phase angle of an inductive impedancecan be compensated by the negative phase angle of a capacitive impedancewhen put in the opposite arm and the circuit is at resonance; i.e., no potentialdifference across the detector and hence no current flowing through it. Theunknown inductance then becomes known in terms of this capacitance.With reference to the picture, in a typical application R1 and R4 are knownfixed entities, and R2 and C2 are known variable entities. R2 and C2 areadjusted until the bridge is balanced.Charlie Chong/ Fion Zhanghttp://en.wikipedia.org/wiki/Maxwell_bridge 139. R3 and L3 can then be calculated based on the values of the othercomponents:Charlie Chong/ Fion ZhangR1 L3http://en.wikipedia.org/wiki/Maxwell_bridgeC2R2R3R4 140. A Maxwell bridge (in long form, a Maxwell-Wien bridge) is a type ofWheatstone bridge used to measure an unknown inductance (usually of lowQ value) in terms of calibrated resistance and capacitance. It is a real productbridge.The Q, quality factor, of a resonant circuit is a measure of the goodness orquality of a resonant circuit. A higher value for this figure of merit correspondsto a more narrow bandwidth, which is desirable in many applications. Moreformally, Q is the ratio of power stored to power dissipated in the circuitreactance and resistance, respectively:Q = P stored /P dissipated = I2XL / I2RQ = XL /Rwhere:X = Capacitive or Inductive reactance at resonanceR = Series resistance.Charlie Chong/ Fion Zhanghttp://www.allaboutcircuits.com/vol_2/chpt_6/6.html 141. 3.4 Display - Complex Impedance Plane (eddy scope)Electrical Impedance (Z), is the total opposition that a circuit presents to analternating current. Impedance, measured in ohms, may include resistance(R), inductive reactance (XL), and capacitive reactance (XC).Eddy current circuits usually have only R and (XL) components. As discussedin the page on impedance, the resistance component and the reactancecomponent are not in phase, so vector addition must be used to relate themwith impedance. For an eddy current circuit with resistance and inductivereactance components, the total impedance is calculated using the followingequation.Charlie Chong/ Fion Zhang 142. ReactanceCharlie Chong/ Fion ZhangXc was assumed nil 143. You will recall that this can be graphically displayed using the impedanceplane diagram as seen above. Impedance also has an associated angle,called the phase angle of the circuit, which can be calculated by the followingequation.The impedance plane diagram is a very useful way of displaying eddy currentdata. As shown in the figure below, the strength of the eddy currents and themagnetic permeability of the test material cause the eddy current signal onthe impedance plane to react in a variety of different ways.Keywords:Impedance plane diagramCharlie Chong/ Fion Zhang 144. Eddy current inspectionCharlie Chong/ Fion Zhang 145. Phasor DiagramCharlie Chong/ Fion ZhangAlSteel 146. If the eddy current circuit is balancedin air and then placed on a piece ofaluminum, the resistance componentwill increase (eddy currents are beinggenerated in the aluminum and thistakes energy away from the coil,which shows up as resistance) andthe inductive reactance of the coildecreases (the magnetic field createdby the eddy currents opposes thecoil's magnetic field and the net effectis a weaker magnetic field to produceinductance). If a crack is present inthe material, fewer eddy currents willbe able to form and the resistance willgo back down and the inductivereactance will go back up. Changes inconductivity will cause the eddycurrent signal to change in a differentway.Charlie Chong/ Fion Zhang 147. Impedance Plane Respond - Non magnetic materialsCharlie Chong/ Fion Zhang 148. Eddy current inspectionCharlie Chong/ Fion Zhang 149. The resistance component R will increase(eddy currents are being generated in the aluminum and this takesenergy away from the coil, which shows up as resistance) The inductive reactance XL of the coil decreases(the magnetic field created by the eddy currents opposes the coil'smagnetic field and the net effect is a weaker magnetic field toproduce inductance).Charlie Chong/ Fion Zhang 150. If a crack is present in the material, fewer eddy currents will beable to form and the resistance will go back down and theinductive reactance will go back up.Charlie Chong/ Fion Zhang 151. Changes in conductivity will cause the eddy current signal tochange in a different way.Charlie Chong/ Fion Zhang 152. DiscussionTopic: Discuss on Changes in conductivity will cause the eddy current signalto change in a different way.Answer: Increase in conductivity will increase the intensity of eddy current onthe surface of material, the strong eddy current generated will reduce thecurrent of the coil, show-up as R &XLCharlie Chong/ Fion Zhang 153. Magnetic MaterialsCharlie Chong/ Fion Zhang 154. When a probe is placed on a magneticmaterial such as steel, something differenthappens. Just like with aluminum(conductive but not magnetic), eddycurrents form, taking energy away from thecoil, which shows up as an increase in thecoils resistance. And, just like with thealuminum, the eddy currents generate theirown magnetic field that opposes the coilsmagnetic field. However, you will note forthe diagram that the reactance increases.This is because the magnetic permeabilityof the steel concentrates the coil'smagnetic field. This increase in themagnetic field strength completelyovershadows the magnetic field of theeddy currents. The presence of a crack ora change in the conductivity will produce achange in the eddy current signal similar tothat seen with aluminum.Charlie Chong/ Fion Zhang 155. The eddy currents form, taking energy away from the coil, whichshows up as an increase in the coils resistance. The reactance increases. This is because the magnetic permeabilityof the steel concentrates the coil's magnetic field. This increase in the magnetic field strength completely overshadowsthe effects magnetic field of the eddy currents on decreasing theinductive reactance.Charlie Chong/ Fion Zhang 156. This increase in the magnetic field strength completely overshadows themagnetic field of the eddy currents.The inductive reactance XL of the coil decreases(the magnetic field created by the eddy currents opposes the coil's magneticfield and the net effect is a weaker magnetic field to produce inductance).Charlie Chong/ Fion Zhang 157. The presence of a crack or a change in the conductivity will produce achange in the eddy current signal similar to that seen with aluminum. If a crack is present in the material, fewer eddy currents will be ableto form and the resistance will go back down and the inductivereactance will go back up Changes in conductivity will cause the eddy current signal to changein a different way.Charlie Chong/ Fion Zhang 158. Eddy current inspectionCharlie Chong/ Fion ZhangThe increase of Inductive Reactance: this isdue to concentration of magnetic field by theeffects magnetic permeability of steelThe increase in Resistance R: this was due to thedecrease in current due to generation of eddy current,shown-up as increase in resistance R. 159. Exercise: Explains the impedance plane responds for Aluminum andSteelCharlie Chong/ Fion ZhangAl:1. Eddy current reduces coil current show-upas R,XL2. Crack reduce eddy current, reduce theeffects on R & XL3. Increase in conductivity increase eddycurrent, increasing the effects on R & XLSteel:1. Eddy current reduces coil current show-upas R,XL. However net XL increase,as magnetic permeability of the steelconcentrates the coil's magnetic field1231 160. In the applet below, liftoff curves can be generated for several nonconductivematerials with various electrical conductivities. With the probe held away fromthe metal surface, zero and clear the graph. Then slowly move the probe tothe surface of the material. Lift the probe back up, select a different materialand touch it back to the sample surface.Charlie Chong/ Fion Zhang 161. Impedance Plane Respond Fe, Cu, AlQuestion: Why impedance plane respond of steel(Fe) in the same quadrant as the non-magnetic Cuand AlCharlie Chong/ Fion ZhangFeAlCuhttps://www.nde-ed.org/EducationResources/CommunityCollege/EddyCurrents/Instrumentation/Popups/applet3/applet3.htm 162. ExperimentGenerate a family of liftoff curves for the different materials available in theapplet using a frequency of 10kHz. Note the relative position of each of thecurves. Repeat at 500kHz and 2MHz. (Note: it might be helpful to capturean image of the complete set of curves for each frequency for comparison.)1) Which frequency would be best if you needed to distinguish between twohigh conductivity materials?2) Which frequency would be best if you needed to distinguish between twolow conductivity materials?The impedance calculations in the above applet are based on codes by Jack Blitz from "Electricaland Magnetic Methods of Nondestructive Testing," 2nd ed., Chapman and HillCharlie Chong/ Fion Zhanghttp://en.wikipedia.org/wiki/Electrical_reactance 163. HurrayCharlie Chong/ Fion Zhang 164. 3.5 Display - Analog MeterAnalog instruments are the simplest of the instruments available for eddycurrent inspections. They are used for crack detection, corrosion inspection,or conductivity testing. These types of instruments contain a simple bridgecircuit, which compares a balancing load to that measured on the testspecimen. If any changes in the test specimen occur which deviate fromnormal you will see a movement on the instruments meter.Charlie Chong/ Fion Zhang 165. Eddy current Digital meterCharlie Chong/ Fion Zhang 166. Analog meters such as the D'Arsonval design pictured in the applet below,must "rectify" the AC into DC. This is most easily accomplished through theuse of devices called diodes. Without going into elaborate detail over howand why diodes work as they do, remember that they each act like a one-wayvalve for electrons to flow. They act as a conductor for one polarity and aninsulator for another. Arranged in a bridge, four diodes will serve to steer ACthrough the meter movement in a constant direction.An analog meter can easily measure just a few microamperes of current andis well suited for use in balancing bridges.Charlie Chong/ Fion Zhang 167. Exercise: Using the equations within the applet, calculate appropriate valuesfor C and R2 for a set of probe values. Then balance the bridge using yourcalculated values. The analog meter should swing close to the left end if itsscale indicates little or no current across the bridge. Across the bridge shouldbe minimized (straight line).Charlie Chong/ Fion Zhanghttps://www.nde-ed.org/EducationResources/CommunityCollege/EddyCurrents/Instrumentation/Popups/applet4/applet4.htm 168. Movie Time http://www.giniko.com/watch.php?id=216Charlie Chong/ Fion Zhang 169. Introduction to Eddy Current Theory https://www.youtube.com/watch?v=djFvnFy3rJcCharlie Chong/ Fion Zhang 170. Eddy Current Math https://www.youtube.com/watch?v=V-IW6cFIt9ECharlie Chong/ Fion Zhang 171. Standard depth penetration https://www.youtube.com/watch?v=G2Yh7tZpKboCharlie Chong/ Fion Zhang 172. Eddy Current Curve https://www.youtube.com/watch?v=Bojm5F_4ay4Charlie Chong/ Fion Zhang 173. Introduction to Eddy Current Machine https://www.youtube.com/watch?v=S34yt8-zgnsCharlie Chong/ Fion Zhang 174. Conductivity Measurement - https://www.youtube.com/watch?v=pvTUomSYEt8Charlie Chong/ Fion Zhang 175. Crack Detections - https://www.youtube.com/watch?v=1YUSn___VxQCharlie Chong/ Fion Zhang 176. 4.0 Probes (Coils)4.1 Impedance MatchingEddy current testing requires us to determine the components of theimpedance of the detecting coil or the potential difference across it. Mostapplications require the determination only of changes in impedance, whichcan be measured with a high degree of sensitivity using an AC bridge. Theprinciples of operation of the most commonly used eddy current instrumentsare based on Maxwell's inductance bridge, in which the components of theimpedance of the detecting coil, commonly called a probe, are compared withknown variable impedances connected in series and forming the balancingarm of the bridge. Refer back to Bridges.Charlie Chong/ Fion Zhang 177. Maxwell inductance bridgeCharlie Chong/ Fion Zhang 178. The input to the bridge is an AC oscillator, often variable in both frequencyand amplitude. The detector arm takes the form of either a meter or a storagecathode-ray oscilloscope, a phase-sensitive detector, a rectifier to provide asteady indication, and usually an attenuator to confine the output indicationwithin a convenient range. Storage facilities are necessary in the oscilloscopein order to retain the signal from the detector for reference during scanningwith the probe.The highest sensitivity of detection is achieved by properly matching theimpedance of the probe to the impedance of the measuring instrument.Thus, with a bridge circuit that is initially balanced, a subsequent but usuallysmall variation in the impedance of the probe upsets the balance, and apotential difference appears across the detector arm of the bridge.Question:Is it the potential difference appears in the CRT?Charlie Chong/ Fion Zhang 179. Although the Maxwell inductance bridge forms the basis of most eddy currentinstruments, there are several reasons why it cannot be used in its simplestform (i.e. Hague, 1934), including the creation of stray capacitances, such asthose formed by the leads and leakages to earth. These unwantedimpedances can be eliminated by earthing devices and the addition ofsuitable impedances to produce one or more wide-band frequency (i.e. low Q)resonance circuits. Instruments having a wide frequency range (i.e. from 1kHz to 2 MHz) may possess around five of these bands to cover the range.The value of the impedance of the probe is therefore an importantconsideration in achieving proper matching and, as a result, it may benecessary to change the probe when switching from one frequency band toanother.Charlie Chong/ Fion Zhang 180. Q Values for frequency (applicable to mechanical sonic or electrical)Charlie Chong/ Fion ZhangThe word Q does nothave any impact on thequality elements in aspecific testing method(UT, ET etc.), it is simply aterm used to describesthe bandwidth of afrequency in questioned.http://community.calrec.com/q-in-60-seconds/ 181. Q Values for frequency (applicable to mechanical sonic or electrical)Charlie Chong/ Fion Zhanghttp://www.eeweb.com/blog/rodney_green_2/a-history-of-hf-radio-receivers-part-2 182. 4.2 Coil (Probe) DesignThe most important feature in eddy current testing is the way in which theeddy currents are induced and detected in the material under test. Thisdepends on the design of the probe. As discussed in the previous pages,probes can contain one or more coils, a core and shielding. All have animportant effect on the probe, but the coil requires the most designconsideration.A coil consists of a length of wire wound in a helical manner around thelength of a former. The main purpose of the former is to provide a sufficientamount of rigidity in the coil to prevent distortion. Formers used for coils withdiameters greater than a few millimeters (i.e. encircling and pancake coils),generally take the form of tubes or rings made from dielectricmaterials. Small-diameter coils are usually wound directly onto a solid former.Charlie Chong/ Fion Zhang 183. The region inside the former is called the core, which can consist of either asolid material or just air. When the core is air or a nonconductive material,the probe is often referred to as an air-core probe. Some coils are woundaround a ferrite core which concentrates the coil's magnetic field into asmaller area. These coils are referred to as "loaded" coils.The wire used in an eddy current probe is typically made from copper or othernonferrous metal to avoid magnetic hysteresis effects. The winding usuallyhas more than one layer so as to increase the value of inductance for a givenlength of coil. The higher the inductance (L) of a coil, at a givenfrequency, the greater the sensitivity of eddy current testing.Keywords:Air coreLoaded coreMagnetic hysteresis effectsCharlie Chong/ Fion Zhang 184. Magnetic hysteresis effectsCharlie Chong/ Fion Zhang 185. It is essential that the current through the coil is as low as possible. Too higha current may produce: a rise in temperature, hence an expansion of the coil, which increases thevalue of L. magnetic hysteresis, which is small but detectable when a ferrite core isused.The simplest type of probe is the single-coil probe, which is in widespreaduse. The following applet may be used to calculate the effect of the inner andouter diameters, length, number of turns and wire diameter of a simple probedesign on the probe's self inductance. Dimensional units are in millimeters.Charlie Chong/ Fion Zhang 186. Eddy current inspectionCharlie Chong/ Fion Zhanghttps://www.nde-ed.org/EducationResources/CommunityCollege/EddyCurrents/ProbesCoilDesign/Popups/applet1/applet1.htm 187. A more precise value of L is given by:ro is the mean radius of the coil.rc is the radius of the core.l is the length of the coil.n is the number of turns.r is the relative magnetic permeability of the core.o is the permeability of free space (i.e. 4 pi x 10-7 H/m).K is a dimensionless constant characteristic of the length and the external and internalradii.Charlie Chong/ Fion Zhang 188. 4.3 Probes - Mode of OperationEddy current probes are available in a large variety of shapes and sizes. Infact, one of the major advantages of eddy current inspection is that probescan be custom designed for a wide variety of applications. Eddy currentprobes are classified by the configuration and mode of operation of the testcoils. The configuration of the probe generally refers to the way the coil orcoils are packaged to best "couple" to the test area of interest. An example ofdifferent configurations of probes would be bobbin probes, which are insertedinto a piece of pipe to inspect from the inside out, versus encircling probes, inwhich the coil or coils encircle the pipe to inspect from the outside in. Themode of operation refers to the way the coil or coils are wired and interfacewith the test equipment.The mode of operation of a probe generally falls into one of four categories:(1) absolute, (2) differential, (3) reflection and (4) hybrid.Each of these classifications will be discussed in more detail below.Charlie Chong/ Fion Zhanghttp://www.vegastel.eu/index.php/en/sukuriniu-sroviu-metodas-en/eddy-current-probes/693-eddy-current-probe-selection-information 189. Keywords:Configurations of probes(1) Bobbin probes,(2) Encircling probes,Mode of operation(1) absolute, (2) differential, (3) reflection and (4) hybrid.Charlie Chong/ Fion Zhang 190. 4.3.1 Absolute ProbesAbsolute probes generally have a single test coil that is used to generate theeddy currents and sense changes in the eddy current field. As discussed inthe physics section, AC is passed through the coil and this sets up anexpanding and collapsing magnetic field in and around the coil. When theprobe is positioned next to a conductive material, the changing magnetic fieldgenerates eddy currents within the material.The generation of the eddy currents take energy from the coil and thisappears as an increase in the electrical resistance of the coil. The eddycurrents generate their own magnetic field that opposes the magneticfield of the coil and this changes the inductive reactance of the coil.By measuring the absolute change in impedance of the test coil, muchinformation can be gained about the test material.Charlie Chong/ Fion Zhang 191. Absolute coils can be used for flaw detection, conductivity measurements,liftoff measurements and thickness measurements. They are widely used dueto their versatility. Since absolute probes are sensitive to things such asconductivity, permeability liftoff and temperature, steps must be taken tominimize these variables when they are not important to the inspection beingperformed. It is very common for commercially available absolute probes tohave a fixed "air loaded" reference coil that compensates for ambienttemperature variations.Charlie Chong/ Fion Zhang 192. Absolute Probes (Single-Coil Probes)The earliest form of eddy current instruments operated with a single-coilprobe that was wound to a specific value frequency. Many newer models ofeddy current instruments have kept this circuitry as a popular option for userswhile also incorporating more sophisticated functions. When these probes areused, a balance coil is also required which may be set from within the eddycurrent instrument or is commonly found within the probe housing, the cableconnector or in a separate adapter. A problem can arise when the probeinductance value is not close enough to the value of the balance coil causingthe instrument not to balance correctly. The result is poor performance (noisyor insensitive) or no response at all (signal saturation).Charlie Chong/ Fion ZhangBalancing coil 193. The Principle:The generation of the eddy currents take energy from the coil and thisappears as an increase in the electrical resistance of the coil. The eddycurrents generate their own magnetic field that opposes the magnetic field ofthe coil and this changes the inductive reactance of the coil.Variations:The change in inductive reactance could be increasing or decreasingdepending on the magnetic permeability of material.Commons:Irrespective of magnetic permeability, the resistance always increase.Charlie Chong/ Fion Zhang 194. 4.3.2 Differential ProbesDifferential probes have two active coils usually wound in opposition,although they could be wound in addition with similar results. When the twocoils are over a flaw-free area of test sample, there is no differential signaldeveloped between the coils since they are both inspecting identical material.However, when one coil is over a defect and the other is over good material,a differential signal is produced. They have the advantage of being verysensitive to defects yet relatively insensitive to slowly varying properties suchas gradual dimensional or temperature variations. Probe wobble signals () are also reduced with this probe type. There are also disadvantages tousing differential probes. Most notably, the signals may be difficult to interpret.For example, if a flaw is longer than the spacing between the two coils, onlythe leading and trailing edges will be detected due to signal cancellation whenboth coils sense the flaw equally.Charlie Chong/ Fion Zhang 195. Differential Probes with two coils wound in different directionsCharlie Chong/ Fion Zhang 196. Differential (Bridge Type) ProbesIn this configuration the probe coils are located in an electrical "bridge" (seefig. below). The instrument balances the bridge and any change in balance isdisplayed as a signal. In this arrangement, the same coil produces the eddycurrents and detects the impedance changes caused by the defects (or anyother variables). Almost all instruments are able to operate with this type ofcoil arrangement.Charlie Chong/ Fion Zhang 197. 4.3.3 Reflection ProbesReflection probes have two coils similar to a differential probe, but one coil isused to excite the eddy currents and the other is used to sense changes inthe test material. Probes of this arrangement are often referred to asdriver/pickup probes. The advantage of reflection probes is that the driver andpickup coils can be separately optimized for their intended purpose. Thedriver coil can be made so as to produce a strong and uniform flux field in thevicinity of the pickup coil, while the pickup coil can be made very small so thatit will be sensitive to very small defects.Charlie Chong/ Fion Zhang 198. Reflection ProbesCharlie Chong/ Fion Zhang 199. Reflection Type ProbeThese probes are also known as send-receive or driver-pickup. In thisconfiguration, the eddy currents are produced by a coil connected to theinstrument's oscillator (driver). The signals received back in the probe aredetected by separate coils called pickups (see Fig. 3 and Fig. 4). All newimpedance plane instruments and also many older models are able tooperate in both differential (bridge) and reflection modes.Charlie Chong/ Fion Zhang 200. 4.3.4 Hybrid ProbesAn example of a hybrid probe is the split D, differential probe shown below.This probe has a driver coil that surrounds two D shaped sensing coils. Itoperates in the reflection mode but additionally, its sensing coils operate inthe differential mode. This type of probe is very sensitive to surface cracks.Another example of a hybrid probe is one that uses a conventional coil togenerate eddy currents in the material but then uses a different type of sensorto detect changes on the surface and within the test material. An example of ahybrid probe is one that uses a Hall effect sensor to detect changes in themagnetic flux leaking from the test surface. Hybrid probes are usuallyspecially designed for a specific inspection application.Charlie Chong/ Fion Zhang 201. 4.3.5 Differential (Bridge) or Reflection?This is a common question asked by those involved in trying to select the bestprobe for an inspection. The answer is "It depends." Let us consider bothsystems.Gain: Reflection probes will give a higher gain, particularly if they are "tuned"to a specific frequency, but normally the difference is on average about 6 dB.It is true that this doubles the signal, but if you consider that the instrumentsare able to give this increase of gain easily, it is not so important.Nevertheless, in critical applications this increase is very welcomed.Frequency range: Reflection probes do not need to balance the driver to thepickup coils. This means that they will give a wider frequency range. As longas the driver produces eddy currents, the pickup will detect them and somesignal will be displayed. This may not provide good information at certainfrequencies, but the probe is still working!Charlie Chong/ Fion Zhang 202. Bridge type probes used to give a limited frequency span in the olderinstruments, as these had to balance an electrical bridge using its other arms(X and R controls). In modern instruments, the bridge is normally formed withfixed precision resistors, or a fixed transformer inside it. The signals detectedin this manner are electronically processed without any "mechanical"adjustments, and this means a greater ability to balance over a widerfrequency range.Drift: Probe drift is mostly caused by temperature change in the coils. Thismay be caused by varyingambient temperature, or the heat produced by the oscillator current, or both.There are design parameters that can be optimized to reduce drift, such aswire diameter and ferrite selection, but reflection probes are normally a goodchoice to avoid this problem even more.Charlie Chong/ Fion Zhang 203. In a reflection probe, the driver current does not flow through the pickup coils;in fact, the magnetic field received back from the specimen is normally muchsmaller and, consequently, the current flowing in the pickups is also reduced.Most probe types (pencil, spot, ring, bolt hole, etc.) can be made as bridge orreflection. Keep in mind that a reflection probe is almost invariably moredifficult to manufacture and therefore more expensive.Charlie Chong/ Fion Zhang 204. 4.3.6 Absolute, Differential (Bridge) and Reflection ProbesThis is an area where some confusion exists. Many users have called a probe"differential" when the signal displayed gives an up and down movement or afigure 8 type signal. This is caused by the two coils sensing the defect insequence. When both sensing coils are on the probe surface, theycompensate for lift-off and as a result no line is visible (see Fig. 5).Charlie Chong/ Fion ZhangFig. 5 205. In contrast, an absolute or bridge display is produced by a single sensing coil(see Fig. 1 through Fig. 4), giving a single, upward movement with a nearhorizontal lift-off line. Others have called a probe "differential" simply whenthe coils were connected differentially such as in a bridge circuit. The problemwith this definition is that probes can be connected differentially in a reflectionsystem as well as when using two pickups (such as most scanner-driven bolthole probes). In this case, the two pickup coils are positioned close to oneanother and contained within a driver coil (see Fig. 6).Fig. 6Charlie Chong/ Fion ZhangThe best way out of this confusion isoften to specify the probe asabsolute, bridge, reflection, bridgedifferential or reflection differential asneeded. It makes more sense toqualify the description according tothe displayed signal, since this iswhat really matters and not manypeople are concerned as to how thecoils are connected internally. 206. 4.3.7 Shielded and Unshielded ProbesProbes are normally available in both shielded and unshielded versions;however, there is an increasing demand for the shielded variety.Shielding restricts the magnetic field produced by the coils to the physicalsize of the probe. A shield can be made of various materials, but the mostcommon are: ferrite (like a ceramic made of iron oxides), Mu-metal, and mildsteel. Ferrite make the best shielding because they provide an easy path forthe magnetic field but has poor conductivity. This means that there is littleeddy current loss in the shield itself. Mild steel has more losses but is widelyused for spot probes and ring probes due to its ease of machining whenferrite is not available in certain sizes or shapes. Mu-metal is sometimes forpencil probes as it is available in thin sheet; however, it is less effective thanferrite.Note: Mu-metal () is a nickel-iron alloy, composed ofapproximately 77% nickel, 16% iron, 5% copper and 2% chromium ormolybdenum, that is notable for its high magnetic permeability.Charlie Chong/ Fion Zhanghttp://en.wikipedia.org/wiki/Mu-metal 207. Shielding has several advantages: first, it allows the probe to be used neargeometry changes, such as edges, without giving false indications; next, itallows the probe to touch ferrous fastener heads with minimal interference;last, it allows the detection of smaller defects due to the stronger magneticfield concentrated in a smaller area.On the other hand, unshielded probes allow somewhat deeper penetrationdue to the larger magnetic field. They are also slightly more tolerant to lift-off.Unshielded probes are recommended for the inspection of ferrous materials(steel) for surface cracks, and in particular with meter instruments. Thereason for this is that the meter response is too slow to allow the signal from ashielded probe to be displayed at normal scanning speeds due to the smallersensitive area.Charlie Chong/ Fion Zhang 208. 4.3.8 AdaptersTo connect a probe with a connector different from the type used on theinstrument, it is necessary to use an adapter. An adapter consists of twodifferent connectors joined and wired to match the inputs and outputs asnecessary. It is normally housed in a short body that can be positioned at theinstrument's input. Sometimes, it is also possible to have a "cable adapter,"which is made to match a connector located at the probe body. Depending onthe instrument's wiring, it may be possible to have a single adapter for bothbridge and reflection probes. In other cases, it is necessary to have twoseparate adapters or use a switchable type.Charlie Chong/ Fion Zhanghttp://www.vegastel.eu/index.php/en/sukuriniu-sroviu-metodas-en/eddy-current-probes/693-eddy-current-probe-selection-information 209. 4.4 Probes - ConfigurationsAs mentioned on the previous page, eddy current probes are classified by theconfiguration and mode of operation of the test coils. The configuration of theprobe generally refers to the way the coil or coils are packaged to best"couple" to the test area of interest. Some of the common classifications ofprobes based on their configuration include surface probes, bolt hole probes,inside diameter (ID) probes, and outside diameter (OD) probes.Charlie Chong/ Fion Zhang 210. Eddy current probesCharlie Chong/ Fion Zhang 211. Eddy current inspection displayCharlie Chong/ Fion Zhanghttp://www.ibgndt.com/eddyliner-s-eddy-current-testers-hardness-case-depth-structure.php 212. Eddy current inspection systemhttp://idea-ndt.en.alibaba.com/product/488266329-212374104/Automatic_ERW_pipes_eddy_current_and_ultrasonic_testing_systems_and_equipments.htmlCharlie Chong/ Fion Zhang 213. 4.4.1 Surface ProbesSurface probes are usually designed to be handheld and are intended to beused in contact with the test surface. Surface probes generally consist of acoil of very fine wire encased in a protective housing. The size of the coil andshape of the housing are determined by the intended use of the probe.Most of the coils are wound so that the axis of the coil is perpendicular to thetest surface. This coil configuration is sometimes referred to as a pancake coiland is good for detecting surface discontinuities that are orientedperpendicular to the test surface. Discontinuities, such as delaminations, thatare in a parallel plane to the test surface will likely go undetected with this coilconfiguration.Charlie Chong/ Fion Zhang 214. Wide surface coils are used when scanning large areas for relatively largedefects. They sample a relatively large area and allow for deeper penetration.Since they do sample a large area, they are often used for conductivity teststo get more of a bulk material measurement. However, their large samplingarea limits their ability to detect small discontinuities.Pencil probes have a small surface coil that is encased in a long slenderhousing to permit inspection in restricted spaces. They are available with astraight shaft or with a bent shaft, which facilitates easier handling and use inapplications such as the inspection of small diameter bores. Pencil probesare prone to wobble due to their small base and sleeves are sometimes usedto provide a wider base.Keywords:Wide surface- deeper penetrationNarrow probe detect smaller discontinuitiesNarrow probe prone to wobbleCharlie Chong/ Fion Zhang 215. Surface ProbeCharlie Chong/ Fion Zhanghttp://advantech.my/Products%20-%20ET.htm 216. Surface ProbeCharlie Chong/ Fion Zhang 217. Surface ProbeCharlie Chong/ Fion Zhang 218. Surface ProbeCharlie Chong/ Fion Zhang 219. 4.4.2 Bolt Hole ProbesBolt hole probes are a special type of surface probe that is designed to beused with a bolt hole scanner. They have a surface coil that is mounted insidea housing that matches the diameter of the hole being inspected. The probeis inserted in the hole and the scanner rotates the probe within the hole.Charlie Chong/ Fion Zhang 220. Bolt Hole ProbesCharlie Chong/ Fion Zhanghttp://www.phtool.com/pages/eddy.asp 221. Bolt Hole ProbesCharlie Chong/ Fion Zhanghttp://www.olympus-ims.com/en/applications/eddy-current-probes-guide/ 222. Birring NDT Series, Eddy Current Testing # 5,Inspection of Fastener Holes using a Rotary Probe https://www.youtube.com/watch?v=X4yqOLUYrBsCharlie Chong/ Fion Zhang 223. 4.4.3 ID or Bobbin ProbesID probes, which are also referred to as Bobbin probes or feed-throughprobes, are inserted into hollow products, such as pipes, to inspect from theinside out. The ID probes have a housing that keep the probe centered in theproduct and the coil(s) orientation somewhat constant relative to the testsurface. The coils are most commonly wound around the circumference of theprobe so that the probe inspects an area around the entire circumference ofthe test object at one time.Charlie Chong/ Fion Zhang 224. Configuration: Bobbin ProbeCharlie Chong/ Fion Zhang 225. Configuration:Bobbin ProbeCharlie Chong/ Fion Zhang 226. 4.4.4 OD or Encircling CoilsOD probes are often called encircling coils. They are similar to ID probesexcept that the coil(s) encircle the material to inspect from the outside in. ODprobes are commonly used to inspect solid products, such as bars.Charlie Chong/ Fion Zhang 227. Configuration: Encircling probesCharlie Chong/ Fion Zhang 228. Configuration: Encircling probesCharlie Chong/ Fion Zhang 229. Configuration: Encircling probesCharlie Chong/ Fio