Basic Principles of Eddy Current Inspection

Basic Principles of Eddy Current Inspection

Basic Principles of Eddy Current InspectionEddy current inspection is one of several NDT methods that use the principal of electromagnetism as the basis for conducting examinations. Several other methods such as Remote Field Testing (RFT), Flux Leakage and Barkhausen 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, a magnetic field develops in and around the conductor. This magnetic field expands as the alternating current rises to maximum and collapses as the current is reduced to zero. If another electrical conductor is brought into the close proximity to this changing magnetic field, current will be induced in this second conductor. Eddy currents are induced electrical currents that flow in a circular path. They get their name from eddies that are formed when a liquid or gas flows in a circular path around obstacles when conditions are right.

One of the major advantages of eddy current as an NDT tool is the variety of inspections and measurements that can be performed. In the proper circumstances, eddy currents can be used for

Crack Detection

Material Thickness Measurements

Coating Thickness Measurements

Conductivity Measurements For:

Material Identification

Heat Damage Detection

Case Depth Determination

Heat Treatment Monitoring

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

Some of the limitation 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 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 and probe scan direction are undetectable

History of Eddy Current TestingEddy current testing has its origins with Michael Faraday's discovery of electromagnetic induction in 1831. Faraday was a chemist in England during the early 1800's and is credited with the discovery of electromagnetic induction, electromagnetic rotations, the magneto-optical effect, diamagnetism, and many other discoveries. In 1879, another scientist named Hughes recorded changes in the properties of a coil when placed in contact with metals of different conductivity and permeability. However, 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. Eddy current testing is now a widely used and well-understood inspection technique.

Present State of Eddy Current InspectionEddy current inspection is used in a variety of industries to find defects and make measurements. One of the primary uses of eddy current testing is for defect 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 design and test parameters must be established with a good understanding of the flaw that is trying to be detected. Since eddy currents tend to concentrate at the surface of a material, they can only be used to detect surface and near surface defects.

In thin materials such as tubing and sheet stock, eddy currents can be used to measure the thickness of the material. This makes eddy current a useful tool for detecting corrosion damage and other damage that causes a thinning of the material. The technique is used to make corrosion thinning measurements on aircraft skins and in the walls of tubing used in assemblies such as heat exchangers. Eddy current testing is also used to measure the thickness of paints and other coatings.

Eddy currents are also affected by the electrical conductivity and magnetic permeability of materials. Therefore, eddy current measurements can be used to sort materials and to tell if a material has seen high temperatures or been heat treated, which changes the conductivity of some materials.

Eddy current equipment and probes can be purchased in a wide variety of configurations. Eddyscopes and a conductivity tester come packaged in very small and battery operated units for easy portability. Computer based systems are also available that provide easy data manipulation features for the laboratory. Signal processing software has also been developed for trend removal, background subtraction, and noise reduction. Impedance analyzer are also sometimes used to allow improved quantitative eddy-current measurements. Some laboratories have multidimensional scanning capability that are used to produce images of the scan regions. A few portable scanning systems also exist for special applications such as scanning regions of aircraft fuselage.

Research to Improve Eddy current measurementsA great deal of research continues to be done to improve eddy current measurement techniques. A few of the these activities, which are being conducted at Iowa State University are described below.

Photoinductive Imaging (PI)

A technique known as photoinductive imaging (PI) was pioneered at CNDE and 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 photoinductive (PI) imaging is 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 study cracks, welds, and diffusion bonds in metallic specimens. The PI technique is being studied as a way to image local stress variations in steel.

Pulsed Eddy Current

Research is currently being conducted on the use of a technique called pulsed eddy current (PEC) testing. This technique can be used for the detection and quantification of corrosion and cracking in multi-layer aluminum aircraft structures. Pulsed eddy-current signals consist of a spectrum of frequencies meaning that, because of the skin effect, each pulse signal contains information from a range of depths within a given test specimen. In addition, the pulse signals are very low-frequency rich which provides excellent depth penetration. Unlike multi-frequency approaches, the pulse-signals lend 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 and quantified in multi-layer aluminum structures. More recently, studies carried out on three- and four-layer structures show the ability to locate cracks emerging from fasteners. Pulsed eddy-current measurements have also been applied to ferromagnetic materials, recent work has been involved with measuring case depth in hardened steel samples

Properties of ElectricitySince eddy current inspection makes use of electromagnetic induction, it is important to know about the scientific principles of electricity and magnetism. For a review of these principles, the Science of NDT materials on this Internet site may be helpful. A review of the key parameters will be provided here.

Electricity

It 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 certain conditions can move from atom to atom. The direction of movement between atoms is random unless a force causes the electrons to move in one direction. This directional movement of electrons due to some imbalance of force is what is known as electricity.

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 of electrons, having one coulomb of charge, moves past a given point in one second. A coulomb is the charge carried by 6.25 x 10^18 electrons or 6,250,000,000,000,000,000 electrons.

Electromagnetic ForceThe force that causes the electrons to move in an electrical circuit is called the electromotive force, or EMF. Sometimes it is convenient to think of EMF as electrical pressure. In other words, it is the force that makes electrons move in a certain direction within a conductor. There are many sources of EMF; the most common being batteries and electrical generators.

The VoltThe unit of measure for EMF is the volt. One volt is defined as the electrostatic difference between two points when one joule of energy is used to move one coulomb of charge from one point to the other. A joule is the amount of energy that is being consumed when one watt of power works for one second. This is also known as a watt-second. For our purposes, just accept 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 energy each second it is on.

Insulator:Anything that insulates, esp., a nonconductor, usually a device of glass or porcelain for insulating and supporting electric wires.Conductor:A substance or thing that conducts electricity, heat, sound, etc.ResistanceResistance is the opposition of a body or substance to the flow of electrical current through it, resulting in a change of electrical energy into heat, light, or other forms of energy. The amount of resistance depends on the type of material. Materials with low resistance are good conductors of electricity. Materials with high resistance are good insulators.

Current Flow and Ohm's LawOhm's law is the most important, basic law of electricity. It defines the relationship between the three fundamental electrical quantities: current, voltage, and resistance. When a voltage is applied to a circuit containing only resistive elements (i.e. no coils), current flows according to Ohm's Law, which is shown below.

I = V / R Where:

I = Electrical Current (Amperes)

V = Voltage (Voltage)

R = Resistance (Ohms)

Ohm's law states that the electrical current (I) flowing in an circuit is proportional to the voltage (V) and inversely proportional to the resistance (R). Therefore, if the voltage is increased, the current will increase provided the resistance of the circuit does not change. Similarly, increasing the resistance of the circuit will lower the current flow if the voltage is not changed. The formula can be reorganized so that the relationship can easily be seen for all of the three variables.

The Java applet below allows the user to vary each of these three parameters in Ohm's Law and see the effect on the other two parameters. Values may be input into the dialog boxes, or the resistance and voltage may also be varied by moving the arrows in the applet. Current and voltage are shown as they would be displayed on an oscilloscope with the X-axis being time and the Y-axis being the amplitude of the current or voltage. Ohm's Law is valid for both direct current (DC) and alternating current (AC). Note that in AC circuits consisting of purely resistive elements, the current and voltage are always in phase with each other.

Exercise: Use the interactive applet below to investigate the relationship of the variables in Ohm's law. Vary the voltage in the circuit by clicking and dragging the head of the arrow, which is marked with the V. The resistance in the circuit can be increased by dragging the arrow head under the variable resister, which is marked R. Please note that the vertical scale of the Oscilloscope screen automatically adjusts to reflect the value of the current.

See what happens to the voltage and current as the resistance in the circuit is increased. What happens if there is not enough resistance in a circuit? If the resistance is increased, what must happen in order to maintain the same level of current flow?

Induction and InductanceInductionIn 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 just the opposite. They noticed that a moving magnetic field would induce current in an electrical conductor. This process of generating electrical current in a conductor by placing the conductor in a changing magnetic field is called electromagnetic induction or just induction. It is called induction because the current is said to be induced in the conductor by the magnetic field.

Faraday also noticed that the rate at which the magnetic field changed also had an effect on the amount of current or voltage that was induced. Faraday's Law for an uncoiled conductor states that the amount of induced voltage is proportional to the rate of change of flux lines cutting the conductor. Faraday's Law for a straight wire is shown below.

Where:

VL = the induced voltage in voltsd/dt = the rate of change in magnetic flux in webers/second

Induction is measured in unit of Henries (H) which reflects this dependence on the rate of change of the magnetic field. One henry is the amount of inductance that is required to generate one volt of induced voltage when the current is changing at the rate of one ampere per second. Note that current is used in the definition rather than magnetic field. This is because current can be used to generate the magnetic field and is easier to measure and control than magnetic flux..

InductanceWhen induction occurs in an electrical circuit and affects the flow of electricity it is called inductance, L. Self-inductance, or simply inductance is the property of a circuit whereby a change in current causes a change in voltage in the same circuit. When one circuit induces current flow in a second nearby circuit, it is known as mutual-inductance. The image to the right shows an example of mutual-inductance. When an AC current is flowing through a piece of wire in a circuit, an electromagnetic field is produced that is constantly growing and shrinking and changing direction due to the constantly changing current in the wire. This changing magnetic field will induce electrical current in another wire or circuit that is brought close to the wire in the primary circuit. The current in the second wire will also be AC and in fact will look very similar to the current flowing in the first wire. An electrical transformer uses inductance to change the 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 is responsible for inductance, it is only present in AC circuits and that high frequency AC will result in greater inductive reactance since the magnetic field is changing more rapidly.

Self-Inductance and Inductive ReactanceThe property of self-inductance is a particular form of electromagnetic induction. Self inductance is defined as the induction of a voltage in a current-carrying wire when the current in the wire itself is changing. In the case of self-inductance, the magnetic field created by a changing current in the circuit itself induces a voltage in the same circuit. Therefore, the voltage is self-induced.

The term inductor is used to describe a circuit element possessing the property of inductance and a coil of wire is a very common inductor. In circuit diagrams, 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 is induced in a wire carrying a changing current. The alternating current running through the coil creates a magnetic field in and around the coil that is increasing and decreasing as the current changes. The magnetic field forms concentric loops that surrounds the wire and joins up to form larger loops that surround the coil as shown in the image below. When the current increases in one loop the expanding magnetic field will cut across some 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.

By studying this image of a coil, it can be seen that the number of turns in the coil 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 flux increases the amount of induced voltage. Therefore, Faraday's Law must be modified for a coil of wire and becomes the following.

Where:

VL = the induced voltage in voltsN = the number of turns in the coild/dt = the rate of change in magnetic flux in webers per second

The equation simply states that the amount of induced voltage (VL) is proportional to the number of turns in the coil and the rate of change of the magnetic flux (d/dt). In other words, when the frequency of the flux is increased or the number of turns in the coil is increased, the amount of induced voltage will also increase.

In a circuit, it is much easier to measure current than it is to measure magnetic flux so the following equation can be used to determine the induced voltage if the inductance and frequency of the current are known. This equation can also be reorganized to allow the inductance to be calculated when the amount of inducted voltage can be determined and the current frequency is known.

Where:

VL = the induced voltage in voltsL = the value of inductance in henriesdi/dt = the rate of change in current in amperes per second

Lenz's LawSoon after Faraday proposed his law of induction, Heinrich Lenz developed a rule 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 its magnetic field opposes the change in magnetic field that induced the current. This means that the current induced in a conductor will oppose the change in current that is causing the flux to change. Lenz's law is important in understanding the property of inductive reactance, which is one of the properties measured in eddy current testing.

Inductive ReactanceThe reduction of current flow in a circuit due to induction is called inductive reactance. By taking a closer look at a coil of wire and applying Lenz's law, it can be seen how inductance reduces the flow of current in the circuit. In the image below, the direction of the primary current is shown in red, and the magnetic field generated by the current is shown in blue. The direction of the magnetic field can be determined by taking your right hand and pointing your thumb in the direction of the current. Your fingers will then point in the direction of the magnetic field. It can be seen that the magnetic field from one loop of the wire will cut across the other loops in the coil and this will induce current flow (shown in green) in the circuit. According to Lenz's law, the induced current must flow in the opposite direction of the primary current. The induced current working against the primary current results in a reduction of current flow in the circuit.

It should be noted that inductive reactance will increase if the number of winds in the coil is increased since the magnetic field from one coil will have more coils to interact with.

Since inductive reactance reduces the flow of current in a circuit, it appears as an energy loss just like resistance. However, it is possible to distinguish between resistance and inductive reactance in a circuit by looking at the timing between the sine waves of the voltage and current of the alternating current. In an AC circuit that contains only 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 same time. When there is inductive reactance present in the circuit, the phase of the current will be shifted so that its peaks and valleys do not occur at the same time as those of the voltage. This will be discussed in more detail in the section on circuits.

Mutual Inductance(The Basis for Eddy Current Inspection)

The magnetic flux through a circuit can be related to the current in that circuit and the currents in other nearby circuits, assuming that there are no nearby permanent magnets. Consider the following two circuits.

The magnetic field produced by circuit 1 will intersect the wire in circuit 2 and create current flow. The induced current flow in circuit 2 will have its own magnetic field which will interact with the magnetic field of circuit 1. At some point P on 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.

Self Inductance:The property of an electric circuit or component that caused an e.m.f. to be generated in it as a result of a change in the current flowing through the circuit.Mutual Inductance:The property of an electric circuit or component that caused an e.m.f. to be generated in it as a result of a change in the current flowing through a neighboring circuit with which it is magnetically linked. The coils in the circuits are labeled L1 and L2 and this term represents the self inductance of each of the coils. The values of L1 and L2 depend on the geometrical arrangement of the circuit (i.e. number of turns in the coil) and the conductivity of the material. The constant M, called the mutual inductance of the two circuits and it is dependent on the geometrical arrangement of both circuits. In particular, if the circuits are far apart, the magnetic flux through circuit 2 due to the current i1 will be small and the mutual inductance will be small. L2 and M are constants.

We can write the flux, B through circuit 2 as the sum of two parts.

B2 = L2i2 + i1M

An equation similar to the one above can be written for the flux through circuit 1.

B1 = L1i1 + i2M

Though it is certainly not obvious, it can be shown that the mutual inductance is the same for both circuits. Therefore, it can be written as follows:

M1,2 = M2,1How is mutual induction used in eddy current inspection?

Eddy Current:A current induced in a conductor situated in a changing magnetic field or moving in a fixed one.When alternating current is passed through the coil, a magnetic field is generated in and around the coil. When the probe is brought in close proximity to a conductive material, such as aluminum, the probes changing magnetic field generates current flow in the material. The induced current flows in closed loops in planes perpendicular to the magnetic flux. They are named eddy currents because they are thought to resemble the eddy currents that can be seen swirling in streams.

Magnetic Permeability:The ratio of the magnetic flux density, B, in a substance to the external field strength. Ferromagnetic:A term used to describe materials, such as iron, nickel, and cobalt, which have a high magnetic permeability. It should be noted that if a sample is ferromagnetic, the magnetic flux is concentrated and strengthened despite opposing eddy current affects. The increase inductive reactance due to the magnetic permeability of ferromagnetic materials makes it easy to distinguish these materials from nonferromagnetic materials.

In the applet below, the probe and the sample are shown in cross-section. The boxes represent a 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 the effects of the shared magnetic field. The liftoff value can be set to 0.1 or less and the current value can be varied from 0.01 to 1.0. The strength of the magnetic field is shown by the darkness of the lines.

Circuits and PhaseA circuit can be thought of as a closed path in which current flows through the components that make up the circuit. The current (i) obeys Ohm's Law, which is discussed in section 2.1. The simple circuit below consists of a voltage source (in this case an alternating current voltage source) and a resistor. The graph below the circuit diagram shows the value of the voltage and the current for this circuit over a period of time. This graph shows one complete cycle of an alternating current source. From the graph, it can be seen that as the voltage increases so does the current. The voltage and the current are said to be "in-phase" since their zero, peak, and valley points occur at the same time. They are also directly proportional to each other.

In the circuit below, the resistive component has been replaced with an inductor. When inductance is introduced into a circuit, the voltage and the current will be "out-of-phase," meaning that the voltage and current do not cross zero, or reach their peaks and valleys at the same time. When a circuit has an inductive component, the current (iL) will lags the voltage by one quarter of a cycle. One cycle is often referred to as 360 degree, so it can be said that the current lags the voltage by 90 degrees.

The resistive and inductive components are of primary interest in eddy current testing since the test probe is basically a coil of wire, which will have both resistance and inductive reactance. However, for the sake of completeness, capacitance also needs to be mentioned. This simple circuit below consists of an alternating current voltage source and a capacitor. Capacitance in a circuit caused the current (ic) to lead the voltage by one quarter of a cycle (90 degrees current lag).

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.

Depth of Penetration & Current DensityEddy currents are closed loops of induced current circulating in planes perpendicular to the magnetic flux. They normally travel parallel to the coil's winding and flow is limited to the area of the inducing magnetic field. Eddy currents concentrate near the surface adjacent to an excitation coil and their strength decreases with distance from the coil as shown in the image. Eddy current density decreases exponentially with depth. This phenomenon is known as the skin effect.

Skin effect arises when the eddy currents flowing in the test object at any depth produce magnetic fields which oppose the primary field, thus reducing net magnetic flux and causing a decrease in current flow as depth increases. Alternatively, eddy currents near the surface can be viewed as shielding the coil's magnetic field, thereby weakening the magnetic field at greater depths and reducing induced currents.

The depth that eddy currents penetrate into a material is affected by the frequency of the excitation current and the electrical conductivity and magnetic permeability of the specimen. The depth of penetration decreases with increasing frequency and increasing conductivity and magnetic permeability. 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 (). The word 'standard' denotes plane wave electromagnetic field excitation within the test sample (conditions which are rarely achieved in practice). Although eddy currents penetrate deeper than one standard depth of penetration they decrease rapidly with depth. At two standard depths of penetration (2), eddy current density has decreased to 1/e squared or 13.5% of the surface density. At three depths (3) the eddy current density is down to only 5% of the surface density.

Semiconductor:A crystalline solid, such as silicon or germanium, with an electrical conductivity intermediate between that of a conductor and an insulator. Since the sensitivity of an eddy current inspection depends on the eddy current density at the defect location, it is important to know the strength of the eddy currents at this location. When attempting to locate flaws, a frequency is often selected which places the expected flaw depth within one standard depth of penetration. This helps to assure that the strength of the eddy currents will be sufficient to produce a flaw indication. Alternately, when using eddy currents to measure the electrical conductivity of a material, the frequency is often set so that it produces three standard depths of penetration within the material. This helps to assure that the eddy currents will be so weak at the back side of the material that changes in the material thickness will not affect the eddy current measurements.

The applet below illustrates how eddy current density changes in a semi-infinite conductor. The applet can be used to calculate the standard depth of penetration. The equation for this calculation is

Where: = Standard Depth of Penetration (mm) = 3.14f = Test Frequency (Hz) = Magnetic Permeability (H/mm) = Electrical Conductivity (% IACS)(Note, however, that the applet uses the relative permeability so there is a permeability of free space term in the equation. i.e. relative permeability multiplied by the permeability of free space puts the material permeability in to H/mm units.)

Phase Lag Phase lag is a parameter of the eddy current signal that makes it possible to obtain information about the depth of a defect within a material. Phase lag is the shift in time between the eddy current response from a disruption on the surface and a disruption at some distance below the surface. The generation of eddy currents can be thought of as a diffusion process meaning that the eddy currents below the surface take a little longer to form than those at the surface. Therefore, subsurface defects will be detected by the eddy current instrument a little later in time than surface defects. 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 phase angle, the current shifted with respect to the voltage.)

Phase lag is an important parameter in eddy current testing because it makes it possible to estimate the depth of a defect and with proper reference specimens, determine the rough size of a defect. The signal produced by a flaw depends on both amplitude and phase of the eddy currents being disrupted. A small surface defect and large internal defect can have a similar effect on the magnitude of test coil impedance. However, because of the increasing phase lag with depth, there will be a characteristic difference in the test coil impedance vector.

Radian:A unit in circular measure, an angle subtended at the center of a circle by an arc of equal length to the radius. One radian is equal to 57.296. At one standard depth of penetration, the phase lag is 57 degrees or one radian. This means that the eddy currents flowing at one standard depth of penetration () below the surface, lag the surface currents by 57 degrees. At two standard depths of penetration (2) they lag the surface currents by 114 degrees. Therefore by measuring the phase lag of a signal, the depth of a defect can be estimated.

In the applet below, the relationship between the depth of penetration and the phase lag is explored. The equation at the bottom of the applet can be used to calculate the depth of penetration by choosing an inspection frequency (f), and, the magnetic permeability (u) and electrical conductivity for the test material. These values may also be selected for a particular material by selecting one of the set materials in the dialog box.

Eddy Current InstrumentsThe most basic eddy current testing instrument consists of an alternating current source, a coil of wire connected to this source, and a voltmeter to measure the voltage change across the coil. An ammeter could also be used to measure the current change in the circuit instead of using the voltmeter.

While it might actually be possible to detect some types of defects with this type of an equipment, most eddy current instruments are a bit more sophisticated. In the following pages, a few of the more important aspects of eddy current instrumentation will be discussed.

Resonant Circuits Every circuit containing capacitance and inductance has a resonant frequency that is inversely proportional to the square root of the product of the capacitance and inductance.

Circuits not containing discreet components for resistance, capacitance, and inductance can still exhibit their effects. For example, a coaxial cable used to interconnect pieces of electronic equipment or equipment to probes, has some capacitance and inductance. These capacitances and inductances distributed throughout the cable are very small, but not negligible in sensitive circuits.

The applet represents an eddy current probe with a default resonant frequency of about 1.0 kHz. An ideal probe might contain just the inductance, but a realistic probe has some resistance and some capacitance. The applet initially shows a single cycle of the 1.0 kHz current passing through the inductor.

BridgesThe bridge circuit shown in the applet below is known as the Maxwell-Wien bridge (often called the Maxwell bridge), and is used to measure unknown inductances in terms of calibrated resistance and capacitance. Calibration-grade inductors are more difficult to manufacture than capacitors of similar precision, and so the use of a simple "symmetrical" inductance bridge is not always practical. Because the phase shifts of inductors and capacitors are exactly opposite each other, a capacitive impedance can balance out an inductive impedance if they are located in opposite legs of a bridge, as they are here.

Unlike this straight Wien bridge, the balance of the Maxwell-Wien bridge is independent of source frequency, and in some cases this bridge can be made to 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.

Exercise: Using the equations within the applet, calculate appropriate values for C and R2 for a set of probe values . Then using your calculated values, balance the bridge. The oscilloscope trace representing current (brightest green) across the top and bottom of the bridge should be minimized (straight line).

In the simplest implementation, the standard capacitor (Cs) and the resistor in parallel with it are made variable, and both must be adjusted to achieve balance. 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 the resistor in parallel with the capacitor, and one of the other two). However, in the latter configuration it takes more trial-and-error adjustment to achieve balance as the different variable resistors interact in balancing magnitude and phase.

Another advantage of using a Maxwell bridge to measure inductance rather than a symmetrical inductance bridge is the elimination of measurement error due to mutual inductance between two inductors. Magnetic fields can be difficult to shield, and even a small amount of coupling between coils in a bridge can introduce substantial errors in certain conditions. With no second inductor to react within the Maxwell bridge, this problem is eliminated.

Display - Complex Impedance Plane (eddy scope)Electrical Impedance (Z), is the total opposition that a circuit presents to an alternating 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 discussed in the page on impedance, the resistance component and the reactance components are not in phase so vector addition must be used to relate them with impedance. For an eddy current circuit with resistance and inductive reactance components, the total impedance is calculated using the following equation.

You will recall that this can be graphically displayed using the impedance plane diagram as seen to the right. Impedance also has an associated angle, called the phase angle of the circuit, which can be calculated by the following equation.

The impedance plane diagram is a very useful way of displaying eddy current data. As shown in the figure below, the strength of the eddy currents and the magnetic permeability of the test material cause the eddy current signal on the impedance plane to react in a variety of different ways.

If the eddy current circuit is balanced in air and then placed on a piece of aluminum, the resistance component will increase (eddy currents are being generated in the aluminum and this takes energy away from the coil and this energy loss shows up as resistance) and the inductive reactance of the coil decreases (the magnetic field created by the eddy currents opposes the coil's magnetic field and the net effect is a weaker magnetic field to produce inductance). If a crack is present in the material, fewer eddy currents will be able to form and the resistance will go back down and the inductive reactance will go back up. Changes in conductivity will cause the eddy current signal to change in a different way.

When a probe is placed on a magnetic material such as steel, something different happens. Just like with aluminum (conductive but not magnetic) eddy currents form which takes energy away from the coil and this shows up as an increase in the coils resistance. And, just like with the aluminum, the eddy currents generate their own magnetic field that opposes the coils magnetic field. However, you will note for the diagram that the reactance increase. This is because the magnetic permeability of the steel concentrates the coil's magnetic field this increase in the magnetic field strength completely overshadows the magnetic field of the eddy currents. The presence of a crack or a change in the conductive will produce a change in the eddy current signal similar to that seen with aluminum.

In the applet below, liftoff curves can be generated for several nonconductive materials with various electrical conductivities. With the probe held away from the metal surface, zero and clear the graph. Then slowly move the probe to the surface of the material. Lift the probe back up, select a different material and touch it back to the sample surface.













Display - Analog MeterIn order to use a DC-style meter movement, such as the D'Arsonval design pictured in the applet below, the alternating current must be "rectified" into DC. This is most easily accomplished through the use of devices called diodes. Without going into elaborate detail over how and why diodes work as they do, remember that they each act like a one-way valve for electrons to flow. They act as a conductor for one polarity and an insulator for another. Arranged in a bridge, four diodes will serve to steer AC through the meter movement in a constant direction.

An analog meter can easily measure just a few microamperes of current and is well suited for use in balancing bridges.

Probes - Mode of OperationEddy current probes are available in a large variety shapes and sizes. In fact, one of the major advantages of eddy current inspection is that probes can be custom designed for a wide variety of applications. Eddy current probes are classified by the configuration and mode of operation of the test coils. The configuration of the probe generally refers to the way the coil or coils are packaged to best "couple" to the test area of interest. An example of different configurations of probes would be bobbin probes, which are inserted into a piece of pipe to inspect from the inside out, versus encircling probes, in which the coil or coils encircle the pipe to inspect from the outside in. The mode of operation refers to the way the coil or coils are wired and interface with the test equipment. The mode of operation of a probe generally falls into one of four categories: Absolute, differential, reflection and hybrid. Each of these classifications will be discussed in more detail below.

Absolute Probes

Absolute probes generally have a single test coil that is used to generate the eddy currents and sense changes in the eddy current field. As discussed in the physics section, AC is passed through the coil and this sets-up a expanding and collapsing magnetic field in and around the coil. When the probe is positioned next to a conductive material, the changing magnetic field generate eddy currents within the material. The generation of the eddy currents take energy from the coil and this appears as an increase in the electrical resistance of the coil. The eddy currents generate their own magnetic field that opposes the magnetic field of the coil and this changes the inductive reactance of the coil. By measuring the absolute change in impedance of the test coil, much information can be gained about the test material.

Absolute coils can be used for flaw detection, conductivity measurements, liftoff measurements and thickness measurements. They are widely used due to their versatility. Since absolute probes are sensitivity to things such as conductivity, permeability liftoff and temperature, steps must be taken to minimize these variables when they are not important to the inspection being performed. It is very common for commercially available absolute probes to have a fixed "air loaded" reference coil that compensates for ambient temperature variations.

Differential probes have two active coils usually wound in opposition, although they could be wound in addition with similar results. When the two coils are over a flaw-free area of test sample, there is no differential signal developed 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 very sensitive to defect yet relatively insensitive to slowly varying properties such as gradual dimensional or temperature variations. Probe wobble signals are also reduced with this probe type. There are also disadvantages to using 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, only the leading and trailing edges will be detected due to signal cancellation when both coils sense the flaw equallyReflection ProbesReflection probes have two coils similar to a differential probe, but one coil is used to excite the eddy currents and the other is used to sense changes in the test material. Probes of this arrangement are often referred to as driver/pickup probes. The advantage of reflection probes is that the driver coil can be made so as to produce a strong and uniform flux field in the vicinity of the pickup coil. The pickup coil can be made very small so that it will be sensitive to very small defects.

Hybrid ProbesAn example of a hybrid probe is the split D, differential probe shown to the right. This probe has a driver coil that surrounds two D shaped sensing coils. It operates in the reflection mode but additionally, its sensing coils operate in the 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 to generate eddy currents in the material but then uses a different type of sensor to detect changes on the surface and within the test material. An example of a hybrid probe is one that uses a Hall effect sensor to detect changes in the magnetic flux leaking from the test surface. Hybrid probes are usually specially designed for a specific inspection application.

Probes - ConfigurationsAs mentioned on the previous page, eddy current probes are classified by the configuration and mode of operation of the test coils. The configuration of the probe 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 of probes based on their configuration include surface probes, bolt hole probes, ID probes, and OD probes.

Surface Probes

Surface probes are usually designed to be handheld and are intended to be used in contact with the test surface. Surface probes generally consist of a coil of very fine wire encased in a protective housing. The size of the coil and shape 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 the test surface. This coil configuration is sometimes referred to as a pancake coil and is good for detecting surface discontinuities that are oriented perpendicular to the test surface. Discontinuities, such as delaminations, that are in a parallel plane to the test surface will likely go undetected with this coil configuration.

Wide surface coils are used when scanning large areas for relatively large defects. They sample a relatively large area and allow for deeper penetration. Since they do sample a large area, they are often used for conductivity tests to get more of a bulk material measurement. However, their large sampling area limits their ability to detect small discontinuities.

Pencil probes have a small surface coil that is encased in a long slender housing to permit inspection in restricted spaces. They are available with a straight shaft or with a bent shaft, which facilitate easier handling and use in applications such as the inspection of small diameter bores. Pencil probes are prone to wobble due to their small base and sleeves are sometimes used to provide a wider base.

Bolt Hole ProbesBolt hole probes are a special type of surface probe that is designed to be used with a bolt hole scanner. They have a surface coil that is mounted inside a housing that matches the diameter of the hole being inspected. The probe is inserted in the hole and the scanner rotates the probe within the hole.

ID or Bobbin ProbesID probes, which are also referred to as Bobbin probes or feed-through probes, are inserted into hollow products, such as a pipe, to inspect from the inside out. The ID probes have a housing that keep the probe centered in the product and the coil(s) orientation somewhat constant relative to the test surface. The coils are most commonly wound around the circumference of the probe so that the probe inspects an area around the entire circumference of the test object at one time.

OD or Encircling CoilsOD probes are often called encircling coils. They are similar to ID probes except that the coil(s) encircle the material to inspect from the outside in. OD probes are commonly used to inspect solid products, such as bar.

Probes - Shielding & LoadingOne of the challenges of performing an eddy current inspection, is getting sufficient eddy current field strength in the region of interest within the material. Another challenge is keeping the field away from nonrelevent features of the test component. Features that could produce a response that complicates the desired signal information. Probe shielding and loading are sometimes used to limit the spread and concentrate the magnetic field of the coil. Of course, if the magnetic field is concentrated near the coil, the eddy currents will also be concentrated in this area.

Probe ShieldingProbe shielding is used to prevent or reduce the interaction of the probes magnetic field with nonrelevent features in close proximity of the probe. Shielding could be used to reduce edge effects when testing near dimensional transitions such as a step or an edge. Shielding could also be used to reduce the effects of conductive or magnetic fasteners in the region of testing.

Eddy current probes are most often shielded using magnetic shielding or eddy current shielding. Magnetically shielded probes have their coil surrounded by a ring of ferrite or other material with high permeability and low conductivity. The ferrite creates and area of low magnetic reluctance and the probe's magnetic field is concentrated in this area rather than spreading beyond the shielding. This concentrates the magnetic field into tighter area around the coil.

Eddy current shielding uses a ring of highly conductive but nonmagnetic material, usually copper, to surround the coil. The portion of the coil's magnetic field that cuts across the shielding generates eddy currents in the shielding material rather than in the nonrelevent features outside of the shielded area. The higher the frequency of the current used to drive the probe, the more effective the shielding will be due to skin effect in the shielding material.

Probe Loading with Ferrite CoresSometimes coils are wound around a ferrite core. Since ferrite is ferromagnetic, the magnetic flux produced by the coil prefers to travel through the ferrite than through air. Therefore, the ferrite core concentrates the magnetic field near the center of the probe. This, in turn, concentrates the eddy currents near the center of the probe. Probes with ferrite cores tend to be more sensitive than air core probes and less affected by probe wobble and lift-off.

Coil (Probe) Design - DiameterThe most important feature in eddy current testing is the way in which the eddy currents are induced and detected in the material under test. This depends on the design of the probe, which can contain either one or more coils. A coil consists of a length of wire wound in a helical manner around the length of a cylindrical tube or rod, called a former. The winding usually has more than one layer so as to increase the value of inductance for a given length of coil.

It is desirable with eddy current testing that the wire is made from copper or other nonferrous metal to avoid magnetic hysteresis effects. The main purpose of the former is to provide a sufficient amount of rigidity in the coil to prevent distortion. Formers used for coils with diameters greater than a few millimeters, e.g. encircling and pancake coils, generally take the form of tubes or rings made from dielectric materials.

The region inside the former is called the core, which can consist of either a solid material or just air. Small-diameter coils are usually wound directly on to a solid core, which acts as the former. The higher the inductance (L) of a coil, at a given frequency, the greater the sensitivity of eddy current testing. It is essential that the current through the coil is as low as possible. Too high a current may produce

a rise in temperature, hence an expansion of the coil, which increases the value of L.

magnetic hysteresis, which is small but detectable when a ferrite core is used.

The simplest type of probe is the single-coil probe, which is in widespread use. The following applet may be used to calculate the effect of the inner and outer diameters of a simple probe design on the probe's self inductance. Dimensional units are in millimeters.

The higher the inductance (L) of a coil, at a given frequency, the greater the sensitivity of eddy current testing. A more precise value of L is given by

L = Kn2 pi [ (ro2 - rc2) - rrc2] o/l

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 4 pi x 10-7 H/m (i.e. the permeability of free space which is effectively equal to the permeabilities of the materials of both the wire and the former).

K is a dimensionless constant characteristic of the length and the external and internal radii.

Coil (Probe) Design - Turns As mentioned in the previous section, an important feature in eddy current testing is the way in which the eddy currents are induced and detected in the material under test.

The winding usually has more than one layer so as to increase the value of inductance for a given length of coil. It is desirable with eddy current testing that the wire is made from copper or other nonferrous metal to avoid magnetic hysteresis effects. The main purpose of the former is to provide a sufficient amount of rigidity in the coil to prevent distortion. Formers used for coils with diameters greater than a few millimeters, e.g. encircling and pancake coils, generally take the form of tubes or rings made from dielectric materials.

The region inside the former is called the core, which can consist of either a solid material or just air. Small-diameter coils are usually wound directly on to a solid core, which acts as the former. The higher the inductance (L) of a coil, at a given frequency, the greater the sensitivity of eddy current testing.

The simplest type of probe is the single-coil probe. The following applet may be used to calculate the effect of the number of turns in the coil on the probe's self inductance.

Impedance MatchingEddy current testing requires us to determine the components of the impedance of the detecting coil or the potential difference across it. Most applications require the determination only of changes in impedance, which can be measured with a high degree of sensitivity using an AC bridge. The principles of operation of the most commonly used eddy current instruments are based on Maxwell's inductance bridge, in which the components of the impedance of the detecting coil, commonly called a probe, are compared with known variable impedances connected in series and forming the balancing arm of the bridge. Refer back to Sec.3.3 - Bridges.

The input to the bridge is an AC oscillator, often variable in both frequency and amplitude. The detector arm takes the form of either a meter or a storage cathode-ray oscilloscope, a phase-sensitive detector, a rectifier to provide a steady indication, and usually an attenuator to confine the output indication within a convenient range. Storage facilities are necessary in the oscilloscope in order to retain the signal from the detector for reference during scanning with the probe.

The highest sensitivity of detection is achieved by properly matching the impedance of the probe to the impedance of the measuring instrument. Thus, with a bridge circuit which is initially balanced, a subsequent but usually small variation in the impedance of the probe upsets the balance, and a potential difference appears across the detector arm of the bridge.

Although the Maxwell inductance bridge forms the basis of most eddy current instruments, there are several reasons why it cannot be used in its simplest form (e.g. Hague, 1934), including the creation of stray capacitances, such as those formed by the leads and leakages to earth. These unwanted impedances can be eliminated by earthing devices and the addition of suitable impedances to produce one or more wide-band frequency (i.e. low Q) resonance circuits. Instruments having a wide frequency range, e.g. from 1 kHz 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 important consideration in achieving proper matching and, as a result, it may be necessary to change the probe when switching from one frequency band to another.

Surface Breaking CracksEddy current equipment can be used for a variety of applications such as the detection of cracks (discontinuities), measurement of metal thickness, detection of metal thinning due to corrosion and erosion, determination of coating thickness, and the measurement of electrical conductivity and magnetic permeability. Eddy currents inspection is an excellent method for detecting surface and near surface defects when the probable defect location and orientation is well known. Defects such as cracks are detected when they disrupt the path of eddy currents and weaken their strength. The images to the right show an eddy current surface probe on the surface of a conductive component. The strength of the eddy currents under the coil of the probe in indicated by color. In the lower image, there is a flaw under the right side of the coil and it can be see that the eddy currents are weaker in this area.

Of course, factors such as the type of material, surface finish and condition of the material, the design of the probe, and many other factors can affect the sensitivity of the inspection. Successful detection of surface breaking and near surface cracks requires:

1. A knowledge of probable defect type, position, and orientation.

2. Selection of the proper probe. The probe should fit the geometry of the part and the coil must produce eddy currents that will be disrupted by the flaw.

3. Selection of a reasonable probe drive frequency. For surface flaws, the frequency should be as high as possible for maximum resolution and high sensitivity. For subsurface flaws, lower frequencies are necessary to get the required depth of penetration and this results in less sensitivity. Ferromagnetic or highly conductive materials require the use of an even lower frequency to arrive at some level of penetration.

4. Setup or reference specimens of similar material to the component being inspected and with features that are representative of the defect or condition being inspected for.

The basic steps in performing an inspection with a surface probe are the following:

1. Select and setup the instrument and probe.

2. Select a frequency to produce the desired depth of penetration.

3. Adjust the instrument to obtain an easily recognizable defect response using a calibration standard or setup specimen.

4. Place the inspection probe (coil) on the component surface and null the instrument.

5. Scan the probe over part of the surface in a pattern that will provide complete coverage of the area being inspected. Care must be taken to maintain the same probe-to-surface orientation as probe wobble can affect interpretation of the signal. In some cases, fixtures to help maintain orientation or automated scanners may be required.

6. Monitor the signal for a local change in impedance that will occur as the probe moves over a discontinuity.

The applet below depicts a simple eddy current probe near the surface of a calibration specimen. Move the probe over the surface of the specimen and compare the signal responses from a surface breaking crack with the signals from the calibration notches. The inspection can be made at a couple of different frequency to get a feel for the effect that frequency has on sensitivity in this application.

Surface Crack Detection Using Sliding ProbesMany commercial aircraft applications involve the use of multiple fasteners to connect the multilayer skins. Because of the fatigue stress that is caused by the typical application of any commercial aircraft, fatigue cracks can be induced in the vicinity of the fastener holes. In order to inspect the fastener holes in an adequate amount of time, sliding probes are an efficient method of inspection.

Sliding probes have been named so because they move over fasteners in a sliding motion. There are two types of sliding probes, fixed and adjustable, which are usually operated in the reflection mode. This means that the eddy currents are induced by the driver coil and detected by a separate receiving coil.

Sliding probes are one of the fastest methods to inspect large numbers of fastener holes. They are capable of detecting surface and subsurface discontinuities, but they can only detect defects in one direction. The probes are marked with a detection line to indicate the direction of inspection. In order to make a complete inspection there must be two scans that are 90 degrees separated from each other.

PROBE TYPES FIXED SLIDING PROBES

These probes are generally used for thinner material compared to the adjustable probes. Maximum penetration is about 1/8 inch. Fixed sliding probes are particularly well suited for finding longitudinal surface or subsurface cracks such as those found in lap joints. Typical frequency range is from 100 Hz to 100 kHz.

ADJUSTABLE SLIDING PROBES

These probes are well suited for finding subsurface cracks in thick multilayer structures, like wing skins. Maximum penetration is about 3/4 inch. The frequency range for adjustable sliding probes is from 100 Hz to 40 kHz.

Adjustable probes, as the name implies, are adjustable with the use of spacers, which will change the penetration capabilities. The spacer thickness between the coils is normally adjusted for the best detection. For tangential scans or 90 degree scanning with an offset from the center, a thinner spacer is often used.

The spacer thickness range can vary from 0 (no spacer) for inspections close to the surface and small fastener heads to a maximum of about 0.3 inch for deep penetration with large heads in the bigger probe types. A wider spacer will give more tolerance to probe deviation as the sensitive area becomes wider but the instrument will require more gain. Sliding probes usually penetrate thicker materials compared to the donut probes.

REFERENCE STANDARDS Reference/calibration standards for setup of sliding probes typically consist of three or four aluminum plates that are fastened together within a lap joint type configuration. EDM notches or naturally/artificially- induced cracks are located in the second or third layer of the standard.

Reference standards used should be manufactured from the same material type, alloy, material thickness, and chemical composition that will be found on the aircraft component to be inspected. Sizes and tolerances of flaws introduced in the standards are usually regulated by inspection specifications.

INSTRUMENT DISPLAY (LIFTOFF)

Liftoff is normally adjusted to be horizontal, but on the CRT liftoff shows up as a curved line rather than a straight line. Sometimes liftoff can be a steep curve and may have to be allowed to move slightly upwards before moving downwards. See Figures 1 and 2.

SCANNING PATTERNS

A typical scan is centralized over the fastener head and moves along the axis of the fastener holes. This scan is generally used to detect cracks positioned along the axis of the fastener holes. For detecting cracks located transverse or 90 degrees from the axis of the fastener holes, a scan that is 90 degrees from the axis of the fastener holes is recommended.

CRACK DETECTIONSIGNAL INTERPRETATION

When the probe moves over a fastener hole with a crack, the indication changes and typically will create a larger vertical movement. The vertical amplitude of the loop depends on the crack length, with longer cracks giving higher indications.

If the crack is in the far side of the fastener, as the probe moves over it the dot will follow the fastener line first but will move upwards (clockwise) as it goes over the crack. If the crack is in the near side, it will be found first and the dot will move along the crack level before coming down to the fastener level.

If two cracks on opposite sides of the fastener hole are present, the dot will move upwards to the height by the first crack length and then come back to the fastener line and balance point. If the second crack is longer than the first one, the dot will move even higher and complete the loop (clockwise) before going down to the balance point. See figures 3 and 4.

VARIABLES:

PROBE SCAN DEVIATION

Most probes are designed to give a narrow indication for a good fastener hole so that the loops from the cracks are more noticeable. Some probes and structures can give wider indications and a similar result can be obtained if the probe is not straight when it approaches the fastener. It is important to keep the probe centralized over the fastener heads. Doing this will give you a maximum indication for the fastener and a crack.

If the probe deviates from the center line, the crack indication will move along the loop that we saw in figure 5 and is now present in figure 6. The crack indication is at "a" when the probe is centralized and moves toward "b" as it deviates in one direction, or "c" as it deviates in the opposite direction. Point "b" gives an important indication even if it loses a small amount of amplitude it has gained in phase, giving a better separation angle. This is because we deviated to the side where the crack is located.

CRACK ANGLE DEVIATION

A reduction in the crack indication occurs when the crack is at an angle to the probe scan direction. This happens if the crack is not completely at 90 degrees to the normal probe scan or changes direction as it grows. Both the fixed and adjustable sliding probes are capable of detecting cracks up to about 30 degrees off angle. See to figures 7 and 8.

ELECTRICAL CONTACT

When inspecting fasteners that have just been installed or reference standards that have intimate contact with the aluminum skin plate, it is not unusual to obtain a smaller than normal indication. In some extreme cases, the fastener indication may disappear almost completely. This is due to the good electrical contact between the fastener and the skin that allows the eddy currents to circulate without finding the boundary and therefore no obstacle or barrier. Because of this effect it is recommended to paint the holes before fastener installation

Crack Detection (Reflection)For crack detection, the simplest type of probe is the single-coil probe, which is in widespread use. Sometimes it is desirable to use a probe consisting of two or more coils arranged in a transformer fashion, and therefore known as a transformer probe. The primary coil induces eddy currents in the test object and the secondary coil acts as a detector. The use of this probe provides an enhanced signal-to-noise ration for detection, advantageous when deep penetration is required, such as seeking internal defects.

The through-transmission method is sometimes used when complete penetration of plates and tube walls is required.

Reflection or Driver/pickup probes have a primary winding driven from the oscillator and one or more sensor windings connected to the measurement circuit. Depending on the configuration of the sensor windings, reflection probes may give a response equivalent to either an absolute or a differential probe. The main advantages of reflection probes are list below:

Driver and pickup coils can be separately optimized for their intended purpose.

They have wider frequency range than equivalent bridge connected probes.

The larger driver coil gives a more even field, resulting in better penetration and liftoff characteristic

Conductivity MeasurementsOne of the uses of eddy current instruments is for the measurement of electrical conductivity. The value of the electrical conductivity of a metal depends on several factors, such as its chemical composition and the stress state of its crystalline structure. Therefore, electrical conductivity information can be used for sorting metals, checking for proper heat treatment, and inspecting for heat damage.

The technique usually involves nulling an absolute probe in the air and placing the probe in contact with the sample surface. For nonmagnetic materials, the change in impedance of the coil can be correlated directly to the conductivity of the material. The technique can be used to easily sort magnetic materials from nonmagnetic materials but it is difficult to separate the conductivity effects from magnetic permeability effects, so conductivity measurements are limited to nonmagnetic materials. It is important to control factors that can affect the results such as the inspection temperature and the part geometry. Conductivity changes with temperature so measurements should be made at a constant temperature and adjustments made for temperature variations when necessary. The thickness of the specimen should generally be greater than three standard depths of penetration. This is so the eddy currents at the back surface of the sample are sufficiently weaker than variations in specimen thickness that are not seen in the measurements.

Generally large pancake type, surface probes are used to get a value for a relatively large sample area. The instrument is usually setup such that a ferromagnetic material produces a response that is nearly vertical. Then, all conductive but nonmagnetic materials will produce a trace that moves down and to the right as the probe is moved toward the surface. Think back to the discussion on the impedance plane and these type of responses make sense. Remember that inductive reactance changes are plotted along the y-axis and resistance changes are plotted in the x-axis. Since ferromagnetic materials will concentrate the magnetic field produced by a coil, the inductive reactance of the coil will increase. The effects on the signal from the magnetic permeability overshadow the effects from conductivity since they are so much stronger.

When the probe is brought near a conductive but nonmagnetic material, the coil's inductive reactance goes down since the magnetic field from the eddy currents and opposes the magnetic field of the coil. The resistance in the coil increases since it takes some of the coils energy to generate the eddy currents and this appears as additional resistance in the circuit. As the conductivity of the materials being tested increases, the resistance losses will be less and the inductive reactance changes will be greater. Therefore, the signals will be come more vertical as conductivity increases as shown in the image above.

To sort materials, using an impedance plane device, the signal from the unknown sample must be compared to a signal from a variety of reference standards.. However, there are devices available that can be calibrated to produce a value for electrical conductivity which can then be compared to published values of electrical conductivity in MS/m or percent IACS (International Annealed Copper Standard). Please be aware that the conductivity of a particular material can vary significantly with slight variations in the chemical composition and, thus, a conductivity range is generally provided for a material. The conductivity range for one material may overlap with the range of a second material of interest so conductivity alone can not always be used to sort materials. The electrical conductivity values for a variety of materials can be found in the material properties reference tables.

The following applet is based on codes for nonferrous materials written by Back Blitz from his book, "Electrical and Magnetic Methods of Nondestructive Testing", 2nd ed., Chapman & Hill (1997). The applet demonstrates how a impedance plane eddy current instrument can be used for sorting of materials.

Conductivity Measurementsfor the Verification of Heat TreatmentWith some materials, such as solution heat treatable aluminum alloys, conductivity measurements are often made verifying that parts and materials have received the proper heat treatment. High purity aluminum is soft and ductile, and gains strength and hardness with the addition of alloying elements. A few such aluminum alloys are the 2000 series (2014, 2024, etc.), 6000 series (6061, 6063, etc.), and 7000 series (7050, 7075, etc.). The 2xxx series aluminum alloys have copper, the 6xxx series have magnesium, and the 7xxx have zinc as their major alloying elements.

Heat treatment of aluminum alloys is accomplished in two phases - solution heat treatment and then aging. In the solution heat treatment step, the alloys are heated to an elevated temperature to dissolve the alloying elements into solution. The metal is then rapidly cooled or quenched to freeze the atoms of the alloying elements in the lattice structure of the aluminum. This distorts and stresses the structure making electron movement more difficult and, therefore, decreases the electrical conductivity. In this condition, the alloys are still relatively soft but start to gain strength as the alloying elements begin to precipitate out of solution to form extremely small particles that impede the movement of dislocations within the material. The formation of the precipitates can be controlled for many alloys by heating and holding the material at an elevated temperature for a period of time (artificial aging). As the alloying elements precipitate out of solid solution, the conductivity of the material gradually increases. By controlling the amount of precipitated particles within the aluminum, the properties can be controlled to produce peak strength or some combinations of strength and corrosion resistance. Sometimes the material must be annealed or put into the softest most ductile condition possible in order to perform forming operations. Annealing allows all of the alloying elements to precipitate out of solution to form a course widely spaced precipitate. The electrical conductivity is greatest when the material is in the annealed condition.

Since solution heat-treated and aged materials are stronger, components that can be made using less material. A lighter or more compact design is often of great importance to the designer and well worth the cost of the heat treating process. However, think of the consequences that could arise if a component that was suppose to be solution heat treated and aged some how left the manufacturing facility and was put into service unheat treated or annealed. This is a real possibility since heat treated aluminum parts look exactly like unheat treated parts. Consider 2024 aluminum as an example. Select tensile properties and its electrical conductivity for various heat treatment conditions are given in the following table.

Properties for Alclad 2024 Aluminum

Heat Treatment ConditionUltimate StrengthYield StrengthElectrical Conductivity

Annealed (O)26 ksi (180 MPa)11 ksi (75 MPa)50 % IACS

Solution Heat Treated and Naturally Aged (T42) 64 ksi (440 MPa)42 ksi (290 MPa)30 % IACS

Solution Heat Treated, Coldworked and Artificially Aged (T861)70 ksi (485 MPa)66 ksi (455 MPa)38 % IACS

It can be seen that the yield strength for the material is 42 kilipounds/square inch (ksi) (290 MPa) in the solution heat treated and naturally aged condition (T42 condition). The yield strength can be increased to 66 ksi (455 MPa) when coldworked and artificially aged (T861 condition). But in the annealed condition, the yield strength is reduced to 11 ksi or 75 MPa). If an annealed part were accidentally used where a part in the T42 or T861 was intended, it would likely fail prematurely. However, a quick check of the conductivity using an eddy current instrument of all parts prior to shipping the parts would prevent this from occurring.

Thickness Measurements of Thin MaterialEddy current techniques can be used to perform a number of dimensional measurements. The ability to make rapid measurements without the need for couplant or, in some cases even surface contact, makes eddy current techniques very use. The type of measurements that can be made include:

thickness of thin metal sheet and foil, and of metallic coatings on metallic and nonmetallic substrate

cross-sectional dimensions of cylindrical tubes and rods

thickness of nonmetallic coatings on metallic substrates

Thickness Measurement of Thin Conductive Sheet, Strip and FoilEddy current techniques are used to measure the thickness of hot sheet, strip and foil in rolling mills, and to measure the amount of metal thinning that has occurred over time due to corrosion on fuselage skins of aircraft. On the impedance plane, thickness variations exhibit the same type of eddy current signal response as a subsurface defects, except that the signal represents a void of infinite size and depth. The phase rotation pattern is the same, but the signal amplitude is greater. In the applet, the lift-off curves for different areas of the taper wedge can be produced by nulling the probe in air and touching it to the surface at various locations of the tapered wedge. If a line is drawn between the end points of the lift-off curves, a comma shaped curve is produced. As illustrated in the second applet, this comma shaped curve is the path that is traced on the screen when the probe is scanned down the length of the tapered wedge so that the entire range of thickness values are measured.

When making this measurement, it is important to keep in mind that the depth of penetration of the eddy currents must cover the entire range of thickness being measured. Typically, a frequency is selected that produces about one standard depth of penetration at the maximum thickness. Unfortunately, at lower frequencies, which are often needed to get the necessary penetration, the probe impedance is more sensitive to changes in electrical conductivity. Thus, the effects of electrical conductivity cannot be phased out and it is important to verify that any variations of conductivity over the region of interest are at a sufficiently low level.

Measurement of Cross-sectional Dimensions of Cylindrical Tubes and RodsDimensions of cylindrical tubes and rods can be measured with either OD coils or internal axial coils, whichever is appropriate. The relationship between change in impedance and change in diameter is fairly constant at all but at very low frequencies. However, the advantages of operating at a higher normalized frequency are twofold. First, the contribution of any conductivity change to the impedance of the coil becomes less important and, it can easily be phased out. Second, there is an increase in measurement sensitivity resulting from the higher value of the inductive component of the impedance. Because of the large phase difference between the impedance vectors corresponding to changes in fill-factor and conductivity (and defect size), simultaneous testing for dimensions, conductivity, and defects can be carried out. Typical applications include measuring eccentricities of the diameters of tubes and rods and the thickness of tube walls. Long tubes are often tested by passing them at a constant speed through encircling coils (generally differential) and providing a close fit to achieve as high a fill-factor as possible. An important application of tube-wall thickness measurement is the detection and assessment of corrosion, both external and internal. Internal probes must be used when the external surface is not accessible, i.e. when testing pipes that are buried or supported by brackets. Success has been achieved in measuring thickness variations in ferromagnetic metal pipes with the remote field technique. See Sec. 5.5 Remote Field Sensing.

Thickness Measurement of Thin Conductive LayersIt is also possible to measure the thickness of a thin layer of metal on a metallic substrate, provided the two metals have widely differing electrical conductivity, e.g. silver on lead where sigma = 67 and 10 MS/m, respectively. A frequency must be selected such that there is complete eddy current penetration of the layer, but not of the substrate itself. The method has also been used successfully for measuring thickness of very thin protective coatings of ferromagnetic metals, e.g. chromium and nickel, on non-ferromagnetic metal bases. Depending on the required degree of penetration, measurements can be made using a single-coil probe or a transformer probe, preferably reflection type. Small-diameter probe coils are usually preferred since they can provide very high sensitivity and minimize effects related to property or thickness variations in the underlying base metal when used in combination with suitably high test frequencies. The goal is to confine the magnetizing field, and the resulting eddy current distribution, to just beyond the thin coating layer and to minimize the field within the base meta

ScanningEddy current data can be collected using automated scanning systems to improve the quality of the measurements and to construct images of scanned areas. The most common type of scanning is line scanning where an automated system is used to push the probe at a fixed speed. Line scan systems are often used when performing tube inspections or aircraft engine blade slot inspections, where scanning in one dimension is needed. The data is usually presented as a strip chart recording. The advantage of using a linear scanning system is that the probe is moved at a constant speed so indication on the strip chart can be correlated to a position on the part being scanned. As with all automated scanning systems, operator variables, such as wobble of the probe, are reduced.

Two-dimensional scanning systems are used to scan a two-dimensional area. This could be a scanning system that scans over a relatively flat area in a X-Y raster mode, or it could be a bolt hole inspection system that rotates the probe as it is moved into the hole. The data is typically displayed as a false-color plot of signal strength or phase angle shift as a function of position, just like an ultrasonic C-scan presentation. Shown below is a portable scanning system that is designed to work on the skins of aircraft fuselage and wing sections.

Listed below are some automated scanning advantages:

minimizes changes in liftoff or fill factor resulting from probe wobble, uneven surfaces, and eccentricity of tubes caused by faulty manufacture or denting

accurate indexing

repeatability

high resolution mapping

Multiple Frequency TechniquesMultiple frequency eddy current techniques simply involve collecting data at several different frequencies and then comparing the data or mixing the data in some way.

Why the need for multiple frequencies? - Some background informationThe impedance of an eddy current probe may be affected by the following factors:

variations in operating frequency

variations in electrical conductivity and the magnetic permeability of a object or structure, caused by structural changes such as grain structure, work hardening, heat treatment, etc.

changes in liftoff or fill factor resulting from probe wobble, uneven surfaces, and eccentricity of tubes caused by faulty manufacture or denting

the presence of surface defects such as cracks, and subsurface defects such as voids and nonmetallic inclusions

dimensional changes, for example, thinning of tube walls due to corrosion, deposition of metal deposits or sludge, and the effects of denting

the presence of supports, walls, and brackets

the presenc

Documents

Basic Principles of Eddy Current Inspection