Basic Principles of Eddy Current Inspection

Eddy 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:

o Material identification o Heat damage detection o Case depth determination o 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 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 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

Eddy 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 other phenomena. 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

Eddy 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 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 analyzers are also sometimes

used to allow improved quantitative eddy-current measurements. Some laboratories have multidimensional scanning capabilities 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 fuselages.

Research to Improve Eddy current measurements

A 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 the case depth in hardened steel samples.

Properties of Electricity

Since 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.

Amperage

The 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, move past a given point in one second. A coulomb is the charge carried by 6.25 x 1018 electrons or 6,250,000,000,000,000,000 electrons.

Electromotive Force

The 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 Volt

The 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.

Resistance

Resistance 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 Law

Ohm'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 Inductance

Induction

In 1824, Oersted discovered that current passing though a coil created a magnetic field capable of shifting a compass needle. Seven years later, Faraday and Henry discovered 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 volts dø/dt = the rate of change of 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.

Inductance

When 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. High frequency AC will result in greater inductive reactance since the magnetic field is changing more rapidly.

Self-inductance and mutual-inductance will be discussed in more detail in the following pages.

Self-Inductance and Inductive Reactance

The 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 surround the wire and join 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 = induced voltage in volts N = number of turns in the coil dø/dt = rate of change of magnetic flux in webers/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 (dø/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 volts L = the value of inductance in henries di/dt = the rate of change of current in amperes per second

Lenz's Law

Soon 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 Reactance

The 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 the 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.

Similarly to resistance, inductive reactance reduces the flow of current in a circuit. 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, 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 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, 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 = M2,1

How is mutual induction used in eddy current inspection?

In eddy current inspection, the eddy currents are generated in the test material due to mutual induction. The test probe is basically a coil of wire through which alternating current is passed. Therefore, when the probe is connected to an eddyscope instrument, it is basically represented by circuit 1 above. The second circuit can be any piece of conductive material.

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 probe's 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.

The eddy currents produce their own magnetic fields that interact with the primary magnetic field of the coil. By measuring changes in the resistance and inductive reactance of the coil, information can be gathered about the test material. This information includes the electrical conductivity and magnetic permeability of the material, the amount of material cutting through the coils magnetic field, and the condition of the material (i.e. whether it contains cracks or other defects.) The distance that the coil is from the conductive material is called liftoff, and this distance affects the mutual-inductance of the circuits. Liftoff can be used to make measurements of the thickness of nonconductive coatings, such as paint, that hold the probe a certain distance from the surface of the conductive material.

It should be noted that if a sample is ferromagnetic, the magnetic flux is concentrated and strengthened despite opposing eddy current effects. The increase inductive reactance due to the magnetic permeability of ferromagnetic materials makes it easy to distinguish these materials from nonferromagnetic materials.

Circuits and Phase

A 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 on the page on current flow. 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, the current does the same. 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 lag the voltage by one quarter of a cycle. One cycle is often referred to as 360o, so it can be said that the current lags the voltage by 90o.

This phase shift occurs because the inductive reactance changes with changing current. Recall that it is the changing magnetic field caused by a changing current that produces inductive reactance. When the change in current is greatest, inductive reactance will be the greatest, and the voltage across the inductor will be the highest. When the change in current is zero, the inductive reactance will be zero and the voltage across the inductor will be zero. Be careful not to confuse the amount of current with the amount of change in the current. Consider the points where the current reaches it peak amplitude and changes direction in the 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 change in current is zero, no magnetic field is generated to produce the inductive reactance. When the inductive reactance is zero, the voltage across the inductor is zero.

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, there is a small amount of capacitance in the circuits so a mention is appropriate. 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 (90o current lead).

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.

Impedance

Electrical Impedance (Z), is the total opposition that a circuit presents to alternating current. Impedance is measured in ohms and may include resistance (R), inductive reactance (XL), and capacitive reactance (XC). However, the total impedance is not simply the algebraic sum of the resistance, inductive reactance, and capacitive reactance. Since the inductive reactance and capacitive reactance are 90o out of phase with the resistance and, therefore, their maximum values occur at different times, vector addition must be used to calculate impedance.

In the image below, a circuit diagram is shown that represents an eddy current inspection system. The eddy current probe is a coil of wire so it contains resistance and inductive reactance when driven by alternating current. The capacitive reactance can be dropped as most eddy current probes have little capacitive reactance. The solid line in the graph below shows the circuit's total current, which is affected by the total impedance of the circuit. The two dashed lines represent the portion of the current that is affected by the resistance and the inductive reactance components individually. It can be seen that the resistance and the inductive reactance lines are 90o out of phase, so when combined to produce the impedance line, the phase shift is somewhere between zero and 90o. The phase shift is always relative to the resistance line since the resistance line is always in-phase with the voltage. If more resistance than inductive reactance is present in

the circuit, the impedance line will move toward the resistance line and the phase shift will decrease. If more inductive reactance is present in the circuit, the impedance line will shift toward the inductive reactance line and the phase shift will increase.

The relationship between impedance and its individual components (resistance and inductive reactance) can be represented using a vector as shown below. The amplitude of the resistance component is shown by a vector along the x-axis and the amplitude of the inductive reactance is shown by a vector along the y-axis. The amplitude of the the impedance is shown by a vector that stretches from zero to a point that represents both the resistance value in the x-direction and the inductive reactance in the y-direction. Eddy current instruments with impedance plane displays present information in this format.

The impedance in a circuit with resistance and inductive reactance can be calculated using the following equation. If capacitive reactance was present in the circuit, its value would be added to the inductance term before squaring.

The phase angle of the circuit can also be calculated using some trigonometry. The phase angle is equal to the ratio between the inductance and the resistance in the circuit. With the probes and circuits used in nondestructive testing, capacitance can usually be ignored so only inductive reactance needs to be accounted for in the calculation. The phase angle can be calculated using the equation below. If capacitive reactance was present in the circuit, its value would simply be subtracted from the inductive reactance term.

or

Impedance and Ohm's Law

In previous pages, Ohm's Law was discussed for a purely resistive circuit. When there is inductive reactance or capacitive reactance also present in the circuit, Ohm's Law must be written to include the total impedance in the circuit. Therefore, Ohm's law becomes:

I = V / Z

Ohm's law now simply states that the current (I), in amperes, is proportional to the voltage (V), in volts, divided by the impedance (Z), in ohms.

Note that the resistance and/or the inductive reactance values must be changed to change the impedance in the circuit. Also note that when there is inductance in the circuit, the voltage and current are out of phase. This is because the voltage across the inductor will be a maximum when the rate of change of the current is greatest. For a sinusoidal wave form like AC, this is at the point where the actual current is zero. Thus the voltage applied to an inductor reaches its maximum value a quarter-cycle before the current does, and the voltage is said to lead the current by 90o.

Depth of Penetration & Current Density

Eddy 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.

The 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 the net magnetic flux and causing a decrease in current flow as the 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 (δ). 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 (2δ), eddy

current density has decreased to 1/e squared or 13.5% of the surface density. At three depths (3δ), the eddy current density is down to only 5% of the surface density.

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: δ = Standard Depth of Penetration (mm)

π = 3.14 f = Test Frequency (Hz) µ = Magnetic Permeability (H/mm) σ = Electrical Conductivity (% IACS)

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 time dependent process, meaning that the eddy currents below the surface take a little longer to form than those at the surface. Disruptions in the eddy currents away from the 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 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 the 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 impedance in a test coil. However, because of the increasing phase lag with depth, there will be a characteristic difference in the test coil impedance vector.

Phase lag can be calculated with the following equation. The phase lag angle calculated with this equation is useful for estimating the subsurface depth of a discontinuity that is concentrated at a specific depth. Discontinuities, such as a crack that spans many depths, must be divided into sections along its length and a weighted average determined for phase and amplitude at each position below the surface.

In Radians

In Degrees

Where: θ=Phase Lag (Rad or Degrees) x=Distance Below Surface (in or mm) δ=Standard Depth of Penetration (in or mm)

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 one radian or 57o. This means that the eddy currents flowing at one standard depth of penetration (δ) below the surface, lag the surface currents by 57o. At two standard depths of penetration (2δ), they lag the surface currents by 114o. Therefore, by measuring 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 phase direction. The angle between the liftoff and defect signals is about twice the phase lag calculated with the above equation. As mentioned above, discontinuities that have a significant dimension normal to the surface, will produce an angle that is based on the weighted average of the disruption to the eddy currents at the various depths along its length.

Eddy Current Instruments

Eddy current instruments can be purchased in a large variety of configurations. Both analog and digital instruments are available. Instruments are commonly classified by the type of display used to present the data. The common display types are analog meter, digital readout, impedance plane and time versus signal amplitude. Some instruments are capable of presenting data in several display formats.

The 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 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

Eddy current probes typically have a frequency or a range of frequencies that they are designed to operated. When the probe is operated outside of this range, problems with the data can occur. When a probe is operated at too high of a frequency, resonance can occurs in the circuit. In a parallel circuit with resistance (R), inductance (XL) and capacitance (XC), as the frequency increases XL decreases and XC increase. Resonance occurs when XL and XC are equal but opposite in strength. At the resonant frequency, the total impedance of the circuit appears to

come only from resistance since XL and XC cancel out. 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.

In eddy current probes and cables, it is commonly stated that capacitance is negligible. However, even circuits not containing discreet components for resistance, capacitance, and inductance can still exhibit their effects. When two conductors are placed side by side, there is always some capacitance between them. Thus, when many turns of wire are placed close together in a coil, a certain amount of stray capacitance is produced. Additionally, the cable used to interconnect pieces of electronic equipment or equipment to probes, often has some capacitance, as well as, inductance. This stray capacitance is usually very small and in most cases has no significant effect. However, they are not negligible in sensitive circuits and at high frequencies they become quite important.

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 component 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 above. 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, which 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, taking energy away from the coil, which 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 increases. 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 conductivity will produce a change in the eddy current signal similar to that seen with aluminum.

Display - Analog Meter

Analog instruments are the simplest of the instruments available for eddy current inspections. They are used for crack

detection, corrosion inspection, or conductivity testing. These types of instruments contain a simple bridge circuit, which compares a balancing load to that measured on the test specimen. If any changes in the test specimen occur which deviate from normal you will see a movement on the instruments meter.

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 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.

Probes - Mode of Operation

Eddy current probes are available in a large variety of 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 an expanding and collapsing magnetic field in and around the coil. When the probe is positioned next to a conductive material, the changing magnetic field generates 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 sensitive 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

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 defects 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 equally.

Reflection Probes

Reflection 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 and pickup coils can be separately optimized for their intended purpose. The driver coil can be made so as to produce a strong and uniform flux field in the vicinity of the pickup coil, while the pickup coil can be made very small so that it will be sensitive to very small defects.

Some absolute and differential "transformer" type eddy current probes.

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

Hybrid Probes

An 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 - Configurations

As 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, inside diameter (ID) probes, and outside diameter (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 facilitates 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 Probes

Bolt 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 Probes

ID probes, which are also referred to as Bobbin probes or feed-through probes, are inserted into hollow products, such as pipes, 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 Coils

OD 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 bars.

Probes - Shielding & Loading

One 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 nonrelevant features of the test component. The impedance change caused by nonrelevant features can complicate the interpretation of the signal. 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 Shielding

Probe shielding is used to prevent or reduce the interaction of the probe's 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 an 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 a 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 will generate 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 the skin effect in the shielding material.

Probe Loading with Ferrite Cores

Sometimes coils are wound around a ferrite core. Since ferrite is ferromagnetic, the magnetic flux produced by the coil prefers to travel through the ferrite as opposed to the 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

The 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. As discussed in the previous pages, probes can contain one or more coils, a core and shielding. All have an important effect on the probe, but the coil requires the most design consideration.

A coil consists of a length of wire wound in a helical manner around the length of a former. 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 (i.e. encircling and pancake coils), generally take the form of tubes or rings made from dielectric materials. Small-diameter coils are usually wound directly onto a solid former.

The region inside the former is called the core, which can consist of either a solid 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 wound around a ferrite core which concentrates the the coil's magnetic field into a smaller area. These coils are referred to as "loaded" coils.

The wire used in an eddy current probe is typically made from copper or other nonferrous metal to avoid magnetic hysteresis effects. The winding usually has more than one layer so as to increase the value of inductance for a given length of coil. 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.

A more precise value of L is given by:

L = Kn2 p [ (ro2 - rc

2) - µrrc2] µo / 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. • µr is the relative magnetic permeability of the core. • µo 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 internal

radii.

• Impedance Matching • Eddy 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 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 that 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 (i.e. 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 (i.e. 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.

Reference Standards

In eddy current testing, the use of reference standards in setting up the equipment is particularly important since signals are affected by many different variables and slight changes in equipment

setup can drastically alter the appearance of a signal. As with most other NDT methods, the most useful information is obtained when comparing the results from an unknown object to results from a similar object with well characterized features and defects. In almost all cases, eddy current inspection procedures require the equipment to be configured using reference standards.

For crack detection, corrosion thinning and other material damage, reference standards are used to setup the equipment to produce a recognizable signal or set of signals from a defect or set of defects. In many cases, the appearance of a test signal can be related to the appearance of a signal from a known defect on the reference standard to estimate the size of a defect in the test component. Signals that vary significantly from the responses produced by the reference standard must be further investigated to the determine the source of the signal.

The reference standard should be of the same material as the test article. If this is not possible or practical, it should be of material that has the same electrical conductivity and magnetic permeability. Component features (material thickness, geometry, etc.) should be the same in the reference standard as those in the test region of interest. If the reference standard is the type with intentional defects, these defects should be as representative of actual defects in the test component as possible. The closer the reference standard is to the actual test component, the better. However, since cracks and corrosion damage are often difficult and costly to produce, artificial defects are commonly used. Narrow notches produced with electron discharge machining (EDM) and saw cuts are commonly used to represent cracks, and drilled holes are often used to simulate corrosion pitting.

Common eddy current reference standards include:

• Conductivity standards. • Flat plate discontinuity standards. • Flat plate metal thinning standards (step or tapered wedges).

• Tube discontinuity standards. • Tube metal thinning standards. • Hole (with and without fastener) discontinuity standards.

• Signal Filtering • Signal filtering is often used in eddy current testing to eliminate unwanted frequencies

from the receiver signal. While the correct filter settings can significantly improve the visibility of a defect signal, incorrect settings can distort the signal presentation and even eliminate the defect signal completely. Therefore, it is important to understand the concept of signal filtering.

• Filtering is applied to the received signal and, therefore, is not directly related to the probe drive frequency. This is most easily understood when picturing a time versus signal amplitude display. With this display mode, it is easy to see that the signal shape is dependent on the time or duration that the probe coil is sensing something. For example, if a surface probe is placed on the surface of conductor and rocked back and forth, it will produce a wave like signal. When the probe is rocked fast, the signal will have a higher frequency than when the probe is rocked slowly back and forth. The signal does not need a wavelike appearance to have frequency content and most eddy current signals will be composed of a large number of frequencies. Consider a probe that senses a notch for 1/60th of a second. In a period of one second the probe could (in theory) go over the notch 60 times, resulting in the notch signal having a frequency of 60 Hz. But, imposed on this same signal, could be the signal resulting from probe wobble, electronic noise, a conductivity shift and other factors which occur at different frequencies.

• Filters Effects • The two standard filters found in most impedance

plane display instruments are the ‘High Pass Filter’ (HPF) and ‘Low Pass Filter’ (LPF). Some instruments also have a‘Band Pass Filter’ (BPF), which is a combination high and low pass filter. Filters are adjusted in Hertz (Hz).

• The HPF allows high frequencies to pass and filters out the low frequencies. The HPF is basically filtering out changes in the signal that occur over a significant period of time.

• The LPF allows low frequency to pass and filters out the high frequency. In other words, all portions of the signal that change rapidly (have a high slope) are filtered, such as electronic noise.

• In the image above, the gradual (low frequency) changes were first filtered out with a HPF and then high frequency electronic noise was filtered with a LPF to leave a clearly visible flaw indication. It should also be noted that since flaw indication signals are comprised of multiple frequencies, both filters have a tendency to reduce the indication signal strength. Additionally, scan speed must be controlled when using filters. Scan over a flaw too slow and the HPF might filter out the flaw indication. Scan over the flaw too fast and the LPF might eliminate the flaw indication.

• Filter Settings • If the spectrum of the signal frequency

and the signal amplitude or attenuation are plotted, the filter responses can be illustrated in graphical form. The image to the right shows the response of a LPF of 20Hz and a HPF of 40Hz. The LPF allows only the frequencies in yellow to pass and the HPF only allow those frequencies in the blue area to pass. Therefore, it can be seen that with these settings there are no frequencies that pass (i.e. the frequencies passed by the LPF are filtered out by the HPF and visa versa).

• To create a window of acceptance for the signals, the filters need to overlap. In the image to the right, the LPF has been adjusted to 60Hz and the HPF to 10Hz. The area shown in gray is where the two frequencies overlap and the signal is passed. A signal of 30Hz will get through at full amplitude, while a signal of 15Hz will be attenuated by approximately 50%. All frequencies above or below the gray area (the pass band) will be rejected by one of the two filters.

• Use of Filters • The main function of the LPF is to

remove high frequency interference noise. This noise can come from a variety of sources including the instrumentation and/or the probe itself. The noise appears as an unstable dot that produces jagged lines on the display as seen in the signal from a surface notch shown in the left image below. Lowering the LPF frequency will remove more of the higher frequencies from the signal and produce a cleaner signal as shown in the center image below. When using a LPF, it should be set to the highest frequency that produces a usable signal. To reduce noise in large surface or ring probes, it may be necessary to use a very low LPF setting (down to 10Hz). The lower the LPF setting, the slower the scanning speed must be and the more closely it must be controlled. The image on the right below shows a signal that has been clipped due to using a scan speed too fast for the selected HPF setting.

• • The HPF is used to eliminate low frequencies which are produced by slow changes, such

as conductivity shift within a material, varying distance to an edge while scanning parallel to it, or out-of-round holes in fastener hole inspection. The HPF is useful when performing automated or semiautomatic scans to keep the signal from wandering too far from the null (balance) point. The most common application for the HPF is the inspection of fastener holes using a rotating scanner. As the scanner rotates at a constant RPM, the HPF can be adjusted to achieve the desired effect.

• Use of the HPF when scanning manually is not recommended, as keeping a constant scanning speed is difficult, and the signal deforms and amplitude decreases. The size of a signal decreases as the scan speed decreases and a flaw indication can be eliminated completely if the scan is not done with sufficient speed. In the images below, it can be seen that a typical response from a surface notch in aluminum without HPF (left image) looks considerably different when the HPF is activated (right image). With the HPF, looping signals with a positive and similar negative deflection are produced on the impedance plane.

• • The use of a minimal HPF setting (1 or 2 Hz) may be used when manually scanning,

provided the operator can largely control the scan speed and becomes familiar with the indication signal changes as scan speed is varied slightly. An good example of such an application would be the manual scan of the radius of a wheel that is rotated by hand, but the speed of rotation can be kept relatively constant.

Surface Breaking Cracks

Eddy 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 current 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 ins 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.

7. Surface Crack Detection Using Sliding Probes 8. Many commercial aircraft applications involve the use of multiple fasteners to connect

the multi-layer 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.

9. 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.

10. 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 orthogonal (90 degrees) to each other.

11. Probe Types 12. 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.

13. Adjustable Sliding Probes These probes are well suited for finding subsurface cracks in thick multi-layer 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.

14. 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.

15. 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.

16. Reference Standards

17. 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.

18. 19. 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.

20. Inspection Variables 21. Liftoff Signal Adjustment

Liftoff is normally adjusted to be relatively horizontal. The term "relatively horizontal" is used here because the liftoff signal often appears a curved line rather than a straight line. Sometimes liftoff can be a sharp curve and may need to be adjusted to run slightly upwards before moving downwards. See Figures 1 and 2.

22. 23. Scan 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.

24. Signal 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.

25. 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.

26. 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.

27. 28. 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.

29. 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.

30.

31. 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 Figures 7 and 8.

32. 33. 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. This condition allows the eddy currents to circulate without encountering a boundary, and therefore, no obstacle or barrier. Because of this effect, it is recommended to paint the holes before fastener installation.

34. Tube Inspection 35. Eddy current inspection is often used to detect corrosion,

erosion, cracking and other changes in tubing. Heat exchangers and steam generators, which are used in power plants, have thousands of tubes that must be prevented from leaking. This is especially important in nuclear power plants where reused, contaminated water must be prevented from mixing with fresh water that will be returned to the environment. The contaminated water flows on one side of the tube (inside or outside) and the fresh water flows on the other side. The heat is transferred from the contaminated water to the fresh water and the fresh water is then returned back to is source, which is usually a lake or river. It is very important to keep the two water sources from mixing, so power plants are periodically shutdown so the tubes and other equipment can be inspected and repaired. The eddy current test method and the related remote field testing method provide high-speed inspection techniques for these applications.

36. A technique that is often used involves feeding a differential bobbin probe into the individual tube of the heat exchanger. With the differential probe, no signal will be seen on the eddy current instrument as long as no metal thinning is present. When metal thinning is present, a loop will be seen on the impedance plane as one coil of the differential probe passes over the flawed area and a second loop will be produced when the second coil passes over the damage. When the corrosion is on the outside surface of the tube, the depth of corrosion is indicated by a shift in the phase lag. The size of the indication provides an indication of the total extent of the corrosion damage.

37. Conductivity Measurements 38. One 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.

39. The technique usually involves nulling an absolute probe in 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 the 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 the variations in the specimen thickness that are not seen in the measurements.

40. 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.

41. 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 opposes the magnetic field of the coil. The resistance in the coil increases since it takes some of the coil's 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 the conductivity increases, as shown in the image above.

42. 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.

43. Conductivity Measurements for the Verification of Heat Treatment

44. With 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.

45. 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, thereby decreasing 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 coarse, widely spaced precipitate. The electrical conductivity is greatest when the material is in the annealed condition.

46. Since solution heat-treated and aged materials are stronger, components 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 supposed to be solution heat-treated and aged somehow 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.

47. Properties for Alclad 2024 Aluminum

Heat Treatment Condition Ultimate Strength Yield Strength Electrical 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

48. It can be seen that the yield strength for the material is 42 kilopounds/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 (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 would prevent this from occurring.

Thickness Measurements of Thin Material

Eddy 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 useful. 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

Corrosion Thinning of Aircraft Skins

One application where the eddy current technique is commonly used to measure material thickness is in the detection and characterization of corrosion damage on the skins of aircraft. Eddy current techniques can be used to do spot checks or scanners can be used to inspect small areas. Eddy current inspection has an advantage over ultrasound in this application because no mechanical coupling is required to get the energy into the structure. Therefore, in multi-layered areas of the structure like lap splices, eddy current can often determine if corrosion thinning is present in buried layers.

Eddy current inspection has an advantage over radiography for this application because only single sided access is required to perform the inspection. To get a piece of film on the back side of the aircraft skin might require removing interior furnishings, panels, and insulation which could be very costly. Advanced eddy current techniques are being developed that can determine thickness changes down to about three percent of the skin thickness.

Thickness Measurement of Thin Conductive Sheet, Strip and Foil

Eddy 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 defect, 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 thicknesses 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 Rods

Dimensions 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, except 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, such as 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.

Thickness Measurement of Thin Conductive Layers

It 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 conductivities (i.e. silver on lead where σ= 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 (i.e. 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 metals.

Thickness Measurements of Nonconducting Coatings on Conductive Materials

The thickness of nonmetallic coatings on metal substrates can be determined simply from the effect of liftoff on impedance. This method has widespread use for measuring thickness of paint and plastic coatings. The coating serves as a spacer between the probe and the conductive surface. As the distance between the probe and the conductive base metal increases, the eddy current field strength decreases because less of the probe's magnetic field can interact with the base metal. Thicknesses between 0.5 and 25 µm can be measured to an accuracy between 10% for lower values and 4% for higher values. Contributions to impedance changes due to conductivity variations should be phased out, unless it is known that conductivity variations are negligible, as normally found at higher frequencies.

Fairly precise measurements can be made with a standard eddy current flaw detector and a calibration specimen. The probe is nulled in air and the direction of the lift-off signal is established. The location of the signal is marked on the screen as the probe is placed on the calibration specimen in areas of decreasing coating thickness. When the probe is placed on the test surface, the position of the signal will move from the air null position to a point that can be correlated to the calibration markings.

Specialized eddy current coating thickness detectors are also available and are often pocket-sized with the probe resembling a small pencil. They are usually operated by a small battery and provide a digital read-out in the appropriate units. Calibration adjustments, some of which are laid down by standards such as BS EN 2360 (1995) and ASTM B 244 and E 376, may be assisted by the use of an inbuilt microprocessor.

Scanning

Eddy 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 indications 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 damage

• accurate indexing • repeatability • high resolution mapping

Multiple Frequency Techniques

Multiple 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 information

The 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 damage

• 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 presence of discontinuities such as edges

Several of these factors are often present simultaneously. In the simple case where interest is confined to detecting defects or other abrupt changes in geometry, a differential probe can be used to eliminate unwanted factors, providing they vary in a gradual manner. For example, variations in electrical conductivity and tube thinning affect both coils of a differential probe simultaneously. However, if unwanted parameters that occur abruptly are affecting the measurements, they can sometimes be negated by mixing signals collected at several frequencies.

An example of where a multi-frequency eddy current inspection is used is in heat exchanger tube inspections. Heat exchanger assemblies are often a collection of tubing that have support brackets on the outside. When attempting to inspect the full wall thickness of the tubing, the signal from the mounting bracket is often troublesome. By collecting a signal at the frequency necessary to inspect the full thickness of the tube and subtracting a second signal collected at a lower frequency (which will be more sensitive to the bracket but less sensitive to features in the tubing), the effects of the bracket can be reduced.

There are a number of commercially available multi-frequency eddy current instruments. Most operate at only two frequencies at a time but some units can collect data at up to four frequencies simultaneously. Multi-frequency measurements can also be made using an impedance analyzer but this equipment is generally not suitable for field measurements. A typical impedance analyzer system is shown below. The interest in pulsed eddy current instruments is largely due to their ability to, in essence, perform multi-frequency measurements very quickly and easily.

Swept Frequency

Swept frequency eddy current techniques involve collecting eddy current data at a wide range of frequencies. This usually involves the use of a specialized piece of equipment such as an impedance analyzer, which can be configured to automatically make measurements over a range of frequencies. The swept-frequency technique can be implemented with commercial equipment but it is a difficult and time-consuming measurement. The advantage of a swept frequency measurement is that depth information can be obtained since eddy current depth of penetration varies as a function of frequency.

Swept frequency measurements are useful in applications such as measuring the thickness of conductive coatings on conductive base metal, differentiating between flaws in surface coatings and flaws in the base metal and differentiating between flaws in various layers of built-up structure. An example application would be the lap splice of a commercial aircraft. Swept frequency measurements would make it possible to tell if cracking was occurring on the outer skin, the inner skin or a double layer. Below is an example of the type of data that can be obtained from swept-frequency measurements.

Data from swept-frequency measurements on two heats of material.

It can be seen that in the etched condition, the material labeled "good" exhibits a much different signal response than the material labeled "bad." It can also be seen that a frequency of around 2.2 MHz provides the largest separation in the curves. Therefore, this frequency should be used if a single frequency is used to sort the parts made from the two metals.

Pulsed Eddy Current Inspection

Conventional eddy current inspection techniques use sinusoidal alternating electrical current of a particular frequency to excite the probe. The pulsed eddy current technique uses a step function voltage to excite the probe. The advantage of using a step function voltage is that it contains a continuum of frequencies. As a result, the electromagnetic response to several different frequencies can be measured with just a single step. Since the depth of penetration is dependent on the frequency of excitation, information from a range of depths can be obtained all at once. If measurements are made in the time domain (that is by looking at signal strength as a function of time), indications produced by flaws or other features near the inspection coil will be seen first and more distant features will be seen later in time.

To improve the strength and ease interpretation of the signal, a reference signal is usually collected, to which all other signals are compared (just like nulling the probe in conventional eddy current inspection). Flaws, conductivity, and dimensional changes produce a change in the signal and a difference between the reference signal and the measurement signal that is displayed. The distance of the flaw and other features relative to the probe will cause the signal to shift in time. Therefore, time gating techniques (like in ultrasonic inspection) can be used to gain information about the depth of a feature of interest.

Remote Field Sensing

Eddy current testing for external defects in tubes where external access is not possible (e.g. buried pipelines), is conducted using internal probes. When testing thick-walled ferromagnetic metal pipes with conventional internal probes, very low frequencies (e.g. 30 Hz for a steel pipe 10 mm thick) are necessary to achieve the through-penetration of the eddy currents. This situation produces a very low sensitivity of flaw detection. The degree of penetration can, in principle, be increased by the application of a saturation magnetic field. However, because of the

large volume of metal present, a large saturation unit carrying a heavy direct current may be required to produce an adequate saturating field.

The difficulties encountered in the internal testing of ferromagnetic tubes can be greatly alleviated with the use of the remote field eddy current method. This method provides measurable through penetration of the walls at three times the maximum frequency possible with the conventional direct field method. This technique was introduced by Schmidt in 1958. Although it has been used by the petroleum industry for detecting corrosion in their installations since the early 1960s, it has only recently evoked general interest. This interest is largely because the method is highly sensitive to variations in wall thickness, but relatively insensitive to fill-factor changes. The method has the added advantage of allowing equal sensitivities of detection at both the inner and outer surfaces of a ferromagnetic tube. It cannot, however, differentiate between signals from these respective surfaces.

Remote Field Testing (RFT)

Remote Field Testing or "RFT" is one of several electromagnetic testing methods commonly employed in the field of nondestructive testing. Other electromagnetic inspection methods include magnetic flux leakage, conventional eddy current and alternating current field measurement testing. Remote field testing is associated with eddy current testing and the term "Remote Field Eddy Current Testing" is often used when describing remote field testing. However, there are several major differences between eddy current testing and remote field testing which will be noted in this section.

RFT is primarily used to inspect ferromagnetic tubing since conventional eddy current techniques have difficulty inspecting the full thickness of the tube wall due to the strong skin effect in ferromagnetic materials. For example, using conventional eddy current bobbin probes to inspect a steel pipe 10 mm thick (such as what might be found in heat exchangers) would require frequencies around 30 Hz

to achieve the adequate I.D. to O.D. penetration through the tube wall. The use of such a low frequency results in a very low sensitivity of flaw detection. The degree of penetration can, in principle, be increased by the use of partial saturation eddy current probes, magnetically biased probes, and pulsed saturation probes. However, because of the large volume of metal present as well as potential permeability variations within the product, these specialized eddy current probes are still limited in their inspection capabilities.

The difficulties encountered in the testing of ferromagnetic tubes can be greatly alleviated with the use of the remote field testing method. The RFT method has the advantage of allowing nearly equal sensitivities of detection at both the inner and outer surfaces of a ferromagnetic tube. The method is highly sensitive to variations in wall thickness and tends to be less sensitive to fill-factor changes between the coil and tube. RFT can be used to inspect any conducting tubular product, but it is generally considered to be less sensitive than conventional eddy current techniques when inspecting nonferromagnetic materials.

RFT Theory of Operation

A probe consisting of an exciter coil and one or more detectors is pulled through the tube. The exciter coil and the detector coil(s) are rigidly fixed at an axial distance of two tube diameters or more between them. The exciter coil is driven with a relatively low frequency sinusoidal current to produce a magnetic field.

This changing magnetic field induces strong circumferential eddy currents which extend axially, as well as radially in the tube wall.

These eddy currents, in turn, produce their own magnetic field, which opposes the magnetic field from the exciter coil. Due to resistance in the tube wall and imperfect inductive coupling, the magnetic field from the eddy currents does not fully counterbalance the magnetic exciting field. However, since the eddy current field is more spread out than the exciter field, the magnetic field from the eddy currents extends farther along the tube axis. The interaction between the two fields is fairly complex but the simple fact is that the exciter field is dominant near the exciter coil and the eddy current field becomes dominant at some distance away from the exciter coil.

The receiving coils are positioned at a distance where the magnetic field from the eddy currents is dominant. In other words, they are placed at a distance where they are unaffected by the magnetic field from the exciter coil but can still adequately measure the field strength from the secondary magnetic field. Electromagnetic induction occurs as the changing magnetic field cuts across the pick-up coil array. By monitoring the consistency of the voltage induced in the pick-up coils one can monitor changes in the test specimen. The strength of the magnetic field at this distance from the excitation coil is fairly weak but it is sensitive to changes in the pipe wall from the I.D. to the O.D.

RFT Theory of Operation (cont.)

The Zones

Direct Couple Zone The region where the magnetic field from the exciter coil is interacting with the tube wall to produce a concentrated field of eddy currents is called the direct field or direct coupled zone. This zone does not contribute a great deal of useful data to the RFT inspection due to problems with rather high noise levels due to the intense varying magnetic field from the excitation coil.

Transition Zone The region just outside the direct couple zone is known as the transition zone. In this zone there is a great deal of interaction between the magnet flux from the exciter coil and the flux induced by the eddy currents. As can be seen in the graph, the interaction of the two opposing fields is strongest near the ID of the tube and fairly subtle at the OD of the tube. The "resultant" field strength (the magnetic field at the sum of the two fields) in this region tends to change abruptly

on the ID due to the interaction of the fields with differing directional characteristics of the two fields.

The receiver coil's signal phase, with respect to the exciter coil, as a function of distance between the two coils is also shown in the graph. When the two coils are directly coupled and there is no interference from a secondary field, their currents are in phase as seen at location zero. In the transition zone, it can be seen that the phase swiftly shifts, indicating the location where the magnetic field from the eddy currents becomes dominate and the start of the remote field.

Remote Field Zone

RFT Probes

The remote field zone is the region in which direct coupling between the exciter coil and the receiver coil(s) is negligible. Coupling takes place indirectly through the generation of eddy currents and their resulting magnetic field. The remote field zone starts to occur at approximately two tube diameters away from the exciter coil. The amplitude of the field strength on the OD actually exceeds that of the ID after an axial distance of approximately 1.65 tube diameters. Therefore, RFT is sensitive to changes in material that occur at the outside diameter as well as the inside diameter of the tube.

Probes for inspection of pipe and tubing are typically of the bobbin (ID) variety. These probes use either a single or dual excitation coil to develop an electromagnetic field through the pipe or tube. The excitation coils are driven by alternating current. The sensing coil or coils are located a few tube diameters away in the remote field zone. Probes can be used in differential or absolute modes for detection of general discontinuities, pitting,

and variations from the I.D. in ferromagnetic tubing. To insure maximum sensitivity, each probe is specifically designed for the inside diameter, composition, and the wall thickness of a particular tube.

RFT Instrumentation

Instruments used for RFT inspection are often dual use eddy current / RFT instruments employing multi-frequency technology. The excitation current from these instruments is passed on to the probe that contains an exciter coil, sometimes referred to as the driver coil. The receiving coil voltage is typically in the microvolt range, so an amplifier is required to boost the signal strength.

Certain systems will incorporate a probe excitation method known as multiplexing. This utilizes an extreme high speed switching method that excites the probe at more than one frequency in sequence. Another method of coil excitation that may be used is simultaneous injection. In this coil stimulation technique, the exciter coil is excited with multiple frequencies at the same time while incorporating filter schemes that subtract aspects of the acquired data. The instrument monitors the pickup coils and passes the data to the display section of the instrument. Some systems are capable of recording the data to some type of storage device for later review.

RFT Signal Interpretation

The signals obtained with RFT are very similar to those obtained with conventional eddy current testing. When all the proper conditions are met, changes in the phase of the receiver signal with respect to the phase of the exciter voltage are directly proportional to the sum of the wall thickness within the inspection area. Localized changes in wall thickness result in phase and amplitude changes. These changes can be indicative of defects such as cracks, corrosion pitting or corrosion/erosion thinning.

RFT Reference Standards

Reference standards for the RFT inspection of tubular products come in many variations. In order to produce reliable and consistent test results, the material used for manufacturing calibration standards must closely match the physical and chemical properties of the inspection specimen. Some of the important properties that must be considered include conductivity, permeability and alloy content. In addition, tube dimensions including I.D., O.D. and wall thickness must also be controlled.

The type of damage mechanisms that are expected to be encountered must also be carfully considered when developing or selecting a reference standard. In order to get accurate quantitative data, artificial discontinuity conditions are typically machined into the standards that will closely match those conditions that may be found in the tubing bundle.

Impedance Matching

Eddy 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 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 that 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 (i.e. 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 (i.e. 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.