Basic Principles of Eddy Current InspectionEddy current
inspection is one of several NDT methods that use the principal of
electromagnetism as the basis for conducting examinations. Several
other methods such as Remote Field Testing (RFT), Flux Leakage and
Barkhausen Noise also use this principle. Eddy currents are created
through a process called electromagnetic induction. When
alternating current is applied to the conductor, such as copper
wire, a magnetic field develops in and around the conductor. This
magnetic field expands as the alternating current rises to maximum
and collapses as the current is reduced to zero. If another
electrical conductor is brought into the close proximity to this
changing magnetic field, current will be induced in this second
conductor. Eddy currents are induced electrical currents that flow
in a circular path. They get their name from eddies that are formed
when a liquid or gas flows in a circular path around obstacles when
conditions are right.
One of the major advantages of eddy current as an NDT tool is
the variety of inspections and measurements that can be performed.
In the proper circumstances, eddy currents can be used for
Crack Detection Material Thickness Measurements Coating
Thickness Measurements Conductivity Measurements For: 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 limitation of eddy current inspection include:
Only conductive materials can be inspected Surface must be
accessible to the probe Skill and training required is more
extensive than other techniques Surface finish and and roughness
may interfere Reference standards needed for setup Depth of
penetration is limited Flaws such as delaminations that lie
parallel to the probe coil winding and probe scan direction are
undetectable
History of Eddy Current TestingEddy current testing has its
origins with Michael Faraday's discovery of electromagnetic
induction in 1831. Faraday was a chemist in England during the
early 1800's and is credited with the discovery of electromagnetic
induction, electromagnetic rotations, the magneto-optical effect,
diamagnetism, and many other discoveries. In 1879, another
scientist named Hughes recorded changes in the properties of a coil
when placed in contact with metals of different conductivity and
permeability. However, it was not until the Second World War that
these effects were put to practical use for testing materials. Much
work was done in the 1950's and 60's, particularly in the aircraft
and nuclear industries. Eddy current testing is now a widely used
and well-understood inspection technique.
Present State of Eddy Current InspectionEddy current inspection
is used in a variety of industries to find defects and make
measurements. One of the primary uses of eddy current testing is
for defect detection when the nature of the defect is well
understood. In general the technique is used to inspect a
relatively small area and the probe design and test parameters must
be established with a good understanding of the flaw that is trying
to be detected. Since eddy currents tend to concentrate at the
surface of a material, they can only be used to detect surface and
near surface defects. In thin materials such as tubing and sheet
stock, eddy currents can be used to measure the thickness of the
material. This makes eddy current a useful tool for detecting
corrosion damage and other damage that causes a thinning of the
material. The technique is used to make corrosion thinning
measurements on aircraft skins and in the walls of tubing used in
assemblies such as heat exchangers. Eddy current testing is also
used to measure the thickness of paints and other coatings. Eddy
currents are also affected by the electrical conductivity and
magnetic permeability of materials. Therefore, eddy current
measurements can be used to sort materials and to tell if a
material has seen high temperatures or been heat treated, which
changes the conductivity of some materials. Eddy current equipment
and probes can be purchased in a wide variety of configurations.
Eddyscopes and a conductivity tester come packaged in very small
and battery operated units for easy portability. Computer based
systems are also available that provide easy data manipulation
features for the laboratory. Signal processing software has also
been developed for trend removal, background subtraction, and noise
reduction. Impedance analyzer are also sometimes used to allow
improved quantitative eddy-current measurements. Some laboratories
have multidimensional scanning capability that are used to produce
images of the scan regions. A few portable scanning systems also
exist for special applications such as scanning regions of aircraft
fuselage.
Research to Improve Eddy current 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 case depth
in hardened steel samples
Properties of ElectricitySince eddy current inspection makes use
of electromagnetic induction, it is important to know about the
scientific principles of electricity and magnetism. For a review of
these principles, the Science of NDT materials on this Internet
site may be helpful. A review of the key parameters will be
provided here. Electricity It is well known that one of the
subatomic particles of an atom is the electron. Atoms can and
usually do have a number of electrons circling its nucleus. The
electrons carry a negative electrostatic charge and under certain
conditions can move from atom to atom. The direction of movement
between atoms is random unless a force causes the electrons to move
in one direction. This directional movement of electrons due to
some imbalance of force is what is known as electricity. 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, moves
past a given point in one second. A coulomb is the charge carried
by 6.25 x 10^18 electrons or 6,250,000,000,000,000,000 electrons.
Electromagnetic Force The force that causes the electrons to move
in an electrical circuit is called the electromotive force, or EMF.
Sometimes it is convenient 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
wattsecond. 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. to
Insulator:Anything that insulates, esp., a nonconductor, usually
a device of glass or porcelain for insulating and supporting
electric wires.
Conductor:A substance or thing that conducts electricity, heat,
sound, etc.
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 LawOhm's law is the most important, basic
law of electricity. It defines the relationship between the three
fundamental electrical quantities: current, voltage, and
resistance. When a voltage is applied to a circuit containing only
resistive elements (i.e. no coils), current flows according to
Ohm's Law, which is shown below.
I=V/RWhere: Electrical Current (Amperes) V = Voltage (Voltage)
Resistance R= (Ohms) I=
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 InductanceInduction
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 d/dt = the rate of
change in magnetic flux in webers/second
Induction is measured in unit of Henries (H) which reflects this
dependence on the rate of change of the magnetic field. One henry
is the amount of inductance that is required to generate one volt
of induced voltage when the current is changing at the rate of one
ampere per second. Note that current is used in the definition
rather than magnetic field. This is because current can be used to
generate the magnetic field and is easier to measure and control
than magnetic flux.. InductanceWhen induction occurs in an
electrical circuit and affects the flow of electricity it is called
inductance, L. Self-inductance, or simply inductance is the
property of a circuit whereby a change in current causes a change
in voltage in the same circuit. When one circuit induces current
flow in a second nearby circuit, it is known as mutualinductance.
The image to the right shows an example of mutual-inductance. When
an AC current is flowing through a piece of wire in a circuit, an
electromagnetic field is produced that is constantly growing and
shrinking and changing direction due to the constantly changing
current in the wire. This changing magnetic field will induce
electrical current in another wire or circuit that is brought close
to the wire in the primary circuit. The current in the second wire
will also be AC and in fact will look very similar to the current
flowing in the first wire. An electrical transformer uses
inductance to change the voltage of electricity into a more useful
level. In nondestructive testing, inductance is used to generate
eddy currents in the test piece.
It should be noted that since it is the changing magnetic field
that is responsible for inductance, it is only present in AC
circuits and that high frequency AC will result in greater
inductive reactance since the magnetic field is changing more
rapidly.
Self-Inductance and Inductive ReactanceThe property of
self-inductance is a particular form of electromagnetic induction.
Self inductance is defined as the induction of a voltage in a
current-carrying wire when the current in the wire itself is
changing. In the case of self-inductance, the magnetic field
created by a changing current in the circuit itself induces a
voltage in the same circuit. Therefore, the voltage is
self-induced. The term inductor is used to describe a circuit
element possessing the property of inductance and a coil of wire is
a very common inductor. In circuit diagrams, a coil or wire is
usually used to indicate an inductive component. Taking a closer
look at a coil will help understand the reason that a voltage is
induced in a wire carrying a changing current. The alternating
current running through the coil creates a magnetic field in and
around the coil that is increasing and decreasing as the current
changes. The magnetic field forms concentric loops that surrounds
the wire and joins up to form larger loops that surround the coil
as shown in the image below. When the current increases in one loop
the expanding magnetic field will cut across some or all of the
neighboring loops of wire, inducing a voltage in these loops. This
causes a voltage to be induced in the coil when the current is
changing.
By studying this image of a coil, it can be seen that the number
of turns in the coil will have an effect on the amount of voltage
that is induced into the circuit. Increasing the number of turns or
the rate of change of magnetic flux increases the amount of induced
voltage. Therefore, Faraday's Law must be modified for a coil of
wire and becomes the following.
Where: VL = the induced voltage in volts N = the number of turns
in the coil d/dt = the rate of change in magnetic flux in webers
per second The equation simply states that the amount of induced
voltage (V L) is proportional to the number of turns in the coil
and the rate of change of the magnetic flux (d/dt). In other words,
when the frequency of the flux is increased or the number of turns
in the coil is increased, the amount of induced voltage will also
increase. In a circuit, it is much easier to measure current than
it is to measure magnetic flux so the following equation can be
used to determine the induced voltage if the inductance and
frequency of the current are known. This equation can also be
reorganized to allow the inductance to be calculated when the
amount of inducted voltage can be determined and the current
frequency is known.
Where: VL = the induced voltage in volts L = the value of
inductance in henries di/dt = the rate of change in 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 inductive
reactance will increase if the number of winds in the coil is
increased since the magnetic field from one coil will have more
coils to interact with.
Since inductive reactance reduces the flow of current in a
circuit, it appears as an energy loss just like resistance.
However, it is possible to distinguish between resistance and
inductive reactance in a circuit by looking at the timing between
the sine waves of the voltage and current of the alternating
current. In an AC circuit that contains only resistive components,
the voltage and the current will be in-phase, meaning that the
peaks and valleys of their sine waves will occur at the same time.
When there is inductive reactance present in the circuit, the phase
of the current will be shifted so that its peaks and valleys do not
occur at the same time as those of the voltage. This will be
discussed in more detail in the section on circuits.
Mutual Inductance (The Basis for Eddy Current Inspection)The
magnetic flux through a circuit can be related to the current in
that circuit and the currents in other nearby circuits, assuming
that there are no nearby permanent magnets. Consider the following
two circuits.
The magnetic field produced by circuit 1 will intersect the wire
in circuit 2 and create current flow. The induced current flow in
circuit 2 will have its own magnetic field which will interact with
the magnetic field of circuit 1. At some point P on the magnetic
field consists of a part due to i and a part due to i . These
fields are proportional to the currents producing them.1 2
Self Inductance: The property of an electric circuit or
component that generated in it as a result of a change in the
current Mutual Inductance: The property of an electric circuit or
component that generated in it as a result of a change in the
current circuit with which it is magnetically linked.1 2
caused an e.m.f. to be flowing through the circuit. caused an
e.m.f. to be flowing through a neighboring
The coils in the circuits are labeled L and L and this term
represents the self inductance of each of the coils. The values of
L1 and L2 depend on the geometrical arrangement of the circuit
(i.e. number of turns in the coil) and the conductivity of the
material. The constant M, called the mutual inductance of the two
circuits and it is dependent on the geometrical arrangement of both
circuits. In particular, if the circuits are far apart, the
magnetic flux through circuit 2 due to the current i will be small
and the mutual inductance will be small. L and M are constants.1
2
We can write the flux,
B
through circuit 2 as the sum of two parts.B2
= Li + iM2 2 1
An equation similar to the one above can be written for the flux
through circuit 1.B1
= Li + iM1 1 2
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: M =M1,2 2,1
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 one above. The second
circuit can be any piece of conductive material.
Eddy Current: A current induced in a conductor situated in a
changing magnetic field or moving in a fixed one.
When alternating current is passed through the coil, a magnetic
field is generated in and around the coil. When the probe is
brought in close proximity to a conductive material, such as
aluminum, the probes changing magnetic field generates current flow
in the material. The induced current flows in closed loops in
planes perpendicular to the magnetic flux. They are named eddy
currents because they are thought to resemble the eddy currents
that can be seen swirling in streams. 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 coating such as
paint that hold the probe a certain distance from the surface of
the conductive material.
Magnetic Permeability: The ratio of the magnetic flux density,
B, in a substance to the external field strength. Ferromagnetic: A
term used to describe materials, such as iron, nickel, and cobalt,
which have a high magnetic permeability.
It should be noted that if a sample is ferromagnetic, the
magnetic flux is concentrated and strengthened despite opposing
eddy current affects. The increase inductive reactance due to the
magnetic permeability of ferromagnetic materials makes it easy to
distinguish these materials from nonferromagnetic materials. In the
applet below, the probe and the sample are shown in cross-section.
The boxes represent a the cross-sectional area of a group of turns
in the coil. The liftoff distance and the drive current of the
probe can be varied to see the effects of the shared magnetic
field. The liftoff value can be set to 0.1 or less and the current
value can be varied from 0.01 to 1.0. The strength of the magnetic
field is shown by the darkness of the lines.
Circuits and PhaseA circuit can be thought of as a closed path
in which current flows through the components that make up the
circuit. The current (i) obeys Ohm's Law, which is discussed in
section 2.1. The simple circuit below consists of a voltage source
(in this case an alternating current voltage source) and a
resistor. The graph below the circuit diagram shows the value of
the voltage and the current for this circuit over a period of time.
This graph shows one complete cycle of an alternating current
source. From the graph, it can be seen that as the voltage
increases so does the current. The voltage and the current are said
to be "in-phase" since their zero, peak, and valley points occur at
the same time. They are also directly proportional to each
other.
In the circuit below, the resistive component has been replaced
with an inductor. When inductance is introduced into a circuit, the
voltage and the current will be "out-of-phase," meaning that the
voltage and current do not cross zero, or reach their peaks and
valleys at the same time. When a circuit has an inductive
component, the current (i ) will lags theL
voltage by one quarter of a cycle. One cycle is often referred
to as 360 degree, so it can be said that the current lags the
voltage by 90 degrees.
The resistive and inductive components are of primary interest
in eddy current testing since the test probe is basically a coil of
wire, which will have both resistance and inductive reactance.
However, for the sake of completeness, capacitance also needs to be
mentioned. This simple circuit below consists of an alternating
current voltage source and a capacitor. Capacitance in a circuit
caused the current (i ) to lead the voltage by one quarter of a
cycle (90 degrees current lag).c
When there is both resistance and inductive reactance (and/or
capacitance) in a circuit, the combined opposition to current flow
is known as impedance. Impedance will be discussed more on the next
page.
Depth of Penetration & Current DensityEddy currents are
closed loops of induced current circulating in planes perpendicular
to the magnetic flux. They normally travel parallel to the coil's
winding and flow is limited to the area of the inducing magnetic
field. Eddy currents concentrate near the surface adjacent to an
excitation coil and their strength decreases with distance from the
coil as shown in the image. Eddy current density decreases
exponentially with depth. This phenomenon is known as the skin
effect. Skin effect arises when the eddy currents flowing in the
test object at any depth produce magnetic fields which oppose the
primary field, thus reducing net magnetic flux and causing a
decrease in current flow as depth increases. Alternatively, eddy
currents near the surface can be viewed as shielding the coil's
magnetic field, thereby weakening the magnetic field at greater
depths and reducing induced currents. The depth that eddy currents
penetrate into a material is affected by the frequency of the
excitation current and the electrical conductivity and magnetic
permeability of the specimen. The depth of penetration decreases
with increasing frequency and increasing conductivity and magnetic
permeability. The depth at which eddy current density has decreased
to 1/e, or about 37% of the surface density, is called the standard
depth of penetration ( ). The word 'standard' denotes plane wave
electromagnetic field excitation within the test sample (conditions
which are rarely achieved in practice). Although eddy currents
penetrate deeper than one standard depth of penetration they
decrease rapidly with depth. At two standard depths of penetration
(2 ), eddy current density has decreased to 1/e squared or 13.5% of
the surface density. At three depths (3 ) the eddy current density
is down to only 5% of the surface density.
Semiconductor: A crystalline solid, such as silicon or
germanium, with an electrical conductivity intermediate between
that of a conductor and an insulator.
Since the sensitivity of an eddy current inspection depends on
the eddy current density at the defect location, it is important to
know the strength of the eddy currents at this location. When
attempting to locate flaws, a frequency is often selected which
places the expected flaw depth within one standard depth of
penetration. This helps to assure that the strength of the eddy
currents will be sufficient to produce a flaw indication.
Alternately, when using eddy currents to measure the electrical
conductivity of a material, the frequency is often set so that it
produces three standard depths of penetration within the material.
This helps to assure that the eddy currents will be so weak at the
back side of the material that changes in the material thickness
will not affect the eddy current measurements. The applet below
illustrates how eddy current density changes in a semi-infinite
conductor. The applet can be used to calculate the standard depth
of penetration. The equation for this calculation is
Where: = Standard Depth of Penetration (mm) = 3.14 f = Test
Frequency (Hz) = Magnetic Permeability (H/mm) = Electrical
Conductivity (% IACS) (Note, however, that the applet uses the
relative permeability so there is a permeability of free space term
in the equation. i.e. relative permeability multiplied by the
permeability of free space puts the material permeability in to
H/mm units.)
Phase LagPhase lag is a parameter of the eddy current signal
that makes it possible to obtain information about the depth of a
defect within a material. Phase lag is the shift in time between
the eddy current response from a disruption on the surface and a
disruption at some distance below the surface. The generation of
eddy currents can be thought of as a diffusion process meaning that
the eddy currents below the surface take a little longer to form
than those at the surface. Therefore, subsurface defects will be
detected by the eddy current instrument a little later in time than
surface defects. Both the signal voltage and current will have this
phase shift or lag with depth, which is different from the phase
angle discussed earlier. (With the phase angle, the current shifted
with respect to the voltage.) Phase lag is an important parameter
in eddy current testing because it makes it possible to estimate
the depth of a defect and with proper reference specimens,
determine the rough size of a defect. The signal produced by a flaw
depends on both amplitude and phase of the eddy currents being
disrupted. A small surface defect and large internal defect can
have a similar effect on the magnitude of test coil impedance.
However, because of the increasing phase lag with depth, there will
be a characteristic difference in the test coil impedance
vector.
Radian: A unit in circular measure, an angle subtended at the
center of a circle by an arc of equal length to the radius. One
radian is equal to 57.296.
At one standard depth of penetration, the phase lag is 57
degrees or one radian. This means that the eddy currents flowing at
one standard depth of penetration ( ) below the surface, lag the
surface currents by 57 degrees. At two standard depths of
penetration (2 ) they lag the surface currents by 114 degrees.
Therefore by measuring the phase lag of a signal, the depth of a
defect can be estimated. In the applet below, the relationship
between the depth of penetration and the phase lag is explored. The
equation at the bottom of the applet can be used to calculate the
depth of penetration by choosing an inspection frequency (f), and,
the magnetic permeability (u) and electrical conductivity for the
test material. These values may also be selected for a particular
material by selecting one of the set materials in the dialog
box.
Eddy Current InstrumentsThe most basic eddy current testing
instrument consists of an alternating current source, a coil of
wire connected to this source, and a voltmeter to measure the
voltage change across the coil. An ammeter could also be used to
measure the current change in the circuit instead of using the
voltmeter.
While it might actually be possible to detect some types of
defects with this type of an equipment, most eddy current
instruments are a bit more sophisticated. In the following pages, a
few of the more important aspects of eddy current instrumentation
will be discussed.
Resonant CircuitsEvery circuit containing capacitance and
inductance has a resonant frequency that is inversely proportional
to the square root of the product of the capacitance and
inductance.
Circuits not containing discreet components for resistance,
capacitance, and inductance can still exhibit their effects. For
example, a coaxial cable used to interconnect pieces of electronic
equipment or equipment to probes, has some capacitance and
inductance. These capacitances and inductances distributed
throughout the cable are very small, but not negligible in
sensitive circuits. The applet represents an eddy current probe
with a default resonant frequency of about 1.0 kHz. An ideal probe
might contain just the inductance, but a realistic probe has some
resistance and some capacitance. The applet initially shows a
single cycle of the 1.0 kHz current passing through the
inductor.
BridgesThe bridge circuit shown in the applet below is known as
the Maxwell-Wien bridge (often called the Maxwell bridge), and is
used to measure unknown inductances in terms of calibrated
resistance and capacitance. Calibration-grade inductors are more
difficult to manufacture than capacitors of similar precision, and
so the use of a simple "symmetrical" inductance bridge is not
always practical. Because the phase shifts of inductors and
capacitors are exactly opposite each other, a capacitive impedance
can balance out an inductive impedance if they are located in
opposite legs of a bridge, as they are here.
Unlike this straight Wien bridge, the balance of the
Maxwell-Wien bridge is independent of source frequency, and in some
cases this bridge can be made to balance in the presence of mixed
frequencies from the AC voltage source, the limiting factor being
the inductor's stability over a wide frequency range.
Exercise: Using the equations within the applet, calculate
appropriate values for C and R2 for a set of probe values . Then
using your calculated values, balance the bridge. The oscilloscope
trace representing current (brightest green) across the top and
bottom of the bridge should be minimized (straight line). In the
simplest implementation, the standard capacitor (Cs) and the
resistor in parallel with it are made variable, and both must be
adjusted to achieve balance. However, the bridge can be made to
work if the capacitor is fixed (non-variable) and more than one
resistor is made variable (at least the resistor in parallel with
the capacitor, and one of the other two). However, in the latter
configuration it takes more trial-and-error adjustment to achieve
balance as the different variable resistors interact in balancing
magnitude and phase.
Another advantage of using a Maxwell bridge to measure
inductance rather than a symmetrical inductance bridge is the
elimination of measurement error due to mutual inductance between
two inductors. Magnetic fields can be difficult to shield, and even
a small amount of coupling between coils in a bridge can introduce
substantial errors in certain conditions. With no second inductor
to react within the Maxwell bridge, this problem is eliminated.
Display - Complex Impedance Plane (eddy scope)Electrical
Impedance (Z), is the total opposition that a circuit presents to
an alternating current. Impedance, measured in ohms, may include
resistance (R), inductive reactance (X ), and capacitive reactance
(X ). Eddy current circuits usually have only R and XL components.
As discussed in the page on impedance, the resistance component and
the reactance components are not in phase so vector addition must
be used to relate them with impedance. For an eddy current circuit
with resistance and inductive reactance components, the total
impedance is calculated using the following equation.L C
You will recall that this can be graphically displayed using the
impedance plane diagram as seen to the right. Impedance also has an
associated angle, called the phase angle of the circuit, which can
be calculated by the following equation.
The impedance plane diagram is a very useful way of displaying
eddy current data. As shown in the figure below, the strength of
the eddy currents and the magnetic permeability of the test
material cause the eddy current signal on the impedance plane to
react in a variety of different ways.
If the eddy current circuit is balanced in air and then placed
on a piece of aluminum, the resistance component will increase
(eddy currents are being generated in the aluminum and this takes
energy away from the coil and this energy loss shows up as
resistance) and the inductive reactance of the coil decreases (the
magnetic field created by the eddy currents opposes the coil's
magnetic field and the net effect is a weaker magnetic field to
produce inductance). If a crack is present in the material, fewer
eddy currents will be able to form and the resistance will go back
down and the inductive reactance will go back up. Changes in
conductivity will cause the eddy current signal to change in a
different way. When a probe is placed on a magnetic material such
as steel, something different happens. Just like with aluminum
(conductive but not magnetic) eddy currents form which takes energy
away from the coil and this shows up as an increase in the coils
resistance. And, just like with the aluminum, the eddy currents
generate their own magnetic field that opposes the coils magnetic
field. However, you will note for the diagram that the reactance
increase. This is because the magnetic permeability of the steel
concentrates the coil's magnetic field this increase in the
magnetic field strength completely overshadows the magnetic field
of the eddy currents. The presence of a crack or a change in the
conductive will produce a change in the eddy current signal similar
to that seen with aluminum. In the applet below, liftoff curves can
be generated for several nonconductive materials with various
electrical conductivities. With the probe held away from the metal
surface, zero and clear the graph. Then slowly move the probe to
the surface of the material. Lift the probe back up, select a
different material and touch it back to the sample surface.
Display - 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 (X ), and capacitive reactance
(X ). Eddy current circuits usually have only R and XL components.
As discussed in the page on impedance, the resistance component and
the reactance components are not in phase so vector addition must
be used to relate them with impedance. For an eddy current circuit
with resistance and inductive reactance components, the total
impedance is calculated using the following equation.L C
You will recall that this can be graphically displayed using the
impedance plane diagram as seen to the right. Impedance also has an
associated angle, called the phase angle of the circuit, which can
be calculated by the following equation.
The impedance plane diagram is a very useful way of displaying
eddy current data. As shown in the figure below, the strength of
the eddy currents and the magnetic permeability of the test
material cause the eddy current signal on the impedance plane to
react in a variety of different ways.
If the eddy current circuit is balanced in air and then placed
on a piece of aluminum, the resistance component will increase
(eddy currents are being generated in the aluminum and this takes
energy away from the coil and this energy loss shows up as
resistance) and the inductive reactance of the coil decreases (the
magnetic field created by the eddy currents opposes the coil's
magnetic field and the net effect is a weaker magnetic field to
produce inductance). If a crack is present in the material, fewer
eddy currents will be able to form and the resistance will go back
down and the inductive reactance will go back up. Changes in
conductivity will cause the eddy current signal to change in a
different way. When a probe is placed on a magnetic material such
as steel, something different happens. Just like with aluminum
(conductive but not magnetic) eddy currents form which takes energy
away from the coil and this shows up as an increase in the coils
resistance. And, just like with the aluminum, the eddy currents
generate their own magnetic field that opposes the coils magnetic
field. However, you will note for the diagram that the reactance
increase. This is because the magnetic permeability of the steel
concentrates the coil's magnetic field this increase in the
magnetic field strength completely overshadows the magnetic field
of the eddy currents. The presence of a crack or a change in the
conductive will produce a change in the eddy current signal similar
to that seen with aluminum. In the applet below, liftoff curves can
be generated for several nonconductive materials with various
electrical conductivities. With the probe held away from the metal
surface, zero and clear the graph. Then slowly move the probe to
the surface of the material. Lift the probe back up, select a
different material and touch it back to the sample surface.
Display - 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 (X ), and capacitive reactance
(X ). Eddy current circuits usually have only R and XL components.
As discussed in the page on impedance, the resistance component and
the reactance components are not in phase so vector addition must
be used to relate them with impedance. For an eddy current circuit
with resistance and inductive reactance components, the total
impedance is calculated using the following equation.L C
You will recall that this can be graphically displayed using the
impedance plane diagram as seen to the right. Impedance also has an
associated angle, called the phase angle of the circuit, which can
be calculated by the following equation.
The impedance plane diagram is a very useful way of displaying
eddy current data. As shown in the figure below, the strength of
the eddy currents and the magnetic permeability of the test
material cause the eddy current signal on the impedance plane to
react in a variety of different ways.
If the eddy current circuit is balanced in air and then placed
on a piece of aluminum, the resistance component will increase
(eddy currents are being generated in the aluminum and this takes
energy away from the coil and this energy loss shows up as
resistance) and the inductive reactance of the coil decreases (the
magnetic field created by the eddy currents opposes the coil's
magnetic field and the net effect is a weaker magnetic field to
produce inductance). If a crack is present in the material, fewer
eddy currents will be able to form and the resistance will go back
down and the inductive reactance will go back up. Changes in
conductivity will cause the eddy current signal to change in a
different way. When a probe is placed on a magnetic material such
as steel, something different happens. Just like with aluminum
(conductive but not magnetic) eddy currents form which takes energy
away from the coil and this shows up as an increase in the coils
resistance. And, just like with the aluminum, the eddy currents
generate their own magnetic field that opposes the coils magnetic
field. However, you will note for the diagram that the reactance
increase. This is because the magnetic permeability of the steel
concentrates the coil's magnetic field this increase in the
magnetic field strength completely overshadows the magnetic field
of the eddy currents. The presence of a crack or a change in the
conductive will produce a change in the eddy current signal similar
to that seen with aluminum. In the applet below, liftoff curves can
be generated for several nonconductive materials with various
electrical conductivities. With the probe held away from the metal
surface, zero and clear the graph. Then slowly move the probe to
the surface of the material. Lift the probe back up, select a
different material and touch it back to the sample surface.
Display - Analog MeterIn order to use a DC-style meter movement,
such as the D'Arsonval design pictured in the applet below, the
alternating current must be "rectified" into DC. This is most
easily accomplished through the use of devices called diodes.
Without going into elaborate detail over how and why diodes work as
they do, remember that they each act like a oneway valve for
electrons to flow. They act as a conductor for one polarity and an
insulator for another. Arranged in a bridge, four diodes will serve
to steer AC through the meter movement in a constant direction. An
analog meter can easily measure just a few microamperes of current
and is well suited for use in balancing bridges.
Probes - Mode of OperationEddy current probes are available in a
large variety shapes and sizes. In fact, one of the major
advantages of eddy current inspection is that probes can be custom
designed for a wide variety of applications. Eddy current probes
are classified by the configuration and mode of operation of the
test coils. The configuration of the probe generally refers to the
way the coil or coils are packaged to best "couple" to the test
area of interest. An example of different configurations of probes
would be bobbin probes, which are inserted into a piece of pipe to
inspect from the inside out, versus encircling probes, in which the
coil or coils encircle the pipe to inspect from the outside in. The
mode of operation refers to the way the coil or coils are wired and
interface with the test equipment. The mode of operation of a probe
generally falls into one of four categories: Absolute,
differential, reflection and hybrid. Each of these classifications
will be discussed in more detail below. Absolute Probes Absolute
probes generally have a single test coil that is used to generate
the eddy currents and sense changes in the eddy current field. As
discussed in the physics section, AC is passed through the coil and
this sets-up a expanding and collapsing magnetic field in and
around the coil. When the probe is positioned next to a conductive
material, the changing magnetic field generate eddy currents within
the material. The generation of the eddy currents take energy from
the coil and this appears as an increase in the electrical
resistance of the coil. The eddy currents generate their own
magnetic field that opposes the magnetic field of the coil and this
changes the inductive reactance of the coil. By measuring the
absolute change in impedance of the test coil, much information can
be gained about the test material. Absolute coils can be used for
flaw detection, conductivity measurements, liftoff measurements and
thickness measurements. They are widely used due to their
versatility. Since absolute probes are sensitivity to things such
as conductivity, permeability liftoff and temperature, steps must
be taken to minimize these variables when they are not important to
the inspection being performed. It is very common for commercially
available absolute probes to have a fixed "air loaded" reference
coil that compensates for ambient temperature variations.
Differential probes have two active coils usually wound in
opposition, although they could be wound in addition with similar
results. When the two coils are over a flaw-free area of test
sample, there is no differential signal developed between the coils
since they are both inspecting identical material. However, when
one coil is over a defect and the other is over good material, a
differential signal is produced. They have the advantage of being
very sensitive to defect yet relatively insensitive to slowly
varying properties such as gradual dimensional or temperature
variations. Probe wobble signals are also reduced with this probe
type. There are also disadvantages to using differential probes.
Most notably, the signals may be difficult to interpret. For
example, if a flaw is longer than the spacing between the two
coils, only the leading and trailing edges will be detected due to
signal cancellation when both coils sense the flaw 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 coil can be made so as to produce a strong and uniform
flux field in the vicinity of the pickup coil. The pickup coil can
be made very small so that it will be sensitive to very small
defects. Hybrid 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 - ConfigurationsAs mentioned on the previous page, eddy
current probes are classified by the configuration and mode of
operation of the test coils. The configuration of the probe
generally refers to the way the coil or coils are packaged to best
"couple" to the test area of interest. Some of the common
classifications of probes based on their configuration include
surface probes, bolt hole probes, ID probes, and OD probes. Surface
Probes Surface probes are usually designed to be handheld and are
intended to be used in contact with the test surface. Surface
probes generally consist of a coil of very fine wire encased in a
protective housing. The size of the coil and shape of the housing
are determined by the intended use of the probe. Most of the coils
are wound so that the axis of the coil is perpendicular to the test
surface. This coil configuration is sometimes referred to as a
pancake coil and is good for detecting surface discontinuities that
are oriented perpendicular to the test surface. Discontinuities,
such as delaminations, that are in a parallel plane to the test
surface will likely go undetected with this coil configuration.
Wide surface coils are used when scanning large areas for
relatively large defects. They sample a relatively large area and
allow for deeper penetration. Since they do sample a large area,
they are often used for conductivity tests to get more of a bulk
material measurement. However, their large sampling area limits
their ability to detect small discontinuities. Pencil probes have a
small surface coil that is encased in a long slender housing to
permit inspection in restricted spaces. They are available with a
straight shaft or with a bent shaft, which facilitate easier
handling and use in applications such as the inspection of small
diameter bores. Pencil probes are prone to wobble due to their
small base and sleeves are sometimes used to provide a wider base.
Bolt Hole 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 a pipe, to inspect from the inside out. The ID
probes have a housing that keep the probe centered in the product
and the coil(s) orientation somewhat constant relative to the test
surface. The coils are most commonly wound around the circumference
of the probe so that the probe inspects an area around the entire
circumference of the test object at one time. OD or Encircling
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 bar.
Probes - Shielding & LoadingOne of the challenges of
performing an eddy current inspection, is getting sufficient eddy
current field strength in the region of interest within the
material. Another challenge is keeping the field away from
nonrelevent features of the test component. Features that could
produce a response that complicates the desired signal information.
Probe shielding and loading are sometimes used to limit the spread
and concentrate the magnetic field of the coil. Of course, if the
magnetic field is concentrated near the coil, the eddy currents
will also be concentrated in this area. Probe Shielding Probe
shielding is used to prevent or reduce the interaction of the
probes magnetic field with nonrelevent features in close proximity
of the probe. Shielding could be used to reduce edge effects when
testing near dimensional transitions such as a step or an edge.
Shielding could also be used to reduce the effects of conductive or
magnetic fasteners in the region of testing. Eddy current probes
are most often shielded using magnetic shielding or eddy current
shielding. Magnetically shielded probes have their coil surrounded
by a ring of ferrite or other material with high permeability and
low conductivity. The ferrite creates and area of low magnetic
reluctance and the probe's magnetic field is concentrated in this
area rather than spreading beyond the shielding. This concentrates
the magnetic field into tighter area around the coil. Eddy current
shielding uses a ring of highly conductive but nonmagnetic
material, usually copper, to surround the coil. The portion of the
coil's magnetic field that cuts across the shielding generates eddy
currents in the shielding material rather than in the nonrelevent
features outside of the shielded area. The higher the frequency of
the current used to drive the probe, the more effective the
shielding will be due to skin effect in the shielding material.
Probe Loading with Ferrite 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 than
through air. Therefore, the ferrite core concentrates the magnetic
field near the center of the probe. This, in turn, concentrates the
eddy currents near the center of the
probe. Probes with ferrite cores tend to be more sensitive than
air core probes and less affected by probe wobble and lift-off.
Coil (Probe) Design - DiameterThe most important feature in eddy
current testing is the way in which the eddy currents are induced
and detected in the material under test. This depends on the design
of the probe, which can contain either one or more coils. A coil
consists of a length of wire wound in a helical manner around the
length of a cylindrical tube or rod, called a former. The winding
usually has more than one layer so as to increase the value of
inductance for a given length of coil. It is desirable with eddy
current testing that the wire is made from copper or other
nonferrous metal to avoid magnetic hysteresis effects. The main
purpose of the former is to provide a sufficient amount of rigidity
in the coil to prevent distortion. Formers used for coils with
diameters greater than a few millimeters, e.g. encircling and
pancake coils, generally take the form of tubes or rings made from
dielectric materials. The region inside the former is called the
core, which can consist of either a solid material or just air.
Small-diameter coils are usually wound directly on to a solid core,
which acts as the former. The higher the inductance (L) of a coil,
at a given frequency, the greater the sensitivity of eddy current
testing. It is essential that the current through the coil is as
low as possible. Too high a current may produce
a rise in temperature, hence an expansion of the coil, which
increases the value of L. magnetic hysteresis, which is small but
detectable when a ferrite core is used.
The simplest type of probe is the single-coil probe, which is in
widespread use. The following applet may be used to calculate the
effect of the inner and outer diameters of a simple probe design on
the probe's self inductance. Dimensional units are in
millimeters.
The higher the inductance (L) of a coil, at a given frequency,
the greater the sensitivity of eddy current testing. A more precise
value of L is given by L = Kn2 pi [ (ro2 - rc2) - rrc2] o/l
ro is the mean radius of the coil. rc is the radius of the core
l is the length of the coil. n is the number of turns. r is the
relative magnetic permeability of the core. o is 4 pi x 10-7 H/m
(i.e. the permeability of free space which is effectively equal to
the permeabilities of the materials of both the wire and the
former). K is a dimensionless constant characteristic of the length
and the external and internal radii.
Coil (Probe) Design - TurnsAs mentioned in the previous section,
an important feature in eddy current testing is the way in which
the eddy currents are induced and detected in the material under
test. The winding usually has more than one layer so as to increase
the value of inductance for a given length of coil. It is desirable
with eddy current testing that the wire is made from copper or
other nonferrous metal to avoid magnetic hysteresis effects. The
main purpose of the former is to provide a sufficient amount of
rigidity in the coil to prevent distortion. Formers used for coils
with diameters greater than a few millimeters, e.g. encircling and
pancake coils, generally take the form of tubes or rings made from
dielectric materials. The region inside the former is called the
core, which can consist of either a solid material or just air.
Small-diameter coils are usually wound directly on to a solid core,
which acts as the former. The higher the inductance (L) of a coil,
at a given frequency, the greater the sensitivity of eddy current
testing. The simplest type of probe is the single-coil probe. The
following applet may be used to calculate the effect of the number
of turns in the coil on the probe's self inductance.
Impedance MatchingEddy current testing requires us to determine
the components of the impedance of the detecting coil or the
potential difference across it. Most applications require the
determination only of changes in impedance, which can be measured
with a high degree of sensitivity using an AC bridge. The
principles of operation of the most commonly used eddy current
instruments are based on Maxwell's inductance bridge, in which the
components of the impedance of the detecting coil, commonly called
a probe, are compared with known variable impedances connected in
series and forming the balancing arm of the bridge. Refer back to
Sec.3.3 - Bridges. The input to the bridge is an AC oscillator,
often variable in both frequency and amplitude. The detector arm
takes the form of either a meter or a storage cathode-ray
oscilloscope, a phase-sensitive detector, a rectifier to provide a
steady indication, and usually an attenuator to confine the output
indication within a convenient range. Storage facilities are
necessary in the oscilloscope in order to retain the signal from
the detector for reference during scanning with the probe.
The highest sensitivity of detection is achieved by properly
matching the impedance of the probe to the impedance of the
measuring instrument. Thus, with a bridge circuit which is
initially balanced, a subsequent but usually small variation in the
impedance of the probe upsets the balance, and a potential
difference appears across the detector arm of the bridge. Although
the Maxwell inductance bridge forms the basis of most eddy current
instruments, there are several reasons why it cannot be used in its
simplest form (e.g. Hague, 1934), including the creation of stray
capacitances, such as those formed by the leads and leakages to
earth. These unwanted impedances can be eliminated by earthing
devices and the addition of suitable impedances to produce one or
more wide-band frequency (i.e. low Q) resonance circuits.
Instruments having a wide frequency range, e.g. from 1 kHz to 2
MHz, may possess around five of these bands to cover the range. The
value of the impedance of the probe is therefore an important
consideration in achieving proper matching and, as a result, it may
be necessary to change the probe when switching from one frequency
band to another.
Surface Breaking CracksEddy current equipment can be used for a
variety of applications such as the detection of cracks
(discontinuities), measurement of metal thickness, detection of
metal thinning due to corrosion and erosion, determination of
coating thickness, and the measurement of electrical conductivity
and magnetic permeability. Eddy currents inspection is an excellent
method for detecting surface and near surface defects when the
probable defect location and orientation is well known. Defects
such as cracks are detected when they disrupt the path of eddy
currents and weaken their strength. The images to the right show an
eddy current surface probe on the surface of a conductive
component. The strength of the eddy currents under the coil of the
probe in indicated by color. In the lower image, there is a flaw
under the right side of the coil and it can be see that the eddy
currents are weaker in this area. Of course, factors such as the
type of material, surface finish and condition of the material, the
design of the probe, and many other factors can affect the
sensitivity of the inspection. Successful detection of surface
breaking and near surface cracks requires: 1. A knowledge of
probable defect type, position, and orientation. 2. Selection of
the proper probe. The probe should fit the geometry of the part and
the coil must produce eddy currents that will be disrupted by the
flaw. 3. Selection of a reasonable probe drive frequency. For
surface flaws, the frequency should be as high as possible for
maximum resolution and high sensitivity. For subsurface flaws,
lower frequencies are necessary to get the required depth of
penetration and this results in less sensitivity. Ferromagnetic or
highly conductive materials require the use of an even lower
frequency to arrive at some level of penetration. 4. Setup or
reference specimens of similar material to the component being
inspected and with features that are representative of the defect
or condition being inspected for. The basic steps in performing an
inspection with a surface probe are the following: 1. Select and
setup the instrument and probe. 2. Select a frequency to produce
the desired depth of penetration.
3. Adjust the instrument to obtain an easily recognizable defect
response using a calibration standard or setup specimen. 4. Place
the inspection probe (coil) on the component surface and null the
instrument. 5. Scan the probe over part of the surface in a pattern
that will provide complete coverage of the area being inspected.
Care must be taken to maintain the same probe-to-surface
orientation as probe wobble can affect interpretation of the
signal. In some cases, fixtures to help maintain orientation or
automated scanners may be required. 6. Monitor the signal for a
local change in impedance that will occur as the probe moves over a
discontinuity. The applet below depicts a simple eddy current probe
near the surface of a calibration specimen. Move the probe over the
surface of the specimen and compare the signal responses from a
surface breaking crack with the signals from the calibration
notches. The inspection can be made at a couple of different
frequency to get a feel for the effect that frequency has on
sensitivity in this application.
Surface Crack Detection Using Sliding ProbesMany commercial
aircraft applications involve the use of multiple fasteners to
connect the multilayer skins. Because of the fatigue stress that is
caused by the typical application of any commercial aircraft,
fatigue cracks can be induced in the vicinity of the fastener
holes. In order to inspect the fastener holes in an adequate amount
of time, sliding probes are an efficient method of inspection.
Sliding probes have been named so because they move over fasteners
in a sliding motion. There are two types of sliding probes, fixed
and adjustable, which are usually operated in the reflection mode.
This means that the eddy currents are induced by the driver coil
and detected by a separate receiving coil. Sliding probes are one
of the fastest methods to inspect large numbers of fastener holes.
They are capable of detecting surface and subsurface
discontinuities, but they can only detect defects in one direction.
The probes are marked with a detection line to indicate the
direction of inspection. In order to make a complete inspection
there must be two scans that are 90 degrees separated from each
other. PROBE TYPES FIXED SLIDING PROBES These probes are generally
used for thinner material compared to the adjustable probes.
Maximum penetration is about 1/8 inch. Fixed sliding probes are
particularly well suited for finding longitudinal surface or
subsurface cracks such as those found in lap joints. Typical
frequency range is from 100 Hz to 100 kHz. ADJUSTABLE SLIDING
PROBES These probes are well suited for finding subsurface cracks
in thick multilayer structures, like wing skins. Maximum
penetration is about 3/4 inch. The frequency range for adjustable
sliding probes is from 100 Hz to 40 kHz. Adjustable probes, as the
name implies, are adjustable with the use of spacers, which will
change the penetration capabilities. The spacer thickness between
the coils is normally adjusted for the best detection. For
tangential scans or 90 degree scanning with an offset from the
center, a thinner spacer is often used. The spacer thickness range
can vary from 0 (no spacer) for inspections close to the surface
and small fastener heads to a maximum of about 0.3 inch for
deep
penetration with large heads in the bigger probe types. A wider
spacer will give more tolerance to probe deviation as the sensitive
area becomes wider but the instrument will require more gain.
Sliding probes usually penetrate thicker materials compared to the
donut probes. REFERENCE STANDARDS Reference/calibration standards
for setup of sliding probes typically consist of three or four
aluminum plates that are fastened together within a lap joint type
configuration. EDM notches or naturally/artificially- induced
cracks are located in the second or third layer of the
standard.
Reference standards used should be manufactured from the same
material type, alloy, material thickness, and chemical composition
that will be found on the aircraft component to be inspected. Sizes
and tolerances of flaws introduced in the standards are usually
regulated by inspection specifications. INSTRUMENT DISPLAY
(LIFTOFF) Liftoff is normally adjusted to be horizontal, but on the
CRT liftoff shows up as a curved line rather than a straight line.
Sometimes liftoff can be a steep curve and may have to be allowed
to move slightly upwards before moving downwards. See Figures 1 and
2.
SCANNING PATTERNS A typical scan is centralized over the
fastener head and moves along the axis of the fastener holes. This
scan is generally used to detect cracks positioned along the axis
of the fastener holes. For detecting cracks located transverse or
90 degrees from the axis of the fastener holes, a scan that is 90
degrees from the axis of the fastener holes is recommended. CRACK
DETECTION 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. If the crack is in the far side of the
fastener, as the probe moves over it the dot will follow the
fastener line first but will move upwards (clockwise) as it goes
over the crack. If the crack is in the near side, it will be found
first and the dot will move along the crack level before coming
down to the fastener level. If two cracks on opposite sides of the
fastener hole are present, the dot will move upwards to the height
by the first crack length and then come back to the fastener line
and balance point. If the second crack is longer than the first
one, the dot will move even higher and complete the loop
(clockwise) before going down to the balance point. See figures 3
and 4.
VARIABLES: PROBE SCAN DEVIATION Most probes are designed to give
a narrow indication for a good fastener hole so that the loops from
the cracks are more noticeable. Some probes and structures can give
wider indications and a similar result can be obtained if the probe
is not straight when it approaches the fastener. It is important to
keep the probe centralized over the fastener heads. Doing this will
give you a maximum indication for the fastener and a crack. If the
probe deviates from the center line, the crack indication will move
along the loop that we saw in figure 5 and is now present in figure
6. The crack indication is at "a" when the probe is centralized and
moves toward "b" as it deviates in one direction, or "c" as it
deviates in the opposite direction. Point "b" gives an important
indication even if it loses a small amount of amplitude it has
gained in phase, giving a better separation angle. This is because
we deviated to the side where the crack is located.
CRACK ANGLE DEVIATION A reduction in the crack indication occurs
when the crack is at an angle to the probe scan direction. This
happens if the crack is not completely at 90 degrees to the normal
probe scan or changes direction as it grows. Both the fixed and
adjustable sliding probes are capable of detecting cracks up to
about 30 degrees off angle. See to figures 7 and 8.
ELECTRICAL CONTACT When inspecting fasteners that have just been
installed or reference standards that have intimate contact with
the aluminum skin plate, it is not unusual to obtain a smaller than
normal indication. In some extreme cases, the fastener indication
may disappear almost completely. This is due to the good electrical
contact between the fastener and the skin that allows the eddy
currents to circulate without finding the boundary and therefore no
obstacle or barrier. Because of this effect it is recommended to
paint the holes before fastener installation
Crack Detection (Reflection)For crack detection, the simplest
type of probe is the single-coil probe, which is in widespread use.
Sometimes it is desirable to use a probe consisting of two or more
coils arranged in a transformer fashion, and therefore known as a
transformer probe. The primary coil induces eddy currents in the
test object and the secondary coil acts as a detector. The use of
this probe provides an enhanced signal-to-noise ration for
detection, advantageous when deep penetration is required, such as
seeking internal defects.
The through-transmission method is sometimes used when complete
penetration of plates and tube walls is required. Reflection or
Driver/pickup probes have a primary winding driven from the
oscillator and one or more sensor windings connected to the
measurement circuit. Depending on the configuration of the sensor
windings, reflection probes may give a response equivalent to
either an absolute or a differential probe. The main advantages of
reflection probes are list below:
Driver and pickup coils can be separately optimized for their
intended purpose. They have wider frequency range than equivalent
bridge connected probes. The larger driver coil gives a more even
field, resulting in better penetration and liftoff
characteristic
Conductivity MeasurementsOne of the uses of eddy current
instruments is for the measurement of electrical conductivity. The
value of the electrical conductivity of a metal depends on several
factors, such as its chemical composition and the stress state of
its crystalline structure. Therefore, electrical conductivity
information can be used for sorting metals, checking for proper
heat treatment, and inspecting for heat damage. The technique
usually involves nulling an absolute probe in the air and placing
the probe in contact with the sample surface. For nonmagnetic
materials, the change in impedance of the coil can be correlated
directly to the conductivity of the material. The technique can be
used to easily sort magnetic materials from nonmagnetic materials
but it is difficult to separate the conductivity effects from
magnetic permeability effects, so conductivity measurements are
limited to nonmagnetic materials. It is important to control
factors that can affect the results such as the inspection
temperature and the part geometry. Conductivity changes with
temperature so measurements should be made at a constant
temperature and adjustments made for temperature variations when
necessary. The thickness of the specimen should generally be
greater than three standard depths of penetration. This is so the
eddy currents at the back surface of the sample are sufficiently
weaker than variations in specimen thickness that are not seen in
the measurements.Generally large pancake type, surface probes are
used to get a value for a relatively large sample area. The
instrument is usually setup such that a ferromagnetic material
produces a response that is nearly vertical. Then, all conductive
but nonmagnetic materials will produce a trace that moves down and
to the right as the probe is moved toward the surface. Think back
to the discussion on the impedance plane and these type of
responses make sense. Remember that inductive reactance changes are
plotted along the y-axis and resistance changes are plotted in the
x-axis. Since ferromagnetic materials will concentrate the magnetic
field produced by a coil, the inductive reactance of the coil will
increase. The effects on the signal from the magnetic permeability
overshadow the effects from conductivity since they are so much
stronger.
When the probe is brought near a conductive but nonmagnetic
material, the coil's inductive reactance goes down since the
magnetic field from the eddy currents and opposes the magnetic
field of the coil. The resistance in the coil increases since it
takes some of the coils energy to generate the eddy currents and
this appears as additional resistance in the circuit. As the
conductivity of the materials being tested increases, the
resistance losses will be less and the inductive reactance changes
will be greater. Therefore, the signals will be come more vertical
as conductivity increases as shown in the image above. To sort
materials, using an impedance plane device, the signal from the
unknown sample must be compared to a signal from a variety of
reference standards.. However, there are devices available that can
be calibrated to produce a value for electrical conductivity which
can then be compared to published values of electrical conductivity
in MS/m or percent IACS (International Annealed Copper Standard).
Please be aware that the conductivity of a particular material can
vary significantly with slight variations in the chemical
composition and, thus, a conductivity range is generally provided
for a material. The conductivity range for one material may overlap
with the range of a second material of interest so conductivity
alone can not always be used to sort materials. The electrical
conductivity values for a variety of materials can be found in the
material properties reference tables. The following applet is based
on codes for nonferrous materials written by Back Blitz from his
book, "Electrical and Magnetic Methods of Nondestructive Testing",
2nd ed., Chapman & Hill (1997). The applet demonstrates how a
impedance plane eddy current instrument can be used for sorting of
materials.
Conductivity Measurements for the Verification of Heat
TreatmentWith some materials, such as solution heat treatable
aluminum alloys, conductivity measurements are often made verifying
that parts and materials have received the proper heat treatment.
High purity aluminum is soft and ductile, and gains strength and
hardness with the addition of alloying elements. A few such
aluminum alloys are the 2000 series (2014, 2024, etc.), 6000 series
(6061, 6063, etc.), and 7000 series (7050, 7075, etc.). The 2xxx
series aluminum alloys have copper, the 6xxx series have magnesium,
and the 7xxx have zinc as their major alloying elements. Heat
treatment of aluminum alloys is accomplished in two phases -
solution heat treatment and then aging. In the solution heat
treatment step, the alloys are heated to an elevated temperature to
dissolve the alloying elements into solution. The metal is then
rapidly cooled or quenched to freeze the atoms of the alloying
elements in the lattice structure of the aluminum. This distorts
and stresses the structure making electron movement more difficult
and, therefore, decreases the electrical conductivity. In this
condition, the alloys are still relatively soft but start to gain
strength as the alloying elements begin to precipitate out of
solution to form extremely small particles that impede the movement
of dislocations within the material. The formation of the
precipitates can be controlled for many alloys by heating and
holding the material at an elevated temperature for a period of
time (artificial aging). As the alloying elements precipitate out
of solid solution, the conductivity of the material gradually
increases. By controlling the amount of precipitated particles
within the aluminum, the properties can be controlled to produce
peak strength or some combinations of strength and corrosion
resistance. Sometimes the material must be annealed or put into the
softest most ductile condition possible in order to perform forming
operations. Annealing allows all of the alloying elements to
precipitate out of solution to form a course widely spaced
precipitate. The electrical conductivity is greatest when the
material is in the annealed condition. Since solution heat-treated
and aged materials are stronger, components that can be made using
less material. A lighter or more compact design is often of great
importance to the designer and well worth the cost of the heat
treating process. However, think of the consequences that could
arise if a component that was suppose to be solution heat treated
and aged some how left the manufacturing facility and was put into
service unheat treated or annealed. This is a real possibility
since heat treated aluminum parts look exactly like unheat treated
parts. Consider 2024 aluminum as an example. Select tensile
properties and its electrical conductivity for various heat
treatment conditions are given in the following table.
Properties for Alclad 2024 Aluminum Heat Treatment Condition
Annealed (O) Solution Heat Treated and Naturally Aged (T42)
Solution Heat Treated, Coldworked and Artificially Aged (T861)
Ultimate Strength Yield Strength 26 ksi (180 MPa) 11 ksi (75 MPa)
Electrical Conductivity 50 % IACS
64 ksi (440 MPa) 42 ksi (290 MPa) 30 % IACS 70 ksi (485 MPa) 66
ksi (455 MPa) 38 % IACS
It can be seen that the yield strength for the material is 42
kilipounds/square inch (ksi) (290 MPa) in the solution heat treated
and naturally aged condition (T42 condition). The yield strength
can be increased to 66 ksi (455 MPa) when coldworked and
artificially aged (T861 condition). But in the annealed condition,
the yield strength is reduced to 11 ksi or 75 MPa). If an annealed
part were accidentally used where a part in the T42 or T861 was
intended, it would likely fail prematurely. However, a quick check
of the conductivity using an eddy current instrument of all parts
prior to shipping the parts would prevent this from occurring.
Thickness Measurements of Thin MaterialEddy current techniques
can be used to perform a number of dimensional measurements. The
ability to make rapid measurements without the need for couplant
or, in some cases even surface contact, makes eddy current
techniques very use. The type of measurements that can be made
include:
thickness of thin metal sheet and foil, and of metallic coatings
on metallic and nonmetallic substrate cross-sectional dimensions of
cylindrical tubes and rods thickness of nonmetallic coatings on
metallic substrates
Thickness Measurement of Thin Conductive Sheet, Strip and 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 defects, except that the signal represents a void of
infinite size and depth. The phase rotation pattern is the same,
but the signal amplitude is greater. In the applet, the lift-off
curves for different areas of the taper wedge can be produced by
nulling the probe in air and touching it to the surface at various
locations of the tapered wedge. If a line is drawn between the end
points of the lift-off curves, a comma shaped curve is produced. As
illustrated in the second applet, this comma shaped curve is the
path that is traced on the screen when the probe is scanned down
the length of the tapered wedge so that the entire range of
thickness values are measured.
When making this measurement, it is important to keep in mind
that the depth of penetration of the eddy currents must cover the
entire range of thickness being measured. Typically, a frequency is
selected that produces about one standard depth of penetration at
the maximum thickness. Unfortunately, at lower frequencies, which
are often needed to get the necessary penetration, the probe
impedance is more sensitive to changes in electrical conductivity.
Thus, the effects of electrical conductivity cannot be phased out
and it is important to verify that any variations of conductivity
over the region of interest are at a sufficiently low level.
Measurement of Cross-sectional Dimensions of Cylindrical Tubes and
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 at all but at very low frequencies. However, the
advantages of operating at a higher normalized frequency are
twofold. First, the contribution of any conductivity change to the
impedance of the coil becomes less important and, it can easily be
phased out. Second, there is an increase in measurement sensitivity
resulting from the higher value of the inductive component of the
impedance. Because of the large phase difference between the
impedance vectors corresponding to changes in fill-factor and
conductivity (and defect size), simultaneous testing for
dimensions, conductivity, and defects can be carried out. Typical
applications include measuring eccentricities of the diameters of
tubes and rods and the thickness of tube walls. Long tubes are
often tested by passing them at a constant speed through encircling
coils (generally differential) and providing a close fit to achieve
as high a fillfactor as possible. An important application of
tube-wall thickness measurement is the detection and assessment of
corrosion, both external and internal. Internal probes must be used
when the external surface is not accessible, i.e. when testing
pipes that are buried or supported by brackets. Success has been
achieved in measuring thickness variations in ferromagnetic metal
pipes with the remote field technique. See Sec. 5.5 Remote Field
Sensing. Thickness Measurement of Thin Conductive 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 conductivity, e.g. silver on lead where sigma = 67 and
10 MS/m, respectively. A frequency must be selected such that there
is complete eddy current penetration of the layer, but not of the
substrate itself. The method has also been used successfully for
measuring thickness of very thin protective coatings of
ferromagnetic metals, e.g. chromium and nickel, on
non-ferromagnetic metal bases. Depending on the required degree of
penetration, measurements can be made using a single-coil probe or
a transformer probe, preferably reflection type. Small-diameter
probe coils are usually preferred since they can provide very high
sensitivity and minimize effects related to property or thickness
variations in the underlying base metal when used in combination
with suitably high test frequencies. The goal is to confine the
magnetizing field, and the resulting eddy current distribution,
to just beyond the thin coating layer and to minimize the field
within the base meta
ScanningEddy current data can be collected using automated
scanning systems to improve the quality of the measurements and to
construct images of scanned areas. The most common type of scanning
is line scanning where an automated system is used to push the
probe at a fixed speed. Line scan systems are often used when
performing tube inspections or aircraft engine blade slot
inspections, where scanning in one dimension is needed. The data is
usually presented as a strip chart recording. The advantage of
using a linear scanning system is that the probe is moved at a
constant speed so indication on the strip chart can be correlated
to a position on the part being scanned. As with all automated
scanning systems, operator variables, such as wobble of the probe,
are reduced. Two-dimensional scanning systems are used to scan a
two-dimensional area. This could be a scanning system that scans
over a relatively flat area in a X-Y raster mode, or it could be a
bolt hole inspection system that rotates the probe as it is moved
into the hole. The data is typically displayed as a false-color
plot of signal strength or phase angle shift as a function of
position, just like an ultrasonic C-scan presentation. Shown below
is a portable scanning system that is designed to work on the skins
of aircraft fuselage and wing sections. Listed below are some
automated scanning advantages:
minimizes changes in liftoff or fill factor resulting from probe
wobble, uneven surfaces, and eccentricity of tubes caused by faulty
manufacture or denting accurate indexing repeatability high
resolution mapping
Multiple Frequency TechniquesMultiple frequency eddy current
techniques simply involve collecting data at several different
frequencies and then comparing the data or mixing the data in some
way. Why the need for multiple frequencies? - Some background
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 denting the
presence of surface defects such as cracks, and subsurface defects
such as voids and nonmetallic