Eddy Current Array Tutorial

Eddy current array (ECA) is a nondestructive testing technology that

provides the ability to electronically drive multiple eddy current coils, which

are placed side by side in the same probe assembly. Each individual eddy

current coil in the probe produces a signal relative to the phase and

amplitude of the structure below it. This data is referenced to an encoded

position and time and represented graphically as a C-scan image. Most

conventional eddy current flaw detection techniques can be reproduced with

ECA inspections; however, the remarkable advantages of ECA technology

allow improved inspection capabilities and significant time savings.

ECA technology includes the following advantages:

A larger area can be scanned in a single-probe pass, while maintaining

a high resolution.

Less need for complex robotics to move the probe; a simple manual

scan is often enough.

C-scan imaging improves flaw detection and sizing.

Complex shapes can be inspected using probes customized to the

profile of the part being inspected.

1.0 Introduction

1.1 General Introduction to Eddy Current (EC) Testing

1.2 History of Eddy Current Testing

1.3 Equipment

2.0 What is Eddy Current Array (ECA) Testing?

2.1 Basic Concepts

2.2 Depth of Penetration

2.3 Multiplexing

2.4 Normalization

2.5 Benefits of Eddy Current Testing

3.0 Probes

3.1 EC Probes

3.2 Eddy Current Array Probes

3.3 Probes Design

3.4 Probes Parameters

3.5 Custom Probes

3.6 Calibration Standards

4.0 Typical Applications

4.1 Rivet Inspection

4.2 Corrosion Detection

1.0 Introduction

1.1 General Introduction to Eddy Current (EC)

Testing

Eddy current (EC) testing is a no contact method for the inspection of metallic

parts. Eddy currents are fields of alternating magnetic current that are created

when an alternating electric current is passed through one or more coils in a

probe assembly. When the probe is linked with the part under inspection, the

alternating magnetic field induces eddy currents in the test part. Discontinuities

or property variations in the test part change the flow of the eddy current and

are detected by the probe in order to make material thickness measurements or

to detect defects such as cracks and corrosion.

Over the years, probe technology and data processing have advanced to the

point where eddy current testing is recognized as being fast, simple, and

accurate. The technology is now widely used in the aerospace, automotive,

petrochemical, and power generation industries for the detection of surface or

near-surface defects in materials such as aluminum, stainless steel, copper,

titanium, brass, Inconel®, and even carbon steel (surface defects only).

1.2 History of Eddy Current Testing

The phenomenon of eddy currents was discovered by French physicist Leon

Foucault in 1851, and for this reason eddy currents are sometimes called

Foucault currents. Foucault built a device that used a copper disk moving in

a strong magnetic field to show that eddy currents (magnetic fields) are

generated when a material moves within an applied magnetic field.

Eddy current testing began largely as a result of the English scientist Michael

Faraday's discovery of electromagnetic induction in 1831. Faraday

discovered that when a magnetic field passes through a conductor (a

material in which electrons move easily)-or when a conductor passes through

a magnetic field-an electric current will flow through the conductor if there is

a closed path through which the current can circulate. In 1879, another

breakthrough was made when another English scientist, David Hughes,

demonstrated how the properties of a coil change when placed in contact

with metals of different conductivity and permeability. However, it was not

until the Second World War that these developments in the transmitting and

receiving of electromagnetic waves were put to practical use for materials

testing.

Beginning in 1933, in Germany, while working for the Kaiser-Wilhelm-

Institute, Professor Friedrich Förster adapted eddy current technology to

industrial use, developing instruments for measuring conductivity and for

sorting mixed-up ferrous components. In 1948, Förster founded his own

company in Reutlingen, a business based on eddy current testing that

continues to this day. Other companies soon followed. Many advances were

made throughout the 1950s and 1960s, especially in the aircraft and nuclear

industries. There have been many recent developments in eddy current

testing, leading to improved performance and the development of new

applications. Eddy current testing is now a widely used and well-understood

inspection technique for flaw detection as well as for thickness and

conductivity measurements.

1.3 Equipment

With thousands of units used throughout the world, the R/D Tech OmniScan

MX is Olympus NDT's most successful modular and portable test unit. The

OmniScan family includes the innovative phased array and eddy current

array test units, as well as the eddy current and conventional ultrasound

modules, all designed to meet the most demanding requirements of NDT.

The OmniScan MX offers a high acquisition rate and powerful software

features in a portable, modular mainframe to efficiently perform manual and

automated inspection.

The OmniScan™ ECA test configuration supports 32 sensor coils (up to 64

with an external multiplexer) working in bridge or transmit-receive mode.

The operating frequency ranges from 20 Hz to 6 MHz with the option of using

multiple frequencies in the same acquisition.

2.0 What is Eddy Current Array (ECA)

Testing?

2.1 Basic Concepts

Eddy current array and conventional eddy current technology share the

same basic principle. Alternating current injected into a coil creates a

magnetic field (in blue). When the coil is placed over a conductive part,

opposed alternating currents (eddy currents, in red) are generated. Defects

in the part disturb the path of the eddy currents (in yellow). This disturbance

can be measured by the coil.

Eddy current array (ECA) technology provides the ability to electronically

drive multiple eddy current coils placed side by side in the same probe

assembly. Data acquisition is performed by multiplexing the eddy current

coils in a special pattern to avoid mutual inductance between the individual

coils.

Most conventional eddy current flaw detection techniques can be reproduced

with an ECA inspection. With the benefits of single-pass coverage, and

enhanced imaging capabilities, ECA technology provides a remarkably

powerful tool and significant time savings during inspections.

Major advantages of ECA testing are the following:

Larger area can be scanned in a single-probe pass, while maintaining a high

resolution

Reduced need for complex robotics to move the probe; a simple manual scan

is often enough

Improved flaw detection and sizing with C-scan imaging

Inspection of complex shapes using probes customized to the profile of the

part being inspected

The OmniScan™ ECA test configuration supports 32 sensor coils (up to 64

with an external multiplexer) working in bridge or transmit-receive mode.

The operating frequency ranges from 20 Hz to 6 MHz with the option of using

multiple frequencies in the same acquisition.

2.2 Depth of Penetration

Eddy current density does not remain constant across the depth of a

material. The density is greatest at the surface and decreases exponentially

with depth (the "skin effect"). The standard depth of penetration equation

(shown to the right) is used to explain the penetration capability of eddy

current testing, which decreases with increasing frequency, conductivity, or

permeability. For a material that is both thick and uniform, the standard

depth of penetration is the depth at which the eddy current density is 37% of

the material surface value. To detect very shallow defects in a material, and

also to measure the thickness of thin sheets, very high frequencies are used.

Similarly, in order to detect subsurface defects, and to test highly

conductive, magnetic, or thick materials, lower frequencies must be used.

Where:

d = Standard depth of penetration (mm)

p = 3.14

ƒ = Test frequency (Hz)

m = Magnetic permeability (H/mm)

s = Electrical conductivity (% IACS)

+ Formula poster

2.3 Multiplexing

Multiplexing is the process by which multiple analog message signals are

combined into one digital signal on a shared medium. When eddy current

array data is multiplexed, the individual eddy current coils are excited at

different times, allowing the system to excite all of the coils in the probe

without ever exciting any two adjacent coils at the same time. An

undesirable effect known as mutual inductance (magnetic coupling between

coils in close proximity) is minimized with the use of an internal multiplexing

system to carefully program the exact time that each coil is excited to

transmit its eddy current signal. The signals are then reassembled before

being displayed as an image. In addition to the enhanced imaging

capabilities of multiplexed data, multiplexing allows any individual coil (data)

channel to be analyzed after inspection. Multiplexing allows an increased

channel resolution, increased coil sensitivity (through the reduction of

mutual inductance), and a reduced noise level. This ultimately leads to an

improved signal-to-noise ratio.

Mutual Inductance is avoided by multiplexing

2.4 Normalization

The main purpose of performing normalization is to standardize sensitivity

for an ECA probe. To do this, the operator scans a sample containing a

calibration defect in order to generate the same eddy current signal for each

channel. For most applications, a defect such as a long transversal notch will

suffice (see illustration to the right). Using the calibration defect, the

operator adjusts the gain and rotation of each channel so that the same

phase and amplitude response is obtained for all channels.

2.5 Benefits of Eddy Current Testing

Benefits of Eddy Current Testing

Eddy current offers the following capabilities:

Quick, simple, and reliable inspection technique to detect surface and near-

surface defects in conductive material

Can be used to measure material electrical conductivity

Measurement of nonconductive coating

Hole inspection with the use of high-speed rotating scanner and surface

probe

Benefits of Eddy Current Array Testing

Compared to single-channel eddy current technology, eddy current array

technology provides the following benefits:

Drastically reduces inspection time

Covers a large area in one single pass

Reduces the complexity of mechanical and robotic scanning systems

Provides real-time cartography of the inspected region, facilitating data

interpretation

Is well suited for complex part geometries

Improves reliability and probability of detection (POD)

3.0 Probes

3.1 EC Probes

Olympus NDT`s standard R/D Tech eddy current probes are available in

different configurations:

Bolt hole probes

Surface probes, in various shapes and configurations

Low-frequency Spot and Ring type probes

Sliding probes

Wheel probes

Conductivity probes

Specialty probes made for specific applications

Reference standards with EDM notches can be manufactured according to

the application specifications.

Probes used to perform eddy current inspections are made with a copper

wire wound to form a coil. The coil shape can vary to better suit specific

applications.

a-The alternating current flowing through the coil at a chosen frequency

generates a magnetic field around the coil.

b-When the coil is placed close to an electrically conductive material, eddy

current is induced in the material.

c-If a flaw in the conductive material disturbs the eddy current circulation,

the magnetic coupling with the probe is changed and a defect signal can be

read by measuring the coil impedance variation.

Surface preparation is minimal. Unlike liquid penetrant or magnetic particle

inspection, it is unnecessary to remove the paint from the surface to inspect

the parts.

3.2 Eddy Current Array Probes

Olympus NDT manufactures R/D Tech ECA probes for a wide range of

applications. Probes can be designed to detect a specific type of flaw or to

follow the shape of the part being inspected. Standard designs are available

to detect defects such as cracks and pitting, and subsurface defects like

cracks in multilayer structures as well as corrosion.

3.3 Probes Design

Surface

Surface probes are made with coils designed to be driven at relatively high

frequencies (typically 50 kHz to 500 kHz). Using higher frequencies results

in less penetration of the eddy current field into the test part, allowing the

area directly below the surface of the part to be inspected. In addition, the

higher frequencies provide a higher resolution for the detection of smaller

defects.

Subsurface

The coils in subsurface probes are designed to be driven at relatively low

frequencies (typically 1 kHz to 20 kHz). Using lower frequencies results in

greater penetration of the eddy current field into the test part, allowing

cracks or corrosion to be detected in thicker structures or in multilayer

structures such as aircraft lap joints. Subsurface probes provide greater

penetration; however, sensitivity to small defects decreases as the frequency

and penetration are increased.

High Frequency, High Resolution

High-frequency, high-resolution probes provide both a high frequency and a

high resolution in order to detectsurface-breaking defects on aluminum

aircraft skins. These probes are made with 32 absolute coils positioned for

the complete coverage of a 26 mm scan area. Although these probes are

used for a specific application, they are very flexible and suitable for various

high-frequency surface-breaking applications being developed.

3.4 Probes Parameters

To achieve optimal inspection performance, there are several important

parameters to consider when designing an effective eddy current probe. Key

factors include inspection coverage, sensitivity, frequency and, of course,

cost. To optimize performance, it is important to carefully balance the

various probe parameters. For example, high-sensitivity probes require

small, high-frequency coils (providing less coverage); probes capable of

greater coverage require larger, lower-frequency coils (resulting in

decreased sensitivity to small defects). As is the case with conventional eddy

current inspections, choosing the correct probe characteristics is essential to

a successful inspection.

Where:

n = Number of channels

r = Resolution (also depends on the coil configuration)

C = Coverage

Probe Structure

Eddy current array probes can be optimized for a specific application by

changing the probe shape and the coil configuration. Most coil configurations

can be expanded into an array configuration.

Single coil probe

The earliest instruments used in aircraft inspection included the Magnaflux

ED-500 and ED520, and the Foerster Defectometer (although not a bridge

type instrument but a resonant circuit type), all of which used single-coil

probes. The probes contain a single coil that is wound to a specific value. No

other coil is needed. More recently, the introduction of the Hocking Locator

and newer models of the Foerster Defectometer have kept this kind of

instrument as a popular option for many users. When these probes are used

with a bridge circuit type instrument, a balance coil is also required. Balance

coils are normally placed in the cable connector or a separate adapter (see

Fig. 1).

Fig. 1

It sometimes creates a problem when the probe inductance value is not

close enough to the value of the balance coil, and the instrument does not

balance correctly. This happens more often when they are not made by the

same manufacturer. The result is poor performance (noisy or insensitive) or

no response at all (signal saturation).

Bridge Type probes

In this configuration the probe coils are located in an electrical "bridge" (see

fig. 2). The instrument balances the bridge and any change in balance is

displayed as a signal.

Fig. 2

In this arrangement, the same coil produces the eddy currents and detects

the impedance changes caused by the defects (or any other variables).

Almost all instruments are able to operate with this type of coil arrangement.

Reflection type probe

These probes are also known as send-receive or driver-pickup. In this

configuration, the eddy currents are produced by a coil connected to the

instrument's oscillator (driver).

Fig. 3

The signals received back in the probe are detected by separate coils called

pickups (see Fig. 3 and Fig. 4). All new impedance plane instruments and

also many older models are able to operate in both bridge and reflection

modes. If you are in doubt, call the manufacturer or give us a call.

Fig. 4

Bridge or reflection

This is a common question asked by those involved in trying to select the

best probe for an inspection. The answer is "It depends." Let us consider

both systems.

Gain. Reflection probes will give a higher gain, particularly if they are "tuned"

to a specific frequency, but normally the difference is on average about 6 dB.

It is true that t his doubles the signal, but if you consider that the

instruments are able to give this increase of gain easily, it is not so

important. Nevertheless, in critical applications this increase is very

welcomed.

Frequency range. Reflection probes do not need to balance the driver to the

pickup coils. This means that they will give a wider frequency range. As long

as the driver produces eddy currents, the pickup will detect them and some

signal will be displayed. This may not provide good information at certain

frequencies, but the probe is still working!

Bridge type probes used to give a limited frequency span in the older

instruments, as these had to balance an electrical bridge using its other arms

(X and R controls). In modern instruments, the bridge is normally formed

with fixed precision resistors, or a fixed transformer inside it. The signals

detected in this manner are electronically processed without any

"mechanical" adjustments, and this means a greater ability to balance over a

wider frequency range.

Drift: Probe drift is mostly caused by temperature change in the coils. This

may be caused by varying ambient temperature, or the heat produced by

the oscillator current, or both. There are design parameters that can be

optimized to reduce drift, such as wire diameter and ferrite selection, but

reflection probes are normally a good choice to avoid this problem even

more.

In a reflection probe, the driver current does not flow through the pickup

coils; in fact, the magnetic field received back from the specimen is normally

much smaller and, consequently, the current flowing in the pickups is also

reduced. Most probe types (pencil, spot, ring, bolt hole, etc.) can be made as

bridge or reflection. Keep in mind that a reflection probe is almost invariably

more difficult to manufacture and therefore more expensive.

Absolute and differential probe

This is an area where some confusion exists. Many users have called a probe

"differential" when the signal displayed gives an up and down movement or

a figure 8 type signal. This is caused by the two coils sensing the defect in

sequence. When both sensing coils are on the probe surface, they

compensate for lift-off and as a result no line is visible (see Fig. 5).

Fig. 5

In contrast, an absolute display is produced by a single sensing coil (see Fig.

1 through Fig. 4), giving a single, upward movement with a near horizontal

lift-off line.

Others have called a probe "differential" simply when the coils were

connected differentially such as in a bridge circuit. The problem with this

definition is that probes can be connected differentially in a reflection system

as well as when using two pickups (such as most scanner-driven bolt hole

probes). In this case, the two pickup coils are positioned close to one another

and contained within a driver coil (see Fig. 6).

The best way out of this confusion is often to specify the probe as bridge-

differential, absolute, or reflection-differential-absolute as needed. It seems

to make more sense to qualify the description according to the displayed

signal, since this is what really matters. Not many people are concerned as

to how the coils are connected internally.

Shielded and unshielded probes

Probes are normally available in both shielded and unshielded versions;

however, there is an increasing demand for the shielded variety. Shielding

restricts the magnetic field produced by the coils to the physical size of the

probe or even less. A shield can be made of various materials, but the ones

mostly used are: ferrite (like a ceramic made of iron oxides), Mumetal®, and

mild steel.

Ferrites make the best shields because they provide an easy path for the

magnetic field but have poor conductivity. This means that there are few

eddy current losses in the shield itself. Mild steel has more losses but is

widely used for spot probes and ring probes due to its machinability and

when ferrites are not available in certain sizes or shapes. Mumetal® is used

sometimes for pencil probes as it is available in thin sheet; however, it is less

effective than ferrite.

Shielding has several advantages: first, it allows the probe to move in (or

close to) geometry changes, such as edges, without giving false indications;

next, it allows the probe to touch ferrous fastener heads with minimal

interference; last, it allows the detection of smaller defects due to the

stronger magnetic field concentrated in a smaller area.

On the other hand, unshielded probes allow somewhat deeper penetration

due to the larger magnetic field. They are also slightly more tolerant to lift-

off. Unshielded probes are recommended for the inspection of ferrous

materials (steel) for surface cracks, and in particular with meter instruments.

The reason for this is that the meter response is too slow to allow the signal

from a shielded probe to be displayed at normal scanning speeds due to the

smaller sensitive area.

Adapters

To connect a probe with a connector different from the type used in the

instrument, it is necessary to use an adapter. An adapter consists of two

different connectors joined and wired to match the inputs and outputs as

necessary. It is normally housed in a short body that can be positioned at the

instrument's input. Sometimes, it is also possible to have a "cable adapter,"

which is made to match a connector located at the probe body. Depending

on the instrument's wiring, it may be possible to have a single adapter for

both bridge and reflection. In other cases, it is necessary to have two

separate adapters or use a switchable type.

3.5 Custom Probes

Custom probes can be ordered to suit specific geometries or applications.

Custom probes are designed and manufactured specifically for the task

required using coils for low-frequency or high-frequency inspections. Your

local Olympus NDT representative will help to answer any questions

regarding custom solutions. Olympus manufactures R/D Tech ECA probes for

a wide range of applications. Probes can be designed to detect a specific

type of flaw or to conform to the shape of the part under inspection.

Standard designs are available to detect defects such as cracks and pitting,

and subsurface defects such as cracks in multilayer structures, as well as

corrosion.

Probes can be made in different shapes and sizes to better conform to the

contour of the part under inspection.

3.6 Calibration Standard

Olympus NDT has the capability and experience to manufacture a wide

range of calibration standards for eddy current array applications. Whether

these calibration standards are defined in an aircraft manual or are entirely

custom-made, Olympus NDT can manufacture to any requirements. Sample

parts can also be manufactured, or real parts can have artificial defects

inserted into the material using spark-erosion and wire-cutting technologies.

4.0 Typical Applications

4.1 Rivet Inspection

Each individual eddy current coil in the probe produces a signal relative to

the phase and amplitude of the structure below it. This data is referenced to

an encoded position and time and represented graphically as a C-scan

image. For rivet applications, the eddy current coils that pass over a

defective rivet generate a unique signal response. For coils that are affected

by a crack initiating from the rivet hole, an amplitude change is represented

in the C-Scan display. For coils that detect no change, the color

representation remains constant in the C-scan display.

4.2 Corrosion Detection

Corrosion detection using eddy current array technology offers major

advantages over conventional eddy current inspection methods. Because

each individual eddy current coil generates a unique electrical signal in

relation to the structure below it, the coils can detect very small changes in

material thickness, along with other parameters, and display these changes

as a color-coded C-scan image. Imaging using eddy current array allows easy

interpretation of the data generated from the probe coils. After it has been

collected, the inspection data can be stored, transmitted, and analyzed.

Color palettes play a very important role in the imaging of eddy current array

data. Color palettes determine how the data will be displayed. Color palettes

are often linked to the amplitude of the eddy current signals; however, when

required, color palettes can also be linked to the phase angle of the signals.

Color palettes range from a gradual rainbow palette to a precise, two-color

"go/no-go" palette. Black and white palettes are also often used.

Eddy Current Probes and Application Guide

Introduction

This paper is intended to provide information to help the user in selecting the

right E.C. probe(s) for a given inspection. Using this data, best results will be

achieved by optimizing the frequency and choosing a suitable instrument.

The subject is divided into three sections -

1. Coil types available

2. Typical applications

3. New developents

1. COIL TYPES AVAILABLE

The early E.C. coils generally had either no core ( air core) or a ferrite center

core only. Meter instruments were used almost exclusively and sensitivity

was comparatively low, but in most cases adequate. Many inspections are

still being done using this type of comparatively large, low sensitivity coils.

Absolute Type Coils

A very widely used type coil is a 100 kHz with a diameter of .1" (2.5mm)

to .2" (5mm) that fits most bridge type older meter instruments (Figure 1).

The sensitivity is acceptable for long cracks that exceed the probe diameter

by a factor of approximately 2, as the field is considerably larger than the

coil size. This also produces a large edge effect.

Figure 1

A suitable alternative now offered is a smaller coil approximately .060"

(1.5mm) diameter with a shield (preferably ferrite) around it. This gives

improved sensitivity particularly to short cracks and good isolation from

edges, bolt heads, etc. 1

The older probes were normally calibrated using an infinitely long (to the

coil) notch, .040" (1mm)or ,020" (.5mm), and their sensitivity will drop off

rapidly with shorter notches. The new probes maintain their sensitivity with a

notch approximately the internal diameter of the ferrite shield and will still

detect a shorter one.

Even meter instruments benefit from this type of coil, but probing speed is

limited as the needle needs time to respond. Display-type instruments allow

for much faster scanning. Larger surface type probes respond in a similar

way to the above. Shielding produces similar improvements when looking for

subsurface cracks or corrosion.

Figure 2

Differential Type Coils

Differential coils have the attraction of built-in lift-off compensation. This has

made them useful for many applications.

The older types of coils had no ferrite shield and they were built just by

placing two coils side-by-side (Figure 3). Later types added individual

shields (Figure 4), but the greatest improvement to the sensitivity was

achieved when both coils were placed within a common shield (Figure 5).

Differential type probes are mostly used in small sizes for surface crack

detection only.

Figure 3

Figure 4

Figure 5

In a probe of this type both coils are wound in opposition. Consequently,

signals that affect both simultaneously will cancel out (such as lift- off).

Normally the air point and the working point will be close, but some

difference is present due to small coil variations.

Normal scan direction is as shown (Figure 6), giving the typical display

presentation. The double indication is, in fact helpful, as it doubles the size of

the defect in the screen (Figure 7).

Figure 6

Figure 7

Sometimes it is necessary to scan in the same direction as the cracks

(Figure 8). This is permissible and the result will be similar for a very short

defect. A larger defect affecting both coils will tend to cancel out because

they are in opposition, but in practice there are enough differences in angle

and depth for this not to happen totally. In any case, the ends of the crack

will show normally.

FIgure 8

The probe body has a line to show the normal scan direction. These coils can

be fitted in pencil, bore hole or molded probes to fit almost any shape.

Bridge and Reflection Coils Older coils were normally connected to the two branches of a bridge

configuration (Figure 9). Later coils have also been used in the reflection

mode where separate coils are used for generating and detecting the eddy

currents (Figure 10).

Figure 9

Figure 10

Bridge coils give generally good performance, particularly if the probe is

designed for a specific application and frequency. Reflection coils will often

give a greater gain and a wider frequency range of operation

but they are more complicated to manufacture. They also have less drift. 2

Reflection probes are also used on special probe designs that rely on the

transmit-receive principle to create a certain size/shape sensitive area (like

the sliding type probes).

2. TYPICAL APPLICATIONS

Surface CracksNormal operating frequency:

Aluminum - 100 kHz - 1 MHz

Steel - 1 MHz - 2 MHz

Inconel Titanium - 2 MHz - 5 MHz

Probes availablePencil types - Absolute or differential . Shielded or unshielded

Surface types - Larger diameter probes can sometimes be used

Sliding probes - For cracks starting under fastener heads

Wheel probes - For bead seat radius, molded

Subsurface Cracks

Frequency must be low enough to penetrate the required depth (use slide

rule or graph attached). Minimum usable frequency is 100 HZ. 3

Probes Available :

Surface probes - Normally absolute shielded. Diameter can be as large as

spacing between fasteners

Encircling probes - Give better penetration. I.D. must be close to fastener

head diameter

Sliding probes - For fast directional inspection (reflection)

Bore Hole Cracks

Frequency as for surface cracks

Probes available:

Absolute - (preferably shielded) in hand operated or scanner versions.

Differential - (shielded or unshielded) - hand operated and scanner versions

Probes can be expanding (contact type) or non-expanding (slightly below

hole size) . Contact probes can be more sensitive to cracks (no lift - off

distance) but they also generate scanning noise. Low frequency types can be

used to penetrate through brass bushings. Automated systems can be

implemented. 4

Coatings

Frequency varies with type of coating

Non-conductive coatings can be considered as lift-off measurements.

Metallic coatings require good penetration, but as they are normally very

thin, fairly high frequencies are usable.

Standard absolute pencil and/or surface probes are suitable.

Conductivity

Using a standard absolute pencil or surface probe and a conductivity sample,

it is possible to identify different alloys with a standard instrument. More

accurate measurements require special

conductivity meters and probes. 5,6

Corrosion/Thickness

Frequency must be able to penetrate thickness required. Use EC slide rule or

graph attached.

Best results are obtained using shielded surface probes. 7,8

3. NEW DEVELOPMENTS

Aluminum Angle Inspection

The problem was to inspect aluminum angle for cracks in the opposite side

(not accesible) in one pass and with high sensitivity. In order to cover the

required width, the coil has a lift-off of about .1" (2.5mm).

A modified version of the sliding probe proved to be the best answer to

several alternatives tried (Figure 11). A .020" maximum (15mm) depth

notch produced with a 1" diameter blade was detectable.

Lift-off variations and edge effects could be identified as such on the display.

Figure 11

Thin Probe

To inspect between two lugs spaced less than .080" (2mm) apart, the probe

had to be shielded and be completely unsensitive at the back (Figure 12).

Using stainless steel for strength, a shielded coil was placed at the tip . A

nylon handle placed at 45° allowed for better access. Such a flat coil proved

very sensitive to small cracks both in steel and aluminum.

Figure 12

Small Sliding Probes

Used to inspect between fasteners in fuselage structures, a sliding probe is

easier to handle than a small round probe. They give directional response

with a frequency range of 1-100 kHz and penetration up to .100" (2.5mm)

Sliding Probe Transparent Lens Adjustment

The sensitive area of this type of sliding probe is the distance between the

driver and pick-up sections. The lens is therefore adjusted to the required

width of inspection (Figure 13).

Figure 13

When scanning along a line of fasteners, this distance can also be optimized

for best discrimination between the fastener indication and the defect . With

no lens present, the probe is at its highest gain, but because of this it also

gives a large indication for the fastener (Figure 14).

Figure 14

By allowing a thin lens, it is noticeable that the fastener indication has

decreased, but the defect has not. Eventually a good compromise can be

reached where the gain is not too high. The defect gives an indication that is

clearly seen and the fastener only causes a comparatively small movement.

A too wide lens will result in a very high gain setting and a decrease in

sensitivity to short defects. 9

Figure 15

Transparent Probe Holders

When calibrating a probe in a test block with small notches, it is difficult to

optimize the indication with an opaque holder. The holder is needed for

stability, so

using a transparent material makes it possible t o see the notch position

(Figure 16) and make the calibration easier. It is also useful when scanning

in confined areas or between fasteners to avoid running too close to the

heads.

Figure 16

High Gain Probes

Sometimes standard types of EC probes are not able to give a large enough

indication for the defect. This may be due to geometry, defect size or a

higher than normal lift-off .

To achieve the extra gain, it is often possible to position a very small hybrid

amplifier circuit within the probe body or in a separate small box close to it

(Figure 17). This pre-amplifier can be tailored to a specific inspection if

needed.

Figure 17

With this extra amplification the instrument gain can be lowered to a

medium setting and the resultant signal will have a better signal-to-noise

ratio than before. Care should be taken to ensure that too large of a signal

does not saturate the instrument input. A quick check for this is to compare

the lift- off with the edge effect. If both lines superimpose on each other,

saturation is present and the gain should be lowered (there should always be

a small angle separation).

A typical application has been the testing of bore holes in disks with a lift-off

of up t o .020" (probe .040" under size), enabling the use of a single probe

size.

Extra gain has been needed to inspect aluminum-titanium sandwich

structures. The titanium allows eddy currents to penetrate to the aluminum

layer if the gain available is sufficient. Unfortunately, when the aluminum is

covering the titanium, the eddy currents do not penetrate this as the

conductivity is much lower than the aluminum, and it behaves like a shield.

If more than one probe is needed, it is possible to multiplex several

probes or coils and use a single instrument. 10

REFERENCES

1. "Shielded E.C. Probes"

Process Laboratory Report

The Physical Sciences Laboratory Branch, Directorate of Maintenance,

Sacramento Air Logistics Center, McClellan A.F.B. Project No. 81-497-085,

Donald M. Bailey

2. "E. C. Development Activities"

Walter J. Harris, Boeing Commercial Airplane Co., ATA 1981

3. "Low Frequency E.C. Inspection of Aircraft Structure"

D. J. Hagemaier and A. P. Steinberg, Douglas Aircraft Company ATA 1980

4. "Update on Automated E.C. Inspection System"

Art Thompson, G.E. Evendale, ATA 1983

5. "Boeing Process Specification" , BAC 5946

6. "Evaluation of Heat Damage to Aluminum Aircraft Structures"

D. J. Hagemaier, Douglas Aircraft Company, ATA 1981

7. "A General Procedure for the Detection and Measurement of Corrosion in

Aircraft Skins Using E.C.

Equipment"

J. Pellicer, Staveley NDT Technologies, Inc., NORTEC Division, ATA 1984

8. "Aircraft Corrosion and Detection Methods"

D. J. Hagemaier, A. H. Wendelbo, Jr., Douglas Aircraft Co., ATA 1984

9. "Sliding Probes with Transparent Lens", Technical Bulletin

J. Pellicer, Staveley NDT Technologies, Inc., NORTEC Division

10. "Application Advancement in E.C. Using Pre-Amplifiers and Multiplexers "

J. Pellicer, Staveley NDT Technologies, Inc., NORT EC Division,

ASNT Spring Conference, Tulsa, OK 1986

Eddy Current Technology

An update for aircraft engine inspections

Eddy current array probes can replace one axis of a two-axis scan

and offer greater flexibility in the eddy current setup.

Eddy current array probe and tooling that requires one pass to

inspect the dovetail

The use of a C-scan image increases the reliability of the inspection

because it guarantees full coverage of the surface inspected.

High-frequency eddy current array probe for surface crack

detection.

The Omniscan MX is a modular, portable, battery operated

instrument. Existing modules can drive ultrasonic phased array and

conventional ultrasound as well as eddy current array. The

Omniscan ECA configuration supports 32 sensor coils (up to 64 with

an external multiplexer) working in bridge- or transmit-receive

mode. The operating frequency ranges from 20 Hz to 6 MHz with the

option of using multiple frequencies in the same acquisition.

Encoders can be connected. Data files up to 200 Mb can be

recorded.

Engine disk dovetail inspection

By Andre Lamarre

Eddy current technology is an electromagnetic technique widely used in the

aerospace, automotive, petrochemical, and power generation industries for

the inspection of metallic structures. In this technique, the probe, which is

excited with an alternative current, induces eddy current into the part under

inspection. Any discontinuities or material property variations that change

the eddy current flow in the part are detected by the probe and considered a

potential defect. Recently, an important improvement of this technique was

realized with the development of eddy current array technology.

Eddy current arrays

Eddy current array (ECA) technology electronically drives and reads several

eddy current sensors positioned side-by-side in the same probe assembly.

Data acquisition is made possible through the use of multiplexing, which

avoids mutual inductance between the individual sensors.

Benefits of eddy current arrays

Compared to single-channel eddy current technology, eddy current array

technology provides the following benefits:

Drastically reduces inspection time

Covers a large area in a single pass

Reduces the complexity of mechanical and robotic scanning systems

Provides real-time cartography of the inspected region, facilitating data

interpretation

Is well suited for complex part geometries

Improves reliability and probability of detection (POD).

Eddy current array probes

An eddy current array probe can be optimized for a specific application by

changing the coil configuration and the probe shape. Different types of

probes can be realized like absolute bridge, differential bridge, absolute

reflection, differential reflection, transmit-receive, shielded, and cross-axis.

Probes can be designed to detect a specific type of flaw or to follow the

shape of the part being inspected. Standard designs are available to detect

surface defects (such as cracks and pitting) and subsurface defects (such as

cracks in multilayer structures and corrosion).

Imaging

Representation of the data plays a major role in the use of eddy current

array. The Omniscan ECA allows the use of C-scan imaging, which is a color-

coded two-dimensional mapping of the inspected surface. The data from

each individual coil are recorded so the impedance plane is always available.

The following image demonstrates the principle of the image representation.

The ECA probe moves over a flaw and each coil produces an EC signal as

shown. The C-scan representation is the color coded image of the amplitude

of the signal. The X axis represents the movement axis while the Y axis

represents the probe axis. So, the C-scan gives the position of the flaw as

well as its size.

Testing and inspection

Eddy current array testing is used in a number of different fields like aircraft

and engine maintenance and manufacturing, power generation, oil and gas,

and tube manufacturing. This article covers the inspection of the engine disk

dovetail.

Aircraft engines are submitted to a lot of stress. Rupture of rotating parts can

cause catastrophic failure of the engine and the aircraft. The engine disk

dovetails or the blade attachment, being submitted to high stress, are

inspected to make sure there is no crack initiation site within the area of the

dovetail. Conventional eddy current are mainly used for this application. The

operator is required to use a tooling that holds a conventional EC probe. The

operator scans the probe along the dovetail length and index position of the

probe approximately 40 times to assure full coverage. The operator also has

to constantly monitor the screen of the conventional eddy current

instrument. This method is long, tedious, and causes fatigue to the operator.

An eddy current array method was developed specifically for this application

resulting in a significant time-savings. A new eddy current array was

developed composed of 32 sensors shaped in a way to fit the dovetail

contour. This probe is attached to a holder designed for the application. The

operator simply puts the probe in place and pushes it only one time through

the dovetail. This one-pass inspection assures full-coverage of the dovetail

areas. The C-scan mapping displayed on the instrument also helps the

operator to localize position of a defect and to size it. The inspection time is

reduced dramatically while the reliability of the inspection is increased

resulting in an important gain for the inspectors.

Conclusion

Eddy current array is a new technology that is used successfully in many

different fields. Portable and easy to operate, the Omniscan ECA makes the

use of EC arrays easy. The main advantages of the ECA technology are

increasing inspection speed, better reliability due to C-scan imaging, and

better reproducibility and probability of detection due to the coverage of the

whole surface assured by the array.

Andre Lamarre is Business Development Director, Aerospace and Defense for

Olympus NDT