eddy current white paper

7/27/2019 Eddy Current White Paper

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GEInspection Technologies Eddy Current

Digital | Eddy Current | Film | Testing Machines | Ultrasonics | X-ray

Whitepaper - Eddy Current

Author - Chris Hocking

1



2

Applications

Page

• Coating Thickness Measurement 3

• Dynamic Automated Inspection 4

• General Crack Detection 5

• Fastener Inspection 6

• Hole Inspection 7

• Material Sorting / Conductivity Measurement 8

• Rail Inspection (railway lines and associated applications) 9

• Thread Inspection 19

• Tube Inspection 20

• Weld Inspection 21

• Wheel Inspection 23



3

Coating Thickness Measurement

Coating thickness measurement is carried out for two

main reasons:

1. Quality Control - determining how thick a

protective coating is.

2. Influence on an eddy current test - different

coating thicknesses can cause a change in

the sensitivity or the frequency needed for the

inspection.

Eddy currents are capable of determining coating

thickness on non conductive coatings on a

conductive base material.

It is also possible to determine the coating thickness

of non-magnetic metallic coatings on metallic bases

providing there is a difference in material properties

(conductivity and magnetic permeability).

Eddy Current Probes

Most eddy current probes can be used for coating

thickness measurement, however, in practice it is

important that the probe mechanically stable. For

this reason it is recommended that flat faced orsprung probes, typically used in material sorting, are

used.

Sprung probee.g. 806P1

Flat faced probee.g. 700P11A

Flat faced probee.g. 720P1F4



4

Dynamic Automated Inspection

Eddy current NDT is highly suited to high speed

dynamic inspection becuase:

• it has high surface inspection speeds (up to 5m

per sec)

• there is no need for a couplant (so there is minimal

surface preperation)

• the probe can scan with a gap between itself and

the test surface. This is most conveniently applied

to conponents that are rotationally symmetrical

such as:

• tapered bearings

• gudgeon pins

• ball pins

• axels

• wheel hubs and so on

Eddy Current Probes

Probes for dynamic automated inspection are

generally differentiated. This means that the probes

contain two or more coils which are electrically

arranged to be in opposition to each other. This

arrangement minimises effects which act on both

coils simultaneously (e.g. material variations,

temperature).



5

Eddy Current Crack Detection

Surface Inspection

Eddy Current NDT is widely used for the detection of

surface breaking defects in ferrous and non ferrous

metals.

Sub-Surface Inspection

By using low frequencies on non ferrous metals,

eddy currents can be used to inspect for sub-surface

defects on materials such as aluminium, stainless

steel, titanium and so on.Defects typically detected by this method include

sub-surface corrosion and cracks.

Surface Inspection Probes (high frequency)

Due to the extremely small size of the probe core

they are able to inspect aras of poor accessibility.

Probes can easily be designed to fit your specific

application. See the Special Design Checklist for more

infomation.

The frequency is chosen to give a good phase

seperation between the lift off signal and the defect

indication. In practise, this means that generally good

results can be achieved on:

• Aluminium with a 2 MHz probe

• Titanium with a 6 MHz probe and

• Ferrous materials with a 100 - 200 Hz probe

Sub-Surface Inspection Probes (low frequency)

The frequency of the test and the probe sizedetermine the depth of the test , with lower

frequency, larger diameter probes giving a deeper

test.

Shielded surface probe

Unshielded surface probe

Examples of sub-surface inspection probes

A high-frequency probe bing scanned across a calibration block

Impedance plane analysis and display of the results of scanning block



6

Fastener Inspection

Eddy Current NDT can be used to detect defects

around fastener holes with the fastener still in place.

Inspections are applicable to all types of fasteners

including screws, bolts, rivits and so on.

Eddy Current Probes

A range of probes are available for the inspection of

fasteners:

• Ring or Doughnut Probe - most simple form of

probe available

• Sliding Probe (Transmit/Receive Probe) - a more

rapid form of inspection

• Hocking FastScan Probe - provides a dual

frequency solution

Please see the table below for a comparison of the

different types of probes:

Sliding probe Righ/Doughnut probe FastScan probe

Ease of use/setup Speed of inpsection Sensitive to defects

in all directions

Sensitivity of

inspection

Ring Probe Excellent Good Excellent Average

Sliding Probe Good Excellent Average Excellent

FastScan Average Average Excellent Excellent



7

Hole Inspection

The inspection of holes in metals is often essential as

the stresses around them are twice as high as in the

rest of the material, causing fatigue cracks to grow.

Eddy Current Probes

Eddy current probes provide an ideal solution as

they can go into extremely small holes. They can be

made to measure to fit any diameter - the smallest

currently has a 1.6mm diameter.

Manual Hole Inspection Probes

• Rigid - suitable for where there is no problems

with access

• Flexible - suitable for areas of poor access as

the probe shaft can bend around problematic

geometry.

Dynamic Rotary Hole Inspection Probes

These are most suited where there are a large

quantity of holes to inspect rapidly and with high

levels of accuracy.

• Set diameter - suitable for applications where the

hole diameter is known in advance. These probes

are extremely robust.

• Flexible diameter - a split tip is provided for

inspections where hole sizes vary or are not

known in advance.

Hocking Probe Drive

The MiniDrive has been specially designed for

dynamic rotary hole inspections. It makes the

inspection of fastener holes, even in confined spaces,

simple and accurate and can help test the largest

number of holes in the shortest amount of time.

Rigid manual probe

Flexible manual probe

Set diameter rotary probe

Flexible diameter rotary probe

MiniDrive with set diameter probe

MiniDrive with Flexible Diameter Probe



8

Materials Sorting / Conductivity Measurement

Electrical conductivity of a material is a measure of

the ease with which electrons will flow within it . A

material having a high conductivity, e.g. copper, will

permit eddy currents to flow more than a material

having a low conductivity, e.g. lead or non-metals.

Conductivity changes in materials can be caused by

variations in:

• Heat treatment

• Chemical Composition

• Temperature

Eddy currents can be used to measure conductivity,

for the purpose of metal sorting or defining areas of

heat damage. Eddy current instruments designed

for measuring conductivity are usually calibrated in

Percentage International Annealed Copper Standard

(IACS). This is a standard whereby pure copper is

said to be 100% IACS, all other conductivities being

compared with it .

Non Ferrous Materials

Conductivity probes may be used for sorting non-

ferrous materials. The main advantage of using

conductivity probes is that a quantitative measure

of conductivity is given. The probes are used for the

following purposes.

• Alloy identification and verification

• Verification of heat treatment during manufacture

and to detect in-service heat damage

• Detection of changes in material grade

• Metal sorting

• Determine the density of powder metal parts

Ferrous Materials

Because of the ferrous materials magnetic

permeability, it is not posssible to get a quantitative

measure of conductivity, so a comparative method

must be used to sort between good and bad

samples.



9

Eddy Current Rail Inspection

The Inspection Problem

The early detection of conditions in rail that may

lead to a break is now a critical activity in the

maintenance of rail worldwide. Understanding of

these mechanisms is constantly improving and

the evolution of a range of complementary NDT

techniques now means that the engineer has a

better choice than ever of tools for the task.

In addition to the maintenance of the rail, there is a

growing requirement for inspection techniques on

the rolling stock itself. The rapid inspection of axels,

wheels and bogies is essential for the safe operation

of the rail network.

This document aims to give you a brief overview of

the different NDT inspection methods currently used

on railways around the world. Eddy Current NDT will

be introduced as a new method of complementing

these inspections along with why and where this

inspection method is needed to ensure rail integrity.

NDT Rail Solutions

NDT Inspections have been used for a number of

years to check our railway tracks for a range of

faults. There are a variety of methods available for

use, the most common of which are mentioned

below.

2.1 Visual Inspection

This form of inspection is widely used, but produces

the poorest results of all the methods. It is now

becoming widely accepted that even surface

cracking often cannot be seen by the naked eye.

2.2 Ultrasonic Inspection

Ultrasonic Inspections are common place in the rail

industry. It’s a relatively well understood technique

and was thought to be the best solution to crack

detection.

However, Ultrasonics can only inspect the core

of materials; that is, the method cannot check for

surface and near-surface cracking where many of

the faults are located. This is where eddy currents

come in.

2.3 Eddy Current InspectionEddy Currents are most effectively used to check for

cracking located at the surface of metals such as

rails. Figure 1 shows the different inspection areas

covered by eddy currents and ultrasonic.

It is important to emphasise at this stage that

ultrasonics and eddy currents are complementary

inspection methods and should not be used

exclusively of one another (fig. 1)



10

Eddy Current Ultrasonic

Good at detecting

surface defects

Poor at detecting

surface defects

Near sub-surface

defects reasonable to

detect

Near sub-surface

defects difficult to

detect

Deep sub-surface defectdetection is impossible

Good sub-surfacedefect detection

Probes are less sensitive

to flaw operation

Signal is strongly

influenced by flaw

orientation

No couplant needed,

stable results

Couplant is needed

between probe and

material - variable

results

Probe can be made wideand profiled to cover

wear face

Defect must be onprobe cnetre line

Faster inspection speeds Slow inspection speeds

2.4 Magnetic Particle Inspection

MPI is also used in the rail industry but there are a

number of problems inherent with the technique.

• The surface of the rail or component must first be

cleaned of all coatings, rust and so on.

• To get a sensitive reading, contrast paint must firstbe applied to the rail, followed by the magnetic

particle coating.

• The same inspection must then be carried out

in two different directions at a very slow overall

speed.

• On top of this, the end results will be less sensitive

than those achieved with eddy currents.

Figure 1 - The different inspection areas covered by ET & UT



11

Type of Cracking and Where it Occurs

A number of main areas have been identified where

cracking occurs):

• Rail Heads

• Switch Blades

• Bolt Holes

• Foot of the Rail

• Thermite Welds

Inspection of Rail Heads

Contact Stresses

Cracking can be found in the head of all types of

track, but is predominantly found on highly canted

curves where stresses develop due to the extra

pressure and wear of the wheel on the rail (see Figure

3).

Water & Lubricants

Water from rain, snow or dew can become trapped

in defects in the rail along with oil and diesel. When

a wheel runs over a track with entrapped fluid in a

crack, a very high localised press at the crack tip will

cause the crack to grow (see Figure 4).

Figure 4 - Trapped fluids causing cracking to worsen

*

Figure 3 - Contact stresses on tight curved track* Area of high stress prone to developing cracking on and

near the surface of the rail head



12

As the wheel approaches the crack the mouth opens

up to draw water in. Then, as the wheel passes over

the crack it closes up the entrance of the crack

mouth, trapping water inside so that the crack tip

stays open allowing further growth.

Tongue Lipping

Tongue lipping develops because surface-breaking

cracks are already present on the rail. Stresses

caused by trains passing over the rail cause the

crack to develop into a tongue which will continue to

grow (see Figure 5).

Ultrasonic inspections cannot reliably detect the

cracks that cause tongue lipping due to their shape,

size and angle. However, eddy currents can. This

means that the cracking that causes tongue lipping

can be identified early enough for preventative

action to be taken.

Companies such as Railtrack in the UK carry out

grinding on all their tracks to try and pre-empt the

problem of tongue lipping. However, this raises a

number of important questions all of which eddy

current inspection can answer:

i. When should the grinding take place?

Regular eddy current inspections will identify

when grinding will need to take place. Without

inspecting the track first , expensive and time

consuming grinding could be carried out for no

reason.

ii. How often should it take place?

Currently many tracks are ground according to

a schedule. However, this doesn’t take into

consideration factors that may cause more

or less cracking to develop than is usual. E.g.

environmental conditions, increased traffic,

abnormal side loading etc.

Figure 5 - Tongue lipping developing from existing cracks



13

iii. Has it solved the problem of cracking?

Without the use of eddy currents it cannot be

determine whether grinding of the rails has even

solved the problem. That is, it cannot be guaranteed

all the cracking has been successfully remove. A

quick post-grinding test will confirm success or

highlight where extra works needs to be carried out.

Squats

Squats and tongue lipping have a number of

factors in common. Both start as surface-breaking

cracks and are bought about by similar causes. The

difference is that squats usually develop at a point

where high contact stresses occur as a result of a

local irregularity in the rail head e.g. at a worn weld.

Advantages of eddy current inspection:

• Faster than visual inspection

• Can identify cracking at a much earlier stage than

ultrasonic testing allowing preventative measure

to be carried out

Wheel Burn

Wheel burns are the result of frictional heating

produced by a spinning wheelset. The effect of veryrapid heating produced by the spinning wheel and

subsequent rapid cooling is to change the structure

of the rail head top layer into ‘martensite’. The

presence of the martensite layer makes the rail un-

testable ultrasonically. This layer is also very brittle

with the result that it tends to spall off very easily.

Additionally, the railhead surface irregularity will

significantly increase dynamic impact forces and the

likelihood of rail breakage will be raised.

Although these areas cannot be tested ultrasonically,

eddy current inspections can be applied. The screen

shot (figure 6 below) shows a Locator 2s instrument

with a WideScan probe clearly picking up cracking

within an area of wheel burn.

Inspection of Switch Blades

Switch blades are subject to a tremendous amount

of stress due to the relatively thin section of metal

carrying the weight of transport usually supported

by much thicker track rail.

Cracking is usually found along the top of the blade,

and along the sides. As with thermite welding, aWideScan probe can be used for the head of the rail,

while a WeldScan probe is suitable for the sides. The

WideScan inspection trolley has a spring mechanism

that lets it automatically adjust to the increasing

blade width (Fig. 7).

WideScan Probe – the eddy current solution

As the stresses discussed so far are occurring on

and near the surface of the material, it is virtuallyimpossible to detect cracking with ultrasonics and

relatively straightforward with eddy current. This

capability has been enhanced with the development

of the patented WideScan probe.

The probe is contoured to the surface of the rail and

runs along the surface transmitting results back to an

eddy current instrument via a probe and cable. The

instruments are able to store the information which

can then be downloaded onto a computer for future

analysis and records (figures 8&9).

Figure 6 - Signal showing the detection of a crack in an area of wheel burn



14

Figure 7 - Inspection of switch blades using WideScan and WeldScan probes

Figure 8 & 9 - WeldScan probe and Phasec 2s



15

The WideScan probe is unique due to the large

surface area it covers. This means that it can detect

cracking over the entire rail head in just one sweep. It

doesn’t matter where on the head the cracking is as

long as it’s surface breaking.

Figure 10 - Phasec 2s and WideScan probe being used by hand to check a section of rail. The equipment can also be attached to an inspectiontrolly (see figure 11) or a vehicle that runs along the track

Figure 11 - Trolley-mounted WideScan probe



16

Inspection of Welded Areas

General Welds

Welds are ground after welding which can lead to a

slightly different profile around the weld than from

that of the rest of the rail. The resulting change in rail

profile has been found to have a significant effect on

the contact stresses between rail and wheel, resulting

in rolling contact fatigue. Subsequent cracking has

been found at the edges of the weld or in the body of

the weld itself.

Thermite Welds

Thermic welding is used on sections of continuously

welded rail (CWR) where two rails are welded

together by means of an exothermic reaction. This

method introduces a weak point in to the structure

of the rail. As a rule they are very rough as the ‘flash’

(surplus weld) has not been removed and so could

damage any fragile inspection probe. However,

‘dressing’ (smoothing down) the weld can be time

consuming and expensive, but may also weaken the

weld itself.

Hocking’s WeldScan range of probes has been

designed specifically to test for surface breaking

cracks in welds.

What makes the probe so special is its ability to

test even very rough surfaces covered with rust or

coatings such as paint and oil.

Inspection of Bolt Holes

Bolt holes are positioned regularly along the length

of the rail and are subject to cracking due to the

stresses placed upon them.

Figure 13 - Eddy Current inspection of bolt holes



17

Rather than removing each bolt to inspect the hole

underneath, an eddy current WeldScan or Pencil

probe can be used to inspect the area around the

bolt to determine whether any cracks are radiating

from the area.

It’s important to note that no surface preparation is

needed for this eddy current inspection, unlike most

other NDT methods, e.g. MPI.

If cracking is detected, the bolt can be removed and a

special bolt-hole probe can be used to check the hole

itself for confirmation of cracking and to determine

the size and position of the crack (see figure 13).

Inspecting the Rail Foot

Fatigue cracking due to the stress of trains travelling

along the track often occurs around the foot of the

rail.

A special WideScan probe can be contoured so that it

exactly fits the foot and checks for surface-breaking,

fatigue cracking. The probe cannot test the areas

around the clips or springs that attach the rail to the

track, but a WeldScan can be used around these

areas.

The arrows in green (figure 14) show the areas of

the foot where the WideScan probe can inspect. The

arrows in white show where WeldScan must be used.

Figure 14 - Inspection of the foot of the rail



18

Summary

Eddy current inspections form a vital part of checking

rails for the cracks and faults that can lead to

serious accidents. Ultrasonic inspections alone do

not cover all areas the rail as the technique cannot

‘see’ surface and near-surface defects. As many of

the cracks appearing in rails are fatigue induced and

thus surface-breaking, it is important to employ eddy

current inspection methods in order to detect them.

Throughout this document, solutions have been

suggested for a number of applications. Figure 15

shows a summary of which eddy current probes we

believe are the most suited to those applications.

Figure 15 - Eddy Current probes for rail inspections



19

Thread Inspection

Threads are commonly used to connect mechanical

items together and forms a likey location for fatigue

cracks.

Eddy Current Probes

In order to inspect the root of the thread, specially

shaped tipped probes must be used. Both male and

female threads may be inspected.

Pencil-type probes are available for hand-held

manual inspection, but for more rapid and

repeatable inspections, saddle and plug-type

probes must be used, suitable to the thread form.

Semi-automated bolt hole inspection is also

available in the form of the Inconel Bolt Hole Tester.

Plug probe Saddle Probe



20

Eddy Current Tube Inspection

Heat exchanger tubing is subject to a number

of problems, the nature of which is usually a

characteristic of the material and application. Typical

problems found include the following:

Corrosion is the most common problem, while

tubing materials are generally chosen to resist

attack from the fluids passing around them, those

fluids are seldom pure. In power station condensers

using sea water as a coolant the most common

problem is hydrogen sulphide produced by bacteria

metabolising sewage, this attacks most copper

alloys. In petrochemical plants impurities such

as hydrogen chloride or ammonia can also give

problems.

Stress Corrosion results when tubes containing

residual stresses are exposed to a corrosive

environment. The grains of the metal tend to

separate when weakened by corrosion, exposing

fresh sites for attack. This can lead to rapid cracking

of the material, usually in a circumferential direction.

Corrosion erosion or impingement attack results fromthe combination of corrosive agents with mechanical

attack from suspended sand, foreign bodies, or from

turbulent flow of the cooling liquid. This prevents the

formation of a protective film on the surface of the

tubing, greatly increasing the corrosion rate in the

exposed areas.

Mechanical damage may come from a variety

of sources. Foreign bodies in the coolant may

cause damage. Poorly designed condensers have

inadequate baffling of steam, leading to erosion of

tubing in the steam inlet areas. Improper operation

of air conditioning systems may allow water to

freeze in evaporators, resulting in “freeze bulging” or

cracking. Many types of heat exchanger are subject

to vibration, resulting in rapid damage to tubes

loosened by corrosion or improper assembly.

Mechanical damage due to vibration is quite

common where Copper Nickel Alloy tubes have

been replaced with thinner, less rigid titanium tubes,

for which the support is marginally adequate. Heat

exchangers designed for such tubing generally have

support plates closer together. In extreme cases

the vibration may be so severe that adjacent tubes

collide, causing wear or cracking.

Periodic eddy current testing of a heat exchanger

assembly allows tubes with such problems to be

identified before they lead to failure. With knowledge

of the problems experienced in the application it

is possible to determine which tubes are likely todeteriorate unacceptably before the next overhaul.

These may then be plugged or replaced; resulting in

a much higher level of confidence in the reliability of

the heat exchanger. In addition to the detection of

such defects, Eddy Current testing can also be used

to monitor other conditions, such as the build up of

external sludge and to verify the degree of expansion

at tube sheets during manufacture.



21

Eddy current inspection of welds for cracks &

corrosion

The Importance of Weld Inspection

The quality of welds is becoming increasingly

important as customer expectations rise. Products

and components are expected to be of a high quality

and not to fail unexpectedly.

Such failures have large financial and social

consequences that can often be avoided with the

proper inspection techniques.

Inspecting welds can also reduce costs by detecting

defects in the early stages of manufacture, reducing

the cost of customer returns and extending the life

of components by detecting and correcting any

defects.

Eddy Current Non Destructive Testing is a reliable,

quick and inexpensive way to carry out preventative

maintenance and ensure safety. Hocking’s range of

eddy current equipment has a world-wide reputation

for its reliability and accuracy, while their service,

support and training ensure that you make the mostof your equipment.

Eddy Current Testing on Welds

In welding inspection there is a need to detect

surface breaking defects. For magnetic material e.g.

carbon steel, generally magnetic particle inspection

is used. However, eddy current inspection offers a

number of advantages:

• No consumables used - e.g. ink & contrast paint

• Ability to test areas with poor access

• No surface preparation required - e.g. paint

doesn’t need to be removed, saving time in

preparation for the inspection and in any

recoating of surfaces

• Improved sensitivity - ability to detect smaller

defects using specially developed Hocking

WeldScan probes.



22

WeldScan Probes

Hocking NDT have developed the WeldScan range

of probes specifically to check welds for cracks and

corrosion. Examples of areas where it is currently

used include:

• Offshore platforms

• Buildings

• Bridges

• Amusement park rides

• Ships, boats, submarines etc.

• Cranes

• Traffic signals

• Aircrafts

WeldScan probes have been designed to be

extremely hard-wearing so that they can handle the

rough weld surface while still picking up any faults in

the weld.

Advantages of WeldScan probes include:

• Faster then MPI (Magnetic Particle Inspection)

• Portability of equipment - light, handheld and

easily transported• Accepted method of use - see British and

European Standard 1711:2000

• May be used by rope access technicians

• Approved for use by Lloyds Register, DNV and

Bureau Vertias - certifying authorities for ships

and offshore structures.

• Waterproof WeldScan probe range is available for

sub-sea weld inspection

See more information on the WeldScan range of

probes and download a datasheet. You can also

request weld inspection application notes.



23

Wheel Inspection

There are four main solutions for testing wheels

depending on the volume, accuracy required, profile

of wheel and so on.

Solution 1 - Pencil Probe

Standard surface inspection with a pencil probe &

bolt hole inspection with a bolt hole probe.

Solution 2 - Bead Seat Probe

Bead seat probes are made to fit the bead seat area

of wheel and scan a width of about 50mm which

gives a more rapid inpsection than with a pencil

probe. A bolt hole probe is needed to inspect the bolt

holes.

Solution 3 - WheelScan LT

WheelScan Lt provides a semi-automatic solution for

the inspection of small, low volume of wheels.

More information on WheelScan LT

Solution 4 - WheelScan 5

Automatic, fast and highly accurate wheel inspection

solution. The best solution where accuracy and,

therefore safety, cannot be comprimised. TheWheelScan range is commonly used in the airline

industry.

The table below shows the advantages and

disadvantages of each of these options.

Advantages Disadvantages

Solution 1 Low cost.

Suits all types of

wheels.

Very low volume

inspection (about

2 wheels per hr).

Poor surface

coverage due to

human factor.

Solution 2 Low cost.

Slightly faster.

Low volume

inspection.

Reduced

sensitivity to

flaws.

Different probe

needed for each

wheel profile

Solution 3 Portable, faster,

more repeatable.

Suits wide range

of wheel profiles.

Cost.

Doesn’t cover

entire wheel

profile.

Solution 4 Very high volume

inspection (about

10-20 wheels perhour).

Data recording of

wheel test.

Suits all wheel

profiles.

Cost.



24

Aircraft Wheel Inspection

Aircraft wheels are subject to high level cyclic fatigue,

particularly during landing. To ensure the safety of

passengers and the aircraft, it is important that the

wheels are maintained to the highest standard.

Eddy Current wheel inspection is widely accepted

throughout the world as a rapid and reliable means

of maintaining the integrity of aircraft wheels.

Automotive Wheel Inspection

Scanning wheels to prevent failure isn’t restricted

to the aerospace industry. Formula 1 racing teams

have recently started utilising the same technology

to ensure there isn’t a failure during the middle of a

race.



Contact Information

For more information about Eddy Current technology or products, please contact us at the following address:

GE Inspection Technologies Ltd

129-135 Camp Road

St Albans

Herts. AL1 5HL

UK

Tel: +44 (0)1727 795500

Fax: +44 (0)1727 795400

Email: [email protected]

Web: www.GEInspectionTechnologies.com

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