Eddy Current Rail Inspection

5/26/2018 Eddy Current Rail Inspection

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RAIL INSPECTIONThe Eddy Current Solution

A document outlining the emergence of the eddy current NDT inspectionmethod as an important part of rail maintenance and safety.

H


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1. The Inspection ProblemThe early detection of conditions in rail that may lead to a break is now a criticalactivity in the maintenance of rail worldwide. Understanding of these mechanisms isconstantly 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 thetask.

In addition to the maintenance of the rail, there is a growing requirement forinspection techniques on the rolling stock itself. The rapid inspection of axels, wheelsand bogies is essential for the safe operation of the rail network.

This document aims to give you a brief overview of the different NDT inspectionmethods currently used on railways around the world. Eddy Current NDT will beintroduced as a new method of complementing these inspections along with whyand where this inspection method is needed to ensure rail integrity.


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2. NDT Inspection SolutionsNDT Inspections have been used for a number of years to check our railway tracksfor a range of faults. There are a variety of methods available for use, the most

common of which are mentioned below.

2.1 Visual InspectionThis form of inspection is widely used, but produces the poorest results of all themethods. It is now becoming widely accepted that even surface cracking often cannotbe seen by the naked eye.

2.2 Ultrasonic InspectionUltrasonic Inspections are common place in the rail industry. Its a relatively wellunderstood technique and was thought to be the best solution to crack detection.

However, Ultrasonics can only inspect the core of materials; that is, the methodcannot check for surface and near-surface cracking where many of the faults arelocated. This is where eddy currents come in.

2.3 Eddy Current Inspection

Eddy Currents are most effectively used to check for cracking located at the surfaceof metals such as rails. Figure 1 shows the different inspection areas covered by eddycurrents 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 oneanother.

Fig. 1 The different inspection areas covered by ET & UT

Ultrasonic

(UT)

Eddy Current

(ET)

Couplant

EC Inspection Area

UT Inspection Area

No Couplant

needed

Metal

Probe


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Table 1 Comparison between Eddy Current and Ultrasonic Inspection

Eddy Current Ultrasonic

Good at detecting surface defects Poor at detecting surface defectsNear sub-surface defects reasonable todetect

Near sub-surface defects difficult todetect

Deep sub-surface defect detection isimpossible

Good sub-surface defect detection

Probes are less sensitive to flaworientation

Signal is strongly influenced by flaworientation

No couplant needed, stable results Couplant is needed between probe andmaterial causing variable results

Probe can be made wide and profiled tocover wear face

Defect must be on probe centre line

Faster inspection speeds Slow inspection speeds

2.4 Magnetic Particle InspectionMPI is also used in the rail industry but there are a number of problems inherentwith the technique.

The surface of the rail or component must first be cleaned of all coatings, rust andso on.

To get a sensitive reading, contrast paint must first be applied to the rail,followed by the magnetic particle coating.

The same inspection must then be carried out in two different directions at a veryslow overall speed.

On top of this, the end results will be less sensitive than those achieved witheddy currents.


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3. Type of Cracking & Where It OccursA number of main areas have been identified where cracking occurs (see figure 2):

Rail Heads Switch Blades

Bolt Holes Foot of the Rail Thermite Welds

Foot

Web

Rail

Head

Figure 2 cracking in rails


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4. Inspection of Rail Heads4.1 Contact Stresses

Cracking can be found in the head of all types of track, but is predominantly foundon highly canted curves where stresses develop due to the extra pressure and wearof the wheel on the rail (see Figure 3).

Figure 3 Contact Stresses on Tight Curved Track

4.2 Water & Lubricants

Water from rain, snow or dew can become trapped in defects in the rail along withoil 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

As the wheel approaches the crack the mouth opens up to draw water in. Then, asthe wheel passes over the crack it closes up the entrance of the crack mouth, trappingwater inside so that the crack tip stays open allowing further growth.

Wheel

RailHead

Area of high stress prone to developingcracking on and near the surface of the

rail head

Direction of Traffic

Crack Tip


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4.3 Tongue Lipping

Tongue lipping develops because surface-breaking cracks are already present on therail. Stresses caused by trains passing over the rail cause the crack to develop into atongue which will continue to grow (see Figure 5).

Figure 5 Tongue Lipping Developing from Existing Cracks

Ultrasonic inspections cannot reliably detect the cracks that cause tongue lipping dueto their shape, size and angle. However, eddy currents can. This means that thecracking that causes tongue lipping can be identified early enough for preventativeaction to be taken.

Companies such as Railtrack in the UK carry out grinding on all their tracks to tryand pre-empt the problem of tongue lipping. However, this raises a number ofimportant 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 totake place. Without inspecting the track first, expensive and time consuminggrinding could be carried out for no reason.

ii. How often should it take place?Currently many tracks are ground according to a schedule. However, thisdoesnt take into consideration factors that may cause more or less cracking todevelop than is usual. E.g. environmental conditions, increased traffic,abnormal side loading etc.

iii. Has it solved the problem of cracking?Without the use of eddy currents it cannot be determine whether grinding ofthe rails has even solved the problem. That is, it cannot be guaranteed all the

Crack

present

Tongue

forms

Crack

grows

Breakage

riskdevelops

Rail Head


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cracking has been successfully remove. A quick post-grinding test willconfirm success or highlight where extra works needs to be carried out.

4.4 SquatsSquats and tongue lipping have a number of factors in common. Both start assurface-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 ofa 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

4.5 Wheel Burn

Wheel burns are the result of frictional heating produced by a spinning wheelset. Theeffect of very rapid heating produced by the spinning wheel and subsequent rapidcooling is to change the structure of the rail head top layer into martensite. Thepresence of the martensite layer makes the rail un-testable ultrasonically. This layeris also very brittle with the result that it tends to spall off very easily. Additionally,the railhead surface irregularity will significantly increase dynamic impact forcesand the likelihood of rail breakage will be raised.

Although these areas cannot be tested ultrasonically, eddy current inspections can beapplied. The screen shot (figure 6 below) shows a Locator 2s instrument with aWideScan probe clearly picking up cracking within an area of wheel burn.

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


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5. Inspection of Switch BladesSwitch blades are subject to a tremendous amount of stress due to the relatively thinsection 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, a WideScan probe can be used for the head of the rail,while a WeldScan probe is suitable for the sides. The WideScan inspection trolley hasa spring mechanism that lets it automatically adjust to the increasing blade width.

Figure 7 Inspection of switch blades using WideScan and WeldScan probes

Switch Blade

Cross Section of

Switch Blade

WideScan probe follows

changing diameter of blade head

WeldScan probe

inspects sides of blade


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6. WideScan Probe the eddy current solutionAs the stresses discussed so far are occurring on and near the surface of the material,it is virtually impossible to detect cracking with ultrasonics and relativelystraightforward with eddy current. This capability has been enhanced with thedevelopment of the patented WideScan probe.

The probe is contoured to the surface of the rail and runs along the surfacetransmitting results back to an eddy current instrument via a probe and cable. Theinstruments are able to store the information which can then be downloaded onto acomputer for future analysis and records (figures 8&9).

Figure 10 Phasec 2s and a WeldScan probe being used manually on a section of rail

Figure 10 shows Phasec 2s and a WideScan probe being used by hand to check asection of rail. The equipment can, however, be attached to an inspection trolley (seefigure 11) or vehicle that runs along the track.

Figure 8 WideScan probe

runs down a rail

Figure 9 ocator 2s eddy current

instrument displays the results


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Figure 11 Trolley-mounted WideScan probe

The WideScan probe is unique due to the large surface area it covers. This means thatit can detect cracking over the entire rail head in just one sweep. It doesnt matterwhere on the head the cracking is as long as its surface breaking.

For more information about WideScan, Locator 2s and Phasec 2s, please see ourproduct specific datasheets or visit www.hocking.com/products.


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7. Inspection of Welded Areas7.1 General Welds

Welds are ground after welding which canlead to a slightly different profile aroundthe weld than from that of the rest of therail. The resulting change in rail profile hasbeen found to have a significant effect onthe contact stresses between rail and wheel,resulting in rolling contact fatigue.Subsequent cracking has been found at theedges of the weld or in the body of the welditself.

7.2 Thermite Welds

Thermic welding is used on sections ofcontinuously welded rail (CWR) where tworails are welded together by means of anexothermic reaction. This method introducesa 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 inspectionprobe. However, dressing (smoothing down) the weld can be time consuming andexpensive, but may also weaken the weld itself.

Hockings WeldScan range of probes (Figure 12) has been designed specifically totest for surface breaking cracks in welds.

What makes the probe so special is its ability to test even very rough surfacescovered with rust or coatings such as paint and oil.

Please see our product specific data sheets or visit www.hocking.com/products formore information on WeldScan probes.

Figure 12 Hockings WeldScan

Probe inspecting a ferrous weld


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8. Inspection of Bolt HolesBolt holes are positioned regularly along the length of the rail and are subject tocracking due to the stresses placed upon them.

Rather than removing each bolt to inspect the hole underneath, an eddy currentWeldScan or Pencil probe can be used to inspect the area around the bolt todetermine whether any cracks are radiating from the area.

Its important to note that no surface preparation is needed for this eddy currentinspection, unlike most other NDT methods, e.g. MPI.

If cracking is detected, the bolt can be removed and a special bolt-hole probe can beused to check the hole itself for confirmation of cracking and to determine the sizeand position of the crack (see figure 13).

Figure 13 Eddy Current inspection of bolt holes

WeldScan

Probe

Bolt Bolt Hole

Crack radiating

from bolt hole

Bolt Hole Probe

Inspection Around Bolt

Inspection in Bolt Hole


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9. Inspecting the Rail FootFatigue cracking due to the stress of trains travelling along the track often occursaround the foot of the rail.

A special WideScan probe can be contoured so that it exactly fits the foot and checksfor surface-breaking, fatigue cracking. The probe cannot test the areas around theclips or springs that attach the rail to the track, but a WeldScan can be used aroundthese areas.

The arrows in green (figure 14) show the areas of the foot where the WideScan probecan inspect. The arrows in white show where WeldScan must be used.

Figure 14 Inspection of the foot of the rail

Foot of Rail

Spring


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10. SummaryEddy current inspections form a vital part of checking rails for the cracks and faultsthat 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 thecracks appearing in rails are fatigue induced and thus surface-breaking, it isimportant to employ eddy current inspection methods in order to detect them.

Throughout this document, solutions have been suggested for a number ofapplications. Below is a summary of which eddy current probes we believe are themost suited to those applications.


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11. Typical Rail Inspection KitBelow are details of the typical eddy current equipment used in rail inspections. Formore details on any of the items, please contact Hocking at the numbers shown in

Section 12.

PRN Item

3K003 Locator 2s Starter Kit comprising of:

39I002Locator 2 Single frequencyeddy current instrument

39DH01 Operating Manual

39A030 Lithium-ion battery pack

39A035 Battery Charger/Eliminator

39A001Locator probe lead, Lemo 7way to Microtech 1.5m long

39A024RS232 Lead, Lemo 0B 5 wayto D Type 9 way

39A038 PC Supervisor Software CDROM Disc

CountryDependant

Power Cord, 2m long

Plus:

On request Inspection Trolley

900P003 WideScan Probe

39A005 Lead for WideScan

800P04ND1P WeldScan Probe

801P04ND1PWeldScan Probe with Right-angled Tip

105P4 Pencil Probe39A001 Lead for Pencil Probe


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12. Further InformationFurther information about rail inspection: Eddy Current Testing, A Solution to Detecting Rolling Contact Fatigue in Rail?

A paper presented at Asia Pacific Rail Conference 2003.

Further product information: WideScan Data Sheet Locator 2s Data Sheet Phasec 2s Data Sheet WeldScan Probe Data Sheet Probe Catalogue (includes information on Bolt Hole and Pencil Probes)Please contact Hocking or visit our web site for more details:

Hocking NDT Ltd129-135 Camp RoadSt AlbansHertfordshireAL1 5HLUK

Tel: +44 (0)1727 795500Fax: +44 (0)1727 795400Email: [email protected]: www.hocking.com

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Eddy Current Rail Inspection