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RTI/02/03/2000 Unrestricted

ULTRASONIC TESTING OF RAILS A NEW LEVEL OF PERFORMANCE

David Griffiths

March 2000

INTRODUCTION

Ultrasonic testing of rails has been used for seventy years to reduce the number of in-track failures. Its initial success in eliminating Hydrogen flaking and subsequently driving steelmaking towards cleaner continuously cast steels is well documented.

The rejection levels have however remained static over the last 20 years. This is not because steel makers have not improved in this time. Universal use of continuously cast steels for main line use is apparent. The main reason has been the inability to clearly demonstrate a cause and effect relationship between track failure and internal irregularities below about 1.5mm (1/8”). Fortunately the probability of a well-manufactured rail having an internal defect anywhere near this size is rare. Conversely a manufacturing problem in which defects approaching this size become common usually tempts the manufacturer to deliver the steel because “it’s within the specification”. This has resulted in dissatisfied customers and frustrated suppliers. An example of a Horizontal Split Head (HSH) caused by such a defect, which can be seen as a line on the centre of the fracture, is shown in Figure 1.

Figure 1 HSH Defect

In track testing has fared a similar fate. In 1988, Roney of Canadian Pacific reported to AREA Committee 2 (now AREMA) that the detectability of defects in track was not 100% as would be desirable but something less, sometimes much less. The chart reproduced in

Figure 2 was based on data collected by the Transportation Systems Centre and the AAR.

Figure 2 Defect Detectability (after Roney)

Roney reported that this could be calculated to lead to 0.1 service failures per mile per test. Using US average statistics for traffic this would then be equivalent to one broken rail derailment per thousand track miles per year.

For the iron ore railways in the Pilbara of Western Australia which each have in the order of 500km of track (300 miles) this would translate to one rail related derailment per year between them. This level of performance is not acceptable for this business and a much improved level of performance has been obtained principally by more frequent testing.

Reiff reported in March of this year that the detection probability for a 30% head area defect stands at about 70%, essentially unchanged from the performance reported by Roney in 1988.

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The limitations in capability for both steel plant and in-track testing have now been overcome.

This report describes a technological advance in testing technology which allows testing to detect inclusions at levels equivalent to the theoretical resolving power of the ultrasonic signal. In fact, advanced signal processing using frame-to-frame comparison has broken through this barrier and inclusions smaller than this theoretical limit can be detected. The equipment has also been demonstrated capable of detecting the previously difficult to impossible to detect defects in track:

• Inclusions

• Vertical split Head

• Defects below masking defects.

The equipment is described in the following pages and examples of these difficult defects which have been detected are presented.

TESTING EQUIPMENT

HARDWARE RTI’s Ultra-light High Production Rail Flaw Detection Platform is known as 8000SX. The compact size can be seen in Figure 3.

Figure 3 8000SX system mounted in a Toyota Landcruiser

The 8000SX DSP based Ultrasonic Rail Flaw Analysis system is a quantum leap in technology and is radically different to any Flaw Detection System previously available. The system development to date has been in excess of 12 man-years.

The system uses the latest Digital Signal Processing (DSP) and large-scale gate array technology. To provide very strong signal processing power the typical sample rate is 18 mega samples per second with provision for up to 32 mega samples a second

Each transducer input has its own DSP to process and analyse signals in real time. This is combined with its own unique structural processing architecture, purpose designed specifically for Rail Flaw Detection made possible through the design of purpose specific silicon chips.

The hardware uses surface mount components and on-board High Speed DSP for every channel. The logic incorporates high speed, high-density programmable gate array architecture. Data Transfer to PC is DMA (Direct Memory Access) or Programmed I/O

The system is of compact modular design for ease of maintenance and servicing. It is small enough to fit in a Daihatsu Terios 4WD for use on narrow gauge tracks. For standard and broad gauge track testing it is mounted in a standard Toyota Landcruiser with the only modifications to the vehicle being the Hirail. It can be returned to the workshop as airline acceptable baggage should the need arise.

The 8000SX system electronics and control systems are mounted in the back of the vehicle. It is very compact; the system is rack mounted and houses 32 channels of ultrasonics together with all necessary pneumatics, compressor and water pumps.

The user is presented with super VGA high resolution Graphics representing a B Scan of the left and right rail.

The Rails and defects are scaled and trigonometrically corrected. The operator can zoom in on a defect and view 600mm-length rail for the width of desktop or zoom out for a 20-metre look or any other length view he chooses.

The defect amplitude is represented on the B Scan by different level colours immediately making defects visually obvious.

The operator also has an A Scan Virtual Cathode Ray Oscilloscope displaying analogue signals windowed with B Scan of the rails. This meets the requirements of some operating specifications to have oscilloscope signals available for the operator but in practice the operator does not refer to this signal, instead relying on the defect window as the key information source.

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Figure 4 Operator Screen

The numeric listing of the defect can be displayed as an A-Scan display (Depth vs. Amplitude) as captured by the oscilloscope trace as the car passed over the defect. The signature of the defect from the A-Scan can be used to determine the type of defect but further recognition is available in software. An example of the display is shown in Figure 5.

Figure 5 Defect Displayed on Operator Screen

SIGNAL TO NOISE RATIO The signal to noise ratio of the 8000SX is better than 24db improvement on the earlier ultra-low-noise, 2000SX system, which was considered to be, a leading technology in 1992. That system successfully and repeatably detected defects down to 250 microns, which was considered a world first.

The improvements have been achieved at a number of points in the process. Real Time, Active Digital Signal Rectification has been applied to avoid the undesirable wider return echoes, which are a feature of passive rectification features at increased gain. This feature also permits the system to test closer to the rail surface.

Figure 6 300 µµm inclusion reported by 2000SX and 8000SX systems

The 2000SX system had previously been demonstrated as finding 250µm inclusions.

Seeing and recording multiple reflectors within a slice makes it possible to detect defects that were previously shadowed by large reflections from the surface condition or large adjacent reflectors. This is demonstrated later in this paper.

SOFTWARE The 8000SX Rail Flaw Analysis System uses Windows 98 operating system. The software for all the upper level is written in C++. The gate array software is in high Level VHDL code the DSP software is written in machine language.

The software display mechanism is non-linear based Global Reflection Map (GRM) display which makes it fully compatible with the 2000SX and 8000SX DSP Based Inertial Track Geometry Systems. The display allows the operator to view any point tested, regardless of vehicle location.

The software is multi-functional and is capable of running the 8000SX DSP Based Ultrasonic Rail Flaw Analysis System whilst simultaneously performing background tasks such as printing or reporting.

Up to three windows of either previous Ultrasonic Surveys or a mixture ultrasonic and geometry data can be loaded at the same time and compared on the run whilst still collecting data.

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Figure 7 Simultaneous Display of A scan, GRM and Defect Data

Automatic defect sizing can be accurately done from within the test vehicle using any of the normal sizing techniques. This is because unlike other systems, which only take the peak amplitude sample every frame, the RTI 8000SX technology enables the sampling of 1024 samples per frame. This enables a very accurate picture of the defect to be generated. This is described later.

Current requirements of specifications still require hand sizing of defects but it is envisaged that as the margin between detectability and critical defect size is demonstrated and becomes accepted, the use of in vehicle defect assessment will become the norm. This will significantly improve testing productivity.

Defect recognition and classification is made possible in RTI 8000SX technology by taking 1024 samples per frame. RTI is then able to apply statistical calculation on the data to determine the signature of the defect from which the defect can be classified.

The on board Library facility captures and records all defects for an ultrasonic survey. It holds defects or any other in track reflectors of interest recorded during the run survey. The same library file can be added to, from survey to survey.

RTI has built an extensive library database of all the different type of defects on different lines throughout Australia.

Each RTI 8000SX Channel has a built in programmable digital Time Gain Amplifier (TGA). The gate array architecture has been programmed to have Thirty-two time zones per slice as can be seen in Figure 8. This means that each zone of the rail being searched has its own programmable parameters such as gain, threshold etc For all intents and purposes, each zone is a totally programmable independent channel.

TGA allows compensation for Signal lost over distance travelled. It also permits different levels of sensitivity and different algorithms to be used in different zones.

For example decreased gain at under head radius, increased gain in neutral axis area and different levels of threshold makes for increased detectability through increased sensitivity at targeted areas of the rail without being impeded by the limitations of other parts of the rail.

It is also possible to use individual zone noise reduction algorithms, add flexibility in unwanted signal rejection together with an overall increase in sensitivity.

Figure 8 Thirty Two, Individual Time Gain Controls per Channel

Similarly the 32 divisions per channel adopt a Time Variable Threshold (TVT) as shown in Figure 9. The width of the thresholds in time is set by the zone width. The TVT feature allows for precise discrimination and sensitising of any part of the rail. A positive or a negative threshold can be set for any zone to detect a signal over threshold or below threshold.

Figure 9 Thirty-two, Time Variable Thresholds per Channel

The effect of these radical system architecture changes is that what is now called a single channel would, by previous Rail Flaw Detection terminology, have been classified as 1024 individual channels. In effect by conventional Flaw Detection terminology the RTI 8000SX system has 32,000 Channels.

Automatic gain control, as applied to the 8000SX system makes it possible for the first time to have absolute references for each channel by monitoring cumulative grain data at up to 1024 individual sample points per frame. The benefit of having multiple

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absolute references is full linearity over the entire rail height and transducer gain drift with temperature is completely eliminated. Non-Linearity between transducers is also fully compensated.

The 8000SX architecture was designed keeping in mind, that it in the future it will form the basis of the technology that could be Loco-mounted, without an on-board operator. It is envisaged that such a system could continually test the tracks as the loco travels on revenue traffic, sending back defect data with severity and location to a central PC via radio modem. It is likely that such a system would first be adopted by a captive type rail operation.

STEEL PLANT TESTING Manufacturing feedback is an important aspect of continuous improvement. Systems generally indicate whether a rail passes or fails whatever specification it is set at. When the steelmaking plant is performing fine then rejects are low or zero and there is no learning obtained. When a small change in steelmaking causes a rash of failures there is usually a sudden interest in the feedback from the ultrasonic system. In many steel plants that have significant stocks of blooms between steelmaking and rolling, the feedback loop is weeks or even months so that very large losses can be incurred before a problem is identified and fixed.

Rails are currently tested by manufacturers against a specification that is typically 1.5mm (1/16”) for inclusions, hydrogen flakes or other discontinuities. Such defects are (when the process is under control) extremely rare. Much smaller inclusions are considered under the assessment of rail steel cleanness. These inclusions are present in every rail – oxides, silicates and sulphides. These have been implicated in rail wear and failure but have never been satisfactorily quantified.

This limitation has been because cleanness assessment has relied on destructive sampling and metallographic examination, which is both time consuming and expensive. It also suffers from a major statistical problem in that the examined mass of rail is of the order of a few grams, representing masses of the order of 100 tonnes of product. That is a sampling ratio of the order of 1 in 108. Ultrasonics has not been used, even though sampling rates of the order of 1in 5 are feasible because the ability to detect inclusions of the appropriate size has not been available.

The 8000SX system can detect this size of inclusions in rails so that feedback is fast, cheap and reliable.

DIFFICULT TO DETECT FEATURES

ULTRASONIC DETECTION OF INCLUSIONS AND SHATTERCRACKS

RTI cooperated with BHP Steel at Whyalla in South Australia and their research organization in Melbourne, in 1992 to design & manufacture a DSP inclusion detection system. This was a single channel system designed to accept signals from an existing water jet probe targeting the side of the railhead. The system demonstrated proof of concept and could reliably detect inclusions in the order of 300 microns. When BHP adopted continuous casting it was no longer necessary to demonstrate steel cleanness in order to remain market competitive.

The knowledge gained from that research however, has been developed further and applied to in-track testing so that inclusions can now be detected in track. It is perfectly feasible to rate the rails of different manufacturers by driving over them with a test car.

An example of the detectability is shown in Figure 10.

Figure 10 8000SX & 2000SX Systems with Standard WB45 Transducer looking in rail with 200-300µµm defect high in the rail head

This data was obtained on the bench from rail known to be defective. Destructive sampling of the adjacent rail confirmed the physical defect size. This particular defect was from the same batch of rails that caused the track failure shown in Figure 1.

An etched, vertical section through the rail is shown in Figure 11. The defects reported here were those about 10mm from the running surface.

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Figure 11 Horizontal Section through Defects

Ultrasonic data from this defect was downloaded as a comma delimited file for later investigation. The energy surface diagram shown in Figure 12 was generated from this data. It shows the multi lobed defect quite clearly.

Figure 12 Energy Surface Obtained from Inclusions on Bench.

It was established during the earlier research that inclusions and similar defects have a size distribution that is well represented by the Weibull distribution. This type of distribution is natural to many systems in nature; it is the same distribution that describes the

strength of materials in that materials under load always fail at the weakest point. This is particularly important to inclusions in rail in service because it is the largest inclusions which determine the weakest point and ultimately the rail performance. The data is plotted by determining the Weibull value given by:

W = Log(Log(1/1-P)

where

P= Probability of excedence

For natural systems a line of fit gives a slope and intercept (α and β) in Weibull terms, which are characteristic of the rail.

When calibrated the system produces data from which these two values can be obtained. Comparison between rails is then possible.

Figure 13 Weibull Plot of Energy

It is considered feasible using this method to compare the effects of a process change in steelmaking on the potential rail performance or to compare the relative cleanness of two manufacturers products in track.

The success on the bench allowed the system to be applied in track. Defects in a known defective rail are quite easy to find but defects in track were expected to be much more difficult. There was a “chicken and egg” situation in that it was necessary to find the defects before they could be tested.

During commissioning of the new rail vehicle, testing was conducted over some very old rail. The rail was branded “LORRAINE STEEL Co 1907 and weighed 80lb/yard (41kg/m). Small but clear reflectors were reported with 38° and 70° probes.

Hand testing using a standard hand set could not discriminate any defect from background noise. Repeated vehicle testing confirmed the reports and further down the track larger reflectors were identified which could be hand confirmed. These were not testing artefacts but inclusions. Data from one of these larger inclusions was examined in detail.

An energy surface plot similar to that shown in Figure 12 was obtained and is shown in Figure 14. It should be noted that even though small on hand assessment,

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these inclusions were so big that the 8000SX receiver was saturated for much of the time. A saturated signal precludes the use of a Weibull analysis because the energy distribution is no longer natural.

The stringer nature of the inclusion is reflected in “mountain range” appearance of the energy surface.

Figure 14 Energy Plot for Inclusions in 1907 Rail

A sample of the smaller inclusions was also analysed and is shown in Figure 15. The Weibull plot obtained from the data is shown in Figure 16

The data shown here was obtained with the zero probe aimed normally into the rail but similar results were obtained for the angled probes.

Figure 15 Energy Plot for smaller Inclusions

Whereas it was impossible to Weibull plot the larger inclusions because the receiver was saturated. These smaller inclusions fall precisely into a Weibull distribution.

Figure 16 Weibull Plot for Small Inclusions

The fact that there is a good margin between the threshold and the level for defects detected indicates that the system has the ability to detect much smaller than these if desired.

Further small inclusions have been found during commercial operation of the 8000SX system on Westrail track on modern rail. An example of a particularly affected length of rail is shown in Figure 17.

Figure 17 Multiple Inclusions in Westrail Track

There are multiple reflectors evident. The small ones appear to be the original rail defects and the larger ones, defects exhibiting growth. It is not yet possible to categorise these reflectors into inclusions, hydrogen flaking or any alternative source. This will only be achievable after some of them are examined metallographically, One of the small initiators and one of the reflectors, which appears to be growing, are shown in Figure 18 and Figure 19.

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Figure 18 Small inclusion

In Figure 19 the reflectors have grown sufficiently that the receiver has become saturated (level =1024).

Figure 19 Inclusion with apparent growth

The difference in these reflectors is readily apparent. The initiator shows a reflector in both forward and reverse 38 degree probe indicating a near vertical plane but the fact that one signal is stronger than the other suggests some tilt rather than truly vertical. The growing defect is much more extensive and includes returns from the zero probe as well indicating some horizontal growth.

The initiating inclusions are larger than the threshold identified as having the potential to initiate fatigue failure but are much smaller than the defect threshold set in AS1085, AREMA or European rail specifications. As such, they would not have been cause for rail rejection, had they been detected, but are nevertheless clearly detectable using the 8000SX equipment and could be eliminated in future.

VERTICAL SPLIT HEAD (VSH) An example of a vertical split head (VSH) found in track is shown in Figure 20. Whilst ultrasonic testing had been conducted in this track (not by RTI), this defect had not been detected. The defect ran for several metres before breaking out at the surface. It was then detected visually, fortunately before any serious train incident. This defect was reported to be about 2 metres long when detected visually.

Figure 20 Vertical Split Head

The vertical split head has been a particularly difficult defect to find in-track in the past simply because the probes which run in a wheel on the railhead are looking at the edge of the defect. In analogy a sheet of paper held flat on is very easy to see. Changing to edge on is much more difficult.

Figure 21 shows the ultrasonic response to the 8000SX system of a similar sized defect in the Westrail system. The defect is clearly apparent using this test system.

Figure 21 Screen print of GRM for a Vertical split head.

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This is a particularly large defect that was confirmed by hand probing to be 3.3metres long. It had almost certainly been there for many years, undetected in previous test runs by the 2000SX technology or that of other operators. The defect is discontinuous and was detected by vertical 0ú, 38ú and 70ú probes. No comfort can be obtained from the fact that this undetected defect grew to such a large size without breaking out or causing a derailment. Much smaller defects of this type (less than 500mm) have been reported to cause track failure.

A vertical split head has been detected using the 8000SX system in BHP Iron Ore Goldsworthy line at 100mm indicating that the detection threshold is well below this failure threshold.

The data for this defect can be displayed graphically on request as shown in Figure 22.

Figure 22 Screen print of Graph for the VSH shown in Figure 21

The graph indicates that the defect was reported in the left rail by zero, 70 forward, and both 70 and 38 reverse probes. The zero probe was saturated for much of the defect and in fact produced a second reflection with a pseudo range of 95mm.

SHELLING (SHELL) & HORIZONTAL SPLIT HEAD (HSH)

Shelling is a very common rail defect found in most rail systems. The defect is fatigue cracking usually initiated internally, commonly at inclusions stringers similar to those described above. An example is shown in Figure 23.

Figure 23 Shelling at weld in Rail

This figure is representative of shells, which, whilst undesirable, are not usually considered cause for removal. If they break out as shown in Figure 24 they are visually obvious but have historically, been difficult to detect ultrasonically because the lower reflecting face is very close to the surface. The depth is typically about 5-10mm, that is, at the depth of highest shear stress. In Figure 24 the condition has progressed to complete separation of the running surface from the body of the rail. Shells have been difficult to detect because they are very close to the running surface and have been masked by the ringing of the surface echo.

Figure 24 Shelling with separation

The separation, while resulting in a poor running surface quality and thus even higher contact stress, makes ultrasonic testing the rest of the rail easier since the interface between the shell and the rest of the rail, which returns much of the energy is lost.

Without complete separation, shelling has been responsible for difficult ultrasonic testing of the rest of the rail. So little energy penetrates the remainder of the rail that existing systems have been unable to detect

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defects below the shelling even at sizes of 25% or more of the head area.

A deeper-seated defect, particularly one that spreads across a substantial portion of the head, is reported as a horizontal split head. The origin of this particular defect previously illustrated in Figure 1 is an inclusion line in the centre of the fatigue

All operators have reliably found horizontal split heads. The difficulty associated with them is that they are slow to grow, don’t pose an immediate threat and, like shells, are usually not removed from track.

There is a down side to this though. Traditional ultrasonic reports show this defect as a massive reflector, which so swamps the receiver, that even large defects below the HSH are neglected. An example of such a defect is shown in Figure 25. For track seriously affected by horizontal defects it is acceptable in North America for the track to be reported as “untestable” which then requires the rail to be replaced.

Figure 25 Screen print of GRM for a Horizontal split head.

As described earlier, the data presented here as a GRM can be called to show the full data set for this region and is presented graphically on request. The data for this defect is shown in Figure 26.

Figure 26 Data for the Horizontal split head shown in Figure 25.

Any defect in the horizontal plane whether it be shelling or some other form of rolling contact fatigue or as extensive as a horizontal split head, has the potential to mask a more serious defect below.

DEFECTS BELOW OTHER DEFECTS The most common form of this defect is shelling or horizontal splitting from which a defect with a vertical, transverse component has developed. An example of such a defect that failed in service and was later broken open is shown in Figure 27.

Figure 27 TD shadowed by shelling (after Broek)

For simplistic testing systems, the shell reflects a much larger amount of energy than the TD. This is because it is not only a much bigger and generally more favourable aligned reflector, but also because after reflection at the interface, there is much less energy transmitted through the shell available for reflection by the TD. The TD appears to be very small by comparison.

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Traditional systems compound the problem because they trigger at either the first threshold breaking reflector or the largest reflector, which is always the HSH/Shell. The operator then rejects the defect as non-critical since it is below threshold size for action as a HSH and the underlying defect is also ignored.

These problems are further compounded by the limited near surface detectability of older systems because they are still “ringing” from the top surface echo. In most instances they don’t in fact report the shell/TD either.

The ability of the 8000SX system to analyse the whole of the signal path, behaving as 32 channels avoids all of these problems and even small defects below large defects can be detected.

The near surface problem is avoided by the use of Active Digital Signal Rectification as described earlier. This permits testing within a few millimetres of the rail surface, certainly closer than the depth of highest shear stress which is the usual defect initiating depth. The problem of high energy reflection at the initial defect is overcome simply by having much more energy available from the initial pulse so that even a small fraction of that energy being transmitted through the top defect is sufficient to detect defects below. The capability is enhanced further by the automatic gain control which linearises the energy over the whole beam path, increasing the gain in the “shadowed” region to compensate for the reduced energy being injected past the higher defect.

Confirmation of this capability is illustrated in the example of a TD that has turned down from a HSH which was detected and is shown in Figure 28.

Figure 28 GRM of TD and HSH

This defect data has been examined in detail as part of the RTI flaw signature project. A “depth surface” map, shown in

Figure 29, similar to the energy surface map was created from the data for this defect, A two-dimensional view showing the depth profile. The change in the defect orientation can be seen from the profile but the most striking feature is the sudden increase in depth standard deviation at the transition. This feature will be used as part of the defect recognition development.

Figure 29 Depth Profile of HSH and TD

CONCLUSIONS

A new level of ultrasonic detectability is now available which can be applied at steel plants and in track.

At the steel plant it can reduce or eliminate the metallographic testing of rails for cleanliness and to report on the effects of process changes.

The equipment is capable of measuring cleanliness, non-destructively, in track and thus rating the products of different manufacturers. It will be capable of determining the relationship between track failures and rail cleanliness.

In track, the new technology makes it possible to rate the quality of existing or newly purchased rails and allows the detection of some features not reliably or precisely detectable in the past. These defects are:

• Very small defects <1mm,

• Vertical, longitudinal defects and

• Defects below otherwise innocuous horizontal defects.

The equipment has entered commercial service on standard gauge tracks in Australia in March 2000 and will be used in a narrow (1067mm) operation before the middle of the year. It will be available for steelworks applications, other gauges and as a hand held inspection unit before the end of the year.