Ultrasonic inspection of railway traction and rolling stock axles

The three main techniques used by British Rail P. Farley

Axles of traction and rolling stock in use by British Rail are subjected to in-service ndt using three mandatory ultrasonic techniques; these are an end-wise scan, a near end-low angle scan and a high angle shear wave scan. The use of these three scans ensures that the whole axle is tested, not just the traditional cracking zones.

Railway organisations throughout the world have for many years, effectively contained the problem of fatigue cracks in traction and rolling stock axles by ultrasonic testing. In the main the problem has centred at the inner edges of the roadwheel seatings where the maximum stress concentrations occur on axles of early and questionable design, as compared with modern standards (see Fig. 1).

Accepting wheelseat cracking as a fact of life, railway engineers of the past instituted many different methods of inspection to provide an early warning of the presence of a crack in an axle. To detect a crack which normally occurred 3 to 4 mm under the inside face of the wheel hub, the face of the hub would be machined back to expose the crack to visual inspection, often augmented by magnetic particle testing. Effective though this method was, the number of inspections possible depended on the dimensional tolerances of the wheel pair.

Inevitably this method became destructive with time, resulting in many sound wheel pairs being scrapped. The detection of cracking in the axle body (see again Fig. 1), entailed the use of hot whale oil and whitewash applied after the axle had been cleaned by scraping, and was an early version of the dye penetrant method.

The advent of a practical method of ultrasonic testing in the middle to later 1940s changed the face of axle testing. 1 Ultrasonic techniques were developed for the detection of axle cracks, although by modern standards these techniques were crude due to the crudity of the instrumentation and probes available at that time.

Railway engineers throughout the world quickly accepted ultrasonic testing as the answer to the traditional problem of axle inspection. As the method developed it soon became apparent that two clearly defined schools of thought existed as the to extent to which ultrasonic inspection should be applied to axles. The first, adopted by the old railway companies in Britain suggested that the whole axle should be inspected. The second, favoured by the Continental

Mr Farley is with the Materials and Inspection Engineers Division of the Chief Mechanical and Electrical Engineers Department, British Rail, Derby, England.

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Fig. 1 Many fatigue cracks occur at the inner edges of the road- wheel seatings, particularly in older design axles

railways, proposed that only the traditional cracking zones need to be inspected. 2 The philosophy of overall inspection in which the whole axle is examined whilst particular attention is paid to the highly stressed areas at the wheel- seats, has been strictly adhered to by British Rail (BR) up to and including the present day.

Experience gained over the thirty years or so since those early days, has shown that it can be highly dangerous to assume that an axle will crack only at points pre-determined by stress concentrations. 3 Consider for example the case of incidental damage to the axle which can occur at any time during the life of the axle and for any one of a number of reasons. Such damage has resulted in the initiation of cracking at positions along the length of the axle where cracks were least expected. Manufacturing faults also give rise to other unpredictable sources of cracking. Fig. 2 shows a fatigue crack midway along the axle, caused by a surface breaking hydrogen flake. Fig. 3 illustrates a fatigue crack which occurred as a direct result of the hot branding used to record the axle production data. Both these examples show that cracking can occur where it is least expected and which would have passed undetected by testing based on the 'zonal' philosophy. The point should be made that if these two defects had not been detected by overal testing of the axles concerned they could have resulted in failure in traffic.

In stressing the need to test an axle overall, it is not the author's intention to minimise the importance of detailed

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detected ultrasonically can often be salvaged. Where the axle is one of questionable design but still possesses certain geometric characteristics at the wheelseat/axle body transition, then this axle can be salvaged by machining the crack out and re-shaping the transition to conform to the requirements of the designer. 4

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Fig. 2 A fatigue crack midway along an axle was initiated at a surface breaking hydrogen flake

Fig. 3 A fatigue crack was caused by hot branding used to record axle production data

examination of the inner edges of the wheelseats since this is where most defects do indeed occur particularly in axles having wheelseats of traditional design. An analysis of the statistics relating to the widthrawal from service of axles found to be cracked will show that only 0.2% are cracked in the axle body, the remaining 99.8% are found to have defects at the inner edges of the wheelseats. This suggests that although the probability of failure in the axle body is very small, nevertheless failure is a finite possibility.

Although the withdrawal of axles from traffic with wheel- seat cracks in the early stages of growth may not be considered by some to be essential there is an economic advantage in doing so, since axles in which shallow cracks have been

Axle testing techniques

There are three mandatory ultrasonic testing techniques to which all axles in use by British Rail are subjected during their service life. These are the 'endwise' or 'far end scan', the 'near end-low angle scan' and the shear wave or 'high angle' scan. However, when specific cracking problems arise, these three scans will be supplemented by additional techniques, for w_ hich special instructions will be issued by the Chief Mechanical and Electrical Engineer as and when the need arises.

The endwise or far end scan

This scan, shown in Fig. 4 is applied from the axle end face and is used to inspect the full length of the axle. Of the three techniques it is the most controversial to many experts in the field for three main reasons.

Firstly the sensitivity of the scan has often been questioned due mainly to the use of compressional waves and the long ranges at which relatively shallow cracks have to be detected. Secondly the use of the axle end face as a probe site is not always acceptable due to damage to the surface caused by stamping etc; and lastly the complex signal patterns produced by an endwise scan are difficult to interpret.

Critics of this technique have in the past put forward sound reasoning for their scepticism but BR now has a wealth of knowledge gained through many years of experience in the application of this scan which suggests that most, if not all the problems associated with it have been solved. Cracks of the order of 1 to 2 mm in depth at ranges of 2 m are normally detected using the endwise scan in BR workshops. It must be stressed however, that this level of performance can only be expected from highly trained and experienced personnel.

Some of the problems with the endwise techniques have been ironed out by better communication between ndt personnel and designers. It is interesting to note that when the British Railways Board Non-Destructive Training School opened in 1964, among the first to attend were a team from the BR design staff whose main responsibility lay in the design of axle wheelsets. One of the important innovations resulting from this liaison between the designers and those responsible for ndt was a much smoother axle end face for ultrasonic testing. This may not in itself appear significant but this and

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Fig. 4 The endwise scan is used to inspect the whole length of the axle

288 N D T I N T E R N A T I O N A L . D E C E M B E R 1978

other results derived from such meetings between designers and ndt personnel has been of lasting benefit. For instance, the reduction in the diameter of the turning centre from the traditional 50 mm to one of 25 mm allowed the probe to be sited nearer the longitudinal centre line. Further, the reduction in the amount and position of the wheelseat production data stamped on the axle end resulted in a considerable improve- ment in the probing surface conditions.

The far end scan uses a 0 ° probe capable of producing a sound output sufficient to penetrate mild steel for a distance of 3 m or more, at a wavelength that will detect cracks less than 3 mm with certainty at a range of 2 m. Probes with a frequency of 2.5 MHz are therefore required; these can be either single or double crystal according to preference. However, if a single crystal probe is used, the transducer should not be less than 20 mm in diameter, BR preference is for a double-crystal purpose-built probe fitted with 20 x 10 mm transducers.

To ensure that the entire length of the axle can be displayed on the oscillograph, the time base of the instrument should be calibrated so that one division corresponds to 250 mm, the full scale value (0-10 divisions) thus representing 2.5 m.

Fig. 5 shows the signal response when this scan is applied to a D/E locomotive driving axle. The signal diagram shows the comprehensive (and in this case, predicted) signal pattern expected when the probe is sited on the end of the axle nearest to the gear wheel. Since the object is to detect the intrusive signal from a defect in the axle, it can be seen that the tester must thoroughly understand the sources of all the signals 7 between the TX point (point 1) and the end echo at 2290 mm (point 6). The group of signals beyond this last signal are due to repeated delayed reflections from the axle geometry and can be ignored since they appear beyond the end echo.

By far the most important criterion to be laid down for the application of this scan, is the sensitivity level to be employed. This must ensure that the signal pattern is not subjected to over amplification such that an intrusive signal cannot be identified, and also that the reverse situation cannot occur, where a signal indicating the presence of a shallow defect passes undetected. It has long been the practice of BR to select a signal from a geometric feature on the axle and to set this signal to a given amplitude. The chosen signal is referred to as the control signal and is used to control the sensitivity level for all axles of its type, since the chosen geometric feature will be consistent in all of them. This ensures a defect sensitivity level common to all operators at depots scattered throughout the country resulting in consistent detection levels and conformity in reporting the results of inspections.

Fig. 6 illustrates the detection of a crack in a rolling stock axle by the endwise scan. Fig. 6a shows the normal signals received when the scan is applied, and Fig. 6b shows the result when the same axle is scanned from the opposite end and is found to be cracked in the journal abutment remote from the probe.

The crack signal can be seen at 7.75 scale divisions at a ragne of 1937 rnm, the crack depth was subsequently found

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Fig, 5 Endwise scan oscillograph from a good axle; the signals bracketed under 2 are from the journal bearings, signals 3 are from both the roadwheel and gear wheel, signal 4 is due to reflection from the roadwheel at the far end, signals 5 are caused by the presence of radii formed when the axle diameter is reduced from the wheelseat to the bearing journal, and signal 6 is the axle end

to be of the order of 3 nun. The sensitivity control signal for this particular type of axle is shown arrowed at 8.2 scale divisions (see Fig. 6a) and is derived in this case from a delayed reflection from the abutment itself, the consistent axle design feature referred to earlier.

Detection of cracks by this method is assured given the optimum crack proportions and orientation. However, the wheelset geometry will always be such that problems in detection must arise if the scan is not applied from both ends of the axle. The problem is illustrated in Fig. 6 where initially (Fig. 6a) the crack signal is hidden by the noise caused by the roller bearings nearest to the probing surface. In Fig. 6b however, even when the crack is at a long range from the probe the crack signal is free standing and relatively easy to recognise by the experienced eye. This of course presupposes that the tester is familiar with the origins of all those signals existing as part of the normal pattern.

Consider also the case of a crack found a short distance in from the inner edge of a road wheel, when the wheel is mounted on a raised wheelseat. The acoustic shadow which inevitably results when the sound passes from the axle body into the wheelseat is sufficiently large to hide a crack of about 12 mm depth. A repeat application of the endwise scan in the reverse direction is one way in which a crack, (albeit a large one) can be detected; this point is illustrated in Fig. 7. How- ever, when the crack is not deep enough to cause a recog- nisable change in the normal signal pattern a crack in this position can easily go undetected.

A detailed examination of the inner edge of each wheelseat is therefore essential in order to detect those cracks which can escape detection when the far end scan is applied. Such a detailed examination can be carried out from the same

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l'he near end-low angle scan is illustrated in Fig. 8. The obiect of this scan is to examine in detail the critical areas of an axle at the inner edges of the roadwheel seats.

Originally conceived about 25 years ago, low angle scanning possesses one major advantage over the shear wave scan from the axle body in that the axle end, already exposed and prepared for the application of the far end scan is again used as a probe site. Thus all that is required is a change of instru- ment calibration and probe. The detection capability of this technique is good when applied by well trained and experienced testers. Cracks of about 0.5 mm are regularly detected in BR workshops and depots using this scan, thereby affording not only a wide margin of safety, but also the detection of the shallow cracks required by the designer for eventual salvage of the axle.

Fig. 6 Endwise scan oscillograph of a crack in a rolling stock axle

probing surface, using angled probes, and a time base which has been delayed and expanded such that zero corresponds to 400 mm and the tenth division represents 650 ram. The following section describes the techniques employed by BR as a mandatory requirement in standing orders for routine inspection.

The near end-low angle scan

The detection of wheelseat cracking can be approached in two ways, either by using hollow ground shear wave probes sited on the axle body (see next section) or by using low angle compressional wave probes from the axle end face where access to the axle is gained through the axle box lid opening.

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Fig. 7 Scanning is required from both ends of the axte to avoid cracks being hidden by an acoustic shadow

290 NDT INTERNATIONAL . DECEMBER 1978

Fig. 8 The near end scan is used to examine the crit ical areas at the inner edges of the wheel seats

Fig. 9 shows how the scan is applied and the resultant typical signal pattern. The axle shown is one fitted to a typical electrical multiple unit driving wheelpair. A 15 °, 2.5 MHz double crystal compressional wave probe fitted with 20 × 10 mm barium titanate transducers 6 is used and is sited below the turning centre. The signal pattern is displayed on an expanded time base where each scale division corresponds to 25 mm and the trace delayed so that 0 corresponds to 400 and 10 to 650 mm, giving a defect location accuracy of 2.5 mm or better.

Since the sole object of the scan is to allow a detailed exam- ination of the innermost edge of the wheelseat, the primary signal is that designated W1, or the first wheel signal. Result- mg as it does from the part of the wheel hub closest to the axle surface, the signal I¢] is an ideal aiming point since the typical wheelseat crack lies between 5 and 10 mm inboard of this point. In addition W~ provides an inbuilt sensitivity control when the amplitude of the signal is increased or decreased to suit the detection level required. Fig. 9 shows where W~ is set to an amplitude of 15 mm, the standard used by BR for the type of wheel/axle arrangement shown. How- ever, when the axle is one where a stress relief groove exists in the axle at the interface of the roadwheel and gear wheel (see inset to Fig. 9), then the amplitude of WI must be reduced to 10 nun. Where a stress relief groove exists, contact between the wheel and axle is reduced by the intrusion of the air pocket formed by the groove. Thus the amplitude of W~ is reduced by an amount equal to one third of its original amplitude (15 mm) to 10 mm for a sensitivity level common to both designs, ie with or without groove.

In the early 1960s an important survey was carried out on BR to establish the reliability of using WI as a test sensitivity control signal.

The use of W~ as a control signal relies for its success upon consistent results from the wheelboss face in the presence of four possible variables:

• the quality of the probing surface at the axle end face, • the acoustic transparency of the axle material, • the transparency of the interface between the axle and

wheel, through which the pulse must pass before impinging upon the face of the wheel, and

• the reflecting properties of the wheelboss face.

The BR survey recorded the results obtained when the near- end scan was applied to 1000 wheelsets fitted to diesel dectric locomotives. The results when analysed, showed a remarkable degree of consistency in test sensitivity when IV1

was set to an amplitude of 15 mm at a scanning angle of 7.5 °. The maximum deviation was found to be 3 db, with the average variation being 2 db.

Experience has shown that any angle, matched to the axle geometry can be used for compressional wave scanning up to a maximum angle in steel of 20°; this is illustrated in Fig. 10. Beyond this angle the intrusion of the shear waves is such that interpretation is extremely difficult. The maximum angle in use on BR is 17.5 ° to the normal of the axle end surface. If angles greater than this are required then it is usual to use 17.5 ° and use the high energy portion of the sound pulse, taken as 4 ° either side of the centre line. Thus scanning at an angle of 21 ° is possible and in the case of one particular class of locomotive axle, is regularly in use.

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Fig. 9 The near end scan showing the primary signal, W 1 used as the control signal

N D T I N T E R N A T I O N A L . D E C E M B E R 1978 291

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Fig. 10 Angles up to 21 ° can be used with near end scanning

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Fig. 1 1 The shear wave scan is applied adjacent to the roadwheel seat

The shear wave or high angle scan

The term 'high angle' is traditionally used by BR to describe an axle testing method at a scanning angle greater than 35 ° and which therefore employs shear waves. As used by BR, the purpose of this scan is similar to that of the near end scan but gives a different ~,iew of the defect since the scan is applied from the part of the axle body adjacent to the road- wheel seats (see Fig. 11). Shear wave scanning, when applied to axles in situ may be at times difficult in the physical sense, but it is the most simple to interpret, except in the case of wheelseats to which a gear wheel is abutted. In these instances problems in signal interpretation will arise, particularly to the unwary.

A shear wave scan can only be applied from the body of the axle when a probe site is available, but since this is not always possible, (particularly in cases where the axle is shrouded by a casing), an alternative probe site must be found on the axle end. It is therefore inevitable that in providing adequate ultrasonic testing coverage of all axles, whether these are fitted to tractive or rolling stock vehicles, the majority of testing is carried out from the axle end. Reference to BR Chief Mechanical Engineers standing order for the ultrasonic testing of axles s will show that the vast majority of axles will be subjected to testing at intermediate periods (linked to tyre turning) between major overhauls. Inevitably, this requires axles to be tested in situ under the vehicle, when access to the majority of axles can only be gained via the axle end. An additional benefit of testing from the axle end is of course the minimum of disturbance to the vehicle, since at Depots tyres are reprof'iled by under- floor lathes. At this time the axle box lids are removed for

tyre turning providing an ideal opportunity to carry otzt ultrasonic testing from the exposed end face.

Essentially, the shear wave scan shown in Fig. l I requires a hollow ground probe to be sited at a predetermined position (P) on the prepared surface of the axle body. The critical area at the inner edge of the roadwheel seat is scanned by moving the probe back and forward across the line (P) so that the area inside and outside of the edge of tile wheelseat is fully covered. Before scanning is commenced, three criteria must be determined:

i probe position (P) 7. 12 ii slant range (R) ~ s e e Fig.

iii the test sensitivity

Points (i) and (ii) need to be calculated for each change of probe angle and for each axle diameter (or mean diameter) where this differs. The practice adopted by the DB many years ago using a standard scanning angle of 37 ° provides a simple answer for the determination of (i) and (ii). In this case (P) is obtained by taking 3/4 of the diameter (or mean diameter), whilst the slant range equals 5/4 of the diameter, as shown in Fig. 12.

The third criterion, the test sensitivity, requires consideratioH. As mentioned above, the sensitivity level adopted must assure detection of small cracks in the early stages of growth, whilst not causing the over amplification of insignificant signals which would result in the rejection of good components.

Fig. 13a shows the case of over-amplification of corrosion pits and/or turning marks received from axle surface in the area of the edge of the wheelseat. However, Fig. 13b shows how the intrusive signals can be put to good use by setting these to a predetermined level, in this case to ~ 5 mm, providing an inbuilt control of sensitivity. Any singular and reproducible signal received from the edge of the roadwheel seat when the probe is sited in its correct scanning attitude is then con- sidered to be from a crack. The signal must exhibit the geometric proportions of a singular signal and must rise sensibly above the noise level of the turning or corrosion marks.

The shear wave scan can be employed in two ways, either as a technique applied to all wheelseats where probe access to the

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Fig. 12 Values that need to be determined when using the shear wave technique

292 NDT INTERNATIONAL. DECEMBER 1978

axle body can be obtained or as the means to confirm the existence of a crack detected by either the far-end or near-end scans.

It is not generally accepted by railway ndt personnel that shear wave scanning can effectively be applied to roadwheel seats where an interface between the roadwheel and an adjacent gear wheel exists. It will be seen from Fig. 14 that problems in interpretation will arise when the sound pulse leaks into the gearwheel and reflects from its geometric features 8 and where also the slant range to the crack in the roadwheel seat either coincides, or is similar to one or more of the gear wheel features.

In this case it is possible that the resultant signals may merge. It is therefore essential that the normal signal pattern is plotted and pre-recorded, as with the far end and near end scans detailed earlier. The control of the test sensitivity can also be laid down by nominating one of the normal reflections as the control signal, or indeed the whole pattern of signals may be used.

Conclusion

The author's intention in compiling this paper is to make available to Railway authorities, (other than British Rail) the information and expertise gathered over many years. As stated in the opening section the techniques are those devel- oped by BR for the solution of its own particular problem, ie the testing of axles in situ, from the end faces. The infor-

Fig. 13 (a) shows over amplification of corrosion pits and (b) shows how background noise can be used as a sensitivity control

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b

Fig. 14 Problems of interpretation will arise when a sound pulse leaks into the gearwheel

mation, although specific to axles fitted to wheelpairs in this paper, can be of the utmost value to other industries, such as mining, marine engineering or wherever a problem in the testing of shafts is involved.

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Finally, it must be emphasised that the techniques are those which require the expertise of well trained and experienced testers who are well versed in the arts of ultrasonic signal interpretation.

A c k n o w l e d g e m e n t s

The author wishes to thank Mr K. Taylor, the Chief Mechanical and Electrical Engineer of British Railways, for his permission to publish this paper. Thanks also to the author's colleagues at both the Non-destructive Testing Training School, (at the Railway Technical Centre, Derby, England), in the Research and Development Division of British Rail, and the Central Photographic Unit, BT Films, Derby.

R e f e r e n c e s

1 Byrne, B.R. 'Ultrasonic flaw detection' Paper presented to the Institute of Mechanical Engineers, 1956

2 F 4 ~ t , K. '20 years of ultrasonic axle testing - established methods and more recent developments in the DB and other railways' Raillnternational (January 1970)

3 Wise, S. 'Ultrasonic testing of railway axles' Proceedings of the third international wheelset conference, Sheffield, July 1969

4 Butdon, E.S. 'Axles - a fatigue problem' BRB/LAMA joint conference on axles June 196 7

5 'Chief mechanical and electrical engineers standing order' Engineering instruction G I O Issue 4 (British Rail, February 1977)

6 Byrne, B.R., Johnson, P.C. and Farley P.G. 'Ultrasonic inspection of railway axles' Ultrasonics 4 (July 1966) pp 143-151

7 Farley, P.G. 'Prediction and interpretation of the signal patterns involved in the ultrasonic examination of cylindrical workpieces' Paper presented to the British Institute of NDT, 1976

8 'Handbook of ultrasonic testing' (3rd print) Data sheet No 7 (British Rail, 1969)

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