rail-track inspection using time-of-flight diffraction

Railway Engineering 2004 conference – London 6:7 July 2004 – ISBN

RAIL-TRACK INSPECTION USING TIME-OF-FLIGHT DIFFRACTION O. Zahran and W. Al-Nuaimy Department of Electrical Engineering & Electronics, The University of Liverpool, Brownlow Hill, Liverpool L69 3GJ Tel: +44 151 794 4580, Fax: +44 151 794 4540 Email: [email protected] ABSTRACT Recently there has been tragic loss of life in train derailment accidents worldwide. A major cause of train derailments is defects within the rail track, which may lead to breakage of the track under the stress of high-speed trains. Most of these accidents could have been prevented by better track inspection regimes. Ultrasonic Time-Of-Flight Diffraction (TOFD) is a recent innovation that has proved highly effective for the inspection of steel plates and tubular pipelines and has started to take its way to replace the other ultrasonic testing techniques. TOFD technique has a lot of advantages which make it the preferable technique in material testing. This technique gives accurate sizing, positioning and characterising of weld and other defects with a high probability of detection. Based on the experimental study, TOFD can be used for the inspection of rail-track particularly the fishplate and welds areas of the track, which are considered high failure-rate places, with satisfactory levels of accuracy and reliability. There are some restrictions on applying TOFD technique for rail-track inspection. The proposed solution for these restrictions and the procedures for applying TOFD inspection of rail-track are presented and discussed. INTRODUCTION Worldwide the last few years there has seen tragic loss of life in train derailment accidents. A major cause of train derailments is undetected or incorrectly evaluated defects within the rail track, which may lead to breakage of the track under the stress of high-speed trains. Most of these accidents could have been prevented by better track inspection regimes, which could potentially save human life, money and time. Ultrasonic techniques are still the most popular non-destructive testing methods applied to problems such as the inspection of rail-track. TOFD is a recent innovation that has proved highly effective for the inspection of steel plates and tubular pipelines and has started to take its way to replace the other ultrasonic testing techniques. TOFD technique has a lot of advantages which make it the preferable technique in material testing. This technique gives accurate sizing, positioning and characterising of weld and other defects with a high probability of detection. TOFD has successfully been applied to the inspection of steel plates and tubular pipelines. It is believed that with some modifications, the inspection of rail-track using TOFD will be possible (Zahran et al, 2002, 2003). TOFD TOFD first appeared in 1977 and started to take its way to replace the other ultrasonic testing techniques. This technique has a lot of advantages which make it the preferable technique in material testing (Erhard et al, 1999, Krutzen, 1998, Trimborn, 1997). There are many successful examples for applying TOFD technique, which show that TOFD is a powerful testing tool which gives accurate sizing, and characterising of weld defects. TOFD is based on measurement of the time of flight of the diffracted echoes of ultrasonic waves on the tips of discontinuities (defects) which are directly related to the true position and size of the defect instead

Railway Engineering 2004 conference – London 6:7 July 2004 – ISBN of geometrical reflection from the interface of the discontinuities in traditional methods (Silk, 1997). This technique uses two probes in a transmitter-receiver arrangement as shown in Fig. 1. When ultrasound is introduced into the material, each defect edge works as a point source of diffracted waves. The received signals can be visualized in an A-scan presentation or stacked together to give a 2-dimensional image called B or D-scan presentation as shown in Fig. 2.

Figure 1: TOFD technique

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Figure 2: TOFD A-scan and D-scan presentations The most important advantages of TOFD technique are that, TOFD defect detection does not depend on the defect orientation, in contrast to the other techniques, defect height can be exactly determined, depth sizing is very accurate with a high probability of detection up to 95%, and very low cost (Betti et al, 1999, 2000, Hetch, 1997, Webber, 2001). There are many successful examples for applying TOFD technique which show that the TOFD is a powerful tool for non-destructive testing and it gives accurate sizing, positioning, and characterizing of weld and other defects in shorter time than other techniques. Also, the results of experimental tests indicate that TOFD technique demonstrated a very high accuracy in detection and sizing. It detected 100% of defects detected by pulse echo and radiography techniques. In various international round robins trails for TOFD techniques, they succeeded in detecting and sizing a large number of defects (Trimborn, 1998). TOFD has been successfully applied for testing a wide range of steel plates and pipelines thicknesses. Rail-track has been made also from steel and the thickness of the track is in this range, which make the testing of rail track using TOFD is possible. The procedures for using TOFD technique for rail-track inspection are explained in more details in the following sections.

Railway Engineering 2004 conference – London 6:7 July 2004 – ISBN RAIL-TRACK INSPECTION Usually most of the defects occurred in rail-track are in fishplate and welds areas which indicated in Fig. 3. These areas are considered as high failure rate places (Railtrack PLC, 1997, 1998, 2001). Therefore this study concerned with the inspection of these two areas. Defects in fishplates usually occurred in the track within the outer limit of the fishplates (rail end) are horizontal cracking at the web-head fillet radius, horizontal cracking at the web-foot fillet radius, and star-cracking of fishbolt holes. The cracks around the holes are usually started at 45° and extended. There are usually 4 positions for these cracks which marked by A, B, C and D as indicated in Fig. 4 (Railtrack PLC, 1997, 1998, 2001). The position and separation of the bolt and bond holes are not constant in all tracks. Defects in welds usually occur in the weld material and the part of the track located within 100 mm of the track or within 25 mm of a weld repair. These defects are progressive transverse cracking (Tache Ovale) at a weld, gauge corner cracking (head checking) at a weld, horizontal cracking of web at a weld, cracking through the fishbolt holes at a weld, early-late tap, porosity, lake of fusion, inclusion, black hole, misalignment, hot tear, oxidation, bad trimming-grinding, transverse cracking of rail at weld repair and detachment or shelling of the weld repair. Traditionally the procedure for weld inspection is divided into several sections, with each part of the rail (head, web, ankle and toy) inspected separately.

Figure 3: Rail-track fishplates area and weld areas

Figure 4: Side-view and cross-sectional view of the fishplates area

Railway Engineering 2004 conference – London 6:7 July 2004 – ISBN Fishplate area inspection using TOFD Considering the Railtrack PLC procedures for ultrasonic inspection of rail-tracks (Railtrack PLC, 1997, 1998, 2001) and the British standard for TOFD (British standard, 1993), It can be said that the implementation of TOFD for rail-track inspection (particularly fishplate and areas) can be achieved. The proposed manner for applying TOFD for rail-track inspection is described and discussed. Usually 5MHz probes with diameter 0.6mm have been used in most of TOFD application, but from experimental practice, it has been found that using these probes is impractical, because the receiver can not receive any back wall signals from the base of the track while it can receive a signal from the bottom of the head. This problem can be solved by using 5MHz probes with 12mm diameter. In that case, the ultrasonic energy introduced into the rail-track is sufficient to hit the base of the track and be detected back at the receiver. Furthermore using 12mm probes will decrease the beam spread which will concentrate the ultrasonic energy towards the web of the track and eliminate the reflection from the bottom of the head. Also, using the normal callipers result-in the problem of setting suitable probe centre separation (PCS) because the maximum PCS from the normal callipers is not enough to focus ultrasonic energy on the deep areas at the web. This had the effect of the receiver being unable to receive the reflected backwall signal. Larger calliper has to be used in order to set the PCS as needed with different angles of wedges to achieve the required focus of the introduced ultrasonic energy through the track. The used probe angle will be 45° to reduce the probe centre separation. The calculated beam spread of ultrasonic with 45° probe will be (39°:51°). There are two proposed procedures for inspection of these cracks. One for the top cracks (A and B), and the other is for the bottom cracks (C and D). Inspection of A and B cracks The inspection of top cracks A1, A2, B1, and B2 can be done easily using 45° probe angle with 5MHz and 12mm diameter probes. Strong diffracted signals will be received from the tip of the crack. The probe centre separation will be around 16 cm which is a reasonable separation. The inspection of these cracks is illustrated in Fig. 5, 6, 7, 8.

Figure 5: Detection of A1 crack using 45° probe angle


Figure 6: Detection of A2 crack using 45° probe angle

Figure 7: Detection of B1 crack using 45° probe angle

Figure 8: Detection of B2 crack using 45° probe angle Inspection of C and D cracks The detection of bottom cracks C1, C2, D1 and D2 depending on the diffracted echoes alone may be difficult because the bolt hole will prevent some of the diffracted echoes from reaching the receiver. Therefore, these cracks can be detected by noticing the reflected signal from backwall in addition to

Railway Engineering 2004 conference – London 6:7 July 2004 – ISBN diffracted echoes. These cracks will affect the reflected signal from the backwall and will produce some distortion in the backwall signal. This distortion beside the diffracted echoes will give indication of the presence of the crack and can be considered as the signature of these cracks. The inspection procedure of C and D cracks using 45° probe angle with 5MHz and 12mm diameter probes has encountered some restrictions in the inspection of D1, C2 and D2 cracks because the probe centre separation in this case will be quite large, around 33cm, in order to provide a focus depth of ultrasonic energy near the track base. As illustrated in Fig. 10, due to the probability of presence of two cracks in the two bolt hole in the same time, the crack A2 may prevent or influenced the detection of crack D1. Also, due to the short distance between the second bolt hole and the rail-end, the detection of the cracks C2 and D2 will be difficult (see Fig. 11, 12) because the small gap between the track-end and the other track will cause the loss of the signal.

Figure 9: Detection of C1 crack using 45° probe angle

Figure 10: Detection of D1 crack using 45° probe angle



Figure 12: Detection of D2 crack using 45° probe angle These restrictions can be overcome by reducing the probe centre separation while the focus depth is still the same. This can be achieved by changing the probe angle. If 30° probe angle is used, the probe centre separation is reduced to around 19cm which considered a reasonable separation and the calculated beam spread at that case will be around (25°:35°). As known the common probe angle used in TOFD technique are 45°, 60°, and 70° and the suitable zone for accurate inspection is (45°:80°) (Charlesworth & Temple, 2001). Within this zone, the diffracted echoes are stronger than the others but in this case, the detection does not depend on the receiving of diffracted echoes alone as mentioned earlier. The inspection of these bottom cracks using 30° probe angle will solve the previously explained problems as illustrated in Fig. 13, 14, 15. Welds inspection using TOFD Most of the weld defects in rail-track are common in normal steel plates and have been considered in weld detection in steel plates using TOFD (Lawson, 1996). The inspection of rail weld can be done by placing the transmitter-receiver arrangement probes in both sides of the weld (5MHz Probes with 12mm diameter). The probes are moved laterally from the toe passing through the ankle, the web and the head (D-scan). This movement illustrated in Fig. 16.



Figure 14: Detection of D1 crack using 30° probe angle

Figure 15: Detection of D2 crack using 30° probe angle Applying this movement using the normal wedge material is impractical, because of the complex geometry of the track and the dimension of normal wedges, the ultrasonic signal is lost due to the air gap which happened during the probe movement at the ankle and the beginning of the web as illustrated in Fig. 17. This problem can be overcome by using a smaller wedge with a parabolic shape base or by using elastic material for wedge base.


Figure 16: Plan and cross-sectional views of the probes movement for weld inspection

Figure 17: Weld inspection restrictions due to rail-track geometry Entire track inspection using TOFD The high failure rate places in rail-track are the fishplates and welds areas. After applying TOFD technique to inspect these areas, it will be easy job to inspect the entire track by using an array of TOFD probes. This can be done by placing the probes on the running surface and moving them across the head surface. In this case, there will be two backwall echoes, one from the head base and the other from the track base. Any defect in the head, web or the centre of the base can be detected except the defects in the toes of the rail base. The defects in the toes can be detected by using other pairs of probes on each side and the movement direction of these probes will be in the running surface direction.

Railway Engineering 2004 conference – London 6:7 July 2004 – ISBN CONCLUSION TOFD has been introduced as a new ultrasonic non-destructive testing technique. TOFD has many advantages compared to other techniques which make it the preferable technique in material inspection. This technique gives accurate sizing, positioning and characterising of defects with a high probability of detection. It has been seen that TOFD technique can be used for the inspection of rail-track (particularly the fishplates and welds areas). The procedures for applying TOFD for rail-track inspection, restrictions and the proposed solutions are presented and discussed. ACKNOWLEDGMENTS I would like to express my deep gratitude and appreciation to Phoenix Inspection Systems Ltd. for granting me access to their TD Pocket Scan system in addition to their continued assistance, technical advice and invaluable research support. I would also like to thank Lavender International NDT for providing the TOFD data and their guidance on the interpretation of the TOFD data. REFERENCES 1. Betti, F, Zappavigna, G, Pedrinzani, C, Nardoni, G & Nardoni, P, (2000), Accuracy capability of TOFD technique in ultrasonic examination of welds, NDTnet (www.ndt.net/article/wcndt00/papers/ idn634/ idn634.htm), 2000 2. Betti, F, Guidi, A, Raffarta, B, Nardoni, G, Nardoni, P & Nottingham, L, (1999), TOFD - the emerging ultrasonic computerized technique for heavy wall pressure vessel welds examination, NDTnet (www.ndt.net/article/v04n09/nardoni/ nardoni.htm), Sep. 1999, Vol.4, No. 9. 3. British standard Institution, (1993), The British TOFD standard BS 7706. British Standards Institute, 1993. 4. Charlesworth, J, & Temple, J, (2001), Engineering applications of ultrasonic time-of-flight diffraction, 2001, second edition, RSP, ISBN 0-86380-239-7. 5. Erhard, A, & Ewert, U, (1999), The TOFD method between radiography and ultrasonic in weld testing, NDTnet (http://www.ndt.net/article/v04n09/erhard/erhard.htm), Sep. 1999, Vol.4, No. 9. 6. Hecht, A, (1997), Time of fight diffraction technique (TOFD) - An ultrasonic testing method for all application, NDTnet (www.ndt.net/article/tofd/hecht/hecht.htm), Sep. 1997, Vol.2, No. 9. 7. Krutzen, R, (1998), Evaluation of currently applied ultrasonic sizing techniques for stress corrosion cracks in steam generator tubes, 17th EPRI Steam Generator NDE Workshop, Breckenridge, Colorado, USA, (www.nuson.nl/news/do05pres.html), Aug. 1998. 8. Lawson, S, (1996), Automatic defect detection in industrial radioscopic and ultrasonic images. PhD thesis, The university of Surrey, April 1996. 9. Railtrack PLC, (1997), Railtrack line specification - rail testing - detection criteria. Railtrack PLC documentations, 1A(RT/CE/S/014), October 1997. 10. Railtrack PLC, (1998), Railtrack line specification - rail testing: ultrasonic procedures. Railtrack PLC documentations, 1A(RT/CE/S/055), February 1998. 11. Railtrack PLC, (2001), Railtrack company specification - rail failure handbook. Railtrack PLC documentations, 4(RT/CE/S/057), October 2001. 12. Silk, M, (1997), The rapid analysis of TOFF data incorporating the provision of standards, NDTnet, (www.ndt.net/article/tofd/Silk/Silk.htm), September 1997, Vol.2, No.9.

Railway Engineering 2004 conference – London 6:7 July 2004 – ISBN 13. Trimborn, N, (1997), The time-of-fight-diffraction technique, NDTnet, (www.ndt.net/article/tofd /trimborn/ trimborn.htm), Sep. 1997, Vol.2, No. 9. 14. Trimborn, N, (1998), The performance of the time of flight diffraction (tofd) technique in various international round robin trails and the continuing research work underway. UTonline (http://www.ndt.net/article/tofd/silk/silk.htm), October 1998, Vol.3, No. 10. 15. Webber, S, (2001), Five years of testing using the semi-automated ultrasonic time of fight diffraction system, NDTnet (www.ndt.net/article/apcndt01/papers/898/898.htm), 2001. 16. Zahran, O, Shihab, S & Al-Nuaimy, W, (2003), Automatic segmentation of Time-Of-Flight Diffraction images using Time-Frequency techniques – Application to rail track defect detection, BINDT 2003, 16-18 September 2003, ISBN no. (0-9031-3231-1), 265-270. 17. Zahran, O, Shihab, S & Al-Nuaimy, W, (2002), Recent developments in ultrasonic techniques for railtrack inspection, BINDT 2002, Southport 17-19 September 2002, UK, ISBN no. (0-9031-3230-3), 5560. 18. Zahran, O, Shihab, S & Al-Nuaimy, W, (2003), Time-Frequency techniques applied to TOFD for the automation of rail-track inspection, Railway Engineering 2003, London 30th April – 1st May 2003, UK, ISBN no. 0-947644-50-4.