Automated Ultrasound-Based Inspection of Rails: Review

IJR International Journal of Railway The Korean Society for Railway Vol. 10, No. 2 / December 2017, pp. 21-29

Automated Ultrasound-based Inspection of Rails: Review

Suvi Santa-aho†, Antti Nurmikolu, and Minnamari Vippola

Abstract

The main aim of the report is to review recent progress in the utilisation of ultrasound-based automated inspection systems for rails of railway tracks and to provide a basis for further research in the field of railway track inspection. This paper reviews the ultrasound technologies currently employed in the automated inspection of railway tracks, along with examples of recent field applications. The main research areas that this review focuses on are firstly, the utilisation of ultrasound inspection for rail tracks; secondly, the different automated ultrasound techniques, and, finally, the special features of the ultrasound inspection of railway tracks. In addition, there is a review of the most recent ultrasound-based systems, and future techniques that have not yet been automated or used in routine inspection.

Keywords: Automated inspection, Ultrasound, Track inspection, Nondestructive testing

1. Introduction may incorporate many inspection methods, i.e. ultrasonic testing facilities [3], the induction method [3], (pulsed) Non-destructive testing (NDT) or non-destructive evalu- eddy current testing [2,5,6] and/or image processing tech- ation (NDE) is characterised as the examination of an nology for visual testing of sectional wear measurement, object without affecting the future usefulness of the and rail corrugation measurement using lasers [2]. The tar- inspected component [1]. The inspection of railway tracks get of one example project was to develop a measurement is important in order to find defects before they reach criti- system combining ultrasound, alternating current field cal size and cause failure in the railway track structure [2]. measurement (ACFM) and artificial vision [7]. Inspection The trend is to combine the NDT of the railway track with systems can be integrated with grinding test trains [5]. In preventative maintenance procedures (like rail head grind- addition, the trend in rail flaw detection is towards improv- ing) to optimise the compromise between maintenance ing the efficiency of measurements whilst minimising cost and reliability [2]. track time for maintenance, as Clark [3] suggests. Auto- Many different NDT methods are utilised for railway matic inspection systems that are able to detect rail defects track condition monitoring. Contact ultrasonic testing will increase the ability of inspection and reduce the mea- belongs to the first methods utilised for railway tracks surement time, allowing more accurate and frequent main- along with magnetic induction method [3]. Nowadays and tenance work [8]. probably even more so in the future, railway track NDT is Various mechanisms are responsible for the different being combined with measurement systems consisting of defect types that appear in rails of railway tracks. many different methods, completing the information Commonly, ultrasound inspection is used for finding gained about the defects and increasing the probability of internal defects that are formed during the cyclic load- defect detection [4]. At present, rail inspection vehicles ing of rails [4]. Ultrasound inspection is capable of searching for the inner defects that lie under the surface. The inspection uses a beam of ultrasonic energy † Corresponding author: Tampere University of Technology, Department of Materi- that penetrates the inspected component via a probe als Science, Tampere, Finland and a couplant in between. Ultrasound energy is E-mail : [email protected] reflected back or scattered from boundaries with differ- ⓒThe Korean Society for Railway 2017 ent acoustic emissions. The received signals are col- http://dx.doi.org/10.7782/IJR.2017.10.2.021

Vol. 10, No. 2 / December 2017 − 21 − Suvi Santa-aho, Antti Nurmikolu, and Minnamari Vippola / IJR, 10(2), 21-29, 2017 lected with receiver transducers and studied in order to 2. Basic Principle of Ultrasound find possible defects [9]. Measurements for Railway Tracks The cyclic loading of rails causes wear and wear- related changes and therefore the rails are inspected Ultrasound inspection is based on sound waves, or periodically to ensure that no possible flaws and defects vibrations, created by a piezoelectric crystal in the probe, compromise the traffic. Certain regulations for track that propagate in solids, liquids and gases, at a frequency maintenance exist and, for example, in the United above the range of human hearing, normally above 20 States, inspection for rail ﬂaws is carried out at inter- kHz. Sound waves can propagate in different forms, of vals based on the track class and annual tonnage [10]. which the most generally used in inspections are longitudi- Ultrasound inspection for rails of railway tracks can be nal (compression) or transverse (shear) waves. The differ- carried out manually with hand-held devices or with fix- ence between these waves is the particle motion in relation tures moved by an operator that slide on top of the rail, to the sound wave direction [9]. In addition, surface waves or with automated ultrasound test fixtures towed by hi- and Lamb waves can be used in some special cases in the rail inspection vehicles or inspection trains. Some rail- inspection of thin structures. Sound waves travel with dif- way track inspection techniques can be installed on ferent velocity depending on the material. The factors commercial trains, but at the current state of inspection affecting this are density and the elasticity of the material. method development, ultrasound inspection is still In addition, different waveforms have different speeds in unsuitable for commercial trains [11]. the material; a transverse wave travels at almost half of the The German Institute for Standardization (DIN) has speed compared to a longitudinal wave [9]. Different recently published a draft standard “DIN EN 16729-1 materials have different acoustic impedance values, mean- Railway Applications - Infrastructure - NDT On Rails In ing that the ultrasound energy is refracted from the bound- Track - Part 1: Requirements For Ultrasonic Inspection ary between different media [9]. The discontinuities act as And Evaluation Principles” of ultrasonic inspection for a reflector to the ultrasonic waves, resulting in a portion of railways [12]. The draft specifies reference reflectors on the wave being reflected back to the receiver. These the test track for ultrasound inspection to produce compa- echoes are studied in order to find defects. In cases when rable results with regard to location, type and size of refer- there are no other reflectors, the reflection comes back to ence reflectors. the receiver from the back wall of the inspected compo- This paper reviews the technologies currently employed nent. If the back wall echo is missing, it might be caused along with examples of recent field applications. The main by defects masking it and in this case also there might be a aim of this report is to review the recent progress in the potential defect in the structure [13]. utilisation of automated techniques and to provide a basis The most common method for ultrasound inspection is for further research. The concept of the ultrasound inspec- called the pulse-echo method. The same transducer sends tion method for railway tracks will be presented in Sec- and receives the pulses which are studied; for example the tion 2, the ultrasound equipment and units for automated amplitude and the time that the ultrasound beams travels inspection in Section 3, rail defects and the probability of inside the inspected component. There exist different their detection and special features of the ultrasound probe configurations for the measurements. A normal inspection of railway tracks in Section 4, future inspection beam probe transmits ultrasound wave that is directed systems for railway track inspection in Section 5 and other straight to the face of the transducer. Probes can be aligned ultrasound-based inspection systems for railway track with wedges in different angles as a function of the inspection in Section 6. Conclusions are presented in Sec- inspected component surface. These probes are called tion 7. This review was carried out in liaison with the Life angle probes [1,9,14]. Cycle Cost Efficient Track research programme (TERA) Generally, NDT techniques are used to locate flaws, to implemented in co-operation with the Finnish Transport size them and to characterise material conditions. A basic Agency and Tampere University of Technology (Finland). flaw detection procedure is applied: firstly, the defects The research programme comprises ten sub-areas, one of need to be detected, secondly, the dimensions can be deter- which is research on track rail life cycle. At the moment, mined and, finally, the evaluation of the defects based on research activity on rails is concentrated on the verifica- the acceptance criteria must be performed. Usually, the tion of rolling contact fatigue and surface defects in Finn- ultrasound systems use a threshold level in a gate to record ish rails, as well as suitable analysis methods for rail the ultrasound response in cases where the reflection is inspection. above this level [12]. The preferred system for data presentation of ultrasound

− 22 − Automated Ultrasound-based Inspection of Rails: Review inspection results, i.e. flaw indications, from rails has been probes can be either in direct contact to the surface [5] or the B-scan [3,15,16]. The B-scan based test system pro- there may be a belt between the probe and the vides an image of a defect in the perpendicular direction of surface [21]. The belt system protects the probe unit from the rail. It gives the cross-sectional display of the time-of- mechanical shocks and against damage caused by bad flight data from the surface of the rail to the bottom of the track conditions [21]. In the studies made by Heckel et rail [9]. Heckel et al. [5] reported the use of a Glassy-Rail- al. [5], the sliding plate sled contained ten ultrasonic diagram for interpreting results given by multiple ultra- probes per rail, which were inserted into their positions by sound probes which gives a side view of the rail. The springs with calibrated tension. colours of the Glassy-Rail-diagram represent the ampli- Differentially oriented ultrasound beams created with tude of the reflected signals and the diagram gives a fixed angle probes can be used for searching different regions of resolution independent of the inspection speed [5]. the inspected rail. Usually, a combination of probes emit- An automated railway track inspection with ultrasound ting waves from different angles is used, and the measure- is carried out simultaneously with multiple ultrasound ment is repeated along the length of the rail as the probes to gain maximum inspection volume and to maxi- inspection instrument moves along. Normally, the probes mise the probability of detecting defects [2]. The ultra- are operated in the pulse-echo mode. Also, the transmitter- sound probes are generally located in rotating wheels or in receiver mode with one sending and one collecting probe sliding shoes/runners also called a sliding plate sled [3,13]. can be used for rail head defect inspection [5]. The probes A schematic presentation of these is shown in Fig. 1. can be aligned in the longitudinal axis of the rail and multiple probes may also be located next to each other to provide better coverage [6,22]. Probes can be aligned to point forward and backward on the rail [5,22]. Generally, there is a normal probe for searching the full railway track depth, rail head and horizontal foot defects, [3,5,6,7,18,22] and several angle beam probes pointing forward and backward for detection of web and foot defects. Probes with angles 35º [3,5,7,18], 37º [6,22], 42º [22], 55º [5] and 70º [3,5,6,7,22,28,29] are reported to be used. An angle beam probe of 70 degrees can be utilised for transverse defect detection [18] or welding crack studies [19].

Fig. 1 Schematic presentation of rotating wheel and sliding 2.3 Couplants plate sled with multiple ultrasound probes. The basic ultrasound inspection always requires a couplant between the sensor and the inspected component [23] to couple the piezoelectric transducers to the rail and to 2.1 Roller search units (RSU) ensure a sufficiently high energy transfer and signal-to- Rotating wheels are also called roller search units (RSU) noise ratio [24]. Normally, water, oil or a gel is applied and are a fluid-filled wheel housing the ultrasound probes between the probe and the inspected component. In auto- for rail inspection [3,6,17]. The RSU wheel membrane can mated ultrasound inspection for railway tracks, a water- be composed for instance of flexible polyurethane [18] or based couplant is injected between the probe and track in a pliable urethane membrane [18]. The elastic shell of the all the studied configurations. For example, for the RSU wheel adapts to the surface of the rail and enables a con- inspection unit, a mixture of water and propylene glycol stant contact during the inspection. The filling material of (40%) is used [18] by sparking it before the RSU moves. the wheel is a solution that permits efficient sound energy This blend offers quite a similar sound velocity to the transfer into the rail track steel [18]. The wheels of the membrane of the RSU wheel and the possibility to use it at RSU (with a diameter of e.g. 170 mm [19]) are installed in different temperatures [18]. For the sliding plate sled a carriage with rail position sensors [19]. installation, water is sprayed constantly below the sensors. To verify the contact between the ultrasound probes and 2.2 Sliding plate sled rail, a coupling check between the rail and sliding plate The sliding shoes, sliding plate sled or slide probes have sled can be done with one normal probe [5]. ultrasound probes mounted in a fixture that ensures direct The main benefits of the RSU-based unit are the robust- contact of the probe to the rail surface [4,5,16,20,21]. The ness of the system and its ability to adapt to the rail

− 23 − Suvi Santa-aho, Antti Nurmikolu, and Minnamari Vippola / IJR, 10(2), 21-29, 2017 geometry [3]. One limitation that has been recognised is confirm” technique. The “stop and verify/confirm” tech- the centrifugal force of water inside the RSU wheel, which nique is used, for example in Northern America [3,30,36] distorts the wheel membrane when using too high mea- where two units carry out the inspection. The first does the surement speeds and may cause premature failure of the automated inspection, while the second one stops to carry wheel [21]. The different installation set-ups of the RSU out manual verification of any flaws found. The second and sliding plate sled also affect the measurements. Gener- unit, the chase car, carries the hand-held ultrasonic test ally, the sliding plate sled ultrasound probes are in closer equipment to verify and size the suspected defect [31]. The contact to the rail with flexible mounting [13]. However, maximum testing speed of the “non-stop” method has the RSU wheel probes can be adapted better to rail profile been reported to be 48 km/h in order for the ultrasound wear [2] and irregular rail shape [13]. beam to investigate the entire rail [15].

3. Manual Inspection and Automated 3.1 Measurement intervals Inspection: Ultrasound Equipment and Railway track endurance is facing new challenges due to units for Automated Inspection the high-speed and high axle load vehicles operating on the track material [37]. The railway tracks are inspected Ultrasonic testing is conventionally performed from the periodically to ensure that any possible flaws and defects top of the rail head in a pulse-echo configuration. It do not compromise the traffic. The periodical inspections includes various ultrasonic probes for different kinds of are important in order to find flaws that have grown from defects and defect orientations. Manual ultrasound inspec- a non-detectable size to critical size [30]. Certain regulations are carried out with portable rail detector units or tions for track maintenance exist and, for example, in the portable rail inspection units [2], which employ the RSUs United States inspection for rail ﬂaws is carried out at or sliding plate sleds in a rail testing trolley [25] that is intervals based on the track class and annual tonnage of pushed forward on the railway tracks. Manual measure- accumulated traffic carried by railway tracks [10]. This is ments can be performed on only one rail or both rails with because heavy axle load operation accelerates and a special fixture [26]. In addition, there is a tandem tech- increases track degradation and thus leads to the need for nique, which utilises two angle beam probes for transmis- track maintenance [10]. For example, the Australian Rail sion and receiver that are moved simultaneously [25]. Track Corporation’s guidelines for ultrasound inspection Manual inspections take much more time than automated state that continuous ultrasonic testing should be carried ones and usually there is only a limited period of time to out at intervals of 15 million gross tons (MGT) of axle carry out the measurements due to rail traffic. Thus, auto- loading passing over a rail [31]. mated inspections have replaced manual measurements. The ultrasound unit, composed either of RSUs or a slid- 3.2 Calibration ing plate sled, can be installed in a number of vehicle plat- Calibration of the ultrasound units is an important step forms or trains. The automated inspection of rail is done in the whole inspection. Probes can be calibrated with spe- by integrating an ultrasound module into a hi-rail cial standardized calibration blocks. For railway track vehicle [27], a bogey that is attached to a hi-rail inspection systems, simulated reflectors can be manufac- vehicle [28] or an inspection train [4,18]. Track inspection tured to represent real rail defects. Ultrasonic reference trains [6] are also called ultrasonic measuring cars [29], reflectors on the test track can be used to determine the inspection vehicles [30], test carriages [31], instrumented inspection system’s ability to detect defects from railway carriages [3] or inspection trains with integrated inspec- tracks [12]. For the test track, multiple simulated reflection systems [2,18,32,34]. tors are manufactured in different areas of the rail head, Generally, defects that are first found by automated web and foot [12]. The test track is studied and reported at inspection are subsequently studied by manual ultrasound the same inspection speed as the one used when taking inspection [34,35]. Each defect is tagged with GPS coordi- measurements. Reference reflectors need to be detected at nates, time stamps, kilometre markers and additional event defined ultrasound probe angles [12]. For defect threshold markers in real time for follow-up inspection [5]. Manual verification, a calibration railway track with drilled flat ultrasound testing is carried out to confirm the defect and bottomed holes of different diameters [12,35] or a test to determine its size and position. Further analysis of the block with drilled holes and welds [5] can be utilised. defect is greatly affected by the operator’s judgment and Also, some calibration tools can be used for verifying the level of experience [36]. Two existing techniques are the probe fixture position. Heckel et al. [5] reported that the so- called “non-stop” method [15] and “stop and verify/ probe adjustment for a sliding plate sled ultrasound fix-

− 24 − Automated Ultrasound-based Inspection of Rails: Review ture is carried out with a special calibration device to ure to detect defects during inspection as follows: 1) flaw achieve a gap of 0.2 mm between the rail head and the size being too small, 2) flaws located in areas that were probe shoe in order to keep the abrasion minimal and the not included in the inspection, or 3) surface conditions coupling optimal. Simulation studies made by Cygan et masking the signal and compromising the detection of the al. [16] showed that the centring of the probe to the rail is flaw [30]. The physical limitations of the ultrasound important in order to maximise acoustic field attenuation method itself pose some restrictions, such as flaws in the in the railway track. first few millimetres of the rail head or surface-breaking flaws being poorly detected due to the dead zone of nor- 3.3 Data processing of ultrasound inspection mal probes [4,13]. In addition, certain types of rail head The automated inspection is carried out with a certain ﬂaws are difficult to detect because of their shape or repetition rate of the beam dispatch of the order of a few because their orientation is not optimal in relation to the kHz. Due to the high inspection speed, the large amounts ultrasound beam [3]. The defect must be oriented at 90 of incoming measurement data present challenges for the degrees to one of the search beams utilised in order to be evaluation and reviewing of all the collected data at found in the inspection [13]. Many difficulties have been once [5]. Additionally, all data have to be processed in real observed, as in the detection of transverse fissures with a time for online monitoring. The recognition of the poten- volume that is only 5% of the rail head volume [39]. Ultra- tial defects from the large amount of data gathered is sound beam simulation has been used to model the crucial [13]. Evaluation of the measurement data by the beam [16] for defect searching with real 3D models of the operator can be done with automated classification algo- defects [7,39]. The simulation verified the difficulty to rithms which allow preselection of the data displayed [5]. detect transverse fissures; they are observable with a 70 Post-processing of the data can use for instance algo- degree angle beam only if their orientation is between 5 - rithms based on neural networks [5,30] and fuzzy 37 degrees to the centreline of the railway track [39]. The logic [5]. Also, wavelet decomposition has been utilised to detection threshold for small flaws can be difficult to separate the defect echo from noise [16]. The different achieve, for instance, in old, less clean steel rails with flaws are identified from the ultrasonic reflection patterns inclusions masking the ultrasound beams [13]. for each different flaw. Ultrasound reflections can be The overall sensitivity of the inspection system depends divided into reflections coming from the head, web, base on the quality of the rail surface quality [5]. For example, and potential defects [13]. The identified rail defects are uneven rail surfaces due to shelling, pitting or worn rail evaluated based on a series of defect categories. In the profiles can reduce the quality of the interface between the Australian Rail Track Corporation’s guidelines, the ultrasonic probe wheels and the rail surface [30]. Horizon- response codes define the appropriate response that needs tal surface cracks such as shelling and heavy head to be applied to the railway track [30]. It has been reported checking [40], corrugation [40] and surface initiated roll- that some algorithms have been used to remove the false ing contact fatigue (RCF) damage [30,34,41,42] can pre- crack signals coming from grease spots, rail gaps and vent the ultrasonic beams from reaching the internal rust [21]. In addition, some recognition patterns have been defects and resulting in false negative indications, missing utilised to identify rail ends and bolt holes, for the flaws located beneath them. Furthermore, there are example [13]. The development of faster computer proces- some common rail conditions that can mask ﬂaws or sors will allow faster signal processing and the utilisation impede defect detection. For example, excessive rail of faster software for adaptive recognition of defects [21]. lubrication [10,43], leaf mould [4], grease and dirt [31] and weather conditions like ice [43] may cause problems in 4. Rail Defects and Probability of probe contact. Detection, Special Features of The probability of detection (POD) for different sizes of Ultrasound Inspection of Railway Tracks defects has been studied with ultrasonic reference reflectors manufactured for a test railway track [30]. The proba- 4.1 Rail defects bility of detecting a defect increases as the defect size There are different defect types in rails, originating from percentage of the head area increases. If the defect is 5% various mechanisms. The most common flaws are found of the head area, the mean value of probability of detec- in the rail head due to the cyclic loading of rails caused by tion is 50% [30]. Another problem with the automated sys- changes related to wear and wear [38]. Multiple transduc- tem was the false negative defects that could not be ers need to be employed to cover the rail head area [2]. verified as flaws with manual inspection [44]. One study Reiff and Garcia sum up three general causes for the fail- concluded that non-stop inspection achieved a result of 85

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% verified defects of all suspected defects reported [15]. the defects detected by ultrasonic high speed systems [7] The ultrasound inspection measurement parameters and during the inspection of welds in railway tracks affect the theoretical observation of defects. While using [45]. Aoki et al. [34] have developed a method with higher frequencies, the probability of observing smaller two-phased array probes to inspect the transverse cracks flaws increases but the absorption of the ultrasound energy in the presence of horizontal cracking from the under- to the inspected component also increases [9]. side of the rail head. This method uses two-phased array probes beneath both sides of the rail crown, which can be 4.2 Measurement speed moved automatically [42]. The research on this promis- Generally, the detectability of defects decreases when ing method is still ongoing [34]. the measurement speed increases. With higher speeds, the The next step in phased array inspection is the ultra- lateral resolution for finding defects decreases [5]. The sonic tomography inspection technique. The automated 3- automated inspection testing speed for inspection vehicles D imaging of rail defects has been studied by Phillips et is between 40km/h and 100km/h. However, as the speed al.[36] to obtain more accurate information on flaw size, increases, the sensitivity and resolution of the system dete- shape and orientation. In the ultrasonic tomography tech- riorate significantly [2]. Heckel et al. concluded that a nique, a few ultrasound elements transmit with diverging measurement speed of 60 km/h is close to the physical beams, whereas in the phased array technique all the limits for ultrasonic testing, as the maximum pulse repeti- phased array elements transmit with a time delay [36]. tion rate is limited by the sound velocity in the rail [5]. Divergent beams will increase the inspection volume com- Aharoni and Glinkman also came to the same conclusion pared to the phased array technique [36]. about the maximum measurement speed, putting it at 58 km/h [20]. Aharoni and Glinkman [20] also concluded that 6. Other Ultrasound-based Inspection the maximal scan speed for the RSU is approximately half Systems for Railway Track Inspection of that for the slide probe, due to the wheel medium caus- ing a delay in the ultrasound beam. Generally speaking, the automated inspection carried out Some real time algorithms have been utilized to increase with RSUs or sliding plate sleds with conventional ultra- the pulse repetition rate [5]. Other issues affecting the sound probes are capable of finding only certain types of measurement speed are the surface quality of the rail, defects. Transverse defects in the head cannot usually be which limits the speed, and probe wear must also be taken detected by the normal pulse echo ultrasound method [46]. into consideration [5]. They also have the limitations which were discussed ear- lier, i.e. the need for coupling. Thus, other ultrasound- 5. Future Inspection Methods based inspection systems are also presented here briefly, including laser ultrasonic inspection, guided wave inspec- Research on using ultrasonic phased arrays in railway tion and electromagnetic acoustic transducers, or EMATs. track inspection is currently ongoing in different universi- These methods are still under development and no com- ties [2,7]. The phased array ultrasound method uses an mercially available automated inspection systems are array of transducers operating with different timing to gen- available yet. The new techniques have demonstrated suc- erate an ultrasound wave front. Comparing the phased cessful application in the laboratory but their utilization in array to a single-element transducer allows many inspec- practice appears to be problematic [40]. tions to be made from one location without changing the angle or moving the probe. The main benefits of using a 6.1 Laser ultrasonic, pulsed laser ultrasonic, phased array system are sectoral scanning, showing the laser-based ultrasonic discontinuities at appropriate angles, and increased cover- The laser ultrasonic method is a non-contact inspection age to raise the probability of detection [43]. method in which the ultrasound wave is generated by a No practical systems utilising the ultrasonic phased high energy pulsed laser. The method uses air-coupled array method have been developed for automated high- transducers for wave monitoring [47-49]. Cerniglia et al. speed rail inspection so far, due to the difficulties aris- [48] concluded that the system is capable of detecting ing from the large amount of data requiring defects in the head, web and base of the railway track. analysis [43]. The maximum inspection speed currently Field tests have been carried out at up to 40 km/h but, achievable with the ultrasonic phased array method is according to Nielsen et al. [50], this is the maximum mea- approximately 5 km/h [2]. Manual phased arrays are surement speed with their inspection system. used, for example, in the validation and verification of

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6.2 Guided wave inspection surement speed will be around 60 km/h, due to the physi- Guided wave inspection utilises low-frequency waves in cal limitations of ultrasound travelling in railway track the region of 10-100 kHz, which travel in the longitudinal steel. Calibration of the ultrasound units is an important direction of the railway track and bounce back from a step in the whole inspection. Simulated reflectors can be transverse defect. Guided waves propagate along the rail manufactured to represent real rail defects which can be and thus are ideal for detecting transverse defects[51]. used to determine the inspection system’s ability to detect Rose et al.[52] presented different combinations of guided defects. Some new studies have been made in the utilisa- wave inspection of railway tracks which could have poten- tion of phased array ultrasound measurement and com- tial. Guided wave inspection can be carried out, for exam- puted tomography in the detection of defects. However, ple, with fixed sensors on rails in places like switches these methods need further development before they can where the probability for defect detection is high and the become commercial automated systems with widely use of other conventional NDT methods is restricted. This acceptance in railway track inspection protocols. has been demonstrated by Loveday et al.[53]. The system could be installed on a rail inspection vehicle where the Acknowledgement ultrasonic transducers are mounted on both ends of the inspection vehicle to induce the guided wave energy on This work was supported by the Finnish Transport one end and receive from the other end. One possibility is Agency and Foundation of Emil Aaltonen. also to use sensors on a train.[52] References 6.3 Electromagnetic acoustic transducers, EMATs 1. Shull, P.J. (2002). Nondestructive Evaluation: Theory, Tech- Non-contact electromagnetic acoustic transducers, or niques, and Applications, CRC Press. EMATs, generate and detect ultrasound in electrically con- 2. Final technical activity report. Non-contact Ultrasonic Sys- ducting or magnetic materials. An EMAT does not need a tem for Rail Track Inspection, Project no, 507622, available: coupling between the railway track and the probe but a http://cordis.europa.eu/publication/rcn/13149_en.html, 19.4.2016. lift-off of up to 10 mm is reported by Petcher et al. in the 3. Clark, R. (2004). Rail flaw detection: overview and needs case of railway track inspection[54]. The studies of for future developments, NDT&E International, Vol. 37, pp. et al. Petcher [54] utilised a pitch-catch conﬁguration for 111-118. EMATs, creating guided surface waves. The benefit of 4. Final report. (2008). D4.4.1- Rail Inspection Technologies, EMATs is that they operate without a coupling[43], how- Innotrack, available: http://www.innotrack.net/IMG/pdf/d441. ever, there are still problems if the lift-off is too big[46]. pdf, 19.4.2016. Utilisation of EMATs for railway track inspection has also 5. Heckel, T., Thomas, H.M., Kreutzbruck, M., Rühe, S. been introduced in the studies of Rose et al.[55]. As (2009). High speed non-destructive rail testing with Petcher et al. conclude, the EMAT still requires develop- advanced ultrasound and eddy-current testing techniques, ment before it is suitable for use in a commercial Indian National Seminar & Exhibition on Non-destructive environment [54]. Evaluation, NDE 2009, December 10-12 2009. 6. Meisser, K., Petrusch, R. (2016). Rail track inspection on new rails, An economical alternative, Available: http:// 7. Conclusions www.pethoplan.de/downloads/Rail%20track%20inspection% 20on%20new% 20trails.pdf, 20.4.2016. This study reviewed the recent progress in the inspec- 7. Marques, R.S., (2016). Desenvolvimento de um método de tion of rails of railway tracks using ultrasound-based meth- inspecção de carris ferroviários por ultrasons, dissertation, ods. The paper reviews the technologies currently Available:https://fenix.tecnico.ulisboa.pt/downloadFile/ employed along with examples of recent field applica- 395143172282/Dissertacao.pdf, 17.4.2016. tions. The current automated inspection systems include 8. De Ruvo, P., De Ruvo, G., Distante, A., Nitti, M., Stella, E., many different methods along with ultrasound inspection. Marino, F. (2008). A Visual Inspection System for Rail The two most common automated methods are either to Detection & Tracking in Real Time Railway Maintenance, install the ultrasound probes inside a fluid-filled wheel, or The Open Cybernetics and Systemics Journal, Vol. 2, pp. 57- roller search unit, or attach them to a sliding plate sled. 67. However, there are more restrictions on measurement 9. Hellier, C.J. (2003). Handbook of Nondestructive Evalua- tion, McGraw-Hill. speed in the case of a fluid filled roller search unit. In 10. Kalay, S., LoPresti, P. (2013). Development of Technologies addition, several studies have concluded that the top mea-

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