Proceedings of the National Seminar & Exhibitionon Non-Destructive Evaluation

NDE 2011, December 8-10, 2011

INTRODUCTION

The Indian Railways is the fourth largest railway networkhaving 114,500 km of total track over a route of 65000km.Itcarries over 30 million passengers and 2.8million tons offreight daily. High speed trains and heavy load traffic arebecoming common in the recent days. Majority of the failedin-service rails are attributed primarily to the internal defectsin the web and head region that propagates easily.The annualdamage to life and property due to rail accidents is enormous.The development and implementation of improved railmetallurgies and maintenance programs are expected todecrease rail flaws. Through increased reliability of flawdetection technologies, a decrease in rail failures and accidentsare expected [1].

Main causes of the defects [7] in rail are

1. Manufacturing defects in material such as non-metallicinclusions, hydrogen flakes which could be present inmaterial even after a set of non-destructive testingconducted during quality assurance.

2. Residual stresses induced during manufacturing.

3. Defects associated with welding.

4. Thermal stresses due to large variations in railtemperature.

5. Dynamic load acting on rails while in-service.

STUDIES ON FEASIBILITY OF ULTRASONIC PHASED ARRAY IN RAIL INSPECTION

Girish.N.Namboodiri1, Krishnan Balasubramaniam2, T.Balasubramanian1 and G.Jothinathan2

1 National Institute of Technology, Tiruchirappalli2 Indian Institute of Technology, Madras

ABSTRACT

Phased Array (PA) Ultrasonics which performs electronic and sectorial scanning could be used for detection andsizing of defects in rails. The analysis of PA images concentrates on defects that are internal and volumetric in rail,which is one of the most critical components in the transportation field. Failure in rail welds is an on-going concernfor rail safety. Fatigue fracture is usually the most common cause of rail failures. These fractures are formed due to theimperfections present in the material and also due to crack formations when in-service. A means to detect flaws usingphased array ultrasonic techniques promises to minimize catastrophic failure and provide paths to perform preventativemaintenance. It also provides a quantitative analysis of the severity of the defects, including sizing and classificationof the defect so as to support a decision of accepting a newly welded rail or to repair a defect as part of a maintenanceschedule. Phased Array technology produces certain beam characteristics by time shifting the pulsing and receiving ofeach array element [6]. These characteristics include beam focusing and skewing, which could be useful in testing ofrails.

Keywords: Phased Array, Sectorial Scan, Half-Moon Crack, Thermite Weld, Bolt-Hole Cracks

Defects in rail [7] could be of the following types

1. Horizontal cracks in head: cracks that run parallel to raildepth at a depth of 10-20mm, which could finally leadto crushing of rail head in some cases.

2. Horizontal cracks in the head-web junction: It could leadto rail head separation.

3. Horizontal crack at web-foot junction: These cracks coulddevelop either towards head or foot.

4. Vertical longitudinal splitting of web: due to heavyaccumulation of non-metallic inclusions.

5. Bolt-hole cracks account for about the 50% of the raildefects in joined tracks. Fretting fatigue of the bolt shankagainst the hole surface is believed to be the principalcause of this typology of crack. These cracks are usuallyformed due to fatigue produced as a result of cyclicstresses combined with stress concentrations in the boltholes [3].

6. Transverse cracks in rail foot: These cracks which developin the rail foot grow as half-moon cracks.

7. Weld cracks and other defects like porosity, slaginclusion, lack of fusion which may be introduced dueto improper welding.

Alumino-thermic welding process provides us with flexibilityand low capital cost, but it shows variable quality in finished

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welds, due to process limitations and operator dependency.The flash butt welds usually exhibit high tensile residualstresses at the level of the rail web [10], which are detrimentalfor fatigue service conditions. As a result, it is common tofind failures starting at butt-welded joints.

An attempt has been made to detect these defects usingultrasonic phased array. The advantage of taking a sectorialscan, where a steering through a sequence of angles is possiblewas utilized for the inspection. The capability of these probesto detect cracks effectively and its flexibility could beprofitably used in inspection of defects in rail.

THEORY

Phased array probes consist of a set of piezoelectric elementsarranged in an array. When excited, the elements producesultrasonic waves, which interacts with each otherconstructively or destructively leading to an increase ordecrease in the resultant wave energy respectively. By varyingthe time at which these elements are excited, it is possible touse these effects to both steer and focus the resulting combinedwavefront. This is the basic principle behind Phased arraytesting. Software called Focal Law Calculator is used toestablish suitable specific delay times for firing each group ofelements, so that the required beam shape could be generatedthrough wave interactions. It also takes into account the probeand wedge characteristics as well as geometry and acousticalproperties of the material while establishing the focal laws[6].

Features that are possible using the above principle areelectronic linear scan and sectorial scan. In electronic linearscan, the same focal law and delay are multiplexed across agroup of active elements and the entire elements would beinvolved in the scanning, with only a set of elements activatedat an instant. Scanning will be done at a constant angle andalong the total aperture length. In sectorial scan, the beamwill be moved through a sweep range for a particular focaldepth. The beam steering taking place during S scan maps thecomponents at appropriate angles, optimizes the probabilityof detection of defects [6]. Electronic focusing optimizes thebeam shape and size at expected defect location and also canimprove the Signal to Noise ratio significantly.

EXPERIMENTAL SETUP

Equipment Used was Omniscan MX, which is portable, light-weight, rugged, battery operated. It has fully featured A scan,B scan and C scan displays which helps in better visualisationof the defects. It has advanced real-time data processingcapability also. Filters are available which could be used toenhance the A scans and also to improve the image quality[6].Rail height = 172mm, Material velocity

= 5890 m/s (longitudinal).Probe and Wedge selection were done and their specificationsare shown in Table 1. Number of elements activated was 16,as in sectorial scan only fixed number of elements could beactivated at a time. Focal depth was fixed at 20mm. A rangeof 180mm was selected.

Table 1 : Probe Specifications

Probe Wedge Frequency Number of Pitch ElementNo. (MHz) Elements (mm) Size(mm)

5L64A2 SA2N60L 5 16 0.6 0.6

Probe was placed on the flat area on the running surface(Figure.1), where proper contact was possible. A sectorialscanning was performed with angle ranging from 300_600 insteps of one. A few linear scans at 00 were also performed tohave a better understanding of the defects. An optimum pulserepetition frequency was selected. Couplant used during testsis water.

Data acquisition was done at each instant and analysed withtomoview software and defects were located. True depth, angleand sound path of the signal received from defect was noteddown, which helped to locate the position of defect.Inspections were also focussed on Alumino-thermic and flash-butt welds that were present in the sample.

RESULTS

In Figure 2a shown below, a bolt-hole crack is there whichstarts from near the bolthole surface and propagating at anangle of near 450 from the vertical till the web-railheadjunction. When a linear scan at 00 was conducted, a very niceindication of the crack was seen, which gave a good idea abouthow the crack had propagated. The indications in linear scan(Figure 2c) were from depths 77.55mm, 74mm and 70mmrespectively. In sectorial scan (Figure 2b), indication other thanthat from bolt hole came at depth 75mm, which shows clearlythat it is from the bolt-hole crack.

Cracks can also be formed in the weld. They are thermite-joint horizontal cracks (Figure 3a). Here in the sample rail, acrack is seen in the alumino-thermic weld which is shownbelow. The indication is from a depth of 110mm, which clearlygives an idea that the echo is from the crack in the weld(Figure 3b).

Internal Cracks can develop in the head region (Figure 4a).These horizontal cracks could be easily detected by sectorial

Fig. 1 : Probe position

NDE 2011, December 8-10, 2011 247

Fig. 2 : a) Crack region between bolt holes b) PA sectorial image picking up the crack along with indication from bolt holec) PA linear scan at 0o

Fig. 3 : a) Crack in the thermic weld b) PA image showing the defect

Fig. 4 : a) Head of rail b) Crack indication in sectorial scan c) Crack indication in linear scan

248 Girish.N.Namboodiri et.al : Proceedings of the National Seminar & Exhibition on Non-Destructive Evaluation

2. John Wilson, GuiyunTian, IlhamMukriz, DarrylAlmond,“PEC thermography for imaging multiple cracks fromrolling contact fatigue”, NDT&E International 44 (2011)pp.505–512.

3. G. Zumpano, M. Meo, “A new damage detectiontechnique based on wave propagation for rails”,International Journal of Solids and Structures 43 (2006),pp.1023–1046.

4. R/D Tech, (2004), Introduction to Phased ArrayUltrasonic Technology Applications, R/D Tech Guideline,1st ed., R/D Tech Inc., Quebec, Canada

5. Azar L., Y. Shi, and S.C. Wooh, “Beam FocusingBehaviour of Linear Phased Arrays”, NDT and EInternational, 33(2000), pp.189-198.

6. “Phased Array Testing: Basic Theory for Industrialapplications”, Olympus NDT, First edition, November2010.

7. “Manual for ultrasonic testing of rails and welds”,Research designs & standards organization, Ministry ofRailways, Government of India, Revised-2006, pp.95-97.

8. U. Zerbst , R. Lundén , K.O. Edel , R.A. Smith,”Introduction to the damage tolerance behaviour of railwayrails – a review”, Engineering Fracture Mechanics 76(2009), pp.2563–2601.

9. Y. Fan, S. Dixon, R.S. Edwards, X. Jian, “Ultrasonicsurface wave propagation and interaction with surfacedefects on rail track head”, NDT&E International 40(2007), pp. 471–477.

10. Anders Ekberg, Elena Kabo, “Fatigue of railway wheelsand rails under rolling contact and thermal loading—anoverview”, Wear 258 (2005), pp.1288–1300.

Fig. 5 : a) Alumino-thermic weld with half-moon crack region b) Sectorial image of crack

scan of the head region. Indications shown in Figure 4b werefrom a depth of 30mm and 36mm. In linear scan at 00, a clearindication of the crack was seen (Figure.4c), with each strongsignal picked up by different elements giving different depthsof 24mm, 26.5mm and 28mm respectively.

Half-moon crack is a defect which usually occurs at the bottomof alumino-thermic weld (Figure 5a). The image shown inFigure 5b is indicating that the signal is coming from a depthof 167mm, which means that the crack is of 5mm depth fromthe bottom. The indication clearly tells us that it is an echofrom the Half-moon crack.

CONCLUSION

The results show that this method of inspection was capableof detecting all defects irrespective of orientation and nature.It can detect defects that lie in head, web and foot of the rail.When compared to conventional ultrasonic testing, the resultswere much better. Apart from detecting the defect, its severitycould also be checked. The scanning speed is the onlylimitation as of now. Therefore it could well be used fordetection of defects in the welds used in rails. Further researchwork will be focussed on alumino-thermic welds and flash-butt welds.

REFERENCES

1. Greg Garcia, TTCI, Jinchi Zhang, Olympus NDT,“Application of Ultrasonic Phased Arrays for Rail FlawInspection”, Federal Rail Road Administration, July2006.