Key word: Ultrasonic Testing, POD, rail, online NDT,detectability,safety

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1 More Info at Open Access Database Probability of Detection of Online Ultrasonic Testing of Rail Debdutta Mallik, Saroj Kumar Ojha, Subhash Bansal, B.Lakshminarasimham Jindal Steel & Power Limited, Raigarh (Chattishgarh),India e mail : debdutta.mallik@jspl.com, sk.ojha@jspl.com, subhash.bansal@jspl.com bln@jspl.com Key word: Ultrasonic Testing, POD, rail, online NDT,detectability,safety Abstract : Rail travel is one of the most convenient and affordable mode of transport in India. The quality level required for rail is very stringent due to it s criticality in our day to day life. Manufacturing of rail passes through several stages in the steel plant. The quality levels require assessment of material both in surface and internal condition. Ultrasonic testing has emerged as the most reliable mode of assuring the internal soundness of the material. Detectability of desired minimum size of defect is one of the most important required criteria which are confirmed by calibration of a test piece having artificial flaws. At the same time the factor probability of detection ensures capability as well as reliability of the ultrasonic testing system and testing process also. This paper deals with various aspect of online ultrasonic testing of rail and study on probability of detection thereon. Introduction: Non Destructive Testing has a great importanace and widely used in several seagment of industry for assessing the product whether it is safe or fit for use. Like other hot rolled products, rails(for railway track) also require very reliable NDT method to ensure it's intended purpose of service. For ensuring the internal quality of unsoundness there are method like Radiographic Testing,Ultrasonic testing are used. However, Ultrasonic Testing has been choosen and widely used in rail industry as Ultrasonic Testing is more productive & economical than any other method to verify the internal soundness of rail. Rail is with very complex geometry compare to any other hot rolled product, so design requirent of probe configuration is quite critical to comply the testing condition and standard compare to any other flat sections.since the technique is online(i.e. Automatic),it is very dificult task also to ensure detactability in case of Online/Automatic Ultrasonic testing of rail. Online Ultrasonic testing system of rail detects internal imperfections(either inherent or processing) in rail head,web and foot area(fig.1). The detectable defect size, and the reliability with which it can be detected, are dependent on many factors like surface finish,mechanical set up,probe condition,speed etc. Any failure in testing system means, a rejecetd rail may go undetected and may cause a severe failure and loss of national property and human life which is unrepairable. Online/Automatic Ultrasonic testing of rail has played a vital role towards safety of rail mainly in last two decades.now a days, it has been a mandatory requirement for all rail(for flat bottom railway track) manufacturer to process 100 % produced rails through online ultrasonic testing machine as requirement of standards to ensure safety and reliability. In this context of demand,jindal Steel & Power Limited,India as one of the major producer of rail track has adopted the most latest technology for online ultrasonic testing of rails and has been following more than a decade. In this paper we estimate POD followed by statistical model to verify the capability of on-line ultrasonic testing system of rail and to assess the minimum flaw size that will be reliably detected.

2 Brief Introduction about online NDT system of rail: Flow Chart of online NDT system of rail The online NDT system consists mainly following units: 1. Brushing Machine - To remove loose scale,dirt etc. 2. Flatness Measurement gauge: To check straightness deviation. 3. Eddy Current Testing Machine: To detect surface/sub-surface discontinuities. 3. Ultrasonic Testing Machine: To detect internal discontinuities. 4. Paint Marking Unit: True to location marking for suspected rails by respective units by different colour. Configuration of Online Ultrasonic Testing System: An unique set of probes have been configured as per requirement of different codes & standards at JSPL. The present system fulfills the rquirement of different national and international standard of flat bottom railway rails lke IRS-T-12,Euronorm etc. A set of 16 probes at different location of rail ensure the requirement of all standards either by detcatability or by scanning coverage. A general distribution of all probes is given in Figure.1. Figure.1 Distribution of probes locationwise

Testing Methodology: The primary requirement of online ultrasonic testing of rail is to calibrate the system followed by a test piece having artificial reflector

Since it is an automatic system so the requirement of detectability at desired testing speed is a key factor to ensure a reliable testing.

simulated flaws on calibration rail followe d by standards like IRS T 12(Indian Standard), EN 13674(European Standard),AREMA(American Standard) etc.

3 Testing Methodology: The primary requirement of online ultrasonic testing of rail is to calibrate the system followed by a test piece having artificial reflector as recommended in standard. Since it is an automatic system so the requirement of detectability at desired testing speed is a key factor to ensure a reliable testing.like other automatic testing, online ultrasonic testing of rail requires calibration at desired speed(i.e. Testing speed of production rails) against artifcially simulated flaws on calibration rail followe d by standards like IRS T 12(Indian Standard), EN 13674(European Standard),AREMA(American Standard) etc. Rails marked as suspected are marked by paint at true to location. A typical result view of rejected and accepted rails are given in Fig.2 & Fig.3 respectively. Fig.2 A typical result window of an accepted rail (single indcation) (multiple indication) Fig.3 A typical result window of an rejected rail(single indication and multiple indication)

4 Why Probability of Detection: It is very important to ensure the detactability at desired speed for a reliable and successful testing which is verified by a calibration rail having artificially simulated flaws as per respective standards. Since the system is automatic(i.e. Testing in dynamic condition), so there are several factors like mechanical set up,improper alignment,unstable electronics,rough surface etc may affect the test result mainly when the geometry is complex in nature.so far rail is concerned we can not take a chance to miss the defect. The reliability of any NDT system is commonly characterised in terms of the probability of detection (POD,) of a specified type of defect as a function of defect size, a. The purpose of a POD trial is to obtain an estimate of the POD by acquiring suitable experimental data and conducting an appropriate statistical analysis. There are four possible outcomes in an online ultrasonic inspection of rail and these four options constitute the probability matrix of detection : No flaw exists and the Ultrasonic method indicates a flaw present (False Positive) An item is flawed and the Ultrasonic method detects it (True Positive) An item is flawed and the Ultrasonic method does not detect it (False Negative) No flaw exists and the Ultrasonic method has no indication of a flaw (True Negative) By estimating probability of detection through statistical approach gives an idea about the capability of the system. POD Approach: In view of railway safety, a POD model has been derrived based on the study carried out against 1000 numbers of trial against different size of flaws(each). Some statistical method of modelling were analyised to determine POD curves but it is found that the log odds distribution was the most consistent distribution for determining a POD curve as a function of flaw size a compare to other probabilistic approach. The log odds distribution is mathematically defined as POD(a) = exp(α + β.ln(a)) / 1 + exp(α + β.ln(a)) Eq.1 where POD(a) is the mean probability of detection of cracks size a and α and β are the parameters of the log odds curve from the calculated data

5 PROBABILITY OF DETECTION vs FLAW SIZE POD FLAW SIZE Fig.4 POD curve plotted against artificial flaw simulated in rail Discussion: We have observed a very high signal to noise ratio,good surface finish and stable mechanical & electronic system resulting a very high degree of POD and ensuring the capability of testing. A high degree of POD observed, ensures capability of online Ultrasonic testing system for rail. In the graph(fig.4) it is clearly indicative that POD value is less than upto 1.5mm size of flaw but 1.5mm and onwards the POD value is or more, which concludes we can ensure a complete reliable testing in case detectability of 1.5mm more by this NDT system. Conclusion: As on date, the detectability requirement of national & international standards like IRS T 12, EURONORM 13674,AREMA etc is 1.5mm and above. Since the system is capable to detect the flaw 1.5mm and above with POD value 0.998, it can be concluded that the present online ultrasonic testing system of rail can fulfill the requirement of all national and international standards with high degree of reliability. Acknowledgement : We are thankful to te management of JSPL for permitting to publish this work. References: [1] E. Ginzel - Materials Research Institute, Canada - Introduction to the Statistics of NDT [2] David S. FORSYTH, TRI/Austin, Austin, TX USA, John C. ALDRIN, Computational Tools, Gurnee, IL USA, Build Your Own POD. [3] Raymond B. MABUZA, Department of Nondestructive Testing & Physics, P/Bag X021, Vanderbijl park, 1900,Johannesburg, South Africa, Numerical Analysis of Probability of Detecting Defects in Engineering Materials. [4] ASNT Handbook, Volume 7 [5] ASM Handbook, Volume 17 [6] MIL Handbook, 1823A [7]

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1. SCOPE ELIGIBILITY EXAMINATION CONTENT RENEWAL & RECERTIFICATION PROCEDURE ESSENTIAL READING... Certification Services Division Newton Building, St George s Avenue Northampton, NN2 6JB United Kingdom Tel: +44(0)1604-893-811. Fax: +44(0)1604-893-868. E-mail: pcn@bindt.org PCN/GEN ISO 20807 Appendix