Session 2A (2) – Phased Array Applications Chairman – Prof ...

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14.55 Automated inspection of alumino-thermic rail welds Authors - Tamara Colombier, Elena Jasiuniene, George Kotsikos,

Joe Small and John Rudlin

There are millions of aluminothermic welds made each year to join the rails in Europe’s rail network and millions are already in-service. Occasionally flaws develop during and shortly after welding that can lead to early failure and this can cause disruption to train services and costs to both the train operating companies and the maintenance operators.

Although radiographic inspection is sometimes carried out, this is not very common, and the conventional ultrasonic procedure requires many probes and scans because of the complex geometry. As the welds are made by a casting process, these can be more difficult to inspect than joints made by other welding processes. Hence, the RAILECT project was conceived to provide a convenient means of inspecting the joints using a multiple phased array ultrasonic system.

This paper describes the type of flaws of concern, the modelling of the ultrasonics, the practical realisation of a prototype system and the development of acceptance criteria.

Session 2A (2) – Phased Array Applications Chairman – Prof B Drinkwater

Copyright © TWI Ltd 2010

Automated Ultrasonic Phased Array Inspection of Aluminothermic Rail Welds

T Colombier, TWI Ltd

Cambridge CB21 6AL Telephone +44 1223 899352 [email protected]

J Rudlin, TWI Ltd, UK

E Jasiuniene, Kaunas University, Lithuania G Kotsikos, Newcastle University, UK

Abstract There are millions of aluminothermic welds made each year to join the rails in Europe’s rail network and millions are already in-service. Occasionally flaws develop during and shortly after welding that can lead to early failure and this can cause disruption to train services and costs to both the train operating companies and the maintenance operators. Although radiographic inspection is sometimes carried out, this is not very common, and the conventional ultrasonic procedure requires many probes and scans because of the complex geometry. As the welds are made by a casting process, these can be more difficult to inspect than joints made by other welding processes. Hence, the RAILECT project was conceived to provide a convenient mean of inspecting the joints using a multiple phased array ultrasonic system. This paper describes the type of flaws of concern, the modelling of the ultrasonics, the practical realisation of a prototype system and the development of acceptance criteria. 1. Introduction The majority of field welding in the railway industry is carried out using aluminothermic procedures, such welds are primarily associated with replacements of rails or welds defects, installation of insulated rail joint and track construction activities. In the UK, it is estimated Network Rail produce over 65,000 Thermite welds per year and they have approximately 1.5 million in track. Thermite welding is an effective, highly mobile and cost effective casting method of joining heavy steel structures such as rails, however this is also a very skilled welding process which requires experience and expertise. The many process steps can be altered by the welder and/or the environmental conditions and can result in poorer weld performance and in creation of defects within the weld. Up until this date, only a few techniques of inspection have had some limited success to control the quality of these welds. For instance, manual ultrasonic and radiography techniques are sometimes carried out on the welds but this is not common use in the railway industry. Most of the

Copyright © TWI Ltd 2010 2

time, only visual inspection is performed. Every new weld is inspected using visual techniques for weld defects, profile and geometry. Although there is no requirement to inspect rail welds using ultrasonics MPI or DPI, there is a conventional ultrasonic procedure EN 14730-1:2006 Annex C developed specifically for the inspection of rail welds. However, EN 14730-1:2006 Annex C describes a relatively long and complex inspection. It involves many ultrasonic probes and scans because of the complex geometry of the weld profile and hence this technique is used in very rare occurrence. As the welds are made by a casting process, these can be more difficult to inspect than joints made by other welding processes and as a result visual inspection is currently the only technique regularly used to inspect aluminothermic rail welds. Rail flaw detection has an important part to play in ensuring the safety of the railroads. Recent accidents such as the Hatfield disaster, UK in 2000 caused by broken rails have focused attention on the technologies that enable the detection of flaws in rails and rail welds. The RAILECT project is a European funded project which aims to develop a novel automated ultrasonic phased array system of inspection for rail welds. The project is a 28 month project which is currently still undergoing. It gathers three research centres from two European countries Kaunas University (Lithuania), University of Newcastle (UK) and TWI (UK) who have been working towards the developments of an innovative ultrasonic phased array system for the benefit of four SMEs originated from three different European countries: Optel from Poland, Vermon from France, Spree Engineering and Kingston Computer Consultancy from the UK. Each SME has its own specific products and services that the RAILECT project is aiming to develop. 2. Concept

2.1. Rail profiles and weld dimensions The project focused on three different types of rail profiles CEN 60E1, CEN 60E2 and CEN 56E1 (Figure 1).

Figure 1 Dimensions of the three different rail profiles

CEN 60E1 CEN 60E2 CEN 56E1


These three rail profiles are in accordance with the European standards and are commonly used within Europe. CEN 60E1 and CEN 60E2 are the most recent profiles whereas CEN 56E1 is an older version. New welds follow the most recent profiles but many welds already in service are more likely to be CEN 56E1. Hence, it was decided that ideally the prototype should be able to fit these three rail weld profiles. The idea was to create a versatile prototype adapted for the quality control of both new and already in service aluminothermic rail welds. Although CEN 60E1 and CEN 60E2 differed slightly in terms of the head geometry, this was relatively negligible when compared to the rail profile CEN 56E1. This latter has a smaller web height i.e. 158.75 mm instead of 172 mm for the other two rail profiles. The weld width of concern for the project was 35 mm (Figure 2), although the finished cast range can vary from 35 mm to 90 mm.

Figure 2 Picture of a 35 mm aluminothermic rail weld in service The first iteration of the RAILECT prototype fitted the two most recent rail profiles but not the oldest one. The challenge was at this stage to produce a mechanical system that could be used on the three rail weld designs. A second iteration was allocated to this task and the mechanical frame is currently under development so that this challenge can be addressed.

2.2. Defect types The project focused on three types of defects namely porosity, lack of fusion and shrinkage (Figure 3).

Figure 3 Examples of defective samples. From left to right: porosity, lack of fusion and shrinkage defects.


These defects were considered the most common type of defects leading to potential failure of the rail weld when in service and hence critical to rail safety. The rail foot was more predisposed to the lack of fusion defect whereas the shrinkage defect was more likely to be located in the web of the rail. Unlike porosity, lack of fusion and shrinkage were more likely to be found in specific areas of the rail weld. These observations were critical features to take in account for the design of the ultrasonic phased array system.

2.3. Sample manufacture In total, 20 rail welds were manufactured for the purpose of the project. Five samples were allocated for each type of defect and a set of height reference samples corresponding to non defective welds were produced (Table 1). For each type of welds, three samples were dedicated to the fatigue testing and two samples were reserved for the ultrasonic phased array inspection with the prototype. Additional mechanical testing was performed on the non defective welds for the purpose of the Engineering Critical Assessment.

Table 1 Rail welds manufactured for the purpose of the Railect project

Samples manufactured Weld characteristics 8 Reference samples - no defect 5 Porosity 5 Lack of fusion in the foot 5 Shrinkage in the lower web

Total = 23 samples After being manufactured, every sample was inspected using both X-rays and manual ultrasonic testing in accordance with EN 14730-1:2006 Annex C so that one could be able to compare the capability and reliability of the different techniques of inspection with the ultrasonic phased array system to be developed.

2.4. Industrial requirements The device to be developed was expected to be:

Portable Weatherproof and functional in inclement weather Easy to operate Give clear indications of defective welds

Furthermore, the system had to inspect rail welds that presented the following characteristics:

Normal Gap width: 25 mm wide (± 2 mm each rail) Gap width range: from 26 to 80 mm The finished cast width: 35 mm (Normal Gap) Finished cast range: from 35 to 90 mm Gap between sleepers: from 340 mm minimum - welds are not always placed at

the centre of the spacing between sleepers


The majority of the work performed within the course of the project was carried out for normal gap corresponding to 35 mm cast width. 3. Development of the UT phased array system

3.1. Principle

In order to choose the most appropriate design, the Consortium had to consider a design that could provide the most effective coverage of the weld area and a design where data acquisition and analysis could be feasible and relatively fast. The other objective was to minimise the amount of scanning in order to maintain an easy application and analysis of the signal. The coverage of the weld area was investigated using both modelling and experimental tests.

3.2. Modelling The modelling work investigated primarily the coverage of the weld, and the response from different defect types. This was carried out with both ESBeam tool and CIVA softwares. Figure 4, 5 and 6 show some examples of the modelling output. A 32 element phased array probe was considered for the purpose of the modelling experiments. The probe and focal law parameters were varied during the modelling experiments so that the most appropriate settings for full volumetric weld coverage could be identified.

Figure 4 Modelling experiments using Civa software to show the beam coverage of

the weld body using 2 32 element phased array probes on each side of the weld.

Figure 5 Modelling of the beam coverage of the weld body using ESBeam Tool – 32

element phased array probe.


Figure 6 Modelling experiments using Civa software for the detection of two 2 mm diameter side drilled holes separated by a 2 mm gap in the ankle of the rail foot.

Thus, it was shown that a single probe could be used in pulse echo to cover the whole of the web and the central part of the weld foot from one side, and that three separate probes were needed to cover each side of the foot. Further work showed that the head needed an additional probe for coverage of the flaw situations envisaged; this probe would cover the top of the weld head. This meant a total of 8 probes were necessary to inspect one side of the weld. The system would simply be placed on the other side of the weld to complete the inspection and to be able to perform a volumetric inspection of the weld. Probe frequencies of 2 MHz and 5MHz were investigated in the modelling but the selection of the most appropriate frequency for the present application was carried out during the experimental testing. The inspection of the head an the web of the rail weld could be easily covered using two 32 element phased array probes however, the inspection of the rail foot and toes was more complex because of the constraints given by the geometry and dimensions of the rail in these areas. Only small phased array probes containing a limited number of elements were able to fit onto the rail foot. These constraints lead to a more complicated system design that it was originally thought and as a result various design options were suggested and investigated.

3.3. System Design To specify the probes in more detail, it was also necessary to consider the possible instrumentation that could be available. It was eventually decided to use a system with 32:128 capability as this was commercially available in a portable instrument. This meant that the instrument could be able to drive a maximum of 128 channels with 32


channels that can be used simultaneously, 32 corresponding to the maximum aperture. This enabled the probes to be specified as shown in Figure 7.

Probe Element Frequency F-1 16 5MHz F-2 8 5MHz F-3 8 5MHz F-4 32 5MHz

Figure 7 Half automated ultrasonic system consisting of 8 probes. The system has to be turned over to inspect the other side of the rail weld.

For the purpose of data acquisition and analysis, an Omniscan MX and the phased array software Tomoview 2.7 were used jointly to carry out the inspection, record and analyse the data. The focal laws were set up and saved initially but their application could be controlled to enable an automatic sequence. The data output were collected for analysis and compared with the acceptance criteria by means of a MATLAB program.

3.4. Prototype Design The first prototype is shown in Figure 8. This clamped on the side of the head by means of a hand grip and a screw mechanism was used to grip in the vertical plane. The probes for the foot were held in place to pass the head, and moved to align above the foot by means of removal and replacement of a series of pins. The probes themselves were spring loaded onto the surface.


Figure 8 SolidWorks drawing of the mechanical design of the half automated phased array system.

4. Acceptance criteria

4.1. Mechanical tests The majority of the mechanical testing was performed for the purpose of the ECA calculations. The following tests were carried out:

Hardness testing – BS EN 14730-1:2006 Tensile testing – BS EN 10002-1: 2001 Fracture toughness testing – BS 7448-1:1991 Fatigue testing – BS EN 14730-1:2006

Results for the fatigue testing showed that generally non defective welds failed in the heat affected zone whereas defective welds failed within the weld, stresses being introduced by the defects.

4.2. ECA results ECA calculations were carried out for different areas of the rail weld in accordance with procedures prescribed in BS 7910:2005. Both surface breaking flaw and internal flaws were assessed. Sensitivity analyses were carried out to determine some critical conditions such as flaw geometry and relative distance to surface for internal flaws. A Finite Element model was also created to assess the accuracy of the analytical solutions developed with the ECA calculations. The model is based on the parameters provided by fatigue test activities. The modelling is shown in Figure 9.


Figure 9 Finite Element modelling built from the results of the mechanical testing. 5. Prototype trials

5.1. Laboratory trials

Laboratory Trials were carried out to verify the correct operation of the focal laws and the detection of defects. Some of the results are shown in the Figures below. Figure 10 shows the validation of the model described in Figure 6 related to the inspection of the rail foot. The two side drilled holes were detected using the probe and focal law parameters given by the model.

Figure 10 Sectorial scan performed on the ankle of the rail foot containing two

2 mm side drilled holes in the rail foot - 16 element transducer (5MHz frequency).


Figure 11 shows the scan obtained for the inspection of the web of the rail. In this case, the scan is given by the probe F-4 located on top of the rail at a selected probe offset with regards to the rail weld. Both non defective and defective welds are illustrated on the Figure below. One might notice that the bottom of the rail weld can be easily seen on the scan, this indication could be used as a reference position when carrying out the inspection.

Web

Foot

Web

Foot

Web

Foot

Web

Foot

Web

Foot

Web

Foot

Figure 11 Inspection of the weld web using 32 element phased array transducer, 5MHz frequency - a) beam configuration, b) non defective weld, c) weld

containing porosity.

When compared with the conventional manual UT technique used for the inspection of rail welds, the phased array system was more successful at detecting defects.

5.2. Field trials Trials were carried out on site at the Barrow Hill Railway Centre, Chesterfield (Newcastle University Test Track). Figure 12 shows the demonstration of the first prototype.

a)

b) c)


Figure 12 Demonstration of the RAILECT prototype at the Barrow Hill Railway Centre.

Although it was shown that the system was capable of being deployed on site, the mechanical system operation was too fragile for normal use and it was decided to build a second prototype. The mechanical frame of the second prototype is currently being designed. A simpler clamping system is under development so that a more field and operator friendly mechanical system can be produced. 6. Conclusion The ultrasonic phased array system produced within the course of the RAILECT project proved the feasibility of an automated phased array system for inspection of rail welds. Modelling studies were validated by the experimental trials. Probes locations and parameters such as focal laws parameters were investigated and optimized so that the maximum volumetric coverage of the rail weld could be achieved. If adopted by the railway industry, the system could be one of the first automatic systems of inspection allowing rapid and simple detection of defects in rail welds and enhancing rail safety significantly. Although the system developed validated the RAILECT concept and proved to be reliable and technically valuable, further improvements can be made towards future adaptability to various rail profiles, portability and durability to make the RAILECT ultrasonic system more attractive, durable and handy. 7. Acknowledgments The authors would like to acknowledge the European Community's Seventh Framework Programme managed by REA-Research Executive Agency for the funding of the RAILECT project and Joe Small from Jarvis who provided much practical input on the railway requirements.

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