Development of a Phased Array Inspection System for the Testing of Railway Solid Axles from the End Face

Introduction

AxleInspect is a collaboration between European Small to Medium Enterprise (SME) companies, research organisations and a railway maintenance provider (end user). It is supporting the efforts of the rail industry in achieving better safety standards by developing novel methodologies and techniques for the inspection of solid axles from the end face. As part of the developments in the project, a new semi-automatic PA ultrasonic system is being developed.

The increasing trend for the industry’s business is forecast to continue in the next ten years, since rail transportation is steadily becoming a more attractive option over other means of public transportation. As a result, today’s rail networks across Europe are getting busier with trains travelling at higher speeds and carrying more passengers and heavier axle loads than ever before. The loads and stresses which axles are subjected to are often inadequately defined. Rolling stock, including passenger and freight wagons, is designed to last for thirty years, but is often used for much longer. The combination of these factors has put considerable pressure on the existing infrastructure, leading to increased demands in inspection and maintenance of rail assets. The expenditure for inspection and maintenance has grown steadily over the last few years without being followed by a significant improvement of the industry’s safety records. As a direct consequence one of the immediate key challenges faced by the rail industry is the improvement in the safety of the railway systems, in both environmental and financial terms, by delivering further efficiencies and exploiting technological innovation.

The structural integrity of wheelsets used in rolling stock is of great importance to the rail industry and its customers. Several rail accidents have been directly related to the failure of axles, leading to increased demands for the inspection and maintenance of such components. The risk of axle failure relates to both distance covered and time in use. Therefore, it is preferably for more frequently scheduled inspections to occur during the lifetime of the axle in order to detect any growing fatigue cracks.

Indeed, strict rail standards such as the Rail Way Group Standards exist to ensure that axles are inspected, and at regular and frequent intervals, to ensure safety is maintained. Depending on the train service operator, it is not unusual for axles to be checked by in-service inspection every 30,000km. To be cost effective and to cause minimal disruption to train services, the NDT inspection should not require the disassembly of the wheelsets and supporting bogies from the vehicles.

Currently the rail industry has available a number of methods to inspect axles that suffer from the problem that full access to the axle is required in order to perform a full inspection. This necessitates that for inspection of an axle, the associated bogey must be fully removed from the vehicle, and the wheelset fully disassembled from the bogey (including the removal of ancillary gear such as breaks and bearings). The resulting out-of-service disruption means that inspection cannot be carried out frequently, and can only take place at major overhaul maintenance.

A number of systems have been developed for the inspection of railway axles, mainly during the manufacturing stage, to determine the quality of these components. However, these systems are often expensive and cannot be deployed in-situ due to their large size.

Modelling approach

The developed PA transducer should have the capability of steering and skewing the ultrasonic beam, compensating for defect misalignments with respect to the main ultrasonic beam. This 2D steering capability is very important to the project for three reasons : firstly, since the target is to inspect the axles from their end face, the beam steering is essential in order to cover the required critical areas of the axle, which are the areas of cross-sectional changes and the wheel seat region; secondly, it will result in better defect detection by maintaining the ultrasonic beam perpendicular to the defect face; thirdly, it allows the beam to surpass any access limitations, mainly bolt holes, which are present between the probe and the axle critical areas.

The limit for the number of elements was set to 256 due to the availability of a suitable PA pulser-receiver used to excite the developed PAUT probe (Micropulse 5PA). The frequency was set to 2.25MHz because it is a typical frequency for axle inspection and as defined in the relevant railway inspection standards.

In order to avoid the beam interruption from the boltholes, and to be able to detect defects aligned directly opposite a bolthole, it was decided that the PA transducer would have the capability to operate both in pulse-echo and pitch-catch modes. Pulse-echo would be used for inspection between two boltholes, and pitch-catch for inspection when going over a bolthole. Pulse-echo mode requires only beam steering while pitch-catch mode requires both skewing and steering capability. The elements located on the edges of the transducer would be responsible for the pitch-catch mode operation.

In order to keep the PA element’s number below or equal to 256, and simultaneously acquire better results for focal depth, steering and skewing, an innovative idea has been formed. The linear matrix PA has been constructed by dividing the probe into three areas. The middle area has been constructed by linear array and the two wings have been constructed by matrix array. The beam profiles of the chosen configuration are shown in figure 2. The selected transducer has a focal depth in pulse echo mode of 402mm. In the pitch-catch mode, the probe generates a high amplitude ultrasonic beam and a large focal depth.

The developed PA probe that was manufactured by Vermon SA offers the following capabilities:

• Adaptability to axle end face diameters
• from 110mm to 160mm;
• Operation in pulse-echo mode;
• Operation in pitch-catch mode;
• Excellent beam steering (0-35°);
• Excellent beam skewing abilities (0-15°);
• Large focal length (400mm);
• Great coverage of the critical areas of the axle;

Scanner mechanical design

Phoenix Inspection Systems Limited has undertaken the task of designing the solid axle scanner that will be used to carry out the inspection of the solid axles from the end face. The design and functional requirements to be satisfied by the solid axle scanner are listed below:

• Mount onto a wide range of axles and wheel bearing housings;
• Ability to position the probe to end face diameters from 110-165mm;
• Perform 360° encoded circumferential scan;
• Rigidly hold the large bespoke PAUT transducer and load it onto the inspection surface;
• Provide radial movement to position the transducer as required.

The scanner has been designed in a modular form in order to be adaptable to different train axle box configurations as well as for ease of installation.

The main scanner parts are:

• Rotating ring for 360° circumferential motion;
• Cross slide movement adjusting screw for axial positioning;
• Demountable constant force probe toolpost to maintain surface compliance;
• Static ring;
• Centralising assembly for mounting the scanner;
• PAUT probe;
• Wheel encoder to record the circumferential position.

Figure 3 shows a CAD model of the overall concept where switchable magnetic mounting feet are used to clamp the scanner directly onto the axle in the absence of the axle box. Therefore, the scanner can be used for:

• Inspection of dismantled wheel sets;
• Inspection of fully assembled wheel sets.

Figure 4 shows the CAD model of the solid axle scanner mounted on the axle box. The modular design of the scanner allows it to fit into a range of train bogies and axle box configurations. The lightweight of the scanner allows ease of mounting on the axle box by one operator. The radial position of the probe can be adjusted accurately to inspect different end face diameters and configurations. The scanner allows recording of the circumferential position of the probe during scanning and therefore identifying the location of any defects detected in the axle circumference.

Figure 5 shows different views of the actual solid axle scanner mounted on a full-length railway axle using the switchable magnetic mounting feet. The main parts of the scanner can be seen. Furthermore, the developed PA probe is attached to the scanner in order to carry out the inspection from the end face. The solid axle scanner is easily adjusted to fit in different axle end face diameters. The PA probe can be adjusted depending on the diameter of the axle, and can be tilted to ensure good coupling and compliance with the axle end face.

Experimental set up and results

Railway axle reference samples

As part of the PAUT technique development and testing, two solid axle reference samples with known reflectors were designed and manufactured. Typically, the critical defects for the structural integrity of the axles are transverse cracks. Therefore, machined transverse reflectors with a range of different sizes were introduced in the critical locations of the solid axle.

Reference blocks were manufactured from a full-length solid axle, cut into three sections. Electrical discharge machining (EDM) notches were introduced into the reference samples in regions of most probable failure, which are the wheel seat region and at the cross-sectional transitional areas along the axle (i.e. curvature). In reference sample 1, three surface breaking notches were machined into the surface of the wheel seat, equidistance around its circumference. Figure 6 shows the detailed computer aided design (CAD) drawing of reference sample 1, where three surface breaking notches were introduced at the wheel seat region. In reference sample 2, three notches were machined into both curved surfaces adjacent to the wheel seat, equidistance around the circumference. All the machined notches have varying depth from the axle surface of 5mm, 3mm and 1mm. The width of the notches is 0.3mm.

Experimental results

Figure 7 shows the experimental results acquired from the solid axle reference samples described previously, using the developed solid axle scanner and PA probe. The results shown are from the 1mm deep notch positioned at the three critical areas of the axle. Figure 7d shows the solid axle scanner attached to the solid axle reference samples. Figures 7b to 7d show the PA sectorial scan data obtained from the EDM notches. The geometric reflections from the axle have been identified on the scans and the EDM notches have been detected and positioned accurately. The sectorial scan results were obtained by operating the probe in pulse-echo mode (Figures 7b and 7c) and in pitch-catch mode (Figure 7d). The ultrasonic beam was sweeping between 0 and 30° both in pulse-echo and pitch-catch modes and skewing 15° in pitch-catch mode.

Further laboratory testing on full length axles showed that the developed system is capable of detecting a 1mm deep grinding slots at a distance of 1.2m with respect to the end face surface.

Conclusions

The developed semi-automated solid axle scanner and PA ultrasonic testing technique is a working prototype that satisfies the requirements for the inspection of the railway solid axles from the end face. It can be mounted on a variety of axle box designs and axle end face diameters, and it requires minimal disassembly of the train bogie.

Extensive ultrasonic modelling was carried out to design a PA probe that is capable of meeting the inspection and defect detection requirements as well as being suitable for a variety of axle end face diameters. The probe has been manufactured and tested, and there was an excellent correlation between the experimental and simulation results. The laboratory testing using the innovative PA probe shows that it is capable of detecting a 1mm deep EDM notch positioned at the critical areas of the axle.

Overall, the PAUT testing shows that a high signal-to-noise ratio can be achieved using the completed so

lid axle inspection system. The modular design of the solid axle scanner allows the scanner to be mounted on a wide range of train axle box designs. The scanner is lightweight and can be fitted easily in the train axle box.

Acknowledgements

AxleInspect is a collaboration between the following organizations: Balfour Beatty Rail, Phoenix Inspection Systems Limited, Vermon, Danobat Railway Systems, IK4-Ideko, West Pomeranian University of Technology (ZUT). The project was coordinated and managed by TWI Ltd and is partly funded by the EC (Research for the Benefit of Specific Groups Project, ref: FP7-SME-2011-1-GA- 286573).