Railway Companies Bet on Ultrasonics
New trend-setting Phased Array testing machine checks wheelset axles at the German Railways' works in Wittenberge. An increasing number of high-speed trains are being used by railway companies, especially in passenger services. The higher speed also leads to higher stress on the material and on individual train components.
There is an extraordinarily high safety factor involved in this sector. With an innovative ultrasonic testing machine for the inspection of wheelset axles, based on the Phased Array technology, GE Inspection Technologies with its Krautkramer Ultrasonic products meets the higher requirements and makes another step forward towards safety and reliability. Wheelset axles are critical components in trains and are checked at regular intervals during the routine maintenance work. The requirements have become higher, especially for this inspection: shorter inspection intervals, extended scope of inspection and absolute reliability of the results - irrespective of the subjective assessment of an individual inspector - are demanded. Critical flaws mainly occur in the area of wheel and brake disc seats, as well as in their cross-sectional transition areas.
Problems with previous inspections
In the past, these axles were mainly inspected by hand - a laborious and time-consuming method because a large number of probes with different angles of incidence had to be used in order to cover the complete test area. It goes without saying that this manual inspection was also expensive and subjective due to the fact that human errors could never be excluded. This meant that the checking of wheelset axles always represented a bottle-neck in the maintenance and upkeep of trains. An automatic ultrasonic test could previously only be implemented with difficulty as axles have many different designs, and the coupling faces available are small. The design may even vary from one axle to the next. This meant that the testing machine had to be frequently reset, which led to long change-over times.
The Krautkramer solution
An innovative system, developed by GE Inspection Technologies in co-operation with the German Railways (Deutsche Bahn AG) and the Federal Institute for Materials Testing (Bundesanstalt für Materialprüfung), succeeded in automating this inspection, and consequently in finding solutions to the past problems. This new inspection concept uses Phased Arrays to check the wheelset axles for any cracks.
A Phased Array probe replaces several conventional angle-beam probes; and the angle of the sound beam is electronically controlled. The testing machine can therefore be easily set to most different axle designs, and requires less mechanical work and time for resetting. All it takes to set the machine to a new type of axle or new axle geometry is to load a predefined and saved data set. This enables an extensive automation of the test sequence having a shorter cycle time and requiring less operation - subjective uncertainties involved in the evaluation of the test results are reduced by means of evaluation software. In addition, the software has all state-of-the-art features for documentation and archiving that are nowadays indispensable as a proof of correct test processes. The mechanics of this testing machine consist in a roller stand and a portal, each with four probe manipulators that can be controlled independently of each other. These manipulators, carrying the Phased Array probes, can be rotated through 180 degrees in order to enable inspection in the opposite axle direction.
The test process
Every wheelset to be tested first passes through a station for cleaning the coupling faces. After that, it is rolled into the roller stand. The operator only enters the wheelset type in the PC; in doing this, the corresponding data set for the evaluation electronics is recalled as well as the mechanics control. The testing machine automatically positions the four probe manipulators at the correct position - and the test procedure starts. The machine carries out a test run, or if the axle is equipped with a third brake disc in the middle, two test runs. For the second test run to check the third brake disc seat, the two inner probe holders are automatically rotated through 180 degrees so that the probes scan the brake seating from either side.
The evaluation software
During testing, the "COMPAS" electronic system produces evaluation images for all activated probes. An evaluation image shows in its top area a developed view of the axle surface where the echo amplitudes are shown colour-coded. This display type shows echoes generated by the geometry of the axle (changes of cross-section) as vertical stripes through 360 degrees. This makes it possible to easily differentiate between echoes generated by the axle geometry, and echoes generated by real flaws. A scaled image of the axle type tested is displayed below the evaluation image to make the evaluation of the axle easier for the operator. The flaw position can also be easily determined by this means. The operator compares these images with the reference image of a flaw-free axle, and decides on the sorting (GO - recheck/NO GO). Apart from that, the software enables a comprehensive documentation of the results, every test process can be reconstructed and checked even afterwards. All data is stored into a database so that it is available for a subsequent inspection of the same axle.
Forward-looking
The use of phased arrays and the automation of the inspection bring about numerous benefits for the maintenance of trains. Firstly, the considerably shorter inspection times should be mentioned - the reduction from the previous time of about one and a half hours to a current five minutes for each axle clearly speaks for itself. Added to this are, no less decisive, the shorter change-over times for adjustment to the different axle types, plus the possibility of a complete documentation of the results. The ultrasonic testing machine has been successfully in use at the German Railways' works at Wittenberge since the end of 2001, and it will serve as a model for further testing stands in other repair shops.
