Strain spotter

A portable device that can be used to measure the stress of a structural materials used in everything from oil rigs to cars is set to undergo trials early next year on the UK’s rail network.

The system is being developed by engineers at UK company MAPS Technology under a contract from Network Rail which is planning to use the technology to monitor sections of continuously-welded rails (CWR) for early signs of buckling or cracking.

To minimise the risks of buckling and breaking when exposed to extremes of temperature, CWRs are typically pre-stressed at installation. This is designed to achieve a zero loading condition when the temperature sits halfway between the operating extremes — this is known as stress free temperature (SFT).

But despite the best efforts of the rail industry, there are still certain conditions that cause problems. For example, as rail temperature falls the tensile stress in the rail rises and the risk of a fracture increases; and when temperatures rise, the tracks, unable to expand are forced into compression — leading to the risk of buckling.

While the industry already has methods to monitor SFT, these techniques are unsuitable for curved sections of track or switches and crossings which, most significantly, need to be cut or unclipped when probed, necessitating track closure (possession) and interruption of the service. With the UK’s network running at capacity, anything that speeds up this process, and enables rail operators to keep a close eye on things is desirable. and this is where the new device, MAPS SFT, comes in.

Dr Geoff Eckold, managing director of MAPS — which was spun out of engineering consultant ESR earlier this year — said: ‘MAPS SFT can measure the load in the rail, and relate that to the desirable load that should be in there to make sure it doesn’t buckle in the summer.’

Eckold explained that MAPS (Magnetic Anisotropic Prediction of Stress) is able to measure stress non-destructively using a small probe which is placed over the surface of the material. ‘It’s a magnetic technique that works on ferromagnetic materials — so if a magnet can stick to it, MAPS will measure it,’ said Eckold. ‘The probe is made of a series of coils. It energises the material and is then able to sense the response of the material in terms of the change in magnetic properties,’ he added.

This behavioural change can then be related to the state of stress in the material, which in turn can give an indication of how close the material is to failure. ‘By measuring stress on the surface we can pick up the potential cracking sites before the cracks occur — we don’t detect cracks, we detect the causes of cracks,’ said Eckold. ‘We can pick up where stresses change, so we can spot where stresses are beginning to get more tensile,’ he added.

Eckold claimed that the MAPS technique has significant advantages over most other methods used to monitor stress. These include strain gauges which will only measure stress where they have been placed, destructive hole drilling techniques, X-ray diffraction (which operates rather like a strain gauge at an atomic level, but requires a chunky piece of equipment and very careful surface preparation) and even more exotic techniques such as those offered by the latest Synchrotron and high-energy physics facilities.

Eckold suggested that it is partly thanks to some of the more exotic lab-based techniques that MAPS, which has been under development for a the past 20 years, is now poised to make an impact.

‘MAPS has had this long gestation time and now we’re at the cusp because we know its capabilities. We’ve done validation against synchrotrons and X-rays and can now go forward with confidence,’ said Eckold.

Indeed, the device that’s currently being developed for Network Rail is just one of a number of applications for the technology, and Eckold said that the core technology holds promise across a range of industries. ‘The ability to measure stress non-destructively allows us to address a range of problems,’ he claimed.

For example, in another major project the group is developing a MAPS-based system for monitoring stresses in risers on offshore platforms. The company has also done work looking at the integrity of bearings in wind turbines and is, said Eckold, keen to explore opportunities in the automotive industry.

‘MAPS can look at automotive components under fatigue; by monitoring the stress state during loading we can spot where the stress distribution changes and predict where cracks will ultimately form.’

Meanwhile, the company is adding the final details to its SFT device ahead of trials that are expected to take place early next year.

The resulting system, though currently under wraps, will, said Eckold, look completely different from MAPS devices developed for the steel or offshore industries. ‘You can’t just take any piece of equipment on to the railway,’ said Eckold. ‘It has to be compatible with the systems and the signalling. It’s got to be man-portable, it can’t interfere in any way with the signalling structure and it’s got to deliver data which is compatible with the maintenance requirements.’

The design of the system has also been heavily influenced by the fact that its use will not require a possession of a section of railway.

‘We take measurements on the web (the vertical section between the head and the foot of the rail) and we’ve got to be sure that the means of attaching it to the web is appropriate,’ said Eckold. He said that although users of the device won’t need possession, it doesn’t mean they can just wander on to the track and use it.

‘Under those circumstances any equipment which is near the rail can’t interfere with the system in any way. All of this operational information needs to be fed into the development of any device, which has to be compatible with the rail operators’ existing process for managing SFT,’ he said.