A test in time: preventing failures
04 January 2012
Les Hunt discovers new ways of testing structural fasteners in real time out in the field as well as on the laboratory bench, and describes an innovative method of detecting dangerous flaws in carbon fibre reinforced plastic bike frames using thermal imaging cameras
Consider a large structure that is subject to extreme loads – the supporting tower of a wind turbine, for example. The fasteners that secure this structure to its foundation are nothing less than critical components, so being able to measure the effects of high loads on their integrity might be deemed essential. And being able to do this in real time would be a bonus. Well, a chance discovery during routine tests on a wind farm in Texas has demonstrated that this is indeed possible.
Norbar’s (http://www.norbar.com/) portable USM-3 ultrasonic bolt meter, which can provide a measurement of the elongation of a threaded fastener of length anywhere between 25mm and 15m to an extraordinary resolution of 0.0001mm, was found capable of providing precise real-time measurements of forces affecting the 2m long foundation bolts securing one of the wind farm’s turbine towers.
The foundation bolt was, of course, fully installed and it was not possible to determine its actual elongation or tension without first loosening the bolt. However, for the purposes of demonstration, the initial length of the bolt was recorded using the USM-3 meter and a magnetic transducer which was placed on the bolt end. After a while, the engineer conducting the measurement commented that the measured elongation did not seem to be stable since the value was constantly drifting. While reviewing the data, Norbar’s ultrasonic specialist, Gregory Young, realised what was happening.
“The tower base foundation bolt was located at a 160o angle from the wind direction. As the wind increased, it pushed the tower in the general direction of the foundation bolt, the joint was compressed slightly and the elongation decreased. As the wind decreased the tower rebounded and the bolt’s elongation increased as the compressive forces on the joint decreased.
“This ability to read minute fluctuations in the bolt tension, real time, can be of value to many companies dealing with bolted joint issues. Many sectors such as the automotive industry will also be interested, since real time changes in bolt tension are highly important in automotive engineering.”
Ultrasonic measurement performed by the USM-3 works on a ‘time-of-flight’ principle, similar to sonar, with a small transducer placed on the head or stud end of the fastener sending an ultrasonic sound wave through its length and back again. After recording the initial bolt length with no load on the fastener, subsequent changes in the time of flight are converted by the USM-3 utilising material constants to eliminate the effect of stress and temperature variations on sound velocity, providing an accurate elongation or load measurement.
Meanwhile back in the more equable environment of the test laboratory, Vibrationmaster’s (http://www.vibrationmaster.com/) J150 portable Junker Test Bench can be used to test two fasteners simultaneously and compare them in real time. Fastener manufacturers, in particular, can now simultaneously demonstrate the self-loosening behaviour of two competing bolted joints secured with lock-nuts or wedge washers using this new instrument. Fastener product performance can be compared and contrasted to the DIN 65151 vibration test in minutes, and because the J150 is portable, you can take it along to sales presentations and trade exhibitions.
“Until now, fastener buyers have been forced to rely on published test results and performance reports when specifying new fasteners for their products,” explains Vibrationmaster CEO Morten Schiff. “Now, for the first time, fastener manufacturers can actually demonstrate the resistance to self-loosening behaviour of their bolted joints using wedge washers, lock nuts and other securing elements against competing anti-loosening solutions, live and in front of their clients.”
According to Mr Schiff, an added advantage of the J150 dual demonstrator is that the self-loosening behaviour of one fastener will not impact on the other – the motor delivers the same load to each, providing two completely independent test results.
Thermographic structural testing
Modern bike frames made of carbon fibre-reinforced plastics behave differently from conventional steel frames in failure mode. Instead of bending, carbon fibre-reinforced plastics fracture, exposing riders to danger. Fortunately, defects in carbon fibre-reinforced materials can be detected, before they become a failure issue, using thermal imaging cameras. One company that provides this service is Carl Messtechnik. Thermography specialist Volker Carl takes up the story:
“Carbon based frames can be surprisingly vulnerable to specific types of stress that produce invisible cracks. On the surface, everything might still look fine, but in reality the bike is damaged. It is, however, possible to repair these defects, and although the repair process is quite complex, it can often result in a bike that is even stronger than the original design.”
Mr Carl uses a FLIR (http://www.flir.com/) SC7000 thermal imaging camera, basing his procedure on a method called ‘pulse thermography’. “We trigger a thermal impulse and use the FLIR thermal imaging camera to trace the heat flow. Differences in the heat flow can indicate material defects. The thermal data collected with the thermal imaging camera provides a unique insight into the flaws in carbon fibre-reinforced materials.”
Volker Carl is now working on a new set-up with a FLIR SC645 thermal imaging camera, which is slightly less sensitive, but the difference in price and in maintenance cost will allow him to charge lower fees for the all-important quality tests.