Precise measurement of stress and strain

Tom Shelley reports on advances in stress, strain and vibration measurement

Nowadays, stress, strain and vibration can be measured from a distance with resolutions down to picometres in some instances. Increasingly, conventional techniques can be made to yield results much more quickly, and with greater accuracy, than was previously possible. The breakthroughs mainly come from computing, both in terms of hardware being able to number crunch faster and software algorithms that extract data from noise. Nigel Peart, regional sales manager of Polytec, claims its laser Doppler vibrometry technology, "can measure the 3D displacement across a component down to several picometres." Strain is measured directly and stress inferred from parameter input by the user. Polytec has recently sold a system for looking at turbines blades in the aerospace industry as a result of very extensive tests alongside strain gauges. "The breakthrough is mainly the software used to process the information," he says. "The only hardware change is a very high resolution camera used mainly to position the laser spots." Rob Wood, application specialist with Dantec Dynamics, says it can use four laser diodes to produce a speckle pattern on an item of interest. This is heated with a lamp, which induces a small amount of stress. Dantec then use a video camera to detect movements down to nm. The technique is referred to as laser shearography. Areas studied range from square millimetres to square metres with one of the largest users being the Royal Air Force, which use it to inspect radar domes. Dr David Hollis, applications consultant with LaVision showed a technique called StrainMaster Digital Volume Correlation in which volume images of components from X-ray CT and MRI scans are imported and used to produce 3D displacement and strain maps. Applications range from studies of bending and delamination in composites, to geology applications. X-ray diffraction has been used to reveal residual stresses in metals for the last hundred years, but in the past it has been a time consuming process, examining film images and carefully measuring the magnitudes and positions of spots. The Canadian company Proto Manufacturing has, however, developed fixed and portable systems that it claims to be the world's fastest. Residual stresses in welds and surrounding heat affected zones remains a major source of failure. They are however, unlikely to be able to deliver results as fast as those produced by Vision Research, which produce Complementary metal–oxide–semiconductor (CMOS) cameras with frame rates in the latest Phantom V710 camera of up to 1.4million frames per second or 7,530 frames per second at its full resolution of 1280x800 pixels. Optional sub-microsecond shuttering goes down to 300nanoseconds in 18ns increments to prevent motion blur. Where physical access is possible, an alternative to the strain gauge is to use Bragg gratings etched into optical fibres. Each grating along the optical fibre can measure strain. Paul Beardsworth, senior sales engineer with HBM, says that up to 40 gratings can be used on a single strand of optical fibre that can be two or three kilometres in length. Each grating works at a different wavelength and may be bonded to a structure in a similar manner to traditional strain gauges. Particularly suitable applications include those in the offshore oil and gas rigs and overhead electricity cables. Fibre optic strain gauges benefit from having very long fatigue lives and on composite structures should outlast traditional strain gauges. Fibre optic strain gauges can be made thin enough to be incorporated into fibre reinforced composites without compromising integrity. Helen Sofianos, a market developer with Belgian Fibre Optic Sensors and Sensing Systems, says a fibre optic sensor with its coating was only 250µm in diameter and added that they were able to supply sensors down to 80µm in diameter. They are currently in service monitoring bridges, but are considered to be highly suitable for incorporation into aerospace composite structures. Conventional strain gauges continue to be improved. Andy Wagstaff of Vishay says its hybrid strain sensors are etched insitu on steel elements. Such solutions are specially developed for specific application and are only suitable for very large volume production runs. With regards processing strain gauge sensor data, Mantracourt next generation wireless data collection systems and USB interface allows strain gauge devices measuring weight, pressure, force or torque to be directly connected to a laptop computer. Although designed as a 1:1 interface they can be connected to multiple sensors by using a suitable hub.