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Comparison of strain/stress measurements on free form surfaces using ESPI and strain gauge technique

: Lemos, J. de; Mischo, H.; Pfeifer, T.


Albertazzi, A. ; Society of Photo-Optical Instrumentation Engineers -SPIE-, Bellingham/Wash.; International Measurement Confederation -IMEKO-, Budapest:
Laser metrology for precision measurement and inspection in industry : 13 - 15 October 1999, Florianópolis, Brazil
Bellingham, WA: SPIE, 2001 (SPIE Proceedings 4420)
ISBN: 978-0-8194-4127-0
ISBN: 0-8194-4127-9
ISSN: 0038-7355
International Symposium on Laser Metrology for Precision Measurement and Inspection in Industry <1999, Florianopolis/Brazil>
Fraunhofer IPT ()
contouring; DMS; ESPI; extension; material analysis; speckle; speckle interferometry; strain; strain gauge; stress

Electronic Speckle-Pattern Interferometry (ESPI) is classically used in high resolution measurements of deformation, but ESPI can also be used for contouring of surfaces even with discontinuities. Furthermore, measurements of both shape and deformation had been carried-out. It has been shown that performing both measurements is necessary to obtain the actual strain information at the specimen topography, and the development of such techniques was presented as an alternative to other strain measurement methods, among who wire resistance strain gauges is the most popular one. But, despite of its reliability and popularity, the strain gauges technique has some disadvantages. Its set-up is very timeconsuming: in order to assure tight contact between gauge and measured surface, one has to take care about the specimen surface cleanness and roughness and the gluing step takes hours; mounting the electric circuit also requests for time and work due to the small voltage signs, demanding high se nsible signal amplifiers and bridge-circuit units able to perform compensation of temperature variation and protection from noise. Beside that once the given strain information results from the integration over the whole area covered by the gauges, this technique gives no lateral resolution. In opposition to this latter problem, optical techniques are able to provide deformation information in a matrix of some thousand points over the measured area, even including sensitivity in the out-of-plane direction. Therefore, ESPI deformation testing is currently state-of-the-art. But only in the case of plane specimen the correct strain distributions can be directly determined. Otherwise, the contour of the surface has to be taken into consideration. So, an ESPI sensor prototype able to perform such deformation and contouring measurements with easy to handle set-up was developed . In order to achieve detailed information about the reliability of the ESPI strain/stress sensor, several tests hav e been performed. For its large acceptance and common know-how, the technique of strain gauges was chosen as the comparison pattern in this task. Beside a short introduction to the principle of optical strain/stress measurements, this paper will also describe the results of the work performed in quantitative comparison of several technical applications using both techniques, strain gauges and the ESPI sensor.