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Fiber optic DSPI strain gauge system for engineering applications

Faseroptisches DSPI-System und dessen Anwendung
: Höfling, R.; Aswendt, P.

Pfeifer, T.:
3rd International IMEKO-Symposium on Laser Metrology for Precision Measurement and Inspection in Industry '94
Düsseldorf: VDI-Verlag, 1994 (VDI-Berichte 1118)
ISBN: 3-18-091118-2
International Symposium on Laser Metrology for Precision Measurement and Inspection in Industry <3, 1994, Heidelberg>
Fraunhofer IWU ()
automatic fringe evaluation; automatische Streifenauswertung; Dehnungsmessung; faseroptisches Meßsystem; fiber optic measuring system; speckle metrology; Specklemeßtechnik; strain measurement

This paper describes a newly developed measuring system based on Digital Speckle Pattern Interferometry (DSPI) that provides a field of strain data with high accuracy. An all-fiber optic arrangement has been designed yielding a small-sized, convenient device. The work aimed for sensitivity and resolution comparable with common strain gauges. Pure in-plane sensitivity of the speckle pattern is achieved by dual-beam illumination. Two such dual-beam interferometers are put together to provide strain in orthogonal directions. To avoid any mechanical components in the measuring head, two laser diodes have been chosen as switchable light sources and are directly connected to the interferometer by a fiber pigtail. Beam splitting and phase shifting equipment is integrated into the fiber optics as well so that miniaturization becomes possible. A small CCD camera is added to pick up the speckle pattern of the laser illuminated object. The measuring principle consists in the automatic evaluation of speckle correlation fringes obtained from two different states of loading. The paper explains the fringe evaluation by image processing algorithms especially designed for strain analysis. As a result, the plane strain tensor components E sub xx, E sub xy, E sub yy are calculated for each of the 512 x 512 picture elements. The evaluation takes 3 minutes from the picture recording to the presentation of results. Using this strain tensor data, other mechanical quantities, such as principal strain and stress, can be easyly derived. Measurable strain depends on the object size and ranges between 10 and 800 myE for an area of 10 x 10 qmm for example. A step-by-step loading procedure allows to measure strain values as high as desired.