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Development of a non-contacting extensometer using digital speckle correlation

A new extensometer has been developed which needs no attachment of line markers or mechanical attachment on a specimen. An expanded beam from a laser diode is incident on the marker position of a specimen which is imaged by a lens on a C-MOS image sensor. The resultant laser-speckle patterns are analyzed by two-dimensional digital correlation at the rate of 20 frames per second. It provides speckle displacement by means of a phase-only-correlation device which uses only phase of Fourier transform of the image. In-plane displacement of the marker position is tracked by moving a head containing the laser and the image sensor under the feedback control that compensates for the speckle displacement detected. Two positions on the specimen are tracked by a pair of the heads. From rubber specimens which had a marker distance of 20 mm and were elongated at the velocity of 500 mm/sec we observed good agreement in load-strain curves with the results from the conventional methods using mechanical trackers.