Noncontacting Extensometer Using Digital Speckle Correlation

摘要

A new extensometer has been developed which needs no attachment of line markers or mechanical tracers 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 CMOS image sensor. The resultant laser speckle patterns are analyzed by two-dimensional digital correlation at the rate of 20 frames per second. Speckle displacement is calculated by means of a phase-only-correlation device which uses only the phase of the fourier transform of the image. In-plane displacement of the marker position is tracked by moving a head containing a laser diode, an imaging lens and an 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 separately. From rubber specimens having a marker distance of 20 mm and elongated at the velocity of 500 mm/min, we observed good agreement in load-strain curves with the results from the conventional techniques using mechanical trackers.