High Accuracy and Wide Temperature Measurement by Non-Contact Speckle Extensometer

A new extensometer that has been developed needs neither markings nor mechanical clips to be attached onto the specimen and is perfectly noncontacting to automatise tensile test. Separate laser beams are incident at the positions where the markings were attached in the conventional extensometer. The spots on the specimen are imaged on a separate charge-coupled device (CCD) to produce a speckle pattern, the displacement of which is detected by real-time digital speckle correlation. Each of the optical systems is installed in a separate movable probe head that is moved under feedback control to nullify the speckle displacement detected. For detecting a small amount of specimen displacement, less than a few millimetres, the optical head is fixed to output speckle displacement. A resolution of 1 mu m was attained by comparison with a linear laser encoder. A larger displacement was detected from displacement of the probe head that cancels the detected speckle displacement by feedback control of the head movement. We also conducted tensile test in a temperature chamber controlled in a range between -40 and 100 degrees C, by placing the probe heads outside a window of the chamber.