Noncontacting strain measurements at high temperatures by the digital laser speckle technique

This paper presents a newly developed laser-based noncontacting strain sensor suitable for temperatures up to 1200 degrees C, which was adapted to a commercial tensile testing machine equipped with an electrical furnace. The principle of the strain sensor is based on tracking laser speckles through a digital correlation technique. Furthermore, the description of the signal processing and the optical arrangement is presented. Based on the experimental data, it can be shown that this simple, laser-based strain sensor can be used successfully for the determination of mechanical and thermal strains up to temperatures of about 1200 degrees C. Using a special data-processing procedure, it was feasible to minimize decorrelation effects caused by changes in the specimen surface due to, for example, slipband and microcrack formation, surface oxidation and phase transformation and, thus, measure large mechanical strains. The strain resolution for the selected setup was about 20 microstrains depending on the testing parameters.