Laser-speckle strain sensor using structured Illumination

The contact-based strain sensor has been broadly employed to measure mechanical deformation of a variety of materials. However, its measurement accuracy is highly sensitive to the sensor location on the specimen, and it can be easily broken and damaged during sensor attachment/detachment and because of environmental factors. Herein, we propose a non-contact optical strain sensor based on laser speckle imaging (LSI). The LSI is highly sensitive to the local modulation of the refractive index, scattering features, and deformation of specimen. We utilize and combine this feature of LSI with structured illumination to achieve high-contrast and high-sensitivity LSI (and strain measurement) even with strong ambient light. Quantitative measurement of strain is performed through two-dimensional cross correlation of LSI images acquired during the deformation. The sensor exhibited a 1.8× higher signal-to-noise ratio compared to that in the conventional LSI technique. The measurement accuracy was also assessed by performing a tensile test with aluminum specimens. The measurement results presented a correlation coefficient of 0.99 upon comparison with the conventional strain gauge