FBG-LPFG-Based Sensor to Monitor 3-D Strain in Ice During Freezing–Melting Processes

The three-dimensional strain (3-D strain) and the principal strain in ice are very important for glacier movement research and glacier-related disaster prediction. The internal 3-D strain of ice is sensitive to temperature, and it is very complex during the whole freezing and melting process, including phase transformation. In this study, we present a 3-D strain sensor (3D-S Sensor) based on fiber Bragg grating (FBG) and long-period fiber grating (LPFG) that can measure the 3-D strain in ice. The LPFG (the temperature sensitivity coefficient is 56.3 pm/°C) was fabricated by the laser etching method, which was used for the temperature compensation of FBGs to improve the accuracy of strain measurement. The 3-D principal strain can be calculated by the value of six direction strains from FBGs. A freeze–melt experiment is designed to verify the performance of 3D-S Sensor. The experimental results show that the sensor can measure the complex 3-D deformation of ice from a more precise angle. Among the six directions of strain measured by the sensor, the strain value in the ${Z}$ -direction is the largest ( $\varepsilon _{z}$ = $6232.9 \mu \varepsilon {)}$ . The first principal strain in ice is close to the negative direction of the ${Z}$ -axis ( $\theta 1_{Z}$ = $160.7^{\circ }{)}$ , and the maximum value is $6638.2 \mu \varepsilon $ . The measured results are consistent with the actual deformation of ice, indicating that the sensor can effectively measure the 3-D strain and principal strain inside the ice.