Industrial equipment, such as wind turbine foundations and oil and gas pipelines in cold regions, may undergo extrusion/expansion deformation during freezing and thawing of frozen soil, which affects their power response and safe operation. Measuring the internal deformation of frozen soil can immediately reflect the strain situation of industrial equipment to reduce the risk of equipment operation. We designed a 6-dimensional strain sensor (6-S Sensor) based on Fiber Bragg Gratings (FBG) to obtain the spatial principal strain distribution. The strain range, linearity, and average error of the sensor were -4000~8000 με, 0.997, and 2.94%, respectively. The sensor accurately measured the frozen and thermal expansion of frozen soil at different temperatures in the laboratory. The maximum frozen expansion was 6471.38 με, which occurred in the X direction. The accuracy of spatial principal strain monitoring for the sensor was evaluated through uniaxial compression. The stability of the sensor was verified by the monitoring experiment under natural temperature for half a month. This study provided a pioneering method for monitoring the internal spatial principal strain of frozen soil.

Hydraulic engineering built in the cold region, such as reservoirs and hydropower stations, is often threatened by static ice pressure from nature. Therefore, it is of vital significance to research the pressure variation in the growth and melting processes of the ice layer for the design and protection of hydraulic structures in cold regions. This paper introduces an optical fiber sensor system based on the fiber loop ring-down spectroscopy technology and field-programmable gate array (FPGA) pulse modulation technology. An electro-optic modulation scheme that relied on FPGA to generate optical pulses with adjustable pulse width and period is proposed, which is more suitable for the in-situ observation. In addition, the temperature stability and repeatability of the system are also discussed. This system was applied to the real-time detection of static ice pressure on the sidewall and bottom of the polyvinyl chloride (PVC) pipe during the ice growth and melting processes. The results indicate that the system has favorable stability and sensitivity, and the relationship obtained between the static ice pressure and temperature could provide some references for the field application in the future.

To perceive spatial principal strain in frozen environments, the 6-axis strain sensor was proposed in previous research. When monitoring ice instability, it is necessary to demodulate the cascaded 6 FBGs of the sensor over a wide wavelength range. The edge filtering demodulation method based on dual-peak resonance LPFG spectral control is proposed, which achieves accurate demodulation of the center wavelength of cascaded 6 FBGs over a large wavelength range (each demodulation range is 15 nm, and the total range is 90 nm). We have etched a dual-peak resonance LPFG using CO2 laser and designed a demodulation system based on piezoelectric ceramics (PZT). By precisely controlling the bending degree of LPFG through PZT, the spectrum of the LPFG can be regularly changed. The linear mathematical model of the demodulation system has been established, which theoretically verifies the feasibility of the proposed demodulation method. To solve the nonlinear error caused by the linearity poor at the edge of the demodulation interval, the BP-neural network demodulation model was established. The average error is smaller, which can reach 0.014 nm, with an MSE of 0.0005. This work can provide useful technical support for the widespread application of FBG multi-dimensional strain sensors.