Elimination of Thermal Strain Interference in Mechanical Strain Measurement at High Temperature Using an EFPI-RFBG Hybrid Sensor With Unlimited Cavity Length

摘要

Mechanical strain measurement at high temperature is a challenge due to thermal strain interference caused by material thermal expansion. In this paper, a novel hybrid sensor possessing unlimited cavity length was developed to eliminate thermal strain interference in mechanical strain measurement at high temperature. The sensor was composed by an extrinsic Fabry-Perot interferometric (EFPI) with unlimited cavity length which performs large strain measurement, and a gold-coated regenerative fiber Bragg grating (RFBG) with high reflectivity to fulfil temperature sensing. Sensing performance was tested on a ceramic cantilever. Firstly cantilever and the sensor was heated from room temperature to 800 degrees C without mechanical load. Pure thermal strain as large as was measured by EFPI. At the same time temperature was measured by RFBG with a sensitivity of (10.98+0.00831T) pm/degrees C. Thermal strain data was fitted as a quadratic equation of temperature with a thermal strain-temperature coefficient of (6.407+0.00542T). This relation still stands when mechanical load is introduced along with heating. In that case, temperature could be recorded by RFBG to get thermal strain, and total strain (containing thermal strain and mechanical strain) be measured by EFPI. By subtracting thermal strain from total strain, mechanical strain could be obtained. To verify the method mechanical load of 300, 500 and 600 $mu arepsilon $ was measured under randomly heating or cooling the cantilever. Deviation between measured and actual loaded mechanical strain was less than 10%, indicating the reliability of the sensor in eliminating thermal strain interference in mechanical strain measurement at high temperature.