低温影响下FBG永久变形计算方法研究

温度对光纤布拉格光栅（简称FBG）变形监测有着重要的影响，导致FBG技术在季冻土路基监测中存在很大的局限性。研究针对FBG传感器交叉敏感问题，提出了温度补偿计算方法，分析了不同荷载及负温下FBG监测梁变形规律；设计室内路基模型，验证了FBG在季冻土永久变形测试中的适用性。结果表明：FBG波长数值与温度呈线性相关，温度越低，波长越小，对变形监测结果影响越大；利用温度补偿计算对FBG监测梁波长数据进行修正，可消除低温引起的波长误差；基于室内模拟土箱变形测试，消除温度影响后FBG与拉线位移计实际测量的变形结果吻合度较好，体现出FBG监测梁基本可随土协同变形，可实现低温土中变形监测；冻融循环影响下，FBG测试结果与实际变形结果接近，误差在5%内，经历4、5次冻融循环后路基土箱永久变形基本趋于稳定。

Calculation method of permanent deformation of FBG under the influence of low temperature

Temperature has a significant effect on the deformation monitoring of fiber Bragg grating (FBG), which leads to great limitations of FBG technology in the monitoring of seasonal frozen soil subgrade. The temperature compensation method to solve the cross-sensitivity problem of FBG sensors is presented in this paper, and the deformation law of FBG-based monitoring beam under different loads and negative temperatures is analyzed. A subgrade model for laboratory tests is proposed, and the applicability of FBG in the monitoring of permanent deformation for seasonal frozen soil is verified. The results show that the wavelength of FBG is linearly correlated with temperature. The lower the temperature, the smaller the wavelength, and the greater the influence on the monitored deformation. The wavelength of the FBG monitoring beam is calculated using the temperature compensation method, so that the wavelength error caused by low temperature can be eliminated. Based on the deformation testing results of the subgrade model, the deformation measured using the FBG is similar to that measured using the cable displacement meter when the influence of temperature is eliminated. This means that the FBG monitoring beam can deform synchronously with soil and the soil deformation monitoring under low temperature can be achieved. Under the influence of freeze-thaw cycle, the FBG testing result is close to the actual deformation, and the error is within 5%. After 4 to 5 freeze-thaw cycles, the permanent deformation of the subgrade model tends to be stable.