基于光纤光栅的结构体裂缝三维应变传感

Mechanism of Three-dimension Sensing with Fiber Gratings for Structure Cracks Detection

利用光纤光栅的反射谱设计了一种用于混凝土纵向裂缝三维应变传感信号的检测及分析处理的方法.利用ANSYS软件自底向上采用构造法构建混凝土三维断裂模型,分析径向均匀作用力下光纤光栅三轴的应力大小,由三维受力模型拟合光栅传感器的三轴应变函数,给出径向作用力下x和y偏振方向谐振波长与三轴应力间的关系,并且由传输矩阵法计算三轴应力作用下光栅反射谱的变化规律;理论分析和模拟计算光纤光栅传感光谱反射峰的分裂规律.结果表明:在均匀的20N作用力下,10cm长光栅x偏振方向的波长偏移量最大值为10.1nm,y偏振方向的波长偏移量最大值为12nm,此时光纤光栅的谐振峰产生明显分裂,形成两个谐振峰,随着载荷的不断增大,两个谐振峰不断地向两边分开,反射峰的分裂点从短波长向长波方向移动,分裂出来的两个谐振峰中短波谐振峰的反射率高于长波谐振峰,但短波谐振峰的带宽小于长波谐振峰带宽;当光栅长度增加至15cm时,在相同的作用力下,光栅两个反射峰的半高宽度均增加,但两个反射峰的间距几乎不变,灵敏度达0.14nm/N.本文研究可将单根光栅结构传感器的二维传感扩展到三维传感.

A method using for the detection and analysis of the three-dimension strain sensing signals of the longitudinal concrete cracks was proposed by utilizing the reflectance spectrum of fiber gratings. The fracture model of three-dimension finite element using bottom-up method based on ANSYS was given. The three-axis stress applied on the fiber grating under uniform and radial force was presented, the three- axis strain function was derived on the basis of above-mentioned stress field, and the relation curve between resonance wavelength of x-polarization, y-polarization and three-axis stress was given. The variation of reflective spectrum of fiber grating of 10 cm length is calculated using transfer matrix method under three-axis strain based on theoretical analysis and numerical calculation, the results show that, the peak wavelength shifts of x-polarization and y-polarization are 10. 1 nm and 12 nm respectively under uniform and radial force of 20N. At the same time,the reflection spectra of the fiber Bragg grating split into two main peaks and the spectral difference of the two peaks increases linearly, and the split point shiftes to long-wavelength, the reflectivity of shorter wavelength is higher than the long-wavelength but the bandwidth of shorter wavelength is less than the long-wavelength when the load increases. The （Full Width Half Maximum, FWHM） of the two splitted reflection peaks increases but the spectral peaks space remains unchanged almost when the length of grating increases to 15 cm under same force compared to grating of 10 em length. The sensitivity of the grating proposed in the paper is up to 0.14 nm/N. The two-dimension sensing is extended to three-dimension sensing based on single grating with the method proposed in this paper.