Studies on Identification of Mechanical Loading Applied to Composite Plates Using Distributed Fiber Optic Sensors

本研究的目的是建立模态域保偏光纤在任意三维应力状态下的模式耦合的基本理论，并通过与一些可能应用于智能材料/结构的实验进行比较来验证其有效性。从理论上推导了不同加载条件下埋入材料中的保偏光纤的模式耦合，以论证嵌入材料的性质以及加载类型如轴向拉伸、纯弯曲、扭转和横向压缩的影响。通过将光纤中心附近的应力展开为泰勒级数，推导出实用的模式耦合方程公式，并考虑到模式耦合在横向压缩载荷作用下的敏感响应，研制了载荷传感模块。为了实现稳定和定量的测量，建立了如下设计准则：(1)保护无涂层光纤，(2)加装传感头，(3)垂直于光纤轴线加载光纤，(4)将光纤主轴与加载方向的夹角固定为45度，(5)将加载长度固定为1 mm。设计并制作了不同类型的载荷传感器模块进行性能测试，成功研制出实用的微型载荷传感器模块，并对裂纹或缺口引起的应力集中进行了检测。在I型和II型加载下，发现传感器输出与裂纹长度之间的灵敏度不同。当光纤的基模拍长等于裂纹长度时，灵敏度极高。然而，当裂纹长度小于光纤基模拍长的5%时，灵敏度变低。

The purpose of the present study is to develop a fundamental theory of mode couplings in modal-domain polarization-maintaining optical fibers subjected to an arbitrary three-dimensional stress state, and to demonstrate its validity through comparison with some experiments for possible applications to smart materials/structures.Mode couplings in polarization-maintaining optical fibers embedded in materials under different loading conditions were theoretically derived to demonstrate the effects of embedded material properties as well as types of loadings such as axial tension, pure bending, torsion and transverse compression. Practically useful formulas were derived for mode coupling equations by expanding the stresses into Taylor series around the center of the optical fiber.Considering a sensitive response of mode couplings under transverse compressive loadings, load sensor modules were developed. For stable and quantitative measurements, the following design criteria were establishied : (1)to protect the optical fiber without coatings, (2)to attach a sensor head, (3)to load the optical fiber normal to the fiber axis, (4)to fix the angle between fiber principal and loading directions to 45 degree, and(5)to fix the loading length to 1 mm. Different types of load sensor modules were designed and fabricated for performance tests, and practical miniature load sensor modules were successfully developed.In addition, some attempts were made to detect stress concentrations due to cracks or notches. Different sensitivities between sensor outputs and crack lengths were noticed under Mode I and Mode II loadings. The sensitivity were found to be extremely high when the beat length of fundamental modes of the optical fiber was equivalent to the crack length. However, the sensitivity became low when the crack length is less than 5% of the beat length of fundamental modes of the optical fiber.