Development And Application Of Fiber Bragg Grating Sensors Packaged By Resin-based Composites

Fiber Bragg grating(FBG)is small,anti-interference and light weight,making it widely used in the fields of aerospace,underground pipelines and large-scale civil engineering to realize the online status monitoring and safety evaluation.At the same time,FBGs can be arrayed for multi-point distributed measurement,or simultaneously monitor multiple physical parameters(such as temperature,stress,pressure,etc).However,the diameter of a fiber grating is relatively small.The diameter of ordinary single-mode fiber is only 10 ?m,which is easily brittle fracture and inactivation in actual engineering applications.Therefore,the bare fiber grating requires encapsulation protection.The commonly packaging method of FBG sensors is to fix the fiber Bragg grating on the resin substrate or grooved rigid substrate and then a substrate FBG sensor is fabricated,or packaging the FBG in a metal tube to make a tube-type FBG sensor.However,there are many defects such as large strain transmission loss,bad compatibility with the matrix material,poor test stability and test accuracy when the FBG sensors prepared by above packaging method.In recent years,composite materials are widely used in various industrial fields.Using composite materials to package FBG sensors or implanting FBG sensors into composite materials to fabricate smart composite structures can significantly ameliorate the insufficient of traditional FBG packaging technology.Thus,it has important theoretical significance and practical value to research composite fiber Bragg grating packaging technology.Firstly,in this paper,the glass fiber/epoxy composite laminate is used as the bottom substrate,and the glass fiber fabric is used as the covering layer.The FBG is encapsulated between the bottom substrate and the covering layer by means of a vacuum-assisted infusion process of liquid epoxy,and then,the FBG sensor packaged in glass fiber/epoxy composite substrate is fabricated(A set of comparison is made between the curing process of the composite substrate and the liquid epoxy resin.The substrate and the liquid epoxy resin of the experimental group ? are fully cured without the second curing process.While the substrate and liquid epoxy resin of the experimental group ? are pre-cured,and then the preformed sensor is placed on the vulcanizing machine for secondary curing process).Subsequently,temperature and strain calibration experiments are carried out on the above two kinds of FBG sensors.Results reveal that the FBG sensor fabricated by the experimental group ? are better than the FBG sensor fabricated by experimental group ? in the aspects of test repeatability,stability and accuracy.Secondly,the paper introduces the preparation technology of smart composite structure,which embed the bare FBGs in the CFRP laminates and cured with the CFRP structure to fabricate the FBG sensors.The embedded FBG sensors can online monitor the temperature and strain evolution of the CFRP laminate during its curing process and feedback the health status of the composite structure during subsequent service.Four kinds of methods are used to protect fiber grating sensors when the fiber grating direction is not parallel to the carbon fiber direction.Based on the monitoring signals of the FBG sensors,the feasibility analysis of the four kinds of protection schemes is carried out.Results reveal that,through the method in which two narrow prepreg strips on both sides of the sensors and opening grooves on the neighboring layers are used to protect the sensors,the survival rate and testing accuracy of the FBG sensors can be significantly improved without changing the mechanical properties of composites with specific layer parameters.Finally,based on the applications of FBG sensors in practical engineering,the composite substrate FBG sensors are used to online monitor the stress evolution of the filter press rack during its servitude,and the comparative analysis is carried out between the experimental monitoring results and the ANSYS finite element simulation results.In addition,FBG sensors are used to online monitor the whole curing process of liquid epoxy resin.Then,a new method of implanting FBG sensors into the outer surface of the plastic pipe is put forward to provide technical support for underground pipeline construction of cityies.