Crack Identification For Pressure Pipeline Based On Distributed Optical Fiber Sensing Technology

The aviation pipeline system is one of the most important part of aviation fuel system,hydraulic system and refrigeration system.The normal operation of the system directly affects the safety,stability,reliability and economy of the aircraft.During its service,the pipeline will be subjected to coupling loads,such as internal pressure,vibration and shock load,which will lead to cracks unavoidably.Therefore,it is indispensable to take regular monitoring of piping system to ensure its safe service.Due to the fact that aircraft pipelines usually have the characteristics of complex structure,large number,high density,and cross distribution,the conventional non-destructive testing methods are not suitable for crack identification.Therefore,it is of great demand to develop a new non-destructive technique for crack recognition of aircraft piping system.Owing to its distributed,high precision and long distance characteristics,Rayleigh-based optical fiber sensing techniques make it possible for the measurement of large and complex structures.Based on the techniques,a nondestructive testing method for surface crack identification of pressure piping is developed in this thesis.The distributed optical fiber sensor(DOFS)is used to monitor the surface crack of pipeline,by analyzing the response,DOFS is able to recognize and locate the crack and estimate its length.The main work is summarized as follows:Firstly,a large number of numerical simulations were conducted to study the effects of crack depth,internal pressure and pipe diameter on the strain distribution near cracks by finite element analysis software ANSYS.The dependence of crack depth,pipe diameter and internal pressure to the circumferential strain was discussed.The key influence factors and distribution characteristics of strain were obtained,according to which the position and length of the surface crack can be determined.Secondly,based on the distributed optical fiber sensing technology,we develop a surface crack identification system for the pressure pipeline.On the basis of which,the surface crack identification experiment is carried out for circumferential cracks identification.The feasibility and accuracy of distributed optical fiber sensing technology to identify surface cracks in pressure piping are explored and the influence of the spacing of distributed optical fiber sensors on the crack recognition is investigated in detail.Then a method for identifying the circumferential cracks on the surface of pressure piping is proposed.Finally,the surface crack identification experiment is carried out on the pipe with circumferential crack,45 degree crack and weld circumferential crack.The experimental results show that the accurate position of cracks can be achieved by analyzing the strain data near the cracks,and the crack length can be estimated for surface circumferential crack.In this thesis,the numerical simulation and experimental study of the surface crack pressure pipeline are conducted.Moreover,a surface crack identification method based on distributed optical fiber sensing technology is proposed.This method can effectively achieve crack recognition,positioning,and length estimation.