The Research Of Optical Frequency Domain Reflection Optical Fiber Sensing Technology In Composite Material Non-uniform Strain Measurement

As the composite material has the advantages of higher strength,better fatigue properties and stronger stiffness compared with traditional materials,it was widely used in the fields of aerospace,automobile and machinery manufacturing,etc.As the same time,the anisotropy of composite material can result in the damage of cracking,fiber fracture,degumming and delamination.The damage of materials' structure is generally accumulated and results in non-uniform strain of composite material,so it is important for the structural health monitoring of composite materials.The most adopted traditional method for strain measurement by resistance strain gauge was limited by their sensing principles that they could not be embedded into composite materials and measure strain distribution within a small range.The Fiber Bragg grating(FBG)with the advantages that it is small enough to be embedded into composite materials is widely used in the field of health monitoring for composite material.When measuring the non-uniform strain in the composite material,the reflection spectrum of the fiber Bragg grating will appear multiple peaks and chirp.The traditional grating demodulation system can't realize the measurement of non-uniform strain field.In this paper,the optical frequency domain reflect(OFDR)demodulation technique is used to measure the distribution of non-uniform strain field within the composite material.The non-uniform strain distribution along FBG tallies with the theoretical simulation and the distribution of non-uniform strain field is detected successfully.The main contents of this paper are as follows:(1)Analyzing the importance of composite materials health monitoring.Researching the spatial resolution,measurement sensitivity and conversion gain of the optical frequency domain reflection system.Studying the principle of non-uniform strain field measurement for OFDR system.(2)According to the principle of OFDR fiber grating demodulation.the test system was set up.And the software part of the system is completed by using LabVIEW which is a graphical programming language to realized the laser and data acquisition card control and data signal processing.(3)Based on the influence-principle of non-linear scanning of the light source of the OFDR system,it was carried out that the correction of the non-linear scanning of the light source is realized by using the method of the reference optical path.After the system corrected,the intensity of the spectrum becomes larger,the spectral width becomes smaller,the spatial resolution becomes higher and the peak chirp disappears.Analyzing the influence of the line width of the laser on the system,the line width of the laser is measured by the delay self-homodyne method.The length of the laser is measured by using the longer delay fiber which is 6 times than the coherence length of the laser.(4)From the perspective of the measurement of non-uniform strain caused by the damage of composite materials,it was carried out that the signal of the optical fiber Bragg grating embedded in the composite material plate is demodulated by using the optical frequency domain reflection system,got the strain distribution along a FBG in 6 tension points.The non-uniform stress in composite material was detected with high spatial resolution successfully.Compared the measuring result through the system with the result through finite element analysis,the distribution of non-uniform strain on the grating with 11.4 mm long length is effectively detected.