| Carbon fiber reinforced polymer?CFRP?has the advantages of high specific strength,small thermalexpansion coefficient and vibration absorption.Compared with the traditional metal shaft,CFRPdrive shaft hasmany advantages of light weight,high strength,size stability and hightransmission efficiency,thus have been widely used in helicopters,large-scale wind turbines,marine vessels,heavy machine tools and other high-end equipment.However,the curing residual stresses generated during the curing process of CFRP shaft,induced by the mismatch of thermal expansion coefficients between reinforced fiber and resin,the shrinkage in the resin polymerization reaction or other reasons,can cause defects such as deformation of fiber and internal delamination in the CFRP shaft even before service.Moreover,these defects are hidden inside the shaft and difficult to check,thus becoming potential hazards and reducing the mechanical performance of the CFRP shaft in service.So it is significant to monitor the curing process of CFRP shaft with efficient non-destructive testing technology.Fiber Bragg Grating?FBG?has the advantages of small diameter?80-125??m,being easilyembedded,high temperature resistance,immunity to electromagnetic interference,and electrical insulation.It can be embedded between the layers of CFRP shaft just like fiber and has very small influence on the mechanical properties of structure.What's more,it can realize multi-point measurements with one fiber,which is very suitable for anisotropy composites.Therefore,in this thesis,FBG sensors were used to monitor the curing process of CFRP shaft.The main contents are as the followings:Firstly,the principle of monitoring curing process of CFRP shaft using FBG sensors was analyzed and elaborated combined with composite structure design basis and sensing principle of FBG.The CFRP shaft with 22 layers was designed and the distribution policy of FBG sensors was determined.Four FBG sensors were embedded between 0°layers to monitor the curing process of CFRP shaft in which two of them were embedded between 1st and 2st,one of them was used to monitor temperature and the other one was used for temperature and strain monitoring,and other two were embedded between 9st and 10st in the same way.A new separated mandrel was designed to embed and extract fibers during the autoclave molding process of the shaft.This mandrel can protect fibers from being damaged during the embedding process and demoulding,and also provide technical basis for the in-service mechanical monitoring of CFRP shaft.In order to measure the residual strain inside the CFRP shaft accurately,temperature and stress calibration tests were conducted on the embedded FBG sensors based on the distribution policy.Stress calibration tests were conducted on FBGa and FBGb,and results showed that the stress sensitivity coefficients of them were very close and both had high linear correlation coefficients with 99.99%.The stress sensitivity coefficients of FBGa and FBGb were1.33×10-5pm/Paand1.37×10-5pm/Pa respectively,which showed basic consistency with theoretical value with 7.0%and 4.2%deviation respectively.A new packaging technology was used on FBG1and FBG2 to exclude the strain factor.Tensile tests were conducted on packaged FBG1 and FBG2 to validate the reliability of the packaging technology.Results showed that the proposed packaging technology can be used to make FBG temperature sensor.Then,temperature calibration tests were conducted on FBG1and FBG2 using water bath cooling method.Results showed that the temperature sensitivity coefficients of them were very close and both had high linear correlation coefficients with 99.99%and 99.98%respectively.The temperature sensitivity coefficients of FBG1 and FBG2 were 8.58pm/?and 8.52pm/?,which showed basic consistency with theoretical value with 2.7%and 3.4%deviation respectively.Lastly,the cure monitoring of CFRP shaft based on FBG sensors was conducted.The CFRP shaft specimen was manufactured with the separatedmandrel through manualply stacking and autoclave molding curing method.Two temperature-compensation FBG sensors and two temperature-strain monitoring FBG sensors were embedded into the lamina of the CFRP shaft during the ply stacking.The strain of the layers of CFRP shaft was monitored during stacking process and the inner temperature and stain of the CFRP shaft were monitored during the curing process.Results showed that FBG sensors can reflect the strain change during the stacking accurately and that residual strain was obtained after the temperature compensation.The residual stress of-4 MPa was monitored between 1st and 2st when temperature decreaced to 85?and the stress monitored between 9st and 10st was nearly 0 when temperature decreased to 55?.The testresults showed that the strain in the inner surface and the middle layer were different in this CFRP shaft with diameter thickness tratio of 28.4.The process to embed the FBG sensors into the CFRP shaftwith the separated mandrel wasfeasible,and the FBG sensors were capable of reflecting the temperature and straincharacteristics of the CFRP shaft during the curing process.This research providesinstructions for the design and manufacture of the CFRP drive shaft. |
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