Damage Monitoring And Repair Of Fiber Reinforced Composites Based On Multi-Walled Carbon Nanotubes

Fiber-reinforced polymer(FRs are increasingly utilized in civil engineering,aerospace and high-speed train fields due to their advantages of lightweight,high strength,fatigue resistance and good design ability.However,defects like cracks、delamination and warpage will emerge after forming process due to improper parameter control or interaction between the composite and mold.In the service process,the composite structure is prone to delamination or other damage under the effect such as long-term fatigue load and impact load.The formation and propagation of damage that not visible like delamination will seriously affect the safety and service life of composite structures.Therefore,it is significance to monitor the damage evolution of composite during the forming and in-service process and resolve it reasonably.At present,the effective monitoring methods for invisible damage in composite are also deficient.Traditional methods are not suitable for health monitoring of composite due to its not only time-consuming and labor intensive,but also needs to predict the damage location in advance.In recent years,carbon nanotubes(CNTs)have been widely used to enhance the mechanical performance of fiber reinforced composite due to their excellent mechanical and electrical properties,and providing a broader application prospect for in-situ electrical resistance measurements.In this paper,a high-performance sensor based on multi-walled carbon nanotubes(MWCNT)is developed for the problems of health monitoring during forming and in-service process and delamination repair of FRP composites.The influence of resin state and interface damage on the resistance of sensor is investigated.The sensor is used to conduct molding process and damage self-monitoring of composite.Meanwhile,an in-situ repair method is also proposed for the delamination damage of carbon fiber reinforced composite.The damage evolution of repaired CFRP panel under compression load after modified by MWCNT is analyzed.The main contents and conclusions as follow:(1)Firstly,MWCNT@GF sensor was manufactured via coated MWCNT on glass fiber surface by physical vapor deposition.In order to evaluate the monitoring performance of MWCNT@GF sensor on the molding of glass fiber reinforced epoxy composites(GFRPs),the sensor was placed on the interlaminate of glass fiber.Subsequently,the resistance change of MWCNT@GF sensor was recorded during molding process and the effect on resistance of dry spots was also analyzed.The experimental results indicate that three stage of resistance change of MWCNT@GF sensor is corresponding well with resin flow-penetration-curing process.Moreover,the resistance respond of the sensor will be affected by dry spot significantly.This demonstrate that MWCNT@GF sensor has potential to monitor the molding process and dry spot defects of GFRPs.(2)Glass fiber/epoxy composites T-joint was fabricated after embedded MWCNT@GF sensor in the middle layer of glass fiber.Bolt fasten and fiber winding fasten were designed to enhance the interlayer performance.Subsequently,tensile test was carried out to obtain the bearing capacity of T-joint and the resistance change of MWCNT@GF sensor was also measured during the tensile process.The monitor ability of MWCNT@GF sensor for delamination damage of T-joint was analyzed.The results indicate that fiber winding fasten can improve the interlayer performance of T-joint without increase its weight significantly.The resistance responds monitored in tensile test was corresponding well to the delamination damage of T-joint.This demonstrates that MWCNT@GF sensor have excellent ability to insitu monitor the delamination damage.(3)A repair method is designed for the delamination damage of carbon fiber reinforced epoxy composites(CFRP)panel.Firstly,the tensile performance of CF burned by fire was explored.KH560 and MWCNT were used to modify the surface of burned CF.The ability of MWCNT interfacial sensor to monitor the CF/resin interfacial damage during tensile test was investigated.Subsequently,the resin in delamination region of CFRP panel was carbonized by fire and the burned CF was modified by KH560 and MWCNT.The healing agent was injected in ablative region and the repair process complete when its solidification.Compression test was performed to evaluate the repair efficiency of repaired panel.Moreover,the resistance change of MWCNT interfacial sensor was also recorded in compression test to explore its ability to monitor the evolution of compression damage.The results indicate that the interfacial shear strength of CF/resin and repair efficiency of the panel can be improved by KH560 and MWCNT.The resistance change under the compression load can be divided into two stages corresponding to different damage mechanisms.This demonstrates that MWCNT interfacial sensor can not only improve the repair efficiency of delamination damage of CFRP panel but also supply a good ability to in-situ monitor the compression damage.