Research On Key Technique Of Special Optical Fiber Grating Used In Thermoplastic Composite Molding Process Monitoring

Composites have received wide attention and numerous applications in many fields.Resin matrix composite is a more used composites,divided into thermosetting composites and thermoplastic composites.During the molding process,composites are affected by the process parameters such as temperature,strain and pressure.After removing the mold may appear warping,rebound and other defects.Therefore,an accurate monitoring technique is needed for real-time monitoring for molding process of fiber reinforced composites,especially embedded real-time monitoring.This provides an important reference role for regulating the molding process and stabilizing the quality of the finished products.Traditional electromechanical sensors have disadvantages of large size and slow response,which are not suitable for embedded monitoring.Optical fiber Bragg grating(FBG)has many unique advantages: easy to reuse,light weight,little impact on composite materials,etc.,which is suitable for composites molding process monitoring.In this paper,based on the research of common fiber Bragg grating used in composite molding process monitoring technique,the characteristics of special grating were further studied and applied to research about monitoring for the molding process of thermoplastic composites.The research is divided into two aspects.On the one hand,the special gratings,such as polyimide fiber gratings,are used for sensing to monitor the molding process.On the other hand,the high performance spectrum of the special gratings are used to demodulate the reflected light signal of gratings used in the molding process.Through the application of special gratings,the system performance is further improved.Compared with other monitoring techniques,polyimide fiber grating sensing system can monitor the high temperature above 350 ? and the large dynamic strain of 20000 ?? produced during the auto fiber placement in-situ molding process of thermoplastic composite.Therefore,the monitoring range of temperature and strain is expanded,and monitoring accuracy of the system is improved.In addition,signal demodulation technique based on high performance spectrum of special grating greatly increases the demodulation speed and cost performance of the system.In this paper,composite components are manufactured by auto fiber placement(AFP)in-situ molding and hot pressing molding.AFP process was monitored by fiber Bragg grating and polylamide fiber grating.The tunable F-P filter demodulation technique and the edge filter demodulation technique are used to demodulate wavelengths of gratings.According to wavelength data,temperature and strain are calculated.The main contents of this paper are as follows:(1)The general situation of thermoplastic composites and the necessity of molding monitoring are described.The basic situation of research topics is summarized from the aspects of molding process,application and research status of thermoplastic composites.(2)The development process of gratings is introduced.The sensing mechanism of temperature and strain of gratings is analyzed theoretically.The principle and characteristics of four common demodulation techniques are described in detail.(3)The fast demodulation of wavelength is realized,and demodulation rate is 100 Hz.The real-time monitoring of temperature and strain evolution in the AFP in-situ molding process of thermoplastic composites was realized by using polyimide gratings.Polyimide gratings can monitor the maximum temperature of 400 ?,and its monitoring strain is more than 20000 ? ?.In the AFP in-situ molding process,the highest temperature is 290.23 ? and the highest strain is509.36 ? ?.The temperature resolution of sensing system is 0.1 ? and the strain resolution is 1 ??.By analyzing temperature and strain evolution in the whole molding process,internal status of thermoplastic composites is analyzed.The results show that the feasibility and accuracy of polyimide gratings sensing system for monitoring the evolution of parameters in the AFP in-situ molding process of thermoplastic composites is proved by comparing the temperature measuring results of thermocouple and gratings.(4)The multi-parameter real-time monitoring for hot pressing process of composites and rapid molding process of fluid resin is realized by gratings.The monitoring results are compared and verified by using traditional sensors.According to monitoring results of gratings,internal structure evolution during the molding process is analyzed.The feasibility of gratings in real-time monitoring for hot pressing process of composites and rapid molding process of fluid resin is proved.(5)An optical fiber sensing system based on efficient edge filtering demodulation is built by using special grating,which is used for real-time monitoring of key parameters in the molding process of composites.The edge filter demodulation system is used to demodulate the center wavelengths of the reflected light of gratings quickly.The intensity demodulation of the reflected light signal of gratings is carried out by using the linear edge of the gratings with triangle high performance spectrum and the long period gratings in a certain band.Two kinds of special gratings are used for the edge filter demodulation,and the system is used in temperature sensing experiment.The monitoring results are showed that the output voltage of the system has a good linear relationship with temperature.The edge filtering demodulation system is used for the verification experiment that the demodulation of wavelengths of gratings in the molding process of composites,and monitoring accuracy of the edge filtering demodulation system is verified by comparison.