Variable Temperature Raman Scattering Study On Micromechanics Of Carbon Fiber Reinforced Polyimide Composites

The thermal expansion of carbon fiber(CF)and the micro-mechanical properties of interface are crucial factors for fiber reinforced composites.Variable temperature Raman spectroscopy,due to its real-time and microcosmic detection,has a unique advantage in studying interfacial micromechanical behaviors.In this work,such Raman technology coupled with mapping scan function was used to investigate the performances of CF and real-time micro-mechanical properties of fiber/matrix interface in heating/cooling process.The main contents and results are shown as followed:(1)The thermal expansion and Raman stress sensitivities of CF have been investigated by Raman spectroscopy.The Raman spectra of CF are obtained under different temperatures of 0~500?,suggesting that G' band position of CF decreases linearly with the increase of heating temperature.Typically,the Raman shift rate is-2.62 cm-1/(100?).The CF with heterogeneous structure exhibits an excellent Raman stress sensitivity,and the G' band shifts to lower wave number direction at the rate of-28.27 cm-1/(1%strain).Moreover,the micro-mechanical investigation of interface region is based on the G' band shift.(2)The Raman sensitivities of CNT-PI films during heating and cooling process have been evaluated by Raman mapping scanning.The performances of CNT-PI films containing different CNT contents were studied.It is found that PI film with 3%CNT possesses the tensile strength of 138.11 MPa and the decomposition temperature is as high as 580?.In addition,internal stress of CNT-PI films have changed significantly while heating.For the case of 3%CNT-PI,the film endures compressive stress at 25~150?,stress transition at 150~300? and tensile stress over 300?.Heating can effectively eliminate the thermal residual stress,but extremely high temperature(over 350?)will cause stress concentration.The micromechanics of CF composites interface can be reflected by CNTs which act as a stress sensor.(3)The micromechanics of CF/PI films is further studied by variable temperature Raman scanning technology.It is found that the interface microregion is strongly affected by thermal motion of the resin,and G' band of CF shifts to lower wave number direction for about 2.5 and 10.1 cm-1 at 25~150 and 150~300?,respectively,which is similar to thermal motion trend of the resin.Herein,the stress of around CF is near 0 MPa at 200?,which indicates that CF/PI films are easier to eliminate thermal residual stress than the pure resin.Meanwhile,the heat transfer between CF and PI resin is a heterogeneous thermal diffusion.And the thermal stress of CF cannot completely recover.(4)The interfacial micro-mechanics of CF/CNT-PI films during heating/ cooling process has been investigated by variable temperature Raman mapping scattering.The results indicate that CF/CNT-PI films contain compressive stress and the stress declines from fiber to matrix at 200?.Especially,the stress of internal region is approximate 0 MPa,implying that the residual stress eliminates at this temperature.During cooling process,a thermal diffusion layer emerges in the interface region due to different thermal conductivities of CF and the resin.The stress transfer between CF and matrix is less than 100%.There is thermal stress hysteresis in the matrix,which makes the CF interface region continue to be subjected to thermal stress,and the stress cannot fully recover to the initial state.But CFs possess excellent enhancement effect.The application of variable temperature Raman technology can effectively acquire some important micromechanics data on CFreinforced composites,e.g.thermal diffusion and stress distribution and transfer.These results might provide a basis for the safety and efficacy of the use of composite materials.