In Situ Characterization Of Mechanical Properties Of Carbon Fiber/epoxy Composites By Nanoindentation

Composite materials are composed of complex multiphase systems ofheterogeneous materials. The mechanical behavior of composites is closely related notonly with in situ properties of components including the matrix and the reinforcement,but also with properties of the interlayer between the matrix and the reinforcement.The interfacial crack and sliding phenomena in the practical application of compositesoften occurs. Therefore, there is a very important significance to study the in situproperties of components including the epoxy matrix, carbon fiber and interlayer ofthe carbon fiber composites.Due to the micro-scale of the component and the interface, it is difficult for thetraditional testing methods to measure the mechanical properties of the correspondingregion in the microstructure. Nanoindentation technique is an effective approach tomeasure the in situ micro/nano-mechanical properties of heterogeneous materials. Thelatest system of nanoindentation combined with nano-mechanical microscope systemhas achieved the measurement of the nano-mechanical test and can obtain the in situmechanical properties in a specific domain and the3-D indentation morphologysimultaneously. In recent years, the application of nanoindentation has received muchattention by many researchers on the experiments of mechanical properties ofcomposites, especially the interlayer of the carbon fiber composites.The present thesis pays attention to examining the mechanical behavior ofcomposites at nano/micros scale by nanoindentation technology. The main contentsare as follows:(1) A systematic study on the in situ properties of components including theepoxy matrix, the carbon fiber and the interlayer was carried out. The variation ofmechanical properties of components between intrinsical properties and in situproperties is analyzed, and the main reason causing the variation is obtained. Furtherexperiments and analysis indicate that surface residual stress of mechanical grindingof the specimen in the sample preparation process and the interaction of the twomaterials in the composite are the main reasons.(2) The mechanical properties of interface of carbon fiber/epoxy composites areevaluated effectively by nanoindentation technique, including the development oftesting device, and the research on testing technique of interfacial shear performance.The research shows that optimal thickness of composite specimen is3micrometer inindividual push-out tests on perpendicularly aligned fibers. The load value and the displacement are respectively,90~100micro Newton and2micrometer, while thesingle fiber debonds from matrix by experiments, and the interfacial shear strength is137MPa~152MPa based on calculation.(3) Based on nanoindentation technique, the creep behavior on the in situproperties of composites components is tested effectively. Experiment results indicatethat the creep behavior of each component is not influenced by the loading rate.Meanwhile, the variation of creep behavior on different components is studied at thesame loading speed. Moreover, creep behavior of composite at the transition zonefrom the fiber to the epoxy matrix is researched in the present nanoindentation tests.The result shows that creep behavior in the interface exhibits dispersion, which iscaused by the heterogeneity of interfacial phase in material preparation and thedifference influence extent from carbon fiber and epoxy matrix around the interface.The present work contributes to understanding reinforcement mechanism ofcomposites and provides in situ data of components for the theoretical model ofeffective properties of the composites.The financial support from the Natural Science Foundation of China under grant#10872011and#11172012, the Doctoral Foundation of Ministry of Education ofChina under grant#20101103110005and the Municipal Natural Science Foundationof Beijing under grant#3092006are gratefully acknowledged.