A Study On Nano-phase Interlaminar Toughening Of Carbon Fiber/Epoxy Composites

Carbon fiber/epoxy(CF/EP)composites have been widely used as structural materials in many aerospace applications owing to their superior specific strength and specific elastic modulus.However,interlaminar delamination,the most common failure mode of these composite materials,frequently occurs due to the inherent brittleness of the epoxy matrix.Many strategies such as stitching or Z-pinning,matrix toughening and interleaving have been developed to overcome this problem.Interleaving as the most popular method,where a discrete interlayer(either thermoset or thermoplastic)is inserted between adjacent plies of composite laminates,has provided significant improvements in interlaminar fracture toughness.In this thesis,three types of interleaving methods were investigated.First,a PA66/PCL hybrid film was used as the interleaf to improve the interlaminar fracture toughness of CF/EP composite laminates.It was found that mode I and II fracture toughness of the laminates interleaving with PA66/PCL significantly increased by 98% and 101%,respectively.However,the PA66/PCL interleaved laminates displayed little change in the flexural strength and modulus.In the second part,ternarybulk composites composed of carboxylated CNTs,PSF and epoxy were studied to lay a basis for the anaylsis of the CNT/PSF interleaved laminates.It can be found that the CNT-COOHs were selectively distributed in the epoxy phase of the ternary composites.Rheological measurements indicate that by adding CNT-COOHs into EP/PSF binary systems,the curing-reaction rate and complex viscosity increased.As a result,the phase separation of the composites was suppressed.Furthermore,the introduction of CNT-COOHs into the epoxy/PSF system was found to yield a significant improvement in the toughness with no adverse effect on the tensile strength and Young's Modulus.Subsequently,a novel CNT/PSF sandwich paper was used as the interleaf in the CF/EP composite laminates.Mode I fracture toughness of 5% and 10%CNT/PSF interleaved laminates were 41% and 53% higher than that of Control.Similarly,5% and 10% CNT/PSF interleaved laminates also produced 25% and 34% increases in mode II fracture toughness.Also,the integration of 5%CNT/PSF interleaves into CF/EP laminates improved the flexural strength and modulus by 19% and 8%.With 10%CNT/PSF interleaving,the flexural strength and modulus of laminates increased by 27% and 29%,respectively.In the third part,a special type of hierarchical ‘interleave',obtained by direct growth of ZnO nanowires(NWs)on dopamine modified carbon fiber surface,was used.The formed CF/PDA/ZnO NW hybrid structure increases the mode I and II fracture toughness of laminates by 63% and 22%.The laminates with CF/PDA/ZnO NW also show an increase of 9% for both the flexural strength and modulus compared to the Control.