A Study on Concomitant Toughening and Strengthening of Carbon Fiber/Epoxy Composites

Abstract

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, often 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 great improvements in interlaminar fracture toughness. However, decreases of in-plane properties usually occur. In order to obtain the composite laminates with excellent overall performance, it is important to develop and investigate the concomitant strengthening and toughening methods. In this thesis, three types of interleaving methods were investigated. In the first method, 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 the mode I and II fracture toughness values of the laminates interleaving with PA66/PCL significantly increased by 98% and 101%, respectively. However, the PA66/PCL interleaved laminates displayed little change in flexural strength and modulus. Thus, concomitant strengthening and toughening of the laminates could not be achieved. In the second method, ternary bulk composites composed of carboxylated CNTs, PSF and epoxy were initially studied to offer a basis for the anaylsis of CNTs/PSF interleaved laminates. It was found that CNT-COOHs were selectively distributed in the epoxy phase of the ternary composites. Rheological measurements indicated that by adding CNT-COOHs into EP/PSF binary systems, the curing-reaction rate and complex viscosity were increased. As a result, the phase separation behavior of the composites was suppressed and the phase separation process stopped at an earlier stage. 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 CNTs/PSF sandwich paper was used as an interleaf in the CF/EP composite laminates. Mode I fracture toughness values of 5% and 10%CNTs/PSF interleaved laminates were 41% and 53% higher than that of the Control. Similarly, the same interleaved laminates also produced 25% and 34% increases in their respective mode II fracture toughness. Moreover, the integration of 5%CNTs/PSF interleaves into the CF/EP laminates improved the flexural strength and modulus by 19% and 8%. With 10%CNTs/PSF interleaving, the flexural strength and modulus of the laminates increased by 27% and 29%, respectively. Hence, concomitant toughening and strengthening of carbon fiber/epoxy composites was achieved. However, minor problems existed, that is, the CNTs in the bulk ternary composites and the laminates generally suffer from partial inhomogeneous dispersion. In the third method, a kind of a hierarchical ‘interleaf’, that is, by direct growth of ZnO nanowires (NWs) on dopamine modified carbon fiber surface, was used. The resultant CF/PDA/ZnO NW hybrid structure increased both mode I and II fracture toughness of the laminates by 63% and 22%, respectively. The laminates with CF/PDA/ZnO NW also showed an increase of 9% for both flexural strength and modulus compared to the Control. Clearly, concomitant toughening and strengthening of carbon fiber/epoxy composites was also achieved with this method. Among these three methods, 5wt%CNT/PSF, 10wt%CNT/PSF and CF/PDA/ZnO NW interleaves could endow the carbon fiber/epoxy composites with concomitant toughening and strengthening effect. However, the best overall toughness and mechanical properties was achieved by 10wt%CNT/PSF interleaving.