Compressive and tensile response of CFRP cylinders induced by multi-walled carbon nanotubes

Compressive and tensile response of filament wound carbon fiber reinforced epoxy (CFRP) cylinders induced by adding various multi-walled carbon nanotubes (MWCNTs) were systematically evaluated in details. The addition of various MWCNTs significantly enhanced the compressive modulus of CFRP cylinders while barely affecting tensile modulus. Specifically, MWCNTs-NH2 endowed highest enhancement on compressive modulus of the cylinders by 63.7% due to their uniform dispersion and excellent interfacial adhesion compared with MWCNTs-COOH and raw MWCNTs. Moreover, specific energy absorption of CFRP cylinders was also enhanced by MWCNTs as a result of their positive effect on microcrack initiation and the corresponding enhancement was more obvious under compressive load than tensile load. Acoustic emission detection illustrated that MWCNTs effectively triggered abundant microcracks by inducing local stresses fluctuations around the fibers, but prevented macrocrack formation by their crack bridging effect, subsequently delaying cylinder collapse at ultrahigh AE energy. Additionally, Raman peak shift revealed that all layers of MWCNTs showed their whole performance under compression, while only the outer nanotube layers played a role through interfacial friction with the resin matrix under tension, which resulted in significant enhancement of the compressive modulus and related energy absorption compared to corresponding tensile values.