Low-velocity impact response of MWCNTs toughened CFRP composites. Stacking sequence and temperature effects

Despite impressive specific mechanical properties, carbon fiber reinforced polymer (CFRP) composites are strongly susceptible to impact events due to their low interlaminar fracture toughness which fosters delamination phenomena detrimental to the residual mechanical properties of the laminate. The incorporation of nano-fillers in the neat matrix is a suitable way to improve both Mode I and Mode II CFRP fracture toughness. In view of this, the present work assessed the toughening effect of multi-walled carbon nanotubes (MWCNTs) on the impact and post-impact response of carbon fiber reinforced epoxy composites evaluating the effect of two key design parameters: ply stacking sequence and operating temperature. Two stacking configurations, i.e., cross-ply (CP) and quasi-isotropic (QI), and three testing temperatures, i.e. −40 °C, room temperature and +80 °C, were considered. The results highlighted a higher damage tolerance of QI laminates characterized by a superior residual flexural strength than CP for all impact energies and temperatures. Moreover, the toughening effect played by MWCNTs on QI samples was disclosed for all temperatures with an improvement in the normalized residual flexural strength up to 10%. The enhanced damage tolerance must be ascribed to the bridging effect and the pull-out phenomena provided by CNTs which delay matrix cracks opening and propagation.