Pullout of rough multiwall carbon nanotubes: A parametric study

A finite element study of MWCNT pullout from a ceramic matrix with a rough fibre–matrix interface is presented. MWCNT pullout differs in several ways from pullout of solid, micro-scale fibres, and has not been studied before. Interfacial roughness, due to missing CNT walls and other fabrication defects, is represented as an axisymmetric sinusoidal fibre–matrix interface with Coulomb friction. We find roughness resists pullout and aids push-out of the fibre over different portions of the pullout process, so a cracked nanocomposite with multiple MWCNTs would experience both crack closing and opening stresses. The net average pullout stress due to fibre bundles is then quite low relative to peak values, reducing energy dissipation relative to expectations of simpler models. During pullout, significant stress concentrations develop that can cause premature fibre fracture. Our results indicate that utilising interface roughness is not an effective strategy to improve composite strength and toughness.