Effect of long multi-walled carbon nanotubes on delamination toughness of laminated composites

Two continuum-mechanics-based mechanistic models are proposed to characterize the pull-out behavior of a long multi-walled carbon nanotube (MWCNT) from its surrounding matrix based on available experimental observations. One model is based on the mechanism of debonding and its propagation along the MWCNT-matrix interface due to weak interfacial shear strength; and the other is based on the sword-in-sheath mechanism after the breakage of the outermost layer in a MWCNT. Both models are then employed to describe the bridging tractions between the two delaminated laminates in a double cantilever beam (DCB) test specimen, and then to study numerically the effect of the long MWCNTs or nano pins on the delamination toughness of laminated composites. The present numerical results reveal that the MWCNT's length, density, and maximum pull-out displacement as well as the interfacial friction shear stress are important parameters affecting the delamination toughness.