When does nanotube grafting on fibers benefit the strength and toughness of composites?

Abstract The answer to this question is revealed by our dual scale models, which say that the composite is the strongest and the toughest not when nanotubes themselves absorb energy but when they serve a supporting role in facilitating matrix to do the energy absorption instead. Toughening by means of only nanotube debonding/pull-out is not the most optimal strategy. The model concurrently considers five energy absorbing mechanisms acting at different scales. These mechanisms are nanotube debonding from the matrix, nanotube breakage, nanotube detachment from the (microscopic) fiber, fiber debonding and finally microdamage in the matrix. Our virtual experiments show that the benefit of CNT grafting largely depends on the tensile strength of CNTs and their interfacial cohesion with the matrix. A map of failure modes is constructed in the parametric space of these two CNT properties to help establish a window of properties beneficial for the strength and toughness improvement. The maximal enhancement is obtained when matrix is allowed to absorb energy through micro-scale damage and CNT debonding at the nano-scale has only partial contribution to it. Recommendations for the CNT tensile strength and CNT/matrix interfacial strength, necessary for creation of strong and tough hierarchical composites, are formulated.