SCALE EFFECTS IN THE POST-CRACKING BEHAVIOUR OF CNT-EPOXY COMPOSITES: PREDICTING CRACK JUMPS AND DUCTILE-TO-BRITTLE TRANSITIONS
The scale effects on the global structural response of fibre-reinforced brittle-matrix specimens subjected to bending are discussed in the framework of Fracture Mechanics by means of the Updated Bridged Crack Model (UBCM). This analytical model assumes the composite as a bi-phase material, in which both the brittle matrix and the reinforcing fibres contribute to the global toughness. In particular, the bridging mechanism of the reinforcing layers can be described by an appropriate cohesive softening constitutive law, which takes into account the progressive slippage of the fibre inside the matrix. In addition, the discontinuous phenomena, i.e., crack jumps (snap-back) and snap-through instabilities, which experimentally characterize the post-cracking behaviour of the composite, can be captured in a quantitative way. Furthermore, UBCM predicts different post-cracking regimes depending on two dimensionless numbers: the reinforcement brittleness number, NP, and the pull-out brittleness number, Nw. Finally, UBCM simulations of non-smooth crack evolutions are compared to experimental results reported in the scientific literature, in which carbon nanotube-epoxy specimens are tested in bending.