Experimental and numerical study of mode I interlaminar behavior of carbon nanotube reinforced glass-epoxy composite: A multiscale approach

In this paper, the effect of adding carbon nanotubes (CNTs) between glass-epoxy composite laminate in the mode I interlaminar behavior was investigated by experimental and numerical methods. Mode I interlaminar fracture toughness of epoxy matrix modified by 0.5 wt%, and 1 wt %, CNTs were compared with an unmodified epoxy matrix using the double cantilever beam (DCB) specimen. By adding a 1% weight ratio of CNT, the mode I interlayer fracture toughness increases by about 50%. A multiscale approach was used for modeling the DCB test. At the micro-scale, toughness mechanisms, including CNT pull-out, its breaking and bridging, and the sepa-ration of the nanotube from the surrounding resin were modeled. The separation of the nanotube from the surrounding resin was modeled with a cohesive element. The cohesive element around the nanotube was embedded in the three-dimensional elements of the resin and according to the weight percentage of the nanotube to the resin, a representative volume element (RVE) was created. The orientation, length, and diameter of CNT were considered randomly in the RVE. In the mesoscale, according to the percentage of glass fibers to resin, an RVE was created randomly. By using the damage model whose parameters were extracted from the micro-scale, crack growth was simulated. According to the Hill-Mendel conditions, the relationship between the meso and macro scale was established and the amount of effective quantities of traction and separation from the micro-scale was used as the parameter of the cohesive element in the macro-scale. To consider the bridging effect of glass fibers and its effect on interlayer fracture toughness, random distribution of interlayer beam elements was created. At the macro-scale, the DCB test simulated and cohesive elements were considered for modeling lower-scale mechanisms and beam elements for fiber bridging simultaneously. A favorable agreement between the experimental and simu-lation results was obtained.