Fracture behavior of epoxy nanocomposites modified with polyol diluent and amino-functionalized multi-walled carbon nanotubes: A loading rate study

The synergistic effects of reactive polyol diluent and amino-functionalized multi-walled carbon nano-tubes on fracture of two-and three-phase (hybrid) epoxy nanocomposites are investigated under quasi-static and dynamic loading conditions. Digital image correlation method with a drop-tower and high-speed camera are used for dynamic tests. The crack-tip deformation histories and fracture parameters for stationary and growing cracks are extracted. Tests show improved crack initiation toughness in modified-epoxies relative to the neat resin with the highest enhancement in hybrid nanocomposites. The dynamic crack initiation toughness values are found to be consistently lower than the static counterparts. Fractographic examinations reveal distinct rate-dependent morphologies. Polymer matrix composites (PMCs) have been widely used in a variety of engineering applications. Among the various matrix systems, epoxy resins are commonly used due to their physical, thermo-mechanical and dielectric attributes. However, their brit-tleness often leads to structural damages due to poor crack growth resistance. A common approach to improve damage tolerance is to incorporate rubber particles [1–3], thermoplastics [4–6] or stiff fill-ers [7–9] that alter the overall mechanical and fracture performances favorably [10,11]. The rubbery or thermoplastic phases, however, generally improve ductility but diminish stiffness of ther-moset resins whereas the rigid inorganic inclusions such as silica and alumina improve stiffness and strength at the expense of duc-tility. In addition to the type of fillers, other studies [12–17] on particulate PMCs suggest that fracture toughness can be affected by various factors such as filler size, shape, volume fraction, filler– matrix adhesion strength and the loading rate. An alternative approach to counteract the reduction in thermo-mechanical properties of PMCs is simultaneous addition of compliant and stiff phases where each contributes its inherent characteristics to produce optimum stiffness, strength and toughness. Previous works on such hybrid composites [18–20] have shown enhanced fracture toughness and energy absorption. In the recent years, a number of polyol based reactive diluents has been considered as a good choice for improving the fracture resistance of brittle epoxies [20,21]. In addition to the toughening effect, the lower viscosity and the extended pot life of polyols generally increase the level of filler loading as well as the resin wetting action without a substantial decrease in curing rate and thermal stability. These properties make polyols suitable for modification of epoxy resins to achieve improved peel and impact strengths, and facilitate processing of particle-filled and fiber-reinforced PMCs. For instance, the use of polyether polyol as a toughener for epoxy resins by …