Fracture Behavior of Pure Epoxy Resin and Effect of Inclusions: A Size Effect Study

Abstract

The fracture behavior of pure epoxy resin is studied under varying loading rates. The study continued by toughening the epoxy resin with multi-walled carbon nanotubes (MWCNTs) and alumina nanoparticles (ANPs). The size effect testing methodology is adopted to prepare geometrically similar specimens with geometrically similar notches. The Single Edge Notched Tensile (SENT) specimens are scaled geometrically in the ratio of 1:2:3:4 while maintaining a constant notch depth-to-width ratio (a/D) of 0.25 and a constant gauge length-to-width ratio (L/D) of 4. The tensile testing of an unnotched specimen determines Young’s modulus. The experimental investigation tested 414 specimens, including unnotched and SENT specimens, under mode-I tensile loading conditions. The fracture behavior of pure epoxy resin is studied under displacement loading rates of 0.01, 0.05, 0.5, 5, 50, and 500 mm/min. The nominal strength of the SENT specimens followed type II energetic strength scaling laws. With the increase in loading rate, the mechanism of failure transitioned from brittle mode i.e. Linear elastic fracture mechanics (LEFM) criteria characterized by negligible fracture process zone (FPZ) to quasi-brittle mode characterized by large FPZ and back to brittle mode. The toughening effect of MWCNTs and ANPs on the fracture behavior of epoxy resin is studied individually for weight fractions of epoxy as 0%, 0.1%, 0.2%, 0.3%, 0.4%, and 0.5%. The addition of nanofillers resulted in a non negligible FPZ. The mode-I fracture toughness, fracture energy, and FPZ size increased with the amount of MWCNTs, peaked at the optimum weight fraction, and then decreased at a higher weight fraction. On the other hand, the fracture parameters increased monotonically as the quantity of ANPs increased. As ANP content increases, the brittleness number shifts from the energy region, explained by LEFM, to the transition region between the energy and strength regions. Whereas in the case of MWCNT toughening, further addition after optimum content resulted in the shifting back of the brittleness number towards the energy region. The decrease in notch width adopted from the varying loading rates study to the toughening effect study resulted in reduced fracture parameters.