Improvement of interlaminar fracture toughness of composites at cryogenic temperature by adding carbon nanotubes

Abstract

Carbon fiber reinforced polymeric composites have been considered as promising materials for aerospace application such as cryotanks of launch vehicles and spacecraft structures. Fiber reinforced composites, however, have usually shown fracture toughness degradation at cryogenic temperatures due to embrittlement of polymer matrices and thermally induced stresses. Therefore, there are two key requirements to apply composites for cryogenic application. First, interlaminar fracture characteristics should be investigated with comparison to the one at room temperature, and second, crack resistance nature should be enhanced against crack-induced failures. These two issues are the purposes of this study, and thus mode-I interlaminar fracture characteristics and improvement of crack resistance for carbon/epoxy composites were investigated at a cryogenic temperature. As an evaluation of the cryogenic characteristics, toughening effect of a carbon/toughened-epoxy was investigated both at RT and at -150 ℃. It was shown that the degree of enhancement of interlaminar fracture toughness by the toughened epoxy was reduced at the cryogenic temperature, which is mainly attributed to embrittlement of the epoxy resin. An attempt was performed to improve the crack resistance by employing MWNT nanofillers into the epoxy resin formulation. As a result, it was clarified that the addition of MWNTs brought about rough fracture surfaces in the matrix region, and thus higher fracture energy was dissipated during the crack propagation. The effects of MWNT nanofiller on matrix dominant properties, i.e. ILSS, transverse tensile property, were also evaluated through experiments and micromechanics-based finite element analysis. Through the FEA, it was found that the principal direction, maximum principal stress region and interfacial stresses were changed due to the induced thermal stresses, which resulted in a change of composite facture mode. A chemical surface treatment process was s...