Influence of external environments on fracture toughness of epoxy resin

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Abstract
Crack propagation in an epoxy resin in the presence of organic solvents was investigated. Fracture toughness (KIC, a critical stress intensity factor) of the epoxy resin in various external environments was measured using a double cantilever beam specimen. Fracture toughness for initiation (KICi) of the resin in the presence of organic solvents was larger than that in the absence of solvents, and the epoxy resin showed a minimum value of KICi in the presence of the organic solvent whose solubility parameter was about 11 (cal/cm3)1/2. This was due to large plastic deformation at a crack tip and the yield strength was lowered by exposure to organic solvents. The former increases KIC, while the latter decreases KIC. Fracture surfaces of the resin fractured in solvents suggest that a crack grew slightly when accompanied by a large plastic deformation, and then propagated at high speed.