Temperature dependence of fracture toughness of epoxy resins cured with diamines

The fracture toughness (critical stress intensity factor, K_Ic) of epoxy resins cured with four diamines has been measured as a function of temperature over the range from –35° C to above T_g. It was found that K_Ic for each epoxy-diamine system did not vary below room temperature, and in the higher temperature range K_Ic increased with increasing temperature to a maximum and then decreased. The temperature which maximized K_Ic, agreed with the temperature at which the flexural modulus of the epoxy resins abruptly dropped. This temperature was therefore considered as T_g. This temperature was found to be about 20° C lower than the heat deflection temperature under load (1.82 M Pa) of the resins.