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