The torsional creep of carbon fibre reinforced epoxide resins

The torsional creep of composite specimens containing 60% by volume of unidirectional HT-S carbon fibre, and of unreinforced epoxide resin has been studied. Measurements were made at temperatures of 25, 50, 75 and 90° C, in two environments — air and water. Torsional preloads of up to 40% of the ultimate torsional strength were applied. All the specimens showed primary and secondary creep behaviour during the 170 h test period, and a few resin ones tertiary creep. The effects, on the secondary creep rate, of varying the proportion of hardener in the matrix and the cure schedule were marked. The lowest creep rate for a given set of test conditions was obtained when the stoichiometric amount of hardener was used and the maximum cure given. Using specimens of this optimum type, more detailed studies of creep were performed. In all cases the activation energy for secondary creep lies between 5 and 6.3 kcal (g mol)^–1 indicating that the same basic mechanism occurs in each instance. The shear stress,, was found to be proportional to the logarithm of the secondary creep rate.