A constitutive equation for creep in glassy polymers and composites

Abstract

The creep of polymethyl methacrylate was investigated in four-point flexural loading mode. Measurements were taken at temperatures from 8\sp\circC to 55\sp\circC, time periods up to 450 hours and stresses ranging from 5 to 25 MN/m\sp2. The data obtained were successfully superposed vertically; the data reduction, in this way, was expressed in the form of a constitutive equation: e(t, T, S) = e\sb0 (ref). exp $\lbrack-(\Delta$H\sb0 $-$ $\beta$S)/R. (1/T $-$ 1/T\sb{\rm ref})).exp ($\beta$/RT. (S $-$ S\sb{\rm ref})). t\sp{\rm n} which shows that the creep strain (e) may be obtained as a product of separable functions that express the effect of time (t), temperature (T) and stress (S). Subscript ref. indicates the chosen reference state. The creep behavior follows a power law time dependence with an exponent equal to 0.24. The apparent activation energy of the creep is independent of temperature (Arrhenius behavior), stress dependent and decreases with increasing stress