## 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$\sp\circ$C to 55$\sp\circ$C, 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

Topics: Chemical engineering
Year: 1988
OAI identifier: oai:scholarship.rice.edu:1911/13296