STRAIN RATES AND VOLUME CHANGES DURING SHORT-TERM CREEP OF PC AND PMMA

Polycarbonate, PC, and poly(methyl methacrylate), PMMA, were subjected to short-term unaxial creep tests in tension. Measurements were made of the axial and lateral strains. The creep curves were fitted to poly nomials [GRAPHICS] (usually of 4th order), both for longitudinal and t ransverse strains, for the purpose of calculating axial and transverse strain rates. Viscous Poisson ratio, mu [GRAPHICS] was calculated and plotted vs. time. Simultaneously, deformational Poisson ratio nu = ep silon33/epsilon11 and volume strain epsilon(nu) = epsilon11 + 2epsilon 33 were measured and plotted. The volume strain increased with time du ring the initial stage of the creep process. After reaching a maximum value, it started to fall. The values of both mu and nu increased mono tonically with time. By comparing the three plots it was seen that, wh ile there was no relationship between the volume reversal and the defo rmational ratio nu, values of the viscous ratio mu < 0.5 coincided wit h an increase in epsilon(nu). For mu > 0.5, the opposite was true. It was concluded that for the purpose of characterizing the volumetric re sponse of materials, such as PC and PMMA, in creep the viscous Poisson ratio represents a suitable and highly indicative parameter. Theoreti cal calculations of the volume strain behavior of the standard linear solid model and the generalized model of Voigt-Kelvin type were in agr eement with the experimental data, predicting an increase or decrease in epsilon(nu) with time for mu < 0.5 and mu > 0.5, respectively.