Michael J. Kubát

Strain rates and volume changes during short-term creep of PC and PMMA

AbstractPolycarbonate, 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 polynomials

Volume changes during flow of solid polymers

t = \sum\limits_{i{\text{ }} = {\text{ }}0}^n {a_i \varepsilon ^i }

Some phenomenological relaxation rate equations based on Bose–Einstein similar kinetics

(usually of 4th order), both for longitudinal and transverse strains, for the purpose of calculating axial and transverse strain rates. Viscous Poisson ratio, μ

A Kohlrausch type equation based on a simplified cooperative model

\mu = - \frac{{\dot \varepsilon _{33} }}{{\dot \varepsilon _{11} }}

Stochastic approach to cooperative relaxation processes in solids

was calculated and plotted vs. time. Simultaneously, deformational Poisson ratio ν = -33/911 and volume strain ɛν = ɛ11 + 2ɛ33 were measured and plotted. The volume strain increased with time during the initial stage of the creep process. After reaching a maximum value, it started to fall. The values of both μ and ν increased monotonically with time. By comparing the three plots it was seen that, while there was no relationship between the volume reversal and the deformational ratio ν, values of the viscous ratio μ<0.5 coincided with an increase in ɛν. For μ > 0.5, the opposite was true. It was concluded that for the purpose of characterizing the volumetric response of materials, such as PC and PMMA, in creep the viscous Poisson ratio represents a suitable and highly indicative parameter.Theoretical calculations of the volume strain behavior of the standard linear solid model and the generalized model of Voigt-Kelvin type were in agreement with the experimental data, predicting an increase or decrease in ɛν with time for μ < 0.5 and μ > 0.5, respectively.

The clustering mechanism underlying a kinetic model related to Bose-Einstein statistics

This paper presents the results of an experimental study of the stress relaxation behaviour of PE where the focus was on determination of the volume changes taking place during the relaxation process. The dimensions of the samples were followed using a specially designed non-disturbing extensometer. The extensometer data were confirmed in experiments where the volume was measured with a specially designed liquid stress dilatometer. The bulk of the results was obtained with LDPE and LLDPE. High density polyethylene was shown to behave similarly.The decrease in volume, corresponding to an increasing Poissons ratio, during relaxation was approximately linear with log time. Volume vs. stress diagrams were linear; the values of the apparent bulk modulus calculated from them were only slightly higher than those obtained from the stress-strain curves. Grüneisen parameter was measured and compared with reported values. Possible similarities between the volume change during stress relaxation and that occuring during the process of physical ageing are discussed.