Stress relaxation experiments in polycarbonate: A comparison of volume changes for two commercial grades

Abstract

Stress relaxation experiments were performed on two grades of polycarbonate at room temperature. The tests were performed in uniaxial extension and compression at deformations from the small strain, linear viscoelastic regime to the highly nonlinear viscoelastic regime just below yield. Simultaneous to the control of axial strain, both stress and lateral strain were measured, the latter providing the volume change for the samples. The volume change measurements in tension show initial dilatation of the samples followed by a volume relaxation that at the largest strains in one grade of polycarbonate leads to densities greater than those of the undeformed sample. In the case of the compression measurements, the volume decreases upon deformation, but, rather than relaxing back towards the undeformed volume, the samples continuously densify. The differences in the tensile dilatation for the two grades of polycarbonate suggest that the volume behavior may be related to the propensity to yield vs. fracture.

Stress-induced densification of glassy polymers in the subyield region

“…When dissolution of voids driven by diffusive flow dominates volume recovery caused by stress relaxation, the specific volume of a polymer decreases with time. This scenario of densification is confirmed by experimental data for polycarbonate, 1 which demonstrate that the rate of decrease in the specific volume at tension essentially exceeds that at compression. The aim of this work is to provide a quantitative analysis of this phenomenon.…”

“…The objective is to derive a constitutive model that can predict the following phenomena observed in uniaxial relaxation tests. 1 (1) Under tensile loads, the specific volume increases immediately after application of forces and slowly decreases afterward. (2) Under compressive loads, the specific volume decreases at the initial instant and proceeds to decrease with time, despite the stress relaxation.…”

Effect of yielding on the viscoelastic response of amorphous glassy polymers

“…We begin with experimental data measured in the sub-yield region at room temperature. For a detailed description of the experimental procedure, see Colucci et al 41 Figure 1 demonstrates fair agreement between experimental data and results of numerical simulation with strain-independent parameters W * and ⌺ * (yielding is neglected). Figure 2 shows that eqs.…”

Mechanical rejuvenation and overaging in the soft glassy rheology model

“…For instance, polymer glasses subject to a stress relaxation experiment well below the glass transition temperature exhibit rapid densification for certain strains [20]. Molecular simulations of simple structural glasses show that a small amplitude strain cycle at zero temperature can decrease the inherent structure energy [10].…”