Constitutive law describing the phenomenology of subyield mechanically stimulated glasses

The principal features of the volumetric as well as the viscoelastic response of mechanically stimulated glasses can be summarized as follows: (i) the time-aging time shift factors contract upon increasing the probe stress (i.e., the stress apparently modifies the volume recovery kinetics), (ii) the volume recovery baseline remains unaltered (i.e., the underlying structure of the stimulated glass remains unchanged). Here we present a series of numerically simulated results concerning the responses of glassy polycarbonate that simultaneously fulfill these apparent contradictions. The problem was tackled coupling a modified Kalroush, Aklonis, Hotchinson, Ramos equation with the constitutive law for linear viscoelasticity within the domain of the reduced time. It was argued that the relaxation times under isobaric conditions depend on the temperature, the dimensionless volume, and the isotropic components of the stress tensor. Simulations are obtained with a minimum of experimental (PVT and linear viscoelastic) data inputs. Different loading protocols consisting of complex combinations and/or sequences of large and small mechanical stimuli were tested. Volumetric as well as viscoelastic behavior are systematically reported. A tentative explanation of the origin of the time-aging time contraction was finally proposed while some additional features concerning the volumetric response emerged.