Finite-strain Thermomechanics of Viscoelastic-Viscoplastic Model for Thermoplastic Polymers

In this paper, a phenomenological finite-strain thermomechanically consistent
constitutive theory is reported to describe the thermomechanical response of a
thermoplastic polymer for its use in a Sheet Molding Compound (SMC) over large ranges
of strain rates and temperatures. This generalized framework addresses non-isothermal
viscoelasticity and viscoplasticity, and self-heating in a straightforward manner to predict
response under complex loading scenarios. Hereditary integrals in bulk and shear describe
the viscoelastic response from below to above glass transition temperatures.
Viscoplasticity is modelled using Perzyna non-associative flow rule with quadratic plastic
potential, Drucker-Prager yield function and non-isothermal Chaboche non-linear
kinematic hardening model [1]. Additionally, a phenomenological relationship for the
isotropic hardening stress is developed to capture self-heating [2]. An experimental
campaign is constructed to determine the viscoelastic relaxation spectrum from Dynamic
Mechanical Analyses; to validate viscoelasticity, capture elastic and yielding asymmetry,
and calibrate hardening parameters from uniaxial testing in tension-compression; lastly,
to calibrate and validate cyclic hardening, self-heating and induced irreversibilities from
uniaxial thermomechanical cyclic testing in individual and combined thermal and
mechanical cycles.