EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

This paper deals with ductile damage nonlocal modelling of semi-crystalline polyamides using state regularized formulation towards capturing ductile damage localization (softening) while overcoming spurious mesh sensitivity in the predictive numerical simulation. The mechanical behavior is simulated through a viscoelastic viscoplastic ductile damage model (VEVPD), from which the constitutive laws are derived within an extended thermodynamic framework combining both nonlocal viscoplastic and nonlocal damage internal variables. Typical local models often suffer from mesh dependency and non-physical solutions due to ill-posed differential equations in confronting with the model bifurcation limit. This research proposes a nonlocal gradient enhanced model aimed at capturing the material behavior at high damage levels, in which local models lead to instabilities. To this end, a gradient enhanced free energy is introduced into the thermodynamic framework, and the state variables are regularized through the interaction with their nonlocal counterpart. The model is developed as a UMAT subroutine based on a return-mapping algorithm, which updates stress, state variables, and the associated tangent moduli at each time step. The model is firstly investigated based on the nonlocal scalar damage variable and then is compared with the case that the nonlocal variable is derived from the plastic deformation. Finally, the capabilities of the developed numerical approach are examined via a set of representative examples under uniaxial monotonic strain-controlled tension. A parametric study is performed in order to evaluate the effectiveness of the method on the mesh objectivity and the regularization of the local variables in the softening zone. It is shown that the ductile damage localization and mesh insensitivity are evidenced more efficiently when the nonlocal variable is derived from the plastic strain rather than from the scalar damage variable. The current model is developed under small deformation assumption towards combining it with multiscale approaches for composites.