NUMERICAL MODELING OF TRANSIENT CREEP IN POLYCRYSTALLINE ICE

Transient creep, an important deformation mechanism for polycrystallin e ice at quasi-static strain rates, is characterized by rate and tempe rature sensitivity, by isotropic and kinematic strain hardening, as we ll as by fabric and deformation-induced anisotropy. A physically based constitutive model, using internal state variables, has been develope d by Shyam Sunder and Wu (1989a, b) to describe the multiaxial behavio r of ice undergoing transient creep. To solve boundary value problems using this constitutive theory requires the numerical time integration of a coupled set of stiff and highly nonlinear first-order differenti al equations. A closed-form Newton-Raphson (tangent) formulation, in c onjunction with the alpha-method of integration, is developed to solve the constitutive equations. The fully consistent constitutive Jacobia n matrix that is used to assemble the finite element tangent stiffness matrix is also established in closed form. This algorithm is implemen ted as a subroutine in the finite element program ABAQUS and its predi ctions are verified against experimental data and known solutions. The importance of transient creep is demonstrated by performing simulatio ns of: (1) Arrested subsurface penetration; and (2) in-plane indentati on of a floating ice sheet.