  The sensitivity of crack growth resistance to the choice of isotropic or
kinematic hardening is investigated. Monotonic mode I crack advance under small
scale yielding conditions is modelled via a cohesive zone formulation endowed
with a traction-separation law. R-curves are computed for materials that
exhibit linear or power law hardening. Kinematic hardening leads to an enhanced
crack growth resistance relative to isotropic hardening. Moreover, kinematic
hardening requires greater crack extension to achieve the steady state. These
differences are traced to the non-proportional loading of material elements
near the crack tip as the crack advances. The sensitivity of the R-curve to the
cohesive zone properties and to the level of material strain hardening is
explored for both isotropic and kinematic hardening.
