In this work, distortion gradient plasticity is used to gain insight into
material deformation ahead of a crack tip. This also constitutes the first
fracture mechanics analysis of gradient plasticity theories adopting Nye's
tensor as primal kinematic variable. First, the asymptotic nature of crack tip
fields is analytically investigated. We show that an inner elastic region
exists, adjacent to the crack tip, where elastic strains dominate plastic
strains and Cauchy stresses follow the linear elastic stress singularity. This
finding is verified by detailed finite element analyses using a new numerical
framework, which builds upon a viscoplastic constitutive law that enables
capturing both rate-dependent and rate-independent behaviour in a
computationally efficient manner. Numerical analysis is used to gain further
insight into the stress elevation predicted by distortion gradient plasticity,
relative to conventional J2 plasticity, and the influence of the plastic spin
under both mode I and mixed-mode fracture conditions. It is found that Nye's
tensor contributions have a weaker effect in elevating the stresses in the
plastic region, while predicting the same asymptotic behaviour as constitutive
choices based on the plastic strain gradient tensor. A minor sensitivity to X,
the parameter governing the dissipation due to the plastic spin, is observed.
Finally, distortion gradient plasticity and suitable higher order boundary
conditions are used to appropriately model the phenomenon of brittle failure
along elastic-plastic material interfaces. We reproduce paradigmatic
experiments on niobium-sapphire interfaces and show that the combination of
strain gradient hardening and dislocation blockage leads to interface crack tip
stresses that are larger than the theoretical lattice strength, rationalising
cleavage in the presence of plasticity at bi-material interfaces.