In this thesis, the mechanisms of plastic strain localization in neutron-irradiated polycrystalline materials are extensively examined with their connection to Irradiation-Assisted Stress Corrosion Cracking (IASCC). New theoretical and numerical advances are introduced within the framework of Strain Gradient Crystal Plasticity (SGCP) that predict the formation and evolution of slip and kink bands, commonly termed “clear channels”. A novel enhanced theory (Enhanced SGCP) is proposed, in which the amplitude of the gradient term in the free energy expression is specified as a function of the local strain, thereby implicitly capturing microstructural evolution. This formulation introduces a variable length scale that enables control over the shape and width of the bands. With an appropriately chosen dependence, the bands become not only regularized but also overcoming limitations of classical SGCP models, in which band spreading and loss of plastic strain localization are observed at higher loads. Within the Enhanced SGCP framework, two models are introduced that differ in the chosen gradient contribution to the free energy. The Enhanced MicroSlip-SGCP model regularizes both types of bands, whereas the Enhanced MicroCurl-SGCP model regularizes only kink bands. In both models, the effects of grain size, the length scale, the boundary conditions at grain boundaries on macroscopic hardening, the microscopic evolution of dislocations, and stress concentrations at grain boundaries are examined in detail. Using an in-house computational code, an efficient framework based on the Fast Fourier Transform is established, enabling rapid solution of the highly nonlinear equations of the Enhanced SGCP theory at high resolutions. The results are validated against analytical solutions and a commercial finite-element code. Simulations indicate that, under MicroHard conditions, mechanical stresses at grain boundaries increase markedly, supporting the hypothesis of intergranular cracking consistent with IASCC mechanisms. Qualitatively, the results agree with experimental observations in irradiated stainless steels and zirconium alloys.
Amirhossein Lame Jouybari (Fri,) studied this question.