Enhanced Strain Gradient Crystal Plasticity Theory: Evolution of the Length Scale during Deformation

Amirhossein Lame Jouybari, dr. Samir El Shawish and prof. dr. Leon Cizelj from the Reactor Engineering Division (R4) at Jožef Stefan Institute published the research article “Enhanced Strain Gradient Crystal Plasticity Theory: Evolution of the Length Scale during Deformation” in International Journal of Plasticity, which is a highly renowned international journal in the field of plastic deformation, damage and fracture behaviour of solids.

The authors propose a new theory for accurate description of the formation of localized shear bands in irradiated metals under mechanical loading. Classical models often incorrectly predict the spreading of these bands and their gradual disappearance with increasing load, which does not reflect the actual behavior of materials.

The new theory addresses this issue by introducing a variable higher-order modulus that correlates the formation of shear bands with the evolution of the material’s microstructure during deformation. This ensures that the bands remain confined to narrow regions even at higher levels of strain. The theory is best suited for materials with strain-softening behavior, such as neutron-irradiated metals in nuclear reactors. These materials tend to form localized shear bands, which can lead to stress concentration at grain boundaries and ultimately result in crack formation.

For the numerical implementation of the theory, the researchers developed in-house computational code based on the Fast Fourier Transform method, and its accuracy was validated against analytical solutions. The new theory, together with its numerical implementation, represents a significant advancement in predicting the behavior of metals under demanding service conditions, such as those found in nuclear reactors.

Figure: Simulation results of a 2D polycrystalline aggregate model with 1800 grains. The main plot shows the distribution of plastic strain at a tensile loading of 0.02. The smaller plots display an enlarged view of region A at three different tensile loadings (0.004, 0.012, and 0.02), illustrating both the plastic strain and the rotation angle distributions.