STZ Theory (Falk-Langer 1998)
Elastic: σ = G·γ (for γ < γ_y)
Yield: σ_y = G·γ_y
STZ activation rate:
R ~ exp(-Δ/kT) · sinh(σ·Ω/kT)
Plastic strain rate:
γ̇_p = R(σ) - R(-σ)
Softening: G_eff = G(1 - ρ_STZ)
Flow stress ~ σ_y · exp(-ρ_STZ)
In amorphous materials (metallic glasses, granular matter), plasticity occurs through localized rearrangements of ~20-200 atoms called Shear Transformation Zones (STZs). Unlike dislocations in crystals, STZs are not topological defects — they emerge from stress concentration. Shear bands form when STZs percolate across the sample.