Question

Explain the ggeometrically necessary dislocation(GND) in comparison with statistically stored dislocation(SSD)

Answer 1

To describe the nonuniform deformation, theory os dislocation mechanics was proposed. Dislocations are mainly caused by strain gradient. The strain gradient plasticity theories consider two types of dislocations:

(i) Statisctically stored dislocations (SSDs), & (ii) Geometrically necessary dislocations (GNDs)

Dislocation storage causes material hardening.

Geometrically necessary dislocations (GND)	Statistically stored dislocations (SSDs)
GNDs are stored dislocations required for compatible deformation within the polycrystal.	While SSDs are randomly created. SSDs are stored dislocations generated by trapping each other in a random manner.
During bending	During shear
GNDs concentrate on the vicinity of grain boundary region.	SSDs distribute evenly within the whole grain.
GNDs are accumulated in the regions of high stress gradient.	SSDs account for dislocations to plastic strain in the absence of high stress gradients.
Their origin is nonlocal and cannot be predicted by local constitutive law.	While SSDs density can be described by a local dislocation based constitutive model.
They are related to the curvature of crystal lattice or to the strain gradients. These are additionally incorporated dislocation to compensate for inherent strain gradient in the lattice and are distributed accordingly.	Created during plastic deformation and are statistically homogeneously distributed.