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Various effects can raise ρ. Dislocations can be created at free surfaces, grain boundaries and within grains.

Consider a dislocation line fixed (pinned, e.g. by sessile jogs) at both ends. When a shear stress τ is applied, a force F = τ b acts normal to the line.

Since the dislocation line is pinned at both ends, it bows outwards into a loop, which eventually surrounds the pinning points. Opposing segments annihilate on meeting, forming a loop and recreating a short, pinned dislocation. This can repeat, generating a new loop each time. This is a Frank-Read source.

Frank-Read Source


As deformation continues, ρ tends to rise and dislocation mobility to decrease (as tangles, jogs etc form), making it harder to deform the material.

Dislocation simulation video

This is often termed work hardening.