= Frank's rule
{c}
{title2=$|\mathbf b_3|^2<|\mathbf b_1|^2+|\mathbf b_2|^2$}
The energy criterion associated with <Frederick Charles Frank> states that, for dislocations of comparable length with approximately common line-energy factors, a reaction conserving <Burgers vector> is energetically favourable when the displayed inequality holds. The rule is an energy estimate, not a proof that the geometry or kinetic barriers permit the reaction. Edge/screw character, elastic anisotropy and changed lengths refine it.
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