f: let dislocation slip a distance dS L FOS ds ∫ SP fs dS=txy A). ().I-ds fs make the dislocation move along the contrary directionx y s f c f ⊥ F b S.P.   1= L ( )s F f dS f b S A b f S A xy xy    =  =    s s d ( ) ( ) 1 d if f s make the dislocation move along the contrary direction f b s xy  = − ① f s : let dislocation slip a distance dS