2 f: let dislocation climb a distance dS b·1 f·dS"=-(x·A Jf b b tot② fc : let dislocation climb a distance dS' S A b f S x A      = −   d 1 d ( ) c c  c f b  c = − x ③ f tot 2 c 2 tot s f = f + f b  dS ⊥