The ideal activation criterion compares resolved shear stress with a critical resolved shear stress. Non-glide stresses, notably in some body-centred cubic crystals, can modify this ideal criterion.
In an ideal single-slip tensile test neglecting elastic stretch, the specimen end-to-end vector expressed in crystal coordinates changes as , with unit slip direction and slip-plane normal . It moves towards the slip direction along a great circle. The perpendicular component is unchanged, so . Secondary slip changes this kinematics; equal Schmid factors determine onset only if the systems have equal current critical stresses.
The orientation factor converting the magnitude of uniaxial stress to resolved shear stress. It lies between zero and because the slip-plane normal and slip direction are perpendicular.
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Schmid's Law, named after the German engineer Erich Schmid, is a fundamental principle in the field of materials science and solid mechanics that describes the relationship between the applied stress and the resulting slip in crystalline materials during plastic deformation. It is particularly relevant to the study of single crystal materials. According to Schmid's Law, the critical shear stress required to initiate slip (plastic deformation) in a crystal is directly related to the applied normal stress.