Schmid's Law (also Schmid factor, m) describes the slip plane and the slip direction of a stressed material, which can resolve the most amount of shear stress.
Schmid's Law states that the critically resolved shear stress (τ) is equal to the stress applied to the material (σ) multiplied by the cosine of the angle with the glide plane (Φ) and the cosine of the angle with the glide direction (λ). Which can be expressed as:
where m is known as the Schmid factor
Both factors τ and σ are measured in stress, which is calculated the same as pressure by dividing force by area. Φ and λ are angles usually measured in degrees.
The factor is named after Erich Schmid who coauthored a book with Walter Boas introducing the concept in 1935. In German the law is called the "Schmid'sches Schubspannungsgesetz" (Schmid's shear-stress-law), while the factor is either called "Schmid-Faktor" or "Schmid'scher Orientierungsfaktor" (Schmid's orientation factor).