The simplest relationship between shear stress (τ‎) acting parallel to a plane and normal stress (σ‎) acting perpendicular to the plane at the point of failure. The relationship σ‎ = σ‎_o + τ‎tanϕ‎, where σ‎₀ is the normal stress after failure and ϕ‎ is the angle of failure, implies that σ‎₂ (see principal stress axes) does not affect brittle fracture strength.