MecMovies 4.0 : M12.13: Absolute Maximum Shear Stress

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If $τ_{z x} = 0$ and $τ_{z y} = 0$ , what can we conclude about the $z$ face of the stress element and about the normal stress $σ_{z}$ ?

The $z$ face of the stress element is a principal plane, and therefore, $σ_{z}$ is a principal stress!!

Recall that planes free of shear stress are called principal planes and the normal stress acting on a principal plane is called a principal stress.