For any state of stress at a point p , at least one coordinate system xyz exists for which the state of stress is of the form where σ 1 , σ 2 , and σ 3 are the principal stresses (see Sect. 7.4)....

For any state of stress at a point
p, at least one coordinate system
xyz
exists for which the state of stress is of the form

where
σ
₁,
σ
₂, and
σ
₃
are the principal stresses (see Sect. 7.4). Determine the normal stress on a plane through
p
whose orientation is specified by the unit vector