Answer:
σ1, σ2 and σ3.
Step-by-step explanation:
The three principal stresses are labelled or written as σ1, σ2 and σ3. The σ1 is the maximum principal stress or most tensile stress, σ2 is the intermediate principal stress and σ3 is the minimum or most compressive principal stress. Principal stresses are the maximum and minimum extensional stresses present in an object. The principal directions have no shear stresses connected with them.