Normal Stress
Consider a bar subjected to axial force P, with a cut taken perpendicular to its axis, exposing the internal crosssection of area A. The force
per unit area acting normal to the crosssection is the stress.
The symbol used for normal stress  the stress perpendicular to the material surface  is s (sigma). The stress in an axially loaded
bar is:


 Stress is positive in tension
(P>0 means s>0), and negative in compression
(P<0).
 English units: psi (pounds per square inch), or ksi
(kilopounds per square inch).
 S.I. units: Pa (Pascal, N/m^{2}), or usually MPa (megapascal, 1 MPa = 1,000,000 Pa).
