Imposing a stress on a material will produce a strain.
Stress is a force per unit area, while a strain is a deformation.
Stress at any point will have both an isotropic component, called the pressure p , and a deviatoric component that varies with direction. The variation usually simplifies to a symmetric ellipsoid having three principal and mutually orthogonal axes. The values of these axes are called the principal stresses . A deviatoric stress will have a maximum principal stress pointing in and thus compressive and positive. There is an intermediate principal stress , and a minimum principal stress pointing out and thus tensional and negative.