Explain the terms stress and strain.

Stress

Stress is the ratio of applied force F and cross section A, defined as "force per area".

Direct Stress or Normal Stress

Stress normal to the plane is usually denoted "normal stress" and can be expressed as

σ = F_n / A         (1)

where

σ = normal stress ((Pa) N/m², psi)

F_n = normal component force (N, lb_f)

A = area (m², in²)

Shear Stress

Stress parallel to the plane is usually denoted "shear stress" and can be expressed as

τ = F_p / A         (2)

where

τ = shear stress ((Pa) N/m², psi)

F_p = parallel component force (N, lb_f)

A = area (m², in²)

Strain

Strain is defined as "deformation of a solid due to stress" and can be expressed as

ε = dl / l_o = σ / E         (3)

where

dl = change of length (m, in)

l_o = initial length (m, in)

ε = unitless measure of engineering strain

E = Young's modulus (Modulus of Elasticity) (Pa, psi)