Normal Stress

σ_axial = N / A

Normal Strain

ε_axial =  (l_new - l_old) / l_old

 - elongation or contraction of a line
segment per unit of length (in/in, m/m)

l_new - l_old = Δl = ΔL = δ 

tensile strain

compressive strain

strain gage - an electrical-resistance device for measuring normal strains on the surface of a stressed object

strain rosette - a group of gages arranged in a particular pattern