Normal Strain, 

a) The elongation of a segment is equal to its length divided by the total length L and multiplied by the total elongation δ. Elongation per unit length is called strain 

i) Bar in tension                =      tensile strain (elongation or stretching)       Positive sign

ii) Bar in compression       =     compressive strain (shortening)                     Negative sign

iii) The strain  is called a normal strain because it is associated with normal stresses.

iv) Dimensionless quantity, but sometime mm/m, μm/m and sometimes expressed as a percent (%), especially when the strains are large.