Quick The strain-energy density of a material is defined as the strain energy per unit volume. It is equal to the area under the stress-strain diagram of a material, measured from ε_{x} = 0 to ε_{x} = ε_{1} as described below.
As stated in the lesson Normal Strain, the load-deformation diagram for a rod BC depends upon the length L and the cross-sectional area A of the rod. The strain energy defined by Eq1 from the lesson Strain Energy, therefore, will also depend upon the dimensions of the rod. In order to eliminate the effect of size from the discussion and direct the attention to the properties of the material, the strain energy per unit volume is now considered. Dividing the strain energy U by the volume V (V = AL) of the rod shown in the following figure:
where ε_{1} denotes the value of the strain corresponding to the elongation x_{1}. The strain energy per unit volume, U/V, is referred to as the strain-energy density and is denoted by the letter u. Therefore:
(Eq1)
Strain-energy density = u =
∫
ε_{1}
^{0}
σ_{x }dε_{x}
The strain-energy densityu is expressed in units obtained by dividing units of energy by units of volume. Thus, if SI metric units are used, the strain-energy density is expressed in J/m^{3} or its multiples kJ/m^{3} and MJ/m^{3}; if U.S. customary units are used, it is expressed in inlb/in^{3}. Note that 1 J/m^{3} and 1 Pa are both equal to 1 N/m^{2}, while 1 inlb/in^{3} and 1 psi are both equal to 1 lb/in^{2}. Thus, strain-energy density and stress are dimensionally equal and could be expressed in the same units.
Referring to the following figure:
it is noted that the strain-energy density u is equal to the area under the stress-strain curve, measured from ε_{x} = 0 to ε_{x} = ε_{1}. If the material is unloaded, the stress returns to zero, but there is a permanent deformation represented by the strain ε_{p}, and only the portion of the strain energy per unit volume corresponding to the triangular area is recovered. The remainder of the energy spent in deforming the material is dissipated in the form of heat.