Before revision: Volumetric strain The dilatation ( the relative variation of the volume ) is the first strain invariant or trace of the tensor: Actually, if we consider a cube with an edge length a, it is a quasi-cube after the deformation (the variations of the angles do not change the volume) with the dimensions and V0 = a3, thus as we consider small deformations, therefore the formula.
After revision: Volumetric strain The volumetric strain, also called bulk strain, is the relative variation of the volume , as arising from dilation or compression; it is the first strain invariant or trace of the tensor: Actually, if we consider a cube with an edge length a, it is a quasi-cube after the deformation (the variations of the angles do not change the volume) with the dimensions and V0 = a3, thus as we consider small deformations, therefore the formula.