“…In these theories it was assumed that the potential energy density depends not only on the strain, but also on higher derivatives of the displacement vector. More recently, the generalized non-classical theories have been also applied to modeling of materials at the micro-and nano-scale [10,23] to describing of phenomena like dislocations [21], to analyzing of composites with a high difference of the material properties at a lower scale [2,20,42,45,51] to describing some phenomena in regions with stress concentrations [5], to accounting for boundary and surface energies [14,28] or to removing singularities caused by discontinues of boundary conditions (e.g., [6,24,46,49]). It has been shown in numerous papers (see, for example, [22,34,35,41]) that some restrictions of the classical theory of elasticity can be overcome with such gradient expansion.…”