The aim of the authors is to study a four-element elementary cell corresponding to a densely packed layer of atoms. The authors study the influence of shear strains on stability of a rhombus-like four-element cell extended along its diagonal when it undergoes a homogeneous deformation. The interatomic interaction is described by the Morse potential. It is shown that this system has an unstable post-critical behavior. This means that the load, when shear strains appear in the system, can happen to be 7.9 percent smaller than the fracturing load of an ideal system. As a result the authors arrive at the conclusion that it is necessary to introduce a correction on shear strains when estimating the theoretical strength of crystalline bodies.