The reorientational process in liquid crystals (LC) is usually described in the frame of fluid mechanics (hydrodynamics) by a oclassicalo Oseen-Franck - Leslie equation under several simplifying assumptions. One of them is the linearity of the elastic response of a LC-structure to the induced stresses causing its deformation. This equation provides essential parameters of the reorientation process like threshold values and it's dynamics which correspond well to those observed experimentally. However, the description of the LC-elasticity by a single-value elastic coefficient (i.e., by linear Hook's law) is an approximation, since real processes are in general nonlinear. In this work an attempt is made to check if this linearity assumption can be extended to large deformations far above threshold. We expected in this case nonlinear effects to be present in structural elasticity because elastic forces result from molecular interactions, which are generally nonlinear, i.e., fundamental interactions obviously decrease with deformation (distance) like 1/r2 divided by 1/r6. Such a decrease is also likely for orientational deformations of molecular arrangement in the LC-structure omeano field. So an important question is to what extent the nonlinearity appears in the practical operation of LC-based devices. The elastic nonlinearity, if not negligible, may play an important role in applications. The elastic properties of a liquid-crystalline structure substantially influence the threshold value, sensitivity, and dynamics of the induced reorientation process. The elastic nonlinearity may also contribute to a hysteresis of the reorientation, introducing or enhancing bistable behavior in a LC-structure. Thus the elastic nonlinearity may affect operation of liquid crystalline photonic devices and its detailed examination would help in their improvement and optimization. This paper presents also a simple method of detection of the elastic constant changes in a deformed LC-structure.