99-26U

Within the four-particle cluster approximation for the proton ordering model we study the influence of shear stress $\sigma _6$ on the phase transition, static dielectric, elastic and thermal properties of deuterated KD$_2$PO$_4$ type ferroelectrics. A simplified version of the model of strained KD$_2$PO$_4$ is presented, where induced by strain $\varepsilon _6$ splitting of up-down and single-ionized deuterong configurations is neglected. Thermodynamic potentials and physical characteristics of the crystals in the presence of stress $\sigma _6$ are calculated. The set of the theory parameters providing the best fit to the available experimental data is found. We show that a fair desription of the available experimental data for the physical characteristics of the crystal is possible even without taking into account the splitting of up-down deuteron configurations, and that prdicted by the theory effects induced stress $\sigma _6$ are indentical in both versions of the model.