  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 KD2PO4 type
ferroelectrics. Thermodynamic potentials and physical characteristics of the
crystals in the presence of stress $\sigma_6$ are calculated. Numerical
analysis of the obtained results is performed, and the set of the theory
parameters providing the best fit to the available experimental data is found.
The $Tc-\sigma_6$ phase diagram is constructed; temperature dependences of the
calculated quantities are studied, possible stress dependences are indicated.
Influence of $\sigma_6$ stress is analogous to that of electric field conjugate
to the order parameter.
