Shear testing of mercuric iodide single crystals

A specialized shear testing system has been designed and assembled for the purpose of measuring the response of thin single crystals of mercuric iodide to shear loading. Numerous stress-strain measurements have been made on single crystal samples of thicknesses ranging from 0.3 to 1.0 mm. Shear strains of over 100% are readily achieved. The results are analysed in terms of a two-parameter semiempirical model for yielding that fits the experimental data extremely well. The model employs a normalized Gaussian distribution f(s) for the change f(s)ds in the relative density of mobile dislocations that occurs when the shear stress s is increased from S to s+ds. The Gaussian parameters s0 (the shear stress at which the density of mobile dislocations has reached half its steady state value) and σ (the standard deviation) are determined from a least squares fit of the theoretically computed stress-strain curve to the experimental data. The “onset of yielding” sc is defined as s0−2σ, which is the shear stress at which the density of mobile dislocations has reached about 2% of its steady state value. The utility of the model for studying work hardening and time dependent recovery in single crystal HgI2 is established.