Measurements in the diamond cell at 300 K show that the static yield strength of polycrystalline ruby (${\mathrm{Al}}_2$${\mathrm{O}}_3$:Cr) increases linearly from about 4 GPa at ambient conditions to 5.8(±0.3) GPa at an average pressure of 70 GPa. In all cases our experiments document a creep (or plastic) strength; no fracturing is observed. Our results are in general agreement with dynamic (shock-wave) measurements of yielding at high pressures, and demonstrate that the increase in yield strength is proportional to the increase in the average shear modulus with pressure. Throughout the pressure range studied, the ratio of yield strength to shear modulus is 0.022(±0.002), which is within a factor of 2–5 of the theoretical yield strength. Extrapolating to higher pressures, the maximum difference between normal stresses is less than 10% (18 GPa) at 200 GPa. Thus, even for an infinitely weak sample contained in the diamond cell, the pressure obtained by the ruby-fluorescence technique should be within 10–20 GPa of the true sample pressure up to at least 200 GPa.

• Received 7 February 1990

DOI:https://doi.org/10.1103/PhysRevB.42.2532