Load relaxation experiments were performed on single crystals of NaCl (at -30°C and 100°C), KCl (at room temperature and 60°C) and MgO (at room temperature). The resulting logσ-logε curves were convex upward for NaCl (at 100°C) and KCl (at room temperature and 60°C). It was slightly convex downward for MgO (at room temperature). The curvature for NaCl (at -30°C) could not be determined owing to the experimental difficulty in keeping temperature constant for a long duration. All the logσ-logε curves for a given sample (examined for NaCl at 100°C and KCl at room temperature and 60°C) were superimposed by a translation along a straight line of slope μ.
These behaviors indicate that Hart model is satisfactorily applicable to these materials. The reason why the Gupta-Li analysis was not successful for KCl at room temperature in our previous experiment was also discussed.