The strength of rock salt

Abstract

The strength of rock salt has attracted a very large number of workers because the equilibrium of the crystal lattice appears to be well understood. The relationship between atomic forces and elastic constants has been calculated using Born's theory, and has been shown to agree with experiment. The force necessary to tear a crystal into two parts across a crystal plane has also been calculated using the same assumptions about atomic forces at those which account for the elastic properties. Very great disagreement, however, was found. Thus Zwicky found that a rock salt crystal ought theoretically to stand a stress of 200,000 gm. mm. 2 before breaking while the breaking stress actually observed in samples tested at "room" temperatures is only about 450 gm. mm. 2 . The reason for the discrepancy has been discussed by Joffé and others who concluded that it was due to surface cracks. By pulling rock salt crystals at high temperatures Joffé found that they stretch plastically, and that strength increases with the amount of plastic stretching. He gives a curve in which the diminution in area of cross-section of a specimen is related to the increase in strength. With large amounts of stretching the strength could be raised to 5000 gm. per square millimetre. Thus the strength is increased by plastic distortion just as it is for a metal.