CLEAVAGE AND CREEP FRACTURE OF ROCK-SALT

The dominant Failure mechanism in rock salt at ambient temperature is either cleavage or creep fracture. Since the transition of creep fract ure to cleavage in a compressive stress field is not well understood, failure of rock salt by cleavage and creep Fracture is analyzed in thi s paper to elucidate the effect of stress state on the competition bet ween these two fracture mechanisms. For cleavage Fracture, a shear cra ck is assumed to cause the formation and growth of a symmetric pair of wing cracks in a predominantly compressive stress field. The conditio ns for wing-crack instability are derived and presented as the cleavag e fracture boundary in the Fracture mechanism map. Using an existing c reep fracture model, stress conditions for the onset of creep Fracture and isochronous Failure curves of specified times-to-rupture are calc ulated and incorporated into the fracture mechanism map, The regimes o f dominance by cleavage and creep fracture are established and compare d with experimental data. The result indicates that unstable propagati on of cleavage cracks occurs only in the presence of tensile stress. T he onset of creep Fracture is promoted by a tensile stress, but can be totally suppressed by a high confining pressure. Transition of creep Fracture to cleavage occurs when critical conditions of stress differe nce and tensile stress For crack instability are exceeded. Copyright ( C) 1996 Acta Metallurgica Inc.