Strain behavior of a granite and a graywacke sandstone in tension

Westerly granite and Lance graywacke sandstone were deformed with one of the principal stresses tensile at the same time that the principal strains were monitored. Both rocks exhibited inelastic behavior prior to failure. Pronounced dilatancy and softening in shear took place in the granite. When Young's modulus varied, it did so continuously between compression and tensile stress states with no apparent discontinuity at zero axial stress, implying that cracks played the same role in tension as in compression. Failure was observed whenever the tensile stress exceeded a critical value, in agreement with predictions from Griffiths' failfure criterion. The sandstone exhibited approximately twice as much strain per unit stress change as the granite and showed evidence of grain-to-grain movement as well as crack closure, results consistent with comparisons determined earlier when all stresses are compressive