A new triaxial compression technique has made possible the study of laws of the fracture and flow of rocks under general triaxial stress states, in which all three principal stresses are different. By this new method, the effects of the stress states on fracture and yielding of rocks were experimentally studied. Fracture and flow properties of rocks are markedly affected not only by the least compression σ3, but also by the intermediate compression σ2. The stress states that produce fracture and yielding are determined by the following formulas: τoct=ƒ1(σ1+σ3)  for fracture τoct=ƒ2(σ1+σ2+σ3) for yielding where ƒ1 and ƒ2 are monotonic increasing functions. The new failure criteria, corresponding to the generalized von Mises criteria, are physically interpreted as follows: fracture or yielding will occur when the distortional strain energy reaches a critical value that increases monotonically with the effective mean pressure: (σ1 + σ3)/2 for fracture and (σ1 + σ2 + σ3)/3 for yielding. The ductility defined as the permanent strain just before fracture, markedly increases with the increasing σ3 but decreases with increasing σ2. The influences of σ2 and σ3 on ductility are just the opposite. From these results on the effect of stress states, fracture and flow properties of the earth's upper mantle were deduced.

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Precise testing methods have been applied to an experimental study of the effect of the intermediate principal stress σ2 on failure of a granite, a limestone, and a dolomite rock at room temperature. σ2 affects failure in two ways. Strength at failure increases with σ2 by an amount that is proportional to, although smaller than, the amount of the least compression σ3; this effect of σ2 is much more pronounced for brittle than for ductile material. The angle between faults and maximum compression direction is markedly reduced as σ2 is increased from the value σ3 to σ1; this effect is pronounced for both brittle and ductile material.

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