We calculated the stress field on and around an elliptical open crack located in an elastic medium under various confining pressures. The problems were treated as two-dimensional ones by using complex potentials, and the considerable differences between uniaxial and triaxial crack growth mechanisms were recognized. A uniaxial condition allows a certain elliptical open crack to develop by itself (without any crack-crack interactions) along its major semi-axis, whereas a triaxial condition does not. This is the principal mechanism of transition from uniaxial to triaxial crack growth. Our transition model, derived using a theoretical approach, explains the dependence of experimental results, such as crack distribution and internal friction angle, on confining pressure well. We conclude that uniaxial and triaxial crack growth mechanisms fundamentally differ from each other, and that attention must be paid to experimental conditions when applying experimental results to phenomena in the Earth.
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