Spontaneous growth of interacting cracks in a cruciform pattern

The stability of the growth regime of a system of four equal cracks is studied. The cracks are positioned symmetrically in a cruciform pattern, and they grow toward the center of the symmetry because of nonuniform thermal contraction or incremental injection of incompressible fluid. It is shown that, for either loading condition, a critical stage will be reached at which one pair of coplanar cracks runs together to form a single crack, while the other pair remains stationary. In fact, for the hydraulic loading, the effective fluid pressure acting on the cracks decreases as the cracks grow spontaneously.