The initiation of secondary cracks in compression

Abstract

In rocks and rock-model materials, two types of cracks are observed: wing cracks, and secondary cracks. Wing cracks are tensile cracks that initiate at the tips of pre-existing cracks (flaws) and propagate in a stable manner towards the direction of the maximum compressive stress. Secondary cracks initiate also from the tips of the flaws, propagate in a stable manner, and have been recognized by many researchers as shear cracks. Two initiation directions are possible: one coplanar or quasi-coplanar to the flaw, and the other one parallel to the wing cracks but in the opposite direction. Shear cracks quasi-coplanar to the flaw are observed in most of the experiments; shear cracks parallel to the wing cracks only in few cases. This indicates that the second direction may be material dependent. Secondary cracks play a major role in the cracking process of rocks in compression. Crack coalescence is caused in many instances by secondary cracks. In biaxial compression and for high confinement, cracking is only produced through secondary cracks. Conventional initiation criteria are suitable for predictions concerning tensile cracks, but are inadequate to predict initiation of secondary cracks. An extension of the maximum tangential stress criterion is proposed in which shear crack initiation is analogous to tensile crack initiation, except that the direction and stress level of initiation are determined by the direction and magnitude of the maximum shear stress. This criterion allows for the initiation of more than one kink from pre-existing cracks. This is in agreement with experiments, and with the fact that the stress singularity at the tip of a flaw does not disappear with the initiation of a kink. A limited number of comparisons between experiments and this model show promising results. Stress initiation and angle of initiation for both wing and secondary cracks can be determined within reasonable errors.