ON THE ORIENTATION AND PATTERNS OF WING CRACKS AND SOLUTION SURFACES AT THE TIPS OF A SLIDING FLAW OR FAULT

Sliding along a preexisting flaw can result in the formation of tensil e cracks where stresses concentrate near the flaw tips. These tensile cracks are referred to as wing cracks and are generally oriented obliq ue to the preexisting flaw. Previous studies based on linear elastic f racture mechanics (LEFM) showed that the kink angle depends on the rat io of normal to shear loading on the flaw. We present analytical solut ions for cohesive end zone (CEZ) flaw models and find that the relatio nship between kink angle and load differs significantly from that for LEFM flaws. Furthermore, the remote flaw-parallel normal stress may si gnificantly reduce or increase the kink angle, especially for CEZ flaw s with large end zones. These results suggest that multiple interpreta tions are possible for: some measured kink angles. In some materials, solution surfaces may form at the tip of the sliding flaw. By consider ing the angle between wing cracks and solution surfaces it is possible to determine whether the LEFM or CEZ model is more appropriate and th us to provide a better constrained interpretation' of the boundary con ditions that accompanied sliding. For some CEZ flaws the stress state in the cohesive end zone is nearly homogeneous, possibly promoting for mation of arrays of opening mode cracks and solution surfaces that tog ether form a shear zone. The CEZ flaw model can explain some orientati ons and patterns of cracks and solution surfaces commonly observed alo ng natural faults that cannot be accounted for with the LEFM model.