Stable and unstable fracture of thin brittle disks under a ball-on-3-balls loading

Ball-on-3-balls tests carried out on a coarse-grained alumina exhibited typical features of stable crack extension well-known for macroscopic cracks in materials with an increasing crack growth resistance. In order to interpret this effect unexpected for small natural cracks, stress intensity factors of semi-elliptical surface cracks were computed by application of the procedure proposed by Cruse and Besuner. For these computations an analytical description for the stress distribution in the disk under ball-on-3-balls loading was applied. The stresses strongly decrease with increasing distance from the disk centre. It can be concluded from the results that natural small cracks can propagate stably due to this decrease.