Hertzian cone crack propagation on polycrystalline materials: Role of R-curve and residual stresses

In this work, the angle of the cone crack produced by Hertzian indentation is studied in relation to the shearing contribution of the crack shielding. This shielding is especially relevant in polycrystalline brittle materials, and may be responsible of the disagreement between previously predicted and actual values of the angle of cone crack. The study was conducted by means of an automated finite-element model of crack propagation, in which an analytical expression of the R-curve was included in the crack path determination. In addition, the influence of extrinsic effects, such as macroscopic residual stresses, was studied. The calculations were compared to crack observations on alumina and alumina-based laminates. Experimental results were in agreement with simulations. Based on this model, a method for quantitative assessment of both mode-I and mode-II crack shielding is proposed.