On the mechanics of edge chipping from spherical indentation

Edge chipping is a basic failure mode in brittle materials which is dictated by a wealth of material and geometric variables. Here we examine the effect of indenter bluntness on chipping load and chip dimensions. Soda-lime glass and YTZP plates are subject to surface-normal loading near an edge by a W/C ball or a Vickers tool. The ball radius r is varied from 0.2 to 8.7 mm while the indent distance h is varied from several millimeters down to a few microns. Although cone cracks are a common feature under spherical indentation, the chipping event is dominated by median-radial cracks formed under the contact. The fracture behavior is characterized by a “large” indent distance regime where the median cracks progress stably up to chipping and a “small” one where they grow unstably to form a chip once initiated. Closed-form relations for chipping load PF under spherical indentation is developed with the aid of the test data and non-dimensional arguments. While in the “large” distance regime PF is proportional to h3/2 irrespective of tool bluntness, in the “small” regime PF is proportional to r1/2h3/4. Interestingly, the chip dimensions are virtually independent of ball radius, varying linearly with h. Beyond relevance to structural integrity, the chipping test facilitates a simple means for determining fracture toughness KC as well as the load needed to initiate median cracks in opaque brittle materials. An attempt is made to extend the static analysis to low-velocity impact. The results show that the damage formed during the fracture process has a major influence on dynamic chipping.