A review of the observations of indentation-induced fracture suggests that there is no simple generalization which may be made concerning crack initiation sequences. Here, we investigate the material dependence of the initiation sequence of indentation cracks (cone, radial, median, half-penny, and lateral) using an inverted tester allowing simultaneous viewing of the fracture process and measurement of the indeter load and displacement during contact. Two normal glasses, two anomalous glasses, and seven crystalline materials are examined. Key results include (i) direct evidence that the surface traces of cracks observed at indentation contacts are those of radial cracks, rather than median-nucleated half-penny cracks (at least for peak contact loads <40 N) and (ii) that, in crystalline materials, radial cracks form almost immediately on loading of the indenter, in anomalous glasses at somewhat greater loads, but in normal glasses during unloading. A detailed consideration of the stress fields arising during indentation contact predicts material-dependent initiation sequences, in agreement with observations, particularly those of radial crack formation on loading for materials with large modulus-to-hardness ratios. In addition, a new, unexplored crack system is demonstrated, the shallow lateral cracks, which appear to be responsible for material removal at sharp contacts.

Direct Observation and Analysis of Indentation Cracking in Glasses and Ceramics

Porous calcium polyphosphate scaffolds for bone substitute applications -- in vitro characterization.

Transparent glass-ceramics functionalized by dispersed crystals

Composition dependence of indentation deformation and indentation cracking in glass

The initiation of radial cracks in Vickers indentation of soda-lime glass is found to be strongly rate dependent. For long contact durations the radial cracks pop in during the indentation event, at a reproducible stage of the unloading half-cycle; for short contacts the pop-in occurs after the event, with considerable scatter in delay time. The phenomenon is interpreted in terms of an incubation time to develop a critical nucleus for the ensuing fracture. Increasing either the water content of the environment or the peak contact load diminishes the incubation time. Scanning electron microscopy of the indentation patterns indicates that the sources of the crack nuclei are constrained shear faults within the deformation zone. A qualitative model is developed in terms of a two-step process, precursor faulting followed by crack growth to pop-in instability. Moisture may influence both these steps, in the first by interfacial decohesion and in the second by slow crack growth. No definitive conclusion is reached as to which of the steps is ratecontrolling, although it appears that it is the shear across the fault and not the tension across the crack which is vital in driving the initiation. The implications of these results in connection with the basic mechanical properties of brittle solids, particularly strength, are considered.

Kinetics of shear-activated indentation crack initiation in soda-lime glass

Critical load characteristics for the indentation-induced median crack in ternary lead-zinc-borate and commercial silicate glasses were investigated. The critical load is directly proportional to the fracture toughness/Vickers microhardness term, supporting the Lawn-Evans model; however, the magnitude of the load is considerably greater than that estiated from the model. The relation of critical load to deformation/fracture properties is discussed in terms of the stress field around the indent area and fractography.

Critical Load for Median Crack Initiation in Vickers Indentation of Glasses

New CaO-P2P5 glass ceramics were prepared by crystallizing glass rods or plates unidirectionally in a temperature-gradient furnace. The unidirectionally crystallized glass showed extremely high bending strength (∼500 to 600 MN/m2) even when subjected to a severe abrasion treatment with Sic paper. Scanning electron microscopy of fracture surjaces revealed a typical fiber-reinforced-composite structure. Crystalline phases and crystallographic directions were checked by X-ray diflraction. The fracture mechanism is discussed in terms of microstructure.

N. Shinkai

Papers

Critical Load for Median Crack Initiation in Vickers Indentation of Glasses

High-Strength Ca(PO3)2 Glass-Ceramics Prepared by Unidirectional Crystallization