LAUSR

The most common method of assessing the fracture properties of ceramics and glasses is the observation and measurement of cracks generated at sharp indentation contact impressions. Specifically, the surface traces of radial or half-penny cracks formed adjacent to Vickers indentations are commonly measured. Such cracks are perpendicular to the indented surface and hence often control the strength of the indented component. The popularity of the method stems from: (a) the minimal specimen requirements—a flat, small area of material is merely required; (b) the ease of application—gravity-loaded indentation followed by optical microscopy is usually sufficient; and (c) the simplicity of analysis—a simple, calibrated, power law can relate measured indentation load and crack length to material toughness. The seminal indentation crack length toughness study by Anstis et al1 nearly 40 years ago demonstrated the caveats necessary in implementing (a)-(c) above: a large enough specimen area is required to perform a sufficient number of indentations over a peak load domain wide enough to establish the applicability of the technique; inert conditions must be maintained, usually requiring a silicone oil coating of the surface to impede moisture ingress during both indentation and microscopy; and, although the power-law formulation was well anchored in fracture mechanics,2 the calibration of the resulting indentation crack length parameter with inert toughness was really a correlation that was demonstrated to have an accuracy of about a factor of two. Nevertheless, in the intervening period, Received: 9 October 2019 | Revised: 12 November 2019 | Accepted: 13 November 2019 DOI: 10.1111/jace.16925