Tensile stresses and their implication to cracking at pyramid indentation of pressure-sensitive hard metals and ceramics

Crack initiation and subsequent crack growth can often influence severely the results given by an indentation hardness test and a proper interpretation of indentation parameters becomes difficult. Indentation cracking can, however, also be used in order to determine the fracture properties of the materials. In particular, during pyramid indentation of high hardening and pressure-sensitive hard metals, glasses and ceramics, cracking occurs frequently at loading as well as unloading of the specimens. It is then of fundamental practical importance to analyse how cracking affects global as well as local indentation parameters. In the present analysis, a detailed investigation is presented on indentation induced tensile stresses from which crack initiation and propagation sequences can be inferred. In particular, the influence from pressure-sensitivity is investigated. The analysis was carried out using the finite element method and includes a parametric study of both the Vickers and the Berkovich indentation tests, without explicit modelling of the cracks. This is a first, but very important, step prior to a full fracture mechanics analysis, not undertaken in this work, needed to complete the problem. The pressure sensitivity of the materials was modeled according to the Drucker–Prager plastic potential while a critical tensile hydrostatic stress criterion for macrocrack initiation was adopted. For comparison, some results pertinent to a critical tensile principal stress criterion were also presented.