Creation of Stable Cracks in Hardmetals Using ‘Bridge’ Indentation

Abstract

Bridge indentation involves the initiation of a precrack at thq centre of the top surface of the sample by meajns of diamond indentation (i.e. a Palmqvist crack). Growth of this crack is then induced by pressing either side of it with rectangular punches having their edges parallel with the crack direction. With I increasing load, the crack grows outwards and downwards, and eventually forms a straight-through crack. The present authors have made a study of the effect of experimental variables such as punch spacing, punch contact area, and diamond indentation load on the crack growth. Specimens of a WC–6Co alloy cracked in this way were subsequently used for K _1C measurement. An important result was that the cracks grew to a stable depth, the value of which could be determined by suitable choice of indentation parameters. Numerical analysis using a boundary element method (BEM) was used to explore the stress situation in bridge indentation and provided a qualitative prediction of the experimental results. The K _1C measurements, using four point bending, indicated that plastic deformation associated with the hardness indentation causes an underestimate of the K _1C value, and should therefore be removed by polishing. However, the effect is relatively small if the ratio of crack depth to indentation size is large. PM/0285