Polycrystalline diamond cutters are known to fail during drilling operations by a fatigue crack growth process. This failure mode is characterised by classical clam shell markings on the fracture surface and in some instances multiple fatigue cracks can be seen to have initiated from the wear scar, Fig 1. The removal of large volumes of highly wear resistant polycrystalline diamond material inevitably leads to premature degradation of the cutter’s ability to drill rock. Therefore developing an understanding of the mechanism of fatigue crack growth behaviour in PCD materials is important in the quest for improved cutter lives. Drill bits are subjected during operation to repeated loads, fluctuating or cyclic and rapidly applied loads. Such loading induces fluctuating or cyclic stresses that often result in failure by fatigue. It is difficult to detect the progressive changes in material properties that occur during fatigue and failure may therefore occur with little or no warning. Also, periods of rest, with the fatigue stress removed, do not lead to any measurable healing or recovery from the effects of the prior cyclic stressing. Thus, the damage done during the fatigue process is cumulative and generally unrecoverable.

Discussion

The large range of crack initiation stress intensity values (is related to a) the acuity of the starter notch and b) the grain size distribution. The ability of the laser notch preparation technique to reproduce the same quality of the notch is important in narrowing the starting fatigue stress intensity range. A calculation of the critical flaw sizes for this material using the transverse rupture strengths show that the critical flaw sizes coincides with the upper end of the grain size distribution. Thus larger grain sizes of a PCD material act as the critical flaws on the tensile surface of a TRS bar. This results from the fact that larger crystal particles have a greater probability of containing defects, which are derived from the diamond synthesis process used to produce the source diamond.

In ceramic materials toughness arises from mechanisms of crack tip shielding, where toughness is achieved by mechanical, microstructural and environmental factors which locally reduce the crack driving force, a process referred to as extrinsic toughening. The final fracture stress intensities are larger than the measured fracture toughness using the Brazilian disc method. This is probably due to the fact that the brazilian disc method measures the intrinsic fracture toughness of this PCD material since the crack at the tip of the slot propagated by critical crack propagation (i.e., there was no opportunity for the development of the extrinsic toughness).

If a sub-critical crack were allowed to grow from the slot of the Brazilian test disc before testing, the resulting fracture toughness will be a better measure of the intrinsic and extrinsic fracture toughness combined. It appeared to be very difficult to define and control the threshold level of the material. In these extremely brittle materials this property is very dependent on the test specimen preparation. It was decided to start the cracks using a chevron notch but even small inaccuracies in the machining were detrimental. Hence, the EDM notches were not successful, whereas the laser cut notches improved the reproducibility. In the cases where a successful crack initiation was obtained, a successful stepwise measurement of crack growth under constant K was also successful. In fracture mechanics crack growth is driven by the presence of the crack driving force and opposed by the resistance of the microstructure. The driving force is defined by the stress intensity KI, which defines the stress and deformation fields in the vicinity of the crack tip. Crack advance is thus restrained by lowering the applied load or by toughening the material by microstructural modifications which impede crack advance.