Fatigue test on polycrystalline diamond compacts

Abstract

Abstract We have carried out compressional fatigue tests on notched polycrystalline diamond compacts (PDC). Fatigue cracks were seen to grow in both the sintered tungsten carbide and the polycrystalline diamond parts of the compact. The cracks in the cemented carbide were very fine and sharp, with an unstressed opening of about 1 μm. That the cracks propagated under the far-field compression was mainly a result of the induced residual tensile stresses that arose upon unloading from the compressive load. These cracks grew at a decreasing rate away from the starting notch, no doubt because compressive load was transferred directly across the crack faces by crack closure during the (compressive) loading part of the cycle. The fact that the fatigue crack was propagating into a region of greatly reduced stress may also have contributed to the deceleration in crack propagation. The cracks in the polycrystalline diamond, however, were relatively wide. At the start of crack growth, we believe that failure of the diamond occurred under mainly uniaxial loading, as happens in the case of borehole breakouts. This is characterized by the formation of spalled flakes whose long axes are nearly parallel with the direction of the applied load. As the crack deepened, the size of the fracture zone decreased to an approximately constant width of about 30 μm. Failure at the root of the crack was then more probably in multiaxial compression, in which the diamond grains were crushed and disintegrated over a certain volume, resulting in the detachment of fragments of diamond, and the maintenance of a wide crack.