FATIGUE TEST ON POLYCRYSTALLINE DIAMOND COMPACTS

We have carried out compressional fatigue tests on notched polycrystal line 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 sh arp, with an unstressed opening of about 1 mum. That the cracks propag ated under the far-field compression was mainly a result of the induce d residual tensile stresses that arose upon unloading from the compres sive load. These cracks grew at a decreasing rate away from the starti ng notch, no doubt because compressive load was transferred directly a cross the crack faces by crack closure during the (compressive) loadin g part of the cycle. The fact that the fatigue crack was propagating i nto a region of greatly reduced stress may also have contributed to th e 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 charac terized 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 mum. 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 deta chment of fragments of diamond, and the maintenance of a wide crack.