Abstract
Rotating fatigue test was carried out using the sintering WC-Co in ultra-long life. A sintered alloy has much defects on the boundary. A crack initiates from these defect, and propagates belong the boundary. Fatigue limit does not exist. Because the sintered alloy like ceramics obeys to weakest link. The boundary slips easily, and is weak to sharing force. The fatigue crack propagation rate (da/dN) cannot calculates. But, fracture toughness K_<IC> can calculate from relationship Fractal dimension or Hurst number and yield area. Yield area of fracture in ultralong N_f=2.05×10^7 broad. Origin of fracture is surface defect s. When K_<IC>>K_<max> in surface stands up, the crack in the plane stress propagate. The fatigue strength of WC cemented carbide is obeyed to weakest link model of defects. Because, even if it supposes that it was worked the cyclic stress over the fatigue strength, the fatigue fracture did not always happen.
