Fracture behavior of as-cast pearlitic nodular iron

The fatigue crack propagation and fracture toughness behavior of an ASTM A536, as-cast, pearlitic nodular iron with a bull's-eye structure was studied. The material had an ultimate strength of 76 ksi, a yield strength of 59 ksi, and an elongation of 1.6%. Fracture toughness tests were conducted on compact tension specimens with thicknesses of 0.30, 0.50, 0.70, and 1.00 inches. The conditional fracture toughness, K_Q, was found to be insensitive to the specimen thickness in the above range and to have a value of approximately 30 ksi-in^1/2. The dependence of the fatigue crack growth rate, da/dN, on the stress intensity factor, δK, was determined for stress ratios of 0.1, 0.3, 0.5, and 0.7. At the same °K level, the da/dN rates were higher for the higher stress ratios. The parameters, C and n, of the Paris equation, da/dN=C(δK)ⁿ were determined for each stress ratio. Near-threshold tests were also conduced for the stress ratio of 0.1. The threshold stress intensity factor, δK_TH, was found to be in the range of 6.3 to 7.7 ksi-in^1/2. A statistical model was used to calculate the agreement of the results of two duplicate fatigue crack growth tests.