Book Description
An experimental study of the effects of closure on the near-threshold fatigue crack growth rates in a nickel-base superalloy is presented. Fatigue crack propagation (FCP) tests were conducted on Rene' 95 at 650°C under either constant load amplitude (increasing ?K) or under threshold load shedding (decreasing ?K) test conditions. Particular emphasis was placed on the level of closure in the near threshold regime as a function of load history, specimen size and thickness, and environment. Test results indicated that for decreasing load threshold tests the recorded threshold levels were independent of load shedding rates. Specimen thickness was found to have no influence on FCP rates, threshold values, or closure loads, thereby eliminating plasticity induced closure as a dominant mechanism. Tests performed in vacuum produced closure values identical to those obtained in an oxidizing environment. It was concluded that the primary operating closure mechanism was roughness induced closure.