Hold Time Effects on the Crack Growth Behavior in Inco 718 Alloy

Abstract

The understanding o f hold time effects on the fatigue crack growth rate (FCGR) behavior above 500°C in Inco718 DA is a great challenge to develop an efficient model for predicting crack propagation life. Trapezoidal wave shape signal fatigue tests from 500°C to 650°C with hold times up to 3600 s were carried out on a small grain size (5-15 |im) alloy. FCGRs were measured using potential drop technique. SEM observations were carried out to correlate the measured FCGRs with the trans-or inter-granular aspect of the fatigue fracture surfaces. Using equations developed in the frame of the Local Approach of Fracture (LAF) theory, predicted life and crack growth rates are compared to experimental results. The emphasis is laid on time dependent effects, in particular those associated with oxidation.

Time‐ and cycle‐dependent growth of surface cracks in alloy 718

“…This grain boundary embrittlement does not occur at lower temperatures and higher cycling frequency (e.g., 20 Hz) due to lack of time for diffusion of oxygen ahead of crack tip 10 . As the cyclic frequency is increased in air, cracking transitions from intergranular to transgranular 11,17 . It occurs at increasing frequency with increasing test temperature 12 .…”

“…10 As the cyclic frequency is increased in air, cracking transitions from intergranular to transgranular. 11,17 It occurs at increasing frequency with increasing test temperature. 12 The frequency needed to reach this environmentindependent behavior has an inverse relationship with ΔK.…”