Crack growth micro-mechanisms in the IN718 alloy under the combined influence of fatigue, creep and oxidation

Creep, fatigue and oxidation in crack growth in advanced nickel base superalloys

The elevated-temperture fatigue crack growth behavior in alloy 718, when subjected to a loading frequency lower than the transitional frequency of this alloy, is viewed as fully environment dependent. In this process, the crack growth increment per loading cycle is assumed to be equal to the intergranular oxygen diffusion depth at the crack tip during the cycle effective oxidation time. In order to identify the trend of this diffusion depth an experimental program was carried out on compact tension specimens made of alloy 718 at 650 °C in which fatigue crack growth measurements were made for cyclic load conditions with and without hold time periods at minimum load level. This work resulted in establishing a relationship correlating the intergranular oxygen diffusion depth and the value of the stress intensity factor range ΔK. This relationship, when integrated over the cycle effective oxidation time, results in a closed-form solution describing the environment-dependent fatigue crack growth rate. A comparison is made between the results of this solution when applied to different loading frequencies and the corresponding experimental results. This comparison shows good agreement between the two sets of results. Furthermore, by combining the parabolic rate law of diffusion and the equation for the intergranular oxygen diffusion depth, an explicit expression for the oxygen diffusivity of grain boundaries is derived. It is found that this diffusivity is both a ΔK- and a frequency-dependent parameter.

Depth of intergranular oxygen diffusion during environment-dependent fatigue crack growth in alloy 718

Crack growth in IN718 at high temperature

High temperature fatigue crack growth behaviour of Inconel 718 under hold time and overload conditions

: Fatigue crack growth was studied at 650 deg C as a function of frequency for several ratios of minimum-to-maximum stress intensity, and for two values of maximum stress intensity factor Crack lengths were monitored at low frequencies from compliance calculations based on crack mouth opening displacement measurements At higher frequencies, crack length was measured using a dc electric potential system It was found that fatigue crack growth rate can be characterized in three distinct frequency regions These three regions represent fully cycle dependent, mixed, and fully time dependent crack growth behavior and each region can be modeled by a power law function Observation of micro mechanisms support the existence of these three different regions of crack growth Keywords: Fatigue crack growth, Time dependent growth, Cyclic-dependent growth, Nickel base superalloy, Crack growth rate model, Frequency effects

Effect of Frequency on Fatigue Crack Growth Rate of Inconel 718 at High Temperature

: An experimental investigation was conducted to evaluate the effects of hold-times on the fatigue crack growth rate of Inconel 718 to 649 C using compact tension specimens. Tests were run under computer controlled constant K conditions using compliance to determine crack length. Hold-times ranging from 5 to 50 s were applied at maximum, minimum, and intermediate load levels. The data show that hold-times at maximum load were the most damaging in terms of crack growth rate. Hold times greater than 5 s led to purely time-dependent crack growth behavior which was predictable from sustained load data using K as a correlating parameter. Hold-times at a minimum or intermediate load levels had little or no effect on crack growth rate. A linear cumulative damage model based solely on fatique and sustained load data was found to be adequate for spectrum loading as long as the hold-times were at maximum load. Additional keywords: gas turbine engines; Air Force equipment; aircraft engines. (Author)

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Tusit Weerasooriya

Papers

Effect of Frequency on Fatigue Crack Growth Rate of Inconel 718 at High Temperature

Hold-Time Effects in Elevated Temperature Fatigue Crack Propagation