Hold-time effect on the thermo-mechanical fatigue crack growth behaviour of Inconel 718

Abstract

In-phase TMF crack growth testing with different lengths of the hold time at the maximum temperature of 550C has been conducted on Inconel 718 specimens. Focus has been on establishing a method for TMF crack growth testing and investigating the effect of high temperature hold times on the TMF crack growth of the material. The tests are compared to isothermal crack propagation tests and show good correlation. It is concluded that the controlling effect of the crack growth is an embrittlement of the material. This embrittlement is related to the concept of a damaged zone active in front of the crack tip. The size of this damaged zone will control the crack propagation rate and therefore it does not matter if the load is cycled under isothermal or TMF conditions.

Influence of overloads on dwell time fatigue crack growth in Inconel 718

“…[6][7][8] where it was shown that the crack growth per cycle during unloading-reloading is much higher after a dwell time period compared to pure cyclic loading. Similar observations has been reported to occur during thermomechanical fatigue crack growth tests [9]. Other crack growth mechanisms, such as dynamic recrystallization, strain localization in persistent slip bands, deformation bands, and vacancy diffusion, have also been proposed in references [10][11][12][13][14].…”

“…The use of wrought fine grained polycrystalline nickel base superalloys, such as Inconel 718, are in many situations limited by their susceptibility to fast intergranular cracking during extended dwell times at high temperatures and high tensile stresses [9]. It has been well established that time dependent intergranular cracking of nickel-based superalloys, under both sustained and cyclic loads, is dominated by environmental interactions with oxygen at the crack tip [8,17].…”

Thermomechanical fatigue crack growth in a cast polycrystalline superalloy

“…Crack lengths were determined using the compliance method in a similar manner as outlined in ASTM E 647 for isothermal crack growth tests. This set-up is described in more detail in [1] where it was also found that one can achieve a temperature gradient in the flat gauge section of the specimen, which is always less than 15 • C if the heating and cooling rates are sufficiently low. As a consequence all tests have been conducted with a constant heating and cooling rate of 1 • C/s.…”

“…However, such a Corresponding author: johan.moverare@liu.se damage tolerant approaches demand accurate predictions of the crack growth under the influence of simultaneous cycling of temperature and mechanical loads. Although methods have been developed to predict crack growth rates under isothermal conditions over a wide range of temperatures, frequencies and load ratios, as well as under sustained load dwell times, only very few published studies on crack propagation under TMF conditions are available, see reference [1] for further references. Furthermore, since the translation of isothermally obtained fatigue crack propagation data into a true thermomechanical fatigue context is questionable, the need for more experimental studies in this field is obvious.…”

“…In a previous work, the behaviour of IN792 during TMF of smooth specimens has been investigated [2]. It was found that the majority of the cracks were transgranular in OP TMF-test, while there was some tendency to intergranular crack growth in IP TMF tests at 750 • C. Furthermore it is known from other alloys that the intergranular cracking behaviour in wrought fine grained material is enhanced by longer dwell times [1,[3][4][5]. To what extent the coarse-grained cast material IN792 is sensitive to environmental assisted cracking during dwell times is to be determined in this study.…”