The study of cyclic crack growth resistance in vacuum for the gas turbine engine disks manufactured from the E741 NP granulated nickel alloy

Aeronautical and Space-Rocket Engineering


DOI: 10.34759/vst-2023-2-99-105

Аuthors

Nemtsev D. V.1*, Potapov S. D.2**, Artamonov M. A.1***

1. Lyulka Experimental Design Bureau, branch of the United Engine Corporation – Ufa Engine Industrial Association, 13, Kasatkina str., Moscow, 129301, Russia
2. Central Institute of Aviation Motors, CIAM, 2, Aviamotornaya St., Moscow, 111116, Russia

*e-mail: dmitrij_n@inbox.ru
**e-mail: potapov_sd@ciam.ru
***e-mail: maxartamonov@gmail.com

Abstract

The technology of Ni-based granulated alloys has become widely spread for gas turbine engine disks manufacturing. This technology concedes the presence of internal defects of metallurgical nature. These defects may cause the crack growth under conditions of vacuum at cyclic loading. It is necessary to know the fatigue crack growth (FCG) rate in vacuum to assess the lifetime of the disks.

Special samples of cylindrical shape have been developed to solve this problem. A non-metallic defect is placed in the center of the working path of the samples, serving as a crack initiation source. The defect placement in the sample occurs while the powder filling into the sample capsule. Cyclic fatigue tests are being conducted until the sample destruction.

The two types of samples are used, namely vented and unvented. The vented sample differs by the presence of a through axial hole, which serves for the air supplying to the crack tip and the growth rate testing in the air. The unvented sample is necessary for testing the crack fatigue growth rate in vacuum.

The fractures of the samples are being examined by the fractography. The search for and measurement of fatigue striation spacing and the crack growth fronts reconstruction are performed at this stage. The presence of fatigue striations indicates a stable period of crack growth. The width of the fatigue striation spacing corresponds to the crack growth in one loading cycle, i.e. the fatigue growth rate. Thee fatigue growth rate is necessary for plotting a crack kinetic diagram. The crack growth rate is necessary to build a relationship between the rate and stress intensity factors (SIF), which is being computed after the crack shape reconstruction by the finite element method.

The sample and the defect diameters are being selected so that elastic stresses prevail in the section with the crack at the given maximum load. The ANSYS software was employed for determining optimal sizes of the sample with the finite element method.

Cyclical test of the special samples from the EP741NP alloy at the maximum loading of the cycle and the temperature of 400°C were conducted. Maximum load in the cycle ensures nominal stresses in the section with the crack, which is 0.58 of the proportionality limit at the beginning of the tests.

The average number of cycles to failure for unvented samples is 12.7 times greater than for vented ones. This indicates a significantly slower crack growth rate in vacuum.

Preliminary fractographic analysis of the specimens surface fractures were performed. The areas of fatigue striations location were identified. Fatigue striations are being observed almost throughout the entire crack growth area for the vented samples. This indicates that the crack growth occurred by the stable growth mechanism. For the unvented samples, fatigue striations are located in a narrow zone near the boundary of stable-tearing crack growth region. In this case, the crack growth in vacuum occurred mainly at a low rate, corresponding to the unstable growth mechanism.

Simulation of the fronts of the cracks started based on the obtained data to determine the values of the stresses intensity factors ranges and plotting kinetic diagram of the crack growth rate in vacuum and in the air.

Keywords:

crack growth resistance of gas turbine engine disks, fatigue crack growth rate in vacuum, powder Ni-based superalloy EP741NP granular nickel alloy

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