Abstract
Gas turbine disks contain many notch-like features acting as stress raisers. The fatigue life based on the notch root stress may be overly conservative as the steep stress gradient in front of the notch may give rise to so-called notch support. In the current work, the theory of critical distances was applied to the prediction of the total fatigue life of low cycle fatigued, notched specimens made from alloy 718. The fatigue tests were performed at 450 °C and 550 °C. It was found that, for lives shorter than 5000–10000 cycles, the notched specimens had longer lives than would have been expected based on the notch root strain. For lives longer than 5000–10000 cycles, there were no notch support. The life prediction for notched specimens could be significantly improved by basing the prediction on the strain chosen some distance from the notch (the critical distance). An expression for calculating the critical distance based on the notch root strain was suggested.
Highlights
Materials used for gas turbine disks should have properties such as high yield and tensile strength, reasonable ductility and fracture toughness, good resistance to fatigue crack initiation and low crack propagation rates [1]
A steep stress gradient at the notch can cause the stress to drop significantly with distance from the notch resulting in the observed fatigue life to exceed the predicted life based on the notch root stress, so called notch support [2, 3]
The purpose of the current work is to test the applicability of critical distances to predict the total life of low cycle fatigued notched specimens made from alloy 718
Summary
Materials used for gas turbine disks should have properties such as high yield and tensile strength, reasonable ductility and fracture toughness, good resistance to fatigue crack initiation and low crack propagation rates [1]. One alloy commonly used as disk material in gas turbines is alloy 718 [1]. Turbine disks may have complex shapes where several geometric features act as notches (e.g. the blade attachments). Such notches act as stress raisers and influence the load at fracture as well as the fatigue life. Fatigue life predictions of turbine disks involve taking notch effects into account. Predictions of failure based on the notch root stress may be overly conservative. A steep stress gradient at the notch can cause the stress to drop significantly with distance from the notch resulting in the observed fatigue life to exceed the predicted life based on the notch root stress, so called notch support [2, 3]
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