A B S T R A C T A numerical prediction of the life of a gas turbine model disc by means of the finite-elementtechnique is presented and the solution is compared with an experimental rim-spinning test. The finite-element method was used to obtain the K solution for a disc with two types of cracks, both at the notch root of the blade insert and located in the corner and in the centre. A crack aspect ratio of (a/c) = 1 was assumed. The fracture mechanics parameters J-integral and K were used in the assessment, which were computed with linear elastic and elastic-plastic material behaviour. Using a crack propagation program with appropriate fatigue-creep crack growth-rate data, previously obtained in specimens for the nickel-based superalloy IN718 at 600 • C, fatigue life predictions were made. The predicted life results were checked against experimental data obtained in real model discs.The numerical method, based on experimental fatigue data obtained in small laboratory specimens, shows great potential for development, and may be able to reduce the enormous costs involved in the testing of model and full-size components. a = crack depth c = half crack length at surface C, m = Paris law parameters E = Young's modulus f = frequency J = J-integral K = stress intensity factor K C = critical stress intensity factor N = number of cycles N t = total number of cycles N p = number of cycles in propagation N i = number of cycles in initiation R = radius r 2 = regression correlation coefficient S UTS = True ultimate tensile stress α = Thermal expansion coefficient K th = Threshold stress intensity factor Correspondence: R.A. Cláudio.
Some exhaust systems of naval gas turbines have been periodically repaired due to
thermal-fatigue crack propagation after entering into service. Those structures were made of
austenitic stainless steel grade AISI 316L in thin wall plates, which were bent in rolling machines
and welded with longitudinal and circumferential joints by means of shielded metal arc, TIG or
MIG/MAG welding processes. The plate thickness is about 3.7 mm and the temperature on the
exhaust system is approximately 500°C and 350°C in the critical zones, which are located in the
lower and intermediate regions of the exhaust system.Several cracks were detected at the critical
regions, near the weld toe of butt and T-welded joints. The stress concentration factors induced by
the weld angle, toe radius and rolled surface finishing diminishes the fatigue life strength. Some
cracked material samples were taken out from the exhaust system structure and were analysed with
a Scanning Electron Microscope (SEM/EDS), in order to determine the failure mechanisms
involved in the crack propagation process. Those results are presented in the paper. Several high
temperature fatigue and creep tests were performed with CT specimens. The mechanisms of crack
propagation on the CT specimens were studied by SEM and compared with the fracture surfaces
obtained from the samples taken out from the structure. The carbide precipitation on the grain
boundaries was also studied.
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