A two-dimensional FE model is constructed by using the commercial package ANSYS . The model is designed to represent the coating configuration corresponding to that obtained with microstructure analysis on TBC cross-sections of turbine blade samples after the high temperature cyclic oxidation(Fig.1). Two cases of the TGO profile shown in Fig.1 are analyzed:a regular sinusoidal undulation with constant thickness and an irregular TGO layer with symmetrical sinusoidal penetration into the TC and BC layers.A sine curve with wavelength of 50 um and amplitude of 10 um is used to reproduce the interfaces. The finite element model includes about 25,000 eight-node generalized plane strain elements with the mesh refinement near the interfaces. Symmetry and periodicity boundary conditions are imposed on opposite lateral edges while the nodes at bottom are fixed along the y-direction.
Fig.1. Finite element models of TBCs with regular (a) and irregular(b) TGO layer.
The thermal loading history corresponding to real operating conditions of gas turbine blades consists of three stages: heating at the rate of 30℃/min followed by dwelling for 23 h at 1100℃, and finally cooling down to room temperature at 30℃/min. No temperature gradient within the TBC system is assumed. The oxide grow stresses are considered to be negligible compared with the thermally induced stresses due to creep relaxation. Moreover,a simulation with cooling from 800℃ results in almost the same stress distribution as that one calculated when the cooling starts from T=1100℃. This implies that a stress level developed at high temperatures is comparably small due to low elastic moduli and relaxation induced by creep deformation.
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