On the propagation and coalescence of delamination cracks in thermal barrier systems

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Coatings subject to residual compression eventually fail by buckle-driven delamination. The phenomenon is most vivid in thermal barrier coatings (TBCs) used in gas turbines. The failure evolution commences with the formation of a large number of small cracks at geometric imperfections near the interface. These cracks spread upon thermal exposure, particularly upon thermal cycling, because of the formation of a thermally grown oxide (TGO) beneath the TBC, which introduces normal and shear stress near the interface. Experimental observations indicate that some of these cracks coalesce to form large-scale delaminations susceptible to buckling. The mechanics governing crack coalescence and the consequent failure are addressed in the present analysis. A model is introduced that simulates stresses induced in the TBC by spatial variations in TGO growth. Energy release rates for cracks evolving in this stress field are determined. Two related scenarios are considered, which differ in the way the TGO shape evolves. In both, contact between the crack faces and the consequent wedging action is responsible for ultimate coalescence. The wedging force induces a mode I stress intensity that becomes infinite as the cracks coalesce. The consequence is that, for some TGO shapes, the energy release rate is always nonzero, with a minimum at a characteristic crack length. This minimum establishes a criterion for crack coalescence and failure. Based on these insights, finite element simulations have been used to predict cyclic crack growth rates in a TBC system that correlate well with experimental observations.
Original languageEnglish
Title of host publicationProceedings of the ASME Materials Division 2003 : presented at the 2003 ASME International Mechanical Engineering Congress & Exposition : November 15-21, 2003, Washington, D.C.
PublisherThe American Society of Mechanical Engineers(ASME)
Number of pages10
ISBN (Print)9780791837092
Publication statusPublished - 2003
Externally publishedYes
EventASME 2003 International Mechanical Engineering Congress and Exposition - Washington, United States
Duration: 15 Nov 200321 Nov 2003


CongressASME 2003 International Mechanical Engineering Congress and Exposition
Abbreviated titleASME 2003
Country/TerritoryUnited States


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