PSI - Issue 61

Mehmet N. Balci et al. / Procedia Structural Integrity 61 (2024) 331–339 Balci and Yalcin / Structural Integrity Procedia 00 (2019) 000 – 000

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Nomenclature TBC

Thermal barrier coating DCT Displacement correlation technique , a Half-length of the centre crack The Stress Intensity Factor , The mode I and mode II stress intensity factors ∗ , ∗ The normalized mode I and mode II stress intensity factors , The mode I and mode II energy release rate The total energy release rate ∅ The phase angle , The thickness of the surface coating and the bond coating , 0 Convection due to hot and cold thermal shock 1. Introduction Horizontal and vertical displacement components

Elastic surface coatings, especially, Thermal barrier coatings (TBCs) are recognized as essential key materials for high-performance aero-engine and heavy-duty gas turbine blades due to their excellent thermal protection properties (Clarke et al. 2005; Padture et al. 2002; Clarke et al. 2012). When the surface of the metallic substrate is deposited by these coatings with a thickness of 100-400 , it was reported that the substrate temperature could effectively be reduced by 100-200 ℃ (Padture, 2016; Zhao et al. 2017; Bialas 2008; Zok, 2016). Moreover, they protect metallic substrate from severe chemical corrosion and high temperature oxidation. Thus, utilization of these coatings leads to increase in the efficiency of engines and gas turbine blades. Yildirim and Erdogan (2010) examined the axisymmetric edge cracking of coatings either bimaterial or functionally graded coating systems using enriched finite element method. This papers examines the effect of transient thermal loading and the thermal shock on delamination behaviour of the thermal barrier coatings (TBCs). The coating-substrate system is considered to consist of three layers which are ceramic coating, bond coating and the metallic substrate. In order to analyse the problem, finite element method (FEM) is used. Boundary conditions and loads are determined in thermal and mechanical fields. Delamination behaviour of TBCs is investigated based on two different crack configurations, which are crack at the interface of the surface coating and crack at the interface of the bond coating. Cracks are modelled using singular finite elements and mixed mode I/II stress intensity factors (SIFs) and the energy release rate are calculated through the use displacement correlation technique (DCT). Parametric analyses are carried out to reveal the influences of crack configuration, material properties, thermal shock time and its type either hot or cold and crack length. Fracture analysis of TBC delamination under cold thermal shock loading was examined by Ping-wei et al. (2015). However, authors examined coating system subjected to cold shock only, crack at the bond coating was not discussed and presented results were limited since effects of aforementioned parameters on delamination were not thoroughly presented. Hence, results presented in this study aims to fill this gap and it is found that the crack tip stress field is shear dominant, energy release rate and SIFs for crack configurations are different from each other due to the utilized materials and loading. Possible delamination due to unstable propagation of the crack is also presented for different parameters. 2. Crack model and boundary conditions The addressed delamination problem is depicted in Fig. 1. Two different cases are examined in the study. Case-A shows the delamination problem between the surface coating and bond coating, hence center crack is located at the interface surface between surface coating and bond coating. In Case-B, however, the center crack is located between bond coating and the substrate, which imply delamination problem between bond coating and the substrate. Generally, TBC system involve three layers as illustrated in Fig. 1. The surface coating is generally ceramics based material which is used as thermal insulator, the bond coating protects the substrate material from high temperature oxidation