In-Situ Characterization on Fracture Toughness of Thermal Barrier Coatings Under Tension by J-Integral with Digital Image Correlation at High Temperatures

Background: The elastic–plastic fracture toughness (J_c) is an important mechanical parameter for studying the failure behavior of air plasma-sprayed (APS) thermal barrier coatings (TBC) at high temperatures. Objective: This study aims to: (1) develop an effective test method to characterize the J_c of TBC at high temperatures; (2) acquire accurate J_c data for TBC at high temperatures; (3) analyze the influence of plasticity of top-coat on the J_c characterization. Methods: The elastic–plastic Ramberg–Osgood equation of the ceramic top-coat and the deformation fields of single edge notched tension (SENT) specimens were measured by high-temperature in-situ tension with digital image correlation (DIC) system. The J_c of TBC was calculated by the numerical J-integral with DIC-measured (DIC-J) deformation fields by adopting Ramberg–Osgood equation of the top-coat. The finite element analysis (FEA) method was adopted to analyze the influence of plasticity of top-coat on the J_c characterization. Results: The curves of J_c varying with crack propagation length (Δa) of TBC were obtained and were expressed as J_R = 24.47 × [1 + 1.0446 × (Δa~)^0.7624] J/m² and J_R = 16.52 × [1 + 1.4806 × (Δa~)^0.6742] J/m² at 800 and 1000 ℃, respectively. Conclusions: A high-temperature in-situ tensile test of SENT specimens combined with the DIC-J method was developed to characterize J_c of TBC. The J_c of TBC displays a rising resistance curve behavior, and FEA results indicated that J_c would be underestimated without considering the plasticity of the top-coat at 800 and 1000 ℃.