In situ characterizations of mechanical behaviors of freestanding (Gd0.9Yb0.1)2Zr2O7 coatings by bending tests under different temperatures based on digital image correlation

Ultra-high temperature video extensometer: System development and experimental validation

The video extensometer has been widely advocated for tensile/compressive strain measurement in high-temperature material testing due to its advantages of non-contact measurement, wider measuring range, and larger applicable temperature over traditional clip-on mechanical extensometers. However, existing video extensometers, despite the adoption of active imaging devices, cannot adapt to the rapidly changing thermal radiation from the heated sample and surrounding heating elements during the high-temperature tests. This is because due to the significantly intensified thermal radiation, the decorrelated images degrade the measuring accuracy or even destroy the analysis. To address this problem, we developed an ultra-high temperature video extensometer that can automatically adjust the camera exposure time for reliable thermal strain measurement. Based on the pre-established image quality evaluation criteria, the camera will choose an appropriate exposure time according to the detected thermal radiation within the region of interest, thus ensuring high-quality images for real-time strain measurement. Static tests of tungsten-molybdenum alloy samples at different temperatures were performed to evaluate the noise level of the established ultra-high temperature video extensometer. The effectiveness and accuracy of the developed ultra-high temperature video extensometer were validated through the tensile tests of tungsten-copper, tungsten-potassium, and tungsten-molybdenum alloy samples at high temperatures up to 2000 °C.

A Review on In Situ Mechanical Testing of Coatings

Real-time evaluation of materials’ mechanical response is crucial to further improve the performance of surfaces and coatings because the widely used post-processing evaluation techniques (e.g., fractography analysis) cannot provide deep insight into the deformation and damage mechanisms that occur and changes in coatings’ material corresponding to the dynamic thermomechanical loading conditions. The advanced in situ examination methods offer deep insight into mechanical behavior and material failure with remarkable range and resolution of length scales, microstructure, and loading conditions. This article presents a review on the in situ mechanical testing of coatings under tensile and bending examinations, highlighting the commonly used in situ monitoring techniques in coating testing and challenges related to such techniques.

Measurement of the Elastic Modulus and Residual Stress of Thermal Barrier Coatings Using a Digital Image Correlation Technique

This paper presents an experimental study on simultaneously measuring the elastic modulus and residual stress of a thermal barrier coating (TBC) after different isothermal heat treatments. The elastic modulus and residual stress of TBCs were theoretically analyzed based on composite beam bending theory. Thereafter, an experimental setup was established combining the 3D digital image correlation method with the bending test to obtain the curvature changes in the TBC sample. Finally, the elastic modulus and residual stress of the ceramic layer with different isothermal heat treatments were obtained. The results show that the elastic modulus of the ceramic layer measured under compression is greater than that under tension, and the elastic modulus of the ceramic layer increases first and then tends to be stable as the heat treatment time increases. In addition, the residual stress of the TBCs ceramic layer quickly changes from compressive stress to tensile stress with heat treatment, and the tensile stress increases with the increase in thermal exposure time. Furthermore, the reasons for the change tendency were analyzed according to the variation in porosity and microstructures by processing the scanning electron microscope (SEM) figures. The results demonstrate that simultaneously determining the elastic modulus and residual stress of TBC based on combining the 3D digital image correlation method with the bending test is effective and reliable.