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Refurbishment of thermal barrier coating (TBC) has become a valuable technique to prolong the service life of high-temperature components. This study investigates the effect of the refurbishment process (coating removal and recoating) on the microstructure evolution and physical properties of TBC, including oxidation characteristics, element diffusion behavior, and crack failure mechanisms. The results showed that a certain amount of interdiffusion zone (IDZ) with Cr-rich would be retained in DD6 superalloy substrate after coating removal. The microstructure of the refurbished specimens showed equiaxed β-NiAl phases, while the ordinary specimens have elongated grain shapes with a high aspect ratio. Moreover, mixed oxides in the refurbished TBC specimens were earlier observed during cyclic oxidation, with a greater thickness compared to ordinary TBC, due to the influence of BC layer phase sizes. The growth mechanism of thermally grown oxide (TGO-Al2O3 layer) in the refurbished TBC specimens was also different, resulting from the different mechanisms of mixed oxides growth. Furthermore, under cyclic oxidation with water quenching at 1100 ℃, the cracks in the refurbished specimen tend to occur in the mixed oxides layer, while the cracks in the ordinary specimen occur in the top coat (TC) layer, attributing to the earlier and thicker mixed oxides layer formed in refurbished specimens.

Abstract As concrete and mortar materials widely used in structural engineering may suffer dynamic loadings, studies on their mechanical properties under different strain rates are of great importance. In this paper, based on splitting tests of Brazilian discs, the tensile strength and failure pattern of concrete and mortar were investigated under quasi-static and dynamic loadings with a strain rate of 1–200 s−1. It is shown that the quasi-static tensile strength of mortar is higher than that of concrete since coarse aggregates weaken the interface bonding strength of the latter. Numerical results confirmed that the plane stress hypothesis lead to a lower value tensile strength for the cylindrical specimens. With the increase of strain rates, dynamic tensile strengths of concrete and mortar significantly increase, and their failure patterns change form a single crack to multiple cracks and even fragment. Furthermore, a relationship between the dynamic increase factor and strain rate was established by using a linear fitting algorithm, which can be conveniently used to calculate the dynamic increase factor of concrete-like materials in engineering applications.

The cavity expansion theory is most widely used to predict the depth of penetration of concrete targets. The main purpose of this work is to clarify the differences between the spherical and cylindrical cavity expansion models and their scope of application in predicting the penetration depths of concrete targets. The factors that influence the dynamic cavity expansion process of concrete materials were first examined. Based on numerical results, the relationship between expansion pressure and velocity was established. Then the parameters in the Forrestal's formula were fitted to have a convenient and effective prediction of the penetration depth. Results showed that both the spherical and cylindrical cavity expansion models can accurately predict the depth of penetration when the initial velocity is lower than 800 m/s. However, the prediction accuracy decreases with the increasing of the initial velocity and diameters of the projectiles. Based on our results, it can be concluded that when the initial velocity is higher than the critical velocity, the cylindrical cavity expansion model performs better than the spherical cavity expansion model in predicting the penetration depth, while when the initial velocity is lower than the critical velocity the conclusion is quite the contrary. This work provides a basic principle for selecting the spherical or cylindrical cavity expansion model to predict the penetration depth of concrete targets.

The ablation process of carbon/carbon (C/C) composites was tested under hypersonic flowing propane flame. The microstructures of C/C composites were characterized and the numerical analysis was performed. Two typical ablation morphologies of the carbon fibers, which are drum-like and needle-like shapes, were observed depending on the alignments of fibers to the flame directions. Temperature fields in the composites were analyzed using finite element method, and the mechanisms that govern the formation of different ablation behaviors were elucidated. For paralleled fiber bundles, the highest temperature situates in the middle parts underlying the ablation pits, where the drum-like shape is formed. For perpendicular fiber bundles, the highest temperature appears at the turning point between the transverse section and the surface of fiber, which leads to the gradual ablation from the fiber surface toward the axis, and eventually the formation of the needle-like shape.