Laser damage experiment in the high-speed wind tunnel is an important method for studying the mechanism of high-speed targets exposed to laser irradiation. There is no substantive progress in the instantaneous ablative behavior of laser-irradiated surfaces owing to the high-temperature radiation coupled with factors such as laser radiation and high-speed wind tunnel environment interference. The conventional methods are used to obtained data, such as the final ablation morphology, ablation depth, or average mass ablation rate, after the experiment. However, the traditional methods cannot provide instantaneous and reliable failure evolution process or real-time experimental data. The temperature of the specimens under laser irradiation was extremely high. For instance, the temperature of ceramic-based composites can exceed 3000 ℃ under a high laser power density. The experimental data on the instantaneous ablative morphology of high-temperature targets exposed to laser irradiation and supersonic tangential airflow have not been reported until now. In the present study, we propose an in-situ observation technology suitable for obtaining the instantaneous laser-irradiated ablative morphology of different materials. The real-time ablative behaviors of the metals and composite materials under supersonic tangential airflow were captured. The ablative characteristics of the specimens were analyzed using image processing methods, and instantaneous ablative data were obtained.
Titanium alloy, nickel-based superalloy, ceramic-based C/SiC, and carbon fiber-reinforced polymer CFRP composites are studied in this paper. First, an in-situ observation platform suitable for laser-irradiated extreme high-temperature environments was established, which mainly composed of a high-speed camera, auxiliary lighting system, attenuating filter, and narrow band-pass filter. Subsequently, laser damage experiments were conducted in a supersonic wind tunnel. The experiment employed a supersonic wind tunnel facility at the State Key Laboratory of High-Temperature Gas Dynamics (LHD) of the Institute of Mechanics, Chinese Academy of Sciences. It operates on the oxygen-hydrogen combustion principle and can provide a free stream of Mach number 1.8-4.0 in the test section. It comprises heaters, nozzles, air supply systems, consoles, and a measurement system. In the experiments, the tangential supersonic airflow was set to Mach number 3.0. The total temperature and pressure of the gas flow were 815 K and 1850 kPa, respectively. Finally, the optical flow method was used to analyze the ablative characteristics and particle motion velocity of each material, and the instantaneous ablation rate was obtained using the PIV method combined with the structural characteristics of the composite material layup.
The burn-through behaviors of titanium alloy and nickel-based superalloy were obtained. The burn-through time under the coupled action of laser and tangential airflow are 1.32 s and 1.44 s, respectively. The final perforation diameters are 7.23 mm and 5.72 mm, respectively. The difference in the flow pattern and burn-through time is attributed to the instability of the melt surface. According to the Kelvin-Helmholtz theory, the mechanism of the burn-through behavior is mainly related to the surface tension and density of the material. Although the melting point of the titanium alloy TC4 (1670 ℃) is higher than that of the nickel-based superalloy GH625 (1340 ℃), the high-density nickel-based superalloy exhibits better resistance to laser breakdown under tangential airflow condition. For the C/SiC composite, the ablative evolution process of the microscopic structure and the formation and migration of silicon dioxide droplets in the edge region of the laser irradiation are clearly visible in the experimental images. The results show that the in-situ observation technology can also be used to observe the ablative behavior of composite materials. Different braided structures can influence the ablative behavior and ablation depth. The ablation depth of the 2D C/SiC composite was 1.13 mm, whereas that of the 3DN C/SiC composite was 1.23 mm. Compared with the 2D C/SiC composite, the 3DN C/SiC composite exhibits higher thermal conductivity in the thickness direction, resulting in a significantly higher temperature than that of the 2D C/SiC composite; therefore, its thermochemical ablation rate is also higher than that of 2D C/SiC. The instantaneous ablation depths of the CFRP were obtained using PIVlab. The results showed apparent nonlinear behavior. The laser ablation depth of a CFRP composite under supersonic tangential airflow is related to the laser power density and airflow velocity. The ablation depth is 0.36 mm when the laser power density is 1273 W/cm2, and the airflow velocity is Mach number of 1.8. When the airflow velocity increases to Mach number of 3.0, the ablation depth increases to 0.47 mm. When the laser power density increased to 2546 W/cm2, the ablation depth increased to 1.07 mm. These results indicate that the laser power density has a strong influence on the laser ablation depth.
In this study, an in-situ observation technology of laser-irradiated high-temperature is proposed, and the instantaneous ablative morphology of metals and composite materials exposed to laser and supersonic tangential airflow is obtained. Real-time ablative data were calculated using image processing methods. The flow of molten metals in the wake zone and the diffusive characteristics of the heat-affected zone were obtained using the Horn-Schunck optical flow method. The ablative behaviors of the composites were related to the braided structure of the reinforced phase. The mechanical ablation effect of the 2D C/SiC composite is mainly sheet-like ablation, whereas the behaviors of the 3DN C/SiC and CFRP composites are mainly fiber-by-layer ablation. The instantaneous ablation depths of the CFRP composites were obtained using PIV method. The results show that the in-situ observation technology proposed in this study has broad application prospects in extreme high-temperature engineering, especially in the study of laser damage effects.
