Experimental research was conducted on the relationship between the graphitic crystallite in the ablation glass fiber reinforced epoxy composite and the microwave transmission decay at a frequency of 10GHz. Ablation samples were prepared by intense laser irradiation of 100W •cm-2 for different time. The microstructure and component of the ablation samples were characterized by means of X-ray diffraction and Raman spectra. The electromagnetic characteristics were investigated by vector network analyzer at 10GHz. When laser of 100W•cm-2 irradiated the samples shorter than8seconds, the microwave transmission decay remained small. When the laser irradiated the samples 8 seconds, the graphitic crystallites were detected and the microwave transmission decay escalated. With increasing irradiation time, the size and quantity of graphitic crystallites , as well as the microwave transmission decay increased. In brief, we concluded that the generation of graphitic microcrystallites induced by laser irradiation attributed to the microwave transmission decay.
In view of the surface temperature distribution and temperature rise measurement demand of metal target irradiated by high power laser, the thermosensitive phosphor surface temperature measurement technology was studied. The principle of two-color temperature measurement was introduced. After solving the key technologies of optical system optimization design, temperature distribution inversion calculation and high precision calibration, a compact thermosensitive phosphor surface temperature measuring system was developed. The temperature measurement range from room temperature to 1500 K, the space measurement range was greater than that of diameter 50mm, and the spatial resolution was better than 0.5mm. The thermosensitive phosphor surface temperature measurement technique was used to measure the surface temperature distribution and temperature rise of stainless steel targets irradiated by high power laser, and the results were compared with the results of thermocouple and numerical simulation. It is proved that the surface temperature measurement system can realize the measurement of surface temperature field distribution of high power laser irradiated target, and has high temperature measurement precision.
To determine the mechanism of melt removal by transverse gas flow, a lateral visualization technique of hydrodynamics on melt pool was developed and experimental apparatus were built. The intensity distribution of the focused beam was confirmed to be in top-hat shape with the 15mm×40mm rectangular. The interface of liquid-solid and free surface of molten metal was observed by a high velocity video camera with acquisition rate of 1kHz. Gas flow blew from left to right and the velocity varied from 15m/s to 90m/s to investigate the evolution of hydrodynamics. Experiment results showed that surface wave was generated at the initial stage and molten metal was removed out from the melt pool by shear stress. When some amount molten metal was removed from melt pool, gas flow separated at the leading edge and reattaches downstream of melt pool. Thus a stagnation point was formed at the downstream edge and a recirculation zone was generated on the left side of stagnation. With recirculation gas flow constrain, the molten metal only can be entrained into main stream and then be swept away. The molten material was removed out by shear stress on the right side of stagnation.
Considered the pyrolysis and the internal gas convection, a three-dimensional mathematical model based on the conservation of energy, had been developed for predicting the pyrolysis behavior of composite subjected to laser. By taking carbon-fiber /epoxy composite for examples, the effects of irradiation time and power density on the mass loss and the thermal ablation rate were obtained from an irradiation experiment with the tangential nitrogen and low power density. The numerical results agree well with the experimental results on the whole. The results reveal that as the irradiation time continues, the thermal ablation rate begins to decrease gradually and then tends to be steady after a rapid increase. The oxidation can be effectively restrained due to a nitrogen protection. However, laser power density will enhance the oxidation effect, which causes that the mass loss approximately shows a linear rise and the thermal ablation rate tends to be steady after 180 W/cm2.
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