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.
The energy deposition efficiency of high energy laser irradiation on painted metal was studied. Experiments of uniform 1 070 nm continuous wave (CW) laser beams irradiating an entire 10 mm quadrate surface of gray epoxy painted iron were performed to investigate the laser-heating-induced changes of the surface absorption behavior. Laser power densities were set as 54-233 W/cm2 and temperature rising processes of the painted iron specimens were monitored. By means of heat transfer inversion calculation, the absorptivity data during laser irradiation were obtained, of which the good accuracy was further proven by optical measurements. It is found that the absorptivity showed strong temperature-dependence properties in spite of the different laser power densities. During temperature increasing from 20 to 1 300°C, the temperature-dependent absorptivity experienced four stages, i.e. slowly decline, rapidly decline, slowly increase, and rapidly increase.The visualized color of gray coating layer changing to white at 510°C and physical broken at 1 050°C were the main reasons for absorptivity rapid changes.
Pyrolysis, carbonization and other reactions would occur in laser irradiated Glass fiber reinforced epoxy resin composite (GFREC). Different laser irradiation time experiments were carried out to get some ablation samples. With the help of X-ray diffraction (XRD) and Raman spectroscopy(RS, graphite microcrystals were found in the severe ablation GFREC. The temperature of laser irradiated GFREC was calculated by using energy conservation equation and pyrolytic decomposition equation, and we found the temperature of laser induced graphite production was about 800°C. And then the information of graphite microcrystals was characterized by 5th order Gaussian function curve-fitting analysis. Results showed that the sizes of graphite microcrystals were several nanometers.
Kevlar/nomex honeycomb sandwich structures are widely used by many apparatuses and vehicles in many domains. Since there are large quantities of epoxy resins in the structure, it is considerable to study the process that the structure is heated and produces pyrolysis gases which diffuse among the honeycomb. In this paper, the process of a laser beam irradiating a kevlar/nomex honeycomb sandwich is studied for building a mathematical model. The process is divided into two parts. One part focuses on the pyrolysis gas producing, the other one focuses on the gas diffusing among the honeycomb. The pyrolysis gas producing model is built according to experiment analysis, as a Boltzmann formula. The gas diffusion model is also built in the form of ODE equations. Validation experiment is carried out, demonstrating the model correct and accurate. Finally, the two models are combined together. By comparing with experiment, the laser irradiating and pyrolysis gas diffusing model is demonstrated to be appropriate to the case that kevlar laminas are bonded to the nomex honeycomb.
Reflectivity is an important physical quantity governing the effects of laser interaction with metal. It directly determines the energy deposition and the relevant thermal-mechanical response of metal. However, metal’s reflectivity is not constant, which will be significantly affected by the surface states, such as roughness and oxidation. Industrial grade polycrystalline iron substrates with different surface roughness were prepared. Samples were ground and polished using progressively finer SiC paper and polisher. The surface morphology and roughness were determined by a 3D microscope. The surface roughness values of these samples varied from 0.08 μm to 2.65 μm. The reflectivity of samples at 1064nm were measured by an integral sphere equipped an InGaAs detector. The reflectivity values increased from 0.52 to 0.70 as the roughness decreasing. Later on, samples were irradiated by 1064nm continuous wave fiber laser in the intensity of 2 W/cm2. The real time reflectivity during laser illumination was also measured. As the samples’ temperature rising, the surface oxidation reaction took place. The reflectivity evolved because of the molecular absorption and interfere effect of the oxides. The phenomena of reflectivity evolution were more obvious as samples smoother. Although the initial reflectivity of samples was different, samples show the same final reflectivity after laser irradiation.
This research aimed to study the relationship between the ablation and the tensile strength change of Carbon-fiber/epoxy composite under laser irradiation. It was performed on Carbon/epoxy laminates using both experimental and numerical modeling methods. The sample thickness was 2mm. A 1070 nm continuous wave laser was chosen as the irradiation source. In order to obtain different ablation states of the sample, the power and exposure time of the laser were changed to set up different experimental conditions. The tensile strength of ablated samples was measured to investigate the degradation tendency of tensile strength. The results show that the degradation curve of tensile strength versus laser intensity and irradiation time follows the inversed power-law. This is to say, the tensile strength decreases rapidly as the laser intensity and irradiation time increases. The mathematical model has been developed to analyze the temperature of surface and the changing tendency of tensile strength. The modeling results match experimental data well.
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.
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