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 purpose of this investigation is to study laser ablation properties of glass fiber composite under obliquely impinging air jet. The continue wave near infrared laser ablation of fiber-reinforced composite materials under air jet with different velocity and inclination was studied experimental. The laser intensity is uniform distribution and the weight of sample was measured before and after experiment. Effective ablation heat was defined as the ratio of laser energy to weight loss. The experimental results showed that the process of the glass fiber reinforced epoxy composite ablation under laser and air jet can be divided into two stages. The first stage is mainly thermal ablation. The effective ablation heat increased as the inclination angle increased under same laser intensity. The effective ablation heat decreased with laser irradiation duration under same inclination angel and laser intensity. the second stage is mainly mechanical damage. the remain materials was broken and removed by air jet impingement, thus the effective ablation heat decreased evidently.
For mechanical properties temperature dependence test in high temperature rising rate, a method of heating specimen using two-sided laser irradiation is proposed. Simulation about different materials in laser heating about different laser power density and different laser duration were conducted. The result shows that two-sided laser irradiation can obviously improve the temperature field of the specimen in normal laser irradiating. The simulation shows that this method is better for metal than composite materials because of the heating pyrolisys may have a bad influence on the thermal balance. Based on this method, the tensile strength temperature dependence of the T700 carbon fiber laminate was obtained. The result is close to the parameter obtained in electrical mechanical machine by furnace heating. The research can make a reference for the mechanical properties test of composite materials or metal in high temperature with high heating rate.
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.
Based on the overhanging beam three-point bending method, the experimental system was set up to measure the variety of shear stiffness of Nomex honeycomb sandwich panel in laser irradiation. The shear stiffness of the specimens under different laser power density was measured. The result shows that the thermal effect during the laser irradiation leads to the degradation of mechanical properties of Nomex honeycomb sandwich panel. High temperature rise rate in the specimen is another main reason for the shear stiffness degeneration. This research provides a reference for the degradation of mechanical properties of composite materials in laser irradiation and proposes a new method for the study of laser interaction with matter.
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.
The pyrolysis responses of kevlar/epoxy composite materials are valuable to study in a case of high temperature rising rate for its widely application. Distinguishing from the Thermal Gravimetric Analysis method, an apparatus is built to research the pyrolysis responses of kevlar/epoxy composite materials irradiated by laser in order to offer a high temperature rising rate of the sample. By deploying the apparatus, a near real-time gas pressure response can be obtained. The sample mass is weighted before laser irradiating and after an experiment finished. Then, the gas products molecular weight and the sample mass loss evolution are derived. It is found that the pressure and mass of the gas products increase with the laser power if it is less than 240W, while the molecular weight varies inversely. The variation tendency is confusing while the laser power is bigger than 240W. It needs more deeper investigations to bring it to light.
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.
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.
This paper shows some tentative results with which laser induced damage on dielectric thin films is analyzed by using the transmission spectrum. The damage characters were extracted in high-resolution images of damaged films, and transmission spectrum of damaged thin films was measured. The mathematical model of transmission was built based on the matrix optics theory with the optical properties, which include the effective refractive index, effective extinction coefficient, effective thickness and wavelength, and so on. Changes of optical properties in different damaged degree were analyzed by the transmittance spectrum. Through which laser-induced damage mechanisms had been analyzed with the micro-examinations of films.
An investigation was conducted to determine the relationship between heat transfer coefficient and molten pool’s geometry. It was accomplished by performing an experimental and numerical investigation using a cylinder dimple with two different serials of geometry: (1) cylinder dimples with fixed print diameter D=50mm and different depth, and (2) cylinder dimples with fixed depth d=10mm and different print diameter. The airflow speed varies from 50m/s to 250m/s in the turbulent regime. The results consist of flow characteristics, mainly velocity profile and heat transfer characteristics, including heat transfer coefficient and Nusselt number along flow direction, were obtained. The comparison was held against the smooth surface. Results showed that a centrally-located vortex was formed due to the flow separation. For heat transfer coefficient, such augmentations are present near the downstream edges and diminutions are present near the upstream edges of dimple rims, both slightly within each depression. It was found that the convection heat transfer coefficients with different geometry parameters have similar distribution along flow direction. A uniform piecewise linear function was built to describe the heat transfer characterizes for different molten pool print diameter.
The tests of carbon-fiber/epoxy composite laminates, subjected to a tangential gas-flow and 1070 nm continuous wave laser,are carried out to acquire the ablation laws of samples on the conditions of different gas-flow. Simultaneously, considered the images from camera of large dynamic range, the damage laws of samples are also obtained for various laser power densities. Experimental results reveal that, without airflow on sample surface, the smoke caused by laser heating can be quickly on fire which causes a burn damage on the surface of samples so that the mass loss is most of all. However, the tangential airflow can remove away the smoke which has a weakening effect on the energy of incidence laser. So the ablation depth has an obvious increase in laser irradiation area. Unlike airflow, nitrogen flow can obviously restrain oxidation ablation on surface so that the ablation damage in laser irradiation area is relatively not severe. On the other hand, as laser power density increases, the mass loss of samples continues to rise but isn’t proportional. And the ablation heat with the increase of power density shows a complex change. Below power density of 390 W/cm2, the mass loss mainly depends on the pyrolysis of epoxy while the ablation heat has a gradual decrease. Along with power density increasing but less than 1330 W/cm2 , the oxidation ablation of carbon fibers will be a leading factor and the ablation heat shows a little increase. Above power density of 1330 W/cm2 , the carbon fibers turn up the phenomenon of sublimation. What’s more, airflow removed effects will be enhanced in high temperature. In this case, the ablation heat again has a trend of decrease.
In order to research the dynamic process of energy coupling between an incident laser and a carbon fiber/epoxy resin composite material, an extinction characterization analysis of soot, which is produced by laser ablating and located in an air flow that is tangential to the surface of the composite material, is carried out. By the theory analyses, a relationship of mass extinction coefficient and extinction cross section of the soot is derived. It is obtained that the mass extinction coefficients of soot aggregates are the same as those of the primary particles when they contain only a few primary particles. This conclusion is significant when the soot is located in an air flow field, where the generations of the big soot aggregates are suppressed. A verification experiment is designed. The experiment employs Laser Induced Incandescence technology and laser extinction method for the soot synchronization diagnosis. It can derive a temporal curve of the mass extinction coefficient from the soot concentration and laser transmittance. The experiment results show that the mass extinction coefficient becomes smaller when the air flow velocity is higher. The reason is due to the decrease of the scatter effects of the soot particles. The experiment results agree with the theory analysis conclusion.
The laser coupling effect of material is a fundamental factor to influence laser interaction with matter. The coupling coefficient, which is the material absorptance of the input laser energy, depends on the surface conditions of materials, such as temperature, incident angle, surface airflow, oxidizing environment, and so on. To measure the laser coupling characteristics of materials, two typical online measuring apparatuses were developed in our laboratory. One is based on a conjugated hemi-ellipsoidal reflectometer, which is suitable to measure the laser coupling coefficients of different temperature in vacuum and air environments. The other is based on an integrating sphere and a simple airflow simulator, which can be applied to online measure the laser absorptance of materials subjected to surface airflow. The laser coupling effects on two types of structural materials, which are alloy steels and composite materials, are given in this paper. With the conjugated ellipsoidal reflectometer, the laser coupling effects on a typical alloy steel are investigated in different temperatures under the vacuum and air environment, and the experimental results are analyzed. According the results, metal oxidization plays a key role in the laser coupling enhancement effects. Especially, when the metal is subjected to high power laser irradiation in the high subsonic airflow, metal oxidization which is an exothermic reaction enhances the laser damage effect and the convective heat loss is negligible. Finally, the laser coupling effects on a typical composite material subjected to airflow are studied by using the integrating sphere with an airflow simulator, and the experimental results of laser absorptance during the laser ablation are presented.
The experimental setup was established for studying damage effects on silicon photoelectric detector materials induced by 800nm and 150fs repetitively-pulsed laser. The detector is irradiated by single shot and multiple shots respectively. The laser damage thresholds of silicon photoelectric detector material were measured. The surface morphologies of the material damaged by laser were analyzed. The surfaces damaged by laser with different energy were compared. The thresholds vary with the number of laser shots. According to the accumulation theory, the damage threshold is the power function of the shot number. Experimental results show that threshold of single shot that damages the silicon photoelectric detector is 0.156J/cm2. The laser damage threshold decreases with the increasing number of laser pulses, but the minimum value exists. The damage is mainly caused by the mechanical effect rather than thermal effect. In fact, the thermal effect during the interaction is so small that it can’t even be observed. Resistivity of the silicon photoelectric detector irradiated by femtosecond laser decreases and finally tends to a constant value.
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