A method of double-sided laser irradiation is presented for temperature control in high temperature mechanical properties test of composite materials. Based on the finite element method (FEM), a numerical model of temperature distribution of materials was established. The effects of specification, laser heating area and laser intensity,laser heating time on temperature uniformity during heating were analyzed. The results show that the laser heating area, thickness of the specimen, laser intensity and laser heating time have a decisive effect on the temperature uniformity. The limit temperature control precision reaches 2% for carbon fiber reinforced polymer(CFRP), and the heating time can be controlled in minutes. The method is especially suitable for composite materials that cannot be heated by electric induction in the traditional heating experiment of high heating rate. Furthermore, an experimental scheme of double-sided irradiation heating using a single laser beam was designed. Experiment results illustrated that the temperature control precision was high before the material appears obvious flame. This method has the advantages of rapid heating rate, high testing efficiency and high testing temperature. It can make a reference for mechanical properties test of composite materials at elevated temperature with rapid heating rate
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.
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.
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.
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.
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.
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.
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.
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.
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 research the influence of initial temperature on the laser ablation impulse coupling coefficient, we have established an experimental setup that can response micro impulse of μNs level. The setup mainly contains the impact pendulum with tiny-damp and the micro-angle detector based on laser diffuse reflection. The different initial temperatures were realized by continuum laser irradiation with different intensities and time duration. Experiments were carried out to show the effect of initial temperature on the impulse coupling coefficient by Nd:YAG laser pulses. The results show that the impulse coupling coefficients rapidly increased with the increases of the laser intensity. When incident laser intensity is about 5.2J/cm2, the coefficient increased to the maximum. Then the impulse coupling coefficients decreased gradually. Theoretical analysis and the experimental results show that the impulse coefficient increase at the first phase result from increase energy of plasma, while the coefficient decrease at the second phase derived mainly from the target softening under high temperature.
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.
This paper describes the combined irradiation effect of 2.7μm pulsed and 10.6μm continuous wave (CW) lasers on TiO2/SiO2 dielectric films. There were four combined irradiation time sequences, two of which were irradiation one after another and two were irradiation overlap. Single laser irradiation on the films was also carried out for analyzing the different results. The transmission spectrums of the films in visible and NIR region were measured before and after laser irradiation, which was taken as one of the measures of the damage degree. Typical damage morphologies under different conditions and the standing electromagnetic wave field of two laser wavelengths in the film samples were given. Comparing the irradiation results of different time sequences, especially when the pulsed laser ahead or behind of CW laser, conclusion can be drawn that the overlap of pulsed and CW laser had the most serious damage degree of the 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.
Based on the invariable stretching stress preloading device, the experimental investigation on damage effect of 30CrMnSiA steel structure specimen by continuous laser was carried out. While the stress is between 660MPa and 1140MPa, the average laser power density is between 110W/cm2 and 330W/cm2, we can get the conclusions as follow: (1) When the preloaded stretching stress increases, the necking phenomenon becomes unobvious. (2) When the preloaded stretching stress increases, both the rupture time and rupture temperature decrease monotonously. (3) When the laser power density increases, the rupture time decreases monotonously. (4) When the preloaded stretching stress or laser power density increases, the rupture threshold decreases monotonously. (5) The strain of the specimen before necking is small, and the specimen metamorphoses during necking period and the deformation of specimen is centralized in the necking region.
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.
A special waveform of CCD being irradiated by intense laser is explained and simulated. Its specialty is that reference level is altered and becomes equal with saturated data level, which can answer for CCD’s black video induced by laser and named as excessive saturation effect. Alteration of reference level has been explained by signal charges injection into the measuring well during reference time. In CCD, wells barriers are largely lower than channel stop. After that all transfer wells are crammed, many remained signal charges getting rid of clock’s control can be hold in channel, and move along it in thermal diffusion and self-induced drift. They can fill up the measuring well ahead of clock’s permission and alter reference level to saturated data level. Based on the explanation, the waveform is simulated on an equivalent circuit of CCD’s charge measurement structure, which is built on the platform of Multisim2001. The voltage sources and switches are used to manipulate the charge and discharge of a capacitor, which simulates the charge injection and resetting of measuring well. The clocks controlling switches represent the injection and reset clocks in CCD. To simulate clock’s impact on output, other capacitor is used to connect it to capacitor that represents the measuring well. The equivalent circuit is validated by the simulated normal waveform. Then, altering the clock and charging the capacitor ahead, the excessive saturation waveform is simulated, which validates the explanation to excessive saturation effect.
Two methods were described to quantitatively evaluate the damage of optical filters, which were through detecting the change of transmission coefficient and damaged area of optical filters. Based on the quantitative evaluation results, the laser-induced damage of optical filters was classified five damage degrees, which were undamaged, color changed, slight melt, middle melt and serious melt. The laser-induced damage was uncertain event because there were many uncertain factors to affect the laser-induced damage degree of optical filters. In view of the advantage of the Bayesian network in processing indefinite information, this paper emphatically studied the laser-induced damage degree assessment method of optical filters based on Static Bayesian network. A Bayesian model was constructed to assess the damage degree of filters. Upon our laser-induced damage experiments on the optical filters, the results of the quantitative evaluation were compared with the assessment results of Bayesian network model, which indicated that Bayesian network method was available to assess the laser-induced damage degree of optical filters.
In this paper we build an experimental apparatus to measure the reflectivity and temperature of the foil in different conditions. The experimental results show that the growth of oxide film can be divided into 3 stages which corresponding to logarithmic, linear and parabolic rate law. A mathematical model is introduced to explain the phenomena observed in experiment. Numeral calculations are made for 30CrMnSi steel while cw-laser wavelength is 1.07μm. The numerical solutions are in agreement with the experimental data.
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