While the effectiveness of innovative high-energy laser demonstrators against lightweight drones is daily being shown in several countries around the world, the understanding of the phenomena occurring during the intensive interaction of the laser beam with the target, which will also later lead to a more efficient and safer use, is far from being exhaustive. This paper focuses on the vulnerability of different kinds of rotating drone propellers submitted to high-energy laser irradiations up to 10 kW. The near-infrared spectral response and the high-temperature thermal behavior are first presented, emphasizing that both type of propellers mainly differ in color only (glossy white and matt black). The high-energy laser trial setup is further detailed with a special focus on the propeller test bench and on a dual-wavelength radiometric device specifically developed to record the temperature of a high-speed rotating target (5000 rpm). Finally, the experimental outcomes are discussed, illustrating the major role of the color of the propeller of course, but also of the laser pointing area and its distance from the rotation center of the propeller.
Composites are subject to a wide variety of physicochemical processes when exposed to laser radiation. Pyrolysis, flow of reaction volatiles and char formation have a considerable influence on the heating process. In this contribution, some of these processes and their impact on the thermal and optical properties of glass-fiber reinforced polymers (GFRP) will be discussed with focus on charring and its impact. Modelling of the thermal and mechanical behavior with all processes occurring will be difficult to realize. With prioritization and concentration on the main processes of pyrolysis, the spatio-temporal thermal evolution was calculated by numerical FEM calculations. Based on the Arrhenius equation, the thermo-dynamical conversion rate was calculated. This transfer from virgin composite to charred material also affects the optical behavior. Absorption and scattering were considered by the photon diffusion equation in the unmodified material, whereas in charred material the laser radiation was assumed to be absorbed already within a small penetration depth.
Whereas laser weapon systems are foreseen as a new possibility to counter unmanned aerial vehicles (UAVs), a better understanding of the complex phenomena occurring during the interaction of a high-energy laser beam with a flying structure is required to use those new innovative defense devices in an efficient but also a secure way. This paper first presents multiple material characterizations performed on glass fibers-reinforced plastics (GFRP), from which near-infrared spectroscopic data and high-temperature thermodynamic results are later implemented into multiphysics simulations. Numerical outcomes from the models are then compared to experimental recordings arising from laser trials carried out with varying power densities (75, 150 and 300 W/cm2) and with illumination times of several seconds. A very good agreement is shown between temperature data collected during laser experiments and temperature values from computations.
Composite materials (GFRP) have shown a significant increase in their use for aerospace and military applications in recent years. Concurrent maturation of the technology in near Infrared High Energy Lasers has proven to be promising for use in directed energy weaponry. The binding matrix of many common aerospace composite materials are made of polymeric materials. The thermodynamics of GFRP as well as the thermo-mechanical behavior of such heterogeneous materials submitted to a typical laser weapon irradiation need therefore to be further explored to understand precisely the deterioration process induced by the illumination. Front and side views during the HEL (High Energy Laser) interaction process with the GFRP target material at different time delays clearly evidence the complicated nature of smoky plume and flame combustion creation during the irradiation process. During the energy deposition rise of the continuous laser wave, the GFRP target heats up due to absorption in the outer epoxy layer. Further increase of the energy reveals a smoky plume optically thin directly above the surface of the GFRP-target. Time resolved emission spectroscopy was used to investigate the chemical decomposition and surface temperature in the low and high temperature regime during the laser matter interaction process. The spectral emission in the visible range is dominated by continuum emission which provides the distribution temperature in time based on Planck`s law. In the low temperature regime (infrared wavelength bands from λ = 2μm up to λ = 11μm) a four channel infrared detector system was designed to retrieve the distribution temperature. The experimental, time resolved signal from four different infrared detectors at center wavelengths of λ = 3.348 μm, λ = 4.49 μm, λ = 7.41 μm and λ = 10.57 μm are used to reconstruct the Planck-function by a fitting routine with the temperature as parameter. The calibration of the system was made with a conventional Black-body source PY5. The obtained temperature distribution in time was then compared to measurements with a conventional pyrometer. With the concept of this four channel detector we overcome the difficulties of acquisition speed and single band nature of conventional pyrometers.
Laser experiments under combined mechanical tensile stress have been carried out to evaluate the operational vulnerability of fiberglass composite structures to a laser irradiation. It follows that the optical propagation of the laser beam inside the semi-transparent reinforced laminate is one of the key issues of this study as it rules every phenomenon occurring at a later stage. The Radiative Transfer Equation (RTE) has been used together with spectrometric measurements to assess the initial coupling of the monochromatic laser beam with the optically diffusing material. However, the partial absorption of the high-energy laser beam quickly leads to very-high temperatures on the irradiated area that can induce several phase transitions of the polymer. These changeovers and their influence on the optical and thermal properties of the system have been investigated with conventional methods (TGA and DSC) and reproduced with a time- and temperature-dependent kinetic scheme based on the Arrhenius equation. Finally, a 1D multiphysics model has been developed to reproduce the temperature evolution recorded during laser trials. Based on a time-explicit scheme, this computational approach shows a fairly good agreement and allows for a further understanding of the multiple phenomena occurring under a laser irradiation.
From their prior emergence in the military domain but also nowadays in the civilian area, unmanned air vehicles constitute a growing threat to the todays civilization. In this respect, novel laser weapons are considered to eradicate this menace and the vulnerability of typical aeronautic materials under 1.07μm-wavelength irradiations is also investigated. In this paper, Kubelka-Munk optical parameters of laminated glass fiber-reinforced plastic composites are first assessed to build up a basic analytical interaction model involving internal refraction and reflection as well as the scattering effect due to the presence of glass fibers. Moreover, a thermo-gravimetric analysis is carried out and the kinetic parameters of the decomposition reaction extracted from this test with the Friedman method are verified trough a comparison with experimental measurements.
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