Modeling of the Free-Surface Shape in Laser Cutting of Metals. 1. Effect of Polarization of the Gaussian Beam on the Shape of the Surface Formed

A three-dimensional problem of describing the shape of a surface formed owing to interaction of laser radiation with a substance in processes of laser cutting of metals is considered. The effect of radiation polarization (linear, elliptic, and circular) on the absorption factor is analyzed. For calculating the latter, a generalized formula is proposed, which takes into account the spatial orientation of the plane of incidence of radiation. The influence of laser-radiation parameters on the surface shape and cut depth is studied numerically. In the case of generation of a beam with the TEM₀₀ mode, it is shown that the use of elliptic polarization of radiation with a certain ratio of semi-axes, aligned with the beam direction, is preferable.     

Mechanism of formation of a high-temperature zone in the case of laser-induced mechanical breakdown of transparent polymers

The mechanical breakdown of transparent polymers that have absorbed millisecond laser radiation on inclusions present in the working material is considered. It is shown that the distribution by dimension of absorbing particles in the polymer obeys a cubic law corresponding to the aerosol distribution in the atmosphere. The process by which an inclusion is heated by laser radiation and by which a gas cavity forms around it due to pyrolysis around the material is studied. A mechanism is proposed for light absorption based on carbon black formation from gas-phase hydrocarbons both in the breakdown zone and in the growing macrocrack. The use of macrokinetic parameters of high-temperature pyrolysis for describing laser-induced mechanical breakdown yields satisfactory agreement between the experimental and calculated data.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 1, pp. 127–134, January–February, 1976.In conclusion, the authors wish to express their appreciation to Yu. G. Ivanov for assistance in the experiments.     

Measurement of amplification coefficients

This study will perform an analysis of errors occurring in the measurement of the coefficient of amplification of the optical medium of a laser by the method of irradiation with monochromatic laser radiation. These errors are related to lack of correspondence in parameters of the probe laser and the laser to be measured.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 3, pp. 3–12, May–June, 1974.     

Laser heating of its erosional low-temperature plasma in the process of gasdynamic motion

A study is made of the gasdynamical and optical properties of erosional laser plasma jets in the presence and absence of laser radiation. It is shown that in processes of plasma formation during the action of laser radiation of moderate intensity (q 10⁷ W/cm²) on absorbing materials, the heating of the disintegration products by the attacking laser radiation plays an important role. The temperature distribution is obtained along the plasma jet which forms during the laser attack counter to its propagation in a quartz tube confining the dispersion. The temperature maximum is found at the exit from the tube, is caused by the heating of the erosional laser plasma by the incident laser radiation in the process of its one-dimensional gasdynamical motion, and indicates the screening of the surface from the laser radiation. It is established that the screening is affected by the gasdynamical structure of the plasma jet and by the spacing of the plasma clusters corresponding to the regular pulses of laser radiation.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 1, pp. 13–18, January–February, 1974.The authors are grateful to M. A. El'yashevich for his interest in the work.     

Sound generation by a laser beam in a liquid with absorbing particles

Sound generation by a laser beam within a liquid, related to liquid boiling at absorption centers, has been studied relatively little. Acoustic effects upon liquid boiling in a laser beam were first reported in [1]. A change in the index of refraction of a liquid has been observed under the action of acoustical radiation of microbubbles formed on absorbing particles in the zone irradiated by a laser beam [2–4]. However direct measurements of acoustical radiation from a set of microbubbles uniformly distributed over the volume of laser beam-liquid interaction has yet to be performed, to the authors' knowledge. The present study is a preliminary report of results of an experimental study of this class of opticoacoustical phenomena.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 1, pp. 9–12, January–February, 1984.In conclusion, the authors consider it their pleasant duty to thank G. A. Askar'yan for evaluating the study and S. V. Luk'yanov for assistance in measuring the particle size distribution.     

Free molecule flow over a nonconvex wedge at a finite mach number

We consider the problem of the effect of multiple molecular reflection on the aerodynamic characteristics of a nonconvex wedge with transverse free molecule flow. The molecular reflection from the body surface is assumed diffuse with partial thermal accommodation [1–7]. For these reflection conditions the aerodynamic characteristics are calculated and are then compared with the characteristics obtained using simpler assumptions.     

Rigid-Plastic Model of Chipping During Metal Cutting

A rigid-plastic model of shearing chip formation is proposed. The model is based on the Mohr-Coulomb yield criterion and allows one to determine the size of chipped elements in orthogonal cutting of plastic and brittle metals.__________Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 46, No. 4, pp. 179–186, July– August, 2005.     

Modeling the dynamics of refractive index variation associated with the absorption of radiation of wavelength λ=10.6 mm by water vapor

The analysis of the dynamics of the changes in the relative index, necessary for determining the best conditions for ensuring minimum energy losses when CO₂ laser radiation is transmitted through the atmosphere, is investigated.Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No.3, pp. 125–134, May–June, 1992.     

Water vapor cooling when radiation of wavelength λ=2.8 μm is absorbed

Investigations of the processes arising under the influence of electromagnetic radiation on resonantly absorbing gaseous media have now been widely developed. Particular interest is shown in the penetration of a pulse of laser radiation through the atmosphere. The main component absorbing the radiation of both CO₂ and HF lasers (wavelengths, respectively, 10.6 and 2.8 m) in the earth's atmosphere is water vapor [1]. Numerous experimental investigations show that the integrated coefficient of laser radiation absorption by water vapor is fairly large [1–3], while at the same time the energy absorption leads to the heating of the medium in a channel around the beam and, as a consequence, to its defocusing. However, all these investigations were carried out with continuous sources of laser radiation or with pulses of fairly great duration. It will be shown below that gas cooling in the channel around the beam is possible when a pulse of radiation with wavelength 2.8 m whose duration is less than the vibrational-translational (V-T) relaxation time of the energy absorbed by the H₂O molecules passes through a stationary medium containing water vapor.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 3, pp. 141–151, May–June, 1986.     

Calculation of reflection of a blast wave by a plane

Some results are given of calculation of the reflection of a blast wave by a rigid flat surface. A model of the explosion with a simple energy dissipation mechanism is considered, radiation being taken into account in the approximation of radiative heat conduction. The pressure distribution on the surface and the flow pattern in the region of propagation of the incident and reflected shock waves are obtained.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 5, pp. 179–182, September–October, 1982.We thank L. A. Chudov for helpful discussions of the work.     

The Stress—Strain State of Thin-Walled Structures Exposed to Radiation

Mathematical models of radiation-induced physical processes in materials are analyzed. An approximate method is proposed to determine thermal and radiation strains depending on the energy spectrum and components of radiation. The stress–strain state of thin plates is studied with allowance for radiation effects     

Simulations of hot, dense iron plasma opacity at 89 eV and comparison with experiment

Predictions of hot, dense iron plasma opacity at 89 eV photon energy are compared with experimental determinations from the transmission of laser-heated iron to extreme ultra-violet (EUV) laser radiation. The EUV laser was pumped using six beams of an Nd-Yag laser in a refraction compensating geometry, while another beam irradiated a tamped solid iron target with an intensity of 10¹⁴ W cm⁻². The Ehybrid hydrodynamic and atomic physics code was used to predict temperatures, densities and ionisation throughout the evolving iron plasma. The iron opacities were deduced taking into account free–free, bound–free and bound–bound absorption. Bound–bound absorption was considered using atomic data generated by the Opacity Project. Reasonable overall agreement between theory and experiment was obtained for the iron layer transmission. The simulations indicated the dominance of bound–bound absorption throughout most regions of the iron plasma, but also the potential importance of photoionisation from core levels where energetically possible.     

Onsager reciprocity relation for rarefied gas flow in a laser radiation field

The Onsager relations are established in general form on the basis of the fundamental properties of the transport equations describing the motion of a rarefied gas in a laser radiation field. Expressions are obtained for the transport coefficient corresponding to the lightinduced drift effect and for the cross coefficient. The relation between them is found.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 1, pp. 163–167, January–February, 1991.     

Initiation of Combustion in a Supersonic Hydrogen-Air Mixture Flow by CO<Subscript>2</Subscript>-Laser Radiation

Features of the ignition kinetics of an H₂/air mixture in the supersonic flow behind an inclined shock front are analyzed when asymmetric vibrations of a small amount (<1%) of O₃ molecules specially introduced into the initial mixture are excited by 9.7 μm wavelength radiation. It is shown that this radiation leads to intensification of the chain reactions and makes it possible to organize combustion at small distances from the front (of the order of 1 m) of even relatively weak shocks at small values of the laser radiation energies absorbed by the gas. This method of initiating combustion in a supersonic flow is 10–100 times more efficient than the thermal method.__________Translated from Izvestiya Rossiiskoi Academii Nauk, Mekhanika Zhidkosti i Gaza, No. 2, 2005, pp. 157–167.Original Russian Text Copyright © 2005 by Lukhovitskii, Starik, and Titova.     

Noncontacting strain measurements during tensile tests

The application of an innovative noncontracting Doppler laser extensometer is presented. True axial strain has been measured during tensile tests conducted on stainless-steel metal sheets over a range of strain rates (from 10^–4 to 10² 1/s) and temperatures (from –40°C to 400°C). The laser radiation scattered at the surface of the specimen is recorded during the duration of the experiment. The signals are then used to determine the evolution of the axial strain, which is subsequently combined with the load signal to construct the stress-strain curve for the material. Excellent agreement has been obtained between the total elongation predicted by the laser measurements and the actual values measured from the specimens. This technique offers several advantages over traditional strain-measuring technologies.     

Measurement of the velocity profile in liquid streams of large volumes using a laser Doppler velocimeter

The use of the laser Doppler velocimeter (LDVM) in fluid mechanics already has a solid past history and work on its development is proceeding rather intensively [1, 2], This method has been used successfully to measure various parameters of flows in channels or chutes of small size [3–6]. Fundamental difficulties due to possible local variations in the index of refraction along the path of the laser beam [7], in addition to technical difficulties, have not been eliminated for its use in other cases of practical importance, particularly in basins or reservoirs of large size. The aim of the present paper is the investigation of the velocity structure of streams in large volumes using an LDVM on the example of a turbulent submerged water jet. In order to estimate the effect of the thickness of the water layer on the applicability of the method we tested three schemes: based on direct [8] and back scattering [9] and a scheme with reflection of forward-scattered light from a mirror [10]. The results of the investigation of a submerged turbulent jet using an LDVM are presented.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 5, pp. 170–173, September–October, 1977.     

Experimental investigation of vibroconvection by the photochromic visualization method

The photochromic visualization method has been used to investigate vibroconvection in fluids. The experimental apparatus and the results of observing the flow in rectangular cells are described. The investigations were carried out for various vibration amplitudes, vibrating body dimensions and vibration frequencies. The position and orientation of the vibrating body were varied. A technology for using continuous low-power laser radiation to study slow flows has been developed.Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No.6, pp. 81–87, November–December, 1993.     

Different modes of heating when high-power radiation fluxes interact with a material

The gas-dynamic and thermal processes which occur when a high-power flux of laser radiation interacts with a material are investigated. Fluxes for which the sublimation energy can be neglected compared with the thermal and kinetic energy of the vapors formed are considered. The electron thermal conductivity is considered as well as the hydrodynamic dispersion. The properties of different modes of propagation of temperature waves in a moving medium are studied. The case of an infinitely large absorption coefficient is given particular attention.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 5, pp. 41–48, September–October, 1972.The authors thank A. A. Samarskii for useful discussions.     

Thermo-gravitational convection in a continuous optical discharge

The relative importance of such processes as the free convective motion of the gas, the absorption of the laser radiation and radiative heat transfer is discussed. The burning of a continuous optical discharge under experimental conditions [6] is theoretically investigated. The two-dimensional problem of the convective motion of the gas in an optical discharge burning in a vertical CO₂ laser beam inside a cylindrical chamber is solved. The principal characteristics of thermogravitational convection of the radiating air under conditions of local thermodynamic equilibrium at atmospheric pressure are studied on the temperature interval from 300 to 20 000°K.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 4, pp. 124–129, July–August, 1989.     

Dynamics of cavitational demolition in the interaction of a shock wave with a free surface

The problem of cavitation in the tension wave associated with the reflection of a shock wave from a free fluid surface is considered. A method of calculating the cavitation zone dynamics which makes it possible to determine the structure of the cavitation front, including for large space scales, is developed. A procedure for determining the dispersity of the fragment-drops of dispersed fluid, which takes into account the initial size distribution of the cavitation nuclei and the parameters of the incident shock wave, is proposed.Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No.6, pp. 73–80, November–December, 1992.