Analytical Model for Determining The Effective Size of an Evaporation Region in Pulsed Laser Ablation

This paper describes the study of the effect of spatial inhomogeneity of surface temperature on the size of a crater forming in the case of pulsed laser ablation. It is assumed that the surface temperature is linearly dependent on laser radiation energy. Analytical expressions determining the effective radius of an evaporation region, characteristic temperatures of the surface on which evaporation occurs, and evaporation depths in the case of the Gaussian distribution of laser radiation energy are derived. It is shown that the analytical dependences obtained are in good agreement with known numerical calculation results.     

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.     

一种新型的精细网格制作工艺研究

本文提出了一种新型的精细网格制作工艺，它的特点概括为：在不同的材料表面上，利用两种选择性金属镀层制作精细网格，使得本工艺制作的精细网格具有广泛的适用性。选择适当的两种金属，能够提高网格的反差；选择抗氧化能力强的金属镀层，能够使网格适用于高温环境。     

Inversion of speckle interferometer fringes for hole-drilling residual stress determinations

Speckle interferometric fringe patterns record stress-relief displacements induced by the drilling of blind-holes into prestressed objects. The quantitative determination of residual stress state from such stress patterns is difficult because of the ambiguity in the order of the observed fringes. The plane stress magnitudes are provided directly from selected fringe positions using a stochastic, iterative least squares minimization approach. The inversion requires prior knowledge of the experimental geometry and an appropriate uniaxial stress-relief displacement basis function derived from three-dimensional finite element calculations. Superpositioning of the rotated and scaled displacement basis functions allows the stress-relief relaxation for any biaxial state of stress to be determined. In this paper, fringe patterns were forward modeled from a large ensemble of calculated biaxial stress-relief displacement fields. Inversion of these noise-free fringe patterns reproduced the biaxial stresses with negligible error. Analysis of more realistic fringe patterns that include speckle noise gave stress magnitude errors that diminished rapidly with the number of selected points to better than 3 percent for 100 points. Sensitivity of the optical method is influenced by a number of factors, but the ensemble of model fringe patterns studied indicates that the stress magnitudes (nomalized with respect to the material's Young's modulus) from 3×10^–4 to 10^–2 can accurately be determined with visible laser radiation. The method is amenable to automation and can easily be extended to study near surface gradients in the residual stresses or applied to other optical recording techniques such as moiré and phase-shifting interferometry.     

Failure properties of a PBAN propellant in multiaxial stress fields

Experimental data have been obtained which duplicate the stress and strain field in a solid-propellant rocket-motor grain under pressurization. Development of a hollow ellipsoidal specimen has made possible the acquisition of these data. This test also allows a more thorough evaluation of the failure mechanism in multiaxial stress and strain fields. Failure data were obtained in the tensile-tensile-compressive (++?) stress and biaxial tensile-compressive (+?) strain octants by applying pressure to the internal surface of an ellipsoidal specimen. Data were obtained in the triaxial compressive (???) stress and biaxial tensile-compressive (+?) strain octants by simultaneous application of pressure to both internal and external surfaces of the ellipsoidal specimen and a gradual reduction of external pressure until failure. A stress analysis of the specimen is presented and data-reduction techniques are discussed. Data obtained with a hollow spherical specimen and with the newly developed hollow ellipsoidal specimen are compared. These data are obtained in the same stress and strain octants; however, the relative magnitudes of the parameters and, therefore, the positions within the stress and strain octants are different. This results in a change in the magnitudes of the failure parameter. Data from uniaxial specimens of the same propellant also are presented.     

Effect of the recoil pressure induced by evaporation on motion of powder particles in the light field during laser cladding

A model is proposed, which takes into account acceleration of powder particles by a force induced by recoil of material vapors from the irradiated region of the particle surface. Results of a numerical analysis of heat and mass transfer in the case of motion of individual stainless steel powder particles in a gas flow and in a light field of laser radiation under conditions of laser cladding are presented. Acceleration of particles is found to depend on their diameter, carrier gas velocity, powder material properties, laser radiation power, and degree of attenuation of the power density in the laser beam in the direction of its action on the substrate. The calculated results are compared with experimental data on light-propulsion acceleration of individual particles (of aluminum, aluminum oxide, and graphite) under the action of pulsed laser radiation.     

低碳钢试件弹塑性边界的白光相关检测

金属试件的弹塑性边界检测对判断该试件是否失效有着重要意义,因此一直是研究人员较感兴趣的研究课题。利用激光和白光作为照明光源的方法都曾被提出以检测金属试件的弹塑性边界以及塑性变形。为避免激光测量的诸多缺点,本文提出一种用白光作为照明光源并结合数字图像处理来检测低碳钢试件弹塑性边界的方法。该方法将数字图像相关和低碳钢试件表面弹塑性变形前后对照明光强的反射特性结合起来以判断试件是否进入塑性屈服。文中通过对加载过程中低碳钢试件表面序列图像的处理,准确地判断了该试件是否进入塑性屈服,验证了该方法的可行性。     

Focusing of radiation from a source of longitudinal waves,located on a free surface

Propagation of waves from a source located on a free surface inside a circular conical horn is studied within the framework of a three-dimensional axisymmetric acoustic approximation. The horn axis is assumed to be orthogonal to the free surface. The influence of the horn geometry on the efficiency of radiation focusing in an arbitrary circular cone is studied. Criteria, objective functions, and control parameters for efficiency estimations and horn optimization are proposed. A method of optimizing the radiating system consisting of the source on the free surface and the horn on the basis of the problem geometry is developed. Geometric parameters ensuring the best focusing of radiation of the source-horn system in a circular cone for an arbitrary transmission angle are determined.     

Application of fourier analysis to the laser based interferometric strain/displacement gage

The laser based interferometric Strain/Displacement Gage (ISDG) measures the in-plane surface deformation between two small reflecting surface markers. When illuminated with a coherent beam of light, the reflected beams from the two markers form an interference pattern, and monitoring the shift of the fringe pattern allows strain in the gage section of a specimen to be directly measured. A minimum on the fringe pattern can be isolated and tracked as the test proceeds, but this technique utilizes only a small part of the optical signal and often requires a complex programming scheme. This paper presents the application of Fourier transform and phase shifting techniques to the use of the ISDG during microsample tensile testing. The Fourier transform samples the entire fringe pattern and greatly improves the optical signal to noise ratio, and the phase shifting fringe pattern analysis has proven to be more robust and less affected by speckle or optical noise than fringe pattern minimum tracking. This results in the ability to measure larger deformations with a system resolution of ∼5 microstrain and an uncertainty of ±15.5 microstrain. An example involving the microsample tensile testing of a MEMS related LIGA Ni specimen is included to demonstrate the utility of these new techniques.     

DIC-Based Surface Motion Correction for ESPI Measurements

Electronic Speckle Pattern Interferometry (ESPI) provides a sensitive technique for measuring surface deformations. The technique involves comparison of the speckle phase angles within surface images measured before and after material deformation. This phase angle comparison requires that the speckle positions be consistent in all images. A lateral shift between image sets of just one pixel substantially degrades ESPI measurements, while a shift of two or more pixels typically causes complete decorrelation and compromises the measurement entirely. To prevent such rigid body motions, the specimen and the optical system must be rigidly fixed. This requirement typically impedes use of the ESPI method in applications outside laboratories or where it is necessary to remove the specimen from the optical setup between ESPI measurements. Here, Digital Image Correlation (DIC) is used to track speckle motion caused by specimen displacement between ESPI phase stepped image sets. The measured image set can then be mathematically shifted to restore the original speckle locations, thereby recorrelating the ESPI measurement. Examples are presented where ESPI measurements are successfully made with specimen shifts over 60 pixels.     

Calculation of the kinetics of propagation of a powerful light beam in a transparent dielectric with impurities

A large number of studies have been dedicated to the interaction of powerful optical radiation with transparent dielectrics in the prebreakdown or breakdown regimes. However, the mechanism by which the material is destroyed has not been determined decisively, as indicated by the constant flow of new publications on this theme. Attempts to obtain destruction as the result of electron avalanche [1] give threshold power values orders of magnitude greater than experiment [2]. In connection with this, in recent years the accent has been to deal with the concept of microimpurities of foreign particles or inhomogeneities within the medium having dimensions so small that their presence and concentration is difficult to monitor. As it absorbs optical radiation, the microimpurity (inhomogeneity) is heated and warms the areas of the medium adjacent to itself, which areas then commence to absorb light with significantly more intensity than they did in the initial state [3]. As a result, increase in absorption within the medium commences, terminating in breakdown or destruction of the material around the inhomogeneity. In [4, 5] it was noted that an important role may be played in such a destruction mechanism by thermoelastic stresses in the medium, which factor was not considered in [3]. In [4, 5] it was proposed that the basic effect of thermoelastic stresses reduces to development of microfissures in the medium. However, thermoelastic stresses can lead to yet another effect — narrowing of the forbidden zone of the medium and increase (together with the analogous action of temperature growth) in the coefficient of absorption of the medium. In the present study, the kinetics of interaction of optical radiation with a dielectric medium containing spherical metal particles as an impurity will be calculated, and it will be shown that thermoelastic stresses produce a significant contribution to the increase in light absorption by the medium around a particle.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 5, pp. 60–66, September–October, 1978.     

Role of bubble boiling in the interaction of intense radiation with matter

The model of stable evaporation [1–4] is widely used to study the interaction of intense radiation with matter. In this model the radiation flux normally incident on a planar surface of the body is constant in magnitude, which also guarantees stationarity of parameter distributions in a coordinate system related to the surface. An assumption of the model is that evaporation occurs at the surface only. As noted in [1, 2], however, in establishing metal evaporation the surface is found to be in a liquid overheated state. Consequently, the evaporation mechanism can be complicated by bubble boiling. This process is usually neglected due to the fact that the surface tension coefficient of metals is large (if the temperature is not too close to the critical temperature), and, consequently, the probability of bubble formation is low [2]. Quantitative estimates are needed to justify this statement. Such estimates were carried out in [5, 6], where it has been shown that there exists a certain intensity flux q_*, above which surface evaporation is modified by bulk boiling. A number of inaccuracies, however, were admitted in [5, 6], which, as shown below, strongly distort the boundaries of the evaporation mechanisms in several cases. The purpose of the present study is to remove these inaccuracies and calculate the quantity q_* more correctly.Translated from Zhumal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 3, pp. 35–44, May–June, 1979.The author is grateful to M. N. Kogan and N. K. Makashev for useful discussions.     

Effect of a laser beam on plexiglas

An analysis is given of the optical effects during the focusing of a laser beam inside a transparent dielectric (plexiglas). Two types of damage were established. One is connected with the appearance of microcracks and the other with large plane cracks. Transition from one type of damage to the other is observed when the focal length of the lenses (energy density in the specimen) is varied and the pulse length is changed from 10^–3 to 10^–8 sec. The effect of a laser beam on metals and ionic crystals was investigated in [1–3]. A number of effects associated with structural changes and a specific material damage were found. There is also some interest in the effects of laser light on transparent dielectrics (glasses and polymers). In the present paper we report results of an investigation of the effect of laser beams on plexiglas.The authors are grateful to G. I. Barenblatt and B. Ya. Zel'dovich for valuable advice in formulating the problem and discussions of the results and to V. V. Kireev, G. F. Kuzmin, and O. E. Marin for assistance in the experiments.     

Nonlinear wave propagation in optical fibres

A generalised mathematical theory leading to the evolution equation of nonlinear pulses propagating in an optical fibre waveguide is presented. The magnitudes of the optical self-steepening and third-order dispersion coefficients are calculated and their relative importance is assessed. The possibility of self-steepening is carefully examined and represented on a pulse-displacement-distance parameter diagram. It is concluded, for specimen single mode fibre data, that, even for a 1% deviation from zero group dispersion, the shock term interacts with the group dispersion to produce a pulse distortion in the form of a velocity change limited by the group dispersion rather than selfsteepening. Also, for such a fibre, self-steepening is so small that vast runs down the fibre are required for it to be seen. Any observation of self-steepening in fibres will require much closer tuning to zero group dispersion and will need the suppression of the third-order term.     

The effect of ultrasonic treatment on mechanical behavior of titanium and steel specimens

The effect of surface ultrasonic treatment on plastic deformation and mechanical properties of polycrystalline titanium and low-carbon steel specimens under tension was studied. The deformation pattern was investigated using optical, transmission electron, scanning electron and scanning tunneling microscopy. It was shown that the material within hardened surface layers is characterized by ultrafine-grained structure. This structure results in plastic flow localization at different scale levels. Localized deformation meso-bands induce weak work hardening of the material. Plastic flow macro-localization causes a drop of the external deforming stress. The peculiar mechanisms of deformation localization within the specimen surface layer govern formation of a banded dislocation substructure in the bulk of the specimen.     

Model of explosive pulsed vaporization of dielectric liquids by intense laser irradiation of their surfaces

The mechanism of explosive vaporization interaction of laser radiation with matter is studied theoretically. It is shown that, in dielectric liquids with a free surface, periodic explosive boiling is possible if the laser radiation intensity exceeds the rate of heat transfer from the region of laser radiation absorption. Analytical expressions are obtained to estimate the pulsating boiling period and the thickness of the surface liquid layer dispersed by fluctuation vapor bubbles during each boiling. The degree of absorption of laser radiation by the aerosol formed above the liquid surface is estimated. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 49, No. 6, pp. 17–24, November–December, 2008.     

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.     

Nondestructive detection of damage in aluminum: Electromagenetic discharge imaging

Electromagnetic Discharge Imaging (EDI) is applied for the nondestructive detection of imperfections and inhomogeneities in solids. These imperfections were introduced mechanically by drilling holes of different sizes and chemically by applying corrosive agents to aluminum specimens. The results are influenced by a host of variables associated with the waveform generator, the dielectric air gap and the specimen type. Correlation is thus established between the electromagnetic discharge image and the degree of material damage. Detection of material damage was also made possible even when the specimen surface was a layer of opaque insulation material.     

Heat Transfer and Evaporation from a Plane Surface into a Half-Space upon a Sudden Increase in Body Temperature

The one-dimensional time-dependent problem of evaporation from a plane body surface into a half-space filled by a gas (condensed phase vapor) upon a sudden increase in the body surface temperature is studied. The evaporation coefficient is the problem parameter and may take arbitrary values within the limits from zero to unity. The problem is formulated for the kinetic equation and solved by the finite-difference method. It is shown that a deviation of the evaporation coefficient from unity considerably modifies the gas phase flow pattern. However, the evaporation rate divided by the rate of evaporation into a vacuum at the given surface temperature is only weakly dependent on the evaporation coefficient.