Finite element analysis of evaporative cutting with a moving high energy pulsed laser

A computational model for simulation of pulsed laser-cutting process has been developed using a finite element method. An unsteady heat transfer model is considered that deals with the material-cutting process using a Gaussian wave laser beam in a pulsed mode. An iterative scheme is used to handle the geometric nonlinearity due to the melting region. The convergence study with mesh refinements and time steps first identifies optimal mesh and time step for the present analyses. Numerical analyses are carried out on the amount of material removal and groove smoothness with laser power (LP) and number of pulses (NPS) while other laser cutting parameters are fixed. The results show that there exist threshold values in number of pulses and laser power in order to achieve two predetermined conditions: (1) amount of material removal and (2) smoothness of groove shape. These values form an envelope called threshold curve that separate the acceptable region from unacceptable one for quality pulsed laser cutting. The effect of velocity also leads to another threshold curve which is determined from both number of pulses and velocity. Finally, the convergence of results in error domain is shown oscillating due to geometric nonlinearity.     

Dynamic simulation on the preparation process of thin films by pulsed laser

An ablation model of targets irradiated by pulsed laser is established. By using the simple energy balance conditions, the relationship between ablation surface location and time is derived. By an adiabatic approximation, the continuous-temperature condition, energy conservation and all boundary conditions can be established. By applying the analytical method and integral-approximation method, the solid and liquid phase temperature distributions are obtained and found to be a function of time and location. The interface of solid and liquid phase is also derived. The results are compared with the other published data. In addition, the dynamics process of pulsed laser deposition of KTN (Kta_0.65Nb_0.35O₃) thin film is simulated in detail by using fluid dynamics theory. By combining the expression of the target ablation ratio and the dynamic equation and by using the experimental data, the effects of laser action parameters on the thickness distribution of thin film and on the thin film component characteristics are discussed. The results are in good agreement with the experimental data.     

Cryogenic mechanical response of multilayer satin weave CFRP composites with cracks

We deal with the thermomechanical response of multilayer satin weave carbon-fiber-reinforced polymer (CFRP) laminates with internal and/or edge cracks and temperature-dependent material properties subjected to tensile loading at cryogenic temperatures. The composite material is assumed to be under the generalized plane strain. Cracks are located in the transverse fiber bundles and extend to the interfaces between two fiber bundles. A finite-element model is employed to study the influence of residual thermal stresses on the mechanical behavior of multilayer CFRP woven laminates with cracks. Numerical calculations are carried out, and Young’s modulus and stress distributions near the crack tip are shown graphically. Russian translation published in Mekhanika Kompozitnykh Materialov, Vol. 44, No. 4, pp. 479–492, July–August, 2008.     

Varying Workpiece Dynamics in Milling Stability Analysis

Dynamic stability of a milling process with varying workpiece dynamics is investigated. The milling tool moves along the workpiece with a prescribed feed rate, whereby the contact point shifts. Furthermore, the workpiece dynamics is affected by material removal. The resultant varying workpiece dynamics is taken into account by parametric model order reduction including modal truncation. (© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Development of a slurry abrasion model using an Eulerian–Eulerian ‘two-fluid’ approach

The aim of the present study was to develop a numerical model that predicts the quantity and location of erosion damage in slurry systems susceptible to erosive wear. Unlike many forms of erosion which often need to be prevented, hydro-erosion is deliberately introduced during the manufacturing process of automotive diesel injector nozzles to smooth out imperfections in the spray-hole geometry. This model was integrated into a commercial CFD code, ANSYS CFX^®, and took into account the change in geometry by dynamically updating the mesh to model the removal of material. A slurry jet erosion apparatus was developed to determine experimentally the parameters influencing the erosion process. Simplified planar geometries with four different angles of inclination were investigated and subject to typical hydro-erosive conditions similar to that used during the smoothing process of injector spray-hole geometries. In addition, the effect of slurry viscosity, particle size and concentration were studied. Results were used to calibrate the developed erosion model which showed encouraging trends in comparison with experimental studies for predicting the location and quantity of erosive wear.     

Mesures de vitesses dans un écoulement laminaire axisymétrique par une technique optique de visualisation en lumière cohérente

An optical technique, based on holography, for measuring the distribution of velocities inside a laminar flow of a transparent liquid is described. A suspension of micro-sized scattering particles in the fluid is illuminated by a coherent light sheet issuing from a doubly pulsed ruby Laser. By temporal integration on a photographic plate of the interference patterns between the two scattered fields and two successive reference fields successively, we get a double exposure hologram of the two successive maps of scatterer distribution. The displacement of the scatterers during the time interval separating the two Laser pulses is measured by observation of the real holographic image through a microscope. This technique leaves the flow nearly undisturbed by the very weak mechanical action of the Laser pulses; it allows very accurate measurements over a large range of velocities. Here we report experimental results about two cases of axisymmetrical flow — confined in a circular pipe, then free jet out of the pipe — for several values of the hydrodynamic parameters.     

MODELING THE EFFECTS OF URBAN HEAT ISLANDS GENERATED MESOSCALE WIND ON AIR POLLUTION DISPERSION IN A PATCHY ATMOSPHERE

Abstract The paper presents a steady‐state two‐dimensional mathematical model to study the dispersion of air pollutants continuously emitting from a point source in a patchy atmosphere. The first patch, which extends to some downwind distance from the source (present in first patch), is the urban center, whereas the second patch is its adjacent countryside. For simulating the effects of urban heat islands on pollutant distribution, a local mesoscale wind is introduced in the first patch along with the usual large‐scale wind. Keeping in mind the real‐life situations, the meteorological, deposition, and removal parameters are taken to be different in different patches. The concentration of pollutants in both the patches has been computed under different stability conditions of the atmosphere through finite difference scheme. The results of the model show that the urban heat island (mesoscale wind) has a tremendous impact on air pollution distribution, both over the urban center as well as its adjacent countryside (rural). The model solutions also reveal about the concentration distribution of pollutants in different parts of the atmosphere in different atmospheric conditions.     

Ablation resistance of certain epoxy compounds

The rate of ablation in oxidative and inert media at flow velocities up to 200 m/sec has been investigated for certain epoxy compounds containing powdered copper and zinc dust. The causes of ablative destruction and the factors tending to improve the ablation resistance are examined. The contributions of mechanical erosion and thermal shock are evaluated.Mekhanika Polimerov, Vol. 4, No. 1, pp. 72–77, 1968     

碳纤维及浸润树脂自然老化性能的模型预测

对目前土木工程加固常用的两种碳纤维片材及树脂耐久性进行了对比性试验研究.截至自然老化3年时,已进行了16组共80个试件的试验.考察在我国自然环境条件下抗拉强度的老化.根据复合材料的中值老化剩余强度半经验数学模型,对其进行拟合,回归出CFRP复合材料及其树脂的剩余强度中值方程的相关参数.预测复合材料自然老化剩余强度.对复合材料进行寿命预估,供进一步研究和工程设计参考使用.     

Nondestructive Evaluation of Graphite-Epoxy Composites by the Laser Ultrasonic Method

A laser ultrasonic method for nondestructive evaluation of the structure of composite materials is proposed. Specimens of graphite-epoxy composites with compaction-type defects and air cavities are investigated. The method is based on the laser thermooptical generation of wide-band acoustic pulses - optoacoustic (OA) signals - in the material investigated. The acoustic pulses backscattered by structural ingomogeneities and defects are registered by a wide-band piezotransducer, which makes it possible to detect acoustic pulses in the frequency range from 0.1 to 30 MHz. Since the generation and detection of acoustic pulses takes place on the front surface of the specimen, this method allows us to carry out nondestructive evaluation with one-sided access to the object under study. The spectral and correlation analyses of backscattered OA signals are used for mathematical processing of the experimental data. The method developed makes it possible to determine the type of defects and the depth of their location.     

Thermoelasticity with second sound—exponential stability in linear and non‐linear 1‐d

We consider linear and non‐linear thermoelastic systems in one space dimension where thermal disturbances are modelled propagating as wave‐like pulses travelling at finite speed. This removal of the physical paradox of infinite propagation speed in the classical theory of thermoelasticity within Fourier's law is achieved using Cattaneo's law for heat conduction. For different boundary conditions, in particular for those arising in pulsed laser heating of solids, the exponential stability of the now purely, but slightly damped, hyperbolic linear system is proved. A comparison with classical hyperbolic–parabolic thermoelasticity is given. For Dirichlet type boundary conditions—rigidly clamped, constant temperature—the global existence of small, smooth solutions and the exponential stability are proved for a non‐linear system. Copyright © 2002 John Wiley & Sons, Ltd.     

Multi-frequency mode-locked lasers

A new theoretical model is constructed which describes the operation of multi-frequency, pulsed mode-locked laser cavities. The model, which is a combination of multi channel interactions in the canonical master mode-locking model subject to three different gain models which account for both self- and cross-saturation effects, results in mode-locking dynamics which qualitatively describe the observed experimental dual-frequency laser operation. Specifically, the combination of self- and cross-saturation in the gain allows for mode-locking at two frequencies simultaneously, which can be of significantly different energies and pulsewidths. The model gives a framework for understanding the operation and stability of the increasingly important and timely technology of dual- and multi-frequency mode-locked laser cavities. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Wave Propagation in a beam with random material properties

Modern composite materials, e.g., carbon fibre reinforced plastics (CFRP), exhibit a complex micro structure due to their fabrication process. The latter, being usually omitted in mechanical models through the homogenization of elastic properties, has a strong influence on the propagation of ultrasonic guided waves [1, 2]. Though it is possible to model the wave phenomena deterministically, taking into account a realistic distribution of fibres and polymer matrix, it is desirable to develop an improved model for the finite element analysis (FEM), which consider the stochastic properties in a more general way. In the current work, an approach for the simulation of waves in a isotropic beam with random material properties is presented. For the numerical computations with the FEM the Young's modulus was discretized by the Karhunen-Loève Expansion (KLE). Numerical investigations on the excited and propagating guided waves are presented. (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Modeling of thermal response and ablation in laminated glass fiber reinforced polymer matrix composites due to lightning strike

Thermal response and ablation of laminated glass fiber reinforced polymer matrix composites subjected to lightning strike are studied. The associated nonlinear time-dependent heat transfer model includes specific features of lightning arcs observed in physical measurements such as lightning channel radius expansion, non-uniform lightning current density, and associated heat flux. Moving spatially and temporally non-uniform lightning-current-induced heat flux boundary and moving boundary due to material phase transition caused by rapid surface ablation are also included. To predict moving phase boundary in the laminated anisotropic composites, an element deletion method is developed and embedded into finite element analysis (FEA), which is performed using ABAQUS. The Umeshmotion + ALE method based on the user subroutine Umeshmotion and arbitrary Lagrangian–Eulerian (ALE) adaptive mesh technique is also used, when applicable (i.e., moving phase boundary is confined within a top layer of the composite laminate). Heat transfer analysis is performed for a non-conductive laminated glass fiber reinforced polymer matrix composite panel representing the SNL 100-00 wind turbine tip. Thermal response of the panel subjected to pulsed and continuing lightning currents at three different lightning protection levels, LPL I, LPL II, and LPL III, is studied. Temperature-dependent anisotropic thermal properties of the composite panel are included in the analysis. The FEA results include temperature distributions and ablation zone profiles. The results show the Umeshmotion + ALE method is sufficient for the pulsed lightning current at all three LPL levels since the moving phase boundary, i.e. the ablation front, is found to be confined within the top layer of the laminate. For the continuing lightning currents at all three LPL levels, the Umeshmotion + ALE method is not applicable since the moving phase boundary comes to rest at depths exceeding the thickness of the top layer of the composite laminate.     

Analysis of Dynamic Stability for Milling Processes with Varying Workpiece Dynamics

Dynamic stability of a milling process along of a flexible cantilever beam is investigated. The moving process point and the material removal leads to a significant change of the dynamical behavior and, therefore, to a shift of the stability boundary in the space of technological parameters. Modal reduction is applied to the beam model in order to enable an efficient stability analysis. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Highly efficient TW multipass Ti:sapphire laser system

An efficient chirped-pulse amplification, Ti: sapphire laser system, has been developed using mainly domestic components. The gain-narrowing effect has been significantly overcome by shaping spectrum of seeding pulses. With a novel aberration-free stretcher and two stage multi-pass amplifiers, pulses with duration of 25 fs and 36-mJ energy have been obtained at 10 Hz repetition rate, using only less than 290 mJ green Nd:YAG pump energy. This corresponds to a 1.4 TW peak power and 32% main amplification efficiency. The energy stability of the laser systems is better than ± 3%.     

Prestress Losses in the Stabilizing Cables of a Composite Saddle-Shaped Cable Roof

The paper deals with a square composite saddle-shaped cable roof 30 × 30 m in the plan, which is formed by two orthogonal cable groups joined with a compliant support contour. The kinematic invariability of the roof is achieved by prestressing the cable net. From the viewpoint of material consumption, the cable roof has rational geometrical characteristics. A hybrid composite cable on the basis of CFRP, GFRP, and steel is considered as a material for contour cables. The load-carrying and the stabilizing cables are made of steel. Prestress losses in the stabilizing cables are determined for three variants of prestressing. The possibility of reducing the consumption of cable materials by prestressing each cable of the net by an individual force is revealed.     

Meshfree isoparametric point interpolation method (IPIM) for evaporative laser drilling

An isoparametric point interpolation method (IPIM) has been developed to analyze evaporative laser machining. The method is based on isoparametric representation of the unknown in the local domain. It also uses a simple strong form, but shows to be powerful enough to handle highly localized boundary conditions. Solution in a typical influence domain is approximated by a polynomial function. The problem is geometrically nonlinear because the domain is not known a priori due to material removal in machining. An iterative scheme is introduced to solve the nonlinear problem. The material removal is handled by redistributing points in the domain. This renders the point distribution non-uniform same as random distribution. Three different boundary conditions considered are of essential, convection, and laser irradiation type. The numerical results show very good agreement with those by FEM and BEM.     

ALE-based 3D FE simulation of electromagnetic forming

Electromagnetic forming is a contact-free high-speed forming process. The deformation of the work piece is driven by the Lorentz force which results from the interaction of a pulsed magnetic field with eddy currents induced in the work piece by the field itself. The purpose of this work is to present a fully-coupled three-dimensional simulation of this process. For the mechanical structure, a thermoelastic, viscoplastic, electromagnetic material model is relevant, which is incorporated in a large-deformation dynamic formulation. The electromagnetic fields are governed by Maxwell's equations under quasistatic conditions. To consider their reduced regularity at material interfaces Nédélec elements are applied. Coupling takes the form of the Lorentz force, the electromotive intensity and the current geometry of the work piece. A staggered solution scheme based on a Lagrangian mesh for the work piece and an ALE formulation for the electromagnetic field is employed. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)