Thermoluminescence energy response of TLD-100 subjected to photon irradiation using Monte Carlo N-particle transport code version 5

Useful TL properties of TLD-100 that is an excellent candidate for using in TL dosimetry of ionizing radiation are demonstrated. This study is focused on response of TLD-100 subjected to photon irradiation. The thermoluminescence (TL) response of TLD-100 subject to various photon energy, ranging from 20 keV to 6 MeV, was investigated as energy absorbed in the TL material using Monte Carlo N-Particle transport code version 5 (MCNP5). The input parameters included in this study are experimental geometry specification, source information, material information, and tallies. Tally F6 is used in this simulation. The results from MCNP5 simulation show good agreement with previous experimental data. However, the data obtained from the simulation are greater than the experimental data especially in lower energy ranges.     

Process characterization and statistical analysis of oxide CMP on a silicon wafer with sparse data

Continuous advancements in chemical mechanical planarization (CMP) process, such as new polishing pads, slurry materials, and abrasive particles necessitate optimization of the key process input parameters for maximum material removal rate (MRR) and/or minimum within wafer non-uniformity (WIWNU) using sparse experimental results. In this investigation a methodology is proposed for developing process models and optimization of input parameters (both main and interaction parameters) for maximum MRR and minimum WIWNU. This approach will be equally applicable for polishing other materials, such as copper, dielectrics and low-k materials. Complex relationships exist between several machine-specific and material-specific input parameters and the output performance variables, chiefly MRR and WIWNU. However, only a few of the input parameters are changed on a regular basis. Hence, only those subsets of relationships need to be considered for optimizing the CMP process. In this investigation, CMP process was characterized for polishing a thin layer of silicon dioxide on top of a silicon wafer. Statistical analysis of the experimental data was performed to obtain the order of significance of the input variables (machine and material parameters and their interactions). Both linear and logarithmic regression models were developed and used to determine optimum process conditions for maximizing MRR and minimizing WIWNU. While the main input parameters were responsible for maximum MRR, interaction parameters were found to be responsible for minimizing WIWNU. This may vary for different materials and polishing environments. PACS 81.00.00; 81.05.Gc; 81.65.Ps     

Numerical study of the hydrodynamic drag force in atomic force microscopy measurements undertaken in fluids

When atomic force microscopy (AFM) is employed for in vivo study of immersed biological samples, the fluid medium presents additional complexities, not least of which is the hydrodynamic drag force due to viscous friction of the cantilever with the liquid. This force should be considered when interpreting experimental results and any calculated material properties. In this paper, a numerical model is presented to study the influence of the drag force on experimental data obtained from AFM measurements using computational fluid dynamics (CFD) simulation. The model provides quantification of the drag force in AFM measurements of soft specimens in fluids.The numerical predictions were compared with experimental data obtained using AFM with a V-shaped cantilever fitted with a pyramidal tip. Tip velocities ranging from 1.05 to 105 μm/s were employed in water, polyethylene glycol and glycerol with the platform approaching from a distance of 6000 nm. The model was also compared with an existing analytical model. Good agreement was observed between numerical results, experiments and analytical predictions. Accurate predictions were obtained without the need for extrapolation of experimental data. In addition, the model can be employed over the range of tip geometries and velocities typically utilized in AFM measurements.     

Identification of modal parameters including unmeasured forces and transient effects

In this paper, a frequency-domain method to estimate modal parameters from short data records with known input (measured) forces and unknown input forces is presented. The method can be used for an experimental modal analysis, an operational modal analysis (output-only data) and the combination of both. A traditional experimental and operational modal analysis in the frequency domain starts respectively, from frequency response functions and spectral density functions. To estimate these functions accurately sufficient data have to be available. The technique developed in this paper estimates the modal parameters directly from the Fourier spectra of the outputs and the known input. Instead of using Hanning windows on these short data records the transient effects are estimated simultaneously with the modal parameters. The method is illustrated, tested and validated by Monte Carlo simulations and experiments. The presented method to process short data sequences leads to unbiased estimates with a small variance in comparison to the more traditional approaches.     

Structural health monitoring and damage assessment using a novel time series analysis methodology with sensor clustering

This study presents a novel time series analysis methodology to detect, locate, and estimate the extent of the structural changes (e.g. damage). In this methodology, ARX models (Auto-Regressive models with eXogenous input) are created for different sensor clusters by using the free response of the structure. The output of each sensor in a cluster is used as an input to the ARX model to predict the output of the reference channel of that sensor cluster. Two different approaches are used for extracting Damage Features (DFs) from these ARX models. For the first approach, the coefficients of the ARX models are directly used as the DFs. It is shown with a 4 dof numerical model that damage can be identified, located and quantified for simple models and noise free data. To consider the effects of the noise and model complexity, a second approach is presented based on using the ARX model fit ratios as the DFs. The second approach is first applied to the same 4 DOF numerical model and to the numerical data coming from an international benchmark study for noisy conditions. Then, the methodology is applied to the experimental data from a large scale laboratory model. It is shown that the second approach performs successfully for different damage cases to identify and locate the damage using numerical and experimental data. Furthermore, it is observed that the DF level is a good indicator for estimating the extent of the damage for these cases. The potential and advantages of the methodology are discussed along with the analysis results. The limitations of the methodology, recommendations, and future work are also addressed.     

Validation of estimated isotropic viscoelastic material properties and vibration response prediction

This paper is concerned with finite element (FE) prediction of forced vibrations using a linear viscoelastic constitutive vibration damping modelling technique. A combined numerical and experimental investigation was performed on two bonded aluminium-PMMA (polymethyl methacrylate) plates with different geometry. Three-dimensional FE models were established using experimentally estimated PMMA material properties (elastic and damping) from previously published procedures. The viscoelastic material damping parameters are here validated from the perspective of accurate estimation of constitutive material properties. Vibration responses were predicted from the FE models and measured on the two composite plate structures at a large number of points. Comparisons between the numerical FE simulations and corresponding measured responses show that the estimated material damping properties used as input to the computations are very accurate and may be treated as independent of the geometry and boundary conditions of the plate structures, i.e., as constitutive damping parameters.     

Filtered thermal contrast based technique for testing of material by infrared thermography

A new kind of thermal contrast, called “filtered contrast” is presented, which allows detecting and characterizing material defects using active thermography under some assumptions on physical and thermal parameters of materials. In opposition to known definitions of the thermal contrast, knowledge about defect-free area is not necessary and this contrast is less sensitive to nonuniformity of heat disposal to the material surface. The measurements were performed on an experimental setup equipped with a ThermaCAM PM 595 infrared camera and frame grabber. The step heating was chosen as heat excitation. The results demonstrate usefulness of the 1D model of heat transfer used for determination of depth of subsurface defects. The influence of the parameter of the smoothing filter, required for filtered contrast implementation, thermal parameters of the tested material and defect on expanded uncertainty of determination of defect depth is also presented. Due to significant complexity of the model of heat transfer, the conditions for the “law of propagation of uncertainty” were not fulfilled and a numerical method, i.e., Monte Carlo simulation is applied for the propagation of distributions.     

Comparison between experimental and 2-D numerical studies of multiple scattering in Inconel600® by means of array probes

Ultrasonic non-destructive testing of polycrystalline structures can be disturbed by scattering at grain boundaries. Understanding and modeling this so-called “structural noise” is crucial for characterization as well as detection purposes. Structural noise can be considered as a fingerprint of the material under investigation, since it contains information about its microstructure. The interpretation of experimental data necessitates an accurate comprehension of complex phenomena that occur in multiple scattering media and thus robust scattering models. In particular, numerical models can offer the opportunity to realize parametrical studies on controlled microstructures. However, the ability of the model to simulate wave propagation in complex media must be validated. In that perspective, the main objective of the present work is to evaluate the ability of the finite-element code ATHENA 2D to reproduce typical features of multiple wave scattering in the context of ultrasonic non-destructive evaluation, with an array of sources and receivers. Experiments were carried out with a 64-element array, around 2 MHz. The sample was a mock-up of Inconel600® exhibiting a coarse grain structure with a known grain size distribution. The numerical model of this microstructure is based on Voronoi diagrams. Two physical parameters were used to compare numerical and experimental data: the coherent backscattering peak, and the singular value distribution of the array response matrix. Though the simulations are 2-D, a good agreement was found between simulated and experimental data.     

The exact solution of the diffusion trapping model of defect profiling with variable energy positrons

We report an exact analytical solution of so-called positron diffusion trapping model. This model have been widely used for the treatment of the experimental data for defect profiling of the adjoin surface layer using the variable energy positron (VEP) beam technique. However, up to now this model could be treated only numerically with so-called VEPFIT program. The explicit form of the solutions is obtained for the realistic cases when defect profile is described by a discreet step-like function and continuous exponential-like function. Our solutions allow to derive the analytical expressions for typical positron annihilation characteristics including the positron lifetime spectrum. Latter quantity could be measured using the pulsed, slow positron beam. Our analytical results are in good coincidence with both the VEPFIT numerics and experimental data. The presented solutions are easily generalizable for defect profiles of other shapes and can be well used for much more precise treatment of above experimental data.     

Investigations on the effect of process parameters on the growth of vertically oriented graphene sheet in plasma-enhanced chemical vapour deposition system

A multistage numerical model comprising the plasma kinetics and surface deposition sub-models is developed to study the influence of process parameters, namely, total gas pressure and input plasma power on the plasma chemistry and growth characteristics of vertically oriented graphene sheets (VOGS) grown in the plasma-enhanced chemical vapour deposition system containing the Ar + H₂ + C₂H₂ reactive gas mixture. The spectral and spatial distributions of temperature and number densities, respectively, of plasma species, that is, charged and neutral species in the plasma reactor, are examined using inductively coupled plasma module of COMSOL Multiphysics 5.2 modelling suite. The numerical data from the computational plasma model are fed as the input parameters for the surface deposition model, and from the simulation results, it is found that there is a significant drop in the densities of various plasma species as one goes from the bulk plasma region to the substrate surface. The significant loss of the energetic electrons is observed in the plasma region at high pressure (for constant input power) and low input power (for constant gas pressure). At low pressure, the carbon species generate at higher rates on the catalyst nanoislands surface, thus enhancing the growth and surface density of VOGS. However, it is found that VOGS growth rate increases when input plasma power is raised from 100 to 300 W and decreases with further increase in the plasma power. A good comparison of the model outcomes with the available experimental results confirms the adequacy of the present model.     

Investigation of material properties of thin copper films through finite element modelling of microindentation test

Technology processes of thin metal films deposition are entailed with changes in material’s microstructure. As a result, deposited films often are characterized with material properties, which are different from these of the original bulk material. Determination of these material characteristics is of big importance for practice. In the present work the material properties of thin bright copper film with known depth were investigated. The film was deposited electrochemically over substratum composed of metallurgic brass alloy (CuZn36). Based on the results from microindentation test the load-displacement curve is obtained after the indenter is unloaded and the imprint diameter is measured. Consequently the process of indentation was modelled numerically. The numerical simulation is based on the finite element model of the indentation process. As a result of the simulation the load-displacement curve was obtained numerically for a certain set of material parameters. The trial-error approach is applied to find most appropriate set which fit the experimental load-displacement curve. At the end results, which were obtained through numerical simulation give good coincidence with the experiment. Therefore the proposed method can be successfully applied for identification of material parameters of the accepted model. The proposed trial-error approach is appropriate for investigation of thin films with known thickness, deposited on a substrate with known material characteristics.     

Numerical Study of Mechanical Properties of Nanoparticles of β-Type Ti-Nb Alloy under Conditions Identical to Laser Sintering. Multilevel Approach

A multilevel approach is used to numerically investigate physical and mechanical properties of titanium-based bcc alloys and their behavior under conditions identical to selective laser sintering. Plastic properties of P-Ti-Nb alloy are calculated within the first principles approach. An algorithm is proposed and tested to optimize the calculations and reduce their number by more than 5 times. A molecular dynamics method is employed to study structural changes of titanium and niobium powder particles during sintering and to calculate adhesion characteristics of nanoparticles of the produced alloy depending on the external action. The simulation results are in good agreement with the known experimental data and can be used as input data both for numerical models of a higher spatial scale and for the optimization of production parameters of titanium alloys by additive technologies.     

On the influence of frequency-dependent elastic properties in vibro-acoustic modelling of porous materials under structural excitation

The aspects related to modelling the frequency dependence of the elastic properties of air-saturated porous materials have been largely neglected in the past for several reasons. For acoustic excitation of porous materials, the material behaviour can be quite well represented by models where the properties of the solid frame have little influence. Only recently has the importance of the dynamic moduli of the frame come into focus. This is related to a growing interest in the material behaviour due to structural excitation. Two aspects stand out in connection with the elastic-dynamic behaviour. The first is related to methods for the characterisation of the dynamic moduli of porous materials. The second is a perceived lack of numerical methods able to model the complex material behaviour under structural excitation, in particular at higher frequencies. In the current paper, experimental data from a panel under structural excitation, coated with a porous material, are presented. In an attempt to correlate the experimental data to numerical predictions, it is found that the measured quasi-static material parameters do not suffice for an accurate prediction of the measured results. The elastic material parameters are then estimated by correlating the numerical prediction to the experimental data, following the physical behaviour predicted by the augmented Hooke?s law. The change in material behaviour due to the frequency-dependent properties is illustrated in terms of the propagation of the slow wave and the shear wave in the porous material.     

Predictions and experimental studies of the tail pipe noise of an automotive muffler using a one dimensional CFD model

The tail pipe noise from a commercial automotive muffler was studied experimentally and numerically under the condition of wide open throttle acceleration in the present research. The engine was accelerated from 1000 to 6000 rpm in 30 s at the warm up condition. The transient acoustic characteristics of its exhaust muffler were predicted using one dimensional computational fluid dynamics. To validate the results of the simulation, the transient acoustic characteristics of the exhaust muffler were measured in an anechoic chamber according to the Japanese Standard (JIS D 1616). It was found that the results of simulation are in good agreement with experimental results at the 2nd order of the engine rotational frequency. At the high order of engine speed, differences between the computational and experimental results exist in the high revolution range (from 5000 to 6000 rpm at the 4th order, and from 4200 to 6000 rpm at the 6th order). According to these results, the differences were caused by the flow noise which was not considered in the simulation. Based on the theory of one dimensional CFD model, a simplified model which can provide an acceptable accuracy and save more than 90% of execution time compared with the standard model was proposed for the optimization design to meet the demand of time to market.     

Fundamental amplitude noise limitations to supercontinuum spectra generated in a microstructured fiber

Broadband supercontinuum spectra are generated in a microstructured fiber using femtosecond laser pulses. Noise properties of these spectra are studied through experiments and numerical simulations based on a generalized stochastic nonlinear Schrödinger equation. In particular, the relative intensity noise as a function of wavelength across the supercontinuum is measured over a wide range of input pulse parameters, and experimental results and simulations are shown to be in good quantitative agreement. For certain input pulse parameters, amplitude fluctuations as large as 50% are observed. The simulations clarify that the intensity noise on the supercontinuum arises from the amplification of two noise inputs during propagation – quantum-limited shot noise on the input pulse, and spontaneous Raman scattering in the fiber. The amplification factor is a sensitive function of the input pulse parameters. Short input pulses are critical for the generation of very broad supercontinua with low noise. PACS 42.50.Lc; 42.65.Re; 42.81.Dp; 02.60.CbAn Erratum to this article can be found at     

Diode laser welding of ABS: Experiments and process modeling

The laser beam weldability of acrylonitrile/butadiene/styrene (ABS) plates is determined by combining both experimental and theoretical aspects. In modeling the process, an optical model is used to determine how the laser beam is attenuated by the first material and to obtain the laser beam profile at the interface. Using this information as the input data to a thermal model, the evolution of the temperature field within the two components can be estimated. The thermal model is based on the first principles of heat transfer and utilizes the temperature variation laws of material properties. Corroborating the numerical results with the experimental results, some important insights concerning the fundamental phenomena that govern the process could be extracted. This approach proved to be an efficient tool in determining the weldability of polimeric materials and assures a significant reduction of time and costs with the experimental exploration.     

Modeling the Effect of Thermophysical Properties on Pyrolysis of Intumescent Fire-retardant Materials

Thermophysical properties of intumescent fire-retardant (IFR) materials are important input parameters to simulate the pyrolysis process of IFR materials in fire scenarios. In this article, the effects of the thermophysical properties on pyrolysis of IFR materials are simulated based on a pyrolysis model of IFR materials. The selected thermophysical properties here are the specific heat capacity of the virgin material, thermal conductivity of the virgin material and char layer, heat of decomposition, density of virgin material, intumescent temperature, and surface emissivity of virgin material and char layer. Simulated mass loss rates (MLR) for the IFR materials at an incident heat flux of 50 kW/m² are investigated for the varied thermophysical parameter values. The results show that changes in these property values can affect the pyrolysis behavior of materials profoundly. Comparison with experimental results indicates that the simulations of MLR are in reasonably good agreement with the experiments.     

Investigation of the intracavity frequency doubling of a gain-switched InGaAs/GaAs pulsed diode laser

A mathematical model describing the dynamic emission of the intracavity frequency doubling (IFD) of a gain-switched two-section InGaAs/GaAs diode laser has been presented. One section is electrically pumped to provide gain, while the second section is unpumped (reverse biased) to provide a saturable absorber. The three-dimensional physical problem is reduced to one-dimensional model using adiabatic approximation, which allows the separation of the wavefunction. The suggested model in this paper allows studying the impact of the variations of the input InGaAs/GaAs/KTP diode laser parameters on the output pulsation characteristics. The proposed mathematical model is solved numerically using fourth-order Runge-Kutta method. The numerical calculations reflect the influence of the variation of the applied gain current and saturable bias current on the output laser pulse characteristics. The numerical results show good consistency with the available experimental data in references.     

Mathematical modeling of ruby laser as a pumping source of a “self Q-switched” distributed feedback dye laser

A mathematical model describing the dynamic emission of the ruby laser as a pumping source of a distributed feedback dye laser (DFDL) has been adapted. The suggested model allows the temporal behavior investigation of the ruby laser and the DFDL on mode characteristics and, moreover, investigating the affect of laser input parameters on the output laser pulses in the ruby laser and in the DFDL.The numerical solutions of a coupled nonlinear rate equations system of the adapted model that predict the generation of picoseconds pulses, with neglecting the effect of refractive index variation, are discussed (feedback process is achieved only by optical gain). The model estimates the density of the emitted radiation, energy density of the first excited state, and the output power of the DFDL. The adapted mathematical model is in good agreement with the available experimental data.