An experimental jet impingement facility is being constructed for conducting detailed analyses of local heat transfer coefficients beneath an array of impinging jets. The test configuration employs uniform, 18.65 mm impingement holes over a range of average jet Reynolds numbers (10,000 - 50,000). Air is heated by the heater mesh technique developed by Gillespie [1]. Liquid crystal thermography techniques are employed to attain a time-temperature profile of jet effectiveness. The use of a three-chip CCD camera system provides a real-time image history of the transient process. The facility enables a more detailed assessment of local heat transfer correlations for impingement cooling.

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Polymeric material in an electronic package expands upon absorbing moisture in uncontrolled humid conditions. Due to differential swelling among polymeric materials, hygroscopic mismatch stresses occur in electronic packages. In this paper, finite element analysis has been carried out using Pro/Mechanica commercial software. Six commercially used underfill materials have been analyzed. For each underfill material, thermal, hygroscopic, and combined thermal and hygroscopic analyses have been carried out. The local moisture concentration along the critical interface, i.e., interlayer dielectric (ILD), is critical for the failure of electronic packages. The most favorable underfill material is one with the least saturated moisture concentration (Csat) and the least coefficient of moisture expansion (ß).

Present work studies the mixing in turbulent flow behind a backward-facing step, which is a classical example of mixing of jet in cross flow. In this study the cross flow was an unconfined cross flow flowing through a duct, which has geometry of backward-facing step with varying cross section. A slot jet with different injection angles was studied. Spatial unmixedness based on helium mass concentration was used as an indicator for determining the degree of mixing. Velocity ratio, defined as ratio of jet velocity to the cross flow velocity, was varied during the analysis. Three different values of velocity ratio were considered during the study. Computational fluid dynamics analysis was performed using commercial CFD software CFX 5.6. Results were extracted in the form of helium mass concentration fields, concentration profiles, concentration contour plots and velocity vector plots. From the results obtained it can be concluded that, angle of jet inclination is the important controlling parameter in mixing in flow behind a backward-facing step.

Carbon nanotubes (CNTs) are extremely thin, hollow cylinders made of carbon atoms. Although their diameters are about 10,000 times smaller than a human hair, the mechanical properties of CNTs exceed those of any previously existing materials. Buckling and collapse of CNTs have been observed experimentally, however, the analytical study on the buckling and instability of thin-wall and long CNTs under compression and bending is limited. The model is currently available for isotropic material only, which is not able to explain the phenomenon observed in anisotropic CNTs buckling experiment. In this paper, a new anisotropic analytical model of buckling and collapse has been developed for the CNTs application. Both anisotropic effect and nonlinear ovalization effect are taken into consideration in the new model. Finally, the predicted critical buckling stresses will be validated against the existing isotropic analytical model results.

In this work, software tools are employed to analyze the performance of a thermal switch that has integrated temperature sensing and the actuation of a switching device in tandem. The design employs a shape memory alloy (SMA) sensor to perform autonomous switching within a predetermined range of temperature. A fabricated Autonomous Shape Memory Alloy Thermal Switch (ASMATS) is discussed along with computational studies conducted to ascertain its performance. Behavior of such a device in a cryogenic environment is investigated. Fluid Flow and Heat Transfer equations are solved and the total heat flux obtained across the heat sink is determined. Thermal and pressure gradients of the cryogenic fluids are predicted by simulating conditions of forced convection across the device. The transient heat transfer problem that results in the thermal switch under forced convection is modeled and solved numerically. A conclusive result is obtained on the effectiveness of the designed ASMATS. A design recommendation for enhancement of the heat sink effect is made.

Chromite has one of the most important crystal structures, the spinel structure; therefore its high pressure behavior has significant implications to the foundation of a wide range of materials. It is also found throughout our earth's interior. These facts make its high pressure studies interesting for materials scientists, physicists, geoscientists, and crystallographers alike. We performed Synchrotron X-ray diffraction measurements of chromite using a symmetrical diamond anvil cell to 41 GPa. We fit the data using the third order Birch-Murnaghan equation of state, and determined the bulk modulus KOT to be 197±19 GPa with K?OT=66±12 when fit to 16 GPa, and KOT to be 260±22 GPa with K?OT =51±9 GPa when fit to 25.3 GPa. Further investigation revealed a pressure induced phase transformation to another structure at 28.2 GPa. The structural identification of this new phase is in progress.

The stability of stationary analytic solutions of the one-dimensional Ginzburg-Landau equation is studied. The Jacobi condition for stability fails numerically, thus an alternative stability criterion, based on critical perturbations, is developed. The degree-of-stability parameter is introduced to quantify the effective stability of long-lived unstable solutions. For nanofilms, the existence of functionally graded nanophases is demonstrated. Numerical simulations indicate that graded nanophases can be produced by dissolving material from both surfaces of a nanofilm. Stability under finite perturbations and post-bifurcation evolution are investigated numerically.

Tungsten disulfide (WS2) is one of the most important solid lubricant materials. However, its high pressure properties such as structural stability and compressibility are not yet available for its broad engineering applications. Synchrotron x-ray diffraction was used in conjunction with a diamond anvil cell to investigate properties of a WS2 sample in the range of 0-25 GPa. Since WS2 is a known soft material, no pressure medium was used to generate hydrostatic pressure. Existence of the hydrostatic pressure within the test chamber was verified by the consistency of the ruby spectrum. No phase transformation was observed in the pressure range tested. The bulk modulus was determined by fitting the pressure-volume data to the third order Birch-Murnaghan equation of state with KOT = 61±1 GPa and K'OT = 9.0±0.3. It is also found that c- direction of the hexagonal structure is much more compressible than a-direction.

A new continuum approach to martensite crystallography is developed for temperature and stress-induced martensitic transformations. This allows taking into account internal stresses, interface friction, and nonequilibrium evolution of all crystallographic parameters under multiaxial loading. A representative volume is considered consisting of austenite (A) and twinned martensite (M) divided by a plane interface. The assumption of homogeneous stress and strain fields in A and each M variant is adopted. Plastic slip along slip systems of A and M is taken into account. The stresses and strains in A and each M variant are described by algebraic equations. All crystallographic parameters (volume fractions of each M variant and the orientation of A-M and variant-variant interfaces) are described by thermodynamically consistent kinetic equations. A computational algorithm is developed and numerical study of fcc?bcc stress-induced transformation is performed. Our approach significantly generalizes the crystallographic theory of M.

A finite element approach is suggested for the modeling of multivariant martensitic phase transitions (PT) in elastic materials at nanoscale in the 3-D case. The approach is based on the Landau theory with a new thermodynamic potential [5, 6] that captures the main features of macroscopic stress-strain curves. The model consists of a coupled system of the Landau-Ginzburg and linear static or dynamic elasticity equations. Distributions of different martensitic variants are the result of the solution of the aforementioned system of equations for order parameters. An explicit finite element algorithm suggested allows decoupling the Landau-Ginzburg and elasticity equations for small time increments. The numerical approach is implemented into the finite element program 'FEAP' [11]. Numerical examples of modeling of evolving microstructure during multivariant martensitic PT in 3D static and dynamic formulations are solved and analyzed.

This research has formulated an expression for the relation of crack length with loading cycles for a single crack throughout crack nucleation to long crack propagation. The expression uses Tanaka and Mura model for crack nucleation, Tryon and Cruse's model is used for small crack simulation, and Paris Law for long crack propagation. Through making assumptions of crack length of nucleation and using Taylor Series, the relation of final crack length and loading cycles has been achieved. According this expression, the crack length can be determined by loading cycles at any stage of crack propagation.

Failure occurs at the interface of a bimaterial wedge because of stress singularities at the interface corners. Magnitude of the singular stress field induced due to this singularity depends upon the value of the notch stress intensity, influence coefficient and order of singularity. Uncertainties of these singularity parameters require finding out the stress near singularity probabilistically. The probabilistic analysis of a bimaterial fracture specimen with a bimaterial interface under various tensile loading has been performed. In this study the singular parameters (order of singularity and influence coefficients) were determined numerically and maximum singular stress around singularity was found deterministically and probabilistically. Also the influence of the geometric parameters and material properties on the maximum singular stress was investigated during sensitivity analysis.

A new thermodynamic approach for sublimation inside of elastoplastic material is developed. Using continuum thermodynamics, a driving force for sublimation, X, is derived. A thermodynamically equilibrium relationship between pressure and temperature was obtained from the condition X=0. Kinetic criterion for thermally activated sublimation is derived. Thermodynamics and kinetics of the appearance of the gas bulb inside of elastoplastic material was considered. For this purpose, the problem for deformation of elastoplastic sphere by internal and external pressure is solved analytically for large strain. Thermodynamically equilibrium and kinetic relationship between sublimation pressure and temperature were obtained. Kinetic relationship is based on consideration of a critical gas nucleus. Results are specified for sublimation in HMX energetic crystal.

A new approach for the solution of linear elastodynamics problems on structured meshes with space-time finite elements is suggested. Weak formulations for elastodynamics problems are based on continuous and discontinuous Galerkin methods. The focus of this paper is the development of a numerical technique to effectively solve elastodynamics problems on structured meshes. A new modification of the continuous Galerkin method is proposed that allows increasing the accuracy of the method and excluding the spurious high frequency oscillations at some range of the element Courant number. Another advantage of the suggested approach is the simplicity of the final finite element equations. Therefore, the new technique can easily be integrated into existing finite element codes. Convergence of the numerical results for several test elastodynamics problems solved with new and known methods is studied.

Due to the presence of flaws in a material, the stress generated due to externally applied load can be multiplied by several times at the tip of U-shape micro fracture specimen. This amplified stress can then exceed the yield stress or the fracture stress of the material, at least at the crack tip. When this occurs, the crack will grow as long as the stress is applied, until such time when the material's cross-section can no longer support the load on it. Consequently the failure of the specimen occurs. Due to uncertainty in geometry, material and loading condition probabilistic analysis is important to find the probability of failure of the specimen. In this study stress field in the region surrounding the U-shaped crack of a micro fracture test specimen with a radius of curvature was analyzed to find out the reliability of the specimen. The classical stress analysis (stress-strength) cannot predict failure scenario as the specimens develop elastic stresses that exceed the material strength, even at the load levels which are below critical failure load. According to linear elasticity, stress concentration factor or stress intensity, KI characterizes the stress state around crack. Under this condition, failure at notch tip occurs only if the stress intensity factor, KI, reaches critical stress concentration factor or fracture toughness, KIC. In this study maximum bending stress at the U-shape crack tip was determined for finding failure probability, pf and sensitivity analysis of the fracture test specimen under bending loading.

Knowledge of the variables that influence the tensile properties of the human Anterior Cruciate Ligament (ACL) is required to standardize the laboratory test results and to understand ACL injury mechanisms. The geometrical parameters of seventeen unpaired human cadaveric ACLs were measured using a 3D scanner and the ACLs were tested in tension along their anatomical axis to determine their tensile properties. Pearson's correlation test and multivariate regression analysis were used to determine the effect of donor variables and ACL size on the tensile properties. A strong correlation between age and tensile properties were found. The height of the donor and the length of the ACL were found to affect most of the tensile properties of the ACL based on correlation analysis. The regression equations were able to closely predict the ultimate load of the ACL. The results of correlation analysis and regression equations can be used to understand the laboratory test results of the tensile test on the ACL and to predict the tensile properties of the ACL if body anthropometry and ACL size of the person is known.

A n-phase system with strain-induced structural changes (SC) which include phase transitions (PTs) and chemical reactions (CRs) is considered. A simple strain-controlled kinetic equations for (SC) are thermodynamically derived. They consider the possibility of direct and reverse SCs and the difference in plastic strain in each phase due to the different yield stress of the phases. A stationary solution for three-phase system is found and analyzed as well as kinetics of SC. The model is applied to explain various mechanochemical phenomena observed under compression and shear of materials in diamond or Bridgman anvils. Stationary solution explains zero pressure hysteresis observed experimentally. Also an explanation was obtained why a nonreacting matrix with a yield stress higher (lower) than that for reagents significantly accelerates (slows down) the reactions, but does not change the stationary solution. Obtained solution is applied for SC in Si and Ge.

Molybdenum disulfide (MoS2) has been identified as one of the most important solid lubricants and its high-pressure properties for engineering purposes are not available. In this paper a high-pressure experiment to study the compression behavior of MoS2 using an energy dispersive synchrotron X-ray diffraction method and the diamond-anvil cell technique was carried out to 38.8 GPa. Fitting of the experimental pressure-volume data to the third order Birch-Murnaghan equation of state yields the values of the bulk modulus KOT = 42.0 ± 0.5GPa and its pressure derivative KOT' = 15.4 ± 0.3.

In general, a suspension can be modeled by springs and dampers subject to vibrations produced by irregularities in the track and the speed of the vehicle. In a mountain bike these vibrations are principally produced by stones and pieces of wood. In this investigation we dynamically analyze a double suspension bicycle using ADAMS® as a computer tool. The principal objective of this project is to find out how is the suspensions performance. A complete model is developed including the cyclist, the bicycle and the track; analyzing the suspension comfort as well as its dynamical efficiency. This is the first investigation that integrates on the same model: the bicycle, cyclist and terrain.

In the present research project the plasma diagnostics are carried out for the coating process optimization using a novel inverted cylindrical magnetron (ICM) sputtering system. This ICM technique offers the advantage of very high deposition rates and has the potential for scale-up of deposition recipes for alumina films for applications in the cutting tool industry. In this paper we present the preliminary spectroscopic results recorded on Ti-Ar-N and Al-Ar-O plasmas. Our initial emission scans were acquired from 4000-8200 Å, for conditions used to deposit TiN and Alumina. For Ti target, the total pressure was 2 mTorr, power supplied was 8 kW, argon flow was 85 sccm, and nitrogen flow was 13 sccm. For Al target, argon and oxygen flow rate was 85sccm and 40 sccm respectively at 2 mTorr and 3.6kW. A two-lens optical system imaged the plasma onto the entrance slit of a 0.5m monochromator. Signal amplification was accomplished using a Hamamatsu PMT. The spectral response of the system was calibrated using a quartz-halogen tungsten lamp whose operating temperature was measured with a Pyro-micro disappearing filament optical pyrometer. Results indicate a Boltzmann temperature of about 5050K using the spectroscopic data for the Ar emission lines. The uncertainty, however, is high because the two states are only 0.2eV apart in energy, but this value is considerably lower than the average electron energy of the plasma (typically in the 1-2 eV range 11,000-22,000 K).

Polycrystalline cBN (PCBN) tools used for hard turning of ferrous alloys are brittle and cannot be manufactured in complex geometries. cBN coatings produced by plasma and ion-assisted techniques are unstable and exhibit poor surface adhesion. An alternative two-stage deposition technique consisting of Electrostatic Spray coating (ESC) of cBN powder and Chemical Vapor Infiltration (CVI) of TiCN on a WC-Co substrate is proposed. The aim of this paper is to show the feasibility of the CVI aspect of the composite coating. Turning tests on 4340 steel show that the observed tool wear rate is comparable to that of conventional PCBN tools.

Tool wear is an important indicator in machining. Evaluation and prediction of tool wear thus becomes vitally important in the optimization of the machining process. Because of the complexity of the process, implementing accurate empirical or mathematical models is difficult. Artificial Neural Networks have been used for modeling highly non-linear and multi-variant data. The applicability of this technique to model tool wear has been explored in this paper. Several multi-layered neural networks with different configurations have been trained using back-propagation algorithm with the sole aim of correlating physical characteristics of tool-coatings and the corresponding tool wear. Data obtained in the machining of a Ti-6A-l4V alloy with different commercial coated tools has been used to train and test these models. The results are presented and analyzed