Effect of Stress State on Growth of Interfacial Intermetallic Compounds Between Sn-Ag-Cu Solder and Cu Substrates Coated with Electroless Ni Immersion Au

Excessive intermetallic compound (IMC) growth in solder joints will significantly decrease the reliability of the joints. IMC growth is known to be influenced by numerous factors during the component fabrication process and in service. It is reported that, other than temperature and holding time, stress can also influence the IMC growth behavior. However, no existing method can be used to study the effect of stress state on IMC growth in a controlled manner. This paper presents a novel method to study the effect of stress on interfacial IMC growth between Sn-Ag-Cu solder and a Cu substrate coated with electroless Ni immersion Au (ENIG). A C-ring was used and in-plane bending induced tensile and compressive stresses were applied by tightening the C-ring. Results revealed that in-plane compressive stress led to faster IMC growth as compared with in-plane tensile stress.     

Material removal and surface damage in EDM of Ti3SiC2 ceramic

Material removal and surface damage of Ti₃SiC₂ ceramic during electrical discharge machining (EDM) were investigated. Melting and decomposition were found to be the main material removal mechanisms during the machining process. Material removal rate was enhanced acceleratively with increasing discharge current, i_e, working voltage, u_i, but increased deceleratively with pulse duration, t_e. Microcracks in the surface and loose grains in the subsurface resulted from thermal shock were confirmed, and the surface damage in Ti₃SiC₂ ceramic led to a degradation of both strength and reliability.     

Risk-Based Allocation of Distribution System Maintenance Resources

Asset management is an important activity in present day distribution system planning and operation. Utilities are constantly striving to optimize the use of resources available for maintenance while ensuring system reliability is within satisfactory limits. This paper proposes a method to allocate maintenance resources to various distribution system assets. To determine the effects of maintenance, a predictive reliability assessment tool is developed. This paper describes the model on which this tool is based. The results obtained from reliability assessment can be used along with an optimizer to allocate resources to various maintenance tasks in a distribution system.     

Prediction of compressive strength and optimization of mixture proportioning in ternary cementitious systems

This paper discussed the application of the method of the simplex-lattice design for predicting the properties of cement-based composites. On the basis of the compressive strength, its use was demonstrated on ternary paste systems composed of cement, silica fume and fly ash with constant water to binder ratio and a mass fraction of mineral admixtures not exceeding 30%. The regression model between compressive strength of paste and binder proportion was built up. The F-test method was utilized for validation of the regression model. The nonlinear programming system with upper and lower bound was solved. This allowed assessment of optimum mixture proportions and corresponding maximum compressive strength. It was shown that: (1) the 3rd-order regression model constructed by using the simplex-lattice design method could accurately predict the compressive strength in ternary paste system made of cement, silica fume and fly ash (the total mass fraction of all mineral admixtures was up to 30%); (2) to solve the nonlinear programming with the constraints of upper and lower bound played an important role in getting the optimum compressive strength and its corresponding mixture proportion at different ages; (3) the combination of the simplex-lattice design method and the optimization theory could be valuable tool for optimization of cement-based composites' properties.     

Almost-global tracking of simple mechanical systems on a general class of Lie Groups

We present a general intrinsic tracking controller design for fully-actuated simple mechanical systems, when the configuration space is one of a general class of Lie groups. We first express a state-feedback controller in terms of a function-the "error function"-satisfying certain regularity conditions. If an error function can be found, then a general smooth and bounded reference trajectory may be tracked asymptotically from almost every initial condition, with locally exponential convergence. Asymptotic convergence from almost every initial condition is referred to as "almost-global" asymptotic stability. Error functions may be shown to exist on any compact Lie group, or any Lie group diffeomorphic to the product of a compact Lie group and R/sup n/. This covers many cases of practical interest, such as SO(n), SE(n), their subgroups, and direct products. We show here that for compact Lie groups the dynamic configuration-feedback controller obtained by composing the full state-feedback law with an exponentially convergent velocity observer is also almost-globally asymptotically stable with respect to the tracking error. We emphasize that no invariance is needed for these results. However, for the special case where the kinetic energy is left-invariant, we show that the explicit expression of these controllers does not require coordinates on the Lie group. The controller constructions are demonstrated on SO(3), and simulated for the axi-symmetric top. Results show excellent performance.     

A Customized Radiation Sensor for Ionization Collection

The measurement of absorbed doses is fundamental to radiation biology and oncology. A customized parallel plate radiation sensor was designed and fabricated as a precursor to investigating novel materials, such as carbon nanotubes, as a substitute for conventional metallic conducting plates or active volume medium. This sensor contains two thick and large-area electrodes that provide the sensor with a good signal-to-noise ratio. The 6 MV and 15 MV photon beams produced by a Varian Clinac 21 EX medical linear accelerator were used in the experiments. The linear accelerator was calibrated such that 1 monitor unit (MU) produces 1 cGy of dose in water with depth of 5 cm for a calibration geometry of source-to-axis distance equal to 100 cm and 10times10 cm² field size at the point of measurement. Ionization measurements were performed by varying the bias voltages, electrode separations, exposures, and angles of the incident beam to characterize the sensor. Signal saturation characteristics of the sensor with different electrode separations and exposures were investigated. This sensor displayed excellent linear response to exposure up to 600 MU. An analytical modeling using the pencil beam model and simulations based on device configuration were given to explain the results. In oblique incident beam experiments, the prototype sensor showed an accurate response compared to simulation results for a small field size of 1times1 cm². The sensor was tested to be suitable in the study of ionization collection efficiencies for different materials     

Simulation of vorticity dominated flows using a hybrid approach. II: Numerical method

A family of numerical methods for the solution of the incompressible Navier–Stokes equations in cylindrical coordinates is developed. The formulation for hybrid spectral-finite difference discretizations in domains with cylindrical boundaries presented in Kollman, in press [Kollmann W, Simulation of vorticity dominated flows using a hybrid approach: I formulation, in press] forms the basis of the solvers. The solution methods use the Fourier-spectral approach for the azimuthal and a set of finite-difference operators for the radial and axial directions. The convective terms are discretized with a linear combination of upwind-biased and central difference operators applied to the non-conservative and conservative formulations, respectively. All other spatial derivatives are discretized with central operators. The time integration is specified as a minimum storage, state space, fourth order Runge–Kutta method. The convergence of the solvers as the formal accuracy of the finite-difference operators varies is tested for an axi-symmetric flow for fixed discretization and time integrator. The results show satisfactory convergence with respect to order of accuracy and the convective operators.     

On a responsive replenishment system: a fuzzy logic approach

In today's competitive business environment, it is important that customers are able to obtain their preferred items in the shops they visit, particularly for convenience store chains such as 7–Eleven where popular items are expected to be readily available on the shelves of the stores for buyers. To minimize the cost of running such store chains, it is essential that stocks be kept to a minimum and at the same time large varieties of popular items are available for customers. In this respect, the replenishment system needs to be able to cope with the taxing demands of minimal inventory but at the same time keeping large varieties of needed items. This paper proposes a replenishment system which is able to respond to the fluctuating demands of customers and provide a timely supply of needed items in a cost–effective way. The proposed system embraces the principle of fuzzy logic which is able to deal with uncertainties by virtue of its fuzzy rules reasoning mechanism, thereby leveraging the responsiveness of the entire replenishment system for the chain stores. To validate the feasibility of the approach, a case study has been conducted in an emulated environment with promising results.