Mechanical Engineering

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Start date: 1990End date: 1996

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    Physics-of-Failure Methodology for Accelerated Thermal Cycling of LCC Solder Joints
    (1995) Rothman, Timothy Paul; Dasgupta, Abhijit; Mechanical Engineering; University of Maryland (College Park, Md); Digital Repository at the University of Maryland
    This research presents a case study were existing physics-of-failure models and Bayesian statistical methods are used in conjunction to quantify the test-time compression achieved during accelerated temperature cycling tests on leadless solder joints. Different combinations of substrate materials and package styles are evaluated with physics-of-failure models and calculable information is obtained from a relatively small population of test specimens under accelerated stresses, because the critical variables are identified, and their influences on the stress magnitude are quantified. Bayesian statistical analysis is employed to obtain an acceleration transform, determine the confidence on the calculations, and determine which outliers are contaminating the database. In addition to accelerating the stress levels, the total test time is further minimized by tailoring the stress drivers in each sample such that multiple stress levels can be achieved under a single loading, which eliminates the need for repeating the test at multiple load levels. This research presents the details of how the models and statistical methods are applied, the results of evaluating different material combinations and package styles, problems encountered during the test, and a summary of the acceleration transforms obtained from the test. Analytical predicative models for life predictions are essential and will obviously result in significant savings of cost and time. The methods used in this are general enough to be applied to screening, qualification, and reliability enhancement tests of a wide range of new or existing electronics assemblies.
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    A MANUFACTURING PROCESS AND MATERIALS DESIGN ADVISOR
    (1996) Kunchithapatham, Arun; Magrab, Edward B.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)
    A computer assisted tool, called the DESIGN ADVISOR, has been developed to help obtain a compatible set of candidate materials and manufacturing processes in a fast and straightforward manner. Information for seventeen manufacturing processes includes animations, written descriptions, still pictures and geometric design rules; information for forty-two materials includes written information and data. The DESIGN ADVISOR determines candidate manufacturing processes based on user-specified levels of one or more of seven manufacturing attributes; namely, surface condition, dimensional accuracy, complexity of shape, size, production run or production rate, and cost. The determination of candidate materials is based on the user-specified levels of one or more of eleven material attributes; namely, yield strength/density, fracture toughness/density, elastic modulus/density, high temperature strength/density, density, magnetic properties, electrical resistivity, thermal distortion, thermal insulation, solvent resistance, and cost. It also determines the suitability of candidate manufacturing processes with the candidate materials, and ranks the suitability of each candidate material within each candidate manufacturing process.
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    Simultaneous Measurements of the Velocity and Vorticity Vector Fields in the Turbulent Near Wake of a Circular Cylinder
    (1993) Nguyen, Phuc Ngoc; Marasli, Barsam; Wallace, James M.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)
    Hot-wire measurements of all components of the instantaneous velocity and vorticity vectors in the wake of a circular cylinder are presented. The experiments were performed at x / d = 20 and 30, at Reα = 2000, using a miniature 12-sensor probe for the simultaneous velocity-vorticity measurements and a 4-sensor probe for the velocity-only measurements. A calibration and a data reduction method for multi-sensor hot-wire probes are introduced. The calibration technique is independent of the number of sensors and requires minimal information about probe geometry. The data reduction scheme involves the solution of an overdetermined nonlinear algebraic system of equations in a least-squares sense. The measurements indicate that the Karman vortices are accurately resolved. Statistical characteristics of the velocity aJ1d vorticity fields in this flow, including moments, probability distributions and one-dimensional spectra components are documented for the first time. Conditional analysis of the measurements at x/ d = 30 with respect, to the passage of the Karman vortices are presented. The vortex center and the saddle regions are identified and characterized. The instanteaneous velocity and vorticity signals are decomposed into mean, coherent and incoherent parts using the triple decomposition technique. A significant percentage of the incoherent fluctuations are observed to be phase-locked to the Karman vortices. The enstropy balances are computed for three decomposed parts. The generation of incoherent enstrophy due to incoherent vortex stretching is detected to be the most dominant term and is balanced by the viscous dissipation.
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    Heat Transfer and Pressure Drop of Liquid Cooled Offset Fin Heat Exchangers
    (1993) Hu, Sen; Herold, Keith E.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)
    An experimental and modeling study of heat transfer and pressure drop in liquid-cooled offset fin compact heat exchangers (cold plates) is described. Liquid coolants used in the testing are water and PAO (polyalphaolefin), for which the Prandtl number ranges from 3 to 150. Attention was focused on the Reynolds number range 10 - 2000 which spans most liquid cooled applications. From the data and from comparisons with previous air-cooled data, it was found that the Prandtl number has a significant effect on the Colburn factor of the offset fin geometry but little effect on the friction factor. A numerical heat transfer analysis was performed to investigate the surface temperature distribution and uniformity of heat flux in the cold plates. The results demonstrate good agreement with surface temperature measurements. The model results were used to guide data reduction procedures. In particular, significant end effects are predicted. Through experience with the heat transfer model, these end effects were isolated. The numerical model predicts approximately uniform heat flux over the central section of the cold plates. Predictive models were developed based on a surface contribution analysis of energy and momentum balances in a unit cell of the offset fin geometry. The Prandtl number effects on heat transfer can be viewed from two perspectives: fin perspective and array perspective. The fin perspective allows explanation of the Prandtl number dependence of the periodic fully developed Nusselt number. The array perspective is analogous to the usual thermal entry region in duct flow. Thermal development from the array perspective yields higher Nusselt numbers in the entry region. The surface contribution model shows significant Prandtl number effects on offset fin heat transfer performance. The models have estimated uncertainty of ±20%. The models have been validated for heat transfer and pressure drop for Prandtl number ranging from 0.7 to 150 and Reynolds number from 10 - 2000.