Mechanical Engineering

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Start date: 1980End date: 1999

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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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    The Stress Field Surrounding the Tip of a Crack Propagating in a Finite Body
    (1987) Chona, Ravinder; Irwin, George R.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)
    The goal of this dissertation was to establish the relationship between a parameter descriptive of the trajectory of a smoothly curving crack, such as the curvature of the crack path, and the local stress state in the close vicinity of the crack tip. The behavior of fast -running cracks propagating along straight and smoothly curving paths in fracture specimens of various geometries was examined using dynamic photoelasticity and representations of the running crack stress field we redeveloped in terms of the coefficients of a set of infinite series, for both opening and shear mode loading conditions. Analysis of the isochromatic patterns, using local collocation methods based on this stress field representation, allowed the stress state in the neighborhood of the propagating crack-tip to be modelled with a high degree of accuracy and results were obtained for the variations with crack tip position of both the singular and leading non- singular stress field coefficients of interest. The results obtained for quasi-static and rapid crack propagation under opening mode conditions in a ring segment revealed the importance of retaining terms of order (at a minimum) r^1/2 even when only the singular term was to be determined accurately. Furthermore, it was found that the non-singular stress field coefficients varied similarly in both static and dynamic situations, with some variations in magnitude that could be attributed to crack speed. The results from the curved crack experiments also showed systematic variation of the non-singular terms, but more importantly, it was found that the instantaneous curvature of the crack path was related to the magnitude of the lowest order non-singular stress component (the coefficient of the r^1/2 term) associated with the local shear mode of deformation in the vicinity of the tip of the running crack. Furthermore, the results established that the only singularity associated with a crack propagating along a smoothly curving path in a brittle, isotropic material was that associated with the opening mode stress intensity factor, K1, and that the shear mode singularity, KII, was identically equal to zero.
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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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    Expert System for Process Selection and Operation Optimization
    (1986) Kar, Amit; Pandelidis, Ioannis; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)
    The working of an Expert System, PRODUCER, conceived as part of a CIM system for a plant manufacturing discreteparts, is presented. PRODUCER starts out by determining if the desired part can be produced. Based on part attributes it then decides on a specific manufacturing process, e.g., Welding, Casting, Forging or Machining. Having selected the process, PRODUCER establishes the particular operation. With the operation decided, PRODUCER proceeds to find all the feasible combinations, of equipment and tools, that could produce the desired component. The turning operation has been chosen to demonstrate PRODUCER's capabilities. PRODUCER then sets about the task of identifying the most optimal pair of machine-tool and cutting-tool, which will provide the highest Metal Removal Rate, MRR. This is accomplished at two levels. At the higher level, PRODUCER generates constraints, representing physical limitations of the cutting process, for each machine and tool combination. These constraints are then passed on to an Optimization program. This is a Fortran program, which operates at a lower level, and returns the optimum values of the process control variables, for each machine-tool and cutting-tool combination. PRODUCER finally yields the highest maximization of the MRR. In doing so it also identifies the particular machine-tool and cutting-tool associated with this global optimum. PRODUCER, essentially a knowledge-based production system, implemented in the First Order Predicate Logic language of Prolog, also enables intelligent adaptive control.
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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.