Mechanical Engineering
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Item Latching Microelectromechanical Shock Sensor Systems: Design, Modeling, and Experiments(2010) Currano, Luke Joseph; Balachandran, Balakumar; Yu, Miao; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Latching shock sensors are acceleration threshold sensors that trigger when the acceleration level exceeds the designed acceleration threshold. The latching mechanism provides a mechanical memory, which keeps the sensor in a triggered, or latched, state until the sensor is reset. The attractive feature of this type of sensor is that it does not require power during monitoring; power is only needed to query and reset the sensor. Several devices have been presented in the literature, but with limited experimental data and models that provide little to no insight into the dynamics of the latching event. The aim of this work is to further the understanding of the physics and design of micromechanical latching shock sensors by conducting a combination of careful experiments and development of original reduced-ordermodels. These efforts enable one to obtain a detailed picture of the latching dynamics for the first time. Latching shock sensors have been designed, fabricated, and experimentally evaluated in this work. The model predictions have been compared to the experimental results to verify the validity, including a quantitative comparison of the position of the shock sensor during a latching event captured via high-speed videography. This is the first time a latching event has been imaged in this class of sensors, and the first time, the model predictions of position versus time histories have been validated through experiments. The models have also been used to conduct detailed numerical studies of the shock sensor, amongst other things to predict a latch "bounce" phenomenon during an acceleration event. To understand more thoroughly how the various design parameters affect the latching threshold of the sensor, various parametric and optimization studies have also been conducted with the reduced-order models to guide designs of future latching acceleration threshold sensors.Item Damage Initiation and Evolution in Voided and Unvoided Lead Free Solder Joints Under Cyclic Thermo-Mechanical Loading(2007-02-05) Jannesari Ladani, Leila; Dasgupta, Abhijit; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The effect of process-induced voids on the durability of Sn-Pb and Pb-free solder interconnects in electronic products is not clearly understood and researchers have reported conflicting findings. Studies have shown that depending on the size and location, voids are not always detrimental to reliability, and in fact, may sometimes even increase the durability of joints. This debate is more intensified in Pb-free solders; since voids are more common in Pb-free joints. Results of experimental studies are presented in this study to empirically explore the influence of voids on the durability of Ball Grid Array (BGA) Pb-free solder joints. In order to quantify the detailed influence of size, location, and volume fraction of voids, extensive modeling is conducted, using a continuum damage model (Energy Partitioning model), rather than the existing approaches, such as fracture mechanics, reported in the literature. The E-P approach is modified in this study by use of a successive initiation method, since depending on their location and size; voids may influence either the time to initiate cyclic fatigue damage or time to propagate fatigue damage, or both. Modeling results show competing interactions between void size and location, that results in a non-monotonic relationship between void size and durability. It also suggests that voids in general are not detrimental to reliability except when a large portion of the damage propagation path is covered with either a large void or with many small voids. In addition, this dissertation also addresses several fundamental issues in solder fatigue damage modeling. One objective is to use experimental data to identify the correct fatigue constants to be used when explicitly modeling fatigue damage propagation in Pb-free solders. Explicit modeling of damage propagation improves modeling accuracy across solder joints of vastly different architectures, since the joint geometry may have a strong influence on the ratio of initiation-life to propagation-life. Most solder fatigue models in the literature do not provide this capability since they predict failure based only on the damage accumulation rates during the first few cycles in the undamaged joint. Another objective is to incorporate into cyclic damage propagation models, the effect of material softening caused by cyclic micro-structural damage accumulation in Pb-free solder materials. In other words the model constants of the solder viscoplastic constitutive model are continuously updated with the help of experimental data, to include this cyclic softening effect as damage accumulates during the damage-propagation phase. The ability to model this damage evolution process increases the accuracy of durability predictions, and is not available in most current solder fatigue models reported in the literature. This mechanism-based microstructural damage evolution model, called the Energy Partitioning Damage Evolution (EPDE) model is developed and implemented in Finite Element Analysis of solder joints with the successive initiation technique and the results are provided here. Experimental results are used as guidance to calibrate the Energy Partitioning fatigue model constants, for use in successive initiation modeling with and without damage evolution. FEA results show 15% difference between the life predicted by averaging technique and successive initiation. This difference could significantly increase in the case of long joints such as thermal pads or die-attach, hence validating the use of successive initiation in these cases. The difference between using successive initiation with and without damage evolution is about 10%. Considering the small amount of effort that has to be made to update the constitutive properties for progressive degradation, it is recommended that softening be included whenever damage propagation needs to be explicitly modeled. However the damage evolution exponents and the corresponding E-P model constants obtained in this study, using successive initiation with damage evolution, are partially dependent on the specimen geometry. Hence, these constants may have to be re-calibrated for other geometries.Item AN EMBEDDED BOUNDARY FORMULATION FOR LARGE-EDDY SIMULATION OF TURBULENT FLOWS INTERACTING WITH MOVING BOUNDARIES(2005-11-01) Yang, Jianming; Balaras, Elias; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A non-boundary-conforming formulation for simulating transitional and turbulent flows with complex geometries and dynamically moving boundaries on fixed orthogonal grids is developed. The underlying finite-difference solver for the filtered incompressible Navier-Stokes equations in both Cartesian and cylindrical coordinates is based on a second-order fractional step method on staggered grid. To satisfy the boundary conditions on an arbitrary immersed interface, the velocity field at the grid points near the interface is reconstructed locally without smearing the sharp interface. The complications caused by the Eulerian grid points emerging from a moving solid body into the fluid phase are treated with a novel ``field-extension'' strategy. To treat the two-way interactions between the fluid and structure, a strong coupling scheme based on Hamming's fourth-order predictor-corrector method has been developed. The fluid and the structure are treated as elements of a single dynamical system, and all of the governing equations are integrated simultaneously, and iteratively in the time-domain. A variety of two and three-dimensional fluid-structure interaction problems of increasing complexity have been considered to demonstrate the accuracy and the range of applicability of the method. In particular, forced vibrations of a rigid circular cylinder including the harmonic in-line vibrations in a quiescent fluid and the transverse vibrations in a free-stream, and the vortex-induced vibrations of an elastic cylinder with one and two degrees of freedom in a free-stream are presented and compared with reference simulations and experiments. Three-dimensional DNS and LES of fluid flows involving stationary complex geometries include the flow past a sphere at $Re=50 \sim 1,000$, the transitional flow past an airfoil with a $10^\circ$ attack angle at $Re=10,000$. Then, the turbulent flow over a traveling wavy wall at $Re=10,170$ are simulated are compared with the detailed DNS using body-fitted grid in the literature. Finally, the simulation of the transitional flow past a prosthetic mechanical heart valve with moving leaflets at $Re=4,000$ has been performed. All results are in good agreement with the available reference data.Item A GRID-FREE LAGRANGIAN DILATATION ELEMENT METHOD WITH APPLICATION TO COMPRESSIBLE FLOW(2004-11-19) Shen, Jun; Bernard, Peter S.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In the computational fluid dynamics research, grid-free methods are getting more attention as an alternative to traditional grid-based methods due to two important reasons. First, grid-free methods can be very easily adapted into applications involving complicated geometries. Secondly, they are less vulnerable to numerical diffusion introduced by spatial discretization than in grid-based schemes. A new grid-free Lagrangian dilatation element method for compressible flow has been developed in this research as an extension of incompressible vortex methods. It differs from grid-based numerical methods in a number of ways. The discretization is represented by a group of Lagrangian particles that are convected with the fluid flow velocities instead of a fixed spatial grid system. The velocity of the flow field, necessary in each time step to move the computational elements, is recovered from the dilatation distribution similar to the 'Biot-Savart' law used in incompressible vortex methods. The Fast Multi-pole Method (FMM) is used to speed up the process and reduce the cost from $O(N^2)$ down to $O(N\log N)$. Each computational particle carries physical properties such as dilatation, temperature, density and geometric volume. These properties are governed by the Lagrangian governing equations derived from the Navier-Stokes equations. While the computational elements are convected in the flow, their properties are updated by integrating their corresponding governing equations. The spatial derivatives appearing in the Lagrangian governing equations are evaluated by using moving least-square fitting. The implementation of several different boundary conditions has been developed in this research. The non-penetration wall boundary condition is implemented by adding a potential velocity field to that recovered from the dilatation elements so as to cancel the normal component at the wall. The zero-gradient of properties at the wall such as temperature and density is enforced by a technique called particle reflection. The inflow and outflow conditions are implemented with the help of the characteristic waves moving up and down-stream. The addition and removal of Lagrangian computational elements at the inlet and outlet are implemented to ensure that the computational domain is fully covered by an approximately uniform distribution of particles with roughly comparable volumes. The new grid-free dilatation method is applied to the compressible oscillating waves in an enclosed tube and a subsonic nozzle flow. Both one-dimensional and two-dimensional results are shown and compared with either the exact solutions or the solutions given by other proven numerical schemes. Good agreement of these results helps to establish the correctness of the present method. Future work will accommodate viscous terms and shock waves, which is given a brief discussion at the end of this thesis.