UMD Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/3
New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a given thesis/dissertation in DRUM.
More information is available at Theses and Dissertations at University of Maryland Libraries.
Browse
2 results
Search Results
Item SPECTRAL METHODS FOR MODELING AND ESTIMATING VIBRATION FATIGUE DAMAGE IN ELECTRONIC INTERCONNECTS(2023) Welch, Jacob Adam; Dasgupta, Abhijit; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The purpose of this thesis is to explore the accuracy of fatigue damage estimation in printedwiring assembly (PWA) interconnects, using purely frequency-domain (also known as spectral) information such as the power spectral density (PSD) of the input excitation. The test case used in this study is the estimation of fatigue damage accumulation rate in the critical interconnects of low profile quad flat-pack (LQFP) components on a PWA, under broad-band random vibration excitation. this study examines whether the fatigue predictions made with this frequency-domain approach are consistent with those obtained from a direct time-domain approach. The frequency-domain response modeling is achieved using a two-stage global-local modeling process using a finite element model (in ABAQUS©), where the dominant modal participation factors for the dynamic response is obtained using a dynamic global simplified dynamic finite element model consisting of shell elements to represent the entire PWA. The PSD of the input excitation is applied as a boundary condition and the PSD of the PWA strain response is recorded at the base of critical components. The corresponding PSD for the dynamic strain response at critical interconnects is estimated with strain-transfer functions (STFs) for each dominant mode, obtained from detailed 3D quasi-static nonlinear local models of the component, adjacent PWB, and the interconnects. The global-local STF provides a relationship between the level of equivalent strain in the critical interconnects and the flexural strain at the adjacent surface of the PWB. The STF for each of the dominant vibration modes is obtained by imposing the corresponding mode-shape predicted by the dynamic global model on the PWB, in the quasistatic local model, using multi-point constraint equations. The PSD of the equivalent strain in the critical interconnect is then estimated via linear modal superposition. A deterministic estimate of the cyclic fatigue damage accumulation rate in the critical interconnect is then conducted with the Basquin high cycle fatigue (HCF) model and linear damage superposition approach, by using three different spectral approaches for representing the strain severity with estimated probability density functions (PDFs). The three approaches include: (i) Raleigh method; (ii) Dirlik method and (iii) Range distribution function created with the Rainflow cycle counting method. Methods (ii) and (iii) are derived from a pseudo time-history created with an inverse Fourier transform. These frequency-domain results are compared to corresponding fatigue damage estimates from a multi-modal time-domain analysis method, to assess the consistency of the two approaches.Item Dynamics of Erythrocytes and Microcapsules(2008-04-25) Dodson, Walter; Dimitrakopoulos, Panagiotis; Bioengineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The erythrocytes are the primary carriers of oxygen and carbon dioxide to and from the systemic tissue. The ability of these cells to deform and navigate through the capillary beds is of fundamental importance for proper functioning of the cardiovascular transport system. The erythrocyte is essentially a capsule, and flow-induced erythrocyte deformation involves the interfacial dynamics of a membrane-enclosed fluid volume stressed in a viscous flow. Elastic capsule dynamics is a complicated problem involving the coupling of fluid and membrane forces; it is also found in a variety of scientific and engineering applications. In this work, we investigate the dynamics of elastic capsules and erythrocytes using the Spectral Boundary Element (SBE) method, a high-order / high-accuracy method for capsule and cellular dynamics. For strain-hardening Skalak elastic capsules in an extensional flow, our investigations demonstrate a shape transition in accordance with experimental observations to a cusped conformation at high flow rates, which allows the capsule to withstand the increased hydrodynamic forces. Our computational methodology reveals a region of bifurcation, in which both spindled and cusped steady-state geometries coexist for a single flow rate. The method is also used to investigate the dynamics of strain-softening Neohookean capsules in the same flow pattern. The strain-softening capsules become highly extended at weaker flow rates than strain-hardening capsules, and do not form steady-state cusped shapes. The SBE method has been extended to model the erythrocyte by using a biconcave disc reference geometry and adaptive prestress to enforce area incompressibility. The method accurately reproduces experimental data from erythrocyte ektacytometry, but allows examination of the erythrocyte dynamics beyond the geometric constraints inherent in ektacytometry and other experimental techniques, including observation of the three-dimensional oscillatory behavior over a range of capillary numbers and viscosity ratios. Our results support a prediction by Fischer, Skalak, and coworkers that the erythrocyte shear modulus decreases at small shear deformations. Our work also suggests that cellular deformation is largely independent of the flow pattern, consistent with the findings of experimental investigators.