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.
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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 Experimental Study of Segmented Constrained Layer Damping in Rectangular and Sinusoidal Beams(2020) Ude, Chinonso Oscar; Wereley, Norman M; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In the aerospace engineering field, structures are constantly subjected to vibrations that are detrimental to the effectiveness and lifespan of the technology in use. In this work the performance of segmented constrained layer damping (SCLD) treatments for reducing vibration amplitudes is experimentally evaluated. In addition, two methods of manufacture and application are presented that employ 3D printed approaches. SCLD performance is evaluated by observing the bending response of cantilevered beams and the axial response of straight and sinuous springs. Measurements show that precise sample construction using a multi-jet modeling 3D printing approach and segment spacing based on a genetic optimization algorithm, leads to SCLD treatments that are effective for reducing vibration in cantilevered beams. Results also show that curved structures can also exploit SCLD treatments to enhance damping in axial springs, but that different algorithms for optimum segment size and spacing would be needed to create treatments that are tailored to the more complex spring structures.Item Stress Response of Tall and Heavy Electronic Components Subjected to Multi-axial Vibration(2017) Sridharan, Raman; Dasgupta, Abhijit; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Electronic assemblies often experience multiaxial vibration environments in use and tall, heavy components are more vulnerable when exposed to multiaxial vibration than are shorter, lighter assemblies. The added vulnerability comes from higher stresses that are a result of nonlinear dynamic amplification which large components are susceptible to under simultaneous multiaxial excitation, termed multi degree of freedom (MDoF) excitation. However, it is still common practice to conduct vibration durability testing on electronic assemblies one axis at a time – in what is termed sequential single degree of freedom (SSDoF) testing. SSDoF testing has been shown to produce lower fatigue damage accumulation rates than simultaneous MDoF testing, in the leads of tall and heavy electronic components. This leads to overestimating the expected lifespan of the assembly. This paper investigates the geometric nonlinearities and the resulting cross-axis interactions that tall and heavy electronic components experience when subjected to vibration excitation along two orthogonal axes – one direction is in the plane of the PWB and the other is along the normal to the PWB. The direction normal to the PWB aligns with the axial direction of the leads, while the in-plane direction aligns with the primary bending direction of the leads. Harmonic excitation was simultaneously applied to both axes to study the vibration response as a function of frequency ratio and phase “difference” along the two axes. The experimental observations were verified with a nonlinear dynamic Finite Element study. The effect of geometric nonlinearity on cyclic stresses seen in the vibrating component are analyzed.Item Response and Durability of Large Radius of Gyration Structures Subjected to Biaxial Vibration(2013) Ernst, Matthew Ross; Dasgupta, Abhijit; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Multiaxial vibration tests were conducted using an electrodynamic shaker capable of controlled vibration in six degrees of freedom. The test specimen consisted of six large inductors insertion mounted on a printed wiring board. Average damage accumulation rate was measured for random excitation in-plane, out-of-plane, and both directions simultaneously. Under simultaneous biaxial excitation, the damage rate was found to be 2.2 times larger than the sum of the in-plane and out-of-plane rates. The conclusion was that multiple-step single-degree-of-freedom testing can significantly overestimate the durability of some structures in a multiaxial environment. To examine the mechanics behind this phenomenon, the response of a simple rod structure was analyzed with the finite element method. Axial vibrations, which produce negligible stress on their own, were found to contribute significant additional stress when combined with transverse vibration. This additional stress contribution was found to be highly dependent on the frequency ratio and phase relationship between the two participating axes.Item ACTIVE VIBRATION ATTENUATING SEAT SUSPENSION FOR AN ARMORED HELICOPTER CREW SEAT(2013) Sztein, Pablo Javier; Wereley, Norman M; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)An Active Vibration Attenuating Seat Suspension (AVASS) for an MH-60S helicopter crew seat is designed to protect the occupants from harmful whole-body vibration (WBV). Magnetorheological (MR) suspension units are designed, fabricated and installed in a helicopter crew seat. These MR isolators are built to work in series with existing Variable Load Energy Absorbers (VLEAs), have minimal increase in weight, and maintain crashworthiness for the seat system. Refinements are discussed, based on testing, to minimize friction observed in the system. These refinements include the addition of roller bearings to replace friction bearings in the existing seat. Additionally, semi-active control of the MR dampers is achieved using special purpose built custom electronics integrated into the seat system. Experimental testing shows that an MH-60S retrofitted with AVASS provides up to 70.65% more vibration attenuation than the existing seat configuration as well as up to 81.1% reduction in vibration from the floor.Item PROGNOSTICS OF SOLDER JOINT RELIABILITY UNDER VIBRATION LOADING USING PHYSICS OF FAILURE APPROACH(2009) Gu, Jie; Pecht, Michael G; Barker, Donald; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Physics-of-failure (PoF) is an approach that utilizes knowledge of a product's life cycle loading and failure mechanisms to perform reliability modeling, design, and assessment. Prognostics is the process of predicting the future reliability of a system by assessing the extent of deviation or degradation of a product from its expected normal operating states. When prognostics is combined with physics-of-failure models, it is possible to make continuously updated reliability predictions based on the monitoring of the actual environmental and operational conditions of each individual product. A literature review showed that the research on prognostics of solder joint reliability under vibration loading is very limited. However, personal portable electronic products are no longer used exclusively in a benign office environment. For example, any electronic component (throttles, brakes, or steering) in an automobile should be able to survive in a vibration environment. In this thesis, a methodology was developed for monitoring, recording, and analyzing the life-cycle vibration loads for remaining-life prognostics of solder joints. The responses of printed circuit boards (PCB) to vibration loading were monitored using strain gauges and accelerometers, and they were further transferred to solder strain and stress for damage assessment using a failure fatigue model. Damage estimates were accumulated using Miner's rule after every mission and then used to predict the life consumed and the remaining life. The results were verified by experimentally measuring component lives through real-time daisy-chain resistance measurements. This thesis also presents an uncertainty assessment method for remaining life prognostics of solder joints under vibration loading. Basic steps include uncertainty source categorization, sensitivity analysis, uncertainty propagation, and remaining life probability calculation. Five types of uncertainties were categorized, including measurement uncertainty, parameter uncertainty, model uncertainty, failure criteria uncertainty, and future usage uncertainty. Sensitivity analysis was then used to identify the dominant input variables that influence model output. After that, a Monte Carlo simulation was used for uncertainty propagation and to provide a distribution of accumulated damage. From the accumulated damage distributions, the remaining life was then able to be predicted with confidence intervals. The results showed that the experimentally measured failure time was within the bounds of the uncertainty analysis prediction.