Theses and Dissertations from UMD
Permanent URI for this communityhttp://hdl.handle.net/1903/2
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 give thesis/dissertation in DRUM
More information is available at Theses and Dissertations at University of Maryland Libraries.
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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 Fatigue Damage Accumulation Due to Complex Random Vibration Environments: Application to Single-Axis and Multi-Axis Vibration(2011) Paulus, Mark E.; Dasgupta, Abhijit; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A combination of experiments and modeling are used to address the vibration durability of structures subjected to different random vibration environments. Presented in this work are a set of experimental data comparing the rate of change of the first natural frequency and the measured time to failure, of simple structural members under repetitive shock (RS) vibration, single-axis electrodynamic (ED) vibration and multi-axis ED vibration. It was found that multi-axis testing is more severe than single-axis testing at the same level. In addition the RS system low frequency amplitude is often too weak to efficiently propagate the crack. Smoothing of the input power spectral density (PSD) or poor line resolution was also shown to change the time to failure of a test. A poor correlation was shown between the PSD and the rate of natural frequency change (RFC) over a wide frequency shift. The change in natural frequency caused the initial PSD to be ineffective in determining the total time to failure. A predictive, analytic methodology to quantify the RFC was developed to predict the fatigue life of a structure experiencing random vibration excitation. This method allows the estimation of fatigue life using the frequency domain, where only the input power spectral density, damping factor and structural information are required. The methodology uses linear elastic fracture mechanics for fatigue crack propagation and accounts for the frequency shifting that occurs due to fatigue crack evolution. The analytic model has been shown to compare favorably to both finite element analysis (FEA) and experimental results, for mild-steel cantilever beams. Monte Carlo simulations have been conducted to assess the sensitivity of the model predictions to uncertainties in the input parameters. In addition a semi-empirical model was developed whereby the input PSD and damping factor are measured from life tests, and the resulting time to failure and the acceleration factors between different vibration environments can be determined. The improved modeling methodology developed by this work are of value not only to structural designers who wish to design for dynamic environments, but also to test engineers who wish to qualify products through accelerated life testing, and to vibration engineers who wish to compare the relative severity of different random vibration environments, in terms of their potential to cause fatigue damage accumulation.