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Ombaba M.M.,University of California at Davis | Logeeswaran V.J.,University of California at Davis | Ionescu A.,DRS Defense Solutions | Saif Islam M.,University of California at Davis
Acta Materialia | Year: 2014

A process of heterogeneously integrating organically modified siliceous aerogel (Ormosil) films onto microstructured substrates is presented. These substrates are architecturally designed to mimic photon detectors for remote sensing applications. Here, ultrasonically homogenized Ormosil sols are drop-cast onto silicon micropyramidal arrays then dried in the ambient to produce highly porous low-density siliceous films with excellent uniformity. The highly facile process yields films endowed with high optical transmittance, high static contact angle of 168°, excellent thermal stability up to 400 °C and, to some extent, excellent adhesion to the microstructured substrates on which they sit. Additional planarization benefits are easily afforded by controlling the substrate arraignment during the ambient drying process which the sol undergoes. In contrast, only conformal films were obtained when sols were spin coated over similar microstructured substrates. In correlating the resultant macroporous films' structural integrity with the underlying substrate topography, this study established that the weak physical bond between the facets of the microstructures and gel acts as crack nucleation points that induce and exacerbate crack propagation within the film. This phenomenon does not manifest itself when thinner films are prepared even on the same microstructured substrates as well as films of similar thickness on planar substrates. Initial studies establish that the non-homogenized sols can yield macroscopic aerogel monoliths with properties akin to those exhibited by supercritically dried monoliths. It is our belief that this study can enlighten the intricacies and pitfalls encountered when fabricating macroscopically monolithic Ormosil films over topographically structured surfaces. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Source


Aasen R.,BAS Engineering | Hays B.,DRS Defense Solutions
69th International Conference on Mass Properties 2010 | Year: 2010

Modern ship design practices require knowledge of a vessel's mass Moment of Inertia (MOI) for various aspects of performance analysis. To find an accurate MOI value of an object, one needs to know the object's actual shape and density to be able to calculate the MOI through integration. Determining the exact MOI for a complete vessel, comprised of thousands of items, is not practical. Instead, engineers simplify the parts of the vessel to point objects or to standard shapes like a box or a cylinder, and calculate an approximation of the MOI. The accuracy of this approximation is dependent on the number of parts the vessel is divided into and how well the shape, orientation and density of each of the simplified items resembles the real objects. The quantification of the inaccuracy involved is seldom addressed. This paper describes a method to find the absolute error range for this simplified MOI calculation by finding the extreme values the MOI approximation can generate, and quantifies the effect that an error in MOI can have on the results of various types of performance analysis. Source


Ma M.,DRS Defense Solutions | Hughes O.F.,Virginia Polytechnic Institute and State University
Proceedings of the International Conference on Offshore Mechanics and Arctic Engineering - OMAE | Year: 2011

Permanent means of access (PMA) of oil tankers and bulk carries consists of a wide platform for walk-through inspection. Since PMA structures have a tall web plate, they are vulnerable to elastic tripping. A previous paper [1] proposed a Rayleigh-Ritz method to analyze elastic tripping behavior of PMA structures. The method is parametric formulated, mesh free, computational efficient, and is able to predict both the flange plate critical tripping stress as well as the web plate local buckling stress; therefore the solution process is suitable for design space exploration. In this paper, multi-objective optimization methods are used to determine the Pareto solutions of a PMA structure based on the proposed tripping algorithm. The objective is to solve a design problem aimed at simultaneously minimizing the weight of a PMA structure and maximizing its critical buckling stress. Three multi-objective methods, Pareto Simulated Annealing (PSA), Ulungu Multi-objective Simulated Annealing (UMOSA) and Multi-objective Genetic Algorithm (MOGA) are presented for a case study. The numerical results show that all three methods can efficiently and effectively solve such optimization problems within a short search time. The critical buckling stress of the final optimal designs is validated by the linear and non-linear buckling analysis of NX-NASTRAN [2]. Copyright © 2011 by ASME. Source


Ma M.,DRS Defense Solutions | Jang B.-S.,Samsung | Hughes O.F.,Virginia Polytechnic Institute and State University
Proceedings of the International Conference on Offshore Mechanics and Arctic Engineering - OMAE | Year: 2011

An efficient Rayleigh-Ritz approach is presented for analyzing the lateral-torsional buckling ("tripping") behavior of permanent means of access (PMA) structures. Tripping failure is dangerous and often occurs when a stiffener has a tall web plate. For ordinary stiffeners of short web plates, tripping usually occurs after plate local buckling and often happens in plastic range. Since PMA structures have a wide platform for a regular walk-through inspection, they are vulnerable to elastic tripping failure and may take place prior to plate local buckling. Based on an extensive study of finite element linear buckling analysis, a strain distribution is assumed for PMA platforms. The total potential energy functional, with a parametric expression of different supporting members (flat bar, T-stiffener and angle stiffener), is formulated, and the critical tripping stress is obtained using eigenvalue approach. The method offers advantages over commonly used finite element analysis because it is mesh-free and requires only five degrees of freedom; therefore the solution process is rapid and suitable for design space exploration. The numerical results are in agreement with NX NASTRAN [1] linear buckling analysis. Design recommendations are proposed based on extensive parametric studies. Copyright © 2011 by ASME. Source


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