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A new analytic plastic potential is developed using a rigorous limit analysis approach. Conditions of homogeneous boundary strain rate are imposed on every cylinder concentric with the cavity. It is shown that, due to the tension-compression asymmetry of the incompressible matrix, the third invariant of the stress deviator has a strong influence on the yielding of the porous solid. New and intriguing results are obtained; namely, for axisymmetric loadings and plane strain conditions, the stress state at yielding is not hydrostatic. In the case when the matrix has the same yield in tension as in compression, the new criterion reduces to Gursons criterion for cylindrical voids. © 2013 Copyright Taylor and Francis Group, LLC.


Lebensohn R.A.,Los Alamos National Laboratory | Cazacu O.,The Reef
International Journal of Solids and Structures | Year: 2012

In this paper, the combined effects of texture and asymmetric single-crystal plastic deformation mechanisms on the dilatational response of voided polycrystals are assessed for the first time. To this end, a full-field dilatational viscoplastic Fast Fourier Transform (FFT)-based approach is used to generate gauge surfaces for porous polycrystals deforming by twinning at single crystal level, which are compared to yield surfaces obtained according to a recent analytical criterion for porous materials. Both approaches are cross-validated, revealing unusual features of the dilatational response, namely, a lack of symmetry of the surfaces with respect to both the hydrostatic and deviatoric axes. This strong sensitivity to the third invariant of the stress deviator is associated to the anisotropy and the tension-compression asymmetry of the plastic response of the matrix. © 2012 Elsevier Ltd. All rights reserved.


Stewart J.B.,Air Force Research Lab | Cazacu O.,The Reef
International Journal of Solids and Structures | Year: 2011

A significant difference between the behavior in tension versus compression is obtained at the polycrystal level if either twinning or non-Schmid effects are contributors to the plastic deformation at the single crystal level. Examples of materials that exhibit tension-compression asymmetry include hexagonal close-packed (HCP) polycrystals and intermetallics (e.g., molybdenum compounds). Despite recent progress in modeling their yield behavior in the absence of voids, the description of coupling between plasticity and damage by void growth in these materials remains a challenge. This paper is devoted to the development of a macroscopic anisotropic yield criterion for a porous material when the matrix material is incompressible, anisotropic and displays tension-compression asymmetry. The analytical yield criterion is obtained based on micromechanical considerations and non-linear homogenization. The matrix plastic behavior is described by the Cazacu et al. (2006) anisotropic yield criterion that is pressure-insensitive and accounts for strength-differential effects. Comparison between finite element cell calculations and theory show the predictive capabilities of the developed anisotropic model in terms of modeling the combined effects of anisotropy, tension-compression asymmetry of the matrix and voids on the overall yielding of the porous aggregate. It is shown that if the matrix material does not display tension-compression asymmetry, the developed criterion reduces to that of Benzerga and Besson (2001). If the matrix is isotropic, it reduces to the isotropic criterion developed in Cazacu and Stewart (2009). © 2010 Elsevier Ltd. All rights reserved.


Grant
Agency: Department of Defense | Branch: Navy | Program: STTR | Phase: Phase II | Award Amount: 749.95K | Year: 2008

The overall goal of this STTR program is to develop an efficient, high-fidelity, CFD-based design tool for the simulation of ship-board flight testing of autonomous UAV systems. In Phase I, an overall flight simulation framework that employs a modular approach to the flight simulation problem was developed. The effort was successful in demonstrating a lower-cost aerodynamic modeling approach that can model a representative UAV rotorcraft recovering to a ship, with accuracy to within 20% of fully-coupled CFD methods and an order of magnitude reduction in simulation time/cost. The proposed Phase II program will address key improvements to the technology developed in Phase I that will permit its use for the simulation of autonomous UAV launch and recovery operations, namely the implementation of detailed rotor and flight controller models into the simulation framework. This development will be carried out in a comprehensive manner with companion experiments at the University of Florida designed to provide experimental data to validate the various components of the simulation software.


News Article | June 13, 2016
Site: www.techtimes.com

The Great Barrier Reef is in danger of losing much of its corals. Mass coral bleaching brought about by climate change damaged as much as 93 percent of the pristine coral reefs, which prodded the Australian government to set aside $1 billion to protect the world heritage site. Prime Minister Malcolm Turnbull announced the creation of a special $1 billion fund, The Reef Fund, which will focus on clean energy projects for the Great Barrier Reef's catchment areas. Turnbull said the budget is part of the $10 billion special account of the Clean Energy Finance Corporation, which will also serve as the fund manager via debt and equity in a period of 10 years. "Climate change is the greatest long-term threat to the Great Barrier Reef and to all coral reefs around the world," said Turnbull. "Australians are passionate about the Great Barrier Reef and the Turnbull government is committed to protecting it for future generations." Aside from preserving the Great Barrier Reef, the government believes that the special fund will also help boost Queensland's economy through job creation, improved farm profitability and mobilization of public and private investments. Turnbull said that improving the quality of water will have a great impact on preserving much of the reef. Clean water will mitigate outbreaks of crown-of-thorns starfish and bleaching. For this reason, the fund will invest on projects that would support clean water such as water and energy-efficient irrigation systems, pesticide sprayers and coastal sewage treatment plants. The fund will also consider investing on solar farms to decrease diesel use and other projects that support renewable energy. Turnbull also mentioned that the fund will add to the existing $461 million fund previously dedicated to the reef preservation. Greg Hunt, the government's environmental minister has acknowledged that the commitment is the largest of its kind that the Great Barrier Reef has received so far. "It's an investment in the legacy for our children, their children and our descendants," said Hunt. Last month, the Labor announced their own rescue plan for the reef by pledging $500 million that will be spread in five years. The Labor is planning on to put much of its money in research, reef management improvement and direct environmental investments. The announcement, however, did not sit well with Labor and Greens. Both are saying that this is a diversionary tactic to distract attention from the coal mines. Greens climate change spokesperson and deputy leader Larissa Waters has qualms about the announcement. "All of this money is taken from the Clean Energy Finance Corporation and the government hasn't specified how much will go to clean energy and how much will go to water quality," said Waters. © 2016 Tech Times, All rights reserved. Do not reproduce without permission.

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