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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. Source


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. Source


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. Source


Banabic D.,Technical University of Cluj Napoca | Barlat F.,Pohang University of Science and Technology | Cazacu O.,The Reef | Kuwabara T.,Tokyo University of Agriculture and Technology
International Journal of Material Forming | Year: 2010

This paper presents synthetically the most recent models for description of the anisotropic plastic behavior. The first section gives an overview of the classical models. Further, the discussion is focused on the anisotropic formulations developed on the basis of the theories of linear transformations and tensor representations, respectively. Those models are applied to different types of materials: body centered, faced centered and hexagonal-close packed metals. A brief review of the experimental methods used for characterizing and modeling the anisotropic plastic behavior of metallic sheets and tubes under biaxial loading is presented together with the models and methods developed for predicting and establishing the limit strains. The capabilities of some commercial programs specially designed for the computation of forming limit curves (FLC) are also analyzed. © 2010 Springer-Verlag France. Source


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.

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