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Lead Investigators
Paul Dawson
prd5@cornell.edu
Matt Miller
mpm4@cornell.edu
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Principal Investigator: Paul Dawson
Sponsor: Office of Naval Research, Structural Materials Program.
Julie Christodoulou, Program Manager
Under this grant, we receive funding for research directed at the development and use of computer simulation tools that contribute to the knowledge and expertise needed to improve naval structural alloys and their processing methods. The research has three thrusts. The topics are: (1) to develop accurate computational methods for intergranular stresses in polycrystalline solids, (2) to build multiscale models for thermomechanical processing, and (3) to expand the polycrystal computational modules library. The goal of the first thrust is to link regions of elevated stress to attributes of the crystallographic neighborhood, including grain boundary orientation and lattice misorientations both within and between grains, to identify defect initiation sites. The goal of the second thrust is to connect thermomechanical histories induced by processing operation (such as friction stir welding) to the resulting microstructures via simulations that model both the continuum (macroscale) deformation patterns and the polycrystalline (microscale) responses. Finally, the goal of the third thrust is to put into the hands of other researchers the computational tools that have been developed by our group under ONR funding over the past decade.
|
Slides for an overview talk on the polycrystal library
can be seen in the file
Tools for Polycrystal Plasticity
(2.36Mb pdf). |
| A presentation given at the TMS spring 2003 meeting that summarizes the results of modeling in situ diffraction tests on AL6XN is given in Diffraction Modeling (4.72Mb pdf). |
|
Other work on this project is described below.
Brian Sidle
Micromechanical Characterization and Simulation of Friction Stir Welded AL-6XN Stainless Steel. Properties of friction stir welded AL-6XN stainless steel are being investigated with the intention of initializing a FEM model of the material system. Initial crystallographic texture in and away from the weld zone is established using EBSD techniques. Gradients in microhardness are measured prior to and after uniaxial deformation of the welded material. The effect of loading on the microstructure and properties of the different material regions is investigate using neutron diffraction techniques. The data from these investigations is then used to initialize and validate a polycrystal plasticity model of the tensile specimens.
Micromechanical Characterization and Simulation of Friction Stir Welded AL-6XN Stainless Steel. Properties of friction stir welded AL-6XN stainless steel are being investigated with the intention of initializing a FEM model of the material system. Initial crystallographic texture in and away from the weld zone is established using EBSD techniques. Gradients in microhardness are measured prior to and after uniaxial deformation of the welded material. The effect of loading on the microstructure and properties of the different material regions is investigate using neutron diffraction techniques. The data from these investigations is then used to initialize and validate a polycrystal plasticity model of the tensile specimens.
Multi-scale material properties of polycrystals
(no file)
Hadas Ritz
The project goal is to find material properties that can be used in finite element simulations modeling mechanical deformation on multiple scales. In-situ neutron diffraction data has been collected on aluminum-magnesium alloys subjected to uniaxial tension; thus we have a set of experimental data with which to compare the results of finite element simulations. The basis of comparison will be overall macroscopic stress-strain response, anisotropic elastic response (varying lattice strains in crystals with different initial orientation), and amount of texture evolution with strain. Through varying simulation parameters such as element type, grain shape, and number of crytals per element we hope to determine a set of material properties applicable in simulations on multiple scales.
The project goal is to find material properties that can be used in finite element simulations modeling mechanical deformation on multiple scales. In-situ neutron diffraction data has been collected on aluminum-magnesium alloys subjected to uniaxial tension; thus we have a set of experimental data with which to compare the results of finite element simulations. The basis of comparison will be overall macroscopic stress-strain response, anisotropic elastic response (varying lattice strains in crystals with different initial orientation), and amount of texture evolution with strain. Through varying simulation parameters such as element type, grain shape, and number of crytals per element we hope to determine a set of material properties applicable in simulations on multiple scales.
See also our website for Polycrystal Plasticity Simulation Tools.
Development of an Accelerated Methodology for the
Evaluation of Critical Properties of Polyphase Alloys by
Simulation and Experiment (AFOSR-MEANS)
Principal Investigators: Paul Dawson and Matt Miller
Sponsor: Air Force Office of Sponsored Research,
MEANS Theme Program
Dr.
Craig Hartley, Theme Program Manager
Strength and stiffness are
material properties
that are critical to the design of aircraft. Engineers need these
properties
early in the design process and the lack of reliable data can rule out
the
use of newer materials. The traditional
laboratory-based
process
to
obtain property data takes substantially longer than the time to
perform
mechanical design, meaning that with current methods it is not
possible
to obtain the needed data once the mechanical design process has begun.
Our
project is aimed at developing a methodology to substantially reduce
the
time need to obtain data for the strength and stiffness of polyphase,
polycrystal alloys. The
methodology relies on a coordinated approach of simulations and
experiments. At the heart of the methodology is a representation
of the material structure based on its features and attributes of those
features. From the material representation, virtual specimens may be
instantiated and tested. Using digital probes, the response of
the specimens to imposed loading is extracted and used to assess the
properties. Experiments provide critical data both to validate the
simulation tools and to provide essential data needed to simulate
the behavior of the virtual specimens.
| Slides for several talks are given below. |
|
Principal Investigator: Paul Dawson
Sponsor: Riso National Laboratory, Denmark
Program Coordinator: Torben Leffers
It is well established that FCC metals and alloys develop two different textures, the Copper type and the Brass type, when subjected to rolling or a plane strain compression deformation. The Copper type texture arises in metals with high-stacking fault energies like copper and aluminum whereas the brass texture arises in FCC alloys/metals with low-stacking fault energies such as brass and silver. Copper texture development is relatively well understood and has been adequately approximated by Taylor model polycrystal simulations. Understanding of the brass texture is somewhat less developed, as the texture community has yet to come to a consensus as to why it arises. It has been shown that a brass type texture is produced when a large enough volume fraction of twins is introduced into the deformation. However, there is some dispute as to whether or not enough twinning is produced to effect bulk texture measurements relatively early in the deformation, when the brass texture is seen to emerge. With this hypothesis in hand the present work attempts to explain brass texture development, without the use of twinning. The present work seeks to examine brass texture development and hopefully prove (or disprove) certain theories as to how it comes to be. Specifically, a FEM elastoviscoplastic polycrystal model is being used to study brass texture development. Metals with low-stacking fault energies tend to exhibit more single slip than high-stacking fault energy metals, which tend towards multi-slip deformation. The implementation of a latent hardening evolution equation in the FEM codes will generate more single slip deformation in the polycrystal. It is hoped that this will create a brass type texture.
A methodology for designing fatigue resistant materials (NSF)
Principal Investigator: Matt Miller
Sponsor: NSF Civil and Mechanical Systems Division,
Program Manager: Jorn Larsen-Basse
In this grant we propose to
create a combined experimental/ simulation framework for studying
mechanical environments conducive to the initiation and propagation of
fatigue microcracks in polyphase metallic alloys as a means to improve
the material selection and design process associated with
fatigue-limited applications. The general framework, which has been
developed under the AFOSR funding is known as the Digital
Material and
includes tools for creating, loading and probing of virtual specimens
that are statistically identical to their physical counterparts. A key
aspect of the Digital Material is the experimental/simulation interface
and the fundamental role played by characterization experiments. The
proposed research is centered around novel sets of in-situ mechanical
loading / synchrotron x-ray diffraction experiments. In these tests,
lattice strains will be measured during cyclic loading and a
methodology will be developed for attaining strains over a vast expanse
of x-ray scattering directions for many different families of lattice
planes. These data are crucial to the development of the simulation
environment associated with the Digital Material representation of the
alloy, which will lead to a greater understanding of mechanical
environments that produce microcracks. In addition, through a
collaboration with researchers working at the forefront of x-ray
detection technology (Professor Sol Gruner's group at Cornell), a
unique set of highly time resolved in-situ experiments will be
conducted that will have exposures on the order of microseconds. By
taking exposures over several cycles out of several thousand cycles, we
will be able to produce lattice strain "snapshots" during cyclic
loading, which will give us a "real time" link to the evolving
mechanical state on the size scale of a metallic grain - exactly what
we need to begin to understand microcrack initiation.
A mechanical loading / synchrotron x-ray diffraction system for in-situ determination of lattice strains (AFOSR-DURIP)
Principal Investigators: Matt Miller and Paul Dawson
Sponsor: Air Force Office of Sponsored Research DURIP program
Program Manager: Craig Hartley
The goal of this project is to
fabricate a mechanical loading / x-ray diffraction system for
employment within a synchrotron x-ray facility such as the Cornell High
Energy Synchrotron Source (CHESS). Using the proposed experimental
system, lattice strains on various planes over a broad range of
scattering vectors can
be measured simultaneously during sophisticated mechanical loading
experiments. High speed area detectors will be employed to capture
time-resolved lattice strain data. Several sets of feasibility
experiments have been conducted using a simple prototype of the
proposed loading frame within the A2 experimental station at CHESS.
This experimental facility will support ongoing sponsored research
being conducted within the Deformation Processes Laboratory
(AFOSR/MEANS, NSF, see above). The system will also greatly expand the
experimental potential of the existing Cornell AFOSR grant and should
serve as a centerpiece to future DOD sponsored research. 