Edge Cracking in Rolling of an Aluminum Alloy Aa2024-O
Kweon, Soondo
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https://hdl.handle.net/2142/72228
Description
Title
Edge Cracking in Rolling of an Aluminum Alloy Aa2024-O
Author(s)
Kweon, Soondo
Issue Date
2009
Doctoral Committee Chair(s)
Beaudoin, Armand J.
Department of Study
Mechancial Engineering
Discipline
Mechanical Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Mechanical
Engineering, Materials Science
Abstract
Edge cracking in the rolling industry has been an endemic problem since rolling was invented as a production method. In the present work, an experimental and computational study is performed to better understand the physics of edge cracking for an aluminum alloy AA2024-O.
A first step in addressing the problem of edge cracking lies in characterizing the kinetics (strain-rate temperature sensitivity) inherent in the constitutive response of the material. A series of experiments covering a wide range of temperature and strain rate were performed and a novel experimental technique, a temperature gradient test, was devised. Two separate regimes of constitutive behavior were noted and cast into corresponding sets of constitutive equations based on the mechanical threshold strength model (MTS), which reproduced well localized behavior of AA2024-O in finite element simulations of the temperature gradient test performed through use of an ABAQUS UMAT.
The stress state at the edge of a rolled slab does not have a high triaxiality value due to the presence of a free surface. Ductile fracture with low triaxiality has not been explained well by damage theories in the past. To evaluate the effect of shear stress as well as hydrostatic stress on the development of damage, specimens of various geometries were designed and tested in order to cover a wide range of triaxiality and shear stress. An emerging experimental technique, Digital Image Correlation (DIC), was utilized to accurately measure the fracture strain of 2-D specimens. Experiments were paired with simulations utilizing J2 plasticity theory to provide a cross-check of the experimental plan, as well as to draw association of the fracture strain with combined hydrostatic and shear stresses. Resulting trends of fracture strain with triaxiality display accelerated damage with shear at low triaxiality. Crystal plasticity simulations were carried out using boundary conditions drawn from the measured displacement field from the DIC test. These simulations reveal the underlying mechanism of the shear damage process: development of tensile hydrostatic stress in grains due to grain-to-grain interaction. These observations serve as the basis for formulation of a crystal-plasticity-based damage model, developed with attention to the physical basis for evolution of damage in a "bulk" shear deformation, and without resort to ad hoc measures of shear deformation.
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