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Toward a damage-based finite element fracture theory and application to ductile metalsWilliams, Thomas Neil 07 August 2010 (has links) (PDF)
In this work, the simulation of monotonic fracture in ductile metals was studied and a method of predicting damage-based fracture propagation was introduced. Traditional methodologies for predicting stable crack growth were investigated, and an error analysis was performed to show the suitability of the fracture simulation method chosen for this study. J2 plasticity was investigated for its applicability in predicting crack advance direction for mode-I and mixed-mode simulations. A two parameter crack advance criterion was introduced, and crack propagation simulations were performed to show the suitability of the new fracture criterion that is dependent on damage. J2 plasticity was modified in an attempt to capture the damage mechanisms occurring in front of the crack tip. The end result of this research is a computational tool that is capable of predicting the crack propagation path based on physical and measurable material parameters without knowledge of the crack trajectory a priori while also allowing the constitutive model for the material response to be readily changed. An error analysis was also performed on the existing equations of crack surface displacements for symmetric cracks emanating from a circular hole in an infinite plate subjected to remote stress and stress applied to a segment of the crack surface. New equations were developed for crack surface displacements for symmetric cracks emanating from the circular hole in an infinite plate subjected to a remote stress.
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A micromechanics based ductile damage model for anisotropic titanium alloysKeralavarma, Shyam Mohan 15 May 2009 (has links)
The hot-workability of Titanium (Ti) alloys is of current interest to the aerospace
industry due to its widespread application in the design of strong and light-weight
aircraft structural components and engine parts. Motivated by the need for accurate
simulation of large scale plastic deformation in metals that exhibit macroscopic plastic
anisotropy, such as Ti, a constitutive model is developed for anisotropic materials
undergoing plastic deformation coupled with ductile damage in the form of internal
cavitation. The model is developed from a rigorous micromechanical basis, following
well-known previous works in the field. The model incorporates the porosity and
void aspect ratio as internal damage variables, and seeks to provide a more accurate
prediction of damage growth compared to previous existing models. A closed form
expression for the macroscopic yield locus is derived using a Hill-Mandel homogenization
and limit analysis of a porous representative volume element. Analytical
expressions are also developed for the evolution of the internal variables, porosity
and void shape. The developed yield criterion is validated by comparison to numerically
determined yield loci for specific anisotropic materials, using a numerical limit
analysis technique developed herein. The evolution laws for the internal variables are
validated by comparison with direct finite element simulations of porous unit cells.
Comparison with previously published results in the literature indicates that the new
model yields better agreement with the numerically determined yield loci for a wide
range of loading paths. Use of the new model in continuum finite element simulations of ductile fracture may be expected to lead to improved predictions for damage
evolution and fracture modes in plastically anisotropic materials.
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