Type of Document Dissertation Author Jaber, Naim A Author's Email Address njaber@vt.edu URN etd-04262001-170912 Title Finite Element Analysis of Thermoviscoplastic Deformations of an Impact-Loaded Prenotched Plate Degree PhD Department Engineering Science and Mechanics Advisory Committee
Advisor Name Title Romesh C. Batra Committee Chair Ali H. Nayfeh Committee Member Martin Klaus Committee Member Michael W. Hyer Committee Member Saad A. Ragab Committee Member Keywords
- Impact Loading
- Failure Mode Transition
- Finite Element Analysis
- Thermoviscoplastic Deformations
Date of Defense 2000-12-13 Availability unrestricted Abstract Four different thermoviscoplastic relations, namely, the Litonski-Batra, the
Johnson-Cook, the Bodner-Partom and the power law are used to model the
thermoviscoplastic response of a material. Each one of these relations
accounts for strain hardening, strain-rate hardening and thermal softening of
the material. The material parameters in these relations are found by solving
an initial-boundary-value problem corresponding to simple shearing
deformations so that the computed effective stress vs. the effective plastic
strain curves match closely with the experimental data of Marchand and Duffy
who tested thin-walled HY-100 steel tubes in torsion.
These four viscoplastic relations are used to analyze dynamic thermomechanical
deformations of a prenotched plate impacted on the notched side by a
cylindrical projectile made of the same material as the plate. The impact
loading on the contact surface is simulated by prescribing the time history of
the normal component of velocity and null tangential tractions. A plane strain
state of deformation is assumed to prevail in the plate and its deformations
are studied for different values of the impact speed. The in-house developed
finite element code employs constant strain triangular elements, one point
integration rule, and a lumped mass matrix. The Lagrangian description of
motion is used to describe deformations of the plate. The coupled nonlinear
partial differential equations are first reduced to coupled nonlinear ordinary
differential equations (ODEs) by using the Galerkin approximation. The ODEs
are integrated by using the stiff solver, LSODE, which adaptively adjusts the
time step size and computes the solution within the prescribed accuracy.
Results computed with the four constitutive relations are found to be
qualitatively similar to each other and the general trends agree with the
experimental observations in the sense that at low speed of impact, a brittle
failure ensues at a point on the upper surface of the notch tip. However, at
high impact speeds, a ductile failure in the form of a shear band initiates
first from a point on the lower surface of the notch tip. The predicted speed
at which the failure mode transitions from brittle to ductile is different for
the four viscoplastic relations.
Results have been computed using the Bodner-Partom law to study the effects of
the notch tip radius and the presence of a circular hole ahead of the
notch-tip. For sharp elliptic notch tips, it is found that there is no failure
transition speed and the ductile failure always preceeded the brittle failure
for the range of the impact speeds studied. For the hole located on the axis
of the circular notch tip, the brittle failure always preceeded the ductile
failure and it initiated at a point on the lower surface of the circular hole.
Files
Filename Size Approximate Download Time (Hours:Minutes:Seconds)
28.8 Modem 56K Modem ISDN (64 Kb) ISDN (128 Kb) Higher-speed Access naim_jaber.pdf 4.91 Mb 00:22:44 00:11:41 00:10:14 00:05:07 00:00:26
|
|
If you have questions or technical problems, please Contact DLA.
