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Symposia
SYMPOSIUM 1
DAMAGE EVOLUTION DURING ACTUATION FATIGUE IN SHAPE
MEMORY ALLOYS
Dimitris C. Lagoudas
Associate Vice Chancellor and Senior Associate Dean
for Engineering Research; Deputy Director, Texas
A&M Engineering Experiment Station (TEES);
Distinguished University Professor,
Texas A&M University
Abstract
Shape Memory Alloys (SMAs) are unique materials able to undergo a
thermomechanically induced, reversible phase transformation. The phase
transformation is associated with a crystallographic reorientation from the
austenitic phase to the martensitic phase, which in the particular case of
NiTiHf means a change from the B2 cubic crystal structure to the B19’
monoclinic crystal structure. An additional property which is unique to
SMAs is that, in addition to traditional fatigue due to cyclic loading,
actuation fatigue (or fatigue due to repeated phase transformation) must
now also be considered due to the crystallographic reorientation
mentioned. Multiple studies have previously attempted to quantify the
actuation fatigue lifetime of SMA structures under varying
thermomechanical conditions, and multiple models have been developed
to predict the actuation fatigue lifetime based on various criteria. However
most of the prior studies have focused on the material at the end of the
fatigue lifetime. It is the purpose of this study to analyze the evolution of weighted mix of two potential functions that are calibrated to the tensile
damage in the microstructure of a SMA component throughout the
actuation fatigue lifetime. This is accomplished by subjecting dogbone
specimens to a static load and then thermally cycling the specimens such
that they undergo repetitive phase transformation, leading to actuation
fatigue. These actuation fatigue experiments are stopped at various points
throughout the predicted actuation fatigue lifetime and the damage to the
specimens is determined through a variety of non-destructive evaluation
criteria, including digital image correlation, x-ray micro-computed
tomography, and mechanical load cycling.
Biography
Dimitris C. Lagoudas currently serves as the Associate Vice Chancellor for
Engineering Research for the Texas A&M University System and as the
Deputy Director of Texas A&M Engineering Experiment Station (TEES), a
Texas State Agency under the Texas A&M University System. He is also
the Senior Associate Dean for Research for the College of Engineering
and a Distinguished University Professor at Texas A&M University. He
served as Department Head of Aerospace Engineering, the inaugural
Chair of the Materials Science and Engineering graduate program and also
as an Associate Vice President for Research at Texas A&M University. He
directed two TEES research centers, one on composite materials and the
second one on multifunctional materials and structures.
Lagoudas’ research focuses on the design, characterization and modeling
of multifunctional material systems at nano, micro and macro levels. His
research team is one of the most recognized internationally in the area of
modeling and characterization of shape memory alloys. He has co-
authored more than 500 scientific publications in archival journals and
conference proceedings and one of the widely used books on shape
memory alloys. He received the 2006 ASME Adaptive Structures and
Material Systems Prize and he is the 2011 recipient of the SPIE Smart
Structure and Materials Lifetime Achievement Award. He is a Fellow of
AIAA, ASME, IOP and SES and was named a University Distinguished
Professor at Texas A&M University in 2013.
SYMPOSIUM 2
A NEW CONSTITUTIVE MODELING APPROACH FOR THE
PSEUDOELASTIC BEHAVIOR OF SHAPE MEMORY ALLOYS
Chad Landis
Professor
Aerospace Engineering and
Engineering Mechanics
The University of Texas at Austin
Abstract
We have developed a new constitutive modeling approach that captures
pseudoelastic transformation induced recoverable deformation in SMAs
including the strong asymmetries in the tensile and compressive
responses. The model is based on a J2-type nonlinear kinematic
hardening framework with the back stress represented through a
and compressive stress-strain responses of the material. A novel feature
of the model is that it is implemented with a single transformation surface
that captures forward and reverse transformation as well as reorientation.
This transformation surface is taken to follow the simple and symmetric
von Mises form, and it will be shown that it is the kinematic hardening
framework that allows the model to capture the rich set of asymmetric
behaviors exhibited by SMAs in the pseudoelastic regime. The
constitutive model has been used to simulate numerically the interaction
of these complex material behaviors with structural nonlinear behavior
observed in experiments. Problems analyzed include the buckling and
recovery of NiTi tubes under axial compression, the propagation of a
well-defined deformation front in uniaxial tension, and the reversible
propagation of curvature localization in NiTi tubes under bending. The
numerical simulations reproduce the structural behavior both qualitatively
and quantitatively demonstrating the fidelity of the constitutive modeling
framework developed.
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