Shape memory alloys (SMAs) show size effect in their response. The critical stresses, for instance, for the start of martensite and austenite transformations are reported to increase in some SMA wires for diameters below 100 μm. Simulation of such a behavior cannot be achieved using conventional theories that lack an intrinsic length scale in their constitutive modeling. To enable the size effect, a thermodynamically consistent constitutive model is developed, that in addition to conventional internal variables of martensitic volume fraction and transformation strain, contains the spatial gradient of martensitic volume fraction as an internal variable. The developed theory is simplified for 1D cases and analytical solutions for pure bending of SMA beams are presented. The gradient model captures the size effect in the response of the studied SMA structures.
Shape memory alloy (SMA) pipe couplers use the shape memory effect to apply a contact pressure onto the surface of
the pipes to be coupled. In the current research, a SMA pipe coupler is designed, fabricated and tested. The thermally
induced contact pressure depends on several factors such as the dimensions and properties of the coupler-pipe system.
Two alloy systems are considered: commercially-available NiTiNb couplers and in-house developed NiTi couplers. The
coupling pressure is measured using strain gages mounted on the internal surface of an elastic ring. An axisymmetric
finite element model including SMA constitutive equations is also developed, and the finite element results are compared
with the experimental results.
Of the factors that mainly affect the efficiency of the wing during a special flow regime, the shape of its airfoil cross
section is the most significant. Airfoils are generally designed for a specific flight condition and, therefore, are not fully
optimized in all flight conditions. It is very desirable to have an airfoil with the ability to change its shape based on the
current regime. Shape memory alloy (SMA) actuators activate in response to changes in the temperature and can recover
their original configuration after being deformed. This study presents the development of a method to control the shape
of an airfoil using SMA actuators. To predict the thermomechanical behaviors of an SMA thin strip, 3D incremental
formulation of the SMA constitutive model is implemented in FEA software package ABAQUS. The interactions
between the airfoil structure and SMA thin strip actuator are investigated. Also, the aerodynamic performance of a
standard airfoil with a plain flap is compared with an adaptive airfoil.
Superelasticity (SE), shape memory effect (SM), high damping capacity, corrosion resistance, and biocompatibility are
the properties of NiTi that makes the alloy ideal for biomedical devices. In this work, the 1D model developed by
Brinson was modified to capture the shape memory effect, superelasticity and hysteresis behavior, as well as partial
transformation in both positive and negative directions. This model was combined with the Euler beam equation
which, by approximation, considers 1D compression and tension stress-strain relationships in different layers of a 3D
beam assembly cross-section.
A shape memory-superelastic NiTi antagonistic beam assembly was simulated with this model. This wire-tube assembly
is designed to enhance the performance of the pedicle screws in osteoporotic bones. For the purpose of this study, an objective design is pursued aiming at optimizing the dimensions and initial configurations of the SMA wire-tube assembly.
Recently, widespread attention has been directed towards scavenging energy from renewable sources such as wind.
Piezoelectric materials are particularly suitable for capturing energy from motion since mechanical deflection of a
piezoelectric specimen results in an electric displacement. This electricity can be stored in batteries or used to power
portable devices. The present work is on the development of a device that can generate electricity from an oscillating
motion using a piezoelectric Macro Fiber Composite (MFC) bimorph. Previously, bimorph vibration was created by a
rotating or reciprocating part hitting the bimorph tip; whereas in the current work, base reciprocation excites the
piezoelectric bimorph. The device includes a fan blade, which aligns with the direction of the wind and moves a rod in
vertical direction. The microfiber composite beams (MFC) are attached to the upper end of the rod. Reciprocation of the
rod acts as a harmonic excitation for the MFC bimorphs. Vibration of the MFCs produces electricity which is stored in a
capacitor to be used to power electronic systems such as different types of remote sensors. Simulation and experimental
results have been compared. In vibration and wind tunnel experiments, comparable amounts of energy were collected
and accumulated in a capacitor.