The focus of the present work is to study the effect of stress and temperature on the accumulated residual
strain during the thermomechanical cycling of Shape Memory Alloys (SMAs). NiTi wires were pseudoelastically
trained at different temperature above the austenitic finish temperature, up to different maximum applied stress
levels. The total residual strain recorded during each training experiment was decomposed into the contributing
plastic strain and retained martensite. The quantity of retained martensite in the trained wire was determined
by a flash heating the trained SMA and recording the recovered strain. Preliminary observations from the
thermomechanical test results suggest that the retained martensite formation is dependent on the maximum
applied stress level during the thermomechanical test and is not dependent on the transformation plateau stress
level of the SMA. On the contrary the transformation plateau stress level or consequently the test temperature is
a critical parameter in dictating the irrecoverable plastic strain generated during the thermomechanical cycling
Shape Memory Alloy are considered as one of the best active
materials for actuation due to their remarkable properties, mainly
their large strain and their power to weight ratio. On the other
hand, they also have undesirable features that limit applications.
One reason is their large thermal time constant. Antagonistic
actuators are generally presented as a possible solution to this
problem. Another reason is the non-linear behavior inherent to the
hysteresis of the phase transition used for the actuation. This
paper addresses this problem in the case of antagonistic actuator
using an adaptive control scheme where the identification is
performed using Laguerre filters. Results obtained on experimental
setup prove that this control scheme is able to handle the complex
non-linearities of hysteretic antagonistic actuator.
Ferroelasticity and ferroelectricity are the non linear behaviours exhibited by piezoceramics, especially in the case of high electric field or stress. Many studies have focused on the role of ferroelastic and ferroelectric switching in fracture of actuators. However, engineering reliability analyses are carried out with tools like finite element software that do not take into account these non linear phenomena. To overcome such a problem, a simplified phenomenological constitutive law has been developed and describe the hysteresis effect of piezoceramics. It is time-independent and relies on the introduction of remnant polarization and remnant strain as internal variables. Two loading surfaces, similar to the ones used in plasticity, provide the evolution laws for the internal variables. Besides, polarization-induced anisotropy in the piezoelectric tensor is taken into account. That constitutive law has been implemented in the commercial software ABAQUS. It has been necessary to develop a finite element with electrical and mechanical degrees of freedom: it is an eight node hexahedron. The stiffness matrix integrates the constitutive law from the four tangent operators given by the constitutive law. The non linear problem is solved by the Newtons method. This finite element tool is used to study the effects of applied voltage on the electroelastic field concentrations ahead of electrodes in a multilayer piezoelectric actuator. The study lies on the experimental observations made by Shindo et al. . Electroelastic analysis on piezoceramics with surface electrode showed that high values of stress and electric displacement arose in the neighbourhood of the electrode tip. Thus, the strain, stress and electric displacement concentrations were calculated and the numerical results showed that ferroelectric switching arose in the area of the electrode tip, causing a change in remnant polarization and remnant strain.
This paper examines the switching process occuring in ferroelectric
and ferroelastic single crystals under electro-mechanical loadings.
Ferroelectrics undergoing a cubic to tetragonal phase transition are
considered. The single crystal energy has three origins: elastic, electric and the incompatibilities of the spontaneous strain and electric displacement fieds between domains. The stress and electric fields fluctuate and present jumps at the domain walls. As a consequence, they induce electro-elastic interaction energy. Thus, it involves dissipation that the present work aims to capture through a micromechanical approach.
This paper presents a study on the fatigue life of shape memory alloy actuators undergoing thermally induced martensitic phase transformation under various stress levels. A microstructural study characterizing specific damage patterns is conducted in the current work. A highly stressed state with formation of different types of microcracks has been observed, showing a superficial micro cracking, responsible for the growth of circular cracks localizing the failure of the specimens. The influence of the interactions between the micro cracking pattern and the corrosion occurrence is also studied. Finally, a spallation oxidation occurring at the surface of the actuator, which damages its properties, is also investigated. The failure pattern observed provides information necessary to introduce a correction to the classical Wohler curve for fatigue life of a material undergoing cyclic loading.
Stress-induced martensitic phase transformation is responsible of very important phenomena like superelasticity or two-way shape memory in shape memory alloys. These phenomena are at the origin of many innovative products in industrial fields like aerospace or biomedical applications. To reach the best design is a very difficult task for applications using shape memory alloys: due to the existence of a phase transformation, these materials can no longer be considered as homogeneous and macroscopic approaches failed to give an accurate description of their behavior. The recent trend using SMA thin film as microactuator in microdevice increase the need of reliable design tools. Moderns concepts developed in micromechanics and finite element analysis are well adapted to deal with these problems. Intra and intergranular stresses building from transformation strain incompatibilities in bulk materials or thin films are well accounted using these tools, even when complex loading conditions or different initial crystallographic texture are considered.