Effect of stress redistribution during thermal actuation of shape memory alloys in notched cylindrical bars

2018 ◽  
Vol 29 (10) ◽  
pp. 2149-2163 ◽  
Author(s):  
Francis R Phillips ◽  
Dimitris C Lagoudas
Keyword(s):  
Phase Transformation ◽  
Shape Memory Alloy ◽  
Shape Memory Alloys ◽  
Shape Memory ◽  
Martensitic Phase ◽  
Reverse Transformation ◽  
Cylindrical Shape ◽  
Austenitic Phase ◽  
Thermal Actuation ◽  
Forward Transformation

Shape memory alloys present a unique ability to undergo a solid-to-solid, diffusionless, reversible phase transformation. The forward phase transformation is commonly associated with transforming from the austenitic phase to the martensitic phase, while the reverse transformation is defined by going from the martensitic phase to the austenitic phase. In thermal actuation loading paths, forward transformation is generally associated with cooling, while reverse transformation is commonly associated with heating. In this article, however, it is shown that reverse transformation may occur during cooling of notched cylindrical shape memory alloy bars. The reversal in phase transformation is associated with the redistribution of stress in the shape memory alloy due to the phase transformation.

Suppression of Martensitic Phase Transformation, Shape Memory and High Damping by Ion Implantation in Ni-Ti Thin Films

2006 ◽  
Vol 319 ◽  
pp. 17-24
Author(s):  
Rolf Gotthardt
Keyword(s):  
Phase Transformation ◽  
Shape Memory Alloys ◽  
Shape Memory ◽  
Memory Effect ◽  
Ion Irradiation ◽  
Amorphous State ◽  
Amorphous Layer ◽  
Martensitic Phase ◽  
Martensitic Phase Transformation ◽  
Damping Capacity

The shape memory effect and the high damping in shape memory alloys are based on the martensitic phase transformation, which takes place essentially without diffusion and any change of order have an influence on its side effects: the memory effect, the superelasticity and the high damping capacity of the martensitic phase. A new method to control the performance of shape memory alloys is presented, which is based on selective modification of specified parts of working components. In this research, ion irradiation has been used to introduce locally disorder into a crystal or even amorphise it. A pre-deformed Ni-Ti, 6μm thin film in its martensitic state has been irradiated with Ni-ions of energy of 5 MeV up to a dose of 1016 ions/cm2. By this treatment, a 2μm thin surface layer has been finally transformed into an amorphous state, in which the martensitic transformation is suppressed. During heating the underlying non-modified layer is contracting and an out-of-plane movement is observed. The amorphous layer is elastically deformed and its energy is used during cooling to bring the film in its original shape. In this way, a reversible movement of the film is created. This new technique not only allows us to design new types of micro-actuators, but also to influence locally the high damping, which can be of great importance for micro-engineering applications.

Spherical Indentation of Superelastic Shape Memory Alloys

2007 ◽  
Vol 334-335 ◽  
pp. 601-604
Author(s):  
Wen Yi Yan ◽  
Qing Ping Sun
Keyword(s):  
Shape Memory Alloys ◽  
Shape Memory ◽  
Bifurcation Point ◽  
Reverse Transformation ◽  
Spherical Indentation ◽  
Indentation Hardness ◽  
Characteristic Points ◽  
Forward Transformation ◽  
Transformation Stress ◽  
Return Point

Spherical indentation of superelastic shape memory alloys (SMAs) has been theoretically analyzed. Two characteristic points on the superelastic indentation curve have been discovered. The bifurcation force corresponding to the bifurcation point relies on the forward transformation stress and the return force corresponding to the return point relies on the reverse transformation stress. Based on these theoretical relationships, an approach to determine the transformation stresses of superelastic SMAs has been proposed. To improve the accuracy of the measurement, a slope method to locate the two characteristic points from the slope curves is further suggested. Additionally, the spherical indentation hardness was also analyzed.

Constitutive Model of Shape Memory Alloys: One-Dimensional Phase Transformation Model

2008 ◽  
Vol 56 ◽  
pp. 84-91
Author(s):  
Tadashige Ikeda
Keyword(s):  
Phase Transformation ◽  
Constitutive Model ◽  
Shape Memory Alloys ◽  
Strain Rate ◽  
Shape Memory ◽  
Strain Rate Effect ◽  
Rate Effect ◽  
Reverse Transformation ◽  
Stress Strain ◽  
One Dimensional

A simple yet accurate macroscopic constitutive model of shape memory alloys has been developed. The features of this model are (1) energy-based phase transformation criterion, (2) one-dimensional phase transformation rule based on a micromechanical viewpoint, (3) dissipated energy with a form of a sum of two exponential functions, (4) duplication of the strain rate effect, and (5) adaptability to multi-phase transformation. This model is further improved to be able to express stress-strain relationships such that the reverse transformation starts at a higher stress than the martensitic transformation starts. Here, the ideal reversible transformation temperature is empirically described by a function of the martensite volume fraction. In this paper, an outline of our model is given, where the improvement is introduced. Then, it is shown that the model can quantitatively duplicate the major and minor hysteresis loops, strain rate effect, and asymmetry in tension and compression on the stress-strain relationship. And that it can also duplicate the stress-strain relationships having the reverse transformation start stress higher than the forward one.

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