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Cleveland, OH, United States

Saleeb A.F.,University of Akron | Dhakal B.,University of Akron | Dilibal S.,University of Akron | Owusu-Danquah J.S.,University of Akron | Padula S.A.,Nasa Glenn Research Center
Mechanics of Materials

The properties of a shape memory alloy (SMA) have been shown to be highly dependent on the chemical composition and thermo-mechanical processing applied to the material. These differences dictate the degree of superelasticity, pseudoplasticity, shape memory effect, and evolution under mechanical/thermal loading cycles, that is observed in the material. Understanding and utilizing these unique phenomena has become essential in many engineering applications. It is, therefore, important to provide two key ingredients in any SMA constitutive model; (i) a sufficiently comprehensive scope in the mathematical formulation to handle different classes of SMA materials; and (ii) a general model parameterization derived from fundamental tests that can be used for a specific SMA as intended for use in a given application. The present work is aimed at a detailed investigation of the interaction aspects between the above items (i) and (ii) in the context of using a recent three-dimensional, multimechanism-based SMA framework to model the experimentally measured responses of four different classes of SMA materials: (a) a commercial superelastic NiTi, (b) a powder metallurgically-processed NiTi-based SMA material, (c) a commercial Ni49.9Ti50.1 actuation material, and (d) a high-temperature Ni50.3Ti29.7Hf20 alloy. To facilitate the parameterization task, the model parameters are classified into two groups, i.e., (1) fixed parameters that are designed to capture the non-linear, hysteretic response under any thermo-mechanical loading condition, and (2) a set of functionally dependent material parameters which account for a number of refinements including asymmetry in tension and compression responses, temperature- and stress-state dependencies, etc. The results of the work showed that the complexity of the characterization is dependent on the SMA feature exploited by the specific application intended, which in turn dictates the amount and type of test data required to accurately predict a given application response. © 2014 Elsevier Ltd. Source

Owusu-Danquah J.S.,University of Akron | Saleeb A.F.,University of Akron | Dhakal B.,University of Akron | Padula S.A.,Nasa Glenn Research Center
Journal of Materials Engineering and Performance

A shape memory alloy (SMA) actuator typically has to operate for a large number of thermomechanical cycles due to its application requirements. Therefore, it is necessary to understand the cyclic behavioral response of the SMA actuation material and the devices into which they are incorporated under extended cycling conditions. The present work is focused on the nature of the cyclic, evolutionary behavior of two widely used SMA actuator material systems: (1) a commercially available Ni49.9Ti50.1, and (2) a developmental high-temperature Ni50.3Ti29.7Hf20 alloy. Using a recently developed general SMA modeling framework that utilizes multiple inelastic mechanisms, differences and similarities between the two classes of materials are studied, accounting for extended number of thermal cycles under a constant applied tensile/compressive force and under constant applied torque loading. From the detailed results of the simulations, there were significant qualitative differences in the evolution of deformation responses for the two different materials. In particular, the Ni49.9Ti50.1 tube showed significant evolution of the deformation response, whereas the Ni50.3Ti29.7Hf20 tube stabilized quickly. Moreover, there were significant differences in the tension-compression-shear asymmetry properties in the two materials. More specifically, the Ni50.3Ti29.7Hf20 tube exhibited much higher asymmetry effects, especially at low stress levels, compared to the Ni49.9Ti50.1. For both SMA tubes, the evolution of the deformation response under thermal cycling typically exhibited regions of initial transients, and subsequent evolution. © 2015, ASM International. Source

Saleeb A.F.,University of Akron | Kumar A.,University of Akron | Padula S.A.,Nasa Glenn Research Center | Dhakal B.,University of Akron
Mechanics of Materials

The main focus in the present work has been on studying the evolutionary responses of Shape Memory Alloy (SMA) materials under isothermal, cyclic loading conditions. To this end, predictions of a recently-developed SMA material model by the authors is used here to carry out the qualitative comparisons to some of the available experimental results in the literature for SMA material responses under different uniaxial and multi-axial conditions of stress-control, both in the pseudoelastic and the pseudoplastic regimes. In the formulation of this model, the significant roles played by the internal state variables, underlying the inelastic mechanisms in the model to regulate the material's evolutionary response under extended cycles, are emphasized. The results presented have led to a number of important conclusions. First, the evolutionary character for pseudoelasticity is markedly different from that occurring in the pseudoplastic regime. Second, the virgin material response under minor loop cycles is dramatically different from its pre-cycled counterpart for which a prior major loop was established. Third, the careful selection of the loading-control variable such as magnitudes of the mean stress and stress amplitude plays a major role in dictating the amount of strain and detailed shapes of the saturated stress-strain loop achieved, as well as the number of cycles required to reach the saturation. Lastly, multi-axial load cycling under conditions of combined tension/compression/shear leads to a faster approach to the saturated states compared to the uniaxial load condition. © 2013 Elsevier Ltd. All rights reserved. Source

Padula II S.A.,Nasa Glenn Research Center | Gaydosh D.,Ohio Aerospace Institute | Saleeb A.,University of Akron | Dhakal B.,University of Akron
Experimental Mechanics

Many of the applications that seek to utilize shape memory alloys for their unique set of properties inevitably must deal, on some level, with the dimensional instability that is inherent to these materials under cyclic thermomechanical loading conditions. As a result, a better understanding of the transient and evolutionary behavior of a shape memory alloy is critical to both the successful design of useful actuation systems and development of accurate material models that can adequately capture the types of dimensional instability that can arise during component design. To this end, a set of experiments were conducted wherein the temperature cycling excursion was held fixed while the applied stress was varied. The results indicated that the extent of strain evolution produced under the initially applied stress has a significant impact on both the amount of transient that is observed as well as the rate of evolution observed under subsequent stress levels. In particular, lowering the applied stress to 50 MPa after cycling under an initial stress of 75 MPa did not stabilize the strain. However, lowering the applied stress to 50 MPa after cycling under an initial stress of 150 MPa produced a nearly saturated strain/temperature response. The thermomechanical observations are discussed in terms of the nature of strain evolution and its connection to the concept of a local/global minimization of the energy of the system, however, the exact mechanisms associated with these strain evolutions were not determined. © 2013 Society for Experimental Mechanics (outside the USA). Source

Padula II S.,Nasa Glenn Research Center | Qiu S.,University of Central Florida | Qiu S.,Intel Corporation | Gaydosh D.,Ohio Aerospace Institute | And 4 more authors.
Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science

Over the past decade, interest in shape-memory-alloy based actuators has increased as the primary benefits of these solid-state devices have become more apparent. However, much is still unknown about the characteristic behavior of these materials when used in actuator applications. Recently, we showed that the maximum temperature reached during thermal cycling under isobaric conditions could significantly affect the observed mechanical response of NiTi (55 wt pct Ni), especially the amount of transformation strain available for actuation and thus work output. The investigation we report here extends that original work to (1) ascertain whether increases in the upper-cycle temperature would produce additional changes in the work output of the material, which has a stress-free austenite finish temperature of 386 K (113 °C), and (2) determine the optimum cyclic conditions. Thus, isobaric, thermal-cycle experiments were conducted on the aforementioned alloy at various stresses from 50 to 300 MPa using upper-cycle temperatures of 438 K, 473 K, 503 K, 533 K, 563 K, 593 K, and 623 K (165 °C, 200 °C, 230 °C, 260 °C, 290 °C, 320 °C, and 350 °C). The data indicated that the amount of applied stress influenced the transformation strain, as would be expected. However, the maximum temperature reached during the thermal excursion also plays an equally significant role in determining the transformation strain, with the maximum transformation strain observed during thermal cycling to 563 K (290 °C). In situ neutron diffraction at stress and temperature showed that the differences in transformation strain were mostly related to changes in martensite texture when cycling to different upper-cycle temperatures. Hence, understanding this effect is important to optimizing the operation of SMA-based actuators and could lead to new methods for processing and training shape-memory alloys for optimal performance. © 2012 The Minerals, Metals & Materials Society and ASM International. Source

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