Anargyros Karakalas
@tamu.edu
Post-Doctoral Researcher at the Aerospace Engineering Department
Texas A&M University
RESEARCH INTERESTS
Shape Memory Alloys Constitutive Modeling
Morphing Structures
Smart Actuators
Finite Element Simulations
Scopus Publications
Scholar Citations
Scholar h-index
Scholar i10-index
Scopus Publications
Matthew C Kuner, Anargyros A Karakalas, and Dimitris C Lagoudas
IOP Publishing
Abstract The authors present the automatic shape memory alloy data analyzer (ASMADA). ASMADA is capable of rapid, robust, and consistent processing of shape memory alloy thermal cycling data acquired under constant stress. This seeks to address two primary issues: the lack of unified analysis procedures in relevant standards and the near-universal manual analysis of such data. ASMADA is compliant with the definitions provided in ASTM standards and calculates up to twenty-three (23) material properties/parameters at speeds ranging from 5 to 35 cycles s−1. These parameters include the four transformation start/finish temperature thresholds, which are calculated using the tangent line method; the transformation region tangent lines are determined using a modified sigmoid function, whereas the single-phase region tangent lines are determined based on the geometry of the cycle data. Additionally, a graphical user interface is provided to make the tool readily accessible and easy to navigate. The capabilities of ASMADA have been tested on experimental data from four different research groups; results from five of these tests are presented to demonstrate the tool’s robustness. This tool was developed in Python and is publicly available at https://github.com/matthewkuner/ASMADA
Giulia Scalet, Anargyros Karakalas, Lei Xu, and Dimitris Lagoudas
Springer Science and Business Media LLC
AbstractThis paper presents a unified modelling effort to describe partial phase transformation during cyclic thermo-mechanical loading in Shape Memory Alloys (SMA). To this purpose, a three-dimensional (3D) finite strain constitutive model considering TRansformation-Induced Plasticity (TRIP) is combined with a modified hardening function to enable the accurate and efficient prediction of partial transformations during cyclic thermo-mechanical loading. The capabilities of the proposed model are demonstrated by predicting the behavior of the material under pseudoelastic and actuation operation using finite element analysis. Numerical results of the modified model are presented and compared with the original model without considering the partial transformation feature as well as with uniaxial actuation experimental data. Various aspects of cyclic material behavior under partial transformation are analyzed and discussed for different SMA systems.
Anargyros A. Karakalas, Theodoros T. Machairas, Dimitris C. Lagoudas, and Dimitris A. Saravanos
Springer Science and Business Media LLC
In this work the time response of pseudoelastic Shape Memory Alloy (SMA) wires is numerically simulated. In particular, the effect of their operation under partial phase transformation is investigated and quantified. Additionally, the effect of the thermomechanical coupling under cyclic operation is evaluated both under adiabatic and natural convection conditions. To this end, proper finite element models are generated considering a low-frequency harmonic sinusoidal excitation. The effect of the partial transformation and thermomechanical coupling on the operation of the SMA is highlighted by comparison with respective results acquired by finite element models which neglect the modified hardening function that accounts for the partial loops. The results suggest that the latent heat produced during forward transformation highly affects the energy dissipation potential of SMAs. The hardening behavior also affects the transformation evolution and therefore impacts the amount of heat generation/absorption. Although both phenomena, when accounted for, result in the prediction of an altered hysteresis area and consequently different dissipation capabilities, the scope of the paper is to highlight their importance on the calculated values of dissipated energy and loss factor. These quantities are of particular interest since they constitute crucial design parameters for the development of smart dampers employing SMA materials.
Christopher Summers, Jonathan M. Weaver-Rosen, Anargyros A. Karakalas, Richard J. Malak, and Dimitris C. Lagoudas
American Society of Mechanical Engineers
Abstract Novel design of more efficient, environmentally friendly, quiet, and cost-effective air transportation could be substantially benefited by introducing highly adaptive, multi-functional systems that are able to mimic the operation of biological systems, like birds. Altering the Outer Mold Line (OML) of an aircraft allows for achieving the optimal response under a wide range of operational conditions. In the framework of the “Adaptive Aerostructures for Revolutionary Civil Supersonic Transportation” project funded by NASA, an articulated panel mechanism controlled by Shape Memory Alloy (SMA) actuators is investigated as a means for reducing the perceived loudness of the sonic boom produced by a commercial aircraft when flying at supersonic speeds. A pair of SMA torque tubes is envisioned to induce the required rotation of the panels in order to achieve the desirable OML shapes. However, design objectives such as minimizing power consumption, mass, and cooling time are often competing and the selection of the optimal dimensions is neither elementary nor straightforward. In the research conducted herein, a case study is defined and realized for the optimal design of the SMA torque tubes as part of a larger morphing structure. In the early stages of design, engineers are often faced with the challenge of making decisions with incomplete information. For example, the designer must know the aerodynamic loads to choose the optimal dimensions, but the aerodynamic loads depend on aircraft dimensions. To enable detailed optimization in the early design stages, parametric optimization can be used to solve for the parameterized Pareto frontier. This parameterized Pareto frontier allows a designer to explore how the traditional Pareto frontier might change as exogenous parameters (the values of which are not yet fully known) change. In this work, the design variables under the control of the engineer are the dimensions of the torque tube, i.e. length, inner diameter, and thickness. The objectives are to minimize cooling time and maximize rigidity. The exogenous parameters outside of the designer’s control include the required actuation stroke and aerodynamic forces. Results show the effects of parameters on the objective tradeoffs and demonstrate how an engineer can choose an optimal solution once the parameter values are known.
Wonjoon Suk, Anargyros A. Karakalas, and Dimitris C. Lagoudas
SPIE
Multifunctional capabilities of Shape Memory Alloys (SMAs) and, more specifically, their inherent characteristic of producing and recovering transformation strain under thermal stimulus, render them ideal for actuator appli- cations. In fact, SMA actuators are widely used in various fields including but not limited to robotics, medical, civil, and aerospace engineering. Moreover, they are also able to be formed in a wide range of shapes that includes, but is not limited to, wires, ribbons, bars, torque tubes and various spring types. This fact combined with their high-energy density, the noise-less, spark-free, and debris-less operation and their compactness renders them ideal for aerospace morphing structures where weight, volume, energy consumption, and other operational specifications have to be strictly met.
In this study, two SMA actuator forms, one linear, i.e., wires of circular cross-section, and one torsional, i.e., torque tubes, are compared in terms of weight/volume, stroke capabilities, developed stresses, cooling requirements, power consumption and overall operation under predefined conditions. The actuators are intended for use in parts of an articulated shape adaptive mechanism envisioned for altering locally the outer mold line of a civil supersonic aircraft. The morphing system is placed on the lower part of the fuselage in order to alter the aerodynamic profile and reduce the sonic boom created during supersonic flight over inhabited areas. The specifications for the design of the actuators are provided and finite element analysis is used to verify the overall response of the SMAs.
Alnto Koualiarella, Apostolos Arvanitidis, Apostolos Argyros, Charoula Kousiatza, Anargyros Karakalas, Dimitris Lagoudas, and Nikolaos Michailidis
Elsevier BV
Abstract Unique thermal shape recovery and chemical stability make Shape Memory Polymers (SMPs) attractive for critical applications in biomedical, aerospace and energy sectors. While additive manufacturing (AM) of SMPs allows fabrication of functionally-graded structures with tailored, intricate design features, the effect of AM on shape recovery characteristics has not received much attention. To demonstrate that shape recovery characteristics can be significantly enhanced through a variation of AM building strategies and process parameters beyond tuning of material compositions alone, an experimental study was developed. In-situ thermo-micro-mechanical testing was applied to capture the shape memory properties, both during shape programming and post that.
Anargyros Karakalas and Dimitris Lagoudas
SPIE
Shape Memory Alloys (SMAs) constitute a class of materials that are distinguished by their highly non-linear, thermo-mechanically coupled behaviour which is related with the phenomena accompanying the diffusion-less, solid-state phase transformation. This transition from the parent phase of Austenite to the product phase of Martensite and vice versa is also bound with the uncommon characteristic of “memory” exhibited when the material undergoes variable thermo-mechanical loadings. When a transformation reversal takes place, the material seems to inherently remember its state and adapts its future response in order to form closed paths, strongly dependent on the induced transformation history. Furthermore, another characteristic trait of SMAs is the asymmetry of their response when under tension or compression. During mixed loading states, such as bending of a beam, the evolution of transformation is observed to be different based on the sign of the load. The aforementioned peculiarities significantly affect the implementation SMAs in the design and realization of smart engineering structures intended for use in a wide range of fields that include but are not limited to aerospace, biomedical, wind energy, civil and automotive. To this end, efficient constitutive modeling of the phenomena related to the phase transformation is essential and of high importance in order to predict the complex performance of these materials. In this paper, emphasis is placed upon the investigation of the combined effect of tension-compression asymmetry and partial transformation on the response of SMA beams subjected to threepoint bending loading conditions. In this context, modeling of tension-compression asymmetry is investigated by using a set of different phase transformation functions based on the principles of computational plasticity, while a modified hardening function is considered to account for partial transformation behaviour. The produced numerical results are compared with respective cases that omit these phenomena in order to quantify their effect in terms of the developed stresses, material state and production/recovery of transformation strain.
Theodoros T Machairas, Alexandros G Solomou, Anargyros A Karakalas, and Dimitris A Saravanos
SAGE Publications
The response of adaptive structures entailing shape memory alloy actuators is investigated both numerically and experimentally in this work. Emphasis is placed on the inclusion of large displacements and rotations, as well as thermomechanical coupling in the simulation of the shape memory alloy actuators. Reduced multi-field beam finite element models for shape memory alloy actuators, encompassing a co-rotational formulation for large displacements and capability to provide the thermomechanically coupled transient response, are briefly overviewed. Prototypes of two adaptive structure configurations are developed, experimentally characterized, and numerically modeled. The measured response of the two prototypes is correlated with respective numerical results that consider both the geometric non-linearity and the thermomechanical coupling of the shape memory alloy actuators. Hence, the influence of these two effects on the predicted response of both the actuator and the adaptive structure is demonstrated. The results quantify also the interactions between geometric non-linearity and thermomechanical coupling terms. As it is shown, better agreement with experimental data is obtained when considering both effects.
Anargyros A Karakalas, Theodoros T Machairas, and Dimitris A Saravanos
SAGE Publications
The present article investigates and explores the effect of partial phase transformation on the response of shape adaptive/morphing structures controlled by shape memory alloy wire actuators subject to variable trajectory and high actuation speed requirements, where the effect of partial transformation becomes more dominant. A modified constitutive model is adopted for the prediction of the thermo-mechanically coupled response on a trailing edge shape adaptive rib prototype intended for active load alleviation in large wind turbine blades, and the simulated behavior is subsequently correlated with experimental results. The experimentally validated model is further used to predict the response of the full-scale camber-line adaptive structure with shape memory alloy Ni51Ti49 wt% actuators in antagonistic configurations, under demanding operational time target trajectories at extreme turbulence conditions. Comparison of the results, with a case that omits partial transformation behavior, reveals substantial improvements in the predicted target trajectories, actuation speed, actuator stresses, and required operational temperature variation. The latter discloses the enhanced potential of shape memory alloy actuators to provide higher transformation rate and possibly higher fatigue life combined with lower energy demands toward the design and realization of efficient morphing structures.
Anargyros A. Karakalas, Dimitris I. Manolas, Theodoros T. Machairas, Vasilis A. Riziotis, and Dimitris A. Saravanos
Wiley
Anargyros A Karakalas, Theodoros T Machairas, Alexandros G Solomou, and Dimitris A Saravanos
IOP Publishing
Anargyros A. Karakalas, Theodoros T. Machairas, and Dimitris Saravanos
SPIE
Multiple applications of shape memory alloys (SMA) involve operation under partial transformation (PT), where reversal of the transformation direction takes place while the material is in a mixed phase state. Typical applications of SMAs include: actuators in adaptive/morphing structures which should repeatedly reach various target shapes or to follow time trajectories at higher time rates; dampers vibrating pseudo-elastically under varying amplitudes of dynamic loads. While the thermo-mechanically coupled behavior of SMAs under full transformation has been studied during the past and various models have been proposed, their response under PT has yet to receive the required attention to fully unravel the potential of these materials. In this paper, an experimental study of SMA wires under PT is presented along with a modified constitutive model. The physical constitutive model of Lagoudas et al.,1 is combined with a new expression of the hardening function to enable the accurate and efficient prediction of PT behaviour. The predicted PT response is correlated with isobaric, thermally induced PT cycle experiments. Very good agreement is obtained with measured partial cycles, especially for PT cycles formed near the middle of the major hysteresis loop. The new constitutive equations are included into a finite element framework to investigate the effect of PT on SMA actuation function in morphing airfoils for active load alleviation in large wind turbine blades, and numerical results are correlated with experimental data. The correlations prove the importance of PT behavior in the actuator performance of SMAs, resulting in substantially more accurate predictions in deformation, stress and temperature.
Theodoros Machairas, Alexandros Kontogiannis, Anargyros Karakalas, Alexandros Solomou, Vasilis Riziotis, and Dimitris Saravanos
IOP Publishing
Alexandros G Solomou, Theodoros T Machairas, Anargyros A Karakalas, and Dimitris A Saravanos
IOP Publishing
Theofanis Tsiantas, Dimitris I. Manolas, Theodore Machairas, Anargyros Karakalas, Vasilis A. Riziotis, Dimitrios Saravanos, and Spyros G. Voutsinas
IOP Publishing
The possibility of alleviating wind turbine loads through blade trailing edge shape morphing is investigated in the present paper. Emphasis is put on analyzing the effect of the trailing edge flap geometry on load reduction levels. The choice of the shape deformation of the camber line as well as the chordwise and spanwise dimensions of the trailing edge flap are addressed. The analysis concerns the conceptual DTU 10 MW RWT. Aeroelastic control of loads is materialized through a standard individual flap controller. Furthermore, a comb ined individual pitch-flap controller is evaluated and found to present advantages compared to the flap only controller. Flapwise fatigue load reduction ranging from 10% to 20%, depending on wind velocity and configuration considered, is obtained. Better performance is achieved by the combined pitch-flap controller.
A. Karakalas, T. Machairas, A. Solomou, V. Riziotis, and D. Saravanos
John Wiley & Sons, Inc.