Engineering Aspects of Shape Memory Alloys

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Views Icon Views. Issue Section:. You do not currently have access to this content. Learn about subscription and purchase options. Product added to cart. Accepted Manuscript Alert. Article Activity Alert. Cited By Web Of Science Acoust June, Close Modal. This site uses cookies. In [ ], the effects of the stress level, of the thermal cycling temperature interval, and of the heat-treatment state on the fatigue-life performances of TiNi wires were investigated. According to the study, the fatigue life is strongly influenced by the temperature interval adopted during the thermal cycling.

The fatigue life of SMA wires is also strongly influenced by the stress and strain reached in their actuated state, as suggested in [ ], where SmartFlex NiTi wires subjected to cyclic tensile loads were experimentally investigated. To reduce the thermal and mechanical overstresses induced in actuated SMAs, bi-stable configurations were investigated [ , , , , ] to develop mechanisms able to shift between a stable de-actuated configuration and a stable actuated configuration.

Hence, the activation of the SMA is needed to actuate and de-actuate the devices, resulting in energy-free actuation states where an electric current is not needed to keep the device in its actuated configuration, reducing the power consumption and the thermal and mechanical overstresses as well. Moreover, in [ ] considerations related to the low energy efficiency of SMA actuators ware addressed.

Different load cases for SMA actuators were compared, resulting in an efficiency that ranges between 0. Adaptive aerodynamics is one of the most promising fields of applications for shape memory alloy components, thanks to their morphing capabilities. To date, several solutions have been investigated to modify the aerodynamic field in aeronautical applications. Shape Memory Polymers SMP were used to modify the wing chord, to tailor the wing to specific scenarios. The adoption of actively cooled SMA for the deployment of flexible control surfaces was investigated in [ 48 ], while in [ 96 ] SMA wires were used to modify the camber of a morphing wing.

Experimental and numerical investigations on a morphing airfoil were carried out in [ 58 ], where SMA springs were used to actuate discrete points of the structure to achieve the desired deformation. In [ ], SMP hinges, adopted to modify the sweep angle of a wing, were numerically and experimentally investigated, while in [ ] the thickness of a flexible skin morphing wing is controlled by means of SMA actuators. A feasibility study of wing flap actuation based on shape memory alloys is presented in [ ]. A wing composed of different telescopic segment deployed by means of SMA actuators is presented in [ 29 ] to improve the aerodynamic performances while reducing the wing volume stowage.

In [ , ], SMAs were used to design and manufacture smart vortex generators, and their performance was investigated by means of experimental wind tunnel tests. In [ 38 ], an application to deploy and stow a flap edge fence is presented. Figure 1 summarizes the present and future applications of SMA concepts for adaptive airplanes aerodynamics. The solutions developed for adaptive aerodynamics can be easily transferred from the aeronautical to the automotive field.

In this work, two preliminary case studies, finalized to the adaptive aerodynamic, are briefly introduced. The presented case studies aim to demonstrate feasibility and inspire future applications to develop SMA-based devices in the adaptive aerodynamics field. Hence, in this preliminary design stage, the SMA characteristics have not been taken into account; instead we focus on the load exerted during the actuation. Indeed, in an advanced design stage, more detailed analyses must be performed, supported by experimental data.

In the proposed case studies, SMA actuators have been adopted to modify the aerodynamic field of vehicle on demand. The presented case studies are focused on the spoiler and on the rear upper panel components. The aim of Case study 1 is to demonstrate the feasibility of SMA-based actuation of a spoiler trailing edge under service aerodynamic loads.

Spoiler and actuated elevator: a cross section; b isometric view unit: mm. The trailing edge actuation will result in a variation in drag and downforce. From preliminary aerodynamic studies, a requirement of at least 10 mm trailing edge displacement should be achieved to guarantee a significant variation of the aerodynamic field. The aim of the second case study see Figure 4 is to demonstrate the feasibility of the actuation of rear upper panels of a vehicle, inducing a variation of the aerodynamic field, with consequent variation of drag and downforce to improve the vehicle performance under service aerodynamic loads.

In order to perform realistic numerical computations, the geometry configuration of an existing sports car has been taken into account. From preliminary aerodynamic studies, a 10 mm displacement of the rear upper panel has been considered sufficient to significantly modify the aerodynamic field. The Shape Memory Alloys are metallic alloys able to recover an initial prescribed shape when subjected to temperature variation.

In particular, an increase of temperature beyond a prescribed threshold generates a phase transition from a martensitic crystal structure to a stable austenitic crystalline structure, as shown in Figure 5. Since the phase transition induces a rearrangement of the microstructural arrays, the SMA mechanical properties including the elastic modulus and the yield strength are significantly modified [ , , , ]. In this phase, a mm long wire, characterized by a 1 mm diameter circular profile representative of a bundle of SMA wires, is considered.

Table 1 reports the mechanical properties of the SMA material system. Ni 52 Ti 48 mechanical properties [ ]. A simplified SMA material model has been adopted. This approach, although not accounting for all the characteristics of the SMA such as the hysteresis, can still be used in a preliminary study to evaluate the load resulting from the SMA actuation. However, in a more advanced design stage, detailed SMA constitutive models [ , , , , ] must be used to assess the behavior of the SMA actuators. In this work, the experimental data found in [ , ] have been used.

In particular, the elastic moduli and the coefficients of thermal expansion needed to describe the behavior of the alloy were derived from a database of experimental tests, exploiting the calibration proposed in [ ]. In particular, the elastic moduli E was experimentally measured from isothermal tensile tests: for each desired temperature, the SMA wires were subjected to a tensile load at a constant temperature.

Hence, the CTE at different temperatures can be expressed as a function of the measured strains and temperatures. In order to numerically replicate the SMA characterization procedure, as a preliminary step to the case studies analyses, a NiTiNOL wire has been clamped at its extremities. This analysis is aimed at the validation of the proposed simplified material model with respect to the literature data.

Indeed, it does not describe the behavior of the SMA wire used as an actuator, due to the different boundary conditions.

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The results of the ABAQUS numerical test, in terms of stress as a function of the temperature, have been found to be in agreement with the data reported in [ ], as shown in Figure 6 , where the numerical test results are compared to the reference ones. Thus, the validated material model has been used in the following test cases, tailoring the number of SMA wires according to the specific application. Numerical test results [ ]. According to the proposed finite element discretization, two solid models Figure 7 a,b have been considered.

The first model represents the fixed part of the spoiler, while the second represents the moving tip see Figure 7. The two solid models have been discretized by means of four-noded shell elements with a reduced integration scheme S4R. In Figure 7 , details of both solid models with corresponding numerical discretization are shown, while the mechanical properties of the adopted aluminum alloy have been reported in Table 3. Case study 1 details : a , b geometries; c FEM.

A hinge placed between the fixed and the moving part allows the rotation of the tip. As already mentioned, two mm-long SMA wires have been introduced to connect the fixed part to the moving tip, through two holes drilled on the fixed part see Figure 7 a. The position of the SMA wires has been chosen to maximize the moment with respect to the hinge. The section of the wires and the properties of the material are defined in agreement with the preliminary study on SMA materials.

In order to guarantee the return of the device to its initial position at SMA deactivation, two elastic beams have been placed between the fixed and moving tip opposed to the SMA wires. Details of the SMAs and beams locations are reported in Figure 8. Two different analyses have been considered.

The first analysis has been finalized to the testing of the SMA wire mechanical behavior and the latter has been finalized to the investigation of the influence of the external aerodynamic load and the elastic load of the beams on the SMA mechanical behavior and on the SMA actuation. Indeed, an equivalent aerodynamic load of 2. Since the weight is one order of magnitude lower than the aerodynamic load, it has been neglected. In Table 4 , the results of both configurations are reported, while Figure 9 shows Case study 1 in actuation mode with the application of both the aerodynamic load and the elastic load from the beams.

According to the numerical results, the investigated device can satisfy the displacement requirement 10 mm. Case study 2 is focused on the actuation of the rear upper panel of a bonnet, by means of properly located mm-long NiTiNOL wires, aimed to increase the drag and the downforce.

The numerical model has been realized by means of eight-noded 3D solid elements with a reduced integration scheme C3D8R. A hinge has been placed between the fixed part of the bonnet and the rear upper panel interfaces to allow its rotation. The rear upper panel can recover its initial position by means of a torsion spring placed in the hinge. As in the previous case study, the weight has been neglected since it is one order of magnitude below the aerodynamic load.

In Figure 10 , the numerical model, including the location of the SMA wires and of the torsion spring, is reported.

Rear upper panel actuation, details of the numerical model, and locations of SMA and torsion spring. Finally, in Figure 11 , the structure in actuation mode is reported. Again, for this configuration, the requirement on displacements 10 mm needed to obtain a significant aerodynamic field variation has been satisfied. However, it is worth noting that modifications to the number and the length of the SMA wire can help to tailor actuation displacements.

Engineering Aspects of Shape Memory Alloys

An overview of SMA-based smart structures has been presented in this work. Extensive studies, analytical, numerical, and experimental, have been found in the literature dealing with shape memory alloys, which can be considered suitable for adaptive aerodynamic applications, thanks to their morphing capabilities. Indeed, the research efforts on SMA-based actuators have been focused on the development of devices able to induce axial, bending, or twisting deformations. Moreover, the adoption of shape memory alloys results in simpler and lighter devices compared to the conventional actuators, significantly reducing the weight and the cost of the components.

Thus, the interest in shape memory alloy applications is increasing even more, as demonstrated by the number of articles published and patents issued, as reported in Figure Articles published and U. Sources: Scopus and Uspto. As a matter of fact, the majority of the applications presented in the literature are confined to the aerospace field, where performance requirements are demanding, as shown in detail in Figure 13 , where the number of articles clearly belonging to the aeronautical or automotive fields are compared.

Articles published in the aeronautical and automotive fields. Sources: Scopus. Hence, based on this literature research, a feasibility study of the development of SMA-based smart actuators for automotive applications, mostly derived from the aerospace experience, has been presented in this paper. In order to focus on the feasibility rather than on the executive design of SMA concepts, in this work a simplified model, descriptive of the NiTiNOL characteristics, has been employed.

Moreover, with the same objective in mind, preliminary, if realistic, aerodynamics loads have been considered for the proposed SMA concepts. Two case studies have been presented: the trailing edge actuation of a spoiler very close to the aerospace background and the rear upper panel deformation of a vehicle. The provided numerical analyses have demonstrated the feasibility of the presented SMA-based smart devices. As a general remark, the key design parameters to be considered in SMA applications, such as the maximum attained force and displacement and the operating range temperature, can be controlled by varying the material, size, and shape of the adopted SMA wires.

Hence, the presented solutions can be improved by tailoring the SMA geometry and material characteristics. National Center for Biotechnology Information , U. Journal List Materials Basel v. Materials Basel. Published online Feb Author information Article notes Copyright and License information Disclaimer. Received Jan 29; Accepted Feb Abstract The development of structures able to autonomously change their characteristics in response to an external simulation is considered a promising research field.

Shape Memory Materials

Keywords: shape memory alloy, smart structure, preliminary design. Introduction The continuously increasing requirements of structures capable of autonomously adapting their shape according to specific varying conditions has led to an increase, in the last few decades, of research studies on morphing technologies. Overview of the State of the Art of SMA Actuators Several investigations on morphing concepts, focusing on morphing components, can be found in the literature [ 46 , 47 , 48 , 49 , 50 , 51 ].

Adaptive Aerodynamic Applications Adaptive aerodynamics is one of the most promising fields of applications for shape memory alloy components, thanks to their morphing capabilities. Open in a separate window. Figure 1. Locations of adaptive aerodynamic applications in the aeronautical field. Figure 2. Locations of adaptive aerodynamic applications in the automotive field. Description of Case Studies 4. Case Study 1—Trailing Edge Actuation The aim of Case study 1 is to demonstrate the feasibility of SMA-based actuation of a spoiler trailing edge under service aerodynamic loads. Figure 3. Case Study 2—Rear Upper Panel Actuation The aim of the second case study see Figure 4 is to demonstrate the feasibility of the actuation of rear upper panels of a vehicle, inducing a variation of the aerodynamic field, with consequent variation of drag and downforce to improve the vehicle performance under service aerodynamic loads.

Figure 4. Numerical Simulation 5. SMA Modeling The Shape Memory Alloys are metallic alloys able to recover an initial prescribed shape when subjected to temperature variation. Figure 5. Table 1 Ni 52 Ti 48 mechanical properties [ ]. Table 2 SMA temperature-dependent mechanical and thermal properties. Figure 6. Figure 7. Table 3 AlT6 mechanical properties. Figure 8. Trailing edge actuation, details of the SMA, hinge, and beam locations. Figure 9. Table 4 Results of Case study 1. Boundary Conditions Max Displacement mm Without load and beam Case Study 2 Case study 2 is focused on the actuation of the rear upper panel of a bonnet, by means of properly located mm-long NiTiNOL wires, aimed to increase the drag and the downforce.

Figure Funding This research received no external funding. Conflicts of Interest The authors declare no conflict of interest. References 1. Qiu J. Smart skin and actuators for morphing structures. Weisshaar T. Induced drag reduction using aeroelastic tailoring with adaptive control surfaces. Bowman J.

Hwang S. Piezoresistive behavior and multi-directional strain sensing ability of carbon nanotube-graphene nanoplatelet hybrid sheets. Smart Mater. John A. Synthesis and characterization of cellulose acetateecalcium carbonate hybrid nanocomposite. Part B Eng.

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Hou Y. Graded conventionalauxetic Kirigami sandwich structures: Flatwise compression and edgewise loading. Wang W. Smart soft composite actuator with shape retention capability using embedded fusible alloy structures. Brinson L. One-dimensional constitutive behavior of shape memory alloys: Thermomechanical derivation with non-constant material functions and redefined martensite internal variable. Khoo Z. Van Humbeeck J. Shape memory alloys: A material and a technology. Icardi U. Preliminary study of an adaptive wing with shape memory alloy torsion actuators.

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