TENSAIRITY STRUCTURE WITH SHAPE-MEMORY WIRE ROPES

Abstract

The present invention relates to a structural element known in the technical field as tensairity, which introduces as distinctive elements with respect to the known art: (i) ropes in the shape-memory alloy (SMA) with superelastic (SE) and shape memory (ME) behaviour; (ii) mechanical tensioners for the adjustment of the initial tension in the ropes; (iii) optionally a control apparatus (processor) is connected to electric circuits that induce flow of intensity variable current through the SMA wire ropes; (iv) optionally devices for real-time monitoring of the temperature and the level of tension in the SMA ropes; (v) optionally devices for real-time monitoring of the tensairity oscillations; (vi) optionally new structural geometries capable of sustaining static actions and multidirectional dynamics.

Claims

1. A tensairity structure (200, 300), comprising a pneumatic element (220, 320) which extends along at least a director curve, one or more ropes (210, 310) connected to the ends of at least one high-slenderness beam rod (230, 330) in connection zones between the ropes, said at least one high-slenderness beam rod (230, 330) being anchored to said pneumatic element (220, 320) along said at least one director curve, the tensairity structure; wherein the at least one high-slenderness beam rod comprises four or more rods (230,330), which are placed along as many director curves of said pneumatic element (220, 330); wherein the tensairity structure comprises four or more pairs of ropes (210, 310), which are made of at least one of a plurality of SMA wires and in a combination of a plurality of threads made of a non-SMA material, and tensioning regulator for regulating the voltage of said four or more pairs of ropes (210, 310) are included.

2. The tensairity structure according to claim 1, wherein said tensioning regulator comprise or are constituted by mechanical tensioners for adjusting the initial tension of said four or more pairs of ropes (210, 310), placed in said connection zones between the ropes and rods.

3. The tensairity structure according to claim 1, wherein said initial tensions include tensions in the linear regime of the SMA wires and tensions in the system of non-linear SMA wires.

4. The tensairity structure according to claim 1, wherein said tensioning regulator comprise a source of electrical current electrically connected to said four or more pairs of ropes such that the electric current induces a temperature change in at least one of said four or more pairs of ropes.

5. The tensairity structure according to claim 1, wherein the elastic modulus of said SMA wires is varied using a source of electrical current electrically connected to said four or more pairs of ropes such that the electric current induces a temperature variation in at least one of said four or more pairs of ropes.

6. The tensairity structure according to claim 4, wherein it comprises a control unit comprising a series of load, strain and temperature sensors applied at corresponding points of said four or more pairs of ropes, as well as an electronic logic unit configured to adjust the flow of current from said current source to said four or more pairs of ropes on the basis of detections of said series of load, strain and temperature sensors.

7. The tensairity structure according to claim 1, wherein said non-SMA material is steel.

8. The tensairity structure according to claim 1, wherein said SMA is the NitiNOL or NiTiCu.

9. The tensairity structure according to claim 1, wherein on said four or more rods (230, 330) a series of accelerometers, preferably micrometric (MEMS), are applied.

10. The tensairity structure according to claim 1, wherein said four or more rods are made of aluminium or in another metal alloy or composite laminate material, for example carbon fibers.

11. The tensairity structure according to claim 1, wherein said pneumatic element is made of PVC or any other waterproof textile composite material.

12. The tensairity structure according to claim 1, wherein said pneumatic element is of cylindrical shape.

13. The tensairity structure according to claim 1, wherein said pneumatic element is of toroidal shape.

Description

[0020] The invention will be now described, for illustrative but not limitative purposes, with particular reference to the figures of the accompanying drawings, in which:

[0021] FIG. 1 shows an example of the basic elements constituting a tensairity, according to the known art;

[0022] FIG. 2 shows a tensairity for multi-directional action, according to the present invention;

[0023] FIG. 3 shows in (a) a three-dimensional view, and (b) a plan view of the toroidal tensairity, according to the present invention;

[0024] FIG. 4 shows a stress-strain cycle of a SMA wire rope SE behavior, according to the present invention;

[0025] FIG. 5 shows the force-displacement cycles of shape memory tensairity, according to the present invention;

[0026] FIG. 6 shows stress-strain cycles of a SE-behaviour SMA wire rope with increasing temperature, according to the experiments given in the literature;

[0027] FIG. 7 shows the effect of memory to a route of a SMA wire rope ME behavior, according to the experimental behavior reported in the literature;

[0028] FIG. 8 shows a system for the active control of the tensairity, according to the present invention;

[0029] FIG. 9 shows cross-sections of mixed shape-memory ropes alloy-other single stranded material, in which the wires in dark gray are in the form of a shape-memory alloy and the remaining ones are made of another material, according to the present invention;

[0030] FIG. 10 shows cross-sections of cables mixed with shape-memory alloys and another, multiple strand material in which the wires in dark gray are in the shape memory alloys and the remainder in another material, according to the present invention;

[0031] FIG. 11 shows cross sections of closed type in which the cylindrical wire ropes represented in dark gray are made of shape memory alloys while the remaining shaped wires, which carry the closure are in another material, according to the present invention; and

[0032] FIG. 12 shows a cyclic tensile test for the rope of diameter equal to 5.7 mm for which there is a high hysteretic dissipation.

DETAILED DESCRIPTION OF EXAMPLES OF THE INVENTION

[0033] Referring to FIG. 2, the tensairity according to the invention has at least four high-slenderness rods 230 solidarized along four lines of the pneumatic element 220. Around the latter, at least four pairs of SMA cables and/or mixed steel-SMA and/or other material-SMA 210 are wound. Each pair of cables has at least one winding around the pneumatic casing and is anchored to the ends of the associated rod. Such realization scheme makes tensairity able to support actions in the two orthogonal directions and in both directions for each one.

[0034] The configuration of FIG. 2 is not restrictive of the geometries that can be generalized using a greater number of beams with various sections and a greater number of ropes. In the simplest non-limiting case, the ropes are connected to the rods through specific anchors similar to terminals, for example friction aluminium terminals.

[0035] In FIG. 3, a different type of toroidal geometry 300 of a tensairity is shown, which is designed in particular for an aerostatic capacity structure (for example, stratospheric platforms). The rods 330 run along the parallels of the toroidal pneumatic element 320 (see Figure (a)) and the SMA ropes 310 are wrapped around it (see FIG. 3 (b)) and connected to the rods.

Structural Damping Increase

[0036] An element of the present invention that allows to extend the application field of tensairity consists of replacing the ropes, usually made of steel, with SMA ropes (superelastic and shape-memory behaviour) or mixed steel-SMA or mixed other material-SMA wherein by other material another metal alloy or polymer materials are intended.

[0037] The presence of the SE shape-memory material greatly increases the damping of the tensairity thanks to the austenite-martensite transformation generated by the traction cycles in the ropes, without showing residual inelastic deformations. The level of dynamic damping confers stability to the structure. Furthermore, it can be widely adjusted by acting on the number and section of the shape-memory wire ropes. The amplitude of the displacement to which such damping is achieved can be varied with the initial level of tension in the ropes. In the connection zone between the ropes and the slender beams, mechanical elements are placed for a pretensioning additional to that already obtained with the pneumatic inflation element. The elements for pretensioning hose inside them the load cells capable of measuring the level of pretensioning applied.

[0038] In FIG. 4 there is shown a stress-strain cycle on which is indicated the pretensioning level .sub.0 which allows the SMA material to dissipate energy due to deformation cycles induced by the actions on tensairity. The numerical value of such pre-tension is extremely variable from alloy to alloy and varies considerably even for a same alloy with different compositions and for different undergone machining processes. In any case, the .sub.0-.sub.0 point to which reference is made in FIG. 4 can be defined as the transition point (transition deformation) by the elastic behavior of the post-elastic behavior. This threshold is also that beyond which it can be assumed that the austenite-martensite transformation begins.

[0039] A prototype of tensairity with SE behaviour SMA material has been made with an aluminium rod, an inflatable cylinder PVC and two wires made of shape memory material (Nitinol). The aluminum rod is constrained at the ends to two supports with a hinge and a carriage in order to be a leaned beam scheme. The wires of Nitinol shape-memory material are pre-tensioned through the inflation of the PVC cylinder and with screw turnbuckles. In general, there will be tensioning means which comprise or are constituted by mechanical tensioners for adjusting the initial tension of four or more pairs of ropes 210, 310, placed in the connection zones between the ropes and the rods. The initial tensions include tensions in the linear regime of SMA wires and tensions in non-linear regime of the SMA wires.

[0040] The (initial or operation) tensioning can also be obtained through the variation of the length of the SMA wire ropes, using a source of electrical current connected to said four or more pairs of ropes, in such a way that the electric current can induce a temperature variation in at least one of said four or more pairs of ropes. In this way, contrary to the known art, the tensioning of tensairity is obtained by the only SMA wire ropes, without having to introduce mechanical tensioners. The tensairity is subjected to cycles of transverse displacement in the center line by measuring the opposing force with a load cell. In FIG. 5, the force-displacement curves are shown, which are obtained for two displacement amplitudes. The dissipated energy is represented by the area internal to the loop and is due to the phase transition of the shape memory material. Upon removal of the load, the structure shows no residual deformations due to the super-elasticity of the consisting Nitinol wires. The curves correspond to an equivalent viscous damping of about 4% (curve with width of 2.5 mm) and 3% (curve with width of 5 mm). Unlike the tensairity according to the invention, the tensairity realized according to the known art shows negligible dampings (less than 1%) for the purpose of a rapid dynamic stabilization of the structure.

Active Control of Tensairity

[0041] A further new innovative aspect here proposed is in the fact that the tangent stiffness of the SE-behaviour SMA ropes can also be increased twice by varying the temperature of the rope (by the Joule effect) making the electric current flow. In FIG. 6, the stress-strain cycles to vary the temperature of the material are shown. The tangent modulus of elasticity at the origin of the Nitinol is doubled for a temperature increase of about 50 C. from the ambient temperature of 20 C. [11-12].

[0042] The ropes in ME behavior may be used as active elements able to vary their action on the tensairity during operation. These cables can be shortened (up to 8% of the value of the length if the Nitinol or NiTiCu is used as a SMA alloy) varying its length through the Joule heating. One wants to take advantage of the so-called one-way effect of the shape memory material for applying in real time an additional state of tension in the tensairity in cases where this is necessary (e.g., loss of pretension as a result of the visco-elastic relaxation or to exercise active control over tensairity). The one-way memory effect is illustrated in FIG. 7 [11-12]. The rope is put into operation in the tensairity in its elongated configuration indicated by p1. Subsequently, it is heated by the Joule effect beyond the Af (Austenite finish) temperature, generating the recovery of the deformation imposed initially. The rope being anchored to the tensairity structure, on the latter a voltage indicated by the point p2 is applied that also preserves itself when the rope temperature returns to room temperature in p3. The proposed tensairity, compared to existing technology, has the possibility to vary in real time both the tangent and the geometric stiffness by a control apparatus that regulates the temperature in the SMA ropes (selectively with respect to the single wire rope according to the needs assessed by a logic control unit referenced as CPU in FIG. 8) according to the operating conditions. It is also possible to ensure an adequate level of structural damping against multidirectional dynamic actions.

[0043] The control system destined to make the tensairity active is schematically represented in FIG. 8. The SMA or mixed SMA-other material ropes are equipped with a distributed network of sensors for the continuous or real-time measurement of temperature and strain level. In the case of distributed network, one can have strain gauges and thermocouples while in the case of continuous network optical fibers can be used.

[0044] In the area of anchorage between the ropes and the beams, elements with high slenderness are positioned, which are adapted to provide an additional pretensioning compared to that obtained with the pneumatic inflation element. Within these elements, load cells are embedded which are capable of measuring the voltage level present in the ropes. The ropes are connected to an electromotive force generator which allows the passage of electric current. They are also wrapped in a coating that insulates them from the rest of the structure. Another possibility is to equip the coating, in addition to insulating material, also of high electrical conductivity material and to make the electric current flow in the latter. Finally, the high slenderness beams are equipped with a distributed network of accelerometers.

[0045] The network of sensors of extension, temperature, acceleration and the load cells send their measurements to an acquisition control unit which in turn sends this information to the central processing unit termed CPU. The CPU processes the information in real time using specific algorithms that combine mechanical simulations, identification processes and control cycles and adjusts, through the generator of the electromotive force, the current intensity inside the ropes.

Types of SMA and Mixed Ropes

[0046] The ropes used for tensairity structures can be realized in different formations that differ according to the number of strands, the number of wires constituting each strand, the relative position of the steel/SMA wires, and the winding angles of the strands and wires in the single strand. The usable shape-memory alloys are different: nickel-based (NiTinickel and titanium; NiAlnickel and aluminum), based on copper (CuSncopper and tin; CuMncopper and manganese; CuAlNicopper, aluminum and nickel; CuAlZncopper, aluminum and zinc), iron based (FeTiiron and titanium; FePtiron and potassium; FeMnSiiron, manganese and silicon).

[0047] In FIGS. 9 and 10 some cross sections of single strand and multiple strand cables are respectively given. The position of the steel wires or other material and of wires made of shape memory alloy is optimized depending on the application being considered to produce the desired mechanical behavior. Other material includes metallic or polymeric materials. The open spiral strands are named according to the number of wires that make up the various layers starting from the center towards the outside. For the multi-strand ropes, the number of strands of each layer and the number of wires constituting the single strand are indicated. The SMA wires are in dark gray whilst the remaining ones are in other material. The SMA wires are placed (a) in the central core of the rope, (b) in the outer layers, (c) arranged alternately with other material wires. For constructional reasons, the last layer always consists of steel and/or other material, for example polymer. The alternating arrangement between wires of different material and/or the SMA is formed both between the layers composing the spiral rope and within the same layer. The alternately placed strands ropes adopt the same criterion of the wires in the spiral alternated ropes and are also made in configurations that use, as a single strand, the alternating ropes of spiral type.

[0048] The cables shown in FIGS. 9 and 10 may be of a various windings spiral type, beam (parallel wires) type and polygonal (braided strands) type. In FIG. 11, three possible sections of a closed rope (usually utilized in stays) are shown, in which the central cores are made of shape memory materials.

[0049] The ropes of FIGS. 9 and 10, in the case where it is necessary, can also be realized entirely in a shape-memory alloy by virtue of the high performance that may be required by a specific application of the tensairity.

[0050] A mixed one between the ropes of spiral type was made by the inventors in two different diameters. The first rope of 5.7 mm total diameter is made of stainless steel of the AISI 302 type and shape memory alloy of nickel and titanium (Nitinol) with austenitic initial transition temperature=10 C. characterized by pseudoelastic behaviour at room temperature. The second rope of 19.5 mm total diameter is made with the same shape-memory alloy but with stainless steel of the AISI 304 type. The cross section of both ropes is represented in FIG. 9 (see (1+6+12+18+24)b). It consists of 61 wires arranged in a central core and 4 subsequent layers (1+6+12+18+24) wound in the opposite direction. The layers containing 12 and 18 threads are made of NiTiNOL while the remaining ones are made of steel. This layout is optimized for a specific application that requires the SMA phase transition for bending stresses applied to the cable. FIG. 12 shows a tensile test performed with a MTS machine in the laboratory of materials and structures of the Department of Structural and Geotechnical Engineering, University of Rome La Sapienza. The experimental response curve shows a formidable hysteretic dissipation capacity. The residual deformation is due to the presence of friction and not to inelastic deformation in the material.

[0051] With regard to the choice of the type of shape memory alloys in the construction of the cables, it is necessary to distinguish between ropes used in tensairity as actuators (in which it is important to activate the effect of memory at temperatures compatible with the operating environment) and the ropes used to increase the inherent damping of the structure and change the tangent stiffness (in which the super-elastic or pseudoelastic effect) is used. For the first group, it is preferable the use of Nitinol alloys (nickel-titanium), while for the second group the use of binary alloys of Copper-Aluminium type or ternary Copper-Aluminium-Zinc type is preferred which offer the advantage of lower costs as they consist of less expensive metals of nickel and titanium. However, such a criterion is not general because the choice of the shape memory alloy to be used for the SE and ME effects depends on the type of application of tensairity (depending on which the economic aspect is established) and especially by the performance level which one wishes to reach for the specific functionalities.

[0052] The stranding process of mixed ropes requires ad hoc thermal processes with respect to standard methods for wire ropes due to the pseudo-elasticity of NiTiNOL whose wires tend to recover their original shape, not so preserving the winding impressed by the stranding process. To get ropes that preserve the shape performing a double heat process is needed. Moreover, the outmost layer made of another material has, as its main purpose, to enclose the shape memory material, thus promoting the compactness and the radial resistance of the rope. The mixed rope in another material and shape-memory alloy has, in addition, the advantage of greatly reducing production costs by virtue of the lower use of shape memory alloy and to facilitate the production process compared to that of ropes that consist entirely of wires in the shape-memory alloy.

Benefits

[0053] The proposed tensairity allows to extend the application of these structures to areas characterized by the presence of multidirectional dynamic actions. This is possible thanks to the additional damping induced by the presence of the shape-memory wire ropes. The latter make the tensairity adaptive according to the operating conditions. It is in fact possible to vary the tangent stiffness and geometric stiffness by modifying, by the Joule effect, the temperature of the SMA respectively superelastic- or shape-memory behaviour ropes. The control system is composed by a processor connected to a sensor network that monitors voltage and temperature in the ropes and oscillations in tensairity. The system, based on the information processed by the processor, is able to adjust the flow of electrical current in the ropes, thus the temperature, and consequently the tension.

[0054] The main application areas are the stratospheric/space structures and roofings for large areas.

Bibliography

[0055] [1] Luchsinger, R H, Pedretti, A., Steingruber, P., & Pedretti, M. (2004). The new structural concept Tensairity: Basic principles. Progress in structural engineering, mechanics and computation, 323-328. [0056] [2] Luchsinger, R H, Pedretti, M., & Reinhard, A. (2004). Pressure induced stability: from pneumatic structures to Tensairity (R). Journal of Bionics Engineering, 1 (3), 141-148. [0057] [3] William L. Heyniger, Garret Corp, U.S. Pat. No. 2,936,056 A, Variable length inflatable escape chute 1957. [0058] [4] Bradley Sallee, Tracor Aerospace, Inc., U.S. Pat. No. 5,311,706 A, Inflatable truss frame, 1991. [0059] [5] Bradley Sallee, Tracor Aerospace, Inc., U.S. Pat. No. 5,579,609 A, Rigidizable inflatable strucuture, 1994. [0060] [6] Gary L, Bailey, Ross S. Woods, Obi Corporation, U.S. Pat. No. 6,463,699 B1, Air beam construction using differential pressure chambers, 2001. [0061] [7] Mauro Pedretti, US 20060260209 A1, Flexible compression member for a flexible pneumatic structural element and pneumatic element means for erecting structures, 2004. [0062] [8] Jean-Marc Daniel TurcotUS 20080295417 A1, Inflatable beam and truss structure, 2008. [0063] [9] Mauro Pedretti, US 20110209416 A1, Pneumatic node for compression elements, 2011. [0064] [10] Saul Griffith, Peter S. Lynn, Other Lab, Llc U.S. Pat. No. 8,640,386 B1, stiffening of an air beam, 2012. [0065] [11] Suzuki, Y., K. Otsuka, and C M Wayman. Shape memory materials (1998): 137-138. [0066] [12] Otsuka, K., and C M Wayman. Mechanism of shape memory effect and superelasticity Shape memory materials (1998): 27-49.

[0067] In the foregoing, preferred embodiments and variants of the present invention have been suggested, but it is to be understood that those skilled in the art can make modifications and changes, without so departing from the related scope of protection, as defined by the claims have been described attached.