Variable-stiffness mesh fabric

Abstract

Variable-stiffness mesh fabric comprising a plurality of cells, wherein each cell can be interconnected with another cell through at least one interconnection element. Each cell of the mesh fabric has, in operating conditions, a substantially constant stiffness, and can operatively engage actuator means comprising a shape-memory material, which can vary the stiffness of the mesh fabric as a function of a phase change.

Description

[0001] DESCRIPTION annexed to PATENT APPLICATION FOR INDUSTRIAL INVENTION entitled: “Variable-stiffness mesh fabric”

[0002] In the name of: Politecnico di Milano, of Italian nationality, with head office in Piazza Leonardo da Vinci 32, 20133 Milano, Italia.

[0003] Appointed inventors: Luca Michele MARTULLI, of Italian nationality, c / o Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano, Italia; Marco ROSSONI, of Italian nationality, c / o Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano, Italia; Paolo PARENTI, of Italian nationality, c / o Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano, Italia; Luca PATRIARCA, of Italian nationality, c / o Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano, Italia; Luca MARIOTTI, of Italian nationality, c / o Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano, Italia.

[0004] Field of the invention

[0005] The present invention relates to a variable-stiffness mesh fabric. Flexible mesh fabrics are light, pliable structures which can be manufactured easily. Due to their very nature, however, such structures are not stiff, behaving more like a textile than like a plate. Yet some applications, e.g. aerospace applications, require light and pliable structures which are also capable of assuming a very stiff configuration.

[0006] Background art

[0007] US 2018 / 0345651 A1 shows a 3D-printed flexible mesh fabric for aerospace applications. Said mesh fabric comprises a plurality of cells printed by means of an additive manufacturing process. Each cell of the mesh fabric can be interconnected with the other cells through a plurality of lacing elements with which such cells are equipped.

[0008] However, the mesh fabric described in US 2018 / 0345651 A1 has zero flexural stiffness. US 2023 / 0052509 A1 shows a 3D-printed mesh structure with variable stiffness. The mesh structure is formed of rings, e.g. having an octahedral shape, interlocked with one another. The structure, which is normally flexible, can be activated, i.e. stiffened, only when subjected to external pressure. Such external pressure can be applied in several ways: for example, by placing the structure in a vacuum within a suitable envelope connected to a pump, or through the use of other actuation systems. Because the activation of the structure requires the use of an external device, the solution proposed by US 2023 / 0052509 A1 needs considerable amounts of energy and is quite bulky. Moreover, activation is manual, i.e. it requires intervention by an operator.

[0009] CN 114054771 A illustrates a wearable jewel obtained by SLM (Selective Laser Melting) three-dimensional printing technology; the wearable jewel comprises a flexible lattice structure.

[0010] EP 3403522 A1 shows a leaf-like jewelry article comprising a plurality of connection elements linked with each other along a lying plane, each connection element comprising stems and openings intended to connect to adjacent connection elements.

[0011] JP 4273902 B2 shows a flexible shape-memory alloy actuator for applications in the fields of blood pressure measurement units, massage devices, etc.

[0012] KR 20220135397 A illustrates a wearable mesh fabric containing a shape-memory alloy. The fabric is flexible and can be used for massage devices.

[0013] Other solutions are known from the scientific literature. For example, Ransley et al. (Smart Mater Struct 2017;26:08LT02) have proposed a chain formed of rings made of 3D-printed polymeric material and actuated by springs made of shape-memory material and mounted within the structure of the chain rings, which springs are automatically activated as a function of temperature, thus changing the rings’ geometry. The solution proposed by Ransley et al. can only be used for applications involving very light loads. Moreover, assembling the chain proposed by Ransley et al. is a particularly difficult task.

[0014] Object of the invention

[0015] The present invention aims at overcoming the problems suffered by the prior art. In particular, it is one object of the present invention to provide a variable-stiffness mesh fabric which requires little activation energy and which is not bulky due to the absence of any external actuation devices.

[0016] It is another object of the present invention to provide a variable-stiffness mesh fabric which allows for both automatic, i.e. passive, activation control (in other words, a variable-stiffness mesh fabric that can “activate” itself autonomously to change its own stiffness without requiring, for example, operator intervention) and manual, i.e. active, activation control, e.g. by an operator.

[0017] It is another object of the present invention to provide a variable-stiffness mesh fabric the activation of which can be controlled by means of a temperature in the environment where the mesh fabric is located.

[0018] It is another object of the present invention to provide a variable-stiffness mesh fabric which has a simple structure and which can be manufactured easily.

[0019] It is another object of the present invention to provide a variable-stiffness mesh fabric which can be easily manufactured by means of additive manufacturing processes.

[0020] It is another object of the present invention to provide a variable-stiffness mesh fabric which offers good scalability and which requires low assembly costs and times.

[0021] It is a further object of the present invention to provide a variable-stiffness mesh fabric which ensures effective and accurate control over the global stiffness level of the mesh fabric.

[0022] It is yet another object of the present invention to provide a variable-stiffness mesh fabric which is suitable for applications involving any type of load, e.g. heavy loads. These and other objects will become apparent to a person skilled in the art in the light of the following description.

[0023] Summary of the invention

[0024] The following will describe, by way of non-limiting example, some preferred embodiments of the present invention. Other embodiments, although not described herein but conceivable by a person skilled in the art, are included as well. The present invention relates to a variable-stiffness mesh fabric comprising a plurality of cells. Each cell of the variable-stiffness mesh fabric according to the invention has a first surface and a second surface. The first surface comprises at least one interconnection element.

[0025] Each cell of the plurality of cells is interconnected with at least one other cell through said at least one interconnection element.

[0026] Each cell of the plurality of cells has at least one housing and has, in operating conditions, a substantially constant stiffness.

[0027] In other words, in operating conditions each cell undergoes negligible deformations, i.e. its dimensions are substantially stable.

[0028] The mesh fabric of the present invention is characterized in that each cell can operatively engage actuator means comprising a shape-memory material. Such actuator means comprise at least one elongate body made of shape-memory material slidably engaged in the at least one housing of each cell and rigidly constrained to one end of the mesh fabric. Such actuator means can vary the stiffness of the mesh fabric as a function of a phase change of the shape-memory material.

[0029] The term “operating conditions” refers herein to the load, temperature, pressure, stress time, etc. conditions to which the variable-stiffness mesh fabric of the present invention is subjected. Such operating conditions may of course vary according to the application.

[0030] According to one embodiment, the cells of the variable-stiffness mesh fabric of the invention are obtained by means of an additive process, i.e. by 3D printing.

[0031] According to one embodiment, the entire variable-stiffness mesh fabric is obtained by means of an additive printing process, wherein each cell is printed already interconnected with another cell through said at least one interconnection element. According to one aspect, the cells of the mesh fabric are made of polymeric, metallic, ceramic or composite material, or any appropriate combination thereof, depending on the intended application, and generally any material capable of bearing a load.

[0032] According to one aspect, when a polymeric material is used for making the cells, said polymeric material is suitable for additive printing, such as PLA, nylon, ABS, PEEK, PEKK, etc. According to one aspect, the cells of the mesh fabric are made of a material having, in operating conditions, a substantially constant stiffness.

[0033] In other words, in operating conditions the material used for making each cell essentially works within the elastic range and undergoes no plastic deformation.

[0034] According to one embodiment, said at least one interconnection element comprises a plurality of lacing elements.

[0035] According to one aspect, the plurality of lacing elements can be associated with either the first surface or the second surface of the cells of the mesh fabric, or with both.

[0036] According to one aspect, the first and second surfaces of the cells of the mesh fabric of the invention have a roughly square or rectangular shape.

[0037] According to one aspect, the thickness of the cells of the mesh fabric is smaller than the width and / or length dimensions of the first and / or second surface of the cells. For example, the thickness of the cells may be equal to or smaller than approximately one third of the width and / or length dimensions of the first and / or second surface of the cells. In general, the ratio between thickness and area of the cells is optimized as a function of the intended final application of the mesh fabric.

[0038] According to one embodiment, each cell may comprise two, three, four or more lacing elements.

[0039] According to one embodiment, a ring or any other structure suitable for interconnecting the cells can be added to the plurality of lacing elements.

[0040] According to one embodiment, all the cells of the mesh fabric have the same type of lacing elements.

[0041] According to an embodiment which is alternative to the preceding one, the cells of the mesh fabric have different types of lacing elements.

[0042] According to a preferred embodiment, the plurality of cells of the variable-stiffness mesh fabric according to the invention is 3D-printed with the plurality of lacing elements already interconnected, and the actuator means are added at a later time. According to one embodiment, the cells of the mesh fabric are mutually spaced apart as a function of the stiffness performance to be attained.

[0043] According to one embodiment, the gap between the cells of the mesh fabric is less than 1 mm, preferably less than 0.9 mm. According to a preferred embodiment, the gap between the cells of the mesh fabric is less than 0.5 mm, even more preferably less than 0.4 mm.

[0044] According to one variant, the actuator means comprise a plurality of elongate bodies made of shape-memory material.

[0045] According to one variant, each cell has a plurality of housings.

[0046] According to a preferred embodiment, the at least one housing is made as at least one through hole, and the at least one elongate body is a wire made of shapememory material.

[0047] According to one aspect, the at least one through hole crosses the cell throughout its length and / or width.

[0048] According to one aspect, the at least one through hole crosses the cell throughout its thickness.

[0049] According to a preferred variant, the at least one through hole lies in a middle plane of the cell.

[0050] According to one variant, each cell has two or more through holes.

[0051] According to one embodiment, the at least one end of the at least one wire made of shape-memory material is constrained to at least one cell situated on an edge row and / or column of the mesh fabric.

[0052] According to one embodiment, the at least one end of the at least one wire made of shape-memory material is constrained to at least one cell by means of a fastening device, e.g. a wire clamp.

[0053] According to one embodiment, the mesh fabric of the present invention comprises a plurality of wires made of shape-memory material which can engage the cells of the mesh fabric along one or more directions. For example, the wires may engage the cells along rows or columns, or in both directions.

[0054] According to one embodiment, the plurality of wires made of shape-memory material comprise wires lying in the same plane.

[0055] According to an embodiment which is alternative to the preceding one, the plurality of wires made of shape-memory material comprise wires lying in different planes. According to one embodiment, the plurality of wires made of shape-memory material comprise wires made of the same shape-memory material. According to an embodiment which is alternative to the preceding one, the plurality of wires made of shape-memory material comprise wires made of different shapememory materials.

[0056] According to one embodiment, the at least one wire made of shape-memory material comprises a material which can change phase as a function of temperature. According to one embodiment, the at least one wire made of shape-memory material can be actively actuated, e.g. it can be heated by means of an electric command or the like.

[0057] For example, the wires made of shape-memory material may be conductive, so that they will heat up when electric current flows through them. By adjusting the current intensity through the wire, it is possible to control the heating, i.e. the temperature, of the wire, and hence how much it will shorten and, as a consequence, how much the cells will be compressed, i.e. how much the mesh fabric will stiffen.

[0058] According to an embodiment which is alternative to the preceding one, the at least one wire made of shape-memory material may be self-actuated, e.g. upon an external temperature stimulus which can be provided by simply immersing the mesh fabric into an environment having the desired temperature or by placing a heat source close to the wire.

[0059] According to a preferred embodiment, the at least one wire made of shape-memory material comprises an alloy of nickel and titanium.

[0060] The present invention further relates to the use of a variable-stiffness mesh fabric as described above.

[0061] According to one embodiment, the present invention concerns a variable-stiffness mesh fabric for aeronautical or aerospace applications.

[0062] According to one embodiment, the present invention concerns a variable-stiffness mesh fabric for orthopaedic applications.

[0063] According to one embodiment, the present invention concerns a variable-stiffness mesh fabric which can be used as a component of an exoskeleton.

[0064] In general, the present invention concerns a variable-stiffness mesh fabric for structural applications requiring the ability to withstand loads, even heavy ones. The following will describe, by way of non-limiting example, some preferred embodiments of the present invention. Other embodiments, although not described herein but conceivable by a person skilled in the art, are included as well.

[0065] Brief description of the drawings

[0066] The following description will refer to the accompanying drawings, provided merely by way of non-limiting example, wherein:

[0067] - Figure 1 a shows an embodiment of a cell of a variable-stiffness mesh fabric according to the present invention;

[0068] - Figure 1 b shows two cells of the type illustrated in Figure 1 a, mutually interconnected by their respective lacing elements;

[0069] - Figure 1 c shows a mesh fabric with 16 cells of the type illustrated in Figure 1 a, mutually interconnected by their respective lacing elements and arranged in 4 rows and 4 columns;

[0070] - Figure 2 shows a mesh fabric made up of 4 cells (configuration with 1 row and 4 columns) assembled by means of 2 wires made of shape-memory alloy fastened to the edge of the mesh fabric by means of wire clamps;

[0071] - Figure 3 shows a diagram representing the load-displacement curves, at four different temperatures, of a first mesh fabric sample;

[0072] - Figure 4 shows a diagram representing the load-displacement curves, at four different temperatures, of a second mesh fabric sample.

[0073] Detailed description of some preferred embodiments of the invention

[0074] In the following description, expressions such as Tight”, “left”, “over”, “under”, “upper”, “lower”, “horizontal”, “vertical”, and the like, may be used merely for illustration purposes to refer to particular arrangements of elements shown in the annexed drawings, without however any limiting meaning.

[0075] The present description concerns a variable-stiffness mesh fabric according to the invention, in particular obtained by means of an additive manufacturing process.

[0076] The variable-stiffness mesh fabric of the invention, when activated, behaves like a plate, i.e. like a rigid structure capable of reacting to complex force systems (normal, shear and bending forces and moments), thus being able to withstand heavy loads with just small deformations and displacements.

[0077] Figure 1a shows a cell 1 having a roughly parallelepiped shape, with a first surface 10 and a second surface 11 , both of which are roughly square.

[0078] The cell 1 is relatively thin, its thickness being approximately equal to or smaller than one third of the width and / or length of the first surface 10 and / or of the second surface 11 .

[0079] The four lateral surfaces 20 of the cell are roughly rectangular in shape.

[0080] On its first surface 10, the cell 1 has an interconnection element shaped as a plurality of lacing elements 30, which can connect to respective lacing elements 30 of an adjacent cell.

[0081] In the particular embodiment illustrated in Figure 1 a, said lacing elements 30 are four in number and have each a tubular shape and a particular bent conformation starting from a common point or stub located near the centre of the first surface 10 and developing in a right-handed helix fashion towards each one of the four comers of the first surface 10.

[0082] In the embodiment illustrated in Figure 1 a, the cell 1 has also two through holes 40, the ends of which are located approximately in a central region of the lateral surfaces 20, lying approximately near a middle plane of the cell.

[0083] The through holes may also lie, with reference to the cell thickness, in slightly offset planes, i.e. at different heights, so that the actuator means, generally consisting of wires made of shape-memory alloy (SMA), can be easily arranged inside the cells constituting the mesh fabric, along both the columns and the rows of the latter (in other words, along both the weft and the warp of the mesh fabric) without interfering with each other.

[0084] In other words, the number and position of the wires in the mesh fabric may vary as needed depending on the type of actuation and / or application.

[0085] Figure 1 b shows two cells of the type illustrated in Figure 1 a, interconnected by respective lacing elements 30. The mesh fabric 100 is 3D-printed with the lacing elements 30 already interconnected; therefore, no intermediate operations are necessary for assembling the cells. This is useful to avoid the risk of the cells scattering during the assembly, while also making it possible to print large mesh fabrics, if allowed by the printer in use. Figure 1 c shows, by way of example, a mesh fabric 100 with 16 cells arranged in 4 rows and 4 columns and already interconnected. The mesh fabric can be printed from polymeric or metallic material, thus extending the possible fields of application of the device.

[0086] After printing, a further processing step may be envisaged, if necessary and particularly when using polymeric material, in order to finish off the through holes. As aforementioned, wires made of shape-memory alloy (SMA), e.g. Nitinol (NiTi), which is an alloy that can exhibit a phase transition between a martensitic phase and an austenitic phase at a certain critical temperature, are made to pass through the through holes 40 of the interconnected cells 1 of the mesh fabric 100. The wires (not shown in Figures 1 a, 1 b and 1 c) are fastened to the mesh fabric by means of wire clamps or other fastening devices at the external edges of the mesh fabric, thus engaging cells located at the edges of the mesh fabric.

[0087] Figure 2 illustrates a mesh fabric made up of four cells arranged in line (configuration with 1 row and 4 columns), with the SMA wires fastened to the ends of the mesh fabric by means of suitable wire clamps 50.

[0088] In general, SMA wires run along the entire length of each row and / or column of cells constituting the mesh fabric.

[0089] The wires are mounted loose between the ends of the mesh, thus allowing the mesh fabric some flexibility. In this case, the SMA is in the martensitic phase, and the mesh fabric is in the “inactive” configuration. At temperatures higher than the assembly temperature, and generally higher than the critical temperature, the SMA will transform into the austenitic phase and the wires will start to contract, thus pressing the cells one against the other proportionally to the wire temperature and to the mesh geometry, and particularly to the gap between the cells prior to activation. The shortening of the wire, countered by the mesh cells, results in a general stiffening of the whole mesh fabric. In this case, the mesh fabric is considered to be in the “active” configuration. Since the shortening of the wires is proportional to their temperature, the stiffness of the mesh fabric in the active configuration is variable with temperature.

[0090] In this respect, SMA wires are commercially available which have different temperatures of transformation between the martensitic phase and the austenitic phase: by appropriately selecting the SMA wires it is thus possible to build variablestiffness mesh fabrics which can be activated and / or deactivated at different temperature ranges.

[0091] The performance of the 3D-printed mesh fabric of the present invention was measured in the laboratory.

[0092] An EOS Formiga P 110 Velocis 3D printer was used to manufacture two mesh fabric prototypes called C1 and C2, consisting of just one row of four polymeric cells. During assembly, metallic shims were inserted in the gap between the cells of each prototype, which were then removed immediately after fastening the alloy wires. This was done to investigate the effect of the gap between the cells. More specifically, one prototype (C1 ) was assembled with an inter-cell gap of 0.8 mm, while the other one (C2) was assembled with an inter-cell gap of 0.35 mm. Each cell had surfaces of equal width and length (15 mm) and a thickness of 5 mm. The cells were made from nylon 12. Commercial SMA wires (NiTiNol) of the type designated as 5S00097 - NITI / 54 / W / 0.5 / T (supplier: SAES Getters SpA) were used, which had a diameter of 0.5 mm and a transition start temperature of 80°C.

[0093] Both mesh fabric prototypes were subjected to bending tests at three room temperature (RT) points as inactive configurations, and at 80C°, 90C° and 100C° as active configurations.

[0094] Both mesh fabric prototypes C1 , C2 were tested by conducting three-point bending tests in ambient chamber in order to evaluate their stiffness. The machine employed for the tests was an MTS Alliance RF / 150 with a 150kN cell. The three-point tool consisted of three rollers with a diameter of 5 mm. Two of them were rigidly fixed to the bottom base, while the top roller was driven by a piston. Two thermocouples were used for controlling the temperature in the chamber. One of these was positioned on the metal base of the ambient chamber, which is that part of the latter which has the highest thermal inertia. The other one was placed into a lateral hole of one cell of the prototype for monitoring its local temperature.

[0095] The stroke of the top roller was 2 mm for all tests, except those at 80°C, where a stroke of 3 mm was used. The stroke start was made to correspond to the relative displacement under a preload of 10 N. The load application speed was 5 mm / s. Figures 3 and 4 show the load-crossbar displacement curves of the two samples C2 and C1 , respectively. The differences in the force values achieved are due to different values of the initial gap between the cells: the sample with thinner initial shims (C2, having an initial inter-cell gap d=0.35 mm) reaches higher forces, and vice versa. This indicates that the stiffening is inversely proportional to the initial inter-cell gap. As shown, the curves of the tests carried out at room temperature (RT) are flat or anyway indicate retarded stiffening. This is a sign of almost null stiffness of the device. On the contrary, the curves of the tests carried out at high temperatures show a significant stiffness from the very start of the stroke of the top roller. Furthermore, the forces reached at the end of the stroke gradually increase as test temperature increases. This demonstrates that the stiffness in the active configuration is dependent on the prototype’s temperature.

Claims

CLAIMS1 . Variable-stiffness mesh fabric (100) comprising a plurality of cells (1 ), each cell having a first surface (10) and a second surface (11 ), and at least one interconnection element (30) integral with the first surface (10); wherein each cell (1 ) of the plurality of cells is interconnected with at least one other cell through said at least one interconnection element (30); and wherein each cell of the plurality of cells has at least one housing (40) and has, in operating conditions, a substantially constant stiffness, characterized in that each cell (1 ) can operatively engage actuator means comprising a shapememory material, said actuator means comprising at least one elongate body made of shape-memory material slidably engaged in the at least one housing (40) of each cell (1 ) and rigidly constrained to one end of the mesh fabric, said actuator means being capable of varying the stiffness of the mesh fabric (100) as a function of a phase change of the shape-memory material.

2. Variable-stiffness mesh fabric (100) according to claim 1 , wherein the plurality of cells (1 ) are obtained by means of an additive printing process, and wherein each cell is printed already interconnected with another cell through said at least one interconnection element (30).

3. Variable-stiffness mesh fabric (100) according to any one of claims 1 or 2, wherein said at least one interconnection element (30) comprises a plurality of lacing elements.

4. Variable-stiffness mesh fabric (100) according to claim 1 , wherein the actuator means comprise a plurality of elongate bodies made of shapememory material.

5. Variable-stiffness mesh fabric (100) according to any one of claims 1 or 4, wherein each cell (1 ) has a plurality of housings (40).

6. Variable-stiffness mesh fabric (100) according to any one of claims 4 or 5, wherein the at least one housing (40) is made as at least one through hole, and the at least one elongate body is a wire made of shape-memory material.

7. Variable-stiffness mesh fabric (100) according to claim 6, wherein at least one end of the at least one wire made of shape-memory material is constrained to at least one cell (1 ) by means of a fastening device (50).

8. Variable-stiffness mesh fabric (100) according to any one of claims 6 or 7, wherein the at least one wire made of shape-memory material comprises an alloy of nickel and titanium.

9. Use of a variable-stiffness mesh fabric (100) according to any one of claims 1 to 8 for aerospace applications.

10. Use of a variable-stiffness mesh fabric (100) according to any one of claims 1 to 8 for orthopaedic applications or as a component of an exoskeleton.

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