WO2009053946A2

WO2009053946A2 - Structures with adaptive stiffness and damping integrating shear thickening fluids

Info

Publication number: WO2009053946A2
Application number: PCT/IB2008/054416
Authority: WO
Inventors: Christian Fischer; R. Cristian Neagu; Pierre-Etienne Bourban; Véronique MICHAUD; Christopher J.G. Plummer; Sébastien Lavanchy; Jan-Anders Edvin Manson
Original assignee: Ecole Polytechnique Federale De Lausanne (Epfl)
Priority date: 2007-10-26
Filing date: 2008-10-25
Publication date: 2009-04-30
Also published as: WO2009053946A3

Abstract

The invention relates in particular to a method to process composite structures with tailored stiffness and damping performance incorporating a smart material, namely a shear thickening fluid (STF), preferably at the interface between two elements belonging to the same structure and moving relatively to each other. The composite structure incorporating STFs according to this invention may advantageously be used in applications such as sports equipment, aeronautics, aerospace, consumer goods or in any other suitable field where the dynamic properties of the said structure need to be tailored.

Description

Structures with adaptive stiffness and damping integrating shear thickening fluids

FIELD OF THE INVENTION This invention relates to structures incorporating adaptive materials to control the overall mechanical behaviour of the structures. Such structures may be used in active structural vibration damping in sports equipment, space structures, consumer goods, or in any other suitable field.

BACKGROUND Adaptive composites

Composite materials have been in use, in particular for aeronautic and space applications for the past 40 years mostly thanks to their high specific mechanical properties. Recently, the field of composite materials research has evolved from the initial search for very high specific properties alone, driven by aerospace application, to the need to maintain high properties while reducing manufacturing time and production costs, driven by automotive and other large scale applications, to recently include the need to integrate added functionality in the composite part. Since conventional structural composite materials cannot fulfill this last requirement alone, adaptive or smart composite materials, which integrate actuators and sensors, hence represent a next step in composite materials development. This evolution is driven in part by the need from various applications to gain efficiency not only by reducing structural weight, but by integrating functions directly into the structure. Examples span from civil engineering applications with fiber optic sensor monitoring of bridges or structures, to sports with piezo-electric fiber systems to actively damp skis or tennis rackets, to health monitoring or active flutter reduction of airplanes. It is also driven by the fact that it is now possible to integrate actuators or sensors directly into the composite material, since they have achieved a high enough degree of miniaturization to not disrupt the structural integrity of the composite part, in service but also during processing. Finally, there is a growing interest to use adaptive materials to suppress or tailor the vibration properties of composite structures, in particular for large moving structures to prevent flutter or other damaging events, or to increase comfort. A potential class of such materials is fiber reinforced polymer composites containing thin Shape Memory Alloy (SMA) wires as actuating elements [I]. SMAs have been available for 40 years and have found applications as actuators but have been only recently manufactured as high quality wires with diameters below 0.2 mm. The main drawback of these materials is that their transformation is induced by temperature or stress, greatly limiting their response time, controlled by heat transfer kinetics. Also, the pre-strained wires have to be maintained in place during processing, if processing occurs at a temperature above the transition temperature of the alloy. Alternatively, passive activation may be exploited, through an external temperature change. As an example, as martensitic phase exhibit high damping properties, whereas the parent austenitic phase does not, a natural temperature change may increase the damping properties of the material whereas most viscoelastic composite materials on the other hand decrease their damping properties at low temperatures [2].

A promising concept in terms of activation frequency is the recent development of Ferromagnetic SMAs. As an example, NiMnGa alloys can achieve magnetically controlled strains up to 10 %, with frequencies in the range of 200 Hz or more. Other more traditional examples include composite materials with embedded piezo-electric fibers [3]. These provide low activation stresses, but may act as sensors or actuators, with a very high activation frequency. They can now be fairly easily implemented, and they do not require any temperature change, hence any external source of energy, if mechanical stress is applied on the sensor. Examples of these mechanisms are found in military research in the US, and in sports applications. A number of other materials are currently under investigation, including electro-rheological, magneto-rheological and shear thickening fluids, electro-active polymers and shape memory polymers.

Vibration control in composites

Damping issues have become increasingly important over the last few decades in a wide range of fields, including aeronautics, automotives and sports. At the same time, polymer composites have gained in importance thanks to their combination of low density, excellent stiffness and damping properties. Passive methods such as surface damping treatments, constrained layer damping, and hybridization (for example the use of mixed polyethylene fibre- and carbon fibre-based laminate structures), may be used to enhance the damping properties of composites, and the effects of fibre length, orientation and surface treatment on the damping performance of composites have also been investigated. Vibration and damping characteristics are certainly affected by temperature, especially in passive systems involving viscoelastic materials. Vibration of composite viscoelastic sandwich plates has been studied for example under thermal environment considering the initial stresses and temperature dependent shear modulus. Shifting of vibrations modes with temperature was observed [4].

Integration of adaptive materials into composite structures to better control vibration and stiffness is another area of considerable interest. A well-known example is that of laminates containing pre-deformed shape memory alloy (SMA) wires, which modify the materials properties as their temperature is varied e.g. by Joule heating Electro-rheological (ER) and magneto-rheological (MR) fluids, often referred to as "smart fluids", provide further means of modifying vibrational properties via external stimuli. However, all these systems and technologies currently used for active damping and in smart structures have in common that they require an external power source to be activated.

In summary, a significant effort has been made in recent years to develop new ways to tailor the dynamic properties of structures under various conditions, such that the structure adapts itself to its environment. However, the methods commonly used to actively damp composite structures all require an external power source to be activated. This is typically linked to relatively complex networks of sensors, and generally requires time to be initiated. There is therefore a need for "smart" structures adapting their stiffness and damping properties independently of external stimuli, such as e.g. Joule heating, a magnetic or an electric field.

SUMMARY OF INVENTION

The present invention relates to flexible composite structures containing shear thickening fluids (STFs) for active vibration and damping control. STFs are typically composed of a concentrated stabilized dispersion of rigid sub-micron particles in a carrier fluid.

The composite structures according to the invention incorporate at least a portion of STF, which is enclosed between interfaces or containment layers. The STF can be tailored such that its storage and loss shear (or tension/compression) can be varied as a function of the amount of shear the said STF is exposed to, thereby permitting continuously variable control of the stiffness and damping characteristics when the composite structure is exposed to dynamic loading, where structure elements containing the STF can be plates, panels, beams and bars, as well as mechanical composite systems containing such structural elements.

The STF structural elements are comprised of interfaces or containment layers which guarantee the structural integrity and also provide that stress is transferred from their faces into the STF when the composite structure is deformed. By tailoring the STF composition, as well as the geometry of the element containing the STF, the amount of deformation required to activate the STF (i.e. to make it shear-thicken) can specifically be tailored for any of the structure elements mentioned. The thickness of the interfaces or containment layers may be kept constant by using continuous or discontinuous fibres, spheres, or any form of suitable spacer. Under specifically tailored conditions, the STF thickens, resulting in an increase in viscosity (thus in shear moduli) inducing changes in the structure's dynamic properties, i.e. either simultaneously increasing the latter' s damping and stiffness, or acting on either damping or stiffness. In the most common case, the STF is activated when reaching a certain amount of shear rate or stress, i.e. on a specific amount and/or speed of the relative displacement between the faces of the containment layers. Since the STF is activated under specific deformation conditions, structural vibration and damping control using STFs does not require any external power source, as it is the case for other active damping methods.

The integration method presented in the following paragraphs use this basic concept, i.e. the STF is contained at interfaces in a structure element, and is subjected to shear deformation when the structure element (or a composite structure containing such elements) is being deformed due to external dynamic loads. Using the integration method, the STF is placed and contained at an interface between gliding pistons and the composite structure, at the interface between fibre bundles of a freely moving fibre fabric, at the interface between fibre and matrix of the composite, inside the matrix material to create a two-phase morphology in the composite, or in pores of cellular composites.

The invention involves a method to integrate STF in a structure that will exhibit desired dynamic behaviour, i.e. damping and stiffnes, as a result of a controlled structure on the micro and macroscale, i.e. by design of specific interfaces or containment layers, which changes the properties of the STF when the structure interacts with external dynamic forces.

DESCRIPTON OF THE FIGURES

The invention will be better understood below with a detailed description including examples illustrated by the following figures:

Figure 1: Rheological behaviour of a STF composed of 67.75 %w/w monodisperse spherical silica particles in polyethylene glycol (PEG) with an average molar mass of 200 g/mol.

Figure 2: General approach to integrate STFs into composite structures is illustrated.

Figure 3: Schematic of a sample structure where glass fibre-epoxy rods are used as pistons glued at one end of the PMMA host structure and freely moving at the structure's free end (top), and longitudinal cross-section of the structure's free end (bottom).

Figure 4: Compliance vs. frequency for the STF integration design based on the glass-epoxy rods with different set tip displacements (a) without STF and (b) with STF at the rod- matrix interface.

Figure 5: Schematic of the sample structure where a glass fibre plain weave is used to shear the impregnated STF on relative motion of the bundles (top). A zoom on the ±45° glass fibre weave is shown at the bottom of the figure.

Figure 6: Compliance vs. frequency for the STF integration design based on the impregnated glass fibre weave with different set tip displacements (a) with a dry weave and (b) with the STF-impregnated weave.

Figure 7: Relative damping increase for (i) both designs without any STF, (ii) the weave based design and (iii) the piston-based design, both with STF.

Figure 8: Cross- and longitudinal sections of the DMA sample composed of a silicone matrix, glass fibre-epoxy rods and STF at the matrix-rod interfaces.

Figure 9: Loss factor, tan£, obtained from shear mode measurements by DMA.

Figure 10: Loss factor, tan£, obtained from shear mode measurements by DMA. Figure 11: Schematic of the concept comprising wires (parallel fibers) with STF at interface.

Figure 12: Schematic of the concept comprising wires (twisted fibers) with STF at interface.

DETAILED DESCRIPTION

The present invention relates to a method for the placement and containment of an adaptive material inside a composite structure, namely shear thickening fluids (STFs), at the interface between two or more elements, or containment layers, which are designed in a way that external dynamic forces result in the elements having opposed deformation to activate the STF by relative displacement between the named elements. The mechanical properties of the STF, e.g. storage and loss moduli, can be varied as function of the shear deformation at the interface containing the STF thereby permitting control of the dynamic behaviour of the entire structure. Thus, the aim consists of integrating STFs with strongly nonlinear rheological behaviour into a composite structure leading simultaneously to changes in stiffness and damping under dynamic loading as the strain and/or frequency are varied.

Figure 1 shows the typical behaviour of a shear- thickening fluid (STF). As the shear rate is continuously increased, the suspension shows a shear-thinning behaviour, before reaching the critical shear rate, at which the viscosity dramatically increases. After the transition, the suspension exhibits a solid-like behaviour, i.e. being several orders of magnitude stiffer and more viscous than before the transition.

Figure 2 schematically depicts the present invention which comprises a method to integrate an adaptive material, such as STF, at the microscale of a structure to tailor its vibration characteristics. A general approach of how STFs are integrated into a composite structure deals with the creation of interfaces or containment layers, where the STF is placed, that will be used to activate the STF by inducing its shear deformation. For instance, Figure 2 shows a matrix (1) containing freely moving fibres/rods (2) in a cavity as the structure is deformed. Spacers (3) can be used to keep the fibres/rods at a constant distance from the structure's matrix. The STF is placed and contained within the structure at the interface between the rod and the matrix (4), where it is exposed to shear when the entire structure is loaded. It is important that the STF is placed in a strategic configuration or location where a high enough shear rate to activate, i.e. thicken, the STF can be achieved. Controllability of the properties of the STF is demonstrated in Figure 1. According to the present invention the frequency and magnitude of excitation when the structure interacts with an external dynamic force are used to control the state of the STF inside the structure to change the stiffness and damping characteristics of the structure. The invention is thus a method to integrate STF in a structure that will exhibit desired properties and functions as a result of a controlled structure on the micro and macroscale which changes the properties of the STF when the structure interacts with an external dynamic force. The method can be applied to a variety of composite structures as depicted hereafter.

This method is applied as shown in Figure 3 to a structure composed of a host structure (5) with cavities distributed equally over its width (6), ground into one of its surfaces. Each cavity contains a rod (7) that runs through its length, and is glued into the structure at one end (8). At the other end, each rod enters a hole in which it can move freely, and is kept at a constant distance from the surrounding structure, using spacers (9). The STF is placed at the interface between the rods and the structure, respectively. For the test configuration used to measure the structure's dynamic properties, the entire structure is firmly clamped at one end (10), while different combinations of tip displacement and frequency can be applied at the structure's free end.

Another structure where the method is applied is shown schematically in Figure 5, and is based on impregnating different fabrics/textiles with STF. It is important to place them inside the structure at strategic location where high local shear strains occur at the interface between the fibre bundles when the fabric/textile is strained. This is a known phenomenon that is crucial in the context of woven composites processing. Thus, when impregnating a fabric with STFs, the latter thickens when high enough strains are locally reached. A different application based on this concept is already used in the design of body armour using Kevlar® mats [5]. The latter is based on fibre-pullout, i.e. extremely high shear rates are locally reached within very short time when the impregnated fabric is impacted or stabbed. However, this type of application does not require a reversible deformation of the fabric, contrary to our application. The sample structure in Figure 5 contains a STF impregnated fabric. More specifically, it is composed of a structure (11) with a cavity ground into one of the structure surfaces (12). The textile fabric (13) is glued at both extremities of the cavity (14), while the longitudinal boundaries are coated with silicone (15). The cavity containing the fabric is then closed by gluing a layer onto it. Since the fabric is placed off the structure's neutral axis, and on the side subjected to tension when bending the structure, the weave is stretched, provoking shear between the fibre bundles. When unloading the structure, the silicone borders serve to (i) avoid pull-out of single bundles and (ii) guarantee an instantaneous re-positioning of the weave to its initial position. The method is as well applied to integrate STF on the microscale at the fibre-matrix interface of any long and/or short fibre composite. Samples composed of a matrix (16), rods (17), and the STF placed at the interface (18) between matrix and rod or fibre is given in Figure 8. When such a structure is forced into a shear mode of deformation at low amplitudes, the STF remains in its low-viscosity state, i.e. very low storage modulus, having none or little effect on the structure's dynamic properties. However, when the shear amplitude is increased, the STF thickens, drastically increasing its viscosity and storage modulus and the stress transfer between matrix and rod, which translates in an increase of the stiffness and the damping of the structure. Several factors can be tuned to control the effective stiffness and damping properties of a structure with STF at the fibre-matrix interface. These include among others the fibre volume fraction, the ratio of the matrix and fibre, the ratio of the fibre radius and interface thickness, the shear modulus of the matrix as well as the loading conditions and the vibration frequency. Such an element could be pre-processed and integrated as such into a greater structure, acting as a tuneable damping element.

In a similar fashion it is proposed that the method can be applied to integrate STF as shown in Figure 11 and Figure 12. STF is integrated in a bundle of several straight fibres/rods/wires (20), which could be coated with a protecting layer to contain the STF therein, and thereafter embedded in a matrix (19) to obtain a composite, or in a fibre composite to form a hybrid composite material. The principle of activating the STF is the same as before, i.e. shear stresses will be transferred from the matrix to and at the interfaces (21) between the fibres/rods/wires at the same time as the energy is dissipated when the STF thickens, which will lead to a change of the stiffness and damping behaviour of the material. This configuration allows for a specific tailoring of the surface area at the interfaces, which increases with the number of fibres or rods or wires, and the shear strain which will depend on the spacing between the fibres or rods or wires. An extension of this proposed way of integrating STF is to use twisted fibre/wire bundles as shown in Figure 12. Moreover, the STF can be integrated at the interface (24) between the twisted fibre/wire bundles (23) and then embedded in a matrix or composite (22) It is believed that the twist of the fibre/wire bundles can further increase the damping capacity, as they can be designed to optimize the shear due to certain loading conditions but in particular for flexural mode of flexible composites. Thus material design factors like fibre/wire geometry, count, size and stiffness can be varied in combination with the STF type, the orientation in the composite structure to create elements with tailored stiffens and damping properties. The method can also be used to create an interface of STF by creating a material with a two- phase morphology, where one phase is the liquid and the other phase is a solid material. It is therefore proposed that STF can be integrated in a matrix to form a two-phase structure that can be reinforced by any type of long and/or short fibres. The idea is to integrate the STF in a matrix material in a way that a two-phase morphology is created. For instance, similar methods that are used to modify epoxy resins by liquid rubbers to increase their toughness could be utilised. The carrier fluid of the STF can be modified in a way to form a suspension with a resin, that on curing leads to a liquid STF phase in a solid matrix. An idea is to use ultraviolet (UV) curable polymers as the matrix. It must be made sure that the polymerisation reaction propagates by free radicals, which should not lead to a polymerisation of a STF with a carrier fluid that does not contain any double bonds. Moreover, UV curing is a extremely fast crosslinking method and depending on the light intensity the gel point of the material, i.e. the transition from liquid to solid, can be around seconds or even tenth of a second. It is expected that as the curing proceeds phase separation occurs at some stage, leading to the formation of a two-phase morphology. In thermoset, phase separations usually generates a morphology of spherical rubber particles in the continuous phase, which leads to a toughening mechanism due to cavitation of the rubber particles followed by void growth and consequent shear yielding of the matrix. For our purpose the STF phase, interfaced inside the matrix, will act as centre for dissipation of mechanical energy by shear deformation of the cavity in which the STF is contained, which will form the interface that allows control of the STF properties. The matrix can be reinforced by different fibres and the degree of shear deformation can be influenced by judicious arrangement of the fibres, e.g. in cross ply configuration with an angle that maximises the shear. Hence, again the dynamic behaviour in terms of stiffness and damping will be modified and controlled, but here other properties such as toughness and impact resistance might be improved simultaneously.

The method considers as well the integration of STF into porous or cellular structures, such as foams or cellular composites where the deformation of the structure induces relative displacements of micro-elements such as cell walls promoting the activation of the STF. MATERIAL SYSTEMS

In all examples presented in the following sections, the STF was composed of 67.75 %w/w monodisperse silica particles (spherical, diameter of 500 nm) in polyethylene glycol (PEG) with an average molar mass of 200 g/mol. The proposed methods can be used with other STF composition, providing other ranges of stiffness and damping properties to the final products. Moving elements such as the mentioned pistons, fabrics or foam can be of any material type.

Example 1 - Design based on STF at the piston-matrix interface

The method is used to integrate STF as schematically presented in Figure 3. The PMMA host structure has three cavities distributed equally over the beam' s width, Each cavity contains a glass-epoxy rod that runs through its entire length, and is glued into the structure at one end. At the other end, each rod enters a hole (total length of 4 cm) in which it can move freely, and is kept at a constant distance from the surrounding matrix, using PTFE (poytetrafluoroethylene) spacers. The STF is placed at the interface between the rods and the matrix, respectively. For the test configuration used to measure the structure's dynamic properties, the entire beam is firmly clamped at one end of the structure, while different combinations of tip displacement and frequency can be applied at the structure's free end. This method is widely known as vibrating beam testing (VBT) [6, 7].

Compliance vs. frequency curves are used to measure a structure's dynamic behaviour. Here, the applied displacement at the tip of the structure was varied from one measurement to the next, in order to vary the strain at the interface between the rods and the matrix. Without any STF integrated into the structure, the dynamic response of the structure was independent of the applied tip displacement (Figure 4a). In the case where an STF was placed at the rod- matrix, the peak intensity continuously diminished, while the resonance frequency of the structure (indicated by the peaks in Figure 4b) decreased. The combination of these phenomena translate into an increase in structural damping [8, 9]. Using the half -power bandwidth of the amplitude response to estimate the damping, the structure loss factor more than doubled when the tip displacement was increased from 0.2 mm (structure loss factor ξ = 0.026) to 4 mm (ξ= 0.055). Example 2 - Design based on STF-impregnated fabric

The method based on the STF impregnating a fabric as shown schematically in Figure 5. A woven fabric( ±45° glass fibre weave) impregnated with STF was placed into the cavity of 200 x 32 x 0.5 mm³ beam made of PPMA. The plain weave had a thickness of 250 μm, and was glued at both extremities of the cavity, while the longitudinal boundaries were coated with silicone. The cavity containing the weave was then closed by gluing a PMMA layer onto it. The test configuration used to measure the structure's dynamic properties implied that the entire beam was firmly clamped at one end of the structure..

Figure 6 shows compliance vs. frequency plots for various applied tip displacements for the dry fabric (Figure 6a), and the STF impregnated fabric (Figure 6b), respectively, using identical scales in both cases for easy comparison. Once more, the sample devoid of STF showed a dynamic behaviour that was independent of the applied tip displacement, while results for the flexural compliance varied significantly with the applied tip displacement when STF was added to the structure. Again, these results show that the structural damping significantly increases when the tip displacement is increased.

Figure 7 shows the damping increase for Example 1 and 2, respectively, when varying the applied tip displacement. Both are compared with the case where no STF is used at the piston- matrix and the bundle-bundle interface, respectively.

Example 3 - Design based on STF at fibre-matrix interface

A third example of the method comprises the matrix (16) and the fibres or rods (17), while the STF is placed at the fibre-matrix interface (Figure 8). A soft matrix composite sample was used in this example, to guarantee high enough shear strains at the matrix-fibre interface with the force amplitudes attainable with the experimental setup. Thus, the samples were composed of a silicone matrix (Elastosil M4470 from Wacker Inc.) and glass fibre-epoxy rods (Swiss Composites) of a diameter of 2 mm, and stiff glass fibre-epoxy rods were used as the continuous phase. The dynamic behaviour of both the matrix material and the composite with a STF at the rod-matrix interface was investigated. Dynamic mechanical data were obtained with a DMA Q800 from TA Instruments, using (i) the shear sandwich mode, and (ii) the torsion mode at various frequencies and force amplitudes. The experimental DMA results of the loss factor, tan<5^", vs. frequency,/, are shown in Figure 9 for the shear mode, and in Figure 10 for the torsion mode. According to the experimental results (Figure 9), tan £ exhibited a strong dependence on both the force amplitude, F_a, and the frequency. The peaks observed at low frequency for the high force amplitude curves suggested that the STFs had thickened and thus significantly affected the overall damping of the composites, momentarily increasing the tan<5Of the composite by a factor of 10 compared to the results obtained at low force amplitudes. A similar behaviour was observed for the torsion mode Figure 10.

REFERENCES

1. Boiler, C, Encyclopedia of materials, in Science and Technology E. S. Ltd, Editor.

2001. p. 1376-1382. 2. Scherrer, P., Bidaux, J. -E., Kim, A., Manson, J. -A. E., Gottardt R., Passive vibration damping in an alpine ski by integration of shape memory alloys. Journal de Physique

IV, 1999. 9: p. 393-400. 3. Nelson, LJ. , Smart piezoelectric fibre composites. Materials Science and Technology,

2002. 18(11): p. 1245-1256. 4. Vangipuram, P. and N. Ganesan, Buckling and vibration of rectangular composite viscoelastic sandwich plates under thermal loads. Composite Structures, 2007. 77(4): p. 419-429. 5. Wagner, N. and E. Wetzel, Advanced body armor utilizing shear thickening fluids .

2004: USA. 6. Oborn, J., H. Bertilsson, and M. Rigdahl, Damping properties of a mica-filled latex

IPN structure and applications in constrained-layer damping. Science and

Engineering of Composite Materials, 1999. 8(6): p. 327-342.

7. Oborn, J., H. Bertilsson, and M. Rigdahl, Styrene-ethylene/butylene-styrene blends for improved constrained-layer damping. Journal of Applied Polymer Science, 2001. 80(14): p. 2865-2876.

8. Balachandran, B. and E.B. Magrab, Vibrations. 2004, Pacific Grove: Brooks/Cole. 582.

9. Clough, R.W. and J. Penzien, Dynamics of structures. 2. ed. 2002, New York etc.: McGraw-Hill. 738.

Claims

1. Deformable composite structure made of at least two elements and at least one STF element, said STF element being located at least at the interface of said two elements.

2. Structure according to claim 1 wherein one of said two elements comprises a fibre or rod.

3. Structure according to claim 2 wherein said one element comprises a bundle of fibres or rods.

4. Structure according to claim 3 wherein said bundle is twisted.

5. Structure according to claim 3 or 4 wherein the fibres or rods are at least partially surrounded by STF.

6. Structure according to one of the previous claims wherein one of said two elements comprises a textile fabric.

7. Structure according to the previous claim wherein the textile fabric is impregnated with STF.

8. Structure according to one of the previous claims wherein the STF element is made of a two-phase morphology material, where one phase is a liquid and the other phase a solid.

9. Structure according to one of the previous claims wherein one said two elements comprises a cavity.

10. Structure according to the previous claim comprising several deformable cavities.

11. Structure according to one of the previous claims wherein said two elements have a relative displacement.

12. Structure according to the previous claim comprising gliding pistons incorporated into a surrounding structure.

13. Structure according to one of the previous claims wherein said STF is composed of

67.75 % w/w monodisperse silica spherical particles of 500 nm diameter in polyethylene glycol with an average molar mass of 200 g/mol.

14. Use of a STF element in a deformable composite structure, as defined in one of the previous claims, as a structural component to control the overall mechanical behaviour of said composite structure.

15. Use of a STF element according to the previous claim for the control of the dynamic behaviour of said structure, in particular its vibration amplitude and frequency.

16. Use of a STF element according to claim 14 or 15for increasing simultaneously stiffness and damping properties of said structure.