Structural damping using encapsulated Shear Thickening Fluids

Smart structures with tunable damping and stiffness characteristics are of high interest to aerospace applications, but often require an external power source to be activated. This can be avoided by using highly concentrated silica suspensions, which exhibit a shear-thickening behavior, linked to a dramatic increase in viscous dissipation. These materials are however liquid at rest, and sensitive to humidity, so they are difficult to implement as such into structural applications. In the present work, highly concentrated solutions of monodisperse silica particles in PEG were selected for their strong thickening effect at rather low critical shear strain. Damping properties were characterized by measuring the energy dissipated per cycle at low frequency (

Shear Thickening Fluid Impregnated Foam Composites for Vibration Control and Impact Protection

Mathieu Soutrenon

Highly concentrated suspensions of silica particles present shear thickening properties, shifting from a low to a high viscosity state, instantly and reversibly at a critical shear rate. Their viscosity increases by up to 3 orders of magnitude, changing from a honey-like liquid to a brittle solid material. This sharp rise in viscosity is associated with a large absorption of energy that can be used in damping applications. These shear thickening fluids (STF) are among the few passive materials that can simultaneously bring both an increase in damping and in stiffness properties. In mechanical design, this leads potentially to a weight reduction of the overall system. This thesis work is aimed to develop and test a composite material solution for vibration control and impact protection, using STF as the damping element. A main challenge was to overcome the fact that STF are liquid at rest, and very sensitive to their environment. The proposed damping material was therefore constituted of foam impregnated with STF and encapsulated with silicone (STF/foam). A comparison of properties with other damping materials such as Smactane®was emphasized along the thesis. Storage and processing conditions of STF constituted of highly concentrated suspensions were shown to affect their rheological properties similarly to other sensitive parameters such as particle concentration. Rigorous processing methods were thus required to produce STF with defined and stable rheological properties. The use of sonication was preferred to disperse the aggregates of particles in concentrated colloidal suspensions due to the packing of the dry powder. Contact with air or humidity deteriorates the shear thickening properties of STF. Storage of STF in a freezer at −24 ◦C or under nitrogen prevents this. The use of a physical barrier to air and humidity such as silicone to protect STF is also shown to greatly reduce the ageing problem. The energy dissipated during a cyclic strain cycle was used as a measure of the damping property of the STF, and compared to that of other materials. It was measured with dynamic rheological measurements performed on suspensions with variations of particle concentration, size and test frequency. STF were shown to dissipate a large amount of energy during their transition, which increases with the increase of frequency and particle concentration and decrease with the increase of particle size. In specific cases of large strain amplitude, they potentially dissipate more energy than viscoelastic materials such as rubber. An open cell melamine resin foam was used as a lightweight scaffold to provide a three dimensional structure to the STF. The foam distributes the load and locally shears the STF. The shape of the foam is easily adjustable. To impregnate the foam with the STF, a system similar to vacuum infusion was used. The foam was placed under vacuum in a mold and the STF heated at 70 ◦C to reduce its viscosity and shear thickening properties was sucked in the foam with the depressurization. The STF/foam systems were then encapsulated in silicone to protect the STF/foam damping pads from contamination and avoid outgassing. During compression tests at low frequencies and large deformations, we demonstrated that STF could be activated in this type of structure, with mechanical properties comparable to those of a viscous honey-like fluid at rest and silicone when the STF is in the solid state. The same behavior was observed during impact solicitations. STF/foam presents a very low coefficient of restitution and absorbs the shock wave associated with impacts. Shock tests done separately confirm the shock absorption properties of STF/foam even if the STF is not activated. The combination of mechanical and damping properties of the foam/STF systems remain lower than for very high performance damping materials like Smactane®. The reasons are their low mechanical compression and shear properties at rest and the need to overcome a threshold strain or stress to activate the STF. However, the large dissimilarity between the states before and after the transition makes these materials interesting for impact or large strain amplitude applications such as sole for running shoes or several space structures.

EPFL2013

Adaptive composite structures for tailored human-materials interaction

Christian Fischer

Controlling the static and dynamic properties of sports equipment has since long been of great concern in the sports engineering industry. More recently there has been interest in better understanding and improving the interplay between such properties and the perception of the athlete using the equipment. The aim of this thesis was first to develop a new approach to identifying the corresponding requirements for a particular type of sports equipment. The tailoring of a novel material with the potential to fulfill these requirements, and the integration of this material into composite structures was then investigated. Alpine skiing was used as a case study for this thesis. During this first project phase, the importance of specific ski properties was identified. Perceived characteristics were linked to physically measurable properties, and the level of differences in mechanical properties discernable by skiers was examined. For this purpose, four pairs of slalom skis that differed in flexural rigidity by a maximum of 20 % and in torsional rigidity by 60 % were tested by advanced and expert skiers under specific snow conditions. Among other results, Spearman correlations showed that expert skiers considered a high torsional rigidity to have a positive effect on characteristics such as quietness at high speed or energy restitution in carved turns, and advanced skiers judged stiff skis in torsion to positively influence the grip during short turns. Furthermore, it was also shown that athletes are able to discern small differences in damping and vibration of the skis. These results demonstrated that a quantitative approach to perceived properties is possible, and that it should be considered for the future design of sports equipment. Stiffness and damping usually counter-balance one another. The need for high values of both properties, as for example in skis and other types of sports gear where shocks induce large deformations, presents a considerable challenge for materials scientists. Shear-thickening fluids (STFs) are exceptional in this perspective, because they reversibly increase their moduli by 2-3 orders of magnitude, and simultaneously increase their energy absorption, when a critical shear stress or strain is reached. The dynamic shear-thickening behaviour of two different types of STFs was investigated, with the aim of integrating them into dynamically loaded structures. Dynamic frequency sweeps showed that the complex viscosity at different imposed strain amplitudes followed a unique power-law type behaviour up to the onset of strain thickening. Similar behaviour was also observed in the post-transition state, i.e. the viscosities again superimposed at frequencies beyond the transition frequency. Furthermore, the shear-thickening effect considerably decreased with increasing frequency. Viscosity increased by 3 orders of magnitude at 0.1 rad/s, and by 1 order of magnitude at 100 rad/s, suggesting that shear-thickening would totally vanish at frequencies beyond those measurable by conventional methods. Depending on the composition of the STFs, they either thickened at a constant shear stress throughout the frequency domain (monodisperse STFs) or required a minimum shear strain to be activated at high frequencies (polydisperse STFs). Results from dynamic rheological measurements for the STFs were correlated with results from vibrating beam tests (VBT) on model sandwich structures containing layers of the same STF sandwiched between solid beams. At the highest STF concentration, continuously increasing the excitation amplitude led to a 9-fold increase of the damping of the 1st resonance frequency. Since the increased viscosity of the STF interlayer resulted in improved stress transfer between the stiff polymer layers, the flexural rigidity simultaneously increased by over 20 %. Also, it was shown that the extent of damping and stiffening of the structure could be tailored by varying the type of STF. Finally, methods to integrate STFs into composite structures at different scales have been developed. At macro-scale, a STF damping device showed that the latter could strongly affect the dynamic behaviour of a ski. At meso-scale, the most promising design was based on glass fibre-epoxy rods integrated into the structure such that they function as small pistons when the structure is bent. For a tailorable deformation and frequency domain, the STF at the rod-matrix interface thickened, increasing the stress transfer between the rods and the matrix, and thus leading to changes in the structure's dynamic response. Dynamic mechanical analysis of soft matrix composites, containing STF at the rod-matrix interface, demonstrated that STFs could be integrated at the micro-scale of composite materials, which was confirmed by a micro-mechanical model. In summary, this thesis has provided a tool to include human perception into the design of sports equipment, and has built up a broad knowledge of the dynamic behaviour of an adaptive material, which can be included into a material-structure-athlete loop with the aim of evaluating its potential for any specific application in sports where comfort, feel and control of the performance should be improved.

EPFL2007

Impact properties of shear thickening fluid impregnated foams

Véronique Michaud, Mathieu Soutrenon

Concentrated colloidal suspensions of silica particles in polyethylene glycol exhibit a shear thickening behavior: above a critical shear rate in a confined environment, they show a steep increase of viscosity. This reversible transition from a low to a high viscosity state is associated with a large energy absorption that could be harnessed for impact protection. As these suspensions are liquid at rest, however, shear thickening fluids (STFs) are difficult to use in practical applications. Furthermore, their specific rheological properties exist within a narrow range of concentration, so they tend to disappear when the material is in contact with air and humidity. In this work, a soft foam scaffold was impregnated with STF to provide a three-dimensional shape to the assembly at rest, while a silicone was cast around it to serve as a physical barrier to the external environment. A method to quickly impregnate the foam was proposed. Impact tests were carried out on the STF/foam/silicone composite pads using a free fall impact tower. Compared to rubber or pure silicone, larger energy absorptions, up to 85%, were observed, which could be repeated for multiple impacts. The transmitted shock waves were also reduced, showing the potential of this system for impact protection of structures.