Dynamic properties of sandwich structures with integrated shear-thickening fluids

The integration of shear-thickening fluids (STFs) into composite structures has been investigated with the aim of tuning part stiffness and damping capacity under dynamic deformation. Results from oscillatory rheological measurements for a STF based on concentrated fused silica in polypropylene glycol were correlated with results from vibrating beam tests on model sandwich structures containing layers of the same STF sandwiched between polyvinyl chloride (PVC) beams. Above a critical amplitude, the relative motion of the PVC beams provoked shear thickening of the silica suspensions, and the vibration and damping properties were significantly modified. These changes were related to the rheological response of the STF through analytical calculations of strains in the STF layers, an approach that was verified experimentally by replacing the STF with a slow-curing epoxy resin. The potential for integrating STFs into structures exposed to dynamic flexural deformation, with the aim of controlling their vibrational response, has thus been demonstrated.

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

Processing of Sustainable Cellular Biocomposites

Carole Boissard

Petroleum-based polymers and composites are commonly used in a wide variety of application fields. However ecological concerns, increasing oil prices and dwindling natural resources explain why nowadays industrial and public interest is more than ever focused on new bio-based – “greener”- products. In order to find viable substitutes for petroleum-based materials, major efforts are devoted to the development of sustainable solutions combining fully compostable matrices and fibres from renewable resources. Polylactide (or polylactic acid) (PLA) is currently one of the most versatile biodegradable polymers derived from renewable resources such as non-food crops of corn or sugar cane. Its transformation into foamed cellular structures would decrease its density and at the same time may improve its specific mechanical properties. This could provide cost-effective and sustainable products for packaging and construction. In the context of developing “green” foams, the focus in the present work has been on a solvent-free process based on physical foaming with supercritical carbon dioxide (ScCO2) as a foaming agent. This foaming process includes thermally driven but kinetically controlled phenomenon. Solubility and diffusion of the gas in the molten polymer, cell nucleation and growth, as well as stabilization of the final foam structures were studied for neat and composite materials. At low processing temperatures and low depressurization rates, homogeneous structures were obtained thanks to a balance between cell growth mechanisms and cell stabilization, thus limiting uncontrolled cell coalescence. The density of the resulting polylactide foams ranged from 0.12 to 0.3 g/cm3 with corresponding compression moduli of 6 to 73 MPa respectively. The processing windows were strongly dependent on the rheological melt properties of the PLAs. PLA is generally considered to be a high strength, high modulus thermoplastic but its brittleness has limited its industrial use. PLA was therefore combined with wood fibres (WF) and microfibrillated cellulose (MFC) in order to improve its mechanical performance without compromising its environmental impact. Two compounding methods, a wet mixing papermaking-like process and a solvent casting process were investigated for the production of WF/PLA and MFC/PLA biocomposites. Degradation of PLA occured during wet-mixing, due to the water used as a dispersant, and to several heating steps applied during compounding. Improvement of the drying and processing conditions and/or the replacement of water by isopropanol limited this degradation. The presence of cellulose fibres considerably modified foaming mechanisms; gas solubility was decreased and the coefficient of diffusion increased, leading to lower quantities of CO2 available for foaming. In addition, stiffness of the fibre network impeded full expansion of the polymer matrix. Thus smaller cells were created. Densities from 0.15 (neat PLA) to 0.37 and 0.55 g/cm3 and moduli ranging from 35 (neat PLA) to 125.5 and 373 MPa were obtained by the addition of 5wt% WF and MFC respectively. Addition of a chain extender enabled viscosity of the matrix to be increased causing increased orientation of the fibre network during flow and hence fibre alignment in the cell walls. Density was thus decreased by 95 % to 0.017 g/cm3 and compression modulus decreased to 0.32 MPa for 5wt% WF composite. Cell wall shrinkage was observed in the neat PLA foams obtained with the chain extender. This was reduced by adding wood fibres while maintaining similar density and modulus. Cellulose fibres act as an extra scaffold in the cellular structures. Industries are currently considering the use of these materials for applications in packaging e.g. as cushioning materials, or in architecture as display panels, or as sandwich core materials for the replacement of expanded polystyrene, polyvinyl chloride or polyurethane foams.

EPFL2012

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