1. Introducing auxetic behavior to syntactic foamsNejc Novak, Miha Kolar, Nima Movahedi, Matej Vesenjak, Zoran Ren, Thomas Fiedler, 2020, original scientific article Abstract: This paper proposes an innovative multi-material approach for introducing auxetic behaviour to syntactic foams (SFs). By carefully designing the size, shape, and orientation of the SFs, auxetic deformation behaviour was induced. Re-entrant hexagon-shaped SF elements were fabricated using expanded perlite (EP) particles and a plaster of Paris slurry first. Then, an auxetic pattern of these SF elements was arranged within a stainless-steel casting box. The empty spaces between the SF elements were filled with molten aluminium alloy (A356) using the counter-gravity infiltration casting technique. The cast auxetic composite had a bulk density of 1.52 g/cm3. The cast composite was then compressed under quasi-static loading to characterise its deformation behaviour and to determine the mechanical properties, especially the Poisson’s ratio. The cast composite deformation was auxetic with a Poisson’s ratio of −1.04. Finite Element (FE) simulations were conducted to understand the deformation mechanism better and provide means for further optimisation of the geometry.
Keywords: auxetic cellular structure, syntactic foams, experimental tests, mechanical properties, Poisson’s ratio
Published in DKUM: 28.03.2024; Views: 222; Downloads: 24
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2. Computer Modelling of Porous Composite Structures with Advanced Pore MorphologyAljaž Kovačič, 2016, doctoral dissertation Abstract: Advanced pore morphology (APM) structures are composite metal foams, which are assembled from a large number of small spherical elements with cellular structure, and are bonded into a composite with polymeric adhesive. The result of such composition is a wide spectrum of achievable mechanical behaviour in APM structures. To explore their full potential, efficient computational models are needed, which allow for simple parameter variation. Unfortunately, the current computer models do not allow for efficient simulations of porous composite structures with advanced pore morphology, as they employ complex discretisation approaches.
A new approach to simulation is presented in this work, based on the discrete particle method (DPM), where every element of APM structure is discretised with a single node. This enables more efficient simulations of APM structures, while still allowing for simple variation of structural parameters. The DPM method was augmented with constitutive models of normal and tangential contact behaviour of APM elements and bonds between them, which were formulated based on an extensive experimental study of APM structure's geometry and mechanical behaviour. Consequently, the models enable simulations of large APM structure's behaviour by modelling the contact behaviour of individual elements.
The implementation of new models was verified on a set of analytically solvable examples, and the accuracy of the models was validated with very good correspondence between computational and experimental results. Moreover, the models were validated on a wide set of examples, also taking into account the various strain rates and the absence of the bonds.
The applicability of new models was demonstrated in a comprehensive parametrical study, where the influential structural parameters and properties were identified for low and high strain rate deformations. The study also demonstrated the possibility of customising the mechanical behaviour with property gradation, and with introduction of regular, as well as geometrically complex APM element assemblies. The possibility of coupled discrete particle method and finite element method simulations was also addressed.
The newly developed models represent a breakthrough in the field of computational investigation of APM structures, and provide for simpler and more efficient investigations of APM structures in the future.
Keywords: Metal foams, advanced pore morphology, composite materials, mechanical properties, contact modelling, discrete particle methods, computer simulations
Published in DKUM: 11.03.2016; Views: 2635; Downloads: 245
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3. Aryl acrylate based high-internal-phase emulsions as precursors for reactive monolithic polymer supportsPeter Krajnc, Dejan Štefanec, Jane F. Brown, Neil R. Cameron, 2005, original scientific article Abstract: Water-in-oil high-internal-phase emulsions (HIPEs), containing 4-nitrophenyl acrylate and 2,4,6-trichlorophenyl acrylate as reactive monomers, were prepared and polymerized, and highly porous monolithic materials resulted. The novel materials were studied by combustion analysis, Fourier transform infrared spectroscopy scanning electron microscopy, mercury porosimetry, and N2 adsorption/desorption analysis. With both esters, cellular macroporous monolithic polymers were obtained: the use of 4-nitrophenyl acrylate resulted in a cellular material with void diameters between 3 and 7 m and approximately3-m interconnects, whereas the use of 2,4,6-trichlorophenyl acrylate yielded a foam with void diameters between 2 and 5 m, most interconnects being around 1 m. The resulting monoliths proved to be very reactive toward nucleophiles, and possibilities of functionalizing the novel polymer supports were demonstrated via reactions with amines bearing additional functional groups and via the synthesis of an acid chloride derivative. Tris(hydroxymethyl)aminomethane and tris(2-aminoethyl)amine derivatives were obtained. The hydrolysis of 4-nitrophenylacrylate removed thenitrophenyl group, yielding a monolithic acrylic acid polymer. Furthermore,functionalization to immobilized acid chloride was performed very efficiently, with more than 95% of the acid groups reacting. The measurement of the nitrogen content in 4-nitrophenyl acrylate poly(HIPE)s after various times of hydrolysis showed the influence of the total pore volume of the monolithic polymers on the velocity of the reaction, which was faster with themore porous polymer.
Keywords: organic chemistry, macroporous polymers, monolithic polymer supports, emulsion polymerisation, foams, functionalization of polymers, high-internal-phase emulsions
Published in DKUM: 01.06.2012; Views: 2329; Downloads: 98
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