THE Edinburgh discussion of the Faraday Society on September 24-26 on the subject of “The Structure of Liquids and Solutions” was a particularly successful and satisfactory conference. The choice of subject had been very happily timed. The theory of the structure of liquids, which has been in recent years approached from a number of different angles, appears now to have reached a stage in which a larger synthesis is possible, and the bringing together in Edinburgh of many of those who have most notably contributed to these theories has markedly helped towards such a synthesis.

Structure of Liquids

Triply periodic minimal surface (TPMS) shell lattices are attracting increasingly attention due to their unique combination of geometric and mechanical properties, and their open-cell topology. However, uniform thickness TPMS shell lattices are usually anisotropic in stiffness, namely having different Young’s moduli along different lattice directions. To reduce the elastic anisotropy, we propose a family of variable thickness TPMS shell lattices with isotropic stiffness designed by a strain energy-based optimization algorithm. The optimization results show that all the six selected types of TPMS lattices can be made to achieve isotropic stiffness by varying the shell thickness, among which N14 can maintain over 90% of the Hashin-Shtrikman upper bound of bulk modulus. All the optimized shell lattices exhibit superior stiffness properties and significantly outperform elastically-isotropic truss lattices of equal relative densities. Both uniform and optimized types of N14 shell lattices along [100], [110] and [111] directions are fabricated by the micro laser powder bed fusion techniques with stainless steel 316 L and tested under quasi-static compression loads. Experimental results show that the elastic anisotropy of the optimized N14 lattices is reduced compared to that of the uniform ones. Large deformation compression results reveal different failure deformation behaviors along different directions. The [100] direction shows a layer-by-layer plastic buckling failure mode, while the failures along [110] and [111] directions are related to the shear deformation. The optimized N14 lattices possess a reduced anisotropy of plateau stresses and can even attain nearly isotropic energy absorption capacity.

Elastically-isotropic open-cell minimal surface shell lattices with superior stiffness via variable thickness design

Cellular materials, such as foams, can be used as load bearing members in civil construction and as protective energy absorbing structures for personnel and equipment. In the present study, novel lightweight closed-cell structures were designed, and their mechanical properties and collapse mechanisms were investigated through a combination of experimental validation and finite element (FE) simulations. Selected porous structure designs were manufactured from acrylonitrile butadiene styrene (ABS) using additive manufacturing technology. These 3D printed structures were subjected to quasi-static loading to determine the dependence of their elastic and plastic responses from their topological features. Deformation mechanisms were elucidated through quasi-static compression experiments and FE modelling. The appropriate distribution of the base material in the designed closed-cell structures inherits the merits of uniform stress distribution and large deformations that lead to reaching high strengths and desirable energy absorption efficiencies. The effects of relative density and cell shape were studied in detail from elastic loading through the large plastic strain densification regions. The effects of cellular architecture on deformation mechanisms and energy absorption capabilities demonstrated the possibility of enhancing energy absorption efﬁciencies with appropriate design criteria. Based on the experimental and numerical analyses, the most efficient energy absorbing closed-cell structure was proposed.

Novel design of closed-cell foam structures for property enhancement

Randomly packing spheres of equal size into a container consistently results in a static configuration with a density of ∼64%. The ubiquity of random close packing (RCP) rather than the optimal crystalline array at 74% begs the question of the physical law behind this empirically deduced state. Indeed, there is no signature of any macroscopic quantity with a discontinuity associated with the observed packing limit. Here we show that RCP can be interpreted as a manifestation of a thermodynamic singularity, which defines it as the “freezing point” in a first-order phase transition between ordered and disordered packing phases. Despite the athermal nature of granular matter, we show the thermodynamic character of the transition in that it is accompanied by sharp discontinuities in volume and entropy. This occurs at a critical compactivity, which is the intensive variable that plays the role of temperature in granular matter. Our results predict the experimental conditions necessary for the formation of a jammed crystal by calculating an analogue of the “entropy of fusion”. This approach is useful since it maps out-of-equilibrium problems in complex systems onto simpler established frameworks in statistical mechanics.

/pdf/a-first-order-phase-transition-defines-the-random-close-3bse3jweg3.pdf

A first-order phase transition defines the random close packing of hard spheres

Elastically isotropic open-cell uniform thickness shell lattices with optimized elastic moduli via shape optimization

Recent advances in the cataloguing of three-dimensional nets mean a systematic search for framework structures with specific properties is now feasible. Theoretical arguments about the elastic deformation of frameworks suggest characteristics of mechanically isotropic networks. We explore these concepts on both isotropic and anisotropic networks by manufacturing porous elastomers with three different periodic net geometries. The blocks of patterned elastomers are subjected to a range of mechanical tests to determine the dependence of elastic moduli on geometric and topological parameters. We report results from axial compression experiments, three-dimensional X-ray computed tomography imaging and image-based finite-element simulations of elastic properties of framework-patterned elastomers.

/pdf/experimental-investigation-of-the-mechanical-stiffness-of-2kxvwwjmxi.pdf

Experimental investigation of the mechanical stiffness of periodic framework-patterned elastomers

/pdf/elastomersof-periodic-framework-patterned-experimental-411q23h1fx.pdf

elastomersof periodic framework-patterned Experimental investigation of the mechanical

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Experimental investigation of the mechanical stiffness of periodic framework-patterned elastomers

elastomersof periodic framework-patterned Experimental investigation of the mechanical