Unusual volumetric behavior of low-density open-porous materials

Modeling open-porous materials, such as aerogels, requires an accurate computational representation of their microstructure. The latter is typically characterized by a highly complex morphology with strongly varying pore sizes and shapes. To capture these features, a three-dimensional Laguerre-Voronoi tessellation informed by a preceding random closed packing of polydisperse spheres is applied. The resulting interconnected microstructure is discretized using beam elements within a representative volume element, which is subsequently analyzed under periodic boundary conditions and compression loading [1]. However, some of the obtained results exhibit unusual and counterintuitive characteristics. In particular, the volume of the representative volume element shows almost no increase, even under uniaxial or pressure-free tension loading. Such behavior differs from the monotonic volumetric expansion commonly assumed for compressible materials and highlights the strong coupling between deviatoric and volumetric deformation in the underlying open-porous microstructure. A noticeable increase in volume is observed only under triaxial loading conditions, such as hydrostatic tension, which is consistent with the stability requirement associated with a positive definiteness of the tangent bulk modulus. From an experimental perspective, however, realizing such hydrostatic tensile states is extremely challenging, if not impossible. In the case of open-porous materials, neither reduced air pressure nor vacuum conditions are suitable for generating hydrostatic tension due to the interconnected open-pore network. Consequently, these materials can generally be characterized by an overall volume-decreasing deformation behavior.