Generalized tessellations of superellipitcal voids in low porosity architected materials for stress mitigation

Stress concentration is a crucial source of mechanical failure in structural elements, especially those embedding voids. This paper examines periodic porous materials with porosity lower than 5%. We investigate their stress distribution under planar multiaxial loading, and presents a family of geometrically optimized void shapes for stress mitigation. We adopt a generalized description for both void geometry and planar tessellation patterns that can handle single and multiple voids of arbitrary void shape at a generic angle. The role of void shape evolution from diamond to rectellipse on the stress-distribution is captured at the edge of voids in a representative volume element (RVE) made of non-equal length periodic vectors. Theoretical derivations, numerical simulations along with experimental validation of the strain field in thermoplastic polymer samples fabricated by laser cutting unveil the role of geometric parameters, e.g. superellipse order, aspect ratio and rotation angle, that minimize stress peak and ameliorate stress distribution around voids. This work extends and complements classical theory by providing fundamental insights into the role that tessellation, void shape and inclination play in the stress distribution of low-porosity architected materials, thus introducing essential guidelines of broad application for stress-minimization and failure mitigation in diverse sectors.

Water uptake in epoxy matrix with voids: Experiments and modeling

The influence of voids on the moisture uptake of epoxy matrix has been studied. Specimens with void contents from 0 to about 50% were prepared. Void geometry and content were analyzed using microscopy and density methods. Void-containing samples were characterized by differential scanning calorimetry and dynamic mechanical analysis which verified consistency of chemistry. The moisture uptake of specimens immersed in distilled water at 40℃ was monitored. The rate of absorption and saturation moisture content increased with increasing void content. Mass balance calculations revealed that only 6–8% of the void volume is occupied by water at saturation. The moisture uptake of void-free and void-containing specimens was non-Fickian. The Langmuir model provided better fits to the experimental results, although the high void content specimens showed substantial deviations from the Langmuir diffusion model. The moisture diffusivity of the void-free and void-containing specimens agreed reasonably with the Maxwell inclusion model.

Prediction of the yielding behaviour of ductile porous materials through computational homogenization

Purpose The purpose of this paper is to predict the yield locus of porous ductile materials, evaluate the impact of void geometry and compare the computational results with existing analytical models. Design/methodology/approach A computational homogenization strategy for the definition of the elasto-plastic transition is proposed. Representative volume elements (RVEs) containing single-centred ellipsoidal voids are analysed using three-dimensional finite element models under the geometrically non-linear hypothesis of finite strains. Yield curves are obtained by means of systematic analysis of RVEs considering different kinematical models: linear boundary displacements (upper bound), boundary displacement fluctuation periodicity and uniform boundary traction (lower bound). Findings The influence of void geometry is captured and the reduction in the material strength is observed. Analytical models usually overestimate the impact of void geometry on the yield locus. Originality/value This paper proposes an alternative criterion for porous ductile materials and assesses the accuracy of analytical models through the simulation of three-dimensional finite element models under geometrically non-linear hypothesis.