This work analyses shear moduli of conventional and auxetic open-cell polymer foams. Shear moduli are i) measured directly and ii) calculated by applying elasticity theory for isotropic solid materials, using Young’s moduli and Poisson’s ratios from compression tests. Zero and negative Poisson’s ratio foams are fabricated from conventional foams using a thermo-mechanical process. Fabricated and conventional foams are compression tested in three orthogonal directions, up to densification at ~60% compression, with full-field strain measure- ments obtained using Digital Image Correlation. Compression testing is followed by shear testing. The measured shear moduli vary from 16±7 kPa for negative Poisson’s ratio foams to 38±2 kPa for zero Poisson’s ratio foams, with conventional foams in between with a mean value of 32±8 kPa. The calculated shear moduli are typically lower than the measured values. The results suggest that the application of elasticity theory to calculate the low strain shear modulus of open-cell foam from Young’s modulus and Poisson’s ratio measured in compression tests is appropriate if the foam is isotropic.
COBISS.SI-ID: 55450627
The auxetic cellular structures with two different chiral geometries (tetrachiral and hexachiral) were fabricated using laser powder bed fusion (LPBF) technology from two different materials (aluminium alloy AlSi10Mg and stainless steel AISI 316L). The specimen's microstructure, porosity and surface texture were observed by X-ray diffraction, optical and electron microscopy and micro-computed tomography. The mechanical properties of all specimens were determined experimentally with a simple tensile test. The results show that hexachiral and tetrachiral auxetic structures have similar initial stiffness, whereas the former provides a more ductile response with more than four times larger failure strain. The AISI316L steel provides for a much more ductile response than AlSi10Mg at comparable normalised load-carrying capability. The samples' deformation behaviour was analysed with the digital image correlation and tracking method, where it was shown that hexachiral samples exhibit a lower Poisson's ratio. The experimental results were used to validate the corresponding computational models, providing a more detailed analysis of deformation behaviour. They allow for cost-effective parametric studies and the development of new optimised chiral geometries.
COBISS.SI-ID: 62109187