1. Behaviour of cellular metamaterials under impact loading conditions : doctoral disertationYunus Emre Yilmaz, 2024, doctoral dissertation Abstract: This doctoral thesis investigates cell-size-graded metallic and non-metallic Triply Periodic
Minimal Surface (TPMS) structures' behavior under varying loading rates. Using experimental
tests, analytical calculations, and advanced computer simulations, the research explores the
interplay between material properties, cell size grading, and deformation mechanisms under
different strain rates. The study focuses on enhancing the Direct Impact Hopkinson Bar (DIHB)
setup for accurate force and displacement measurements and pioneering a method for
quantifying inertial forces, critical at high strain rates.
Key findings show that cell-size grading significantly affects deformation patterns, with initial
deformation occurring in regions with smaller and lower stiffness cells across different loading
rates and TPMS geometries. The research also highlights topology's influence on mechanical
response, with photopolymer-based diamond structures showing superior energy absorption
and gas-atomized steel structures favoring gyroid configurations. This underscores the
importance of considering both topology and base material selection during TPMS design. The
study demonstrates the increasing prominence of inertial forces as deformation rates rise,
impacting structural response and failure likelihood in TPMS structures.
These insights inform the design of optimized cellular metamaterials for high-performance
applications requiring superior energy absorption and structural integrity under high loading
rates. The research advances material characterization techniques and computational
modelling capabilities, contributing to the development of next-generation cellular
metamaterials for broader engineering applications.
Keywords: Triply Periodic Minimal Surfaces, TPMS, Cell-size-grading, Impact, High-strainrate, Digital Image Correlation
Published in DKUM: 17.10.2024; Views: 0; Downloads: 16
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2. Mechanical behaviour of photopolymer cell-size graded triply periodic minimal surface structures at different deformation ratesYunus Emre Yilmaz, Nejc Novak, Oraib Al-Ketan, Hacer Irem Erten, Ulas Yaman, Anja Mauko, Matej Borovinšek, Miran Ulbin, Matej Vesenjak, Zoran Ren, 2024, original scientific article Abstract: This study investigates how varying cell size affects the mechanical behaviour of photopolymer Triply Periodic Minimal Surfaces (TPMS) under different deformation rates. Diamond, Gyroid, and Primitive TPMS structures with spatially graded cell sizes were tested. Quasi-static experiments measured boundary forces, representing material behaviour, inertia, and deformation mechanisms. Separate studies explored the base material’s behaviour and its response to strain rate, revealing a strength increase with rising strain rate. Ten compression tests identified a critical strain rate of 0.7 s−1 for “Grey Pro” material, indicating a shift in failure susceptibility. X-ray tomography, camera recording, and image correlation techniques observed cell connectivity and non-uniform deformation in TPMS structures. Regions exceeding the critical rate fractured earlier. In Primitive structures, stiffness differences caused collapse after densification of smaller cells at lower rates. The study found increasing collapse initiation stress, plateau stress, densification strain, and specific energy absorption with higher deformation rates below the critical rate for all TPMS structures. However, cell-size graded Primitive structures showed a significant reduction in plateau and specific energy absorption at a 500 mm/min rate.
Keywords: cellular materials, triply periodical minimal surface, photopolymer, mechanical properties, strain rate, experimental compressive testing, computer simulations
Published in DKUM: 22.05.2024; Views: 216; Downloads: 15
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3. Dynamic characterisation of novel three-dimensional axisymmetric chiral auxetic structureAnja Mauko, Yunus Emre Yilmaz, Nejc Novak, Tomáš Doktor, Matej Vesenjak, Zoran Ren, 2024, original scientific article Abstract: The study presents an extensive mechanical and computational characterisation of novel cellular metamaterial with axisymmetric chiral structure (ACS) at different strain rates. The Direct Impact Hopkinson Bar (DIHB) testing device was used for impact testing up to 21 m/s striker speed, which was insufficient to reach the shock deformation regime. Thus, using computational simulations to estimate the structure behaviour at high strain rates was necessary. Experimental and computational results showed that all ACS structures exhibit a nominal stress–strain relationship typical for cellular materials. As the loading conditions shifted to a dynamic regime, the micro–inertia effect became increasingly pronounced, leading to a corresponding rise in structure stiffness. The Poisson's ratio in all ACS increases gradually, making them superior to traditional cellular materials, which experience a sudden increase in Poisson's ratio during loading. Additionally, the study found that the structures exhibited a rise in the auxetic effect with an increase in strain rate, highlighting the benefits of axisymmetric structures in high-loading regimes. Overall, the obtained results provide valuable insights into the mechanical properties of ACS under different loading regimes and will contribute to further design improvements and the fabrication of novel ACS metamaterials.
Keywords: axisymmetric chiral structure, auxetic, chiral unit cell, impact testing, dynamic characterisation, finite element simulations
Published in DKUM: 15.02.2024; Views: 345; Downloads: 31
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