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1.
Behaviour of cellular metamaterials under impact loading conditions : doctoral disertation
Yunus Emre Yilmaz, 2024, doktorska disertacija

Opis: 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.
Ključne besede: Triply Periodic Minimal Surfaces, TPMS, Cell-size-grading, Impact, High-strainrate, Digital Image Correlation
Objavljeno v DKUM: 17.10.2024; Ogledov: 0; Prenosov: 24
.pdf Celotno besedilo (46,48 MB)

2.
Mechanical behaviour of photopolymer cell-size graded triply periodic minimal surface structures at different deformation rates
Yunus Emre Yilmaz, Nejc Novak, Oraib Al-Ketan, Hacer Irem Erten, Ulas Yaman, Anja Mauko, Matej Borovinšek, Miran Ulbin, Matej Vesenjak, Zoran Ren, 2024, izvirni znanstveni članek

Opis: 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.
Ključne besede: cellular materials, triply periodical minimal surface, photopolymer, mechanical properties, strain rate, experimental compressive testing, computer simulations
Objavljeno v DKUM: 22.05.2024; Ogledov: 216; Prenosov: 19
.pdf Celotno besedilo (9,33 MB)
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3.
Dynamic characterisation of novel three-dimensional axisymmetric chiral auxetic structure
Anja Mauko, Yunus Emre Yilmaz, Nejc Novak, Tomáš Doktor, Matej Vesenjak, Zoran Ren, 2024, izvirni znanstveni članek

Opis: 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.
Ključne besede: axisymmetric chiral structure, auxetic, chiral unit cell, impact testing, dynamic characterisation, finite element simulations
Objavljeno v DKUM: 15.02.2024; Ogledov: 345; Prenosov: 32
.pdf Celotno besedilo (7,73 MB)
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