Yarı sekizgen girintili yapıların mekanik özelliklerinin parametrik olarak incelenmesi
Parametric investigation of the mechanical properties of semi-octagonal re-entrant structures
- Tez No: 979916
- Danışmanlar: DR. ÖĞR. ÜYESİ KAAN YILDIZ
- Tez Türü: Yüksek Lisans
- Konular: Savunma ve Savunma Teknolojileri, Defense and Defense Technologies
- Anahtar Kelimeler: Darbe dayanımı, Enerji sönümleme, Mekanik özellikler, Savunma endüstrisi, Impact strength, Energy damping, Mechanical properties, Defence industry
- Yıl: 2025
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Lisansüstü Eğitim Enstitüsü
- Ana Bilim Dalı: Savunma Teknolojileri Ana Bilim Dalı
- Bilim Dalı: Savunma Teknolojileri Bilim Dalı
- Sayfa Sayısı: Belirtilmemiş.
Özet
Teknolojinin hızla gelişmesi, pek çok mühendislik alanında yenilik ihtiyacını artırmıştır. Bu ihtiyaçların başında ise havacılık ve savunma sanayii gelmektedir. Özellikle malzeme bilimi, söz konusu alanlarda önemli teknolojik ilerlemelere öncülük etmiştir. Hafiflik, yüksek dayanım ve enerji emilimi kapasitesi gibi malzeme özellikleri, savunma ve havacılık uygulamalarında temel gereklilikler arasında yer almaktadır. Bu gerekliliklere yanıt verebilecek yenilikçi çözümlerden biri ise oksetik yapı tasarımlarıdır. Oksetik malzemeler, negatif poisson oranına sahip alışılmışın dışında malzeme özellikleri gösteren yapılardır. Bu özellik, bir malzemeye uygulanan çekme kuvvetine dik yönde genişleme meydana gelmesiyle tanımlanır. Bu sıradışı davranışları ile geleneksel malzemelerden ayrılırlar ve darbe dayanımı, enerji emilimi ve geometrik kararlılık gibi mekanik özellikleri ile öne çıkarlar. Özellikle zırh sistemleri, koruyucu giysiler, uçak ve araç gövdeleri gibi enerji sönümleme ve patlamaya karşı direnç gerektiren uygulamalarda büyük avantaj sağlamaktadırlar. Bu tez çalışmasında, savunma teknolojileri bağlamında oksetik yapıların mekanik performansını artırmaya yönelik sonlu elemanlar analizleri ile parametrik analizler gerçekleştirilmiştir. Araştırma kapsamında, literatürde yaygın olarak kullanılan girintili hücre tipinin bir versiyonu olan yarı sekizgen girintili birim hücreler tercih edilmiştir. Temel geometrik parametreler olarak hücre yüksekliği (h), hücre genişliği (w) ve girinti açısı (θ) çeşitli kombinasyonlarla modellenmiş; bu parametrelerin yapının mekanik davranışı üzerindeki etkileri sistematik bir şekilde incelenmiştir. Değerlendirme kriterleri arasında basma modülü (compressive modulus), basma dayanımı (compressive strength), ortalama ezilme gerilmesi (mean crushing stress), özgül enerji emilimi (specific energy absorption), enerji emilimi (energy absorption) ve yoğunlaşma gerinimi (densification strain) gibi mekanik özellikler yer almıştır. Sayısal analizler, ABAQUS/Explicit çözümleyicisi kullanılarak gerçekleştirilmiş olup, modellerde C3D8R tipi (üç boyutlu, azaltılmış entegrasyonlu, 8 düğüm noktalı) katı elemanlar tercih edilmiştir. Her bir oksetik yapı modeli, -y yönünde %64 oranında eksenel basma deformasyonuna maruz bırakılmıştır. Malzeme modeli olarak polilaktik asit bazlı PLA+ kullanılmış; bu malzemenin mekanik özellikleri ABAQUS ortamına dijital olarak tanımlanmıştır. Temas tanımı için 'sert temas' (hard contact) kullanılmış ve temas yüzeyleri arasında 0,1 sürtünme katsayısı atanmıştır. Yüksek çözünürlükte ve güvenilir sonuçlar elde edilebilmesi amacıyla, analizlerde 0,4 mm kenar uzunluğuna sahip homojen bir mesh ağı kullanılmıştır. ABAQUS ortamından elde edilen kuvvet-deplasman verileri, MATLAB yazılımı kullanılarak işlenmiş ve bu veriler, yapının gerilme- birim şekil değiştirme eğrilerine dönüştürülmüştür. Elde edilen bu eğriler, yapının parametrelerinin yapısal performans üzerindeki etkilerinin karşılaştırılmalı olarak incelemeye olanak tanımış ve çeşitli geometrik parametrelerin, özellikle girinti açıları (θ) ile hücre yüksekliği-genişlik oranı (h/w) gibi faktörlerin mekanik performans üzerindeki etkileri detaylı bir şekilde analiz edilmiştir. Bu analizler, geometrik parametrelerin oksetik yapıların enerji emilimi, deformasyon davranışı ve genel mekanik özellikler üzerindeki etkilerini karşılaştırmalı olarak ortaya koymuştur. Çalışma sonucunda elde edilen veriler, en uygun yapısal özelliklere sahip oksetik tasarımların, savunma ve havacılık sektörlerinde hafif, dayanıklı ve yüksek enerji sönümleme kapasitesine sahip sistemlerin tasarımında önemli bir rol oynayabileceğini ortaya koymuştur. Ayrıca, bu yapıların otomotiv, biyomedikal ve spor güvenliği gibi çeşitli mühendislik alanlarında da kullanılma potansiyeline sahip olabileceği düşünülmekte ve bu alanlarda katkı sağlayabileceği öngörülmektedir.
Özet (Çeviri)
The rapid advancement of technological developments has significantly increased the demand for the design of high-performance, durable, and lightweight structures in the defense and aerospace industries. The optimization of material properties for structures used in these fields is expected to provide substantial advantages, particularly in terms of rigidity, impact resistance, and energy absorption capacity. At the core of these requirements lie critical engineering goals such as reducing fuel consumption, increasing payload capacity, ensuring energy efficiency, and enhancing operational safety. Therefore, reducing weight while maintaining mechanical strength has become one of the fundamental challenges to be addressed in modern engineering. In recent years, the need for lightweight, rigid, and energy-absorbing materials has been rapidly increasing not only in the defense and aerospace sectors but also in areas such as space technologies, automotive engineering, robotics, and biomedical applications. This trend has encouraged the investigation of innovative geometric approaches and smart material behaviors within the fields of materials science and mechanical design. In this context, auxetic structures with a negative Poisson's ratio, which behave entirely differently from conventional materials, have become the focus of intense scientific research in recent years. Auxetic materials exhibit an unusual mechanical response by expanding both in the direction of the applied load and in the perpendicular direction when subjected to stress. This unique property provides several mechanical advantages such as high shear strength, enhanced impact resistance, superior energy dissipation capability, and increased puncture resistance. The concept of auxetic behavior was first introduced in the scientific literature and has since evolved into a broad field of research through both experimental and numerical studies. Honeycomb, lattice, foam, and re-entrant structural types have demonstrated that auxetic behavior can be achieved through various geometric principles. The geometric nature of these structures has the potential to create a mechanical response that is far more complex and, at the same time, more controllable than that of conventional materials. While traditional materials tend to contract laterally under load, auxetic materials expand, which helps to homogenize the stress distribution within the material and prevent the accumulation of local deformations. This behavior delays crack propagation and reduces the risk of sudden failure. These distinctive characteristics offer significant advantages, especially in systems exposed to repeated impacts, collisions, or high-energy interactions. Recent studies have shown that auxetic structures possess high potential not only at the theoretical level but also in practical engineering applications. For instance, numerous studies have been conducted on the use of auxetic geometries in impact-absorbing panels, personal protective armor systems, automotive crash absorbers, and aerospace structural components. In the defense industry, such structures provide great advantages in armor systems, mine protection panels, and military vehicle body designs due to their energy dissipation capability. In addition, their ability to distribute energy in a controlled manner during high-speed impacts enables them to offer next-generation material solutions that can protect both vehicle personnel and overall system integrity. Compared to conventional materials, auxetic structures have the potential to maintain structural integrity more effectively by distributing impact energy over a wider area. This feature is particularly crucial in conditions involving high-energy collisions or explosive effects, where maintaining durability and reliability is vital. Thus, even in environments characterized by intense energy interactions, the robustness and safety of the structure can be significantly enhanced. For these reasons, the investigation and geometric optimization of auxetic materials hold great strategic importance for the future of defense technologies and high-performance engineering structures. In material design, cultural and artistic architectural patterns have also attracted considerable attention. In this study, an original semi-octagonal auxetic unit cell, named SOR (Semi-Octagonal Re-entrant), was developed by drawing inspiration from the geometric motifs of the Jameh Mosque of Isfahan, one of the significant works of Islamic architecture. This approach aims to integrate traditional geometric patterns with modern engineering principles, transforming aesthetic design into a functional structural advantage. Accordingly, the SOR geometry was evaluated not only as a visually inspired element derived from a cultural form but also as an engineering solution with the potential to enhance mechanical performance. As a result, the relationship established between the geometric principles of traditional art and the mechanical characteristics of modern auxetic structures provides an innovative contribution to interdisciplinary design understanding. In this thesis, the mechanical properties of semi-octagonal re-entrant auxetic structures were parametrically investigated, and their potential applications in the defense industry were evaluated. The main objective of the research is to analyze in detail the effects of geometric parameters—such as cell wall thickness, width, and angle—on mechanical performance. In this way, the quantitative relationship between structural strength, energy absorption capacity, and deformation behavior of the material has been revealed. For this purpose, fifteen different cell configurations were designed using SolidWorks, and a comparative evaluation was performed by maintaining the same number of unit cells in each model to ensure similar relative density. Additionally, to preserve the geometric accuracy of the generated models and increase the reliability of the results, all dimensional ratios were kept constant within specific engineering constraints. Geometric diversity was achieved through variations in wall thickness, height-to-width ratio (h/w), and re-entrant angle (θ). The effects of these parameters were assessed in terms of energy absorption, compressive strength, and deformation behavior. During the analysis process, deformation patterns that occurred under loading were closely examined, and the relationship between auxetic behavior and geometric parameters was clarified. The obtained results demonstrated that certain geometric combinations significantly enhance energy absorption capacity and improve structural stability. Proper optimization of these parameters constitutes a fundamental step toward the design of auxetic structures with the desired mechanical properties. Therefore, this study is not only a theoretical investigation but also provides a practical engineering approach for developing high-performance, lightweight, and durable auxetic structures suitable for use in the defense and aerospace industries. Initially, the cell height (h) was set to 12 mm, the cell width (w) to 12 mm, and the angle (θ) to 120 degrees. Using these basic parameters, two different structures with h/w ratios of 1.2 and 0.8 were designed, where the width (w) values were selected as 10 mm and 15 mm, respectively. This configuration enabled the investigation of the effects of the h/w ratio on the mechanical properties. Furthermore, by keeping the h/w ratio constant (1), the angle (θ) was varied by ±10 degrees to examine its influence on the overall behavior of the structure. As a result of these analyses, a total of five distinct designs were obtained. This systematic approach made it possible to understand how each geometric parameter affects the structural performance both individually and in interaction with other parameters. During the design process, the wall thickness was initially set to 1 mm for all models; later, additional variations with wall thicknesses of 0.5 mm and 1.5 mm were introduced. The top and bottom plates were designed with a thickness of 3 mm to enhance structural rigidity and durability. The relative density was taken into consideration in determining the number of cells, ensuring comparable density values across all models for a fair evaluation. Maintaining constant relative density was particularly critical, as density directly affects both energy absorption capacity and mechanical strength. This variation is of critical importance for examining the effects of mass distribution within the material and its energy storage capacity. While an increase in wall thickness is generally expected to enhance stiffness, it also alters the energy absorption behavior and local buckling modes of the structure. Therefore, the thickness parameter has been treated as a key design variable, providing control over both strength and deformation characteristics throughout the optimization process. Parametric analyses revealed that changes in the cell geometry significantly affect the mechanical behavior of the structures. Increasing the h/w ratio resulted in notable improvements in compressive modulus and maximum strength, as well as improvements in energy absorption and specific energy absorption (SEA) values. This indicates that auxetic materials with taller cell structures exhibit more rigid behavior under impact and demonstrate more effective performance in energy dissipation. Conversely, increasing the width (reducing the h/w ratio) led to a decrease in crushing resistance and caused more sudden and irregular changes in structural deformation. These findings clearly indicate that the h/w ratio is a critical design parameter directly affecting the energy dissipation and impact resistance performance of auxetic structures. The angle (θ) parameter, while having a positive effect on rigidity, contributed more limitedly to energy absorption. Angles exhibited fluctuating results in specific energy absorption, indicating that the angle must be optimized in conjunction with other parameters. While the angle alone is not sufficient, when combined with appropriate h/w ratios, it contributes to the mechanical performance of the structure. In particular, certain angular configurations were observed to affect the collapse mechanism of the structure, thus altering the energy absorption efficiency. This suggests that auxetic structures represent a multi-faceted optimization problem and emphasizes that focusing on a single parameter is not sufficient. As a result, the highest-performing structure was determined to be a configuration with a high h/w ratio and moderate angles. This structure represents an ideal design approach for defense and aerospace applications, offering high impact resistance and energy absorption capacity. Furthermore, the deformation modes analyzed demonstrate that the controlled collapse behavior of such auxetic structures provides significant engineering solutions for applications requiring high security, such as armor systems, impact protection panels, and military vehicle bodies. This controlled collapse allows the material to gradually absorb impact energy, preventing localized damage and reducing the forces transmitted to the main structure. This thesis demonstrates that semi-octagonal re-entrant auxetic structures can be geometrically optimized to design lightweight, rigid, and energy-absorbing structures for use in the defense industry. The findings suggest that these structures can provide substantial benefits not only in defense but also in other engineering disciplines such as automotive, biomedical, and sports safety. In conclusion, this work makes an important contribution to understanding the mechanical behavior of auxetic structures and improving their design.
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