Nebojša Bogojević

Auxetic structures are a class of mechanical metamaterials characterized by a negative Poisson’s ratio and enhanced energy absorption capabilities under impact loading. In this study, the influence of impact velocity on the mechanical response of a star-shaped auxetic (SSA) structure is investigated using the finite element method implemented in ANSYS Workbench LS-DYNA. A cellular structure composed of a 6×6 arrangement of SSA unit cells was subjected to compressive impact loading at velocities of 1 m/s, 10 m/s, and 100 m/s. The deformation mechanisms, force–displacement response, kinetic and internal energy histories, and energy absorption characteristics were analyzed. The results show that the deformation patterns obtained at 1 m/s and 10 m/s are nearly identical, exhibiting the characteristic elastic, plateau, and densification stages of cellular structures. Furthermore, the quasi-static condition was satisfied for these loading velocities, as the kinetic energy remained significantly lower than the internal energy throughout the simulations. In contrast, the response at 100 m/s was dominated by dynamic effects, resulting in non-uniform deformation, increased impact forces, and significantly higher energy absorption. The findings provide insight into the influence of impact velocity on the mechanical behavior of star-shaped auxetic structures and may contribute to their application in energy-absorbing systems.

Auxetic materials, characterized by their negative Poisson’s ratio (NPR), exhibit unique mechanical properties that make them highly desirable for lightweight structural applications. This study introduces a novel 3D double arc star-shaped (3D DASS) auxetic structure and systematically investigates the influence of cross-sectional shape (square vs. circular) and thickness variation on its mechanical behavior. We derive expressions for Poisson’s ratio and Young’s modulus using analytical modeling and numerical simulations and validate them across different geometric configurations. The results reveal that while Poisson’s ratio remains nearly constant for both cross-sections, Young’s modulus is significantly influenced by cross-sectional thickness, particularly in circular configurations. These findings provide valuable insights into the design optimization of lightweight auxetic structures and their potential applications in engineering.

This paper presents a numerical investigation of four types of auxetic star-shaped structures: the conventional 2D Arc Star-Shaped (2D-AS), the Improved 2D Arc Star-Shaped (i2D-AS), the newly proposed Tangent 2D Arc Star-Shaped (t2D-AS), and the standard 2D Star-Shaped (2D-SS) structure. By modifying the geometry of the original 2D-AS structure, the t2D-AS variant was developed and analyzed for the first time. All structures were designed to maintain the same geometric parameters, allowing for a consistent comparison of their mechanical properties. Numerical simulations were performed using the finite element method in ANSYS Workbench under periodic boundary conditions, with a focus on evaluating Poisson’s ratio and Young’s modulus. The i2D-AS structure exhibited the highest stiffness due to the use of circular arches, while the t2D-AS structure demonstrated the lowest relative density, making it a promising candidate for lightweight applications. Although its mechanical properties are slightly lower than those of the 2D-SS, the t2D-AS offers a favorable balance between structural efficiency and weight reduction. Future research will focus on the analytical modeling and energy absorption capabilities of the newly proposed structure.

This study focuses on the determination of cavitation rate for samples produced by Direct Metal Laser Sintering (DMLS) of MS1 maraging steel powder without any postprocessing thermal or mechanical treatment (as-built). The test samples, fabricated using an EOSINT M280 machine in a nitrogen atmosphere, were evaluated for their cavitation resistance using the ultrasonic vibration method as proposed by the ASTM G32 standard. Surface characterization to determine the duration of the experiment, as well as during and after the experiment to analyze changes in surface characteristics, was conducted using Scanning Electron Microscopy (SEM). Additionally, chemical analysis was performed using the Energy Dispersive X-ray Spectroscopy (EDS) method before and after the experiment to identify any changes and locations of significant surface damage. The mass loss of the samples over time was measured to calculate the cavitation rate. The goal of this research was to explore the potential applications of laser-sintered MS1 steel powder for manufacturing machine components and parts that are subject to cavitation erosion, due to the numerous benefits of additive manufacturing over traditional conventional technologies.

The processes of metal additive manufacturing (AM) offer advantages over traditional technologies, especially when it comes to producing parts with complex designs. Thanks to the flexibility in manufacturing processes and the relatively high speed of part production, AM technologies can now be found across various industries. A better understanding of the mechanical properties and fatigue behavior of metal parts is crucial for their successful integration in different sectors. Numerous factors play a key role in influencing the fatigue properties of metal parts produced through AM. This study investigates the impact of heat treatment and surface treatment on the fatigue behavior of components made from marginal steel (MS1) and stainless steel (PH1). After subjecting MS1 and PH1 steel to heat treatment, it is expected that the fatigue limits will increase compared to their original state. Additionally, experimental findings show that not only do surface treatments significantly affect fatigue strength when used alone, but also that changing the order of subsequent treatments can either enhance or diminish fatigue strength.

The microstructure of maraging steel produced by direct metal laser sintering (DMLS) is influenced by various factors, including parameters of the DMLS process and post-treating methods. This study investigates the effect of build orientation on the microstructure and fatigue behaviour of M300 maraging steel manufactured using DMLS. Microstructural characterization was conducted using light microscopy (LM) and scanning electron microscopy (SEM). The study also involved analysing fracture surfaces using SEM and performing fatigue tests on additively manufactured samples. The research reveals that the presence of inclusions in the specimens has resulted from trapped powder on the edge of the melted pool or from trapped metal oxides. These defects, especially when located near the surface of the specimens, often represent a weak spot and the source of initial cracks.

The Poisson's ratio represents one of the very important characteristics of mechanical metamaterials. Thanks to additive manufacturing, it is now much simpler to create structures for the study of Poisson's ratio. Particularly, for research purposes, re-entrant structures are of interest as they can exhibit negative values of Poisson's ratio. Due to their capabilities , such structures find extensive applications across various industries. The modified arc-star-shaped (m2D-AS) has been studied analytically and numerically, and the negative Poisson's ratio (NPR) and Young's modulus has been examined for different geometric parameters of the structure.

The present study focuses on the results of cavitation resistance of samples obtained by the Selective Laser Sintering technology. All samples are made from Nylon 12 ? Polyamide 12 powder, known as PA2200 in the industry, produced by EOS GmbH. The powder, which has been used for building of samples, represent mixture of 50% new powder and 50% of the recycled powder. In the work, samples with different section thicknesses (0.4; 0.8 and 1.0 mm) were subjected to the ultrasonically induced cavitation test method to investigate the effect of geometry on their cavitation behaviour. The change in mass loss during different cavitation times was measured on the tested samples. The morphology of cavitation damage was characterized using Scanning Electron Microscopy (SEM). Results obtained in this paper have shown that section thickness of the samples have a significant influence on the cavitation resistance. The highest cavitation resistance was obtained for the samples with 0.4 mm thickness.