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1

Deceleration behavior of multi‑layer cork composites intercalated with a non‑Newtonian material

Sheikhi Mohammad Rauf, Gürgen Selim

Archives of Civil and Mechanical Engineering

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2023

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Vol. 23, no. 1

art. no. e2, 2023

EN

Impact or sudden accelerations are strictly avoided by sensitive systems such as electronic devices, robotic structures and unmanned aerial vehicles (UAVs). In order to protect these systems, various composites have been developed in recent years. Due to its excellent energy absorbing capabilities as well as eco-friendly and sustainable properties, cork is one of promising materials dedicated to protective applications. In this study, we beneft from cork agglomerates in multi-layer design considering its advantages such as high fexural stifnesstoweight ratio and good buckling resistance over monolithic structures. In addition, a non-Newtonian material, namely shear thickening fuid (STF) was incorporated in this design. STF shows rapid increase in its viscosity under loading and thereby enabling a stifer texture that contributes to protective performance. At rest state, STF exhibit fuidic behavior and provides fexibility for composite. In the experimental stage, deceleration behavior of these composites was investigated. According to the analyses, STF exhibits promising results to lower peak decelerations while extending time period of deceleration under impact loading. STF contribution is pronounced by using this material in a closed medium such as in wrapped foam to avoid spilling out of composite during impact. The designed eco-friendly smart composites are suggested to cover internal parts in sensitive systems. Micro-mobility helmet is another prospective application area for cork/STF structures since they provide light-weight, excellent fexibility and good deceleration behavior.

2

Anti‑impact and vibration‑damping design of cork‑based sandwich structures for low‑speed aerial vehicles

Sheikhi Mohammad Rauf, Gürgen Selim, Altuntas Onder, Sofuoğlu Mehmet Alper

Archives of Civil and Mechanical Engineering

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2023

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Vol. 23, no. 2

art. no. e71, 2023

EN

Nowadays, lightweight and eco-friendly composites with improved mechanical properties are highly interesting. Sandwich-structured composites are a type of high-performance structural composite that is lightweight with a high strength-to-weight ratio and excellent specifc energy absorption capabilities. In this study, cork-based sandwich structures resistant to impact and vibrations were designed and produced for the possibility of being used in the protective structures of low-speed aerial vehicles. To identify and match the best combination of different face sheets with a cork core, first, aramid fabric-reinforced polymer (AFRP), carbon fiber-reinforced polymer (CFRP), and glass fiber-reinforced polymer (GFRP) face-sheet composites were produced using the compression molding method (prepreg layup). Then, sandwich structures consisting of AFRP, CFRP, GFRP, and aluminum face sheets with a fixed core layer of cork were designed and assembled. Since the design goal of these structures is to use them in low-speed aerial vehicles, impact deceleration and vibration tests were applied to face sheets and sandwich structures individually, which are the most important factors involved in these structures during fight, particularly in rotary-wing drone applications. A low-energy drop-tower system was used for the calculation of deceleration results. Besides, the vibration properties of the structures were investigated using the modal analysis method and based on the natural frequency responses of the tested face sheets and sandwich structures, damping ratios and structural stiffness were measured. According to the results, compared to other face sheets, CFRP showed better resistance along with the cork core, when the structure was exposed to impact and vibration threats. This study provides useful information on cork core sandwich structures for academic and industrial researchers in choosing the right face sheet.

3

Optimization of micromachining operation for particle reinforced UHMWPE composites

Sofuoğlu Mehmet Alper, Gürgen Selim

Archives of Civil and Mechanical Engineering

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2022

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Vol. 22, no. 3

art. no. e138

EN

Unlike metals, polymers are highly affected by the heat generation during the machining of the workpiece, because the thermal conductivity of polymers are considerably lower than metals, and therefore heat is much more effective in the cutting zone. If the appropriate cutting parameters are not selected, the polymers become excessively deformed and the final part has high surface roughness, dimensionally large burr formation, or dimensional deviations. Machining of polymers ultra-high molecular weight polyethylene (UHMWPE) is quite common in industrial applications. In this study, the effect of SiC fillers on the machinability of UHMWPE polymer composite was investigated. First, different samples were produced using different filler sizes (1 μm, 50 μm, and 100 μm) and different filler amounts (1%, 3%, 5%). Micro-milling tests were carried out at a constant feed rate (70 mm/min), constant cutting depth (0.1 mm) and spindle speeds (1200, 2800, and 4400 rpm). Tool overhang lengths were selected as 10, 15, and 20 mm. During the experiments, the surface/burr shapes, cutting temperatures and cutting forces were observed. In general, it is observed that SiC filler reduces cutting forces and cutting temperatures. In the further stage of the study, Taguchi analysis was performed in the light of different SiC filler sizes, filler amounts, rotational speeds, and tool overhang lengths.

4

Micro-machining of UHMWPE composites reinforced with carbide fillers

Gürgen Selim, Sofuoğlu Mehmet Alper

Archives of Civil and Mechanical Engineering

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2021

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Vol. 21, no. 4

189--202

EN

Polymer matrix composites (PMCs) have become one of the most widely used engineering materials due to both the developments in polymers and advanced fillers. It is expected that polymer composites will take their final shape during the production phase, which means that they are not required to undergo new processes. However, in some applications, machining operations, such as turning, milling, grooving and hole drilling, cannot be avoided and thus, finishing operations must be applied to these materials. Since these materials have complex microstructures, finishing operations may cause situations that adversely affect engineering properties, such as matrix cracking, delamination, debonding, etc. In this study, micro-milling operations were performed for recently developed ceramic reinforced polymer composites. Three different spindle speeds were used while feed rate and cutting depth were kept constant in the operations. The composites were produced from powdered UHMWPE and silicon carbide particles. Several parameters were varied in the production of the composites, such as molding pressure, filler loading and filler size. The investigated outputs were cutting temperature and surface roughness, whereas machined surfaces and chip morphologies were also investigated via microscopy analyses. In the final stage, regression analyses were performed to investigate the relationships between the process parameters. According to the results, ceramic reinforced polymer composites exhibit different machinability properties than fiber-reinforced ones due to hard fillers and low melting point of UHMWPE.

5

Rheological and deformation behavior of natural smart suspensions exhibiting shear thickening properties

Gürgen Selim, de Sousa Ricardo J. Alves

Archives of Civil and Mechanical Engineering

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2020

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Vol. 20, no. 4

166--173

EN

Shear thickening fluid (STF) is a very interesting and promising material in several application fields where a different mechanical is demanded based on loading rates, like body armor and vibration insulators. Cork is a natural cellular material by excellence, filled with well-known beneficial effects in terms of insulation and also interesting crashworthiness properties. In this work, cork grains of very small size (0.5–1.0 mm) are added to two different shear thickening suspensions, one of them a fully natural water and cornstarch, and the other based on fumed silica and polyethylene glycol. The rheology of these eco-friendly suspensions was investigated and the influences of including cork grains were discussed. In addition, microscopic analyses were carried out to observe the deformations at each component during the shear thickening phenomenon. Cork grains reduce the load-carrying capacity in the suspensions due to the deformable characteristics of cork. For this reason, shear thickening properties are suppressed in the mixtures. Despite this, it is possible to state that viscosity increase in the mixtures leads to strong particle contacts, and thereby resulting in particle deformations in the main constituent powder as well as in the cork additives due to their softer structures.

6

Low-velocity impact performance of UHMWPE composites consolidated with carbide particles

Gürgen Selim

Archives of Civil and Mechanical Engineering

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2020

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Vol. 20, no. 2

112--123

EN

Ultra-high molecular weight polyethylene (UHMWPE) is one of important materials utilized against impacting threats. In this work, bulk UHMWPE specimens were fabricated in a compression molding chamber, and molding parameters such as pressure and temperature were varied in the specimen preparation stage to investigate the effect of molding parameters on the impact performance. In addition, silicon carbide fillers were included in the UHMWPE matrix to enhance the anti-impact properties of the specimens. From the results, high molding pressure provides enhanced impact resistance due to improved microstructural consolidation. On the other hand, molding temperature just above the melting point of polymer is much beneficial to the anti-impact behavior of the structures. Carbide fillers lead to an increase in the frictional interaction between the impactor and composites and thereby enhancing the impact resistance of the structures. However, the gain in the protective properties performance is restricted up to a certain amount of carbide loading because at higher filler ratios, the composites change from ductile to brittle characteristics. For this reason, crack growth susceptibility develops in the composites at excessive carbide loadings.

7

A parametric investigation of roller hemming operation on a curved edge part

Gürgen Selim

Archives of Civil and Mechanical Engineering

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2019

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Vol. 19, no. 1

11--19

EN

This study presents the roller hemming operation of a curved edge sheet metal which is critical in terms of dimensional accuracy and deformation. The investigation is focused on the process parameters such as bending angle, sheet thickness and roller diameter to observe their influence on roll-in, deformation, undesired wrinkling formation and hemming force which are directly related to material reliability and visual quality in the operation. A set of numerical simulations was designed and through the regression analysis of the results, the magnitude of each input parameter was investigated. The verification of the numerical model was carried out using experimental results. In the light of the results, the influence of input parameters was discussed for designing proper roller hemming operations for curved edge parts.