Sürdürülebilir Kalkınma Amaçları
Yayınlar
- MakaleYalnız künye2026Electromechanical characterization and deep learning-based force prediction in 3D-printed PLA core–shell composites
Ekmekci, Dursun; Kaplan, Emrah
In this study, composite samples were produced using fused deposition modeling to contain an outer layer of red insulating polylactic acid and an inner core of black conductive polylactic acid. The mechanical properties of the samples were determined using three-point bending experiments, and their electro-mechanical impedance responses during loading were monitored using impedance experiments. To examine the effects of physical aging and crystallization, the composites were heat-treated at 50 °C for 2, 4, and 6 hours. The insulating polylactic acid exhibited brittle fracture with a sharp post-peak force drop, whereas the conductive polylactic acid showed a ductile fracture mode accompanied by substantial energy dissipation. The physically aged composites were stiffer and less ductile, which was attributed to the presence of partially crystallized and partially relaxed molecules, especially around the glass transition region. In support of the observed properties, scanning electron microscopy images confirmed that the fractured layers were smoother compared to the ductile conductive layers, which had surface roughness patterns resembling fibrillated or lamellar structures. Additionally, the impedance signals obtained during the bending experiments were linked with the corresponding mechanical properties using an advanced deep learning model that could model the electric signals to accurately provide the corresponding real-time force values.
- MakaleYalnız künye2023Comparison of Mechanical Properties Obtained for Armor Steels: Conventional Methods vs. Nanoindentation
Ekmekci, Dursun; Cora, Ömer Necati
The mechanical properties of three armor steels, namely Ramor 500, Ramor 550, and Armox Advance, were acquired by nanoindentation and conventional methods, and then the results were compared. To this goal, samples were extracted from different (e.g., undeformed and deformed) zones of the ballistically tested armor plates. Then, nanoindentations were performed at 10-mN maximum load and different maximum indentation depths (50-150 nm). In addition to the hardness and modulus of elasticity values of the examined materials, residual stress values of the samples were calculated through the equations suggested in the literature. These values were then compared with the residual stress measurements obtained with the conventional x-ray diffraction (XRD) - sin2ψ method. Moreover, Charpy V-notch impact tests were performed to calculate the fracture toughness of armor steel samples. Microstructural analysis (e.g., optical microscope, scanning electron microscope, atomic force microscope), surface roughness, and micro-hardness (micro-Vickers) measurements were also obtained. It was noted that hardness values for the Ramor 500 and Ramor 550 armor steels were in the range of 5.5-9 GPa, whereas the hardness values of Armox Advance samples were obtained in the 8.6-10.2 GPa range at a 50-nm indentation depth. The residual stress values obtained with nanoindentation were found to be in quite good agreement with the one obtained through XRD. It was proven that nanoindentation can be used to acquire several mechanical properties through considerably less sample preparation procedure, in an automated, and a more representative manner because of the measurements obtained at multiple points.
- MakaleYalnız künye2023Effect of Plastic Deformation on the Mechanical Properties of Dual-Phase Steels Using Nanoindentation
Ekmekci, Dursun; Uşun, Altuğ; Cora, Ömer Necati
The mechanical properties of two advanced high-strength steels (AHSS), DP 1000 and DP 1200 dual-phase (DP) grades, under deformed and undeformed conditions, were investigated using nanoindentation, and the results were compared with those obtained from the conventional methods. To this goal, 3-point bending tests were applied to induce deformation in the samples. Before and after these tests, nanoindentations were performed at different forces and indentation depths. In addition to the hardness and modulus of elasticity values of the steels, the residual stresses on the samples after deformation were obtained by using the equations suggested in the literature, also with x-ray diffraction (XRD). Finite element (FE) modeling of 3-point bending and nanoindentation were performed to obtain stress–strain curves of the materials numerically. The stress–strain curves obtained by numerical analysis agree well with those reported in the literature. The variations of the hardness and modulus of elasticity values are narrower for deeper indentation (50 nm versus 200 nm), and the effect of deformation is more pronounced on the modulus of elasticity values (10–30% increase with the deformation) while hardness values increased with the effect of deformation, 10% at most.
- MakaleYalnız künye2020Effect of indentation loading type on the mechanical properties of advanced high strength steel grade DP 800
Ekmekci, Dursun; Cora, Ömer Necati
The goal of this study was to acquire mechanical properties of a dual-phase grade advanced high strength steel (AHSS) by means of different microindentation loading conditions. Conventional, cyclic, and multi-step indentations were performed on DP 800 sample; and Young’s modulus, hardness values were obtained by the depth-sensing indentation technique. Effects of different load levels (50–300 mN) and type of indentation on the results were also analyzed through ANOVA. The load levels experimented yielded overall material response rather than its constituents (e.g., martensite and ferrite). Both hardness and Young’s modulus tend to decrease with increasing maximum indentation load level, especially from 50 to 100 mN, which is regarded as indentation size effect.
- MakaleYalnız künye2017Microindentation on the porous copper surface modulations
Ekmekci, Dursun; Yilmaz, Fikret; Kölemen, Ugur; Cora, Omer Necati
This study aimed to investigate the mechanical properties of a surface modulation which was realized through compaction, and then sintering of copper powders. To this goal, Berkovich type of indenter and depth-sensing indentation technique were used in microindentation to measure the hardness and modulus of elasticity values at different features of compact. Indentations were performed with a peak force of 50 mN. Hardness values were obtained in 0.88-1.12 GPa range while the modulus of elasticity was recorded in the 70-111 GPa interval. Even though both modulus of elasticity and hardness values were noted to be different for copper powders and substrate, one-way ANOVA analyses showed that the differences in both modulus of elasticity and hardness values are insignificant. FE modeling of microindentation was also performed and validated. It was shown that the force-displacement values obtained from FE analyses are quite well in agreement with the experimental data.
- MakaleAçık Erişim2015Preparation and characterization of electroless Ni-B/nano-SiO2, Al2O3, TiO2 and CuO composite coatings
Ekmekci, Dursun; Bülbül, Ferhat
Ni-B/SiO2, Al2O3, TiO2 and CuO composite coatings were successfully obtained by the electroless plating technique. Dispersible SiO2, Al2O3, TiO2 and CuO nanoparticles were co-deposited with electroless Ni-B coating onto AISI-304 steel substrates. Deposits were characterized for its structural properties by X-ray diffraction (XRD). XRD results showed a broad peak of Ni-B and low intensity composite nanoparticle peaks. The surface and cross-section morphology of samples were analysed using a scanning electron microscope (SEM). Surface analysis showed that the incorporation in Ni-B matrix of nano-SiO2, Al2O3, TiO2 and CuO particles increases the nodularity of composite coatings. The deposits are composed of a columnar structure grown along the vertical direction of the substrate surface. The results also show that electroless nanocomposite coatings tend to have greater microhardness compared with the coating without nanoparticles. The increase in the microhardness of the nanocomposite coatings reported in this study is attributed to an effect of dispersion strengthening of ceramic particles in the Ni-B matrix.