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    Dynamic behavior of screwed joints for CFRP composite laminate structures under impact loading
    (Elsevier Sci Ltd, 2022) Can, Ahmet; Meram, Ahmet
    The detachable joining techniques in composite laminates compared to metals are limited and assumed one of the major obstacles in developing composite structures. In prior work, the authors successfully investigated the screw joining technique for CFRP composite laminates under static loads. However, the dynamic behavior of this joining technique for composite laminates was unknown. This study was conducted to examine the dynamic behavior of the screwed joints in CFRP composite laminates. For this purpose, impact tests were performed on bolted composite laminates with metric screw sizes ranging from M6 to M14 and different pitch screws. The dynamic response of screwed joints was determined by load deflection, peak force, and failure mechanism. Macro and scanning electron microscope (SEM) images were used to analyze the damage mechanisms in detail depending on screw and pitch size. Pure stripping tests are also conducted to determine the effect of tooth root thickness on the stripping force. The damage mechanism changed from interlaminar delamination to push-out delamination as the pitch increased, and outward fibering was primarily observed in high pitches. The maximum dynamic failure loads for the screwed joints were between 27.55 and 78.12 kN.
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    Dynamic characterization of elastomer buffer under impact loading by low-velocity drop test method
    (Elsevier Sci Ltd, 2019) Meram, Ahmet
    An approach is proposed to establish a discrete model to characterize the dynamic behavior of a thermoplastic polyurethane elastomer buffer under impact loading. To this end, a series of low velocity drop tests with initial impact velocities 1.77-4 m/s was carried out. The impact force versus time histories for the samples under impact loading were measured with varying initial impact velocities. To obtain acceleration, velocity and displacement of the projectile, Newton's second law of motion and the numerical integration method were applied. The stress-strain, coefficient of restitution, loss factor and dissipated energy of samples were determined. Following, the experimental data were used to establish a mathematical model for the elastomer buffer. To reach this aim, Maxwell and Kelvin-Voigt viscoelastic impact models and one-dimensional viscoelastic free decay oscillation model were employed. It was observed that Kelvin-Voigt and one-dimensional viscoelastic free decay oscillation models underestimated the buffer dynamic characteristics. An approach was proposed to calculate the calibrated nonlinear Maxwell viscoelastic impact model. The proposed model consists of a nonlinear dashpot and nonlinear spring. The calibrated nonlinear Maxwell model predicted the elastomer buffer impact behavior in terms of peak force, maximum deflection, dissipated energy and coefficient of restitution with high accuracy.
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    Experimental investigation of screwed joints capabilities for the CFRP composite laminates
    (Elsevier Sci Ltd, 2019) Meram, Ahmet; Can, Ahmet
    The load-carrying capability of the screwed joints for the carbon fiber reinforced polymer (CFRP) composite laminates was investigated by compression and torsion tests. To this end, the M6, M8, M10, and M12 metric internal threads were obtained by tapping directly into the CFRP composite laminates. In order to avoid material damage, the milling and tapping operations were implemented using CNC controlled machine tools. Quasi-static compression and torsion tests were carried out on the simply tapped and Helicoil reinforced screwed joints in the universal tensile-press test machine under uniform test conditions. The applied force and torque values respect to the screw displacements were obtained. Based on the experimental outcomes, the load-carrying capability and failure mechanisms of screwed joints were evaluated. The SEM graphics were utilized to explain the failure mechanism of threads. The load-carrying capability of joints increased with increasing the joint size. Helicoil reinforced specimens showed significantly higher maximum failure load and torque values compared to simply tapped specimens. Although, Helicoil inserts were effective in improving load-carrying of screwed joint, analysis of Helicoil performance proved that the effectiveness of Helicoil decreased with an increase in metric screw size. The computed stripping strength values indicated that Helicoil reinforced screwed joint can be an alternative to riveted and adhesively bonded joints as a detachable joining technique for thick CFRP composite laminates.
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    Experimental Investigation on the Effects of Core/Facing Interface Performance on the Low-Velocity Impact Behavior of Honeycomb Sandwich Panels
    (Springer, 2020) Meram, Ahmet; Cetin, Mehmet Emin
    This paper gives an important contribution by investigating the effectiveness of core/facing interface performance of aluminum honeycomb sandwich panels under low-velocity impact energy. Low-velocity drop tests were conducted on five different panels under 50, 75, and 100 J impact energies (loads). The following procedure is followed to evaluate the impact response of panels: the force-time histories are acquired; the numerical integration method is applied, and force-displacement histories are obtained; and then the damage mechanism and theoretical energy balance modeling are used to analyze the effectiveness of core/facing interface performance on the impact behavior of the panels. Scanning electron microscopy is used to examine the microstructural and the morphology of the core/face sheet interface of the aluminum honeycomb sandwich panels. The effects of voids, interface, and cohesive cracks on the impact behavior of the panels are analyzed. Energy balance modeling proved that energy absorbed in the bending and shear deflections increased as the resistance at the core/facing interface is increased. In addition, changing the initial impact energy from 50 to 100 J produced more than 120% increase in the effectiveness of the panels in terms of energy absorbed in shear and bending deformations.
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    Vibration analysis of a novel magnetic-viscous nonlinear passive isolator via finite element simulation
    (Tubitak Scientific & Technological Research Council Turkey, 2019) Meram, Ahmet; Onen, Umit
    In this paper, the design and the finite element simulation of a novel magnetic-viscous vibration isolator have been presented. The proposed isolator consists of permanent magnets axially aligned in repulsion position and a viscous damper in parallel with them. The nonlinear spring characteristic of the permanent magnets provides a good damping property with this configuration. Explicit finite element analyses have been conducted to examine the dynamic behavior of the isolator. Output displacements and transmissibility ratios were measured for various magnet configurations, dashpot coefficients, and input displacement excitation frequencies to determine the best damping properties. The results of the finite element modeling revealed that the performance of the isolator is highly sensitive to the quantity of magnets. Isolators with four or more magnets can successfully reduce the output displacement. The results indicate that the isolator significantly reduces the displacement transmissibility in frequencies over the resonant frequency region. It is possible to ensure an infinite operating life for magnet-viscous vibration isolators by protecting the magnets against breakage.