Phd Degree / Doktora

Permanent URI for this collectionhttps://hdl.handle.net/11147/2869

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Access Right: Open AccessSubject: search.filters.subject.Impact test

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  • Doctoral Thesis
    The Development of Constitutive Equations of Polycarbonate and Modeling the Impact Behavior
    (01. Izmir Institute of Technology, 2023) Sarıkaya, Mustafa Kemal; Güden, Mustafa; Taşdemirci, Alper
    The Johnson and Cook (JC) flow stress and damage parameters of a polycarbonate were determined by the mechanical tests and numerical simulations. The experimental tests included quasi-static and high strain rate tension and compression, quasi-static notched-specimen tension, quasi-static indentation (QSI), low velocity impact (LVI) and projectile impact (PI). The flow stress equation determined from the experimental average true stress-true strain curve well agreed with the effective stress-strain obtained from the quasi-static numerical tension test. The numerical QSI force-displacement curve based on the experimental average true stress-true strain equation was further shown to be very similar to that of the experiment. The LVI and PI test simulations were then continued with the experimental average true stress-true strain equation using five different flow stress-strain rate relations: JC, Huh and Kang, Allen-Rule and Jones, Cowper-Symonds and the nonlinear rate approach. No strain rate sensitivity in the LVI tests was ascribed to low strain rate dependency of the flow stress at intermediate strain rates and large strains. On the other side, all the stress-strain rate relations investigated nearly predicted the experimental damage types in the PI tests, except the Cowper-Symonds relation which predicted the fracture of the polycarbonate plate at 140 m s-1. The absorbed energy at 160 m s-1 test was determined 1.6 times that of the QSI test, proving an increased energy absorption of the tested polycarbonate at the investigated impact velocities. The verified parameters were finally used to model the damages formed on a canopy against bird strike.
  • Doctoral Thesis
    Impact Behavior of Textile Reinforced Concrete Slabs
    (01. Izmir Institute of Technology, 2021) Batarlar, Baturay; Saatçi, Selçuk
    Reinforced concrete (RC) technology is still the most preferable and common method to build civil engineering structures. In accordance with design purposes and needs, these structures are built to resist various loading scenarios. Throughout the lifespan of RC structures, they may be subjected to high rate loading scenarios due to either expected or unexpected reasons such as impacts caused by vehicular collisions, debris generated by typhoons, tsunami or floods, rock or object falls to protective shelters. Therefore, understanding of impact behavior of RC members plays a vital role not only for design stages but also retrofitting and strengthening purposes thereafter. For this purpose, an experimental program was carried out to reveal the impact behavior of RC slabs strengthened with carbon textile reinforcements. In this program, four slabs specimens, two unstrengthened and two strengthened with two different carbon textile reinforcements, having dimensions of 1.5 m × 1.5 m × 0.2 m were tested by using an advanced impact testing facility at Otto-Mohr Laboratiorum of Technische Universität Dresden. In these tests, all slabs were tested under repeated impact loads by using the same steel striker with a 200 mm - diameter flat contact surface in the velocity range of 25.2 to 30.2 m/s. The results obtained from these tests are presented in terms of midpoint-displacement histories, reaction force histories, slab accelerations, and strain histories of steel reinforcements for each impact. As a result of the test program, it is shown that carbon textile reinforcements have significant effects on enhancing impact capacity as well as limiting maximum and residual midpoint displacements. By using the data obtained from tests, a finite element (FE) modeling study was performed by using the LS-DYNA software tool. In this study, two FE models with different mesh sizes were created and compared with each other to obtain efficient modeling conditions. In the light of the tests and validated models, a parametric study was performed to figure out efficient impact conditions and parameters for carbon textile reinforcements. It is shown that carbon textile reinforcements are more effective for limiting damage levels under low-velocity impacts.
  • Doctoral Thesis
    Finite Element Simulations of Impact Test for Light Alloy Wheels
    (Izmir Institute of Technology, 2016) Pehlivanoğlu, Uğur; Yardımoğlu, Bülent
    Static and dynamic finite element models for the simulation of the wheel impact test defined in ISO7141 were developed for the AlSi7Mg and AlSi11Mg alloy wheels. The dynamic model consists of the striker, the wheel with radial pneumatic tire, and the hub adapter structure. Two types of tire models, composite and simplified, are formed in this study. The finite element model in the dynamic model, referred to as composite tire, involves bead, bead core, casing and crown plies, tread, and side walls. A simplified tire model that does not include bead cores, casing and crown plies is also generated. Although these items are not used in the second model directly, they are considered using their equivalent effects. It is shown that a simplified tire model can be used instead of the composite tire model. The dynamic model is validated by experimental studies. Such studies are related to the plastic deformations at the impact point of the wheel. It is shown that simulation of the failure of the wheel during impact tests can be determined using von Mises and effective plastic strain occurs in the wheel. In total, forty-one experiments are done to see the wheel behaviors and whether it performes according to the standard. The experimental results and the corresponding simulations focusing on von Mises stresses along with effective strains are shown in box plots. Thus, critical values for design are found. The static model consists of the wheel with simplified tire and the lumped model of the hub adapter structure. The stiffness characteristic of the impact point of the wheel is determined by using the static model. It is shown that the maximum von Mises stress that occurs in the wheel due to impact load is found using energy conversions. Significant time can be saved by this manner.
  • Doctoral Thesis
    Experimental and Numerical Investigation of the Quasi-Static and High Strain Rate Crushing Behavior of Single and Multi-Layer Zig-Zag 1050 H14 Al Trapezoidal Corrugated Core Sandwich Structures
    (Izmir Institute of Technology, 2014) Kılıçaslan, Cenk; Güden, Mustafa; Taşdemirci, Alper; Güden, Mustafa; Taşdemirci, Alper
    The quasi-static and dynamic crushing behavior of single, double and multi-layer zig-zag 1050 H14 Al trapezoidal corrugated core sandwich structures in 0°/0° and 0°/90° core orientations and with and without interlayer sheets were investigated both experimentally and numerically at varying impact velocities. The numerical simulations were conducted using the finite element code of LS-DYNA. The effect of fin wall imperfection was assessed through the fin wall bending and bulging. The numerical homogenization of the single layer corrugated structure was performed using MAT26 honeycomb material model. The buckling stress of single- and double-layer corrugated sandwich structures increased when the strain rate increased. The increased buckling stresses were ascribed to the micro inertial effects. The initial buckling stress at quasi-static and high strain rate was numerically shown to be imperfection sensitive. Increasing the number of core layers decreased the buckling stress and increased the densification strain. The panels tested with spherical and flat striker tips were not penetrated and experienced slightly higher deformation forces and energy absorptions in 0°/90° corrugated layer orientation than in 0°/0° orientation. However, the panels tested using a conical striker tip were penetrated/perforated and showed comparably smaller deformation forces and energy absorptions, especially in 0°/90° layer orientation. The homogenized models predicted the low velocity compression /indentation and projectile impact tests of the multi-layer corrugated sandwich with an acceptable accuracy with reduced computational time.