Phd Degree / Doktora

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

Browse

Filters

2014 - 20185

Settings

Author: search.filters.author.Taşdemirci, AlperPublisher: search.filters.publisher.Izmir Institute of Technology

Search Results

Now showing 1 - 5 of 5
  • Doctoral Thesis
    The Penetration Behavior of Repeated Hemisphere Core Sandwich Structures: an Experimental and Numerical Study
    (Izmir Institute of Technology, 2018) Turan, Ali Kıvanç; Taşdemirci, Alper; Güden, Mustafa
    In this study, penetration behavior of novel core structure consisting hemispherical and cylindrical parts was investigated. Core units were manufactured with deep drawing method in two thicknesses to have monolithic form without any sort of assembly method or element. Produced specimens were then subjected to penetration tests at low and intermediate velocities against blunt, conical and hemispherical tipped indenters using special fixtures and apparatuses on conventional testing equipment. Effect of heat treatment on penetration behavior was investigated to observe whether residual stresses arise from manufacturing process changes the penetration behavior. Confinement effects were studied experimentally with a special fixture, allowing tested specimen to be radially confined with other core units as in an armor structure. Finally, experimental work was finished by conducting a case study where core units were subjected to spherical projectile impact up to impact velocities of 180 m.s-1 in a composite sandwich structure. Results show that each indenter geometry showed unique deformation characteristics in testing of both core units and both of the core geometries were able to hold a steel spherical projectile with mass of 110 g without full perforation at impact velocity of 180 m.s-1. Details of experimental results were presented in each chapter. Study also included modeling parts where core units were numerically produced with residual stresses and strains and good correlation was noted where thickness was compared with actual measurements on core units. Test conducted on single core structure in as-received and heat-treated condition were also repeated in numerical environment, where numerical study exhibited good correlation on both forcedisplacement curves and deformation of core units with tests. Correlation achieved with experimental study has led into further investigations of strain rate and micro-inertia where behavior of core units was studied at numerical impact velocities of 300 m.s-1. Results show that both strain rate and micro-inertia increase the local maximums and average of force levels. Effect of strain rate and micro-inertia is clearly distinguished for a threshold displacement level where micro-inertia is further dominant on behavior.
  • Doctoral Thesis
    Modeling of Concrete Under High Strain Rate Conditions Using Nonlinear Finite Element Method
    (Izmir Institute of Technology, 2017) Çankaya, Mehmet Alper; Saatcı, Selçuk; Taşdemirci, Alper
    In this study, a comprehensive experimental and numerical study was undertaken to model concrete under high strain rate conditions. Concrete cylinder specimens, all obtained from the same batch, were tested both under ststic and high strainrate conditions. 15 eylinder specimens were tested under 3.55x10-5, 3.23x10-4, 2.97x10-3 1/s strain rates, whereas three identical specimens were tested using a Split Hopkinson Pressure Bar SHPB) tes setup under 235, 245, 260 1/s strain rates. Used SHPB setup was modified to include quartz crystal stress developed in the specimens werw directly obtained, eliminating common isssues regarding stress readings in a conventional setup. Stress-strain behavior and other material parameters that would be necessary for numerical modeling were obtained under various strain rates. Test samples were modeled using an explicit finite element program LS-DYNA, using Holmquist-Johnson-Cook model with experimentally obtained model parameters. To verify the obtained parameters further, drop tower test on concrete plates were also performed and modeled. Numerical modeling of both SHPB samples and concrete plates were successful in capturing the observed behavior. The study also provided the literature with a reliable test data with complete parameters that can be used for further studies in the area.
  • Doctoral Thesis
    Experimental and Numerical Evaluation of the Blast-Like Loading of Fiber Reinforced Polymer Composites and Aluminum Corrugated Core Composite Sandwiches Through Projectile Impact Testing Using Aluminum Corrugated Projectiles
    (Izmir Institute of Technology, 2015) Odacı, İsmet Kutlay; Güden, Mustafa; Taşdemirci, Alper
    This thesis develops and validates a laboratory scale blast-like testing method that can simulate explosive blast tests in air and under water without using explosives. The study has mainly focused on the shock loading potential of 1050 H14 trapezoidal corrugated core aluminium sandwich structures on E-glass/polyester composite plates and corrugated core composite sandwich structures experimentally, numerically and analytically. The composite plates were modelled using MAT_162 material model in LS-DYNA finite element code. Quasi-static and high strain rate tests were performed to determine the material model parameters of composite and corrugated structure. The resultant parameters were calibrated and validated by comparing the numerical results with the experimental results. The planar shock wave formation and propagation in corrugated core sandwich structures were shown experimentally using a direct impact Split Hopkinson Pressure Bar test set-up. Rigid-perfectly-plastic-locking material model and Hugoniot jump relations revealed the shock loading potential of the tested corrugated core sandwich structures. The shock loading response of composite plates and sandwich structures were investigated by firing the corrugated sandwich projectiles on the targets. These impact tests were also simulated numerically and an analytic model was used to predict the plate deflections. The experimentally, numerically and analytically determined back face deflections were compared with the deflections of the Conwep blast simulations in LS-DYNA. The results have shown that the corrugated core sandwich structures can generate shock loading as in the explosive blast tests and can be used to produce shock loads in laboratory scale experiments.
  • Doctoral Thesis
    Experimental and Numerical Approaches To Evaluate the Crushing Behavior of Combined Geometry Core Sandwich Structures Against Blast
    (Izmir Institute of Technology, 2015) Kara, Ali; Taşdemirci, Alper; Güden, Mustafa
    In this study, novel sandwich structures containing combined geometry structures as core materials were designed and developed for blast protection applications. The proposed combined geometries consist of a hemispherical geometry attached seamlessly to a cylindrical segment. Deep drawing method was used to obtain four different types of combined geometries having two different radii from blanks with two different initial thicknesses. The mechanical properties of the blank material were obtained by conducting tensile experiments at quasi-static and high strain rate regimes. Thereafter, crushing and energy absorption behavior of core units were determined by tests at quasi-static and low velocity regimes, experimentally. Before crushing simulations, manufacturing method was simulated to have realistic residual stress/strain and thickness variations of numerical specimens. Having accurate deformation history, crushing experiments were simulated and a good agreement was reached proving the realistic modeling of the manufacturing effects. The effect of heat treatment on the crushing behavior of combined geometry shells was also investigated both experimentally and numerically and there was a good agreement noted. After, cross-shaped sandwich structures of one type of combined geometry were prepared. Static, low velocity and high velocity crushing behavior of sandwiches were investigated. Study on sandwich structures also included confined experiments in order to account for the interaction between the core units and between the core units and surrounding environment; such a case might be a bigger sandwich in which adjacent cores could exert forces to each other. Numerical study was validated by comparing experimental and numerical results of three different loading regimes for sandwiches. Having well-verified numerical models, numerical study was extended to investigate strain rate and inertial effects on sandwich structures by simulations at high crushing velocities. With complete knowledge on crushing and energy absorption of single geometries and sandwiches, behavior of sandwiches under blast was investigated by using ConWep function. Various types were proposed for arrangements of sandwiches to have higher energy absorption and lower transmitted forces to the protected structures.
  • 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.