Browsing by Author "Seven, Semih Berk"

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Constitutive Equation Determination and Dynamic Numerical Modelling of the Compression Deformation of Concrete
(Wiley, 2021) Seven, Semih Berk; Çankaya, M. Alper; Uysal, Çetin; Taşdemirci, Alper; Saatci, Selçuk; Güden, Mustafa; 03.03. Department of Civil Engineering; 03.10. Department of Mechanical Engineering; 03. Faculty of Engineering; 01. Izmir Institute of Technology
The dynamic compression deformation of an in-house cast concrete (average aggregate size of 2-2.5 mm) was modelled using the finite element (FE), element-free Galerkin (EFG) and smooth particle Galerkin (SPG) methods to determine their capabilities of capturing the dynamic deformation. The numerical results were validated with those of the experimental split Hopkinson pressure bar tests. Both EFG and FE methods overestimated the failure stress and strain values, while the SPG method underestimated the peak stress. SPG showed similar load capacity profile with the experiment. At initial stages of the loading, all methods present similar behaviour. Nonetheless, as the loading continues, the SPG method predicts closer agreement of deformation profile and force histories. The increase in strength at high strain rate was due to both the rate sensitivity and lateral inertia caused by the confinement effect. The inertia effect of the material especially is effective at lower strain values and the strain rate sensitivity of the concrete becomes significant at higher strain values.
The Development of a New Testing Methodology in Dynamic Mechanical Chracterization of Concrete
(Izmir Institute of Technology, 2018) Seven, Semih Berk; Güden, Mustafa; Taşdemirci, Alper; Taşdemirci, Alper; Güden, Mustafa; 03.10. Department of Mechanical Engineering; 03. Faculty of Engineering; 01. Izmir Institute of Technology
Concrete is one of the most used material types in the world. Due to its structural complexity and insufficient testing techniques, the dynamic mechanical behavior of concrete has not yet been revealed sufficiently. This thesis aims to develop reliable and accurate mechanical characterization methodology for concrete using the combination of experimental and numerical methods together. The dynamic mechanical characterization of concrete at quasi-static and high strain rates was performed implementing unique techniques for both experimental and numerical studies. In quasi-static testing, universal compression test machine was used with strain gage mounted specimen for better strain measurements. In high strain rate tests, two modifications were implemented on the conventional Split Hopkinson Pressure Bar (SHPB) test apparatus. The first modification is the usage of pulse shaper to obtain nearly constant strain rate and dynamic stress equilibrium in the specimen. Second, piezo-electric quartz crystal force transducers were implemented on the specimen-bar interfaces to increase accuracy and sensitivity of the force measurement on the front and back forces of the specimen. Experimental results were validated constituting numerical study using finite element tool LS-DYNA. Concrete was modeled using Holmquist-Johnson-Cook (MAT_111) material model. HJC material model parameters were determined using experimental results coupling with the numerical analysis and the mechanical behavior of concrete was constituted. It was concluded that using pulse shaper and quartz crystals pretty useful when testing concrete and other brittle materials at high strain rates. Modification of new specimen geometries on numerical analysis showed better understandings of the effect of geometry on the dynamic stress equilibrium.
Experimental and Numerical Investigation of the Impact Resistance and Impact Damage Tolerance of a Carbon Fiber Reinforced Thermoplastic Polyphenylene Sulfide (pps) Matrix Composite
(01. Izmir Institute of Technology, 2024-07) Seven, Semih Berk; Güden, Mustafa; Taşdemirci, Alper; 03.10. Department of Mechanical Engineering; 03. Faculty of Engineering; 01. Izmir Institute of Technology
The impact resistance and impact damage tolerance of an aerospace grade high performance 5 Harness Satin woven fabric carbon fiber reinforced/polyphenylene sulfide matrix (CF/PPS) thermoplastic composite were investigated experimentally and numerically. The numerical modeling was performed using the experimentally determined parameters of material model MAT-58 and Hashin failure criteria in LS-DYNA using the single shell and stacked shell models. The numerical models of the low velocity impact (LVI) tests showed good correlations with the experimental tests while the stacked shell model showed nearer results with the experimental tests. The stacked shell model also estimated the LVI test delamination areas, which were comparable with the experimental damage areas. The LVI tested coupons were further subjected to the compression after impact (CAI) tests in order to determine the damage tolerance of CF/PPS composite. The CAI tests were modeled using the single shell model. The numerical models of the CAI tests showed very similar trends with the experimental CAI tests. The trends were shown to be more converging in the specimens tested at 3 m/s and above in the LVI tests. Lastly, three high velocity impact (HVI) tests were performed at around 100 m/s. The failure mode of the HVI tests was shown to be very different from that of the LVI tests. The long longitudinal and transverse cracks were formed in the HVI tests. The delamination damage in the HVI tests determined using the stacked shell model was found to be more comparable with the experimental delamination damage determined by the C-Scan.