Browsing by Author "Taşdemirci, Alper"

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The deformation behavior of a multi-layered aluminum corrugated structure at increasing impact velocities
(Izmir Institute of Technology, 2017-12) Sarıkaya, Mustafa Kemal; Güden, Mustafa; Taşdemirci, Alper
The compression impact deformation of a layered 1050 H14 aluminum corrugated sandwich structure was determined both experimentally and numerically under low, intermediate and high velocities to investigate the validity of the perfect and imperfect models. Three-dimensional finite element models of the tested specimens were developed using the LS-DYNA. At increasing velocities from quasi-static velocity to 200 m s-1, the tested corrugated structures showed three distinct deformation modes: between 0.0048 and 22 m s-1 the deformation was quasi-static homogenous mode; between 22 and 60 m s-1 a transition mode and above 90 m s-1 a shock mode. These observations were also confirmed by the camera records and model layer strain profiles. The imperfect models predicted the deformation behavior in homogeneous and transition modes, while the imperfect and perfect models both well predicted the shock mode. Layer strain profiles showed that as the velocity increased, the crushed layer densification strains increased. The numerical models and experiments of direct impact tests showed that distal end crushing stress increased with increasing velocity. The increase of the stress within the homogeneous and transient mode velocities was ascribed to the micro-inertia effect and the tested corrugated structure showed a Type II behavior. The rigid perfectly plastic locking (r-p-p-l) model prediction using quasi-static plateau stress and densification strain and quasi-static plateau stress and numerically determined densification strain at that specific velocity resulted higher velocities and full densification, while the r-p-p-l model based on varying plateau stress and densification strain well predicted in the shock mode.
Development and design of closed-cell aluminum foam-based lightweight sandwich structures for blast protection
(Izmir Institute of Technology, 2008) Ergönenç, Çağrı; Taşdemirci, Alper
Blast performance and energy absorption capability of closed-cell aluminum foam based lightweight sandwich structures were investigated by a coupled experimental and numerical technique to find out the effect of face and core material on the blast response. Split Hopkinson Pressure Bar Testing Method (SHPB) was used to characterize the mechanical properties of constituents of the sandwich structures at high strain rates. A SHPB set-up, a high strain rate testing apparatus which can successfully create blast load at laboratory scales, was built at IZTECH on behalf of a TUBITAK project (106M353). The high strain rate test data were used as an input for the numerical models. Closed-cell aluminum foam was chosen as core material for sandwich structures owing to its high energy absorption characteristic while deforming plastically. Finite element modeling of sandwich structures subjected to blast loading were performed for different core and face thicknesses and face materials in order to investigate their effects on the blast load mitigation.Experimentally and numerically revealed conclusions are; sandwich structures absorbed more energies than the bulk materials from %50 to %150 when appropriate combinations of core and face materials are used. Numerical simulations showed that 6.3 and 7.2 cm thick foam interlayer are the most efficient foam thicknesses for a 9 cm sandwich plate against 10 kg TNT blast load. Another important conclusion is for the same blast threat i.e. 10 kg of TNT, AISI 4340 Steel is the most effective face material.
The development of a new testing methodology in dynamic mechanical chracterization of concrete
(Izmir Institute of Technology, 2018-07) Seven, Semih Berk; Taşdemirci, Alper; Güden, Mustafa
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.
The development of forming simulation methodology of a plate type heat exchanger
(01. Izmir Institute of Technology, 2023-07) Şimşek, İbrahim; Taşdemirci, Alper
In this study, the production process of plate type heat exchangers was developed as a simulation methodology. Within the scope of the study, first, the parameters in the production process were determined. Then, mechanical characterization studies were planned with the AISI 316L stainless steel material used during production and the alternative AISI 304 stainless steel material, and the tests were completed with the support of the relevant stakeholders. The tests were determined according to the requirements of the simulation methodology. In this context, uniaxial tensile test, biaxial hydraulic bulge test and Split Hopkinson tensile tests were performed to obtain the necessary inputs for the mechanical characterization of the material and creating the material model. The material models established with the information obtained from the tests were validated with the modeling of the test setups in the numerical environment. The simulation methodology was developed in the LS-DYNA environment in the light of the process parameters obtained from the production and the data obtained from the mechanical characterization tests. The simulation model created with the developed methodology was verified because of comparison with the sample produced from AISI 316L stainless steel material taken from production. After the verified model was obtained, a simulation model was created with AISI 304 stainless steel. In addition, for the model formed with AISI 316L stainless steel, process parameters optimization study was carried out, and preliminary work activities related to reducing production times were carried out in numerical environment. After these modeling activities, the knowledge of the license plate was increased. In addition, effective plastic stress during the process, springback effect, residual stress values after springback, effective plastic strain, thickness distribution and thickness reduction values were obtained for the plate. By using the forming limit diagram of AISI 316L stainless steel, information about the final formability behavior was obtained.
Dynamic crushing behavior of sandwich panels with bio-inspired cores
(Izmir Institute of Technology, 2017-07) Güzel, Erkan; Taşdemirci, Alper; Güden, Mustafa
In the current study, a new approach was shown to develop an innovative loadcarrying and energy absorbing structure which can fulfill the requirements in the fields of automotive, defense and aerospace. Two different topics which have been in great demand in the recent times were combined: sandwich structures and bio-inspiration. Balanus which is a barnacle living along the seashores and on the ships’ surfaces was taken under examination to design a novel sandwich structure core geometry. The designed geometry was manufactured with deep drawing process. The sandwich structures were produced with different face sheets using a pattern to ensure the repeatability of the crushing tests. Firstly, the advantage of the bio-inspired core over the conventional core geometries was shown with a numerical study. Then, the crushing tests were conducted at both quasi-static and dynamic loading rates. Further, the effects of foam filling, confinement, inertia and strain rate sensitivity on the crashworthiness performance of the proposed structure were investigated. In addition to the experimental studies, numerical analyses were also performed using LS-DYNA 971. In the numerical studies, manufacturing process of the core geometry was also modeled to count in the residual stress/strain so that a good proximity was obtained between the experimental and numerical results. Moreover, the penetration and perforation behaviors were inspected. Utility of the proposed geometry where a high resistance is needed against dynamic crushing was demonstrated. Finally, several suggestions were proposed for the future works to elaborate the present study.
Dynamic crushing behaviour of cactus geometry inspired core structure
(Izmir Institute of Technology, 2019-12) Balya, Ozan; Taşdemirci, Alper; Güden, Mustafa
Cactus geometry inspired core structure was manufactured with the fused deposition modelling method by a 3D printer using Acrylonitrile Butadiene Styrene (ABS) material filament. The characterization of ABS was made by performing compression tests to take some parameters for numerical models. Numerical preliminary studies were carried out by using the areal density concept and direct-impact Hopkinson pressure bar test method to compare the cactus geometry with the conventional ones in point of the specific energy absorption capacity (SEA). It was understood that from the preliminary work, the cactus inspired structure is intriguing to investigate the dynamic crushing behaviour at least. Quasi-static, drop weight and direct-impact Hopkinson pressure bar tests were conducted to comprehend the energy absorption and crushing behavior in all cases, then to investigate the strain rate and inertia effects on the structure. Implicit and explicit numerical models were made by using LS-DYNA software to validate experiments and to set a precedent for future works. It was seen that the result of numerical models is in harmony with that of experiments excluding the non-fracture structure at the quasi-static implicit model. Moreover, although quasi-static compression gave the structure more stable deformation behavior compared to drop weight impact, higher energy absorption capability was observed on drop-weight tests. In addition, the strain rate effect is more forceful in point of loading carrying capacity compared to the inertia effect despite the fact that it provides the development of buckling and damage formation.
Dynamic force measurement techniques Split Hopkinson Pressure Bar testing of low acoustic impedance materials used as armor interlayer materials
(Izmir Institute of Technology, 2012) Turan, Ali Kıvanç; Taşdemirci, Alper
GoreTM PolarchipTM heat insulating Teflon and Dow ChemicalsTM Voracor CS Polyurethane were characterized in this study by conducting compression tests at various strain rates. Quasi-static compression tests were done with a Shimadzu AG-X conventional test machine while two different modified Split Hopkinson Pressure Bar (SHPB) systems were used for dynamic compression tests. Since dynamic testing of soft materials with classical SHPB is problematic due to low signal levels and relatively higher signal to noise ratio, impact end of transmitter bar was modified with insertion of piezoelectric force transducers through the SHPB tests of Teflon, thus enabling the direct measurement of force on specimen. High strain tests of Polyurethane involved oscillations in both incident and transmitter bar signals. To overcome this, EPDM rubber pulse shaper was used through the SHPB tests of Polyurethane. Experimental results were used in numerical study as material model parameters and SHPB tests of both materials were simulated in LS-DYNA. Experimental study concluded strong strain rate dependency in both Teflon and Polyurethane, depicting an increase in maximum stress with the increase in strain rate. Numerical study showed a good correlation with experiments in terms of bar stresses and damage behavior of specimens, offering a solution to more complex problems that can be encountered in future studies.
The dynamic mechanical characterization of a bio-inspired sandwich structure
(Izmir Institute of Technology, 2019-12) Ramyar, Ayda; Taşdemirci, Alper; Güden, Mustafa
In this study, the sandwich structure consisting of novel-3D-printed-polymeric base core was examined in terms of crashworthiness. The designed core structure for energy absorption purpose is inspired by the geometry of the human fingerprint. The fingerprint geometry is a spiral-shaped, asymmetrical and complex structure; therefore, the manufacturing of the geometry is difficult by conventional manufacturing methods. Fused Deposition Modeling (FDM) which is one of the additive manufacturing (AM) methods was used for fingerprint core preparation by layer by layer production technique with low-density material. After the material characterization of 3D printed thermoplastic specimens, optimum geometric parameters of fingerprint were determined via experimental and numerical studies by changing the height and thickness. The fingerprint performed better crushing performance compared to other conventional geometries. Quasi-static and dynamic crushing experiments were conducted, and the results were verified with models by non-linear finite element code LS-DYNA. The results showed that the energy absorption capacity and peak crushing force of fingerprint geometry increases with strain rate increment. However, the deformation behavior of the structure under dynamic loads changes and the material becomes more brittle. This is caused by the change in deformation mechanism due to AM and material itself. It was found that the 3-D printed core structure is suitable to be employed at low-to-medium strain rates due to its multi-stage deformation behavior. It was observed that the bio-inspired sandwich structure consisting of 4 fingerprint-core can absorb 10% more impact energy than fourfold individual 3-D printed core geometry, which indicates the promising potential of the novel sandwich structure for crashworthiness applications.
The effect of material strain rate sensitivity on the shock deformation of an aluminum corrugated core
(Izmir Institute of Technology, 2018-06) Canbaz, İlker; Güden, Mustafa; Taşdemirci, Alper
The effect of the material model on the crushing behavior of a layered 1050 H14 aluminum corrugated sandwich structure was investigated numerically as function of velocity (0.0048, 20, 60, 150 and 250 m s-1) using three different material models; elastic-perfectly plastic (model I), elastic-strain hardening (model II) and elastic-strain and strain rate hardening (model III). Three-dimensional finite element models were developed in the explicit finite element code of LS-DYNA. Between 0.0048 m s-1 and 20 m s-1, the numerically calculated stresses at the impact and distal end were almost the same and in equilibrium, showing a “quasi-static homogenous mode”. The deformation mode at 60 m s-1 was a “transition mode” and between 150 and 250 m s-1 a shock mode in which the layers were crushed sequentially. The numerical study showed that the strain and strain rate hardening models tended to induce non-sequential layer crushing. The collective layer crushing was also more pronounced in the material model II and III than the material model I. For low strain hardening aluminum alloys and similar materials, the effect of strain hardening in increasing plateau stress was more significant than the strain rate hardening at the quasi-static velocity, while both strain hardening and strain rate hardening effect increased with increasing velocity. The stress reduction by the inclusion of imperfections however declined with the velocity since the samples started to deform near the impact end as the velocity increased.
The effect of strain rate on the dynamic mechanical behaviour of concrete
(Izmir Institute of Technology, 2018-07) Uysal, Çetin Erkam; Taşdemirci, Alper; Güden, Mustafa
The fast-growing population of mankind has brought out household needs and working structures that might be subjected to static and dynamic loads. Impact loads and repetitive dynamic loads can produce an overload on the structures in a very short period that causes relentless casualties and unfortunate property losses. The response of the concrete material on strain rate increase is critical. The dynamic characterization of concrete, lack of adequate and consistent study causes disagreement about strain rate sensitivity of concrete, so a consensus has not been reached. In this study, quasi-static (3.55x10-5, 3.23x10-4, and 2.97x10-3 s-1) and high strain rate (140-250 s-1) tests were conducted and the effect of strain rate on the mechanical behavior of concrete was investigated both experimental and numerical. A modified Split Hopkinson Pressure Bar test setup was used, by using an EPDM (Ethylene Propylene Diene Monomers) rubber pulse shaper, non-oscillatory results and nearly constant strain rate were reached, and premature failure was prevented. Modeling the test setup was conducted in Ls-Dyna and the Holmquist-Johnson Cook material model parameters were found. A good agreement between experimental and numerical results was reached. The strength enhancements of concrete material, while increasing strain rate was noticed. Using both experimental and numerical studies, the total strength increase is due to inertia effect and strain rate sensitivity effects were observed.
The effects of light-weight interface material on the stress wave propagation in the multilayered composite armor system
(Izmir Institute of Technology, 2011) Tunusoğlu, Gözde; Taşdemirci, Alper
The main purpose of the current study is to investigate the effect of interlayer material on the ballistic performance of composite armor and stress wave propagation both experimentally and numerically. Three different interlayer materials, EPDM rubber, Teflon and Aluminum metallic foam, were tried. Relatively large pieces of the ceramic around the impact axis in the rubber interlayer configuration were observed while the ceramic layer was efficiently fragmented in Aluminum foam and Teflon interlayer configurations. Accordingly, more significant amount of delamination in composite layer of without interlayer, larger and deeper delamination in EPDM rubber configurations was observed while fewer amounts were observed on Teflon and Aluminum foam configurations .Also, all interlayers caused reduction in the magnitude of the stress transmitted to the composite backing plate, particularly Aluminum foam. However, EPDM rubber did not cause delay in the initial stress build-up in the composite layer, whereas Teflon (~15 ms) and Aluminum foam (~25 ms) caused a significant delay. Also, as ceramic was efficiently fragmented in Teflon and Aluminum metallic foam interlayer configurations, greater amount of projectile kinetic energy was absorbed in this layer, as a consequence, the remaining energy which was transmitted to composite backing plate was decreased. At this point, the effectiveness of Aluminum foam and Teflon were validated with conducting ballistic tests and corresponding numerical simulations and impact chamber tests. After this validation, the ballistic performance of aforementioned materials was compared at equal areal densities. Finally, Aluminum foam was found to be more effective interlayers in reducing the stress values transmitted to the composite backing plate and reduction of the damage imparted to this layer.
Experimental and numerical analysis of the strain rate dependent compressive strength of a cellular concrete
(Izmir Institute of Technology, 2019-12) Akyol, Burak; Güden, Mustafa; Taşdemirci, Alper
Experimental and numerical quasi-static and high strain rate tests, including compression, indentation and direct impact, were performed on a cellular concrete in order to investigate the effect of strain rate on the compressive strength. The results of compression tests indicated three distinct regions of the compressive strength dependence on strain rate. A relatively lower strain rate dependent compressive stress was found in the quasi-static strain rate-regime, 2x10-3-2x10-1 s-1, a relatively high strain rate dependent compressive stress in the dynamic strain rate-regime, 180-103 s-1 and a cut-off strength above 103 s-1. The dynamic increase factor (DIF=dynamic/static fracture strength) varied between 1 and 2.5 from quasi-static to dynamic strain rate-regime with a sharp increase after about 100 s-1. The indentation tests using 25 and 30 mm-diameter indenters in the quasi-static strain rate-regime (uniaxial state of strain) and resulted in moderate DIF values (1-1.13), very similar with those of the quasi-static compression tests (1-1.15). In the indentation tests, the DIF values significantly and also confirmed the numerically determined DIF values of concrete at 1000 s-1 (~1.30) without radial and axial inertia. The compression and direct impact tests in the Split Hopkinson Bar (SHPB) set-up were implemented numerically in LS-DYNA using an anisotropic strain rate insensitive material model, MAT_096 (MAT BRITTLE DAMAGE). The stress readings were performed at the specimen different locations of the SHPB and indicated that radial and axial inertia were dominant between 1 and 30 m s-1 (30-1000 s-1).
Experimental and numerical approaches to evaluate the crushing behavior of combined geometry core sandwich structures against blast
(Izmir Institute of Technology, 2015-07) 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.
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, 2015-09) 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.
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
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.
Foaming of waste glass of a glass polishing factory
(Izmir Institute of Technology, 2012) Attila, Yiğit; Taşdemirci, Alper
The foaming behavior of a glass powder, a residue from a window glass polishing factory in Bursa, was investigated at the temperatures between 700-950°C. As-received glass powder composition, 72.76% SiO2, 11.18% Na2O, 11.31% CaO, 1.74% MgO and 1.61% Al2O3, was well matched with that of soda lime window glass. The expansion of the glass powder compacts started at a characteristic temperature of 690-700 °C and reached a maximum volumetric expansion values at about 866-877 °C. The maximum volume expansion and foam density varied between 700-772% and 0.378-0.206 g/cm3, respectively. The foaming of the compact at 750 °C yielded only crystalline phase of quartz, as the foaming temperature increased over 750 °C, wollastonite and diopsite crystals formed The compressive strength of the foams ranged between 1.9 and 4 MPa and the thermal conductivity between 0.048-0.079 W/K m. Both collapse and plateau stresses increased with increasing relative density, while heating rate was found to be not affect the collapse and plateau stresses. The foamed glass samples showed the mechanical behavior similar to open cell foams. This was attributed to the thicker cell edges and thinner cell walls leading to higher glass material accumulation on the cell edges. The self-foaming behavior of the studied waste glass powder was attributed to the organic compounds within the boron oil which was used as a coolant in the polishing operations.
The investigation of blast response of sandwich panels with bio-inspired cores
(Izmir Institute of Technology, 2017-07) Tüzgel, Fırat; Taşdemirci, Alper; Güden, Mustafa
In this thesis, blast response of sandwich structures with bio-inspired cores having applicable potential for protection against blast loading, balanus, were investigated in detail. The proposed geometry consists of outer shell and inner core which separately manufactured using deep-drawing method. Commonly used blast simulation methods which are pure Lagrangian, Arbitrary Lagrangian Eulerian (ALE), and hybrid (coupled other two approaches) approaches were comparatively investigated as finding their main outstanding features and drawbacks after investigation of blast phenomenon. Calibration study with facesheet of sandwich structure was conducted to demonstrate practically performance of blast simulation methods and tune essential parameters. Well proximity between results was obtained in calibration study. Converge analysis which is especially mandatory in ALE approach was also implemented employing Grid Converge Index (GCI) for selection of mesh density of air and plate in calibration study. Pure Lagrangian approach is conservative approach among the studied blast simulation methods was shown. Direct Pressure Pulse (DPP) experiments were separately conducted for facesheets of sandwich and complete sandwich structures to reveal dynamic performance of them. Equivalent blast loading conditions corresponding to each DPP experiment were found as considering deformation levels of the structures. Therefore, DPP experiment as lab-scale experiment effectively mimicked blast-type loading was revealed. Effect of heat treatment and placement of proposed geometries subjected to blast loading were also examined creating large scaled sandwich structures. Finally, it was demonstrated sandwich structure with balanus cores revealed good blast mitigation performance even at low-scaled distance and would be able to satisfied requirement of defence industry.
The investigation of energy absorption characteristics of TPU TPMS structures subjected to impact loading
(01. Izmir Institute of Technology, 2023-07) Bakıcı, Çetin; Taşdemirci, Alper
In this thesis, the energy absorption capability of a schwarz based TPMS structure both experimentally and numerically was invetigated. In the product, TPU material and FDM printer was used. Instead of the regular schwarz primitive cell structure, which has been frequently examined in the literature, the sandwich structure design was prepared with the geometry selected from the region between two cells was used and its advantages were compared. In the selection of the TPMS structure, both its high energy absorption capability per unit weight and its geometry suitable for mass production in the future was important. A hyperelastic material TPU and a printer suitable for its production were selected to show deformation behaviour of the structure against multiple loading. After material characterization with TPU specimens, the determined printer parameters were kept constant, and single and multiple cell structures were produced. Static and dynamic tests were performed, and single and multiple-cell structures were modeled and validated in the LS-DYNA finite element package program. It was observed that as the strain rate increases, the structures densification point also decreased and the first peak force and the energy absorption per unit weight (SAE) increase. In addition, it was observed that the deformation behaviour of single and multiple-cell structures were rate dependent. It has been observed that the structure with 9 cells absorbs 20% more energy than the structure with unit cell, which is 9 times higher than the unit cell structure due to the interaction of cells. The developed structure was numerically exposed to blast loads following Nato Stanag 4569 standart. In this standart, from the defined of the injury criteria,on the lower and upper tibia joint should experienced force values lower than 2.6 kN and 5.4 kN respectively. From the numerial simulations, it was found that the structure was able to mitigate the blast load transmitted to the during the accaptable limits.
The investigation of static and dynamic compressive deformation behavior of a paper based sandwich material
(01. Izmir Institute of Technology, 2022-12) İmrağ, Berkay Türkcan; Taşdemirci, Alper
In this study, dynamic and quasi-static compression behavior of paper-based honeycomb sandwich structures were investigated. It is known that the mechanical properties of paper-based honeycomb structures change with changing strain rate values. For this reason, dynamic and quasi-static loading conditions should be considered separately when investigating the compressive behavior of the structure. In the material characterization studies, a series of tests were conducted to examine mechanical properties of the paper layer material and sandwich structure. Using data from mechanical tests, numerical models were established in the finite element tool LS-DYNA. Outputs of numerical models were validated with mechanical test outputs. After the validation study, the effects that influence the dynamic compressive behavior of the paper-based honeycomb sandwich structure and their contribution percentages were investigated using the opportunities provided by the FE tool. The results showed a 150.48 % difference between the dynamic and quasi-static compressive behavior of the structure. The numerical results obtained from explicit and implicit solvers also showed good correlation with the experimental results. In addition, the micro-mechanical modeling approach in numerical models made it possible to investigate the effects such as strain rate sensitivity of the paper layer material, entrapped air inside the core cells, and micro-inertia individually. The contribution percentages of the effects were calculated by comparing the numerical and experimental results.
The investigation of the dynamic compression characteristics of a layered glass system
(01. Izmir Institute of Technology, 2023-07) Ağırdıcı, Burak; Taşdemirci, Alper
Layered glass structures are one of the most common material types used in air, land, and sea vehicles. Since these structures are exposed to external impact loads, it is important to determine their dynamic mechanical behavior. In this study, dynamic compression characteristics of the layered glass system were investigated numerically using the LS-DYNA finite element program. The Johnson Holmquist Ceramics material model was used for the glass layer, the Ogden Rubber material model, which is used in material models with high elastic structural behavior was used for the polyvinyl butyral (PVB) interlayer, and the SAMP-1 material model was used for the polycarbonate interlayer. Numerical studies were carried out to investigate the stress wave propagation, the amount of energy released, and the deceleration rate of the penetration velocity. Split Hopkinson Pressure Bar setup was used to numerically load the layered glass systems at high strain rates for a reliably easy controlled wave generation. The layered glass structure consisting of two interlayer types with different thicknesses was loaded in the SHPB system, and the effect of the interlayer material type and thickness on the stress wave propagation was investigated. Then, the projectile impact test was modeled at different impact velocities for a square plate of PVB-layered glass structure. The thickness of the PVB interlayer was kept constant, while the thickness and location of the glass layer varied. From the results, the slowing rate of the projectile, the amount of erosion energy, and the energy balance were determined.