Mechanical Engineering / Makina Mühendisliği
Permanent URI for this collectionhttps://hdl.handle.net/11147/4129
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Research Project Beton için Yeni Bir Statik ve Dinamik Mekanik Karakterizasyon Metodolojisi Geliştirilmesi(2017) Taşdemirci, Alper; Güden, Mustafa; Saatcı, SelçukGünümüze kadar beton malzemesi üzerine yapılan çalısmalarda betonun sekil degistirme hızına baglı olarak mukavemetinin degisimi konusunda bir fikir birligi olusturulamamıstır. Betonun yüksek deformasyon hızı testleri esnasında karsılasılan zorluklar nedeniyle test verilerinden elde edilen sonuçlar farklı sekillerde yorumlanmaktadır. Günümüzde Split Hopkinson Basınç Barı testi bu amaçla en yaygın olarak kullanılan test metodudur. Fakat testler esnasında numunede homojen olmayan gerilme dagılımı meydana gelme riski ve gerilme dalgasında dispersiyon egilimi vardır. Bahsi geçen problemleri asmak amacıyla proje kapsamında dinamik test düzeneklerinde bazı inovatif iyilestirmeler uygulanmıstır. Bunlar piezoelektrik kuartz kristal ve dalga sekillendirici kullanımıdır. Piezoelektrik kuartz kristaller numune çubuk ara yüzeylerine dogrudan yerlestirildigi için gerilme dalgasındaki dispersiyon etkisi minimize edilir. Böylece numunede meydana gelen gerilme tarihçesi daha yüksek hassasiyetle ve farklı noktalardan ölçülebilir. Numune içerisinde gerilme dengesinin saglanması beton gibi gevrek karakterli bir malzemede prematüre kırılma egiliminin önlenmesi açısından oldukça önemlidir. Proje kapsamındaki deneylerde gerilme dalgası sekillendiricisi kullanılarak gerilme dalgasının siddeti ve yükleme hızı kontrol edilebilmistir. Bu sayede numune içerisinde homojen bir gerilme dagılımı saglanmıs ve prematüre kırılma egilimi önlenmistir. Statik ve dinamik mekanik karakterizasyon sonuçları incelenerek beton malzemenin mekanik davranısına uygun bir malzeme modeli seçilmis ve gerekli parametreler belirlenmistir. Belirlenen parametrelerin dogrulukları farklı yükleme kosulları altında test edilmistir. Bu amaçla düsen agırlık testleri icra edilmis ve numunelerde meydana gelen hasarların ve kuvvet tarihçelerinin deneylerle olan uyumları nümerik model sonuçlarıyla tayin edilmistir. Elde edilen sonuçlar incelendiginde betonun mukavemetinde sekil degistirme hızının artısıyla birlikte bir artısın meydana geldigi tespit edilmistir. Bu artısın iki ana sebebi vardır. Bunlardan ilki yüksek hızda meydana gelen hasar esnasında olusan mikro atalet etkisidir. Ikincisi ise beton malzemenin ihtiva ettigi su ve gözenekli yapısından kaynaklanan viskoz davranısıdır. Bu ikinci etkiye malzemenin sekil degistirme hızı hassasiyeti olarak bakılabilir. Yürütülen deneysel ve nümerik çalısma sayesinde bu etkilerin bireysel olarak toplam mukavemet artısındaki etkinlikleri tespit edilebilmistir. Bu sonuç dünya literatürüne oldukça önemli bir katkıdır.Article 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, MustafaThe 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.Article Citation - WoS: 6Citation - Scopus: 7Experimental and Numerical Investigation of the Effect of Interlayer on the Damage Formation in a Ceramic/Composite Armor at a Low Projectile Velocity(SAGE Publications Inc., 2017) Taşdemirci, Alper; Tunusoğlu, GözdeThe damage formation in a multilayered armor system without and with an interlayer (rubber, Teflon, and aluminum foam) between the front face ceramic layer and the composite backing plate were investigated experimentally and numerically. The projectile impact tests were performed in a low-velocity projectile impact test system and the numerical studies were implemented using the nonlinear finite element code LS-DYNA. The results of numerical simulations showed that the stress wave transmission to the composite backing plate decreased significantly in Teflon and foam interlayer armor configurations. Similar to without interlayer configuration, the rubber interlayer configuration led to the passage of relatively high stress waves to the composite backing plate. This was mainly attributed to the increased rubber interlayer impedance during the impact event. The numerical results of reduced stress wave transmission to the backing plate and the increased damage formation in the ceramic front face layer with the use of Teflon and foam interlayer was further confirmed experimentally.Article Citation - WoS: 71Citation - Scopus: 81The Impact Responses and the Finite Element Modeling of Layered Trapezoidal Corrugated Aluminum Core and Aluminum Sheet Interlayer Sandwich Structures(Elsevier Ltd., 2013) Kılıçaslan, Cenk; Güden, Mustafa; Odacı, İsmet Kutlay; Taşdemirci, AlperThe impact responses of brazed and adhesively bonded layered 1050 H14 trapezoidal corrugated aluminum core and aluminum sheet interlayer sandwich panels with 3003 and 1050 H14 aluminum alloy face sheets were investigated in a drop weight tower using spherical, flat and conical end striker tips. The full geometrical models of the tests were implemented using the LS-DYNA. 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 impacted using a conical striker tip were penetrated/perforated and showed comparably smaller deformation forces and energy absorptions, especially in 0°/90° layer orientation. The simulation and experimental force values were shown to reasonably agree with each other at the large extent of deformation and revealed the progressive fin folding of corrugated core layers and bending of interlayer sheets as the main deformation mechanisms. The experimentally and numerically determined impact velocity sensitivity of the tested panels was attributed to the micro inertial effects which increased the critical buckling loads of fin layers at increasingly high loading rates.Article Citation - WoS: 118Citation - Scopus: 144The Effect of the Interlayer on the Ballistic Performance of Ceramic/Composite Armors: Experimental and Numerical Study(Elsevier Ltd., 2012) Taşdemirci, Alper; Tunusoğlu, Gözde; Güden, MustafaThe effect of rubber, Teflon and aluminum foam interlayer material on the ballistic performance of composite armor was investigated both experimentally and numerically. Although, rubber interlayer did not cause any significant delay in the initial stress build-up in the composite layer, Teflon and aluminum foam interlayer caused a significant delay and reduction in the magnitude of the stress transmitted to the composite backing plate. Damage in the ceramic layer was found to be highly localized around the projectile impact zone for the configuration without interlayer and rubber interlayer while aluminum foam and Teflon interlayer spread the damage zone in the radial direction. 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 configuration.Article Citation - WoS: 27Citation - Scopus: 28Split Hopkinson Pressure Bar Multiple Reloading and Modeling of a 316 L Stainless Steel Metallic Hollow Sphere Structure(Elsevier Ltd., 2010) Taşdemirci, Alper; Ergönenç, Çağrı; Güden, MustafaThe high strain rate (600 s−1) compression deformation of a 316 L metallic hollow sphere (MHS) structure (density: 500 kg m−3; average outer hollow sphere diameter: 2 mm and wall thickness: 45 μm) was determined both numerically and experimentally. The experimental compressive stress–strain behavior at high strain rates until about large strains was obtained with multiple reloading tests using a large-diameter compression type aluminum Split Hopkinson Pressure Bar (SHPB) test apparatus. The multiple reloading of MHS samples in SHPB was analyzed with a 3D finite element model using the commercial explicit finite element code LS-DYNA. The tested MHS samples showed increased crushing stress values, when the strain rate increased from quasi-static (0.8 × 10−4 s−1) to high strain rate (600 s−1). Experimentally and numerically deformed sections of MHS samples tested showed very similar crushing characteristics; plastic hinge formation, the indentation of the spheres at the contact regions and sphere wall buckling at intermediate strains. The extent of micro-inertial effects was further predicted with the strain rate insensitive cell wall material model and with the strain rate sensitive behavior of MHS structure similar to that of the cell wall material. Based on the predictions, the strain rate sensitivity of the studied 316 L MHS sample was attributed to the strain rate sensitivity of the cell wall material and the micro-inertia.Article Citation - WoS: 3Citation - Scopus: 5The Effect of Strain Rate on the Mechanical Behavior of Teflon Foam(Elsevier Ltd., 2012) Taşdemirci, Alper; Turan, Ali Kıvanç; Güden, MustafaThe quasi-static (1 × 10−3, 1 × 10−2 and 1 × 10−1 s−1) and high strain rate (7200 and 9500 s−1) experimental and high strain rate numerical compression deformation of a Gore Polarchip™ CP7003 heat insulating Teflon foam was investigated. High strain rate tests were conducted with the insertion of quartz crystal piezoelectric transducers at the end of the transmitter bar of a compression Split Hopkinson Pressure Bar (SHPB) set-up in order to measure the force at the back face of the specimen. A fully developed numerical model of the SHPB test on Teflon was also implemented using LS-DYNA. The simulation stresses showed close correlations with the experimentally measured stresses on the bars. The developed model successfully simulated the high strain rate loading. The damage initiation and progression of experimental high strain rate tests were further recorded using a high speed camera and found to be very similar to those of the simulation high strain rate tests.Article Citation - WoS: 32Citation - Scopus: 37Modeling Quasi-Static and High Strain Rate Deformation and Failure Behavior of a (±45) Symmetric E-glass/Polyester Composite Under Compressive Loading(Elsevier Ltd., 2013) Kara, Ali; Taşdemirci, Alper; Güden, MustafaQuasi-static (1 × 10−3–1 × 10−2 s−1) and high strain rate (∼1000 s−1) compressive mechanical response and fracture/failure of a (±45) symmetric E-glass/polyester composite along three perpendicular directions were determined experimentally and numerically. A numerical model in LS-DYNA 971 using material model MAT_162 was developed to investigate the compression deformation and fracture of the composite at quasi-static and high strain rates. The compressive stress–strain behaviors of the composite along three directions were found strain rate sensitive. The modulus and maximum stress of the composite increased with increasing strain rate, while the strain rate sensitivity in in-plane direction was higher than that in through-thickness direction. The damage progression determined by high speed camera in the specimens well agreed with that of numerical model. The numerical model successfully predicted the damage initiation and progression as well as the failure modes of the composite.Conference Object Citation - WoS: 11Citation - Scopus: 18Experimental and Numerical Investigation of High Strain Rate Mechanical Behavior of a [0/45 - 45] Quadriaxial E-glass/Polyester Composite(Elsevier Ltd., 2011) Taşdemirci, Alper; Kara, Ali; Turan, Ali Kıvanç; Tunusoğlu, Gözde; Güden, Mustafa; Hall, Ian W.Quasi-static (10−3–10−1 s−1) and high strain rate (∼900 s−1) compression behavior of an E-Glass fiber woven fabric reinforced Polyester matrix composites was investigated by using a Shimadzu AG-I testing machine and a Split Hopkinson Pressure Bar apparatus in the Dynamic Testing and Modeling Laboratory of Izmir Institute of Technology. During the experiments, a high speed camera was used to determine deformation behavior. In both directions, modulus and failure strength increased with increasing strain rate. Higher strain rate sensitivity for both elastic modulus and failure strength was observed in the in-plane direction. Based upon these experimental data, a numerical model was developed using the commercial explicit finite element code LS-DYNA to investigate compressive deformation and damage behavior of composites. Excellent agreement was demonstrated for the case of high strain rate loading. Also, the fracture geometries were successfully predicted with the numerical model.Article Karmaşık ve İçi Boş Parçaların Çapraz Kama Haddeleme İşlemi: Sonlu Elemanlar Simülasyonları ile Kalıp Tasarımı(TMMOB Makina Mühendisleri Odası, 2012) Kılıçaslan, Cenk; Odacı, İsmet Kutlay; Çakırcalı, Metin; Güden, MustafaÇapraz Kama Haddeleme (ÇKH), düz plakalar ya da merdaneler üzerine oluşturulmuş kamalar ile iş parçalarının deforme edildiği ve genellikle yüksek sıcaklıarda gerçekleştirilen bir metal şekil verme işlemidir. Ancak işlemde meydana gelen hasar yapısının karmaşık olması ve özellikle karmaşık geometriye sahip parçalar için kalıp tasarımının zor olması bu yöntem için dezavantaj sağlamaktadır. Bu çalışmada içi boş ve karmaşık dış geometriye sahip iş parçasının sonlu elemanlar simülasyonları ile ÇKH kalıbı tasarımı ele alınmıştır. Tasarlanan kalıplar ile gerçekleştirile sonlu elemanlar analizleri, iş parçasının tam olarak şekillenip şekillenmediğini belirlemiş ve sonuca göre mevcut kalıp tasarımı en iyilenmiştir. Simülasyonlarda büyük genişletme açısının içi boş iş parçasında eğilmeye sebep olduğu görülmüştür. Ayrıca iş parçası ile kalıp arasında sürtünmenin yetersiz olduğu bölgelerde çentiklerin kullanılması gerektiği belirlenmiştir.