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Article Citation - WoS: 20Citation - Scopus: 20The Stress Response of Partially Plastic Rotating Fgm Hollow Shafts: Analytical Treatment for Axially Constrained Ends(Taylor & Francis inc, 2006) Eraslan, Ahmet N.; Akis, Tolgaanalytical solutions to estimating the elastoplastic response of rotating functionally graded (FGM) hollow shafts with fixed ends are presented. The modulus of elasticity, as well as the uniaxial yield limit of the shaft material, are assumed to vary nonlinearly in the radial direction. The plastic model is based on Tresca's yield criterion, its associated flow rule, and ideal plastic material behaviour. Elastic, partially plastic, fully plastic, and residual stress states are investigated. It is shown that the elastoplastic stress response of a rotating FGM hollow shaft is affected significantly by the nonhomogeneity of the material. Unlike the case of a homogeneous hollow shaft, plastic deformation may commence at the inner surface, at the outer surface, or simultaneously at both surfaces. Accordingly, each case requires different mathematical treatment to arrive at its partially plastic solution. It is also shown that, by taking a numerical limit, the complete FGM solution presented herein converge to the solution of a homogeneous rotating shaft.Article Citation - WoS: 33Citation - Scopus: 35Exact Solution of Rotating Fgm Shaft Problem in the Elastoplastic State of Stress(Springer, 2007) Akis, Tolga; Eraslan, Ahmet N.Plane strain analytical solutions to estimate purely elastic, partially plastic and fully plastic deformation behavior of rotating functionally graded (FGM) hollow shafts are presented. The modulus of elasticity of the shaft material is assumed to vary nonlinearly in the radial direction. Tresca's yield criterion and its associated flow rule are used to formulate three different plastic regions for an ideal plastic material. By considerina different material compositions as well as a wide range of bore radii, it is demonstrated in this article that both the elastic and the elastoplastic responses of a rotating FGM hollow shaft are affected significantly by the material nonhomogeneity.Article Citation - WoS: 47Citation - Scopus: 46Elastoplastic Analysis of Functionally Graded Spherical Pressure Vessels(Elsevier Science Bv, 2009) Akis, TolgaPurely elastic, partially plastic and fully plastic stress states of internally pressurized functionally graded spherical pressure vessels are investigated analytically in the framework of small deformation theory. The modulus of elasticity and the uniaxial yield limit of the spherical pressure vessel material are assumed to vary nonlinearly in the radial direction. The plastic model is based on Tresca's yield criterion and ideal plastic material behavior. It is shown that, unlike in the case of a homogeneous spherical pressure vessel, different modes of plasticization may take place due to the radial variation of the functionally grading parameters. (C) 2009 Elsevier B.V. All rights reserved.Article Citation - WoS: 9Citation - Scopus: 10Influence of Cement Replacement by Calcinated Kaolinitic and Montmorillonite Clays on the Properties of Mortars(Springer Heidelberg, 2023) Al-Noaimat, Yazeed A. A.; Akis, TolgaThis study aims to investigate the decomposition and pozzolanic reactivity of two different clays (kaolinitic and montmorillonite) from different origins and to determine their effects after calcination on the properties of cement mortars when used to replace Portland cement partially. Mineralogical and chemical compositions of the clay samples were determined using XRD (X-ray Diffractometer) and XRF (X-ray Fluorescence) tests, respectively. TG-DTA (Thermogravimetry-Differential Thermal Analyses) was used to determine the temperature profiles and the burning temperatures of the clays. The density and fineness of the burnt clays were also determined. In order to investigate the optimum material properties, different burning temperatures and replacement levels were considered. It was found that for all temperatures, the two burnt clays possess good pozzolanic activity. The highest compressive strength and lowest water absorption capacity were achieved when the clay determined as kaolinitic was burned at 700 & DEG;C and with 10% replacement level. While for the clay determined as montmorillonite, the optimum properties were obtained at 700 & DEG;C with a 20% substitution level. Kaolinite had better pozzolanic reactivity than montmorillonite, achieving higher strength performance with lower water absorption when partially replaced with cement. Moreover, it had compressive strength values even higher than plain cement.Article Citation - WoS: 11Citation - Scopus: 13Automated Selection of Optimal Material for Pressurized Multi-Layer Composite Tubes Based on an Evolutionary Approach(Springer London Ltd, 2018) Azad, Saeid Kazemzadeh; Akis, TolgaDecision making on the configuration of material layers as well as thickness of each layer in composite assemblies has long been recognized as an optimization problem. Today, on the one hand, abundance of industrial alloys with different material properties and costs facilitates fabrication of more economical or light weight assemblies. On the other hand, in the design stage, availability of different alternative materials apparently increases the complexity of the design optimization problem and arises the need for efficient optimization techniques. In the present study, the well-known big bang-big crunch optimization algorithm is reformulated for optimum design of internally pressurized tightly fitted multi-layer composite tubes with axially constrained ends. An automated material selection and thickness optimization approach is employed for both weight and cost minimization of one-, two-, and three-layer tubes, and the obtained results are compared. The numerical results indicate the efficiency of the proposed approach in practical optimum design of multi-layer composite tubes under internal pressure and quantify the optimality of different composite assemblies compared to one-layer tubes.Article Citation - WoS: 54Citation - Scopus: 58Investigation of Rc Buildings After 6 February 2023, Kahramanmaras, Turkiye Earthquakes(Mdpi, 2023) Mertol, Halit Cenan; Tunc, Gokhan; Akis, Tolga; Kantekin, Yunus; Aydin, Ishak CanTwo major earthquakes struck Pazarcik and Elbistan, towns in Kahramanmaras, Turkiye, on 6 February 2023, approximately 9 h apart. The first earthquake, recorded at 04:17 local time, had a M-w = 7.7, with a focal depth of 8.6 km. At 13:24 local time, a second earthquake occurred with M-w = 7.6 at a focal depth of 7 km, approximately 90 km north of the first one. A total of 11 provinces were severely affected by these earthquakes. As of 15 April 2023, they caused close to 51,000 deaths and almost 215,000 completely destroyed/severely damaged buildings. At some locations, the largest horizontal peak ground acceleration (PGA) values of the first and second earthquakes exceeded the code-generated PGAs by almost 3 and 1.75 times, respectively. A technical team visited these areas within 15 h of the first earthquake. The purpose of this article is to present their observations, findings, and the characteristics of the two earthquakes, with comprehensive site survey results supported by photographs. This study concludes that most of the collapsed and severely/moderately damaged buildings in the region were built between 1975 and 2000, when site inspections were rare or non-existent. In addition to the high PGAs recorded in these earthquakes, it was verified that the design and construction of these buildings did not fully comply with the earthquake codes valid at the time. The collapsed buildings and their damage patterns confirm inadequate development length, violation of bending stirrup ends at 135 & DEG;, deficiencies in construction materials and reinforcement configuration, noncompliance with confinement zones, violation of the strong beam-stronger column analogy, and issues related to building inspection. Based on the extent of the damage, it is strongly recommended that the structural performance inspection of all other buildings located near major fault lines, specifically those constructed between 1975 and 2000, should be completed. Since these earthquakes generated much higher PGAs, which is believed to be one of the main reasons for the extensive damage, a re-evaluation of all other PGAs along major fault lines is also recommended.Article Citation - WoS: 15Citation - Scopus: 16Damage Observation of Reinforced Concrete Buildings After 2020 Sivrice (elazig) Earthquake, Turkey(Asce-amer Soc Civil Engineers, 2021) Cenan Mertol, Halit; Tunc, Gokhan; Akis, TolgaAn earthquake with a magnitude of M-w = 6.8 and a depth of approximately 8.06 km occurred on January 24, 2020, with the epicenter near the town of Sivrice in Elazig province, Turkey. The earthquake resulted in numerous collapses and severe structural damage to the buildings located in the downtowns and villages of Elazig and Malatya provinces. A technical team investigated the earthquake-affected areas, and this study presents their observations and findings with respect to reinforced concrete buildings. It was concluded that almost all of the collapsed or severely-damaged reinforced concrete buildings were constructed between 1975 and 1998. The design and construction of these buildings did not comply with the specifications of the Turkish earthquake codes valid at that time. The failures and severe damage of the buildings are associated with poor material quality, inadequate reinforcement configuration, and framing problems related to their lateral load-carrying systems. Therefore, it is recommended that buildings constructed between 1975 and 1998 in the region be structurally re-evaluated to avoid additional loss of life and property in future earthquakes. (C) 2021 American Society of Civil Engineers.
