• Steel and Composite Structures
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• 제49권6호
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• pp.667-684
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• 2023

In the context of finite element method, a computational simulation is presented to study and analyze the dynamic behavior of regularly perforated functionally graded rotating beam for the first time. To investigate the effect of perforation configurations, both regular circular and squared perforation patterns are studied. To explore impacts of graded material distributions, both axial and transverse gradation profiles are considered. The material characteristics of graded materials are assumed to be smoothly and continuously varied through the axial or the thickness direction according the nonlinear power gradation law. A computational finite elements procedure is presented. The accuracy of the numerical procedure is verified and compared. Resonant frequencies, axial displacements as well as internal stress distributions throughout the perforated graded rotating cantilever beam are studied. Effects of material distributions, perforation patterns, as well as the rotating beam speed are investigated. Obtained results proved that the graded material distribution has remarkable effects on the dynamic performance. Additionally, circular perforation pattern produces more softening effect compared with squared perforation configuration thus larger values of axial displacements and maximum principal stresses are detected. Moreover, squared perforation provides smaller values of nondimensional frequency parameters at most of vibration modes compared with circular pattern.

• Advances in aircraft and spacecraft science
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• 제10권3호
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• pp.223-243
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• 2023

There is a burgeoning demand for minimizing the mass of satellites because of its direct impact on reducing launch-to-orbit cost. This must be done without compromising the structure's efficiency. The present paper introduces a relatively low-cost and easily implementable approach for optimizing structural mass to a maximum natural frequency. The natural frequencies of the satellite are of utmost pertinence to the application requirements, as the sensitive electronic instrumentation and onboard computers should not be affected by the vibrations of the satellite structure. This methodology is applied to a realistic model of Al-Azhar University micro-satellite in partnership with the Egyptian Space Agency. The procedure used in structural design can be summarized in two steps. The first step is to select the most favorable primary structural configuration among several different candidate variants. The nominated variant is selected as the one scoring maximum relative dynamic stiffness. The second step is to use perforation patterns reduce the overall mass of structural elements in the selected variant without changing the weight. The results of the presented procedure demonstrate that the mass reduction percentage was found to be 39% when compared to the unperforated configuration that had the same plate thickness. The findings of this study challenge the commonly accepted notion that isogrid perforations are the most effective means of achieving the goal of reducing mass while maintaining stiffness. Rather, the study highlights the potential benefits of exploring a wider range of perforation unit cells during the design process. The study revealed that rectangular perforation patterns had the lowest efficiency in terms of modal stiffness, while triangular patterns resulted in the highest efficiency. These results suggest that there may be significant gains to be made by considering a broader range of perforation shapes and configurations in the design of lightweight structures.

• Steel and Composite Structures
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• 제20권4호
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• pp.913-930
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• 2016

It is common practice to use Reduced Web Beam Sections (RWBS) in steel moment resisting frames. Perforation of beam web in these members may cause stress and strain concentration around the opening area and facilitate ductile fracture under cyclic loading. This paper presents a numerical study on the cyclic fracture of these structural components. The considered connections are configured as T-shaped assemblies with beams of elongated circular perforations. The failure of specimens under Ultra Low Cycle Fatigue (ULCF) condition is simulated using Cyclic Void Growth Model (CVGM) which is a micromechanics based fracture model. In each model, CVGM fracture index is calculated based on the stress and strain time histories and then models with different opening configurations are compared based on the calculated fracture index. In addition to the global models, sub-models with refined mesh are used to evaluate fracture index around the beam to column weldment. Modeling techniques are validated using data from previous experiments. Results show that as the perforation size increases, opening corners experience greater fracture index. This is while as the opening size increases the maximum observed fracture index at the connection welds decreases. However, the initiation of fracture at connection welds occurs at lower drift angles compared to opening corners. Finally, a probabilistic framework is applied to CVGM in order to account for the uncertainties existing in the prediction of ductile fracture and results are discussed.

• Restorative Dentistry and Endodontics
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• 제37권1호
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• pp.50-53
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• 2012

Cone-beam computed tomography (CBCT) is a useful diagnostic tool for identification of both internal and external root configurations. This case report describes the endodontic management of a lateral incisor with both dens invaginatus and external root irregularity by using CBCT. Nonsurgical endodontic retreatment was performed on the lateral incisor with dens invaginatus. A perforation through the dens invaginatus and external concavity was repaired using mineral trioxide aggregate. After 18 mon of follow-up, there were no clinical symptoms. Recall radiographs appeared normal and showed healing of the periapical pathosis. The understanding of both internal root canal configuration and external root irregularity using CBCT can ensure predictable and successful results.

• Structural Engineering and Mechanics
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• 제77권3호
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• pp.329-342
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• 2021

In this paper, the effect of different steel bar configurations on the quasi-static punching and impact response of concrete slabs was studied. A total of forty RC square slab specimens were cast in two groups of concrete strengths of 40 and 63 MPa. In each group of twenty specimens, ten specimens were reinforced at the back face (singly reinforced), and the remaining specimens were reinforced on both faces of the slab (doubly reinforced). Two rebar spacing of 25 and 100 mm, with constant reinforcement ratio and effective depth, were used in both singly and doubly reinforced slab specimens. The specimens were tested against the normal impact of cylindrical projectiles of hemispherical nose shape. Slabs were also quasi-statically tested in punching using the same projectile, which was employed for the impact testing. The experimental response illustrates that 25 mm spaced rebars are effective in (i) decreasing the local damage and overall penetration depth, (ii) increasing the absorption of impact energy, and (iii) enhancing the ballistic limit of RC slabs. The ballistic limit was predicted using the quasi-static punching test results of slab specimens showing a strong correlation between the dynamic perforation energy and the energy required for quasi-static perforation of slabs.

• Restorative Dentistry and Endodontics
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• 제32권1호
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• pp.37-46
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• 2007

C-shape 근관은 주로 하악 제 2대구치에서 보이는 해부학적 변이 형태로서, 그 특징은 각 근관을 연결하는 fin이나 web이 존재한다. 이런 해부학적으로 복잡함은 근관치료시에 세정과 성형 및 근관충전을 방해하며, 과다한 기구 조작시에 뜻하지 않는 천공을 유발하기도 한다. 이 연구의 목적은 C-shape 치근을 가진 하악 제2 대구치의 해부학적인 형태와 치근의 위치에 따른 최소 근관벽 두께에 대하여 연구하고자 하였다. 발거된 하악 대구치중 C-shape 치근을 가진 치아를 30개 선별하여 방사선 사진 및 협측, 설측, 치근단 부위를 디지털 카메라를 이용하여 촬영하였다. 자가 중합형 교정용 레진 (Orthodontics resin, Densply/Caulk, U.S.A)을 이용하여 치관부를 제거한 치아를 매몰한 후 각각의 시편을 Accutom-50 (Struers, Denmark)에 고정시켜 1 mm 간격으로 절단, 연마한 다음 절단면을 수술용현미경에 연결된 디지털 카메라 (Coolpix 995, Nikon, Tokyo, Japan)를 이용하여 각각의 상을 촬영하였다. 30개 치아에서 촬영한 197개의 치아단면을 Root thickness gauge program을 이용하여 근관형태의 변화를 분류하고 부위별 근관과 치아외벽 사이에 존재하는 상아질의 최소두께에 대해 연구 분석하였다. 1. 근관입구에서 C I이 가장 많이 (73%) 관찰되었으나 치근단부로 갈수록 근관 형태의 다양한 변화를 보여 C II와 C III의 근관형태가 많이 관찰되었으며, 11 개의 (36%)치아에서는 근관입구에서 치근단부까지 근관형태의 변화를 보이지 않았다. 근관입구에서 단지 2개의 치아에서 나타난 C III type은 치근단부로 갈수록 높은 발현빈도를 나타내었다. 2. C-shape 근관은 근관입구 부위에서는 continuous 나 semicolon canal을 가지나 치근단부로 갈수록 2개나 3개의 근관을 갖는 경우가 많다. 3.평균적인 근관과 치질사이의 최소거리는 근첨부의 설측부위에서 나타났다.