• The Journal of Korean Academy of Prosthodontics
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• v.39 no.6
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• pp.599-610
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• 2001

Due to an increasing interest in esthetics and concerns about toxic and allergic reactions to certain alloys, patients and dentists have been looking for metal-free tooth-colored restorations. Recent improvement in technology of new all-ceramic materials and composite materials has broadened the options for esthetic single crown restorations. The aim of this investigation was to study the fracture strength of the metal-free posterior single crowns fabricated using two recently introduced systems, Empress 2 ceramic and Targis-Vectris. Forty premolar-shaped stainless steel dies with the 1mm-wide circumferential shoulder were prepared. Ten cylindrical crowns having a diameter of 8.0mm and total height of 7.5mm were fabricated for each crown system respectively(PFM, Empress staining technique, Empress 2 layering technique, and Targis- Vectris). The crowns were filled with cement and placed on the stainless steel dies with firm finger pressure. The crowns were then stored in distilled water at room temperature for 24 hours before testing. The crowns were tested for fracture strength in an Instron universal testing machine (Instron 6022). With a crosshead speed of 1mm/min the center of the occlusal surface of the crown was loaded using a 4-mm-diameter stainless steel ball until fracture occurred. The fracture surfaces of the crowns were gold coated and examined using scanning electron microscopy(Jeol JSM-840 Joel Ltd., Akishima, Tokyo, Japan). Within the parameters of this study the following conclusions were drawn: 1. The mean fracture strength for PFM crowns was 5829(${\pm}906$)N; for Empress staining technique the fracture strength was 1697(${\pm}604$)N; for Empress 2 Layering technique the fracture strength was 1781N(${\pm}400$)N, and the fracture strength for Targis- Vectris was 3093(${\pm}475$)N. 2. The fracture strength of the PFM crowns was significantly higher than that of the Empress 2 and the Targis-Vectris crowns (P<0.05). 3. The fracture strength of the Targis-Vectris crowns was significantly higher than that of the Empress 2 crowns (P<0.05). 4. No statistical difference was found when Empress staining technique was compared with Empress 2 layering technique. 5. The SEM image of fracture surface of Empress 2 crown showed a very dense microstructure of the lithium disilicate crystals and the SEM image of fracture surface of Targis-Vectris crown showed indentations of Vectris and some fibers tom off from Vectris.

• Journal of the Korean Recycled Construction Resources Institute
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• v.7 no.3
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• pp.279-286
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• 2019

In this study, an experimental study on the tensile properties of steel fiber-reinforced ultra high strength concrete(UHSC) with a standard compressive strength of 180MPa was performed. Steel fibers with a volume ratio of 1% were mixed to prepare direct tensile strength specimens and prism specimens for the three-point bending test. The fabricated specimens were set up in the middle section of the specimen to induce cracks, and the test was carried out according to each evaluation method. First, the stress-strain curves were analyzed by performing direct tensile strength tests to investigate the behavior characteristics of concrete after cracking. In addition, the load-CMOD curve was obtained through the three-point bending test, and the inverse analysis was performed to evaluate the stress-strain curve. Tensile behavior characteristics of the direct tensile test and the three-point bending test of the indirect test were similar. In addition, the tensile stress-strain curve modeling presented in the SC structural design guidelines was performed, and the comparative analysis of the measured and predicted values was performed. When the material reduction factor of 1.0 was applied, the predicted value was similar to the measured value up to the strain of 0.02, but when the material reduction factor of 0.8 was applied, the predicted value was close to the lower limit of the measured value. In addition, when the strain was greater than 0.02, the predicted value by SC structural design guideline to underestimated the measured value.

• Steel and Composite Structures
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• v.29 no.3
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• pp.405-422
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• 2018

In this research, experimental tensile test and manufacturing of carbon nanotube reinforced composite beam (CNTRC) is presented. Also, bending, buckling, and vibration analysis of CNTRC based on various beam theories such as Euler-Bernoulli, Timoshenko and Reddy beams are considered. At first, the experimental tensile tests are carried out for CNTRC and composite beams in order to obtain mechanical properties and then using Hamilton's principle the governing equations of motion are derived for Euler Bernoulli, Timoshenko and Reddy theories. The results have a good agreement with the obtained results by similar researches and it is shown that adding just two percent of carbon nanotubes increases dimensionless fundamental frequency and critical buckling load as well as decreases transverse deflection of composite beams. Also, the influences of different manufacturing processes such as hand layup and industrial methods using vacuum pump on composite properties are investigated. In these composite beams, glass fibers used in an epoxy matrix and for producing CNTRC, CNTs are applied as reinforcement particles. Applying two percent of CNTs leads to increase the mechanical properties and increases natural frequencies and critical buckling load and decreases deflection. The obtained natural frequencies and critical buckling load by theoretical method are higher than other methods, because there are some inevitable errors in industrial and hand layup method. Also, the minimum deflection occurs for theoretical methods, in bending analysis. In this study, Young's and shear modulli as well as density are obtained by experimental test and have not been used from the results of other researches. Then the theoretical analysis such as bending, buckling and vibration are considered by using the obtained mechanical properties of this research.

• Earthquakes and Structures
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• v.12 no.1
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• pp.67-77
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• 2017

In this paper, it is aimed to present a comparative study about the structural behavior of tall buildings consisting of different type of materials such as concrete, steel or timber using finite element analyses and experimental measurements on shaking table. For this purpose, two 1/60 scaled 28 and 30-stories wooden building models with $40{\times}40cm$ and $35{\times}35cm$ ground/floor area and 1.45 m-1.55 m total height are built in laboratory condition. Considering the frequency range, mode shapes, maximum displacements and relative story drifts for structural models as well as acceleration, displacement and weight limits for shaking table, to obtain the typical building response as soon as possible, balsa is selected as a material property, and additional masses are bonded to some floors. Finite element models of the building models are constituted in SAP2000 program. According to the main purposes of earthquake resistant design, three different earthquake records are used to simulate the weak, medium and strong ground motions. The displacement and acceleration time-histories are obtained for all earthquake records at the top of building models. To validate the numerical results, shaking table tests are performed. The selected earthquake records are applied to first mode (lateral) direction, and the responses are recorded by sensitive accelerometers. Comparisons between the numerical and experimental results show that shaking table tests are enough to identify the structural response of wooden buildings. Considering 20%, 10% and 5% damping rations, differences are obtained within the range 4.03-26.16%, 3.91-65.51% and 6.31-66.49% for acceleration, velocity and displacements in Model-1, respectively. Also, these differences are obtained as 0.49-31.15%, 6.03-6.66% and 16.97-66.41% for Model-2, respectively. It is thought that these differences are caused by anisotropic structural characteristic of the material due to changes in directions parallel and perpendicular to fibers, and should be minimized using the model updating procedure.

• Steel and Composite Structures
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• v.35 no.6
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• pp.789-798
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• 2020

The effect of Chitosan (CS), Carbon Nanotube (CNT) and hybrid (CS-CNT) fillers on the natural frequency of drilled composite plate is investigated by experimentally in this study. The numerical validation is also made with a program based on Finite Element Method (SolidWorks). Nine types filled and one neat composite plates are used in the study. The fillers ratios are 1% CS, 2% CS, 3% CS, 0.1% CNT, 0.2% CNT, 0.3% CNT, 1% CS+0.3% CNT, 2% CS+0.3% CNT, 3% CS+0.3% CNT. The specimens cut to certain sizes by water jet from the plates 400 mm × 400 mm in dimensions. Some of them are drilled in certain dimensions with drill. The natural frequency of each specimen is measured by the vibration test set up to determine the vibration characteristic. The vibration test set up includes an accelerometer, a current source power unit, a data acquisition card and a computer. A code is written in Matlab^® program for the signal processing. The study are investigated and discussed in four main points to understand the effect of the fillers on the natural frequency of the composite plate. These are the effect of fillers contents and amounts, orientation angles of fibers, holes numbers and holes sizes. As results, the natural frequency of the plate with 1% CS and 0.1% CNT hybrid filler is lower than those of the plates with other fillers ratios for 45° orientation angle. Besides, in the composite plate with 0° orientation angle, the natural frequency increases with increasing the filler ratio. Moreover, the natural frequency increases until a certain hole number and then it decreases. Furthermore, the natural frequency is not affected until a certain hole diameter but then it decreases.

• Journal of the Korea Concrete Institute
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• v.18 no.5 s.95
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• pp.703-708
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• 2006

The slurry infiltrated fiber concrete(SIFCON) is recognized as one of the most promising new construction materials. Compressive and direct tensile tests are performed to investigate the mechanical property of SIFCON. Hooked-end steel fibers are used in the mix with fiber volume fraction varied from 4% to 10%. The water/cement ratio is kept constant at 0.4. The amount of silica fume added is 10% by weight of cement and 0.5% of water reducing agent is added to improve the workability of the slurry. The test results in this study show that the compressive strength of SIFCON is about 1.59 to 2.68 times in comparison with the cement paste. Tensile strength is showed the enhancement of about 2.51 to 8.77 times. It is also observed that the toughness and ductility of SIFCON are increased significantly with the increasing in fiber volume fraction.

• Journal of the Korean Society of Fisheries and Ocean Technology
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• v.40 no.4
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• pp.337-343
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• 2004

This paper presents an experimental study of friction and wear properties of a unidirectional oriented continuous crbon-fiber reinforced epoxy composite at the ambient temperature. Friction and wear experiments were conducted in the three principal sliding direction of the fiber orientation in the composite were selected against the stainless steel counterpart specularly processed were using a pin -on-disc apparatus. Friction coefficient and specific wear rate at various normal loads and sliding velocities wear determined. When sliding took place against smooth and hard counterpart, the hightest were resistance and the lowest friction coefficient were observed in the anti-parallel direction. The wear track of the worn specimens was examined with a scanning electron microscope(SEM) to observe the damaged fibers on the surface. In addition, SEM observations of the worn surfaces allowed to identify the involved different wear mechanisms.

• Journal of the Korea institute for structural maintenance and inspection
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• v.17 no.1
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• pp.37-45
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• 2013

In this study, the resistance of various reinforced concrete (RC) slabs subjected to contact detonation was assessed. In order to enhance the blast resistance, fibers and external FRP sheets were reinforced to RC slabs. In the experiment, the $2,000{\times}1,000{\times}100mm$ sized RC slabs were fabricated using normal concrete (NC), steel fiber reinforced concrete (SFRC), polyvinyl alcohol fiber reinforced cementitious composite (PVA FRCC), and ultra-high performance cementitious composites (UHPCC). The damage levels of RC slabs subjected to contact detonation were evaluated by measuring the diameter and depth of crater, spall and breach. The experimental results were compared to the analyzed data using LS-DYNA program and three different prediction equations. The diameter and depth of crater, spall and breach were able to be predicted using LS-DYNA program approximately. The damage process of RC slabs under blast load was also well expressed. Three prediction equations suggested by other researchers had limitations to apply in terms of empirical approaches, therefore it needs further research to set more analytical considerations.

• Journal of the Korea institute for structural maintenance and inspection
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• v.18 no.4
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• pp.92-98
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• 2014

Spacer is a construction material for maintaining cover depth and steel installation, however several problems like staining, leakage, and cracking are currently issued due to performance degradation and unsatisfactory dimensional stability of spacer. Plastic composite is widely used for prevention of brittle failure in cement based material, which yields improvement of crack resistance and ductile failure. This study is for development and applicability evaluation of high strength spacer with slag cement for environmental load reduction and plastic composite like polypropylene fiber, nylon fiber, and glass fiber. For this work, unit weight of 4 different plastic fibers are evaluated through preliminary tests. Physical tests including compressive, flexural, and tensile strength and durability tests including absorption, permeability, length change, crack resistance, carbonation, and freezing and thawing are performed. Through various tests, optimum plastic fiber is selected and manufacturing system for high strength spacer with the selected fiber is developed. Dimensional stability of the developed spacer is evaluated through field applicability evaluation.

• Journal of the Korea Concrete Institute
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• v.15 no.3
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• pp.377-385
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• 2003

Flexural characteristics of the R.C beams strengthened with newly-developed grid-type carbon fiber plastics(CFRP-GRIDS) were investigated. The tests were conducted under the four-points load to the failure to investigate the strengthening effects of CFRP-GRIDS on the beams. Results showed that initial cracks appeared in the boundary layers of fibers embedded in the newly-placed mortar concrete slowly progressed to the direction of supports and showed fracture of fiber plastics and brittle failure of concrete in compression in sequence after the yielding of steel reinforcement. Accordingly, the appropriate area of Grid-type carbon-fiber plastics in the strengthening design of deteriorated RC structures should be limited and given based on the ultimate strength design method to avoid the brittle failure of concrete structures.