• Journal of the Korea Concrete Institute
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• v.24 no.5
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• pp.525-533
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• 2012

In this study, bending tests were performed on beam specimens made of UHPCC with the fiber content range of 0~5 vol% to investigate the contribution of fiber content to first cracking strength and flexural tensile strength. Also, four-point bending tests for unnotched beam as well as three-point bending test for notched beam were performed to estimate the effect of the presence of notch on the strengths. The experiment result showed that the increase in fiber content made linear improvement in the flexural tensile strength; whereas first cracking strength was enhanced only when at least 1 vol% of fibers was incorporated. Comparison of the bending test results with and without notch showed that the notch effect varied with the fiber content. The increase in fiber content diminished the effect of stress concentration on the notch tip, reducing the difference in the strengths. With much higher fiber content, the effect of stress concentration almost disappeared and the defection on cracking plane or the size effect dominated the strengths, consequently resulting in higher strengths in the notched beams than the unnotched ones.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.25 no.10
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• pp.1528-1534
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• 2001

In order to improve Leak-Be(ore-Break methodology, more precisely the crack growth evaluation, a round robin analysis was proposed by the CEA Saclay. The aim of this analysis was to evaluate the crack initiation life, penetration life and shape of through wall crack under cyclic bending loads. The proposed round robin analysis is composed of three main topic; fatigue crack initiation, crack propagation and crack penetration. This paper deals with the first topic, crack initiation in a notched pipe under four point bending. Both elastic-plastic finite element analysis and Neuber's rule were used to estimate the crack initiation life and the finite element models were verified by mesh-refinement, stress distribution and global deflection. In elastic-plastic finite element analysis, crack initiation life was determined by strain amplitude at the notch tip and strain-life curve of the material. In the analytical method, Neuber's rule with the consideration of load history and mean stress effect, was used for the life estimation. The effect of notch tip radius, strain range, cyclic hardening rule were examined in this study. When these results were compared with the experimental ones, the global deformation was a good agreement but the crack initiation cycle was higher than the experimental result.

• Computers and Concrete
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• v.9 no.1
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• pp.1-19
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• 2012

The size-effect study of various fracture parameters obtained from two parameter fracture model, effective crack model, double-K fracture model and double-G fracture model is presented in the paper. Fictitious crack model (FCM) for three-point bend test geometry for cracked concrete beam of laboratory size range 100-400 mm is developed and the different fracture parameters from size effect model, effective crack model, double-K fracture model and double-G fracture model are evaluated using the input data obtained from FCM. In addition, the fracture parameters of two parameter fracture model are obtained using the mathematical coefficients available in literature. From the study it is concluded that the fracture parameters obtained from various nonlinear fracture models including the double-K and double-G fracture models are influenced by the specimen size. These fracture parameters maintain some definite interrelationship depending upon the specimen size and relative size of initial notch length.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.18 no.6
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• pp.697-702
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• 2017

A preliminary study was carried out to investigate the feasibility of replacing honeycomb cores with pyramidal truss cores in the doors of urban transit railway vehicles. The doors in current operation are sandwich structures comprising a honeycomb core and reinforcements between two facesheets. The structural requirements of doors for urban transit vehicle are specified in the KRS and KRT and standards, according to which the deflections from three-point bending tests must be limited. To this end, two types of pyramidal truss cores with equivalent mass to a honeycomb core were designed. The structural stiffness of doors with pyramidal truss cores and honeycomb cores were numerically calculated via finite element analysis. The three-point bending models were constructed and simulated, and then the calculated deflections were compared with the requirements specified in the regulations. The results show that doors with pyramidal truss cores satisfied the stiffness requirements, although their deflections were 2.5% larger than that of the honeycomb cores. Therefore, the pyramidal truss cores could replace the aluminum honeycomb cores, and their multi-functional capability could be exploited.

• Journal of Digital Convergence
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• v.17 no.10
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• pp.373-379
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• 2019

Polymethyl methacrylate (PMMA), a self-polymerizing resin for removable orthodontic devices, has been used as a dental orthodontic device for many years because of its advantages such as color stability, volume stability, and tissue compatibility. However, such a removable orthodontic device has a disadvantage that the longer the use in the oral cavity due to the low strength of the PMMA fracture of the orthodontic device resin in use. In this study, zinc nanoparticles (ZNP) were mixed with orthodontic PMMA to introduce strength effect. Rectangular samples ($1.4{\times}3.0{\times}19.0mm$) of orthodontic PMMA (0, 0.5, 1.0, 2.0 and 4.0%) containing ZNP were prepared. The finished specimen was tested for three-point bending strength at a speed of 1 mm / min, and the Vickers hardness was measured three times using a hardness tester. The surface roughness was measured with a surface roughness. As a result, the 3-point bending strength did not change significantly (p>0.05). Surface energy increased significantly. As a result, we successfully synthesized ZNP in this study and prepared the dispersed resin specimen for calibration. It will be possible to develop high-density dental orthodontic resins.

• 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.

• Restorative Dentistry and Endodontics
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• v.44 no.4
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• pp.45.1-45.10
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• 2019

Objectives: Self-adhesive resin cements contain functional monomers that enable them to adhere to the tooth structure without a separate adhesive or etchant. One of the most stable functional monomers used for chemical bonding to calcium in hydroxyapatite is 10-methacryloyloxydecyl dihydrogen phosphate (10-MDP). The aim of this study was to evaluate the influence of the10-MDP concentration on the bond strength and physical properties of self-adhesive resin cements. Materials and Methods: We used experimental resin cements containing 3 different concentrations of 10-MDP: 3.3 wt% (RC1), 6.6 wt% (RC2), or 9.9 wt% (RC3). The micro-tensile bond strength of each resin cement to dentin and a hybrid resin block (Estenia C&B, Kuraray Noritake Dental) was measured, and the fractured surface morphology was analyzed. Further, the flexural strength of the resin cements was measured using the three-point bending test. The water sorption and solubility of the cements following 30 days of immersion in water were measured. Results: The bond strength of RC2 was significantly higher than that of RC1. There was no significant difference between the bond strength of RC2 and that of RC3. The water sorption of RC3 was higher than that of any other cement. There were no significant differences in the three-point bending strength or water solubility among all three types of cements. Conclusions: Within the limitations of this study, it is suggested that 6.6 wt% 10-MDP showed superior properties than 3.3 wt% or 9.9 wt% 10-MDP in self-adhesive resin cement.

• The Journal of Advanced Prosthodontics
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• v.10 no.1
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• pp.25-31
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• 2018

PURPOSE. The aim of the present study was to record the metal-ceramic bond strength of a feldspathic dental porcelain and a Co-Cr alloy, using the Direct Metal Laser Sintering technique (DMLS) for the fabrication of metal substrates. MATERIALS AND METHODS. Ten metal substrates were fabricated with powder of a dental Co-Cr alloy using DMLS technique (test group) in dimensions according to ISO 9693. Another ten substrates were fabricated with a casing dental Co-Cr alloy using classic casting technique (control group) for comparison. Another three substrates were fabricated using each technique to record the Modulus of Elasticity (E) of the used alloys. All substrates were examined to record external and internal porosity. Feldspathic porcelain was applied on the substrates. Specimens were tested using the three-point bending test. The failure mode was determined using optical and scanning electron microscopy. The statistical analysis was performed using t-test. RESULTS. Substrates prepared using DMLS technique did not show internal porosity as compared to those produced using the casting technique. The E of control and test group was $222{\pm}5.13GPa$ and $227{\pm}3GPa$, respectively. The bond strength was $51.87{\pm}7.50MPa$ for test group and $54.60{\pm}6.20MPa$ for control group. No statistically significant differences between the two groups were recorded. The mode of failure was mainly cohesive for all specimens. CONCLUSION. Specimens produced by the DMLS technique cover the lowest acceptable metal-ceramic bond strength of 25 MPa specified in ISO 9693 and present satisfactory bond strength for clinical use.

• Korean Journal of Materials Research
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• v.21 no.8
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• pp.425-431
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• 2011

The industrial manufacturing of YSZ products can be summarized as a three step process: a) hydrolysis of zirconyl chloride and mixing of other solutions, b) precipitation, and c) calcination. The addition of ammonia or OH- is essential in the precipitation process. However, a strong agglomeration was observed in the results of an ammonia or OH- addition. Thus, it is necessary to disperse the powders smoothly in order to improve the mechanical strength of YSZ. In this study, YSZ was synthesized using the urea stabilizer and hydrothermal method. YSZ powders were synthesized using a hydrothermal method with Teflon Vessels at $180^{\circ}C$ for 24 h. The mole ratio of urea to Zr was 0, 0.5, 1, and 2. The crystal phase, particle size, and morphologies were analyzed. Rectangular specimens ($33\;mm{\times}8\;mm{\times}1{\pm}0.5\;mm$) for three-point bend tests were used in the mechanical properties evaluation. The crystalline of YSZ powders observed a tetragonal phase in the sample with a ratio of Zr:urea = 1:2 addition and a hydrothermal reaction time of 24 h. The average primary particle size of YSZ was measured to be 9 nm to 11 nm. The agglomerated particle size was measured from 15 nm to 30 nm. The three-point bending strength of the YSZ samples was 142.47 MPa, which is the highest value obtained for the Zr:urea = 1:2 ratio addition YSZ sample.

• Computers and Concrete
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• v.31 no.5
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• pp.433-442
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• 2023

This study presents the visually observed behavior of fibers embedded in concrete samples that were subjected to a flexural bending test. Three types of fibers such as macro polypropylene, macro polyethylene, and the hybrid of steel and polyvinyl alcohol were mixed with cement by a designated mix ratio to prepare a total of nine specimens of each. The bending test was conducted by following ASTM C1609 with a net deflection of 2, 4, and 7 mm. The X-ray computed tomography (XCT) was carried out for 7 mm-deflection specimens. The original XCT images were post-processed to denoise the beam-hardening effect. Then, fiber, crack, and void were semi-manually segmented. The hybrid specimen showed the highest toughness compared to the other two types. Debonding based on 2D XCT sliced images was commonly observed for all three groups. The cement matrix near the crack surface often involved partially localized breakage in conjunction with debonding. The pullout was predominant for steel fibers that were partially slipped toward the crack. Crack bridging and rupture were not found presumably due to the image resolution and the level of energy dissipation for poly-fibers, while the XCT imaging was advantageous in evaluating the distribution and behavior of various fibers upon bending for fiber-reinforced concrete beam elements.