• Proceedings of the Korea Concrete Institute Conference
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• 2005.05b
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• pp.337-340
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• 2005

In this study, an effect of fiber blending on material property of Hybrid Fiber Reinforced Concrete (HFRC) was evaluated. Also, Compare and evaluates collating and mechanical property by the mixing rate of fiber for HFRC was determine. Modulus of rupture and strength effectiveness of Hybrid Fiber Reinforced Concrete mixed with macro-fiber(steel fiber) and micro-fiber(glass fiber, carbon fiber, cellulose fiber). Test result shows, in the case of mono fiber reinforced concrete. As the steel fiber mixing rate increases to 1.5$\%$, the strength effectiveness promotion rate rises. However, when is 2.0$\%$, strength decreases. In the case of hybrid fiber reinforcement concrete, synergy effect of micro fiber and macro fiber happens and higher Modulus of rupture and strength effectiveness appears than mono-fiber reinforcement concrete. Use of hybrid fiber reinforcement in concrete caused a significant influence on its fracture behavior; consequently, caused increase by mixing rate of steel fiber + carbon fiber and contributed by steel fiber + glass fiber, steel fiber + celluloid fiber in reinforcement effect in order. And was expose that steel fiber(1.5$\%$) + carbon fiber(0.5$\%$) is most suitable association.

• Composites Research
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• v.13 no.4
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• pp.1-10
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• 2000

SiC particulate reinforced metal matrix composites(MMCs) are emerging as candidate materials for the automobile and aerospace industries due to their significant increase in elastic modulus and strength compared to conventional metallic materials. However, in order to make successful application of MMCs, it is very important to understand micro-failure mechanism under cyclic loading because failure mechanism of MMC is dominated by accumulation of micro-failure due to applied loading. In this study, ultrasonic Lamb wave and acoustic emission(AE) have been used to monitor microscopic damage accumulation under cyclic loading for SiC particulate reinforced metal matrix composite(SiCp/A356). It was found that the change in velocity and attenuation of ultrasonic Lamb wave due to the increase of loading cycles could be characterized by three different stages corresponding to the microscopic fracture processes. The characteristic of AE signal at each stage was analyzed and discussed by comparing with the change of ultrasonic characteristic in MMCs.

• Computers and Concrete
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• v.4 no.5
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• pp.377-402
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• 2007

The paper presents quasi-static plane strain FE-simulations of strain localization in reinforced concrete beams without stirrups. The material was modeled with two different isotropic continuum crack models: an elasto-plastic and a damage one. In case of elasto-plasticity, linear Drucker-Prager criterion with a non-associated flow rule was defined in the compressive regime and a Rankine criterion with an associated flow rule was adopted in the tensile regime. In the case of a damage model, the degradation of the material due to micro-cracking was described with a single scalar damage parameter. To ensure the mesh-independence and to capture size effects, both criteria were enhanced in a softening regime by nonlocal terms. Thus, a characteristic length of micro-structure was included. The effect of a characteristic length, reinforcement ratio, bond-slip stiffness, fracture energy and beam size on strain localization was investigated. The numerical results with reinforced concrete beams were quantitatively compared with corresponding laboratory tests by Walraven (1978).

• Advances in materials Research
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• v.9 no.2
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• pp.99-107
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• 2020

Reinforced aluminum alloy base composites have become increasingly popular for engineering applications, since they usually possess several desirable properties. Recently, Warm Accumulative Roll Bonding (WARB) process has been used as a new novel process to fabricate particle reinforced metal matrix composites. In the present study, TiO₂ particles are used as reinforcement in aluminum metal matrix composites fabricated through warm accumulative roll bonding process. Firstly, the raw aluminum alloy 1060 strips with TiO₂ as reinforcement particle were roll bonded to four accumulative rolling cycles by preheating for 5 min at 300℃before each cycle. The mechanical and bonding properties of composites have been studied versus different volume contents of TiO₂ particles by tensile test, peeling test and vickers micro-hardness test. Moreover, the fracture surface and peeling surface of samples after the tensile test and peeling test have been studied versus different amount of TiO₂ volume contents by scanning electron microscopy. The results indicated that the strength and the average vickers micro-hardness of composites improved by increasing the volume content of TiO₂ particles and the amount of their elongation and bonding strength decreased significantly.

• Advances in concrete construction
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• v.10 no.3
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• pp.195-209
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• 2020

This study investigates the behavior of ultra-high-performance fiber-reinforced concrete (UHPFRC) with hybrid macro-micro steel and macro steel-polypropylene (PP) fibers. Compression, direct and indirect tension tests were carried out on cubic and cylindrical, dogbone and prismatic specimens, respectively. Three types of macro steel fibers, i.e., round crimped (RC), crimped (C), and hooked (H) were combined with micro steel (MS) and PP fibers in overall ratios of 2% by volume. Additionally, numerical analyses were performed to validate the test results. Parameters studied included, fracture energy, tensile strength, compressive strength, flexural strength, and residual strength. Tests showed that replacing PP fibers with MS significantly improves all parameters particularly flexural strength (17.38 MPa compared to 37.71 MPa). Additionally, the adopted numerical approach successfully captured the flexural load-deflection response of experimental beams. Lastly, the proposed regression model for the flexural load-deflection curve compared very well with experimental results, as evidenced by its coefficient of correlation (R²) of over 0.90.

• Smart Structures and Systems
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• v.29 no.4
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• pp.535-547
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• 2022

Failure patterns of rock specimens represent valuable information about the mechanical properties and crack evolution mechanism of rock. Several kinds of research have been conducted regarding the failure mechanism of brittle material, however; the influence of brittleness on the failure mechanism of rock specimens has not been precisely considered. In the present study, experimental and numerical examinations have been made to evaluate the physical and mechanical phenomena associated with rock failure mechanisms through the uniaxial compression test. In the experimental part, Unconfined Compressive Strength (UCS) tests equipped with Acoustic Emission (AE) have been conducted on rock samples with three different brittleness. Then, the numerical models have been calibrated based on experimental test results for further investigation and comparing the micro-cracking process in experimental and numerical models. It can be perceived that the failure mode of specimens with high brittleness is tensile axial splitting, based on the experimental evidence of rock specimens with different brittleness. Also, the crack growth mechanism of the rock specimens with various brittleness using discrete element modeling in the numerical part suggested that the specimens with more brittleness contain more tensile fracture during the loading sequences.

• Journal of the Korea Concrete Institute
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• v.14 no.3
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• pp.357-364
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• 2002

The resistance curves (R-curves) for 381 m crack extension of CLWL-DCB specimens had been determined. The average velocities of the crack extension measured with strain gages were 0.70 and 55 ㎜/sec. The measured rotation angle of the notch faces showed the existence of the singularity at least before 171 and 93 mm crack extensions for the 0.70 and 55 ㎜/sec crack velocities, respectively. The maximum slopes of the R-curves occurred between 25 and 89 ㎜ crack extensions for 0.70 ㎜/sec crack velocity and between 51 and 127 ㎜ crack extensions for 55 ㎜/sec crack velocity During the maximum slopes of the R-curves, the micro-crack localization can be expected, and faster crack velocity may form longer micro-cracking and micro-crack localizing zones. The fracture resistance of 0.70 ㎜/sec crack velocity reached a roughly constant maximum value of 143 N/m at 152 ㎜ crack extension, while that of 55 ㎜/sec crack velocity increased continuously to 245 N/m at 254 ㎜ crack extension and then decreased to the value of 0.70 ㎜/sec crack velocity. The R-curve of 55 ㎜/sec crack velocity was similar to that of the small size three-point bend test, and it showed that small size specimen or fast crack velocity could cause more brittle behavior.

• Journal of the Korea Furniture Society
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• v.23 no.3
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• pp.324-331
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• 2012

The domestic pine was used to investigate the change of specific gravity, moisture contents, color and anatomical structure by accelerated weathering test (AWT). According to visual inspection, a few knot separation and looseness as well as considerable surface discoloration was found out. However, the crack and split of surface texture have been never occurred till the last step of AWT. On the whole, as the time of accelerated weathering test has increased, the specific gravity has decreased. Finally, after the 9th week of AWT, the specific gravity was 0.38 that reached to 82% compared to the control specimen. In case of moisture content (MC), it showed rising trend in its early stages, however, after 3th week of AWT it have displayed steady state. A deterioration of cell-wall components was not remarkably observed by scanning electron microscope (SEM), however the ray fractures of AWT specimen were observed more than those of control specimen. The full fracture of epithelial cell around resin canal was observed by optical microscope. The fracture of ray of 2th cycle AWT specimen was first, followed by 1th week and control group. A distortion of tracheid for early spring wood and fracture of epithelial cell were generally observed by a similar level, regardless of duration time of AWT. Therefore, it is obvious that increasing duration time of AWT does not affect the deterioration of micro-structure for wood members from this study. Although a considerable change of anatomical properties was not found, there is a need of further research to understand how will the changes of specific gravity and MC on the physical properties of wood member.

• Transactions of Materials Processing
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• v.17 no.4
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• pp.240-248
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• 2008

The effect of alloying elements(Mn, S, Mo, B) on the high temperature deformation behavior of Fe-29%Ni-17%Co (Kovar) alloy were investigated. And the effect of high temperature oxidation on the hot ductility was also studied. The hot ductility of Kovar alloy was drastically increased with the addition of Mn and lowering of S content. It has been found that the brittle intergranular fracture at high temperature cracking is closely associated with the FeS sulfide along the grain boundary. When Mn was added, the type of sulfide was changed to MnS from FeS and ductile intergranular fracture and transgranular fracture were promoted. The formation of oxide layer was found to have minimized the hot ductility of the Kovar alloy significantly. Grain boundary micro-cracks in the internal oxide region were noted following deformation due to high temperature, one of which acting as a notch that caused the poor hot workability of the oxidized specimen. The addition of Mo to the Kovar alloy could also retard the decrease in the hot ductility of the oxidized specimen through the prevention of notching due to internal oxidation. Hot ductility was remarkably improved by the addition of Boron. The improvement of hot ductility results from the grain boundary migration mainly due to the dynamic recrystallization at lower temperature range ($900{\sim}1000^{\circ}C$).

• Journal of Powder Materials
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• v.27 no.4
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• pp.311-317
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• 2020

In this study, the effects of Co content on the microstructure and Charpy impact properties of Fe-Cr-W ferritic/martensitic oxide dispersion strengthened (F/M ODS) steels are investigated. F/M ODS steels with 0-5 wt% Co are fabricated by mechanical alloying, followed by hot isostatic pressing, hot-rolling, and normalizing/tempering heat treatment. All the steels commonly exhibit two-phase microstructures consisting of ferrite and tempered martensite. The volume fraction of ferrite increases with the increase in the Co content, since the Co element considerably lowers the hardenability of the F/M ODS steel. Despite the lowest volume fraction of tempered martensite, the F/M ODS steel with 5 wt% Co shows the highest micro-Vickers hardness, owing to the solid solution-hardening effect of the alloyed Co. The high hardness of the steel improves the resistance to fracture initiation, thereby resulting in the enhanced fracture initiation energy in a Charpy impact test at - 40℃. Furthermore, the addition of Co suppresses the formation of coarse oxide inclusions in the F/M ODS steel, while simultaneously providing a high resistance to fracture propagation. Owing to these combined effects of Co, the Charpy impact energy of the F/M ODS steel increases gradually with the increase in the Co content.