• 한국안전학회지
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• 제34권6호
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• pp.1-6
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• 2019

Carbon fiber has excellent specific strength, corrosion resistance and heat resistance. And p-Aramid fiber has high toughness and heat resistance and high elasticity, and is used in various fields such as industrial protective materials, bulletproof helmets and vests, as well as industrial fields. However, carbon fiber is relatively expensive, and is susceptible to brittle fracture behavior due to its low fracture strain. On the other hand, the aramid fiber tends to decrease in elastic modulus and strength when applied to the epoxy matrix, but it is inexpensive and has higher elongation and fracture toughness than carbon fiber. Thus the twill hybrid carbonaramid fiber reinforced composite laminate composite was investigated for a delamination fracture toughness under Mode I loading by 2 kinds of MBT and MCC deduction. The specimen was fabricated with 20 hybrid fabric plies. The initial crack was made by inserting the teflon tape in the center plane with a₀/W=0.5 length. The results show that SERR(Strain Energy Release Rate) as the critical and stable delamination fracture toughness were 0.09 kJ/㎡, 0.386 kJ/㎡ by MBT deduction, and 0.192 kJ/㎡, 0.67 kJ/㎡ by MCC deduction, respectively.

• Journal of Welding and Joining
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• 제33권2호
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• pp.32-39
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• 2015

This study examined the welding residual stress and fracture toughness of 78mm thick steel electro gas welding (EGW) and flux cored arc welding (FCAW) welded joints by numerical analyses of the thermal elasto-plastic behavior and fracture toughness(KIC). The residual stress, fracture toughness characteristics and production mechanism on the welded joints were clarified. Moreover, the effects of the welding process (EGW and FCAW) on the welding residual stresses and fracture toughness of welded joints were evaluated. The results showed that the new welding process (EGW) appears to be an effective substitute for the existing welding process (FCAW) in a thick steel plate with high strength.

• Advances in concrete construction
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• 제2권2호
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• pp.145-162
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• 2014

In this study, the fracture toughness $K_{IC}$ of high performance concrete (HPC) was determined by conducting three-point bending tests on eighty notched HPC beams of $500mm{\times}100mm{\times}100mm$ at high temperatures up to $450^{\circ}C$ (hot) and in cooled-down states (cold). When the concrete beams exposed to high temperatures for 16 hours, both thermal and hygric equilibriums were generally achieved. $K_{IC}$ for the hot concrete sustained a monotonic decrease tendency with the increasing temperature, with a sudden drop at $105^{\circ}C$. For the cold concrete, $K_{IC}$ sustained a two-stage decrease trend, dropping slowly with the heating temperature up to $150^{\circ}C$ and rapidly thereafter. The fracture energy-based fracture toughness $K_{IC}$' was found to follow similar decrease trends with the heating temperature. The weight loss, the fracture energy and the modulus of rapture were also evaluated.

• 한국정밀공학회지
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• 제27권8호
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• pp.28-34
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• 2010

Railway wheel and axle is the most critical components in railway system. A wheel and axle failure can cause a derailment with its attendant loss of life and property. The service conditions of railway vehicles have become severe in recent years due to a general increase in operating speeds. Therefore, more precise evaluate of wheelset strength and safety has been desired. Fracture mechanics characteristics such as dynamic fracture toughness, fatigue threshold and charpy impact energy with respect to the tread, plate, disc hole of wheel and the surface of press fitted axle are evaluated. This paper describes the difference of fracture toughness, fatigue crack growth and fatigue threshold at the locations of wheel and axle. The results show that the dynamic fracture toughness, $K_{ID}$, is obviously lower than static fracture toughness, $K_{IC}$ and the fracture mechanics characteristics are difference to the location of wheel tread and hole.

• Nuclear Engineering and Technology
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• 제55권11호
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• pp.4112-4119
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• 2023

This study employs the microshear test method to examine the local mechanical properties of narrow-gap welded joints, revealing the mechanical inhomogeneity by evaluating the microshear strength, stress-strain curves, and failure strain. On this basis, the influence of weld joints micromechanical inhomogeneity on the crack tip opening displacement (CTOD) fracture toughness is investigated. From the root weld layer to the cover weld layer, the fracture toughness at the center of the weld seam demonstrates an increasing trend, with the experimental and calculated CTOD values showing a good correspondence. The microproperties of the welded joints significantly impact the load-bearing capacity and fracture toughness. During the deformation process of the "low-matching" microregions, the plastic zone expansion is hindered by the surrounding microregion strength constraints, thus reducing the fracture toughness. In contrast, during the deformation of the "high-matching" microregions, the surrounding microregions absorb some of the loading energy, partially releasing the concentrated stress at the crack tip, which in turn increases the fracture toughness.

• 한국재료학회지
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• 제28권6호
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• pp.343-348
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• 2018

Recently, the properties of nanostructured materials as advanced engineering materials have received great attention. These properties include fracture toughness and a high degree of hardness. To hinder grain growth during sintering, it is necessary to fabricate nanostructured materials. In this respect, a high-frequency induction-heated sintering method has been presented as an effective technique for making nanostructured materials at a lower temperature in a very short heating period. Nanopowders of W and $Al_2O_3$ are synthesized from $WO_3$ and Al powders during high-energy ball milling. Highly dense nanostructured $W-Al_2O_3$ composites are made within three minutes by high-frequency induction-heated sintering method and materials are evaluated in terms of hardness, fracture toughness, and microstructure. The hardness and fracture toughness of the composite are $1364kg/mm^2$ and $7.1MPa{\cdot}m^{1/2}$, respectively. Fracture toughness of nanostructured $W-Al_2O_3$ is higher than that of monolithic $Al_2O_3$. The hardness of this composite is higher than that of monolithic W.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2003년도 추계학술대회
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• pp.1240-1245
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• 2003

Reduced activation ferritic steel (JLF-1) is considered as a promising candidate material for blanket or first-wall structure of D-T fusion reactors. The fracture tests of fracture resistance curve (J-R curve) and $J_{IC}$ are desirable to investigate the exact fracture toughness of JLF-1 steel, since it has a high ductility. The fracture toughness of JLF-1 steel is affected by the configuration of test specimen such side groove, specimen thickness or specimen size. In this study, the fracture toughness tests were performed with various size(plane size and thickness) and various side groove of specimens. The test results showed the standard specimen with the side groove of 40 % represented a valid fracture toughness. The fracture resistance curve increased with increasing plane size and decreased with increasing thickness. However, the fracture resistance curve of half size specimen was similar to that of the standard specimen.

• Journal of Welding and Joining
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• 제17권2호
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• pp.36-43
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• 1999

The effect of microstructure on the fracture toughness of multi pass weld metal has been investigated. The micromechanisms of fracture process are identified by in-situ scanning electron microscopy(SEM) fracture observation using single edge notched specimen. The notches of the in-situ fracture specimens were carefully located such that the ends of the notches were in the as-deposited top bead and the reheated weld metal respectively. The observation of in-situ fracture process for as-deposited top bead indicated that as strains are applied, microcracks are formed at the interfaces between soft proeutectoid ferrite and acicular ferrite under relatively low stress intensity factor. Then, the microcracks propagate easily along the proeutectoid ferrite phase, leading to final fracture. These findings suggest that proeutectoid ferrite plays an important role in reducing the toughness of the weld metal. On the other hand, reheated regions showed that the microcrack initiated at the notch tip grows along the localized shear bands under relatively high stress intensity factor, confirming that reheated area showing momogeneous and fine microstructure would be beneficial to the fracture resistance of weld metal.

• Composites Research
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• 제14권3호
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• pp.51-56
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• 2001

섬유강화 고분자기지 복합재에 있어 탄성계수, 최대응력, 최대변형률, 파괴특성 등이 압력에 의해 영향을 받는다는 것은 잘 알려진 사실이다. 본 연구에서는 준등방성이며 두꺼운 두께를 갖는 [0$^{\circ}$/$\pm$45$^{\circ}$/90$^{\circ}$]$_{11s}$ 로 적층된 탄소섬유/에폭시 복합재에 있어 압력을 0.1 MPa, 100 MPa, 200 MPa, 300 MPa로 변화시켜 압축 파괴실험을 수행하였으며 이로부터 압력변화에 따른 파괴특성 변화에 대해 검토하였다. 결과로서 가해진 압력이 증가함에 따라 압축파괴인성은 증가함을 알수 있었다. 구체적으로 압력이 대기압에서 300 MPa으로 증가할 때 압축파괴인성 값은 약 44% 증가하였다.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2000년도 추계학술대회논문집A
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• pp.93-98
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• 2000

Unpredictable failures can occur due to the DHC (delayed hydride cracking) or the degradation of fracture toughness by hydride embrittlement in CANDU pressure tube which can result from the absorption of hydrogen or deuterium in the high temperature coolant. To investigate the hydride embrittlement of CANDU Zr-2.5Nb pressure tube, the transverse tensile test and the fracture toughness test were performed from room temperature to $300^{\circ}C$ using three different specimens which have an AR (As Received), 100, and 200 ppm hydrogen. As the amount of absorbed hydrogen was increased, the transverse yield strength and the ultimate tensile strength were also increased. In addition, as the test temperature became higher they were decreased linearly. While, at room temperature, the hydrogenbsorbed specimens represented the embrittlement which resulted in sudden decreasing of fracture toughness, the fracture characteristics became ductile such as AR specimen at high temperatures. Through the observation of fracture surface using SEM, it was found that the stress state of mixed mode could be related to the fissure which was believed to decrease the global fracture toughness.