• 한국건축시공학회:학술대회논문집
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• 한국건축시공학회 2017년도 추계 학술논문 발표대회
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• pp.147-148
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• 2017

In this study, the tensile properties of mono and hybrid fiber reinforced cement-based composite according to fiber blending ratio under the high strain rate was evaluated. Experimental results, the HSF1.5PVA0.5 shown the highest tensile strength because the PVA fiber suppressed the micro cracks in the matrix around the hooked steel fiber and improved the pull-out resistance of hooked steel fiber. Thus, DIF of strain capacity and fracture toughness of HSF1.5PVA were greatly improved. Also, the fracture toughness was greatly improved because the tensile stress was slowly decreased after the peak stress by improvement of the pull-out resistance of hooked steel fiber at strain rate 101/s.

• Corrosion Science and Technology
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• 제20권2호
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• pp.69-76
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• 2021

In this work, we have performed a corrosion failure analysis of a leaking tube connected to an upper header of a condensate pre-heater in a heat recovery steam generator. It was revealed that the leakage position in the tube was the location where the materials were easily vulnerable due to tensile residual stresses induced by the material manufacturing process and welding process. In addition to an imbalance in the module induced by temperature difference during operation of the pre-heater, the weight of the modules and thermal fatigue provoked a type of stress of tensile-tensile fatigue on the tube. Thus, the leakage position of the pre-heater was exposed to the tensile stress on the inner surface of the tube facing the gas, which rendered the unstable oxide layer susceptible to corrosion and the formation of pits on the water side. The cracks propagated along with the degraded microstructure in a transgranular cracking mode under fatigue loading and finally resulted in water leakage.

• Advances in concrete construction
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• 제13권3호
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• pp.265-279
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• 2022

The roughness of substrate concrete interfaces before new concrete placement has a major effect on the interface bond behaviour. However, there are challenges associated with the consistency of the final roughness interface prepared using conventional roughness preparation methods which influences the interface bond performance. In this study, five quantitative interface roughness textures with different roughness tooth angles, depths, and tooth distribution were created to ensure consistency of interface roughness and to evaluate the bond behaviour at a precast and new concrete interface using the splitting tensile test, slant shear test, and double-shear test. In addition, smooth interface specimens and two separate the pitting interface roughness were also utilized. Obtained results indicate that the quantitative roughness has a very limited effect on the interface tensile bond strength if no extra micro-roughness or bonding agent is added at the interface. The roughness method however causes enhanced shear bond strength at the interface. Increased tooth depth improved both the tensile and shear bond strength of the interfaces, while the tooth distribution mainly influenced the shear bond strength. Major failure modes of the test specimens include interface failure, splitting cracks, and sliding failure, and are influenced by the tooth depth and tooth distribution. Furthermore, the interface properties were obtained and presented while a comparison between the different testing methods, in terms of bond strength, was performed.

• 콘크리트학회논문집
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• 제13권5호
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• pp.499-506
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• 2001

본 연구는 8개의 직접인장실험을 수행하여 축방향 부재의 콘크리트강도에 따른 부착특성과 균열거동을 조사하였다. 주변수는 콘크리트강도로 보통강도 24-25 MPa, 고강도 61-63 MPa이다. 직접인장시험체는 2종류로서 짧은 시험체는 횡균열 사이의 인장부재를 모형화하였고, 긴 시험체는 다수의 횡균열이 발생하는 인장부재를 모형화하였다. 이러한 직접인장실험을 수행하여 다음과 같은 결론을 얻었다. 부착강도는 압축강도에 비례하여 증가한다. 그러므로 고강도콘크리트에서 응력교란구간의 길이는 더 짧아지고 횡균열 간격이 더 줄어든다. 콘크리트강도가 25MPa에서 61MPa으로 증가하였지만 쪼갬균열하중은 거의 같게 나타났다. 반면에 횡균열하중은 인장강도에 비례하여 증가하였다. 따라서 고강도콘크리트를 사용할 때에 현행 구조설계기준의 정착길이 산정방법은 재고될 필요가 있고, 피복두께와 순철근간격을 더 크게 하든지 횡보강철근의 의무화하는 것이 현행 기준에 추가되기를 권장한다.

• 대한기계학회논문집A
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• 제26권1호
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• pp.105-112
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• 2002

In recent years, the integrity of reactor Pressure Vessel(RPV) under pressurized thermal shock (PTS) accident has been treated as one of the most critical issues. Under PTS condition, the combination of thermal and mechanical stress by steep temperature gradient and internal pressure causes considerably high tensile stress at the inside of RPV wall. As a result, cracks on inner surface of RPV may experience elastic-plastic behavior which can be characterized by J-integral. In such a case, however, J-integral may possibly lose its vapidity due to the constraint effect. The degree of constraint effect is influenced by the loading mode, crack geometry and material properties. In this paper, in order to investigate the effect of clad thickness and crack geometry on constraint effect, three dimensional finite element analyses were performed for various surface cracks. Total of 27 crack geometries were analyzed and results were presented by a two-parameter characterization based on the J-integral and the f-stress.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 춘계학술대회논문집A
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• pp.26-36
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• 2001

Smart sensors and actuators have recently been developed. In this study, first, small-diameter fiber Bragg grating (FBG) sensors developed by the author, whose cladding and polyimide coating diameters were 40 and $52{\mu}m$, respectively, were embedded inside a laminate without resin-rich regions around sensors and the deterioration of mechanical properties of the composite laminate. The small-diameter FBG sensor was embedded in $0^{\circ}$ ply of a CFRP laminate for the detection of transverse cracks in $90^{\circ}$ ply of the laminate. The reflection spectra from the FBG sensor were measured at various tensile stresses. The spectrum became broad and had some peaks with an increase of the transverse crack density. Furthermore, the theoretical calculation reproduced the change in the spectrum very well. These results show that the small-diameter FBG sensors have a potential to detect the occurrence of transverse cracks through the change in the form of the spectrum, and to evaluate the transverse crack density quantitatively by the spectrum width. On the other hand, shape memory alloy (SMA) films were used to suppress the initiation and growth of transverse cracks in CFRP laminates. Pre-strained SMA films were embedded between laminas in CFRP laminates and then heated to introduce the recovery stress in SMA films and compressive stresses in the weakest plies ($90^{\circ}$ ply). The effects of recovery stresses are demonstrated in the experiments and well predicted using the shear-lag analysis and the nonlinear constitutive equation of SMA films.

• 한국생산제조학회지
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• 제9권5호
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• pp.73-79
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• 2000

The applications of fracture mechanics have traditionally concentrated on cracks loaded by tensile stresses, and growing under an opening or mode I mechanism. However, many cases of failures occur from growth of cracks subjected to mixed mode loading. Several criteria have been proposed regarding the crack growth direction under mixed mode loadings. This paper is aimed at investigation of fatigue crack growth behaviour under mixed mode(I＋II) with variation of angle and pre-crack length in two dimensional branched type precrack. Especially the direction of fatigue crack propagation was predicted and effective stress intensity factor was calculated by finite element analysis(FEA. In this paper, the maximum tangential stress(MTS) criterion was used to predict crack growth direction. Not only experiment but also finite element analysis was carried out and the theoretical predictions were compared with experimental results.

• 한국염색가공학회지
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• 제29권4호
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• pp.181-194
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• 2017

The biodegradability of polylactic acid(PLA) fabrics was evaluated by two methods: enzyme and soil degradation. Three different enzymes were selected to evaluate. Degradation times were measured at optimal enzyme treatment conditions. Biodegradation by enzymatic hydrolysis was compared with soil degradation. As a result, biodegradation created cracks on the fiber surface, which led to fiber thickening and shortening. In addition, new peak was observed at $18.5^{\circ}$ by degradation. Moreover, cracks indicating biofragmentation were confirmed by enzyme and soil degradation. By enzyme and soil degradation, the weight loss of PLA fabrics was occurred, there through, the tensile strength decreased about 25% by enzyme hydrolysis when 21 days after, and 21.67% by soil degradation when 60 days after. Furthermore, the biodegradability of PLA fabrics by enzymatic and soil degradation was investigated and enzymatic degradation was found to be superior to soil degradation of PLA fabrics. Among the three enzymes evaluated for enzymatic degradation, alcalase was the most efficient enzymes. This study established the mechanism of biodegradation of PLA nonwovens, which might prove useful in the textile industry.

• 한국미생물학회:학술대회논문집
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• 한국미생물학회 2001년도 추계학술대회
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• pp.26-36
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• 2001

A novel approach of microbiologically-enhanced crack remediation (MECR) has been initiated and evaluated in this report. Under the laboratory conditions, Bacillus pasteurii was used to induce $CaCO_3$ precipitation as the microbial urease hydrolyzes urea to produce ammonia and carbon dioxide. The ammonia released in surroundings subsequently increases pH, leading to accumulation of insoluble $CaCO_3$. Scanning electron micrography (SEM) and x-ray diffraction (XRD) analyses evidenced the direct involvement of microorganisms in $CaCO_3$ precipitation. In biochemical studies, the primary roles of microorganisms and microbial urease were defined. Furthermore, the role of urease in $CaCO_3$ precipitation was characterized utilizing recombinant Escherichia coli that encoded B. pasteurii urease genes in a plasmid. Microorganisms immobilized in polyurethane (PU) polymer were applied to remediate concrete cracks. Although microbiologically- induced calcite precipitation enhanced neither the tensile strength nor the modulus of elasticity of the PU polymer, cement mortar whose crack was remediated with the cemaden polymer showed a significant increase in compressive strength. Through detailed investigation, MECR showed an excellent potential in cementing cracks in granite, concrete, and beyond.

• 한국안전학회지
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• 제14권4호
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• pp.28-36
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• 1999

In this paper, A circular arc crackered plate in biaxially stretched sheets was investigated in the boundary element method. The applications of fracture mechanics have traditionally concentrated on crack problems under an mode I, straight crack. However, many service failures occur from growth of cracks subjected to mixed mode loadings. A rectangular plate with arc crack or slanted central crack, under biaxial tensile loading, was treated analytically and also solved numerically. The Results from BEM applying different loading conditions, crack length (a/W), arc angle($\alpha$) are presented and discussed. The stress intensity factors are evaluated by the techniques of the J-integral. The decomposition method, used to decouple the stress intensity factors in mixed mode problems, is implemented by a considering a small circular contour path around each crack tip. The BIE method was successfully applied to a circular arc crackerd plate problem, also slanted centre cracked plate under mixed mode.