• Journal of the Korean Society of Fisheries and Ocean Technology
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• v.39 no.4
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• pp.262-268
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• 2003

This paper describes the effect of loading rate, specimen geometries and material properties for Mode I interlaminar fracture toughness of hybrid composite by using double cantilever beam (DCB) specimen. In the range of loading rate 0.2-20mm/min, there is found to be no significant effect of loading rate with the value of critical energy release rate (G_IC). The value of $G_IC$ for variation of initial crack length are nearly similar values when material properties are CF/CF and GF/GF, however, the value of $G_IC/$ are highest with the increasing initial crack length at CF/GF. The SEM photographs show good fiber distribution and interfacial bonding of hybrid composites when the moulding is the CF/GF

• Journal of the Korea Concrete Institute
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• v.19 no.6
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• pp.677-684
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• 2007

This study investigates the failure mechanism of RC beams strengthened with GFRP (glass fiber reinforced polymer) sheets. After analyzing failure mechanisms, the various methods to prevent the debonding failures, such as increasing bonded length of GFRP sheets, U-shape wrappings and epoxy shear keys are examined. The bonded length of GFRP sheets are calculated based on the assumed bond strengths of epoxy resin. The U-shape wrappings are either adopted at the end or center of the CFRP sheets bonded to the beam soft. The epoxy shear keys are embedded to the beam soft to provide sufficient bond strength. The end U-wrappings and the center U-wrappings are conventional, while epoxy shear keys are new details developed in this study. A total six half-scale RC beams have been constructed and tested to investigate the effectiveness of each methods to prevent debonding failure of GFRP sheets. From the experimental results, it was found that increasing bonded length or end U-wrappings do not prevent debonding failure. On the other hand, the beams with center U-wrappings and shear keys reached an ultimate state with their sufficient performance. The center U-wrappings tended to control debonding of the longitudinal GFRP sheets because the growth of the longitudinal cracks along the edges of the composites was delayed. In the case of shear keys, it was sufficient to prevent debonding and the beam was failed by GFRP sheets rupture.

• Journal of the Korean Geotechnical Society
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• v.28 no.7
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• pp.5-16
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• 2012

Soil-nailing is the most popular method of reinforcing for slope stability. In general, two factors are considered as failure modes during the soil-nailing design stages: pullout failure mode and shear failure mode that will occur on the most probable failure plane. In many cases, however, shallow failure can also occur when the ground near the slope face is swept away by the horizontal stress release during the staged top-down excavation. In this paper, an optimized soil-nailing design methodology is proposed by considering the three failure modes mentioned above: pullout failure; shear failure; and shallow failure. The variables to be optimized include the bonded length and number of soil-nailings, and the confining pressure that should be applied at the slope face. The procedure to obtain the optimized design variables is as follows: at first, optimization of soil-nailings, i.e. bonded length and number, against pullout and shear failure modes; and then, optimization of confining pressure at each excavation stage that is needed to prevent shallow failure. Since the two processes are linked with each other, they are repeated until the optimized design variables can be obtained satisfying all the constrained design requirements in both of the two processes.

• Proceedings of the Korea Concrete Institute Conference
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• 2008.11a
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• pp.859-862
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• 2008

The objective of this study offer the equation of the development length for GFRP reinforcement. Pullout test carried out to propose the development length for GFRP reinforcement. Test variables included embedment length (L=15, 30 and 45d$_b$ ), pure cover thickness(C=0.5, 1.0, 1.5, and 2.0d$_b$ ), diameter of reforcement(D10, D13 and D16), and three types, (domestic : K2KR, K3KR, foreign : AsUS) of GFRP reinforcement. The method of test were introduced pure pullout and tests lasted until the GFRP reinforcements were reached final failure. Based on the results through the pullout test, the bond characteristics and average bond stress for GFRP reinforcement were investigated. The equation of development length was proposed based on the regression analysis selected specimens having splitting failure. The equation gained from this study compared with the design equation provided by ACI committee 440.1R-06. The results through this study are capable of the flexural member design with GFRP reinforcement having lab spliced.

• Journal of Aerospace System Engineering
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• v.17 no.2
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• pp.1-8
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• 2023

Power transmission shafts in rotary wing aircraft use a hollow shaft to reduce weight. We can apply linear elastic fracture mechanics to predict crack propagation behavior. This paper predicted crack growth life of a hollow shaft with a circumferential through-type crack by finite element analysis. A 2D finite element model was created by applying a torsion and forming elements considering cracks. We defined the initial crack length and performed the finite element analysis by increasing the crack length to derive stress intensity factor at crack tips. We defined the length just prior to the stress intensity factor exceeding the fracture toughness as the crack limit length. We calculated the crack limit length using a handbook and numerically integrated the crack growth rate equation to derive growth life of each crack. The growth life of each crack was compared to verify the proposed finite element analysis method.

• Journal of the KSME
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• v.37 no.10
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• pp.41-46
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• 1997

철강구조물 부재 내에 노치나 균열이 존재할 수 있고, 외부의 피로하중에 의하여 취약부에서 발생한 균열이 진전하여 전구조물의 최종파손을 야기시킬 수 있다. 부재를 보다 안전하게 사용하고 또한 신뢰성을 확보하기 위해서는 이미 손상된 부재에서 균열의 진전상태를 계측할 수 있는 방법이 확립되어져야 하고, 파괴역학적 파라미터를 이용한 사용재의 균열진전거동특성이 평가되어야 한다. 균열길이의 측정방법은 지금까지 많은 연구자들에 의하여 개발되어져 왔는데 크게 광학현미경을 이용하여 육안으로 직접 균열길이를 측정하는 방법과 컴플라이언스, 초음파, AE 또는 전기적 신호를 통하여 얻어진 결괄부터 균열길이로 환산하는 간접적인 방법으로 대별된다. 대부분의 균열길이의 측정방법은 많은 수작업이 요구되고, 특히 하한계응력확대계수영역의 미세한 균열진전량을 측정하기에는 어려움이 따르고 있다. 이에 대하여 전도체 시험편에 일정전류를 흐르게 하고 균열길이의 증가에 따라 변화하는 전위차로 이를 균열길이로 평가하는 전기적인 측정방법이 있다. 이 방법은 실험장치가 비교적 간단하고 미세한 균열길이의 측정이 용이하여 균열길이의 직접적인 측정이 곤란한 고온역 그리고 충격하중하에서의 균열길이 측정에 이용이 확대되고 있다. 이 글에서는 여러 균열길이 측정방법의 장.단점에 대하여 고찰하고, 그 중 많은 장점을 갖고 있는 직류전위차법의 실험방법을 소개한다.

• Proceedings of the Korea Concrete Institute Conference
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• 2005.05a
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• pp.143-146
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• 2005

최근 들어 90도 표준갈고리의 대안으로 정착판을 지니는 헤드 철근(headed bar)에 대한 관심이 높아지고 있다. 헤드 철근의 정착내력은, 정착판의 지압력과 위험단면에서 헤드까지 정착길이의 부착력으로 발현된다. 실제 구조물에서는 정착되는 부재의 재료 및 기하학적 물성에 의해 다양한 파괴가 발생된다. 따라서 헤드 철근의 정착내력은 단순히 지압력과 부착력의 합으로 산정될 수 없으며, 발생 가능한 모든 파괴양상을 고려한 최소 내력으로 결정되어야 한다. 헤드 철근의 정착내력을 산정하기 위한 기본적인 해석모델로, CCT 절점에 정착된 헤드 철근의 트러스 모델을 제안하였다. 제안된 트러스 모델의 파괴는 부착파괴와 콘크리트의 압축파괴로 구분되며, 재료 및 기하학적 물성에 의해 파괴 양상이 결정된다. 이러한 트러스 모델은 외부 보-기둥 접합부와 같이 보다 복잡한 부위에 정착된 헤드철근의 정착 기구를 설명하는데 활용될 수 있다.

• Journal of the Korea institute for structural maintenance and inspection
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• v.9 no.1
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• pp.283-292
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• 2005

This paper predicts premature failure load of reinforced concrete beams by epoxy-boned partially steel plates. A parametric study is conducted to estimate premature failure load of beams such as with or without stirrups, unplated length ratio, steel and reinforcement ratio, shear span to depth ratio of reinforcement beam. By results of finite element analysis, it turned out that the unplated length played a dominant role in partially plated beams but reinforcement ratio and shear span to depth ratio effected the premature failure load. The approximate expression with regard to combined design variables is compared with experimental results. It shows closely agreement.

• Tunnel and Underground Space
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• v.11 no.3
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• pp.237-247
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• 2001

In this study, the fracture mechanics model as well as the elastic model was reviewed theoretically and four field case studies were conducted to investigate the feasibility of fracture mechanics model for hydraulic fracturing test. There was a difference between the result by fracture mechanics model and the one by elastic model. And the smaller initial crack length is, the larger the difference is. It is considered that the fracture mechanics model can be applied to the specific case of which the crack length is known. In this study, the rock tensile strength is measured using fracture mechanics model, brazilian test and elastic model. The measured tensile strength by the fracture mechanics model is the largest and the elastic model is the smallest. This result is due to the size effect of the each test. And the tensile strength from the elastic model for hydraulic fracturing test can be used to estimate the in-situ rock tensile strength.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.37 no.12
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• pp.1503-1512
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• 2013

In this study, we proposed an indentation evaluation method for fracture toughness using cohesive finite element simulations. First, we examined the effect of material properties (yield strain, Poisson's ratio, and elastic modulus) on crack size during Vickers indentation and then generated a regression formula that explains the relations among fracture toughness, indentation load, and crack size. We also proposed another indentation formula for fracture toughness evaluation using the contact size a and E/H (H: hardness). Finally, we examined the relation between the crack size and the indenter shapes. Based on this, we can generate from the formula obtained using the Vickers indenter a formula for an indenter of different shapes. Using the proposed method, fracture toughness is directly estimated from indentation data.