• Korean Journal of Materials Research
• /
• v.3 no.1
• /
• pp.84-94
• /
• 1993

This paper deals with the fatigue properties of chopped strand glass mat/polyester composite to understand the effect of water absorption on fatigue behavior of GFRP. The fatigue crack in the both no water and a water absorption materials initiated at the initial of cycle. Thereafter, it was divided with two regions that one decreased with the crack extension and the other increased with the crack extension. The absorption of distilled water degrades the bond strength between fiber and matrix, there, by the tendency of fiber pull-out is increased in perpendicular to crack growth deirection and the debonding of fibers increased to the place which is parallel to crack growth direction. Therefore, the reduction of fatigue strength was caused by these factors.

• Journal of the Korean Society of Manufacturing Technology Engineers
• /
• v.9 no.5
• /
• pp.73-79
• /
• 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.

• Proceedings of the Materials Research Society of Korea Conference
• /
• 2009.05a
• /
• pp.42.1-42.1
• /
• 2009

오스테나이트 스테인리스강은 우수한 내식성 및 기계적 특성으로 인해 구조용 재료로 널리 사용되고 있다. 표준원전 경수로의 경우 가압기 밀림관소재로 Nb 안정화 오스테나이트 스테인리스강인 type 347 스테인리스강이 사용되고 있다. 그러나 원전배관에서는 운전중 배관내 온도편차에 의한 열응역과 하중변화에 의한 기계적하중에 의해 피로손상을 받는다. 일반적으로 범용 오스테나이트 스테인리스강(AISI 304, 316)의 피로균열 성장거동에 대한 연구결과는 국내외적으로 다수 축적되어 있으나 type 347 탄소, 질소 함량에 따른 기계적 특성 및 피로균열성장 연구는 매우 미비하다. 따라서 본 연구에서는 탄소와 질소의 함량에 따른 기계적거동을 평가하고, 이에 따른 피로균열전파속도를 관찰하여 스테인리스강의 정확한 피로균열전파속도 곡선을 제시하고자 한다. 실험에 사용된 시편은 두께 5mm, 폭 25.4mm CT시편을 사용하였으며, 1mm의 예비균열을 주었다. 그리고 실험온도는 상온과 원전가동온도인 $316^{\circ}C$에서 실시하였으며, 주파수는 10Hz를 주었다. 실험결과 각 함량에 따른 type 347의 미세조직 관찰결과 기지내에 압연방향을 따라 조대한 석출물의 흐름이 관찰되었으며, 크기나 분포가 큰 차이를 보였다. C+N 함량이 낮은 시편은 주로 $0.1\;{\mu}m$ 이하의 미세한 입자들이 오스테나이트 기지조직의 입내와 입계에 고르게 분포되어 있었다. 그러나 C+N 함량이 높은 시편의 경우에는 $0.1\;{\mu}m$ 이하의 미세한 입자들과 함께 국부적으로 $1\sim10\;{\mu}m$의 조대한 입자들이 분포하고 있는 것이 관찰되었다. 그리고 질소의 함량이 높아짐에 따라 인장강도는 증가하였으며, 피로시험결과 고온에서 실험한 피로균열성장률 곡선이 상온보다 높게 나타남을 확인할 수 있었다. 그리고 질소가 적게 첨가되고 탄소의 함량이 많을수록 피로균열성장률은 ASME 곡선보다 낮게 나타났다.

• Composites Research
• /
• v.22 no.1
• /
• pp.9-14
• /
• 2009

This paper suggested the electric potential method with a bridge circuit concept for the detection of the location and crack growth of carbon fiber reinforced composites to reduce the measurement numbers. 2 pairs of electrodes were fabricated on the center cracked thin composite plates, and potential changes at one pair of adjacent electrodes were observed while external voltage input was applied to the other pair of adjacent electrodes. The effects of the size and interval of electrodes, location and propagating direction of center cracks were investigated by experiments and finite element analyses. Detectable crack size was influenced by the electrode interval rather than the electrode size, and crack detection was enhanced as the size and interval of electrodes were smaller. Besides, output potential changes were larger as the crack grew and was nearer the voltage input electrodes.

• KSCE Journal of Civil and Environmental Engineering Research
• /
• v.14 no.6
• /
• pp.1319-1327
• /
• 1994

Since fatigue cracks in rail can be the source of fractures and subsequent derailments, quantitative evaluation of the fatigue behavior and fracture properities due to the analysis results of laboratory test are drawn on the basis for predicting fatigue life and making a decision of safe inspection interval. Charpy V-notch and fracture toughness behavior were evaluated from the results of Charpy impact test. Fatigue test was performed by using CT type specimen under constant amplitude loading, and finally the effects of the following parameters; crack orientation, temperature, and stress ratio, on the fatigue crack growth behavior were studied.

• Proceedings of the KSME Conference
• /
• 2000.04a
• /
• pp.206-211
• /
• 2000

Fatigue test was conducted on a S45C steel using hour-glass shaped smooth tubular specimen under biaxial loading in order to investigate the crack formation and growth at room temperature. Three types of loading system, i.e fully reserved cyclic torsion without a superimposed static tension or compression, fully reserved cyclic torsion with a superimposed static tension and fully reserved cyclic torsion with a superimposed static compression were employed. The test results show that a superimposed static tensile mean stress reduced fatigue lifetime. however a superimposed static compressive mean stress increased fatigue lifetime. Experimental results indicated that cracks were initiated on planes of maximum shear strain with either a superimposed mean stresses or not. A biaxial mean stress had an effect on the direction which cracks nucleated and propagated at stage I (mode II).

• Tunnel and Underground Space
• /
• v.23 no.6
• /
• pp.470-479
• /
• 2013

Hydraulic fracturing is used as a method for promoting the fluid flow in the rock and, in the energy field such as geothermal development and the development of sales gas, many studies has been actively conducted. In many cases, hydraulic fracturing is not performed in isotropic rock and especially in the case of sedimentary rocks, hydraulic fracturing is conducted in the transverse isotropic rock. The direction of the crack growth on hydraulic fracturing does not necessarily coincides with the direction of maximum principal stress in the transverse isotropic rock. Therefore, in this study, bonded particle model with hydro-mechanical coupling analysis was adopted for analyzing the characteristics of hydraulic fracturing in transverse isotropic rock. In addition, experiments of hydraulic fracturing were conducted in laboratory-scale to verify the validity of numerical analysis. In this study, the crack growth and crack patterns showed significant differences depending on the viscosity of injection fluid, the angle of bedding plane and the influence of anisotropy. In the case of transverse isotropic model, the shear crack growth due to hydraulic fracturing appeared prominently.

• Journal of the Korean Crystal Growth and Crystal Technology
• /
• v.2 no.1
• /
• pp.43-50
• /
• 1992

$ LiNbO_3$ single crystals were grown using the Czochralski Method at various pulling speeds. Macroscopic defects such as cracks, bubbles and cellular structures were observed in some crystals. Cracks and bubbles observed in the crystals depended on the pulling speed and cooling rate. $ LiNbO_3$ crystals of about 15mm diameter could be grown properly at 6-7mm/h pulling speed and $ 20^{circ}C/h$ cooling rate. In order to investigate dielectric properties and optical properties for device application, these properties were measured for the sample cut along a axis and c axis at different temperatures.

• Journal of the Korean Ceramic Society
• /
• v.41 no.9
• /
• pp.677-683
• /
• 2004

Nd:LLM (Nd:LiLa(MoO$_4$)$_2$) single crystals for the laser host material were grown by the Czochralski method. The Nd:LLM grown single crystals cracked easily, and the reasons of cracks are generally related with phase transition, incongruent melting, chemical heterogeneity of composition, geometric thermal structures of imbalance and growth direction. We confirmed that phase transition is not observed by TG-DTA thermal analysis, and the XRD analysis revealed congruent melting in our products. It was confirmed that the volatilization of Li$_2$O composition is the important reason of chemical heterogeneity. The geometric thermal profile of the resistance furnace of our own design was controlled with a crucible height. Also, Nd:LLM crystal affected growth direction, and was the best quality in case of (101) growth direction. The distribution and effective distribution coefficient of Nd$^{3+}$ ion were accomplished by PIXE analysis.s.

• Transactions of the Korean Society of Mechanical Engineers A
• /
• v.37 no.1
• /
• pp.47-54
• /
• 2013

This study investigates the crack growth behavior due to primary water stress corrosion cracking (PWSCC) in the dissimilar metal butt weld of a reactor piping using Alloy 82/182. First, detailed finite element stress analyses were performed to predict the stress distribution of the dissimilar metal butt weld in which the hydrostatic and the normal operating loads as well as the weld residual stresses were considered to evaluate the stress redistribution due to mechanical loadings. Based on the stress distributions along the wall thickness of the dissimilar metal butt weld, the crack growth behavior of the postulated axial and circumferential cracks were predicted, from which the crack growth diagram due to PWSCC was proposed. The present results can be applied to predict the crack growth rate in the dissimilar metal butt weld of reactor piping due to PWSCC.