• Journal of the Korean Society of Propulsion Engineers
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• v.15 no.5
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• pp.82-88
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• 2011

In the present work, FEM analyses are carried out to investigate the fractures occurred within the structural part in the course of combustion experiment. The loss of structural integrity stems from the localized deformation and the damage induced due to a severe change in the thermal load. Moreover, the two-back stress evolution model is proposed using the Armstrong-Frederick and the Phillips' rules to depict the plastic deformation, and the continuum damage mechanics is also incorporated into the present model. It is noted that the present model is able to formulate a wide range of constitutive description with ease. The numerical results depicts that a severe strain localization and damage evolution can be obtained depending on the dominant back stress.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.16 no.8
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• pp.5051-5059
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• 2015

Structural behaviour of the front loader for an agricultural tractor was analyzed for three impact test conditions: drop and catch, corner pull, and corner push. Rigid-body dynamic, transient structural, and static structural analyses were conducted using a commercial finite element software. Analysis of the drop and catch test dealt with the case that the bucket located at the maximum elevation was dropped and catched through three steps. Analysis of the corner pull test dealt with the case that the bucket constrained to the ground by a chain at its corner was raised suddenly. Analysis of the corner push test dealt with the case that the corner of the bucket collided with an obstacle. Results of analyses of the three test conditions showed that maximum stress occurs at the geometrically discontinuous location in the mount and is caused from local stress concentration. Results of the present research can be utilized as a guideline to achieve more reliable and safe structural design of the front loaders.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.36 no.12
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• pp.1529-1534
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• 2012

In this study, tensile tests for 12-${\mu}m$-thick copper thin foils were performed by using the DIC method. The DIC method provided precise stress-strain curves for thin film materials, and a commercial inkjet printer can be simply and effectively used for printing speckle patterns on the specimen of Cu thin films whose surface contrast is too low to apply the DIC method. The mechanical properties of Cu thin foils obtained in this study are as follows: elastic modulus E = 89.2 GPa, 0.2% offset yield stress $S_{0.2%}$= 232.8 MPa, tensile strength $S_u$= 319.2 MPa, elongation at fracture ${\varepsilon}_f$=16.8 %, and Poisson's ratio ${\nu}$= 0.34.

• Journal of the Korean Institute of Gas
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• v.12 no.1
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• pp.1-6
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• 2008

The notched Charpy impact test is one of the most prevalent techniques used to characterize the effect of high impulse loads on polymeric materials. In this study, a method of analysis in nylon plastic materials is suggested to evaluate the critical strain energy release rate for variation of notch angles from the Charpy impact energy measurement. Instrumented Charpy impact tester was used to extract ancillary information concerning fracture parameters in addition to total fracture properties and maximum critical load. The dynamic stress intensity factor of nylon plastic material was calculated for the ASTM Charpy specimen from the obtained maximum critical load. Also, the finite element model was developed to figure out the stress distributions for Charpy specimen with different notch angles subject to 3 point bending load which is equivalent to the load applied in the experiment.

• Journal of the Korea institute for structural maintenance and inspection
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• v.9 no.3
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• pp.178-186
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• 2005

In this study, the PSC girder bridge retrofitted with external tendons is tested to verify the strengthening effects. We measure the variations of the displacement and strain at mid-span of each beam as external tendons are removed in sequence. The structural behavior of the bridge are examined using controlled truck load tests for the systems before and after all external tendons were removed. From the test results, the characteristics of structural behavior of the bridge do not change significantly, but the natural frequency is decreased after the external tendons are removed. The strengthening effects of bridges can be exactly estimated by analytical methods some extent. As a result of this study, when a PSC girder bridge is deteriorated, the bridge can be retrofitted effectively by External Prestressing Strengthening Method, and the strengthening effects can be predicted through accurate structural analysis.

• Composites Research
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• v.26 no.2
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• pp.79-84
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• 2013

The prediction and analysis procedure of fatigue damage crack growth life for a stiffener bonded composite laminate panel including center hole and edge notch damage, was studied. It was performed on the basis of fatigue damage growth test results on a laminated skin panel specimens and the analysis results of stress intensity factor for the stiffener bonded composite panel. According to the comparison between experimental test and prediction results of fatigue damage growth life, it was concluded that the residual strength and damage tolerance assessment can be carried out along to the edge notch crack growth.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2011.04a
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• pp.272-278
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• 2011

In the present work, FEM analyses are carried out to investigate the fractures occurred within the structural part in the course of combustion experiment. The loss of structural integrity stems from the localized deformation and the damage induced due to a severe change in the thermal load. Moreover, the two-back stress evolution model is proposed using the Armstrong-Frederick and the Phillips' rules to depict the plastic deformation, and the continuum damage mechanics is also incorporated into the present model. It is noted that the present model is able to formulate a wide range of constitutive description with ease. The numerical results depicts that a severe strain localization and damage evolution can be obtained depending on the dominant back stress.

• Korean Journal of Materials Research
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• v.10 no.2
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• pp.111-116
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• 2000

Fracture mode and carbide reactions of cast alloy 738LC during thermal exposure and creep at 816$^{\circ}C$/440MPa and 982$^{\circ}C$/152MPa were investigated. Crystallographic transgranular failure was observed in the specimen crept at 816$^{\circ}C$ due to shearing on the slip plane. Because selective oxidation at the grainboundaries which was exposed at the surface leads reduction in surface energy, however, early initiation of crack at the grainboundaries and intergranular failure were observed in the specimen crept at 982$^{\circ}C$/152MPa. As a result of decomposition of MC carbide at the tested temperatures, M(sub)23C(sub)6 carbide precipitated either on the grainboundaries or on the deformation band. The applied stress enhanced decomposition of MC. $\sigma$phase nucleated from Cr(sub)23C(sub)6 then grew to the ${\gamma}$+${\gamma}$\\` matrix. Precipitation of $\sigma$was accelerated by increasing temperature and applied stress.

• Proceedings of the Materials Research Society of Korea Conference
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• 2009.05a
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• pp.42.2-42.2
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• 2009

연속주조공정에서 용강의 통로, 산화방지 및 유체 흐름을 용이하게 하는 역할을 하는 다공성 노즐(porous nozzle)은 용강과의 직접적인 접촉으로 인한 화학 반응 및 용강의 침투현상을 방지하기 위해 불활성 가스를 주입하여 청정강을 제조하는데 이용된다. 공정 중 노즐 막힘으로 인한 배압상승과 열충격에 의한 크랙(crack) 발생이 문제되고 있으며 신뢰성 향상 연구가 요구되고 있다. 따라서 본 연구에서는 기공크기와 기공분포가 고온안정성 및 내열충격성에 미치는 영향을 알아보고, 내구성 시험 및 고장분석을 통하여 노즐의 신뢰성 향상 방안을 고찰 하였다. 기공을 제어한 시편을 제조하여 기공분포에 따른 고온안정성을 확인하기 위해 실제 사용 조건인 용강온도($1550^{\circ}C$)와 보다 높은 온도($1700^{\circ}C$)에서 각각 고온 시험을 수행하였다. 열충격을 스트레스 인자로 한 내구성 시험을 수행한 후 고장원인을 분석하였으며 열화정도를 확인하기 위해 열처리 온도에 따른 차압 및 굽힘 강도 변화를 비교하였다. 또한 결정상 분석을 통해 온도에 대한 상변화를 확인하였고, 시편의 표면 및 파단면의 미세구조 분석을 통해 크랙 발생여부를 확인하였다. 다공성 노즐의 기공분포가 균일 할수록 고온안정성 및 내열충격성이 향상됨을 확인하였고, 이를 통해 Porous Nozzle의 열화원인으로 판단되는 기공 크기 및 분포에 따른 크랙 발생에 대해 열응력 고찰을 수행하였다.

• Journal of the Korean Society for Nondestructive Testing
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• v.28 no.4
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• pp.323-330
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• 2008

High cycle bending fatigue of socket welded small bore pipe was characterized, and also the fatigue crack initiation of small bore pipe was monitored in situ by the acoustic emission (AE) technique. The STS 316L stainless steel specimens were prepared by gas tungsten arc welding (GTAW) process having the artificial defect (i.e., lack of penetration) and defect free at the root. The fatigue failure was occurred at the loc for high stress and root for relatively low stress. The crack initiation cycles ($N_i$) was defined to the abrupt increase in AE counts during the fatigue test, and then the cracks were observed by the radiographic test and electron microscope before and after the fatigue crack initiation cycles. The socket welded pipe damaged by bending fatigue was studied regarding the welding defect, failure mode, and crack initiation cycles for the diagnosis and monitoring.