• 대한기계학회논문집A
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• 제26권11호
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• pp.2305-2311
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• 2002

The pressure tube is a major component of the CANDU reactor, which supports nuclear fuel bundle and heavy water coolant. Pressure tubes are installed horizontally inside the reactor and only selected samples are periodically examined during in-service inspection. In this respect, a probabilistic safety assessment method is more appropriate fur the assessment of overall pressure tube safety. The failure behavior of CANDU pressure tubes, however, is governed by delayed hydride cracking which is the major difference from pipings and reactor pressure vessels. Since the delayed hydride cracking has more widely distributed governing parameters, it is impossible to apply a general PFM methodology directly. In this paper, a PFM methodology for the safety assessment of CANDU pressure tubes is introduced by applying Monte Carlo simulation in determining failure probability Initial hydrogen concentration, flaw shape and depth, axial and radial crack growth rate and fracture toughness were considered as probabilistic variables. Parametric study has been done under the base of pressure tube dimension and hydride precipitation temperature in calculating failure probability. Unstable fracture and plastic collapse are used for the failure assessment. The estimated failure probability showed about three-order difference with changing dimensions of pressure tube.

• 한국공작기계학회논문집
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• 제14권2호
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• pp.75-82
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• 2005

Recently the needs of high reliable substances of high strength and high ductility are gradually increased with the development of aerospace industry. The characteristics of maraging steel has high ductililty, formability, corrosion resistant and high temperature strength and is easy to fabricate, weld and treat with heat, and maintain an invariable size even after heat treatment. e steels are furnished in the solution annealed condition and they achieve full properties through martensitic precipitation aging a relatively simple, low temperature heat treatment. As is true of the heat treating procedures, aging is a time/temperature dependent reaction. Therefore, the objective of this stud)'was consideration of fatigue characteristics according as Nb(niobium) content and time/temperature of heat treatment change. Also the stress analysis, fatigue lift, and stress intensity factor were compared with experiment results and FEA(finite element analysis) result. The maximum ftresses of)( Y, and Z axis direction showed about $2.12\times$10$^{2}$MPa, $4.40\times$10$^{2}$MPa and $1.32\times$10$^{2}$MPa respectively. The fatigue lives showed about $7\%$ lower FEA result than experiment result showing almost invariable error every analyzed cycle. Stress intensity factor of the FEA result was lower about $3.5~ 10\%$ than that of the experiment result showing that the longer fatigue crack ten添 the hi인or error. It considered that the cause for the difference was the modeled crack tip having always the same shape and condition regardless of the crack growth.

• 대한기계학회논문집A
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• 제25권8호
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• pp.1277-1286
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• 2001

Aluminum 5052/Aramid Fiber Reinforced Plastic(Al5052/AFRP) laminates are applied to the fuselage-wing intersection. The Al5052/AFRP laminates suffer from the cyclic bending moment of variable amplitude during the service. Therefore, the influence of cyclic bending moment on the delamination and the fatigue crack propagation behavior in Al5052/AFRP laminate was investigated in this study. Al5052/AFRP laminate composite consists of three thin sheets of Al5052 and two layers of unidirectional aramid fibers. The cyclic bending moment fatigue tests were performed with five different levels of bending moment. The shape and size of the delamination zone formed along the fatigue crack between Al5052 sheet and aramid fiber-adhesive layer were measured by an ultrasonic C-scan. The relationships between da/dN and ΔK, between the cyclic bending moment and the delamination zone size, and between the fiber bridging mechanism and the delamination zone were studied. Fiber failures were not observed in the delamination zone in this study. It represents that the fiber bridging modification factor should turn out to increase and that the fatigue crack growth rate should decrease. The shape of delamination zone turns out to be semi-elliptic with the contour decreased non-linearly toward the crack tip.

• 한국정밀공학회지
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• 제9권1호
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• pp.106-117
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• 1992

In this study, the propagation behaviour and the closure phenomena of physically small surface cracks were investigated by the techinque of the Kikukawa-unloading elastic compliance method using a back face strain gage. The surface cracks initiated and propagated from notched specimens under constant amplitude bending load. The crack shape (aspect ratio) with approximately semi-circular at the early stage was changed to semi-elliptical as the cracks grew larger. The crack depth (a) could be expressed uniquenly by the crack length (c). The dependence of the crack propagation rate on the stress ratio R was strongly related in the lower ${\Delta}K$ range. The deceleration of the surface crack propagation rate was prominent in lower R during the crack length was small. When the propagation rate was rearranged with the effective stress intensity factor range ${\Delta}$K_{eff} the dependence of the crack propagation rate on the stress ratio R was found to be diminshed. These were caused by the crack closure phenomena that was most prominent at the lower propagation rate. The mechanism of crack closure phenomena was dominated by the plasticity-induced mechanism.

• 한국분말재료학회지
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• 제11권1호
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• pp.8-15
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• 2004

FGM은 원하는 물성의 점진적인 변화를 통해 재료에 다양한 특성을 확보할 수 있는 방법이다. 여러FGM의 제조 방법 중 분말야금법과 열용사법이 많이 사용되며, FGM적용은 열ㆍ기계적 물성이 요구되는 응용분야에서 가장 전망이 있고 현재 가장 많은 연구가 진행되어 왔다. 경사기능의 도입은 2층 구조의 재료에 비하여 열팽창 계수의 차이에 의한 층간 잔류응력 집중을 완화 시켜 접합 강도와 열 충격 특성 및 열피로 특성 등의 향상을 가져왔다. 그러나 120$0^{\circ}C$ 이상의 고온에서 사용되어지는 경우, 표면에서 가까이 위치한 금속부의 산화에 의해서 2층 구조보다 저하된 열피로 특성을 보였다. 이러한 점들은 FGM이 가지고 있는 근본적인 문제이므로 응용처의 환경에 대한 고려가 필요하다. 내열 재료뿐만 아니라, 구조용 재료에서도 기계적 물성의 향상을 위해 FGM이 시도되고 있는 등, 현재까지 응용분야의 확대 가능성은 무한하다. 경사 기능 재료를 실제 적용하는데 있어서 문제점으로 지적되고 있는 점들은 1)경사 기능 재료의 신뢰성, 2)경사 기능 재료의 설계, 3)경사 기능 재료의 제조 공정 등의 세 가지 문제점으로 요약할 수 있다. FGM에서의 물성의 변화는 미세구조의 변화 등으로 단순히 rule of mixture로 해석되지 않는다. 이를 위하여 최근 전산 모사를 통해 조성 경사도 및 재료 선정 등에 대한 연구가 이루어지고 있는 추세이며, 새로운 제조 방법 개발 및 개량이 꾸준히 진행되고 있어 향후 FGM의 적용에 대한 전망은 밝다.

• 한국철도학회논문집
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• 제12권1호
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• pp.16-24
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• 2009

선형탄성파괴역학을 적용하여 균열이 발생한 변동하중하의 철도차량 대차틀에 대한 균열성장속도를 예측하였다. 이를 위하여 철도차량 대차틀의 균열발생사례를 분석하여 취약부위를 파악하였으며, 영업노선에서의 실동하중 측정과 구조해석을 통한 정하중 계산으로 대차틀 취약부에서 운행 중 받는 총 하중이력을 생성하였다. 총 하중이력에서 균열닫힘을 고려한 유효하중이력을 계산하였으며, 취약부 3곳에서 균열성장속도를 예측하고 일본에서 측정한 균열진전 사례와 비교하였다. 해석결과 초기길이 40mm의 균열이 급속한 균열성장을 일으키기까지는 약 50만km의 주행거리가 필요하며 이는 약 3.8년의 운행기간에 해당하므로 도시철도의 유지보수기간을 고려하면 임계균열로 도달하기 전에 충분히 감지할 수 있을 것으로 생각된다.

• 한국해양공학회지
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• 제9권1호
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• pp.92-100
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• 1995

The effect of different anisotropy and stress ratio on fatigue crack propagation behavior was investigated under various stress ratio(R=-0.4, -0.2, 0.2, 0.2, 0.4) using pure titanium sheet used in aerospace, chemical and food industry. The rack closure behavior under constant load amplitude fatigue crack propagation test was examined. Fatigue crack propagation rate da/dN was estimated in terms of effective stress intensity factor range, $\Delta$K$_{eff}$, regardless of various stress ratio but was influenced by anisotropy. Also, it was found that the effect of anisotropy was considerably decreased but still not negligible when he da/dN was evaluated by a conventional parameter, $\Delta$$K_{eff}$/E and when the modified da/dN.$\sqrt{\varepsilon}_f$ was evaluated by $\Delta$$K_{eff}$/E. On the other hand, da/dN could be evaluated uniquely by effective new parameter, $\Delta$K$_{eff}$/$sigma_{ys}$, regardless of anisotropy, as int he following equation da/dN=C''[\frac{{\Delta}K_{eff}}{{\sigma}_{ys}}]^{n''}. And effective stress intensity factor range ratio, U was estimated by the following equation with respect to the ratio of reversed plastic zone size, $\Delta r_{p}$ to monotonic plastic zone size, $r_p$ regardless of stress ratio and anisotropy. U=-4.45$(\Delta r_{p}/r_{p})^{2}$+4.1$(\Delta r_{p}/r_{p})$+0.245_{p})$+0.245

• 대한기계학회논문집A
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• 제20권10호
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• pp.3048-3057
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• 1996

Analysis model containing two inclined surface cracks on semi-infinite elastic body is established and analyzed on the basis of linear fracture mechanics to examine mutual interference of two surface cracks. Muskhelishvili's complex stress functions are introduced and a set of singular integral equations is obtained for a dislocation density function. The stress intensity factors at crack tip are obtained by using the Gerasoulis'method. When two surface cracks are parallel and have the same length, the values of $K_1$and $\Delta K_11$(variation of $K_11$) for crack 1 and crack 2 decrease by the mutual interference of two surface cracks as the distance between the two surface cracks shortens. The effect of mutual interference is remarkable in high friction coefficient. In case that two surface cracks are parallel, the values of $K_1$and $\Delta K_11$for crack 2 decrease as the length ratio ot crack 2 to crack 1 becomes small. As the crack inclination angle rises, the value of $K_1$ and the mutual interference of $K_1$for crack 2 increase and the value of$\Delta K_11$ for crack 1 becomes smaller than that for crack 2.

• 대한기계학회논문집
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• 제18권6호
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• pp.1388-1398
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• 1994

The fracture toughness and micro-fracture mechanisms of the porous glass and stainless fiber reinforced glass composite were evaluated by using the acoustice mission(AE) technique, fracture toughness $test(K_{IC})$ and the macroscopic observation of the specimen surface which was being under the loading. At initial portion of the loading, the AE signals with low energy, of which origins were considered as the micro-cracks formated at the crack tip, were emitted. With increasing the applied load, AE signals having higher energies were generated due to the coalesence of micro-cracks and fast fracture. Based on the such relationship between AE emission and loading condition, fracture toughness $K_{IAE}$ could be defined successfully be using the $K_I$ value corresponding to an abrupt change of the accumulated AE signal energies emitted during the fracture toughness test. In spite of its brittleness of glass material, nonlinear deformation behavior before maximum load was observed due to the formation of micro-cracks. Further, the stainless fiber may have attributed to the improvement of fracture toughness and the resistance to crack propagation comparing to noncomposited materials Finally, models of the micro-fracture process combined with the AE sources for the porous glass material and its composite were proposed paying attention to the micro-crack nucleation and its coalescence at the crack tip. Fiber fracture and its Pullout, deformation of fiber itself were also delinated from the model.

• 대한기계학회논문집A
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• 제25권4호
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• pp.592-601
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• 2001

Fiber reinforced metal laminates(FRMLs) are new types of hybrid materials. FRMLs consists of high strength metal(Al 5052-H34) and laminated fiber with structural adhesive bond. The mixture ratio effect of epoxy resin$.$curing agent$.$accelerator on the fatigue behavior of FRMLs was investigated in this study. The epoxy, diglycidylether of bisphenol A(DGEBA), was cured by methylene dianiline(MDA) with or without an accelerator(K-54). Eight different kinds of resin mixture ratios were selected for the test ; five kinds of FRMLs(1) and three others of FRMLs(2). The relationship between da/dN and ΔK with variation of resin mixture ratio was studied. FRMLs(1) and FRMLs(2) indicated approximately 2 times and 2.2 times more improved maximum bending strengths in comparison with those of Al 5052-H34. The resin mixture ratio <1:1> in case of FRMLs(1) indicated the maximum fatigue life, while the resin mixture ratio <1:1:0.2> in case of FRMLs(2) indicated the maximum fatigue life. As results, FRMLs(2) turned out to have more effective characteristics on the fatigue properties and the bending strength than those of FRMLs(1).