• 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.

• 한국해양공학회지
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• 제17권2호
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• pp.47-53
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• 2003

The development of new materials with light weight and high strength has become vital to the machinery, aircraft and auto industries. However, there are a lot of problems with developing such materials that require such expensive tools, as well as a great deal of time and effort. Therefore, the improvement of fatigue life through, the adoption of residual stress, is the main focus. The compressive residual stress was imposed on the surface according to each shot velocity(1800, 2200, 2600, 3000rpm) based on Shot-peening, which is the method of improving fatigue life and strength. By using the methose mentioned above, we arrived at the following conclusions; 1. The fatigue crack growth rate(da/dN) of the Shot-peened material was lower than that of the Un-peened material. In stage I, $\Delta$K$_{th}$, the threshold stress intensity factor, of the shot-peen processed material is high in critical parts, unlike the Un-peened material. Also m, fatigue crack growth exponent and number of cycle of the Shot-peened material was higher than of the Un-peened material. That is concluded from effect of da/dN. 2. Fatigue life shows more improvement in the Shot-peened material than in the Un-peened material. Compressive residual stress of the surface on the Shot-peen processed operate resistance force of fatigue crack propagation.

• Journal of Advanced Marine Engineering and Technology
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• 제25권6호
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• pp.1228-1236
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• 2001

In this study, CT specimens were prepared from ASTH A5l6 steel which was used for pressure vessel plates for moderate and lower temperature service. And we got the fellowing characteristics from fatigue crack growth test carried out in the environment of room and low temperature at $25^{\circ}C$ , $-30^{\circ}C$, $-60^{\circ}C$, $-80^{\circ}C$, $-100^{\circ}C$ and $-120^{\circ}C$ and in the range of stress ratio of 0.1, 0.3 by means of opening mode displacement. At the constant stress ratio, the Threshold stress intensity factor range ΔAKth in the early stage of fatigue crack growth (Region I) and stress intensity factor range $\DeltaK$ in the stable of fatigue crack growth (Region II) was increased in proportion to descend temperature. It assumed that the fatigue resistance characteristics and fracture strength at low temperature is considerable higher than that of room temperature in the early stage and stable of fatigue crack growth region. The straight line slope relation of logarithm da/dN-$\Delta$K in Region II that is, the fatigue clack growth exponent m increased with descending temperature at the constant stress ratio. It assumed that the fatigue crack growth rate da/dN is rapid in proportion to descend temperature in Region II and the cryogenic-brittleness greatly affect a material with decreasing temperature.

• 한국생산제조학회지
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• 제21권6호
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• pp.915-919
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• 2012

In constructing all kinds of equipment and steel structure, parts with discontinuities such as weld defects formed in the welded structure can generate fatigue cracks that results in damage or accidents. In this study, weld zones are investigated with X-rays and the latent images are put on film. Weld zone defects can be verified by developing the film. As weld defects are investigated by radiographic testing and correlated with the welding condition, more appropriate welding conditions can be found. According to the result of X-ray radiographic inspection of butt welding ATOS 80 high-strength steel with a thickness of 12mm, the best conditions for welding without creating weld defects are 4 weld-passes, a protective gas of 20% $CO_2$ and 80% Ar, a protective gas flow of 20L/min, a welding current of 200A, an arc voltage of 24V, a welding speed of 14.4cm/min, a welding rod angle of $50^{\circ}$, a welding gap of 5 mm with a ceramic base, and sand pre-heating to $160^{\circ}$ Celsius prior to welding.

• 동력기계공학회지
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• 제4권4호
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• pp.41-47
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• 2000

Fatigue crack propagation rate and threshold characteristics of the SA516/70 steel which is used for the low temperature pressure vessels, were studied in the room temperature of $25^{\circ}C$ and low temperature ranges of $-10^{\circ}C,\;-30^{\circ}C,\;-60^{\circ}C\;and\;-80^{\circ}C$ with stress ratio of R=0.1. In the logarithmic relationship between the fatigue crack propagation rate($d{\alpha}/dN$) and stress intensity factor range ${\Delta}K$, the linear relationship was obtained up to $d{\alpha}/dN=4.425{\times}10^4mm/cycle$ in the same of room temperature, but in low temperature case, the relationship was extended to the range of low crack propagation rate. The fractured specimens were examined by SEM. Tested results showed that specimen failed at low temperature exhibit the quasi-cleavage fracture formation however considerable ductility proceed final fracture.

• 대한용접접합학회:학술대회논문집
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• 대한용접접합학회 2001년도 춘계학술발표대회 개요집
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• pp.220-223
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• 2001

In this study, CT specimens were prepared hem ASTM SA516 which was used for pressure vessel plates for room and low temperature service. And we got the following characteristics from fatigue crack growth test carried out in the environment of room and low temperature at $25^{\circ}C$, -3$0^{\circ}C$, -6$0^{\circ}C$, -8$0^{\circ}C$, -l$0^{\circ}C$ and -l2$0^{\circ}C$ and in the range of stress ratio of 0.1, 0.3 by means of opening mode displacement. At the constant stress ratio, the threshold stress intensity factor range ΔK$_{th}$ in the early stage of fatigue crack growth ( Region I ) and stress intensity factor range ΔK in the stable of fatigue crack growth ( Region II) was increased in proportion to descend temperature. It assumed that the fatigue resistance characteristics and fracture strength at low temperature is considerable higher than that of room temperature in the early stage and stable of fatigue crack growth region. The straight line slope relation of logarithm da/dN - ΔK in Region II, that is, the fatigue crack growth exponent m increased with descending temperature at the constant stress ratio. It assumed that the fatigue crack growth rate da/dN is rapid in proportion to descend temperature in Region H and the cryogenic-brittleness greatly affect a material with decreasing temperature.e.greatly affect a material with decreasing temperature.

• 한국해양공학회:학술대회논문집
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• 한국해양공학회 2001년도 춘계학술대회 논문집
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• pp.326-331
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• 2001

In this study. CT specimens were prepared from ASME SA5l6 which was used for pressure vessel plates for room and low temperature service. And we got the following characteristics from fatigue crack growth test carried out in the environment of room and low temperature at $25^{\circ}C$, -3$0^{\circ}C$, -6$0^{\circ}C$, -8$0^{\circ}C$, -10$0^{\circ}C$ and -12$0^{\circ}C$ in the range of stress ratio of 0.1 by means of opening mode displacement. At the constant stress ratio, the threshold stress intensity factor range ΔKsub/th/ in the early stage of fatigue crack growth ( Region I) and stress intensity factor range ΔK in the stable of fatigue crack growth ( Region II) was increased in proportion to descend temperature. It assumed that the fatigue resistance characteristics and fracture strength at low temperature is considerable higher than that of room temperature in the early stage and stable of fatigue crack growth region. The straight line slope relation of logarithm da.dN -ΔK in RegionII, that is, the fatigue crack growth exponent m increased with descending temperature at the constant stress ratio. It assumed that the fatigue crack growth rate da/dN is rapid in proportion to descend temperature in Region II and the cryogenic-brittleness greatly affect a material with decreasing temperature.

• 한국정밀공학회지
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• 제14권6호
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• pp.83-89
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• 1997

In order to study on fatigue crack retardation and retardation mechanism in variable loading, the effects of crack tip branching in fatigue crack growth retardation were examined. The characteristics of crack tip banching behavior was considered to micro structure. It was examined that the variation of crack tip branching angle. Crack tip branching was observed along the grain boundary of ferrite and pearlite structure. It was found that the abanching angle ranges from 25 to 53 degrees. Using the finite element method, the variable of crack driving force to branching angle was examined. The effective crack driving force ( $K_{\eff}$ ) decreased as the braching angle increases. The rate of decrease was 33% for the kinked type and 29% for the forked one. It was confirmed that the effect of crack tip branching is a very important factor in fatigue crack growth retardation. Therefore, crack branching effect should be considered building the hypoth- etical model to predict crack growth retardation.

• 한국융합학회논문지
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• 제6권5호
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• pp.85-90
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• 2015

본 논문은 ATV차량의 전면에 부착되는 범퍼의 구조적인 형상에 따른 수명평가를 피로해석을 토대로 연구하였다. 본 연구의 목적은 진동과 같이 반복적인 하중조건에 노출되는 범퍼의 특성의 파악하며, 여러 가지 피로 상황에서 발생되는 크랙의 위치를 사전에 파악하며 이를 보완하기 위함이다. 다목적 차량이 일반적인 포장도로가 아닌 비포장도로와 같은 험로를 주행하게끔 설계됨을 고려한다. 그리고 이러한 주행환경에 대한 수명평가 연구를 통해, 그 범퍼의 취약부위를 파악한다. 본 연구의 해석결과를 토대로 자동차 범퍼의 안전설계를 함으로서 내구성을 향상하고 설계에 있어 융합기술에 접목하여 그 미적인 감각을 나타낼 수 있다.

• 한국해양공학회:학술대회논문집
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• 한국해양공학회 2002년도 추계학술대회 논문집
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• pp.341-346
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• 2002

In this study, an effect that compressive residual stress formed by shot-peening the surface of spring steel(JISG SUP-9) at each shot velocity(1800, 2200, 2600, 3000rpm) on the fatigue crack growth property and threshold stress intensity factor, ${\Delta}K_{th}$, was examined. Followings are the result (1) Compressive residual stress on surface of specimen was determined at each -601 MPa(1800rpm), -638 MPa(2200rpm), -587 MPa (2600rpm), -550 MPa(3000rpm) by shot velocity of shot peening and threshold stress intensity factor, ${\Delta}K_{th}$, fatigue crack growth rate, da/dN, on fatigue crack growth is obstructed by the compressive residual stress was determined at each $5.619\;MPa\sqrt{m}$(Un-peening), $8.319\;MPa\sqrt{m}$(1800rpm), $8.797\;MPa\sqrt{m}$(2200rpm), $7.835\;MPa\sqrt{m}$(2600rpm), $7.352\;MPa\sqrt{m}$(3000rpm) (2) Existing compressive residual stress by effect of shot velocity of shot-peening on relation of crack length. a, and number of cycle, N, was 2 times progressed in case of 2200rpm than specimen of Un-peening on fatigue life. And fatigue life was 1.6 times progressed incase of 3000rpm by Over peening. (3) Fatigue life of Material on Paris' law, $da/dN=C({\Delta}K)^m$, that effect of material constant, C, and fatigue crack growth exponent, m, was influenced by effect of. C and m.