• 한국해양공학회지
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• 제5권2호
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• pp.198-198
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• 1991

Not only difference of fatigue crack growth and propagation behavior resulted from the grain size, the hardness ratio and volume fraction in M.E.F. dual phase steel composed of martensite in hard phase and ferrite in soft phase, but also the effects of the plastic constraint were investigated by fracture mechanics and microstructural method. The main results obtained are as follows: 1) The fatigue endurance of M.E.F. steel increases with decreasing the grain size, increasing the ratio of hardness and volume fraction. 2) The initiation of slip and crack occures faster as the stress level goes higher. These phenomena result from the plastic constraint effect of the second phase. 3) The crack propagation rate in the constant stress level is faster as the grain size gets larger, the ratio of hardness lower and volume fraction smaller.

• 한국정밀공학회지
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• 제21권4호
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• pp.161-169
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• 2004

In this study, we investigated fatigue crack growth behavior of laser welded sheet metal due to a single overload. Fatigue specimens were made using butt joint of cold rolled sheet metal that was welded by $CO_2$ laser. The fatigue crack propagation tests were performed in such a way that fatigue loading was parallel to the weld line while crack propagation was perpendicular to the weld line. Single overload was applied when fatigue crack tip was arrived near the weld line. The distances between the crack tip and the weld line at which a single overload was applied were 6, 4 and 2mm. The effect of specimen thickness and overload ratio on the fatigue behavior was determined. The plastic zone size of crack tip due to the single overload was determined from the finite element analysis. For investigating fatigue crack growth behavior, we used different thickness specimen 0.9mm and 2.0mm, and variable overload ratio applied fatigue crack propagation test. Also we used finite element analysis for investigating the plastic zone size of crack tip when single overload applied

• Journal of Welding and Joining
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• 제18권5호
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• pp.33-40
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• 2000

Fatigue crack propagation life of weld toe crack through residual stress field was estimated with Elber's crack concept. Propagation of weld toe crack is heavily influenced by residual stress caused by welding process, so it is essential to take into account the effect of residual stress on the propagation life of weld toe crack. Fatigue crack at transverse and longitudinal weld toe was studied respectively, which represent typical weld joint in ship structure. Numerical and experimental studies are performed for both cases. Residual stress near weldment was estimated through nonlinear thermo-elasto-plastic finite element method, and residual stress intensity factor with Glinka's weight function method. Effective stress intensity factor was calculated with Newman-Forman-de Koning-Henriksen equation which is based on Dugdale strip yield model in estimating crack closure level U at different stress ratio. Calculated crack propagation life coincided well with experimental results.

• 대한조선학회지
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• 제12권2호
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• pp.43-58
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• 1975

There are many reports on fatigue crack of metallic materials but most of them relate crack propagation rate to stress intensity factor. The problem of crack propagation is not yet clarified, especially the bridge between micro and macro phenomena In this experiment rotating bending fatigue tests have been carried out with smoothed specimen of rolled steel plates including 0.2% carbon under application of three stress conditions to investigate the slip band and the crack propagation behaviour. The results obtained are as follows; 1) The length of cracks which have grown at initial crack tips can be expressed as follows; $l=Ae^{BNr}$(A,B: constant, $N_r$: cycle ratio) $\frac{dl}{dN}=\frac{AB}{N_f}{\cdot}e^{BNr}$($N_f$:fatigue life) 2) The ratio of slipped grain number to total grain number is $S_f=7{\sigma}-5.6$-5.6{\sigma}_c$($\sigma$: stress amplitude) (${\sigma}_c$: fatigue limit) 3) When the fatigue process transfers from Stage I to Stage II, the crack which propagates into specimen changes its direction from that of the maximum shear stress to the direction of perpendicular to principal stress and this is same in the circumferential direction of specimen. the crack propagation behaviors of both sides of a crack are different each other when they approach to the grain boundary.

• 한국해양공학회:학술대회논문집
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• 한국해양공학회 2000년도 추계학술대회 논문집
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• pp.240-245
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• 2000

Fatigue crack propagation rates and characteristics of the SA516-60 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,\; -10^{\circ}C,\; -30^{\circ}C,\; -50^{\circ}C, \;and\; -70^{\circ}C4 with stress ratio of R=0.05. The obtained experimental results are as follows; 1) In the logarithmic relationship between the fatigue crack propagation rate(da/dN) and stress intensity factor K, the linear relationship was obtained up to da/dN 〉$8\times10^{-3}$/mm/cycle in the same of room temperature, but in low temperature case, the relationship was extended to the range of crack propagation rate. 2) The lower limit stress intensity factor of SA516-60 $\DeltaK_{th}$ was 15.8MPa and in the case of low temperature $-50^{\circ}C\; and\; -70^{\circ}C$, the crack propagation rate da/dN which showed a linear relation, reached rapidly to the $\DeltaK_{th}$/. As the results, the crack propagation rates of $-50^{\circ}C\; and\; -70^{\circ}C$ were lower than that of room temperature and according to the testing temperature the rates were decreased rapidly to the $\DeltaK_{th}$/. 3) On the relationship between the stress intensity factor $\DeltaK$ and the crack propagation cycle, the stress intensity factors of low cycle region was rapidly increased at low temperature, but $\DeltaK$ was increased rapidly at room temperature of high cycle. 4) On the relationship between the fatigue crack propagation rate and cycle, the fatigue crack propagation rate showed higher gradient in the room temperature than the low temperature due to the increment in ductility at low temperature.

• 대한기계학회논문집
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• 제13권5호
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• pp.938-946
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• 1989

본 연구에서는 CT시험편을 이용하여 소규모 항복조건하에서 하한계(near threshold)영역과 안전균열성장(제II)영역의 균열진전거동을 파악하고 피로균열 진전속도와 음향방출거동, 파단면 특성을 각각 비교, 고찰하여 각 파라미터간의 상관 관계 및 유용성을 검토하고자 한다.

• 한국안전학회지
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• 제16권1호
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• pp.18-24
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• 2001

The fatigue crack growth behavior of the SA516/70 steel which is used for pressure vessels was examined experimentally at room temperature, $-60^{\circ}C$,$-80^{\circ}C$ and $-100^{\circ}C$ with stress ratio of R=0.05, 0.1 and 0.3. Fatigue crack propagation rate da/dN related with stress intensity factor range ${\Delta}K$ was influenced by stress ratio in stable of fatigue crack growth (Region II) with an increase in ${\Delta}K$. The resistance of fatigue crack growth at low temperature is higher compared with that at room temperature, which is attributed to the extent of plasticity-induced by compressive residual stress according to the cyclic loads. Fractographic examinations reveal that the differences of the fatigue crack growth characteristics between room and low temperatures are mainly explained by the crack closure and the strengthening due to the plasticity induced and roughness induced.

• 한국마린엔지니어링학회:학술대회논문집
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• 한국마린엔지니어링학회 2001년도 추계학술대회 논문집(Proceeding of the KOSME 2001 Autumn Annual Meeting)
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• pp.80-87
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• 2001

The fatigue crack growth behavior of the SA516/60 steel which is used for pressure vessels was examined experimentally at room temperature $25^{\circ}C, -30^{\circ}C, -60^{\circ}C, -80^{\circ}C, -100^{\circ}C$ and -l2$0^{\circ}C$ with stress ratio of R=0.05, 0.1 and 0.3. Fatigue crack propagation rate da/dN related with stress intensity factor range ΔK was influenced by stress ratio in stable of fatigue crack growth (Region II) with an increase in ΔK. The resistance of fatigue crack growth at low temperature is higher compared with that at room temperature, which is attributed to the extent of plasticity-induced by compressive residual stress according to the cyclic loads. Fractographic examinations reveal that the differences of the fatigue crack growth characteristics between room and low temperatures are mainly explained by the crack closure and the strengthening due to the plasticity induced and roughness induced.

• 한국공작기계학회:학술대회논문집
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• 한국공작기계학회 2002년도 춘계학술대회 논문집
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• pp.53-58
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• 2002

This paper aims to synthesize the research on fatigue mechanisms of high strength aluminum alloys which are widely used in motorcars or airplanes to prevent accidents. To measure the data of crack opening ratio, the same materials and methods are used for evaluating the fatigue crack propagation rate as an effective stress intensity factor. But, many researchers have brought different results. An exact crack opening ratio was, therefore, proposed for getting a more accurate fatigue crack propagation rate.

• 동력기계공학회지
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• 제3권2호
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• pp.51-56
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• 1999

Fatigue crack propagation rates and characteristics of the SA516-70 steel which is used for the low temperature pressure vessels, were studied in the room temperature of $10^{\circ}C$ and low temperature ranges of $-10^{\circ}C,\;-30^{\circ}C,\;-50^{\circ}C,\;and\;-70^{\circ}C$ with stress ratio of R=0.05. The obtained experimental results are as follows; 1) In the logarithmic relationship between the fatigue crack propagation rate(da/dN) and stress intensity factor K, the linear relationship was obtained up to da/dN > $8{\times}10^3$ mm/cycle in the same of room temperature, but in low temperature case, the relationship was extended to the range of low crack propagation rate. 2) The lower limit stress intensity factor of SA516-70 ${\Delta}K_{th}\;was\;23MPa\sqrt{m}$ and in the case of low temperature $-50^{\circ}C\;and\;-70^{\circ}C$, the crack propagation rate da/dN which showed a linear relation, reached rapidly to the ${\Delta}K_{th}$. As the results, the crack propagation rates of $-50^{\circ}C\;and\;-70^{\circ}C$ were lower than that of room temperature and according to the testing temperature the rates were decreased rapidly to the ${\Delta}K_{th}$. 3) On the relationship between the stress intensity factor ${\Delta}K$ and the track propagation cycle, the stress intensity factors of low cycle region was rapidly increased at low temperature, but ${\Delta}K$ was increased rapidly at room temperature of high cycle. 4) On the relationship between the fatigue crack propagation rate and cycle, the fatigue crack propagation rate showed higher gradient in the room temperature than the low temperature due to the increment in ductility at low temperature.