• 한국안전학회지
• /
• 제9권3호
• /
• pp.13-21
• /
• 1994

Effects of tensile and compression residual stresses in the welded SS41 and A17075-76 on fatigue crack propagation behavior are investigated when a crack propagates from residual stresses region. We propose the fatigue crack growth equation on tensile and compression residual stresses in welded metal. The results obtained in this experimental study are summarized as follows . 1 ) A fatigue crack growth equation which applied fatigue fracture behavior of the welded metal is proposed. (equation omitted) where, $\alpha$, $\beta$, ${\gamma}$ and $\delta$ are constants, and R$_{eff}$ is effective stress ratio [R$_{eff}$=(Kmin+Kres)/(Kmax+Kres)], Kcf is critical fatigue stress intensity factor. The constants are obtained from nonlinear least square method. The relation between crack length and number of cycles obtained by integrating the fatigue crack growth rate equation is in agreement with the experimental data. 2) The experimental results confirmed that the cause of crack extension and retardation by residual stresses has relation to the phenomenon of crack closure. 3) The relaxing trend of residual stresses by the crack propagation was greater In case of compressive residual stress than that of tensile residual stress in the welded metal.tal.

• 한국강구조학회 논문집
• /
• 제22권6호
• /
• pp.589-597
• /
• 2010

강구조물의 점검시 피로균열이 발견되는 경우에는 보수 보강 시행의 결정과 구조물의 점검주기 결정을 위한 균열의 현재 상태평가와 앞으로의 균열진행 정도에 대한 정량적인 예측이 요구된다. 피로균열에 대한 손상도 평가는 파괴역학 파라미터를 이용한 균열진전속도와 취성파단의 여부 결정이 대표적인 방법으로 파괴역학에 관한 전문적인 지식과 함께 수치해석 프로그램의 활용능력이 필요하다. 이에, 본 연구에서는 피로손상도 평가 방법에 관한 다양성을 확보하고, 파괴역학에 대한 전문적인 지식이 없는 기술자가 피로균열에 의한 강구조물의 손상도를 간편하게 평가 할 수 있는 방법을 개발하기 위한 첫 단계로, 관통균열 시험편을 이용한 균열진전시험을 실시하였다. 그리고 피로균열 표면으로부터 측정된 COD(Crack Opening Displacement)의 크기와 측정위치를 이용하여, 균열진전속도를 추정하기 위한 파라미터를 도출하고, 그 타당성을 분석하였다. 그리고 관통균열이 있는 시험편의 형상, 응력범위, 그리고 응력비에 상관없이 COD를 이용하여 피로균열진전속도를 추정할 수 있는 방법을 제시하였다.

• 대한기계학회논문집
• /
• 제19권2호
• /
• pp.402-407
• /
• 1995

The effects of room temperature and strain aging treatment are discussed on the critical condition for the onset of growth of non-propagating cracks on 0.15% C low carbon steel, with special emphasis on the length of the critical non-propagating crack and on the crack opening displacement(COD) at the crack tip. It is found from the experimental analysis that room temperature and strain aging of a fatigue pre-cracked specimen introduced the closure of a crack tip of the pre-crack and the reduction of crack opening displacement at the wake of crack, together with an improvement in crack growth resistance of the microstructure. This may cause an increase in the endurance limit of the specimen, through the enhancement of effective stress for the onset of growth of the critical non-propagating crack.

• 한국구조물진단유지관리공학회 논문집
• /
• 제15권2호
• /
• pp.179-188
• /
• 2011

강부재의 피로가 주요원인이 되어 구조물이 붕괴되거나 교체되는 사례를 찾아보기 어렵다. 이처럼 사실상 피로에 대해서는 손상을 허용하고 있지만, 발견되는 피로균열에 대한 직접적인 상태평가는 이루어지지 않고 있다. 피로균열에 대한 진전 비진전성의 판단 및 균열진전속도의 평가가 이루어져야만, 합리적인 보수 보강 공법의 선정과 시행시기가 결정될 수 있을 것이다. 본 연구에서는 측정되는 COD(Crack Opening Displacement)를 통한 피로균열진전속도 평가법을 검토하기 위하여, 2종류의 관통 균열 시험편과 면외거셋 용접이음 시험편의 균열진전시험을 실시하였다. 그리고 실용적인 COD의 측정을 위해 변형률 게이지를 이용하는 방법에 대해 검토하였다. 그 결과, COD 측정을 통한 균열진전속도의 합리적인 평가법을 제안하였고, 변형률 게이지를 이용한 성공적인 COD 측정을 실험적으로 증명하였다.

• 대한기계학회논문집A
• /
• 제23권6호
• /
• pp.1039-1047
• /
• 1999

It is necessary to evaluate the fatigue behavior of rail steel under the multi-axial stress state to assure the railway vehicle's safety. For this purpose, the stress analysis to investigate the crack initiation criteria, static failure and fatigue behavior under mixed-mode are performed. The stress analysis results show that the initiation of the transverse fissure depends on the maximum shear stress below the surface. For the mixed mode, the fatigue crack growth behavior which is represented by the projection crack length and comparative S.I.F, ${\Delta}K_v$, shows the more conservative results. Also, its rate is lower than that of the case of the mode I, and this difference decreases with increasing the stress ratio, R.

• International Journal of Naval Architecture and Ocean Engineering
• /
• 제5권4호
• /
• pp.580-597
• /
• 2013

Three materials SUS304, 9% Ni steel and Al 5083-O alloy, which are considered possible candidate for International Maritime Organization (IMO) type B Cargo Containment System, were studied. Monotonic tensile, fatigue, fatigue crack growth rate and Crack Tip Opening Displacement tests were carried out at room, intermediate low ($-100^{\circ}C$) and cryogenic ($-163^{\circ}C$) temperatures. The initial yield and tensile strengths of all materials tended to increase with decreasing temperature, whereas the change in elastic modulus was not as remarkable. The largest and smallest improvement ratio of the initial yield strengths due to a temperature reduction were observed in the SUS304 and Al 5083-O alloy, respectively. The fatigue strengths of the three materials increased with decreasing temperature. The largest increase in fatigue strength was observed in the Al 5083-O alloy, whereas the 9% Ni steel sample showed the smallest increase. In the fatigue crack growth rate test, SUS304 and Al 5083-O alloy showed a decrease in the crack propagation rate, due to decrease in temperature, but no visible improvement in da/dN was observed in the case of 9% Ni steel. In the Crack Tip Opening Displacement (CTOD) test, CTOD values were converted to critical crack length for the comparison with different thickness specimens. The critical crack length tended to decrease in the case of SUS304 and increase for the Al 5083-O alloy with decreasing temperature. In case of 9% Ni steel, change of critical crack length was not observed due to temperature decrease. In addition, the changing material properties according to the temperature of the LNG tank were analyzed according to the international code for the construction and equipment of ships carrying liquefied gases in bulk (IGC code) and the rules of classifications.

• 한국자동차공학회논문집
• /
• 제4권4호
• /
• pp.130-139
• /
• 1996

The silicon carbide particles reinforced aluminium 6061($SiCp/A\ell$) composites are generally known have wild range of applications from automobiles to airospaces. But, by the results of existing study for $SiCp/A\ell$ composites, there are reports that the fatigue life of $SiCp/A\ell$ composites has improved than $A\ell$matrixes and has not improved then $A\ell$ matrixes. Consequently, in order to perform the reliable life prediction for $SiCp/A\ell$, the properties of probability distribution of fatigue crack initiation life & fracture life, crack growth length in constant number of cycles, crack growth rate in constant stress intensity factor range and m & C value in Paris's fatigue crack growth law and the estimation of statistical parameters have been evaluated by the statistics method.

• 한국정밀공학회지
• /
• 제8권2호
• /
• pp.27-35
• /
• 1991

We propose the crack growth rate equation which applied over three regions (threshold region, stable region, unstable region) of fatigue crack propagation. Constant stress amplitude fatigue tests are conducted for four materials under three stress ratios of R=0.05, R=0.2 and R=0.4. Materials which have different mechanical properties i.e. stainless steel, low carbon steel, medium carbon steel and aluminum alloy are used. The fatigue crack growth rate equation is given by $da/dN={\beta} (1-R)^{\delta}\({\DELTA}K-{\DELTA}K_t)^{\alpha} / (K_{cf}-K_{max})$${\alpha}, {\beta}$ , and ${\delta}$ are constants, and ${\Delta}K_t$ is stress intensity factor range at low ${\Delta}K$ region. The constants are obtained from nonlinear least square method. $K_{ef}$is critical fatigue stress intensity factor. The relation between half crack length and number of cycles obtained by integrating the crack growth rate equation is in agreement with the experimental data. It is also experimented with constant maximum stress and decreasing stress ratios, and the fatigue growth rate of each material is in accord with the proposed equation.

• 한국해양공학회지
• /
• 제8권2호
• /
• pp.115-123
• /
• 1994

Under variable amplitude loading conditions, retardation or accelerated condition of fatigue crack growth occurs with every cycle, Because fatigue crack growth behavior varied depend on load time history. The modeling of stress amplitude with storm loading acted to ships and offshore structures applied this paper. The crack closure behavior examine by recording the variation in load-strain relationship. By taking process mentioned above, fatigue crack growth rate, crack length, stress intensity factor, and crack closure stress intensity factor were obtained from the stress cycles of each type of storm ; A(6m), B(7m), C(8m), D(9m), E(11m) and F(15m) which was wave height. It showed that the good agreement with between the experiment results and simulation of storm loads. So this estimated method of crack propagtion rate gives a good criterion for the safe design of vessels and marine structure.

• 대한조선학회지
• /
• 제23권3호
• /
• pp.17-26
• /
• 1986

A new relationship between total acoustic emission counts per cycle and crack growth rate is derived in terms of stress intensity factor to explain the acoustic emission behavior due to fatigue crack growth. The experimental results of the total acoustic emission counts per cycle obtained from aluminium 5,000 series alloy specimens are compared with the theoretical values. Both experimental and theoretical total acoustic emission counts per cycle result in the linear relationship to the crack length.