Search

In this paper, fatigue tests are conducted for the specimens with longitudinal and transverse attachment under variable amplitude axial loading based on storm model. Considered loadings include repeated single storm, 6 or 8 storms randomly, and storms including calm sea condition while the mean stress and the maximum stress of loadings are changed. The effect of three variables are investigated; root mean square(RMS) value of stress amplitude, mean stress shift and maximum stress, which can characterize storm loading on fatigue life. In addition, experiments including calm sea loading are also carried out to investigate the effect of calm sea state. Test results are evaluated and compared with DNV-CN2005 and Matsuoka`s method for the estimation of crack initiation and propagation life. To verify the validity of the criteria, the measured crack initiation lifes are compared with the specific crack length 15mm, which are calculated with beach marks.

This paper deals with the crack propagation life assessment of T-joint welded structure where typical fatigue cracks have been frequently initiated when the marine vessels experience the storm load. Welding residual stresses are calculated to investigate its effects on the fatigue life. Thereafter the residual stress distribution was applied to the AFGROW life prediction program, which incorporated the loading, the welding residual stress, and the geometric shape of the structure. The fatigue tests of the T-joint welded specimen under storm loading show the beach mark clearly generated on the fractured section of the weldment. The crack propagation life estimated based on the beach mark is compared with that of AFGROW to validate the life prediction. Based on the results, the evaluation method of the remaining fatigue life for T-joint fillet weldment of marine vessel`s cargo hold with random load or storm load was established.

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 degrees C) and cryogenic (463 degrees 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.

IMO (international maritime organization) type B CCS (cargo containment system) is considered for LNG (liquefied natural gas) FPSOs. For the three materials of SUS304, Al 5083-O alloy and 9% Ni steel regarded as possible candidates for IMO type B CCS, extensive tensile, fatigue, fatigue crack growth rate and CTOD (crack tip opening displacement) tests were carried out at room, intermediate low (−100°C) and cryogenic (−163°C) temperatures and described in this paper. All materials show a tendency for initial yield and tensile strengths to increase with decrease in the temperature, while change in elastic moduli is not as remarkable. The largest and smallest increase ratios of the initial yield strengths due to temperature reduction are seen in SUS304 and Al 5083-O alloy, respectively. In fatigue tests, fatigue strengths increase due to the temperature decrease for three materials. The highest increase of fatigue strength due to the temperature decrease is observed in Al 5083-O alloy, while 9% Ni steel shows lowest increase. In fatigue crack growth rate test, SUS304 and Al 5083-O alloy show reduction in crack propagation rate, due to the reduction in temperature, while any visible improvement of da/dN is not witnessed in case of 9% Ni steel. In CTOD test, CTODs tend to decrease in case of SUS304 and CTODs for Al 5083-O alloy increase with reduction in temperature.Copyright © 2011 by ASME