Hisao Matsushita

Visual Assessment of Corroded Condition of Plates With Pitting Corrosion Taking Into Account Residual Strength: In the Case of Webs of Hold Frames of Bulk Carriers

Corrosion pits with a conical shape are typically observed in hold frames in way of cargo holds of bulk carriers which carry coal and iron ore. The ratio of the diameter to the depth of the typical corrosion pits is in the range between 8–1 and 10–1 and its diameter might become up to 50mm. The evaluation of residual strength of members with large uneven pitting corrosion is difficult compared with that of members with general corrosion. Therefore, it is of crucial importance to develop a method for the evaluation of residual strength of pitted members. The purpose of the present study is to investigate the effect of pitting corrosion on the ultimate strength of steel plates under various loading conditions and explore a method for the evaluation of residual thickness of pitted plates. In the present study, a series of non-linear FE-analyses has been conducted with steel plates with a variety of random pit distributions under various loading conditions such as uni-axial compression, bi-axial compression, shear and combination of these. In these analyses, random pit distributions were calculated by the previously developed corrosion model. It has been shown that equivalent thickness loss, which is defined as thickness loss of uniformly corroded plates with the same ultimate strength as the randomly pitted plates, is smaller than or equal to 1.25 times the average thickness loss. It has been also revealed that the equivalent thickness loss for the ultimate strength under the above-mentioned loading conditions is smaller than average thickness loss at the minimum cross section, where the average thickness loss at the minimum cross section almost corresponds to the equivalent thickness loss for the tensile strength. Based on these findings, a method for the estimation of equivalent thickness loss of pitted plates has been discussed using the thickness diminution-DOP relationship, where DOP (Degree of Pitting Intensity) is defined as the ratio of the pitted surface area to the total surface area.Copyright

Effect of Pitting Corrosion on Ultimate Strength of Web Plates Subjected to Shear Loading

Pitting corrosion is a great concern when the integrity of ships hull structures is considered. Corrosion pits with a conical shape are typically observed on coated hold frames in way of cargo holds of bulk carriers which exclusively carry coal and iron ore. Therefore, it is important to investigate the effect of pitting corrosion on local strength of hold frames of bulk carriers. In the present study, a series of 3-point bend tests with structural models which consist of web, shell and face plates has been conducted. In these tests, a concentrated load has been vertically applied at the center of simply supported models so that shear load would act on the web plate with artificial pits. In this testing condition, load increased even after shear buckling occurred. When there is artificial pitting, fracture of web plates occurs due to strain concentration at the pits. It has been revealed that the empirical formula, which was developed based on non-linear FE-analyses, overestimates the equivalent thickness for the shear ultimate strength obtained in the present experiment, because the effect of web fracture is not taken into account in the analyses.

Current Wall Thinning Measured on Piping System of Main or Auxiliary Boiler Plant in Ships

A disaster happened in a nuclear power plant in Japan in August 2004, which was caused by failure of condensation water pipe in the secondary line. Shipping industries were concerned for possibility of occurrence of such a disaster in ships due to its construction similarity to marine boiler plant in steam, feed water and condensation piping for main or auxiliary boilers. Nippon Kaiji Kyokai has therefore investigated and gathered data of piping lines corrosion in ships collaborated with major Japanese ship owners right after the disaster. The results show that similar corrosion failure as in the nuclear power plant has occurred in shipboard steam/feed water/condensation water pipes for main and auxiliary boiler plants without causing severe consequences. The wall thickness measurements on actual pipe lines of steam, feed water and condensation water at bend parts, at T-junction, behind orifices, behind valves and at diffusers/reducers with a ultrasonic thickness gauge show a very definite evidence of a reduction in wall thickness of carbone steel pipes. It was confirmed that the amount of actual reduction in wall thickness could be well predicted by Kastner Equation [2–3].© 2008 ASME

Estimation of Fatigue Crack Propagation of Subsurface Cracks by “SCAN”

The authors have proposed an influence function method to calculate stress intensity factor, K, of the surface cracks. This method makes the calculating task easier for arbitrarily distributed surface stresses. They have developed the database of influence coefficients, Kij , for various types of surface cracks through a series of finite element analyses.[1] They also have developed a software system “SCAN” (S urface C rack An alysis), from the database. The K values of surface cracks can be evaluated immediately, and further, fatigue crack propagation can be simulated easily with a personal computer. A fatigue crack often initiates from a defect located at the subsurface of a structural member. In this case, it is important to account for the fatigue life from the initiation of a subsurface crack to its propagation into a surface crack. However, since it is difficult to simulate this process precisely, the authors have proposed a simple model about the transition from a subsurface crack into a surface crack based upon ASME CODE SECTION XI [2] and WES 2805 STANDARD. [3] They have developed a SCAN system – Subsurface Crack Version-. They calculated the fatigue life for some models of subsurface cracks and compared the quantitative differences between two standards.Copyright

On the interaction of structure with ice sheet in medium scale field indentationtests

The consiruction of structure in ice in fested arca requires the proper evaluation ofdesign ice load on structure.For that, it is required to clarify that the scale effect, that indentation ice pressure decreases as an area of structure with ice sheetincreascs, is caused by the nonsimulaneo us failure of ice sheet. Medium scale fieldindenlation test (MSFIT) in JOIA project includes ice in dentation tests, tests onphysical pmperties of ice sheet and image sensing on ice deformaion. In thispaper, the whole plan of MSFIT and the resuhs of ice indemation tests conducted in 1996 winter are reported.