Tatsuro Nakai

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.

Effect of Corrosion and Inclined Angle on Static Strength of Fillet-Welded Joint of 500 MPa-Class Steel

There are reported cases of serious accidents in ship hull structures of aged ships due to corrosion of steel after long period of use. This paper presents basic information on the effect of corrosion on static strength, that is, tensile strength, bending strength and shear strength, of fillet-welded joints of 500MPa-class steel used for ship hull construction. It is based on experimental results using specimens made of material of corroded fillet-welded joints of actual old ships and as-fabricated fillet-welded joint and simulated corroded fillet-welded joint. And also the effect of inclined angles on perpendicular direction shear strength of fillet-welded joint is investigated. Basically, static strength of the fillet-welded joint is affected by the (R) ratio of thickness of steel plates to total thickness of throat of the weld metal. When R was greater than 1.0, fracture occurred at weld metal under tensile stress condition or shear stress condition. However, under bending stress condition fracture at weld metal occurred when R was greater than 1.4. The effect of inclined angles on shear strength of fillet-welded joints is also presented.Copyright