Ship Structural Steel Research Articles

Investigation of strain localization on marine structural steels: Uniaxial tests

Results of a series of uniaxial tests to investigate fracture initiation states of AH32 steel, a ship structural steel, are presented. Strains at fracture are obtained using digital image correlation analysis via DaVis LaVision software, and the corresponding states of stress are obtained via a plastic flow rule and a hardening relationship using the measured strains. The strain fields are calculated to the point of fracture initiation where the fracture initiation is defined as the condition when the first visible crack appears in the digital image of the test specimen. The software provides displacement and strain fields of the entire region and any specified sub-region of the imaged surface, which uses a so-called ‘geometric mask’. The strain values are mask-size dependent due to the local nature of deformation before fracture. Therefore, in the current study, strain fields are obtained for various mask sizes to investigate the effect of mask size on failure strain, i.e., strain localization phenomena, with the minimum mask size chosen at roughly the limit of the continuum scale (O (100 μm)). The evolution of the stress and strain fields as a function of loading is provided for different mask sizes. Moreover, comparison of the failure strain as mask sizes are decreased provides information on strain convergence. In this investigation, non-convergence behaviors are obtained for all experiments, as failure strain increased as the mask sizes are decreased.

Laser welding process evaluation on stake-welded T-joints

Ship structural steel EH36 sheets with thickness of 3 mm were stake-welded with T-joints by using fiber laser beam welding (LBW). Optical microscope (OM) observation, angular distortion measurement and tensile test were conducted to investigate the influence of defocusing distance, welding speed and line energy on the joint properties. The experiment results showed that laser stake-welded T-joints with high quality could be obtained at 1.8 m min−1 speed high, −5 mm defocusing distance and 5 kW power. The loss of liquid metal in the misalignment condition led to severe underfill. In addition, laser line energy and misalignment contributed significantly to welding angular distortion. The welding width between face panel and web panel exerted an important influence on the tensile properties. Moreover, the thermal distortion process induced by fiber laser welding has been schematically illustrated and systematically revealed.

Residual stress redistribution during elastic shake down in welded plates

Residual stresses are a consequence of welding in various structures such as ships and offshore structures. Residual stresses can be relaxed or redistributed according to the load levels during operation. The elastic shakedown phenomenon can be considered as one of the reasons for this change. This paper studies the relaxation/redistribution of weld residual stress during different levels of shakedown in a butt-welded plate chosen according to ship design and welding procedures. Welding was performed on DH36, a ship structural steel. Neutron diffraction was used to measure residual stresses in these plates in the as-welded state and after different levels of shakedown. A mixed hardening model in line with the Chaboche model is determined for both weld and base material. A numerical model is developed to estimate the shakedown limit on butt-welded plate. Further, the redistribution of residual stress in a numerical weld model according to the different levels of shakedown limit is studied. Based on the shakedown limit of the butt-welded plate, a shakedown region is determined, where the structure will undergo elastic shakedown in the presence of an existing residual stress field if the maximum stress on the load section after a few initial cycles is in the shakedown region.

Residual Stresses in Ultrasonically Peened Fillet Welded Joints

Fatigue cracks mostly initiate at areas subjected to high tensile residual stress and stress concentration. Ultrasonic peening is a mechanical method to increase fatigue life by imparting compressive residual stress. In this study residual stresses are characterized in fillet welded ship structural steel plates with longitudinal attachments. As-welded, ultrasonically peened, and specimens peened then subjected to accelerated corrosion testing were measured. Residual stress characterization was performed by the contour method and neutron diffraction.

A study on the fatigue strength characteristics of ship structural steel with gusset welds

This study aims to assess fatigue property by the static overload and average load in the fillet welded joints which is on the ship structural steel having gusset welds. To this end, a small specimen was made, to which the same welding condition for the actual ship structure was applied, to perform fatigue tests. In this study, a method to simply assess changes in welding residual stress according to different static overload was suggested. By measuring actual strain at the weld toe, the weld stress concentration factor and property which is determined by recrystallization in the process of welding were estimated to investigate the relation between overload and fatigue strength.

A study on the fatigue strength characteristics of ship structural steel with gusset welds

This study aims to assess fatigue property by the static overload and average load in the fillet welded joints which is on the ship structural steel having gusset welds. To this end, a small specimen was made, to which the same welding condition for the actual ship structure was applied, to perform fatigue tests. In this study, a method to simply assess changes in welding residual stress according to different static overload was suggested. By measuring actual strain at the weld toe, the weld stress concentration factor and property which is determined by recrystallization in the process of welding were estimated to investigate the relation between overload and fatigue strength.

造船用鋼材の繰返し軟化特性とき裂伝播速度の関係に関する一考察

造船用鋼材の繰返し軟化特性とき裂伝播速度の関係に関する一考察

Hydrogen Effect on Fatigue Strength of Structural Steels

In this paper hydrogen effect on fatigue strength of structural steels is mainly reviewed on the basis of the author's experimental results. The emphasis is focused upon the hydrogen related corrosion fatigue crack propagation behavior of the ship structural steels and their welded joints in sour crude oil environment. It can be concluded that the environment enhancement of the fatigue crack propagation rate in sour crude oil is dependent of hydrogen evolved by reaction of H2S and H2O in the sour crude oil with structural steels.

Fuzzy Model-based Charpy Impact Toughness Assessment for Ship Steels

The present research aims at linking steel composition and microstructure to a prediction of Charpy impact properties of ship structural steel, including Charpy impact energy, transition temperature (ITT) and fracture appearance (brittleness). In this paper, adaptive fuzzy modelling techniques were applied to develop generic models for the prediction of Charpy impact properties of Grade A and AH36 ship steels. Using the proposed fuzzy modelling approach, the rule-based fuzzy prediction models, which involved chemical composition, grain size, tensile strength and Charpy impact energy, were generated and optimised automatically from numerical data. The investigation of the relationship between Charpy impact energy and fracture surface characteristics is also presented in the paper. Numerical analysis shows that using the obtained fuzzy models we are able to predict Charpy toughness for given steel compositions and microstructures, and also reveal useful qualitative information linking composition-microstructure to Charpy impact properties.

The Microbiologically Influenced Corrosion Behavior of C-Mn Ship Structural Steel with Different Manufacturing Processes

The MIC (Microbiologically Influenced Corrosion) of lower deck steel plates in double hull VLCC (Very Large Crude Oil Carrier) cargo tanks has been focused because of severe localized corrosion. Recently, ship companies have reported that TMCP (Thermo-mechanical Control Process) steel plates have been showed more severe localized corrosion than conventional rolled steels. In order to elucidate the MIC resistance of TMCP steels by comparison with conventional rolled steels and normalized steels in environments of double hull VLCC cargo oil tanks, various measurements and corrosion tests were performed such as measurements of polarization curves, immersion tests in bacteria culture medium.All results revealed that three types of steels have almost the same corrosion resistance in bacteria culture medium. Three kinds of steel exhibit almost the same polarization behavior and the corrosion rate. The movement of the open-circuit potential first towards more negative values and later to more positive values is a phenomenon common to all kinds of specimens. This phenomenon resulted from an initial stimulation of the anodes by sulphide ions produced by the bacteria from the reduction of sulphate ions in the medium, followed by the formation of an insoluble partly protective film of ferrous sulphide on the surface of the electrode. The effect of SRB (Sulphate Reducing Bacteria) is clearly showed in the cathodic polarization curve. When the SRB is in a condition of rapid growth, there is a strong cathodic depolarization. However, as the sulphate is depleted and reaction products accumulate, the activity of the bacteria declines and the cathodic polarization curve returns the same form as in the inoculated culture.

Corrosion and Corrosion Fatigue of Ship Structural Steel Plates in Vapor Space of Crude Oil Tankers

Corrosion wastage has important roles for an evaluation of strength of aged hulls. Corrosion fatigue strength is also indispensable factor in health of hull strength with aged upper deck plates. From those backgrounds, corrosion and corrosion fatigue tests were performed under an environment substituted a vapor space of crude oil tankers. The vapor space is filled up with H2S gas comes from crude oils and an inert gas for preventing an explosion of cargo oil tanks (COT).A corrosion rate, 0.2mm/y, was estimated from a continuous corrosion test in the vapor space. The rate was twice as fast as measurement data of ships. The small corrosion diminution of the upper deck plate is caused by cycles of full load and ballast load conditions.S-N curves, Seq-Nc curves, ΔK-Nc curves and crack propagation limits were compared with the data of corrosion environments in seawater and dilute sulfuric acids. Crack initiation lives, Nc, were nearly equal to the lives in dilute sulfuric acids of pH4 and pH2 in a short life range. In a long life range, the lives of Kt=1.2 decreased to the lives in dilute sulfuric acids of pH4 and pH2, and the lives of Kt=>2.0 were nearly equal to the lives in seawater. An aspect ratio, 0.6, was obtained from corrosion pits to propagate as fatigue crack. The stress intensity factor range, ΔKCF=4.70 MPa•m1/2, was the lowest corrosion fatigue strength and the boundary level whether or not the corrosion pits grow into a fatigue crack or progress as corrosion wastage.

Improvement Of Corrosion Fatigue Strength Of Materials By VariousSurface Treatments.

In this paper it is summarized on improvement of corrosion fatigue strength of materials by various kind of surface trcatrnents mainlv based upon the author's experimental data. It is described on the effect of synthetic resins. thermalsprayed coatings and ion plating on corrosion fatigue strength of structural materials. The emphasis is focussed upon the tar epoxy resin coating for ship structural steels and thermal-sprayed coatings for machne structural steels. The mechanism of the improvement of corrosion fatigue strength of materials by surface coatings are also described. It c'm be concluded that the improvement of corrosion fatigue strength is not effective when an interception effect disappear due to the breakage or degradation of the tar epoxy resin coating. It can also be concluded that the effective thermal-sprayed coating to improve corrosion fatigue strength should be of very low porosity and finnly adhesive to the base metal.

Welding of ship structural steel A36 using a Nd:YAG laser and gas–metal arc welding

Laser welding has the potential of offering both technical and economical advantages in many applications in the shipbuilding industry. A limiting factor is currently the power level available with commercial lasers, since steel plates of more than 5 mm in thickness are used in almost every shipbuilding application. In addition, the high hardness of the welds produced using laser welding is a disadvantage compared with the requirements of existing classification society standards. It has been reported that by using a hybrid welding method in which a laser beam and a gas–metal arc weld (GMAW) arc are combined it is possible to weld thicker sections, because the penetration is increased. Hardness values are correspondingly lower than those using the laser process because of the increased energy input. Results of a study of hybrid high power Nd:YAG laser and GMA welding are reported. All plates were welded in a butt joint configuration. When laser and GMAW were combined into a single process, I grooves were used. The air gaps between the welded plates varied. Some tests were made using a partially grooved V joint. In these cases, the root faces were welded by a Nd:YAG laser, and the upper part of the joint was welded by GMAW. It was seen that it is possible to produce defect-free welds using these processes. Macrosections of the welds, hardness values and parameters are presented and discussed.

Corrosion Fatigue Crack Initiation Life of a Ship Structural Steel in Dilute Sulfuric Acids

This paper describes corrosion fatigue for notched specimens in dilute sulfuric acids. The acids are typical corrosion environments of condensate from coal vapor in a coal cargo hold. Fatigue data were consolidated as the relationships between the equivalent stress at a notch root and a fatigue crack initiation life. Life in dilute sulfuric acids was shorter than that in seawater because of the severe dissolution of iron. Long term corrosion fatigue was also estimated from the relationship between the stress intensity factor range to the fatigue crack initiation and the life. Corrosion wastage was considered as an estimate of the fatigue crack initiation life.

Corrosion and Corrosion Fatigue of a Ship Structural Steel in Dilute Sulfuric Acids Substituted for a Corrosion Environment of a Coal Cargo Hold

An acid corrosive environment of a coal cargo hold was reproduced experimentally by using immersions of coal. Immersion tests, crack growth tests, and corrosion fatigue tests were performed under dilute sulfuric acids on a ship structural steel plate.A pH decreased with passage of time in an exudation solution from coal, and the end point was pH 2. 5. Corrosion rates were obtained in exudation solution and in dilute sulfuric acids of pH 4, pH 3 and pH 2. It could be confirmed that the corrosion rate in a dilute sulfuric acid could be substituted for that in the exudation solution from coal.The conditions of the fatigue crack initiation, ΔKCF, were obtained from the observations of corrosion pits by a scanning electron microscope. The ΔKCF was 3.60 and 5.39 MPa m1/2 in H2SO4 at pH 4 and at pH 2, respectively. The crack growth rates estimated under corrosive dissolutions were 5.63 × 10-10 and 1.90 × 10 -9m/cycle for ΔKCF in above environments. Concerning corrosion fatigue, S/ N curves, Seq/Nc curves on modified MIL HDBK 5 and ΔK/Nc curves were discussed in comparison with corrosion fatigue in seawater.

Corrosion fatigue strength of ship structural steel butt welded joints in synthetic sea-water

Summary This paper describes an investigation into the corrosion fatigue strength of a MAG-welded 50 MPa class TMCP steel in fatigue tests carried out under a constant-amplitude load at a stress ratio of 0.1 in air and synthetic sea-water. The tests took place under natural corrosion conditions at 25°C at a cycling rate in air and sea water of 3–12 and 0.17 Hz respectively. The fatigue corrosion initiation life Nc was evaluated from the crack length measured by the DC electric potential difference method. This paper describes the S/N relationships, the macroscopic stress concentration factors and the corrosion initiation behaviour at the toes of butt welded joints. The results obtained may be summarised as follows. 1. The ΔSn/Nc relationship between the nominal stress range and the corrosion initiation life as determined in the tests made in synthetic sea-water is represented by a straight line. In the high applied stress range, the corrosion initiation life as determined in the tests made in air is virtually equal to that determined in the tests made in synthetic sea water. The ΔSn/Nc relationship found in the tests made in synthetic sea-water can be applied as the corrosion initiation life for purposes of ensuring the greatest margin of safety in tests made in air. 2. Even when a weld toe crack is present in the specimens tested in air, there is an applied stress range leading to no fatigue failure after fatigue cycling through 1 × 107 cycles. As distinct from the crack arrested at the weld toe in the tests made in air, the crack found in the specimens tested in synthetic sea-water propagates through corrosion dissolution of the crack tip. 3. The stress concentration factor at the weld toe Kt2 determined from strain gauge data is smaller than the local stress concentration factor Kt1 calculated from the two-dimensional cross section at the corrosion initiation point. Its scatter is also smaller. The ΔSt/Nc relationship of the weld toe in which the bending stress of welding angular distortion is superposed on the tensile stress of the axial force is therefore determined as ΔSt = Kt* 2ΔSn. 4. The corrosion initiation life to failure life Nc/Nf has a value of around 0.84 and 0.91 in air and synthetic sea-water, respectively. This difference is due to the effect of the environment on the crack propagation rate. 5. Fatigue cracks are always initiated in the weld toe bond and propagate at right-angles to the loading direction.

A Study on a Long Term Corrosion Fatigue Strength of a Ship Structural Steel Plate in Synthetic Seawater

In order to evaluate precisely the life and residual-life time of ship structures, it is essential to examine the S/N relationships of their fatigue strengths. The long term corrosion fatigue behaviors, however, have not been fully investigated yet.In the present study, we have measured the corrosion rates and the fatigue crack growth rates of a 500 MPa class steel plate in synthetic seawater, and discussed the long term corrosion fatigue strength obtained under a cyclic frequency of 0.17 Hz within stress ranges less than the fatigue limit tested in air. The results obtained are as follows.(1) The corrosion rate, Rc, in an immersion test was 0.07 and 0.09mm/year for a polished and shotblasted surfaces of the plate, respectively. The effect of the stress increase in corrosion wastage cannot be neglect in the corrosion fatigue tests exceeding five years under the cyclic frequency of 0.17 Hz.(2) The crack growth rate measured in seawater was two to three times larger than that in air in the range where Paris equation may hold. The effect of the environment on a crack growth rate is significant in the range where the crack intensity factor is relatively low.(3) The range of the threshold stress intensity factor, ΔKth, is 2.5 and 1.2 MPa m1/2 in air and in seawater, respectively.(4) The long term corrosion fatigue strength can be calculated roughly from the empirical equation.ΔK= f (Rc, y) CcNnc, where f (Rc, y) is the corrosion wastage rate estimated from the corrosion rate of material and year, Nc is crack initiation life, n is the index given by crack growth rate, and Cc is a constant obtained from the corrosion fatigue tests of about 106 cycles.(5) For the long term corrosion fatigue, according to the results of Komai and coauthor, no fatigue cracks initiate from the corrosion pits after 14 to 24 years in the plate specimen area of 320 mm2 with Rc=0.09 mm/year, and the pits move to the progression of corrosion wastage in corrosion fatigues less than ΔKCF.(6) In order to decide the f (Rc, y) during corrosion fatigue, the tests through five years are necessary.

Temperature Effect on Corrosion Fatigue Strength of Coated Ship Structural Steel

Corrosion fatigue tests were carried out in synthetic seawater, in order to clarify the temperature effect on the corrosion fatigue strength of coated ship steel. To simulate the shipbuilding construction process, KA 32 TMCP steel in 10 mm thickness was sprinkled with water in outdoor twice a day for 20 days. After the exposure, the surface preparation was conducted, and the tar epoxy resin was painted with 50, 100, 200, and 300 μm in thickness. These specimens were subjected to cyclic axial stress at 0.17 Hz in 25, 40 and 60°C synthetic seawater. For the comparison, corrosion fatigue tests of uncoated specimens were also carried out.In the low cycle region, the corrosion fatigue strength of the coated specimen was almost equivalent to that of uncoated. On the contrary, it was improved in the high cycle region. The temperature effect of the corrosion fatigue strength was assessed by introducing corrosion factor Kc, which implies the reduction rate of the corrosion fatigue strength against that at 25°C. The corrosion factor K, indicated 1.031.13 at 4060°C.In addition the maximum depth of the corrosion pits was found to be 3040 μm after 20-day exposure. In the long-life corrosion fatigue, cracks were initiated from such corrosion pits under the coating film before it was destroyed. Finally, effect of corrosion pits on the fatigue crack initiation behavior was considered under the conditions of surface preparation of specimens.

Corrosion Fatigue Strength of Notched Specimens of a Ship Structural Steel

Corrosion Fatigue Strength of Notched Specimens of a Ship Structural Steel

Corrosion Fatigue of Ship-Structural Steel with Cathodic Protection under Intermittent Immersion Condition in Seawater at 60°C

It is important to clarify the effect of cathodic protection (CP) on corrosion fatigue properties of shipstructural steels in the actual environment of ballast tank. In this study, corrosion fatigue tests were conducted on notched base material specimens and welded joint specimens of a high strength steel for shipbuilding in a simulated environment of ballast tank under intermittent immersion condition (IIC) in synthetic seawater (SSW) at 60°C with CP. And the results were compared with those under continuous immersion condition.It was revealed that CP improves the corrosion fatigue life of the steel under IIC, and CP of-1.0 V (vs. SCE) in SSW is remarkably effective, especially in a longer fatigue life region. However, the fatigue lives under IIC both in SSW with CP of-0.8 V and in 3.5% NaCl aqueous solution with CP of-1.0 V are shorter than that in SSW with CP of-1.0 V. It is considered that the improvement in fatigue life under IIC in SSW with CP of-1.0 V is mainly caused by the wedge effect of calcareous deposits which have been produced on the crack surface during the periods of immersion and have been remaining there during the periods of exposure to damp atmosphere. Therefore, an application of linear cumulative damage law using S-N curves for continuous immersion of SSW with CP of-1.0 V and free corrosion condition leads to a very conservative estimation of IIC fatigue life, especially in a lower stress range region.