Kazushige Arimochi

Evaluation of Prestraining and Dynamic Loading Effects on the Fracture Toughness of Structural Steels by the Local Approach

On the occasion of recent great earthquakes, great concern is focused on the prevention of unstable fracture of steel structures against the seismic loading. This paper employs the local approach for the evaluation of prestraining and dynamic loading effects, experienced during an earthquake, on the fracture toughness of structural steels. The prestraining and dynamic loading lead to a similar result: increasing the yield stress and tensile strength and decreasing the fracture toughness. It is shown, however, that the combined effects of prestraining and dynamic loading is not equivalent to the sum of each individual effect. The analysis using the local approach demonstrates that the critical Weibull stress at brittle fracture initiation is independent of prestraining and dynamic loading. Based on the Weibull stress fracture criterion, the prestraining and dynamic loading effects on the fracture toughness can be predicted from static toughness results of the virgin material. As an engineering application, a simplified method is proposed for the estimation of fracture toughness under the seismic condition. This method uses a reference temperature concept: the dynamic fracture toughness at the service temperature T with prestrain is displaced by the static toughness of the virgin material at a lower temperature T -ΔT PD , where ΔT PD is a temperature shift of the fracture toughness caused by prestraining and dynamic loading. The temperature shift ΔT PD is provided as a function of the flow stress elevation in the seismic condition.

Fracture mechanics analysis of Charpy test results based on the weibull stress criterion

Abstract The material fracture toughness is often estimated from the Charpy impact energy using empirical correlations. However, each correlation has a limitation of application, although some are implemented in fabrication standards. This study employs the Local Approach to interpret Charpy test results. Instrumented Charpy tests and fracture toughness tests are performed in the lower-transition range for structural steels of 490 and 780 MPa strength class. Stress fields are addressed by 3D-FEM considering the strain rate effect and temperature rise during dynamic loading. It is shown that the critical Weibull stress at brittle fracture initiation is almost independent of the loading rate. This enables the Charpy results to be transferred to the material fracture toughness. As an alternative to the instrumented test, a simplified procedure is proposed: The evaluation of fracture initiation at 0.6 KV to 0.8 KV leads to a good estimation of brittle fracture toughness in the lower-transition range, where KV is a total impact energy.

Local Approach to Dynamic Fracture Toughness Evaluation

Fracture toughness properties of structural steels of 490-MPa strength class and weld metals under dynamic loading are studied in the light of the near crack tip stress fields. The loading rate in toughness tests range from 0.1 to 500 mm/s. The near-tip stress fields are analyzed by 3D-FEM considering the strain rate effect on the flow properties of materials. Temperature rise caused by high-speed plastic deformation is also included in the FE analysis. The critical CTOD at brittle fracture initiation decreases when increasing the loading rate. This is due to the elevation of a local stress near the crack tip. The local approach is applied to the dynamic fracture toughness evaluation. It is shown that the brittle fracture resistance evaluated in terms of the Weibull stress, an integrated stress over a highly stressed region near the crack tip, is a material property independent of the loading rate. Based on the Weibull stress fracture criterion, the dynamic fracture toughness can be predicted from static toughness results. An engineering procedure to estimate the loading rate effect on the fracture toughness is also presented.

Constraint Correction of Fracture Toughness CTOD for Fracture Performance Evaluation of Structural Components

A correction of CTOD for constraint loss in large-scale yielding conditions is made on the basis of the Weibull stress fracture criterion that eliminates an excessive conservatism in the conventional fracture assessment and material fracture toughness requirement. A CTOD ratio β = δ3P / δWP (

Development of Structural Steel With High Resistance to Fatigue Crack Initiation and Growth: Part 3

In recent years, higher safety and reliability of steel welded structures have been required as it shows growing concern about environmental problems. To prevent fatigue fracture is one of the most important challenges to improve the safety and reliability. A lot of studies how reduce stress concentration at critical areas have been carried out from the viewpoint of structural design as prevention measures while nothing has been studied from the viewpoint of material because fatigue strength of welded joints converges in limited range regardless of material strength. On the other hand, it was found that an appropriate dual phase microstructure could reduce the fatigue crack growth rate remarkably. The newly developed steel plate with high resistance to fatigue crack growth could extend the fatigue life of structures. The developed steels have already been applied to some ships and vessels, and a new bulk carrier applied the developed steels acquired the notation and descriptive note as the valuable ship with resistance to fatigue fracture by Nippon Kaiji Kyokai for the first time in the world. From further studies, it was found the developed steels had also high resistance to fatigue crack initiation as well as the growth even in welded structure. In this study, it was clarified that the fatigue strength of HAZ, where fatigue crack generally initiates, in the developed steel was higher than that in conventional steel and the stress concentration at toe of weld in the developed steel was smaller than in the conventional steel. It was considered the mechanism of suppression of fatigue crack initiation with FEM analysis and fatigue test. The newly developed steel can effectively extend fatigue fracture life of welded structure from the viewpoint of material.Copyright

Development of Structural Steel With Superior Resistance Against Fatigue Crack Growth

The fatigue crack problems that broke out at the end of the twentieth century were fatigue crack properties in hull structures. This motivated research on a lot of technologies against fatigue fracture. It was clarified that the detection of fatigue crack initiation in complex welded structures like hulls is quite difficult, and that the crack length at recognition is mostly long compared to mechanical parts. From these research results, not only stress reduction at critical areas by improvement in design but also newly developed materials with excellent resistance to fatigue crack growth has been desirable for structural integrity. The newly developed structural steel, in which fatigue crack growth resistance is controlled by microstructures, will be introduced in this report. Various fatigue properties of the base steel plates in air and in synthetic sea water are compared. And the fatigue life extension effects by FCA is observed in fatigue tests of welded joints and welded structural models.Copyright

Development of Structural Steel With High Resistance to Fatigue Crack Initiation and Growth: Part 4

For many years, fatigue design has been based on the fundamental that the fatigue strength of welded structures is independent of the steel material and/or strength. Nothing has been studied from the viewpoint of material because fatigue strength of welded joints converges much to the same capacity regardless of material strength. For improvement of fatigue lives, the designers have been advised to improve the geometry of the details, to reduce the nominal stress level or to use some post weld improvement method like toe grinding. In 2001, a new steel material was developed that showed extended fatigue initiation life as well as extended crack growth life, and a new alternative method for fatigue life extension appeared. This steel was denoted FCA (Fatigue Crack Arrester) due to the improved fatigue properties. The improved fatigue strength in welded joints is explained by flat hardness distribution and very fine microstructure at heat affected zone (HAZ). It was clarified that fatigue strength of HAZ in FCA where fatigue crack initiates generally was higher than that in conventional steel. And the improved fatigue crack propagation properties in base steel is explained by a decreased crack growth rate when a fatigue crack passes a grain boundary from a soft phase (feritte) to a hard phase (bainite) that is present in these new dual phase steels. FCA steel has now been used for details of a number of newly built ships, where good fatigue properties are required. In order to establish a general design S-N curve that can be used for the FCA steel, it was agreed in 2007 to start a joint industry project among Kawasaki Shipbuilding Corporation, Det Norske Veritas and Sumitomo Metal Industries. This JIP is now being finished and a design S-N curve has been proposed based on 66 data from small scale testing of specimens made from FCA steel, and 18 data from conventional steel. These test results have been supplemented by some large scale tests of relevant ship details. The discussed results from this JIP including a recommended design S-N curve for FCA steel will be shown in this paper.Copyright

Development of 7%Ni-TMCP Steel Plate for LNG Storage Tanks

Demand of natural gas continues to increase in the recent years due to the rise of environmental issue and the drastic increase of crude oil price. These events led to the increase of constructions of Liquefied Natural Gas (LNG) storage tanks worldwide. The inner tank material for above ground LNG storage tanks have mostly been made of a 9% nickel steel plate over the last 50 years as it has excellent mechanical properties under the cryogenic temperature of −160deg-C. During this period, the LNG storage tanks made of 9%Ni steel plate have been operated safely at the many LNG export and import terminals in the world. Meanwhile, technologies of steel making, refinement, design, analysis, welding and inspection have been improved significantly and enabled enlarging volumetric capacity of the tank 2–3 times. There was a tendency for nickel price to increase in recent years. In such a circumstance lowering Ni content has focused attention on the 9%Ni steel as nickel is an expensive and valuable rare metal and a 7%Ni steel plate was eventually researched and developed by optimizing the chemical compositions and applying Thermo-Mechanical Controlled Process (TMCP). As a result, it was demonstrated that 7%Ni-TMCP steel plate had excellent physical and mechanical properties equivalent to those of 9%Ni steel plate. In order to evaluate fitness of the 7%Ni-TMCP steel plate and its weld for LNG storage tanks a series of testing was conducted. Several different plate thicknesses, i.e. 6,10,25,40 and 50 mm, were chosen to run large scale fracture toughness tests including duplex ESSO tests, cruciform wide plate tests as well as small scale tests. It was concluded that the 7%Ni-TMCP steel plate warrants serious consideration for use in LNG storage tanks. This paper reports details of the research and development of the 7%Ni-TMCP steel plate.Copyright

Consideration on the Toughness Requirement to the Austenitic Weld Metal in the LNG Storage Tanks Subjected to a Partial Height Hydro Test

PD7777 published by British Standard Institute in 2000 proposes an additional fracture requirement to the main steel components of the low temperatures storage tank where a partial height hydrostatic test is allowed instead of the full height hydrostatic test required in BS 7777. In the PD7777 a high level (75J) of fracture toughness is required for the austenitic weld metal in 9%Ni steel plate to prevent the re-initiation of ductile fracture from the arrested brittle crack in the weld. This is to report a study that the J-Cv correlation of austenitic weld metal is determined by the experimental data obtained from the actual weld joints in 9%Ni steel plate in order to assess a rational toughness requirement to the austenitic weld metal employed in the 9% Ni made LNG storage tanks. From this study it is concluded that fracture toughness of 75J is too conservative and 50J is adequate. Further some FEM analyses were performed to verify the validity of the weld surrounded by the large amount of heterogeneous base metal in the yield strength distribution.

Application of large heat input automatic welding process to beam-to-column connections of steel framed structures (1st Report)

Summary Beam-to-column connections of construction steelwork have been conventionally welded by a CO2-based, semi-automatic, multi-layer welding process. Such welding processes, however, face the problems of limited efficiency and the fact that the welding quality often depends on the welders workmanship. The electrogas arc welding process is generally considered difficult to apply to beam-to-column connections of construction steelwork because of the poor weld toughness it confers due to its conventional high heat input. Based on recent developments in structural steel plates for large heat input welding, the authors have developed a one-pass vertically upward large heat input electrogas arc welding process (referred to in this paper as large heat input welding process and in the diagrams as EGW) able to produce welds with a deposition rate of twice the conventional one without any loss of toughness and with far fewer weld defects. The proposed process can also be applied to new improved seismic details...