Noboru Konda

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.

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

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.

Development of a new steel plate possessing self-suppression effect for fatigue-crack propagation and properties of welded joints

Abstract With welded steel structures, fatigue-cracks usually occur at the welded joints. Therefore the structures are designed to reduce stress at the points where fatigue damage is expected. It is, however, impossible to totally prevent fatigue-cracks from occurring. Moreover, in a complex structure, there is a possibility that a simple method such as visual inspection does not enable the detection of the occurrence of a fatigue-crack at the welded joint at an early stage, and hence it can have grown into a long crack by the time it is discovered.

Fracture Assessment Procedure for Steel Structures Subjected to Large Cyclic and Dynamic Strain – Based on WES TR 2808

With a lesson leaned from the Kobe Great Earthquake, an engineering method, WES TR 2808, has been developed in Japan for evaluating the structural integrity under the seismic condition. This paper presents a main story of WES TR 2808, which is characterized by two key ideas. One is a reference temperature concept for fracture toughness evaluation. The material fracture toughness under the seismic condition is replaced by the static fracture toughness without prestrain at a reference temperature of T − ΔTPD , where T and ΔTPD are the service temperature and the temperature shift of fracture toughness by prestraining and dynamic loading, respectively. WES TR 2808 gives ΔTPD as a function of the elevation of flow stress in the seismic condition. Another idea is a correction of constraint loss in structural components. A large amount of plastic deformation causes a loss of constraint in structural components, which leads to a significant difference in the fracture driving force of the components and fracture toughness specimen. In WES TR 2808, the CTOD ratio β = δ3P.Equiv /δStruc was defined on the basis of the Weibull stress criterion, where δStruc is the CTOD of the structural component and δ3P.Equiv is an equivalent CTOD of the fracture toughness specimen at which the toughness specimen presents a compatible Weibull stress with the structural component. The CTOD ratio β is apparently less than 1 in a large-scale yielding condition, and roughly 0.4 for the wide plate component with a surface crack. It was shown that WES TR 2808 is applicable to the fracture performance assessment of column-to-beam connections subjected to large cyclic and dynamic strain. The discussion was given on the engineering judgement in WES TR 2808, which was adopted to make the procedures as simple as possible.Copyright