Tsunehisa Handa

Application of WES2808 to Brittle Fracture Assessment for High Strength Gas Pipelines

This paper presents the results of a preliminary study to establish an assessing method for the tensile strain limit against brittle fracture of pressurized gas pipelines subjected to axial tensile deformation. The basis of the assessment method is the Japan Welding Engineering Society standard WES2808–2003. WES2808 provides a procedure for evaluating the fracture limit using the CTOD design curve relating flaw size, applied strain and fracture mechanical parameter (CTOD). The main characteristics of the method are a consideration of the deterioration of the fracture toughness of material resulting from large cyclic and dynamic straining, a correction of CTOD fracture toughness for constraint loss in structural components in large scale yielding, and an estimation of critical CTOD value from Charpy test results. Modifications of the procedure to enable evaluation of the fracture properties of high strength gas pipelines under biaxial loading conditions are studied.Copyright

Three-Dimensional Dynamic Explicit Finite Element Analysis of Charpy Impact Test

Dynamic explicit finite element (FE) analysis of the Charpy impact test was conducted in this study to investigate the inertial effect on the stress field ahead of the V-notch in a Charpy specimen. The deformation behavior of the Charpy specimen and the constraint effect on the stress field in the plastic zone near the V-notch were numerically simulated using three-dimensional FE analysis, while considering the contact of the specimen with the striker and anvil. The effect of the strain rate on the flow stress and the increase in temperature during impact loading were included in the dynamic analysis. This analysis shows that the impact load exhibits oscillation and the contact stiffness between the specimen and the striker affects the oscillation of the impact load. The analysis was validated by comparison with experimental results obtained using an instrumented Charpy impact testing machine, which measured the impact load and the load point displacement. The oscillation of the load–time curve was recorded. The magnitude and period of the peak inertia load obtained by the FE analysis were almost consistent with the experimental results. The contact stiffness between the specimen and the striker affected the stress field near the V-notch in the specimen. This indicates that the stress field in the Charpy specimen should be analyzed by the dynamic analysis procedure considering the contact stiffness based on the Hertzian contact theory.

DIFFERENCE BETWEEN ASTM E1290 AND BS 7448 CTOD ESTIMATION PROCEDURES

Since the British Standards Institution (BSI) standardized BS 5762 in 1979, a popular way of calculating the Crack Tip Opening Displacement (CTOD) has been the use of the plastic hinge model with an assumed rotational centre. The American Society for Testing and Materials (ASTM) previously accepted the plastic hinge model and standardized E1290 in 1989. However, ASTM revised E1290 in 2002, and has proposed a fracture parameter conversion from the J-integral to CTOD. These two different CTOD calculations probably lead to confusion for Fitness-for-Service (FFS). The Fracture Toughness Study Committee of the Japan Welding Engineering Society (JWES) organized a working group, and the effects of CTOD testing methodology on CTOD values were investigated. In this paper, the results of CTOD round-robin tests in the working group are summarized, and the difference between the critical CTOD values obtained by ASTM E1290–02 and those by BS 7448, which involves the plastic hinge model, is described.

Brittle Crack Propagation/Arrest Behavior in T-Joint Structure of Heavy Gauge Steel Plates

Brittle crack arrestability is extremely important in welded joints of heavy gauge steel plates used in large container ships. Recently, much attention has been focused on potential crack propagation along welds using large heat input. This paper examines the application of a T-joint to the strength deck structure of container ships to enhance crack arrestability. The crack arrest toughness, Kca, for crack arrest was varied. The ESSO test of T-joint components showed that brittle crack was arrested at the T-joint if the steel plate used for the flange had a high Kca value in the range from 4900 to 7300N/mm3/2. FE-analysis of the stress intensity factor K indicated that brittle crack propagation was arrested under the condition that the K-value at the running crack tip was less than the Kca of the material. In the T-joint, it was noted that the K-value around the area of the deepest point of the crack decreased and was finally less than the Kca of the flange plate when the brittle crack penetrated suddenly into the flange plate to a 10mm depth. This phenomenon shows the advantage of using a T-joint for brittle crack arrest in the flange plates of strength deck structures.

Effect of unwelded length on behaviour of brittle crack arrest in T-joint structure

In the large heat input weld joint of heavy gauge steel plate used for large container carriers, a brittle crack possibly propagates straight along a weld joint without diverging to the base metal. This phenomenon is discussed for its application to the large heat input weld of heavy gauge plate to ship structures. In this study, to arrest a brittle crack at the T-joint embedding unwelded face, the effect of the unwelded face on behaviour of brittle crack propagation/arrest in the T-joint structure was investigated and analysed. The ESSO test of the T-joint structure was carried out, supposing that a brittle crack which propagates along a weld joint of hatch coaming rush into T-joint of hatch coaming and strength deck. The test results showed that the brittle crack arrested at the T-joint embedding the unwelded face, and the brittle crack arrested easier, the longer the unwelded face. The results of static FEM analysis showed that the stress intensity factor of the brittle crack was increased by the unwelded face, until the brittle crack reached a flange. However, when the brittle crack propagated into the flange, the stress intensity factor of the brittle crack was decreased by the unwelded face. The crack-arrest effect of the unwelded face appears after the brittle crack propagates into the flange.

Relationship Between Weibull Parameter and Fracture Toughness of Structural Steels

The brittle fracture assessment for structural components excluding an excessive conservatism should be conducted under the concept of fitness-for-service assessment. One of the factors that lead to such a conservative estimation of brittle fracture performance is no consideration of plastic constraint loss in structural components compared to the fracture toughness test specimen. The Weibull stress criterion is expected to correct the CTOD (Crack Tip Opening Displacement) fracture toughness of materials to the critical CTOD for structural components of concern through the same level of Weibull stress, which take into account not only the difference in plastic constraint but also volume of fracture process zone between toughness test specimen and structural components. On the basis of the Weibull stress criterion, the fracture driving force, that is the Weibull stress, is dependent on the Weibull shape parameter m. Furthermore, such dependency is influenced by both the plastic constraint level and the volume of fracture process zone for specimens of interest. The different m-value would result in the different correction ratio of the fracture toughness to the critical CTOD for structural components. Accordingly, the parameter m should be estimated for the appropriate fracture performance evaluation in consideration of constraint loss correction. In this paper, a simple method for estimating the Weibull shape parameter m were introduced. That is the effort to address the factors to affect the m-value in terms of strength class and toughness level of materials based on the data from literatures, which is for efficient and rational estimation of m-value without any experimental and numerical works.Copyright

Effect of toughness distribution in the thickness direction on long brittle crack propagation/arrest behaviour of heavy gauge shipbuilding steel

ABSTRACT Brittle crack arrestability of the heavy gauge steel plates for shipbuilding is now an important issue for the recent mega container ships. In the present work, the brittle crack arrestability of the steel plate with different toughness distributions in thickness is examined in ultra-wide duplex ESSO tests. It is examined whether a running long brittle crack arrests or not in flat temperature condition in ultra-wide duplex ESSO test that are harder mechanical conditions similar to an actual ship hull condition. Test temperatures are selected at which arrest toughness, Kca, evaluated by temperature gradient type standard ESSO test is the same for two test plates. The steel plate with higher toughness in mid-thickness (t/2) than that in quarter thickness (t/4) could arrest a running long brittle crack although the plate with lower toughness in mid-thickness than that in quarter thickness could not arrest it. The typical split-nail shape appeared at the arrested crack front in the plate with higher toughness in mid-thickness than that in quarter thickness. The numerical analyses also demonstrate that the local stress intensity factor at the arrested crack tip is changing sensitively to the crack front shape. It suggests that the higher brittle crack arrestability appears due to the split-nail shape of the arrested crack front enhanced by the inhomogeneous toughness in thickness.

Numerical analysis of strength mismatch effect on stress field in Charpy specimen

The influence of strength mismatch adjacent to the V-notch in Charpy specimen on stress field has been numerically analyzed with a 3D finite element method. It is found that the softer material adjacent to the V-notch provides a “shielding effect” from high-speed straining introduced by impact loading. As this leads to reduction in flow stress ahead of the V-notch, lower opening stress compared with homogeneous specimen is found in the case of the V-notch within the harder material in heterogeneous specimen. By contrast, in the case of the V-notch within the softer material, plastic strain is concentrated near the V-notch and the opening stress is elevated by constraint due to hard material adjacent to the V-notch. The Charpy impact test was conducted with the Charpy specimen extracted from the clad steel consisted of low- and high-strength steels in order to investigate the strength mismatch effect Charpy absorbed energy KV. It is found that KV value is affected by strength mismatch adjacent to the V-notch and the Weibull stress criterion can be applied to the quantitative evaluation of the strength mismatch effect on KV.

Prediction of ductile fracture path in Charpy V-notch specimen for laser beam welds

The V-notch Charpy testing for laser beam welds (LBWs) sometimes presents fracture path deviation (FPD) into a base metal with lower strength at service temperature. The main effort of this work is to propose a damage model to simulate ductile fracture path depending on the feature of LBWs such as strength mismatch and a width of a weld metal. Static loading tests for a Charpy V-notch specimen reveals details of the FPD process; the ductile crack directly starts to deviate from the notch root into a softer base metal in purely shear-slip mode over full thickness of the specimen. The subsequent ductile crack extends toward the base metal in shear-slip mode along with shallow dimples. The proposed damage model can predict the occurrence of fracture path deviation in shear-slip mode for the Charpy specimen with a notch in narrow bead used for experiment.

Fracture toughness and microstructure of 780 MPa class high-tensile strength steel plates: improvement of steel structural performance by laser beam welding for mid-thick section steel plate

Fracture toughness and microstructure of laser weld metal of 780 MPa class steels are investigated and compared with those of SM490A and SM570Q. In SM490A and SM570Q, Charpy energy transition temperature of laser weld metals is 60–90°C higher than that of base metal, and upper bainite microstructures are observed in the laser weld metals. In 780 MPa class steels, difference of Charpy energy transition temperatures between laser weld metal and base metal are only 10–30°C, and no upper bainite microstructures are observed in the laser weld metals. Hardness of the laser weld metals of 780 MPa class steels is identical to that of martensite microstructure. A superior toughness of the laser weld metal of 780 MPa class steels would be owing to the martensite microstructure resulted from a high carbon equivalent.