Yoichi Kayamori

Transformation of BS7448-CTOD to ASTM E1290-CTOD

Experimental and analytical investigations into crack tip opening displacement (CTOD) were conducted to demonstrate the relationship between BS7448-CTOD and ASTM E1290-CTOD. The CTOD test results showed that ASTM-CTOD was occasionally much lower than BS-CTOD both in single edge notch bend specimens and in compact tension (C(T)) specimens for low-strength structural steels, and this tended to be more remarkable in C(T) specimens. In addition, the analytical results of simplified elastic-plastic fracture parameter calculation using the Electric Power Research Institute scheme demonstrated that the ratio of ASTM-CTOD to BS-CTOD was not constant but varied according to CTOD changes. Material factors such as the yield stress, the strain hardening exponent, specimen size and configurations influenced the CTOD ratio, and low strain hardening exponents in the Ramberg-Osgood relation and C(T) specimen configuration significantly decreased the CTOD ratio. An equation that transforms BS-CTOD into ASTM-CTOD is proposed in this study. This equation gives a good estimate of ASTM-CTOD from BS-CTOD.

Plastic Rotational Factor Calculation for Shallow-Notched SE(B) Specimens

The plastic part of crack tip opening displacement (CTOD) is derived from the plastic hinge model for deep-notched single edge-notch bend (SE(B)) specimens in BS, WES and ISO CTOD testing standards, and a typical plastic rotational factor is given by a constant value of 0.4. This value is appropriate for deep-notched SE(B) specimens, but is not suitable for shallow-notched SE(B) specimens. In this study, a new equation of calculating the plastic rotational factor was obtained by using the Electric Power Research Institute (EPRI) scheme. The equation shows the effect of crack length and strain hardening on the plastic rotational factor, and is useful for evaluating CTOD in shallow-notched SE(B) specimens.

Crystallographic orientation analysis of cleavage facets adjacent to a fracture trigger in low carbon steel

The microstructure and crystallographic orientation under a cleavage crack trigger point, which was detected on a fracture toughness specimen of low carbon steel, were investigated. SEM fractographs of an etched cleavage facet reveal that a flat cleavage facet between facet ridges spreading from the trigger point is divided by a grain boundary between ferrite and pearlite. Even a different phase boundary does not occasionally create any steps and ridges on the cleavage facet: it suggests this phase boundary is not an obstacle to cleavage cracking. Electron diffraction analysis for the thin foil sample milled out from this phase boundary demonstrates that the crystallographic orientation of the ferrite grain is consistent with that of the ferrite in the adjacent pearlite block. It is strictly examined that the ferrite/pearlite boundary does not act as a local resistance to cleavage crack growth when the crystallographic orientation of the ferrite in the pearlite block is aligned with an adjacent ferrite grain.

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.

Ductile Crack Propagation Characteristics in Steel Thin Single Edge Notched Tension Specimens

Static and dynamic ductile crack propagation tests were carried out using thin single edge notched tension (SENT) specimens of carbon-manganese steel, each of which had a fatigue pre-crack or a sharp V-notch as a crack initiator. The crack tip opening angle (CTOA) was measured using digital images on the surface of the SENT specimens, and the critical values of CTOA for crack propagation decreased with increasing crack length while initial crack growth was still small. After the initial crack growth up to the distance of the specimen thickness, the critical CTOA remained almost constant. These tendencies were common in static and dynamic crack propagation specimens as well as fatigue pre-cracked and sharp V-notched specimens. There was no particular difference in the static crack propagation characteristics of both fatigue pre-cracked and sharp V-notched specimens. On average, it was observed that higher crack speeds affected the constant values of the critical CTOA by slightly reducing them. The constant CTOA tends to decrease with an increasing global constraint factor, and this suggests that the factor is insensitive to a crack starter, fatigue pre-crack or a sharp V-notch, but relatively sensitive to crack speed.

Development of 6% nickel steel for LNG storage tanks

9% Ni steel has been used for LNG storage tanks for more than four decades although 5.5% Ni steel (N-TUF CR196) was developed in the 1970’s using a special heat treatment method named L-treatment. The reason why the actual application of 5.5% Ni steel has not been attained to LNG storage tanks is mainly because the requirement of fracture properties is not confirmed for the tanks. Under the circumstances of expanding demand for natural gas and double-integrity in LNG storage tanks, we restarted developing low Ni steel for LNG storage tanks by using both conventional and advanced techniques. For the application of low Ni steel to the present LNG storage tanks, both fracture initiation and propagation properties of base metal plates and welded joints should be concerned. The fracture initiation and propagation properties of base metal were compensated with the intercritical reheating process (L-treatment), and the propagation property was additionally enhanced by combining TMCP with L-treatment. In addition, the chemical composition adjustment and the homogenization treatment of solute elements were conducted for improving the fracture initiation and propagation properties of welded joints. 6% Ni steel plates were manufactured by the process of continuous casting, reheating, hot rolling, direct quenching (TMCP), L-treatment, and tempering, and their chemical composition was 0.05C-0.06Si-1.0Mn-6.3Ni-Cr-Mo. As the results of fracture property evaluation including large-scale fracture tests such as the duplex ESSO test and the wide plate tensile test, it was demonstrated that 6% Ni steel has good characteristics regarding brittle fracture initiation and propagation in base metal plates and welded joints.Copyright

Effects of Cleavage Triggers and Constraint on Brittle Fracture in Steel Weld Heat Affected Zone

Cleavage fracture in steel is usually evaluated by using the stress criterion. However, some researchers indicated the necessity of strain in the stress criterion according to the cleavage fracture test results of steel base metal. If so, the cleavage fracture criterion should be also applicable to steel welded joints. In this study, cleavage fracture tests, SEM fracture surface observations and FEA were conducted using the smooth and notched round bar tensile specimens of the simulated HAZ microstructure of JIS SM490A. Cleavage trigger types did not influence the cleavage fracture properties of the specimens obviously. FEA results suggested that the critical condition of cleavage fracture initiation was associated not only with stress but also with strain.

Micromechanical Characterisation of Microstructure in Weld Heat Affected Zone of Structural Steel

Microstructures in the weld heat affected zone (HAZ) can cause a decrease in fracture toughness, and evaluating the effect of microstructures on fracture toughness is helpful in understanding the cause of the fracture toughness decrease. In this study, micro-sized tensile specimens were sampled from base metal and HAZ, and the mechanical properties and fracture behaviours of different steel microstructures were directly investigated by micro-sized mechanical testing.

A Proposal for Two Characteristic Ratios of ASTM-CTOD to BS-CTOD

There have been two different CTOD calculations, geometric CTOD calculation in BS7448 and J-based conversion in ASTM E1290 since the ASTM International revised E1290 in 2002, and it is useful to transform one CTOD into the other CTOD. In this study, analytical and experimental investigations into the ratio of ASTM E1290-CTOD to BS7448-CTOD were carried out, and two ratios of ASTM-CTOD to BS-CTOD, R1 and R2, were characterised in terms of cleavage cracking. These two characteristic CTOD ratios are helpful in simply transforming one critical CTOD to the other.

A Comparison of CMOD-Based CTOD and J-Conversion CTOD in Shallow Crack Specimens

The ratio of J-conversion CTOD to CMOD-based CTOD was experimentally evaluated and analytically estimated in shallow crack specimens. It was demonstrated that the low strain hardening exponent in the Ramberg-Osgood relation reduced the CTOD ratio. A CTOD transformation equation, which was proposed by the authors, can transform CMOD-based CTOD into J-conversion CTOD with reasonable accuracy for 0.15≤a/W≤0.5.