Ductile-brittle Transition Temperature Region Research Articles

Nature of Microcracks in Ferritic Steels Occurred during Fracture under Conditions of Ductile-Brittle Transition Temperature Region

It has been shown by means of EBSD techique that fracture of ferritic steel in ductile-brittle transition temperature region, along with the formation of previously discribed cleavage microcracks, results in the formation of ductile microcracks. It has also been shown that microstructure of plastic zones under brittle and ductile fracture components produced by the main crack propagation differ significantly. Better developed plastic zone under ductile fracture component protects steel from overstress. The plastic zone under brittle fracture surface, apparently, has a reduced local plasticity. Consequently, the cleavage microcracks formation precedes the fracture process. During the main crack formation such microcracks occur in steel microvolumes located both in front of its tip and in adjacent to its edges microvolumes. Further propagation of the main crack is realized in steel which already contains scattered cavities and reduces to ductile fracture of the connections between them.

Impact toughness scattering of bainitic steel in the ductile-brittle transition temperature region

The impact toughness scattering in the ductile-brittle transition temperature (DBTT) region was experimentally examined on mixed and homogeneous grains of low alloy high strength bainitic steel under dynamic loading conditions. The results revealed that the mixed grain microstructure had larger impact toughness scattering than the homogeneous one, and the impact toughness scattering was mainly caused by the scattering in the cleavage fracture stress σf. The value of σf is related to the size of the microcrack formed in the bainitic packet. When a bainitic packet-sized microcrack propagates from one bainitic packet into the adjacent packet, cleavage fracture occurs. The cleavage fracture is controlled by the few coarse packets in the microstructures, and the σf scattering is influenced by the varied distances/relative locations between these coarse packets, and homogenizing the distribution of fine bainitic packet sizes is an effective way to reduce the impact toughness scattering in the DBTT region.

Application of Coupled Damage and Beremin Model to Ductile-Brittle Transition Temperature Region Considering Constraint Effect

For nuclear safety, fracture evaluation of reactor pressure vessels (RPV) under neutron irradiation is key issue. Fracture toughness from a CT specimen is used as a material constant for fracture evaluation, but it is well known that it has a large constraint, which causes lower toughness than that of flawed structures, such as a RPV with a surface flaw. In ductile to brittle transition temperature (DBTT) region ferritic steel which is material of RPV has a large scatter and it becomes important to know the accurate scatter of an irradiated material because of less margin of RPV’s integrity after a long term operation. In this paper to establish a more precise fracture evaluation method in DBTT region for an irradiated RPV with a postulated surface flaw, a coupled model of damage mechanics for ductile fracture and Beremin model for cleavage fracture was applied for correction of the effect of a small ductile growth on the stress-strain field. To confirm the validity of the method, as the first trial, fracture tests using CT specimens were performed in several temperature conditions. The temperature dependence of the parameters of Beremin models were investigated as well.

Effect of Bainitic Packet Size Distribution on Impact Toughness and its Scattering in the Ductile-Brittle Transition Temperature Region of Q&T Mn-Ni-Mo Bainitic Steels

The impact toughness of Q&T Mn-Ni-Mo bainitic steel in the ductile–brittle transition temperature (DBTT) region was measured. The fracture micromechanism associated with the microstructural features was studied, by statistically analyzing the grain sizes of various structures. The results reveal that the cleavage fracture behavior is controlled by bainitic packets. There exists a critical bainitic packet size. When a microcrack formed in the bainitic packet with a size exceeding the critical size propagates into adjacent packets, brittle cleavage fracture occurs. The probability of finding the bainitic packets larger than the critical size dominates the impact toughness and its scattering. It is suggested that refining the bainitic packets and homogenizing their sizes is an effective method of improving the impact toughness and reducing its scattering in the DBTT region.

Scattering of values of impact toughness of low-alloy steel in the ductile-brittle transition temperature region

A considerable scattering of impact toughness of low-alloy steel in two structural states have been studied in the ductile-brittle transition temperature region. It is shown that maximal impact toughness levels have a linear thermal dependence in the ductile-brittle transition temperature region. The increased scattering of impact toughness can be considered a characteristic of the ductile-brittle transition temperature region. Frequency distributions of impact toughness are aggregations of frequency distributions with different characteristics, which implies the existence of several simultaneously active mechanisms of cleavage initiation.

Rational Determination of Lower-Bound Fracture Toughness Curves Using Master Curve Approach

The Master Curve gives the relation between the median of fracture toughness of ferritic steels and the temperature in the ductile–brittle transition temperature region. The procedure used to determine the Master Curve is provided in the current American Society for Testing and Materials (ASTM) E1921 standard. By considering the substitution of the alternative lower-bound curves based on the Master Curve approach for the KIc curves based on reference data sets in the present codes such as ASME Code Cases N-629 and N-631, the statistical characteristic should be well incorporated in the determination of the lower-bound curves. Appendix X4 in the ASTM standard describes the procedure used to derive the lower-bound curves; however, it appears to be addressed without sufficient consideration of the statistical reliability. In this study, we propose a rational determination method of lower-bound fracture toughness curves using the Master Curve approach. The method considers the effect of sample size in the determination of the tolerance-bound curve. The adequacy of the proposed method was verified by comparing the tolerance-bound curve with the fracture toughness database for national reactor pressure vessel (RPV) steels including plate and forging obtained from 4 T to 0.4 T C(T) specimens and 0.4 T SE(B) specimens. The method allows the application of the Master Curve using fewer specimens, which can coexist with the present surveillance program.

Statistical evaluation of fracture toughness with stable crack extension

The scatter of fracture toughness in the ductile-brittle transition temperature region was experimentally examined on 500MPa class low carbon steel. The Weibull distribution of the fracture toughness K(Jc) at −60°C showed the single modal distribution with the shape parameter of around 4. The Weibull distributions of the K(Jc and Ju)s at −20°C and −10°C, however, showed the bilinear distributions with the elbow point. The Weibull slopes below the elbow points were closed to the value of 4, but the slopes showed the transition to be smaller above the elbow points. The bilinear Weibull distributions were discussed from the viewpoints of the constraint loss and the stable crack extension. However, the fracture surface observations revealed that the stable crack extension was the main factor that brought the bilinear Weibull distributions. The bilinear Weibull distributions could be qualitatively simulated by the analytical model with the consideration of the stable crack extension.

Evaluation of Fracture Toughness by Master Curve Approach Using Miniature Specimens

The fracture toughness Master Curve gives a universal relationship between the median of fracture toughness and temperature in the ductile-brittle transition temperature region of ferritic steels such as reactor pressure vessel (RPV) steels. Considering the possible application of the Master Curve method coexistent with the present surveillance program for operating RPVs, the utilization of miniature specimens is quite important for efficient determination of the Master Curve from limited volume of the materials of concern. In this study, fracture toughness tests were conducted for typical Japanese RPV steels using the miniature C(T) specimens with the thickness of 4 mm following the procedure of the ASTM standard. The results showed that the differences in test temperature, evaluation method, and specimen size did not affect the Master Curves, and the fracture toughness indexed by the reference temperature, To, obtained from miniature C(T) specimens were consistent with those obtained from standard and larger C(T) specimens.

Constraint loss correction for assessment of CTOD fracture toughness under welding residual stress. Part I: Methodology using the equivalent CTOD ratio

Abstract This part I of a two-part paper presents a method of assessing the effects of welding residual stress and constraint loss on the cleavage fracture of a wide plate subjected to membrane stress based on the Weibull stress criterion. It has been found that the Weibull stress criterion is efficient for evaluating the fracture instability of wide plates with and without a welding residual stress field. The concept of an equivalent crack-tip opening displacement (CTOD) ratio β r under a welding residual stress field is introduced for assessment of constraint loss effects on CTOD fracture toughness of wide plates. The equivalent CTOD ratio β r is defined as the ratio of the CTOD in the standard fracture toughness specimen to the CTOD in a wide plate with a welding residual stress at the same level of the Weibull stress. Fracture assessment procedures using β r for wide plates are shown within the framework of the failure assessment diagram. It has been found that the excessive conservatism observed in the conventional procedure can be reduced reasonably by applying the proposed method. The companion part II of the paper presents applications of the CTOD toughness correction method using β r to the fracture test data of welded joints and verifies the proposed method in the ductile–brittle transition temperature region.

Method of Constraint Loss Correction for CTOD Fracture Toughness under Welding Residual Stress Field

This paper studied the assessment method for welding residual stress effects and constraint loss effects on brittle fracture of structural component subjected to membrane stress. The methodology of CTOD fracture toughness correction for welded joints is proposed from lower to upper ductile-brittle transition temperature region. The methodology is based on the tensile plastic zone size criterion and the equivalent CTOD ratio derived from the Weibull stress criterion. It has been found that the proposed methodology has given the reasonable fracture assessment results.

1008 超小型試験片を用いたマスターカーブ破壊靭性評価(OS10-2 供用エネルギー機器の経年変化と健全性評価-破壊評価-)

The fracture toughness Master Curve gives a universal relationship between the median of fracture toughness and temperature in the ductile-brittle transition temperature region of ferritic steels such as reactor pressure vessel (RPV) steels. Considering the possible application of the Master Curve method coexistent with the present surveillance program for operating RPVs, the utilization of miniature specimens is quite important for efficient determination of the Master Curve from limited volume of the materials of concern. In this study, fracture toughness tests were conducted for typical Japanese RPV steels using the miniature C(I') specimens with the thickness of 4 mm following the procedure of the ASTM standard. The results showed that the differences in test temperature, evaluation method, and specimen size did not affect the Master Curves, and the fracture toughness indexed by the reference temperature, T_o, obtained from miniature C(T) specimens were consistent with those obtained from standard and larger C(f) specimens.

マスターカーブ法による国産原子炉圧力容器鋼の破壊靭性評価(保全・設備診断技術,<特集>第13回動力・エネルギー技術シンポジウム)

The fracture toughness Master Curve is a universal relationship between the median of fracture toughness and temperature in the ductile-brittle transition temperature region of ferritic steels. The Master Curve approach specified in the ASTM standard theoretically provides the confidence levels of fracture toughness in consideration of the inherent scatter of fracture toughness. In this study, fracture toughness tests for Japanese reactor pressure vessel steels, with various specimen sizes and shapes were conducted. The differences in test temperature, evaluation method, and specimen size and shape did not affect the Master Curves, ascertaining that the Master Curve approach could be well applied to these classs of steels. The adequacy of the lower bounds based on the ASTM standard was also verified by comparing with experimental fracture toughness data.

延性脆性遷移温度領域における溶接残留応力場中のき裂の脆性破壊発生評価法

This paper presents the methodology of the CTOD fracture toughness correction for wide plates' welded joints from lower to upper ductile-brittle transition temperature region. Tensile plastic zone size criterion is adopted in lower transition region. The equivalent CTOD ratio, β r , is adopted in ductile-brittle transition and upper transition region. The equivalent CTOD ratio is defined as the ratio of CTOD in a standard fracture toughness specimen to CTOD in a wide plate with welding residual stress at the same level of the Weibull stress. Fracture assessments for After Weld Notch and Before Weld Notch type welded joints have been conducted by applying the toughness correction methodology. It has been found that an excessive conservatism observed in the conventional procedure is reasonably reduced by applying the proposed methodology.

An updated continuum damage model to investigate fracture process of structures in DBTT region

In the Ductile–Brittle Transition Temperature (DBTT) region, it is not realistic to take unique fracture stress or fracture strain as the fracture criterion to investigate the fracture properties. In this paper, an updated continuum damage model was proposed, in which the fracture energy density, a function of the stress triaxiality, temperature and strain rate in the transition region was taken as the critical damage factor. Uniaxial tension tests were carried out to get the basic material properties at different temperatures, to calibrate the fracture model constants and verify the validity of the damage model. The fracture behaviour of pipes with penetrating cracks under the internal pressure was experimentally investigated with the load–deflection curves and the crack propagation length captured from tests. The J–R curves were obtained from the testing results for different temperatures. Based on the Finite Element Analyses (FEA) with the proposed fracture criterion of the updated continuum damage model, the loading level of pipes with penetrating cracks were estimated and compared with the experimental results. Meanwhile the fracture processes of the pipeline structures in the transition region were reproduced. The experimental and numerical results agreed very well in present calculations. It has been shown that the fracture process in the transition region strongly depends on both the stress and strain states, and could be effectively predicted using the continuum damage model.

A203 マスターカーブ法による国産原子炉圧力容器鋼の破壊靭性評価(材料強度評価-1,OS-6 保全・設備診断技術(3),一般講演,地球温暖化防止と動力エネルギー技術)

The Master Curve gives the relation between the median of fracture toughness and temperature in ductile-brittle transition temperature region. The Master Curve approach theoretically provides the confidence levels of fracture toughness in consideration of the inherent statistical distribution of fracture toughness. In this study, fracture toughness tests for Japanese reactor pressure vessel (RPV) steels, SFVQ1A and SQV2A, with various specimen sizes and shapes were conducted. The differences in test temperature, evaluation method, and specimen size and shape did not affect the Master Curves, thus, it was ascertained that the Master Curve approach could be well applied to them. The adequacy of the lower bounds based on the Master Curve was also verified by comparing with experimental fracture toughness data.

Weibull Statistics as a Basis for Assessment of Ductile-Brittle Transition Behavior

The objective of this paper is to investigate failure characteristics of SA508 carbon steel in ductile-brittle transition temperature region. To achieve this goal, a series of finite element analyses as well as fracture toughness tests are performed for pre-cracked V-notch specimens. An assessment of failure probabilities is, then, carried out employing Weibull stress models with different rank probability options. Finally, a prototype of toughness scale diagram is derived through comparison of estimated fracture toughness data with those for compact tension specimens. The present results can be utilized to found a basis of realistic integrity evaluation on major nuclear components containing defect.

安定き裂進展による破壊靭性のばらつきの遷移

The scatter of fracture toughness in the ductile-brittle transition temperature region was experimentally examined on 500 MPa class low carbon steel. The repeated toughness tests with the three points bend specimens were carried out at -60°C, -20°C and -10°C. The Weibull statistics of the fracture toughness K(Jc) at -60°C showed the single modal distribution with the shape parameter of around 4, which agrees with the analytical prediction from the conjunction between the Weibull stress analogy and the fracture mechanics. The Weibull distributions of the K(Jc)s at -20°C and -10°C, however, showed the typical bilinear pattern with the elbow point. The Weibull slopes below the elbow points were closed to the value of 4, but the slopes showed the transition to be smaller slope at the elbow points. The K(Jc)s above the elbow points induced the larger scatter of the data set at -20°C and -10°C. The fracture surface observations revealed that the K(Jc)s of the elbow points on the Weibull distributions are coincided with the stable extension of around 0.2mm or 0.3mm in length. This transition condition in the Weibull slope were also discussed, compared with the JIc value and the required conditions for δc or Jc prescribed in BSI and ASTM.The bilinear Weibull distributions experimentally obtained were also simulated by the analytical model, in which the Weibull stress criterion was applied to the stably extending crack tip. In spite of the simplified model with the radical approximation, the analytical model could describe the trend of the bilinear Weibull distribution.

Further study on the mechanism of the ductile-to-brittle fracture transition in C-Mn base and weld steel

In the present study, the crack opening displacement (COD) tests of specimens of C-Mn base and weld steel were carried out in the ductile-brittle transition temperature region. The majority of the specimens were fractured and others were unloaded prior to fracture after ductile fracture initiated and extended. The cavities and cleavage microcracks located in the vicinities of tips of fibrous cracks of the unloaded specimens were observed in detail. The finite element method (FEM) calculations of the stress and strain distribution ahead of the tip of an extending fibrous crack were completed. The mechanism of the ductile-to-brittle fracture transition was further investigated. It was revealed that in the ductile-brittle transition temperature region, the ductile fracture process was independent of temperature. The ductile-to-brittle fracture transition was triggered by initiating a catastrophic extension of a cleavage crack ahead of the fibrous crack tip, which occurred in a condition satisfying a combined criterion composed of three items, i.e., e p ≥ e pc for initiating a crack nucleus; σ m √σ ≥ T c for preventing the crack nucleus from blunting; and σ yy ≥ σ f for propagating the crack nucleus. For a specimen in which a fibrous crack occurred and propagated, the critical event for initiating a brittle cleavage fracture was the propagation of a ferrite grain-sized crack into neighboring grains. With extension of a fibrous crack, the behavior of the ductile-to-brittle fracture transition could be analyzed by the effect of the size of an “active zone” on the initiation of the brittle cleavage fracture.

Size Effect on Cleavage Crack Initiation Sites and Fracture Toughness. 1st Report. Result on Ductile-Brittle Transition Temperature Region of SM400C Steel.

Fracture toughness tests were performed at -50°C in the ductile-brittle transition temperature region for a low-strength steel SM400C using 0.3T, 0.5T, 0.8T and 1T compact specimens. All the locations of the cleavage crack initiation sites and fibrous crack shapes were investigated in detail from the fracture surface using by a scanning electron microscope and a binocular stereoscopic microscope. The probabilistic characteristics of cleavage fracture toughness and the thickness effect of cleavage crack initiation sites were analysed systematically. All cleavage crack initiation sites were found within a certain distance from the specimen surfaces. By locating all the sites and investigating fibrous crack shapes, the plane strain regions in specimens were deduced. The criterion for obtaining plane strain at the midplane of the specimens is given by B≥0.004(KC(J)/σys)2+0.01. This criterion indicates the specimen thickness necessary for the plane strain condition to be fairly smaller than the size requirements in ASTM Standard E399.

Brittle fracture initiation in low carbon steels at the ductile-brittle transition temperature region

The fracture process that determines the temperature dependence of toughness at the ductile-brittle transition was investigated for low carbon steels. It was revealed that the resistance against the stretch zone at the crack tip and the stable crack extensions is temperature independent while it depends on the carbon content. Applicability of elastic-plastic analyses of the crack tip fields was examined in terms of the relationship between J integral value normalized by the flow stress and crack opening displacement. Temperature dependence of toughness is determined primarily by the brittle fracture initiation. Locations of the initiation sites were revealed to coincide with the maximum stress triaxiality rather than the maximum tensile stress. It was suggested that the brittle fracture is caused by the deformation-induced initiation and triaxial stress assisted growth of an incipient crack.