Fracture Toughness Master Curve Research Articles

Influence of fatigue precracking and specimen size on Master Curve fracture toughness measurements of EUROFER97 and F82H steels

EUROFER97 and F82H steels are two leading reduced-activation ferritic-martensitic (RAFM) steels for fusion first wall and blanket applications. Exposure to the harsh environment of fusion reactors can result in severe degradation of fracture toughness. Thus, the post-irradiation evaluation of fracture toughness is critical for understanding the material behavior. Due to the space constraints of irradiation facilities and challenges in controlling a uniform irradiation condition for large size specimens, the development of small specimen test techniques (SSTT) is indispensable to evaluate the performance of irradiated materials. In this study, we evaluated specimen size effects on the Master Curve fracture toughness of EUROFER97 and F82H steels. A wide variety of specimens, including 0.5 T compact tension (C(T)) specimens, 0.16 T mini-compact tension (miniC(T)) specimens, and 1.65 mm miniature bend bar specimens, were tested. The testing methodology was based on the Master Curve method in the ASTM E1921 standard. No specimen size effect was observed in 0.5 T C(T) and 0.16 T miniC(T) specimens on the Master Curve reference temperature T0, while 1.65 mm miniature bend bar specimens yielded a higher T0Q. A strong effect of fatigue precracking on T0 for 0.5 T C(T) and 0.16 T miniC(T) specimens was observed, such that testing on specimens with skewed fatigue precrack fronts resulted in lower T0 than for specimens with ASTM standard qualified straight fatigue precrack fronts. The results highlight the importance of experimental quality control in developing SSTT for Master Curve fracture toughness testing. Lastly, we also evaluated and provided recommendations on the minimum number of specimens needed for each specimen type for yielding reliable T0Q values.

Time-temperature superposition principle for the shear fracture behaviour of soft adhesive layers: From bulk to interface

Soft adhesive layers have attracted great attention as a commonly used adhesive technique, particularly because of their shear fracture behaviour at extreme loading rates and temperatures. Therefore, the present study examined the shear fracture properties of a soft adhesive layer through systematic rheological experiments, loading-unloading shear tests and shear fracture tests. Due to a similar viscoelastic characteristic in rheological properties and fracture properties, the time-temperature superposition principle (TTSP) was extended from the bulk to the interface to investigate its shear cohesive failure. A shear fracture toughness master curve was defined using the time-temperature equivalence shift factors derived from the rheological properties. The shear fracture behaviour of the soft adhesive layer could be predicted using data from a narrow range of loading rates/temperatures. The results demonstrated the potential of extending the TTSP approach from the bulk to a soft adhesive layer having viscoelastic character, which might be utilised to predict the shear fracture toughness of soft adhesive materials under extreme conditions.

Data‐driven approach to construct a fracture toughness master curve for ferritic steels based on tensile properties

Data‐driven approach to construct a fracture toughness master curve for ferritic steels based on tensile properties

Characterization of fracture toughness based on yield stress and successful application to construct a lower-bound fracture toughness master curve

The fracture toughness KJc of ferritic steels in the ductile-to-brittle transition region is important for analyzing the structural integrity of structures subjected to large temperature variations. In this study, a σYS-based lower-bound (LB) KJc master curve (MC) was developed to overcome the failure of the ASTM E1921 MC to predict KJc above room temperature (RT). The characteristic KJc, was first identified after finding KJc for the load P5, which is the upper limit load for KIc defined in ASTM E399. It can be approximated as KL = 0.6σYS (W)1/2 for various materials and specimen types. Then, the characteristic σYS was selected to be σYSRT/σYS. Assuming that KJc > KLRT at RT, KLRT was assumed to give an LB, which is empirically true for materials tested in accordance with ASTM E1921. Finally, the MC was formulated as KJcLB = KLRT (σYSRT/σYS)5.5, by applying the critical distance scaling method. KJcLB was successfully a LB for 715 unirradiated KJc data points, including eight heats of reactor pressure vessel (RPV) steel and seven heats of non-RPV steel. This MC can be constructed without conducting fracture toughness tests if σYSRT is known, because a temperature-based σYS MC exists.

Master Curve of 20MnMoNi55 Steel From Miniature CT Specimens

The steels used for manufacturing nuclear reactor pressure vessel are low alloy ferritic steels. There is a range of temperatures at which these steels exhibit transition from ductile to brittle fracture known as the ductile to brittle transition temperature (DBTT). In these range of temperatures, ferritic steels exhibit a scatter in the fracture toughness values having a characteristic statistical distribution which is unique to ferritic steels. The master curve methodology aims to capture this behaviour of ferritic steels through fracture mechanics principles. To obtain master curve, fracture experiments of standard specimens as per ASTM E-1921 must be carried out. Since reactor pressure vessel steels suffer loss in ductility due to irradiation embrittlement during its service life, master curve needs to be generated at periodic intervals using surveillance specimens. Use of standard sized fracture specimens as surveillance specimens are difficult as the space available in a nuclear reactor for keeping these specimens are limited. Also, the radiation dose associated with testing standard sized irradiated specimens can be dangerous for the personnel involved. Under these circumstances, it becomes necessary to carry out such tests using miniaturized specimens. In this work, we will obtain the fracture toughness master curve of 20MnMoNi55 steel (low alloy ferritic steel used for making nuclear reactor pressure vessels) from miniature CT specimens having a thickness of 4 mm.

Modification of fracture toughness Master Curve considering the crack-tip Q-constraint

Engineering structures may have significant lower crack-tip constraint compared with those in the standard fracture toughness test specimens. This paper considers a methodology to extend the application of fracture toughness master curve approach for structures with low crack-tip constraint. This is conducted by considering the Q-parameter as a suitable crack-tip constraint parameter, and study its effects on the reference temperature T0 in the master curve approach. The experimental fracture toughness data from the literature as well as supplemental experiments are used for this purpose. The relation between Q and T0 is investigated by using numerical analyses of experiments covering various types of geometry, loading condition, thickness and crack depth. The effects of loading amplitudes and material properties on the relation between Q and T0 are also studied. Based on this study, a relationship between Q and T0 is derived and experimentally verified. It is shown that the proposed relationship has a considerable ability to predict the apparent fracture toughness behavior in the ductile to brittle region of complex structures without need of conducting additional experimental fracture toughness tests.

Evaluation of Fracture Toughness by Master Curve Approach Using Miniature C(T) Specimens

The fracture toughness master curve shows the 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. The master curve approach specified in the ASTM standard theoretically provides the confidence levels of fracture toughness in consideration with the inherent scatter of fracture toughness. The authors have conducted several fracture toughness tests for typical Japanese RPV steels with various specimen sizes and shapes and ascertained that the master curve can be accurately applied to the specimens with a thickness of 0.4-in. or larger. With respect to using the master curve method with the current surveillance program for operating RPVs, the utilization of miniature specimens is important. Miniature specimens, which can be taken from the broken halves of surveillance specimens, are necessary for the efficient determination of the master curve from the limited volume of the available materials. In this study, fracture toughness tests were conducted for typical Japanese RPV steels, particularly SFVQ1A forged and SQV2A plate materials, using the miniature C(T) specimens with a thickness of 4 mm, following the procedure in the ASTM standard. The results show that the differences in the 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 the standard and larger C(T) specimens. It was also found that valid reference temperatures can be determined with a realistic number of miniature C(T) specimens, i.e., less than ten, if the test temperature was appropriately selected. Thus, the master curve method using miniature C(T) specimens could be a practical method to determine the fracture toughness of actual RPV steels.

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.

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.

Mechanical properties of irradiated ODS-EUROFER and nanocluster strengthened 14YWT

Irradiations to 1.5 dpa at 300–750 °C were conducted to investigate the changes in mechanical properties of an advanced nanocluster strengthened ferritic alloy, designated 14YWT, and an oxide dispersion strengthened ferritic alloy ODS-EUROFER. Two non-dispersion strengthened variants, 14WT and EUROFER 97, were also irradiated and tested. Tensile results show 14YWT has very high tensile strengths and experienced some radiation-induced hardening, with an increase in room temperature yield strength of 125 MPa after irradiation, while results for ODS-EUROFER show a 275 MPa increase following irradiation. Master curve fracture toughness analysis show 14YWT has a cryogenic T o reference temperatures before and after irradiation of about −188 and −176 °C, respectively, and upper-shelf K J Ic values between 175 and 225 MPa√m. The favorable fracture toughness properties and resistance to radiation-induced changes in mechanical properties observed for 14YWT are attributed to a fine grain structure and high number density of Y–Ti–O nanoclusters.

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.

Applicability of the Fracture Toughness Master Curve to Irradiated Highly Embrittled Steel and Intergranular Fracture

Abstract The Heavy-Section Steel Irradiation Program at Oak Ridge National Laboratory has evaluated a submerged-arc (SA) weld irradiated to a high level of embrittlement and a temper embrittled base metal that exhibits significant intergranular fracture relative to representation by the Master Curve. The temper embrittled steel revealed that the intergranular mechanism significantly extended the transition temperature range up to 150°C above To. For the irradiated highly embrittled SA weld study, a total of 21 1T compact specimens were tested at five different temperatures and showed the Master Curve to be nonconservative relative to the results, although that observation is uncertain due to evidence of intergranular fracture.

Attenuation of Neutron Radiation Damage Through a Simulated RPV Wall

Abstract An experiment has been conducted in which a 180-mm thick reactor pressure vessel (RPV) wall has been simulated using eighteen 10-mm slices and irradiated under test reactor conditions to investigate the through wall attenuation of neutron embrittlement. Attenuation of neutron radiation damage through the wall of an RPV is a process that involves a changing neutron flux spectrum. The effect of the changing spectrum has not been fully studied to define the change in fracture toughness properties through the RPV wall. One low copper content base metal and one high copper content Linde 80 weld metal have been irradiated in various positions through the simulated wall to allow quantification of an improved experimentally-based embrittlement attenuation model. Comparisons are made of predicted attenuation changes in toughness properties with measured fracture toughness and Charpy V-notch results for the high copper content weld metal and the low copper content plate. The predictions of through-wall attenuation follow the practice defined in ASTM E 900-02, in which the attenuation of high energy neutron fluence (E>1 MeV) is projected based upon displacements per atom (dpa) change through the wall thickness. The resultant degree of material damage (Charpy V-notch 41 J transition temperature, T41J) using this dpa-based fluence change is estimated also using the ASTM E 900-02 embrittlement model. The irradiation-induced shift in T41J (ΔT41J) is typically assumed to infer the shift in fracture toughness transition temperature to be used for structural integrity assessments for the reactor pressure vessel. This assumption will be checked by measuring Master Curve fracture toughness properties for the high copper content weld metal and the low copper content plate.

Evaluation of fracture toughness master curve shifts for JMTR irradiated F82H using small specimens

Small to ultra-small 1/3 size pre-cracked Charpy and 1.65 × 1.65 × 9 mm deformation and fracture minibeam (DFMB) specimens of the F82H IEA heat were irradiated to 0.02 and 0.12 dpa at 290 °C in the Japanese Materials Test Reactor. Nominal cleavage transition temperature shifts, based on the measured toughness, K Jm( T), data (Δ T m) as well as reference temperature shifts (Δ T 0) found after size-adjusting the K Jm( T) data yielded Δ T m/0 ≈ 27 ± 10 and 44 ± 10 at the two doses, respectively. Using measured yield stress changes (Δ σ y), the C 0 = Δ T 0/Δ σ y = 0.58 ± 0.14 at 0.12 dpa, is in good agreement with data in the literature. The dynamic transition temperature shift, Δ T d, derived from DFMB tests, was ≈30 ± 20 °C at 0.1 dpa, also in good agreement with the estimated Δ T 0 shifts. The Δ T d and Δ T 0 are also in excellent agreement with a Δ T 0 = C 0Δ σ y (dpa, T i) hardening-shift model, where the Δ σ y (dpa, T i) was found by fitting a large database of tensile properties.

Evaluation of ductile–brittle transition temperature before and after neutron irradiation for RPV steels using small punch tests

Small punch (SP) tests were performed to evaluate the ductile–brittle transition temperature before and after a neutron irradiation of reactor pressure vessel (RPV) steels produced by different manufacturing (refining) processes. The results were compared to the standard transition temperature shifts from the conventional Charpy tests and the Master Curve fracture toughness tests in accordance with the American Society for Testing and Materials (ASTM) standard E1921. Small punch specimens were taken from a 1/4 t location of the vessel thickness and machined into a 10 mm × 10 mm × 0.5 mm dimension. The specimens were irradiated in the research reactors at Korea Atomic Energy Research Institute Nuclear Research Institute in the Czech Republic at the different fluence levels of about 290 °C. Small punch tests were performed in the temperature range of RT to −196 °C using a 2.4 mm diameter ball. For the materials before and after irradiation, the small punch transition temperatures ( T SP), which are determined at the middle of the upper small punch energies, showed a linear correlation with the Charpy index temperature, T 41 J . T SP from the irradiated samples was increased with the fluence levels and was well within the deviation range of the unirradiated data. However, the transition temperature shift from the Charpy test (Δ T 41 J ) shows a better correlation with the transition temperature shift (Δ T SP( E) ) when a specific small punch energy level rather than the middle energy level of the small punch curve is used to determine the transition temperature. T SP also had a correlation with the reference temperature ( T 0) from the Master Curve method using a pre-cracked Charpy V-notched (PCVN) specimen.

Fracture Toughness and Atom Probe Characterization of a Highly Embrittled RPV Weld

Abstract The Heavy-Section Steel Irradiation Program at Oak Ridge National Laboratory (ORNL) includes a task to investigate the shape of the fracture toughness master curve for reactor pressure vessel steel highly embrittled as a consequence of irradiation exposure. A radiation-sensitive reactor pressure vessel (RPV) weld with intentionally enhanced copper content, designed KS-01, is characterized in terms of static initiation (KIc, KJc) and Charpy impact toughness in the unirradiated and irradiated conditions. The objective of this project is to investigate the ability of highly embrittled material to maintain the shape of the unirradiated transition fracture toughness curve, as well as to examine the ability of the Charpy 41-J shift to predict the fracture toughness shift at such a high level of embrittlement. Irradiation of this weld was performed at the University of Michigan Ford Reactor. Specimens of KS-01 weld were irradiated to about 0.74 × 1019 neutron/cm2 at 288°C. Irradiation resulted in Charpy ductile-to-brittle transition temperature (DBTT) shift of 169°C. It was anticipated that this shift would result in a fracture toughness transition temperature (at 100 MPa√m) in the irradiated condition near or slightly above the pressurized thermal shock screening criterion for weld metals, ToPTS = 129°C. The fracture toughness characterization of KS-01 weld in the unirradiated and irradiated conditions was mainly performed by testing 1T C(T), although some 0.5T C(T) and precracked Charpy specimens were used in this study. The master curve analysis showed that this material exhibited shift of reference fracture toughness transition temperature, To, of 165°C as result of radiation, which is in remarkable agreement with Charpy DBTT shift. The absolute value of To in the irradiated condition was determined to be equal to 139°C. This weld exhibited a low ductile initiation toughness (JQ) after irradiation. It left a relatively narrow temperature window to examine the shape of the transition region. Irradiated median fracture toughness values up to 148 MPa√m follow the master curve shape. However, low toughness brittle fractures occurred at temperatures further above To (To + 61°C) than expected with a leveling of the KJc data from the master curve shape. The microstructure of the weld was characterized with the ORNL's energy-compensated optical position-sensitive atom probe. Atom probe tomography revealed a high number density (∼3 × 1024 m−3) of Cu-, Mn-, Ni-, Si-, and P-enriched precipitates and a lower number density (∼1 × 1023 m−3) of P clusters.

Application of Master Curve fracture toughness for reactor pressure vessel integrity assessment in the USA

The Master Curve fracture toughness approach has been used in the USA for better defining the transition temperature fracture toughness of irradiated reactor pressure vessel (RPV) steels for end-of-life (EOL) and EOL extension (EOLE) time periods. The first application was for the Kewaunee plant in which the life-limiting material was a circumferential weld metal. Fracture toughness testing of this weld metal corresponding to EOL and beyond EOLE was used to reassess the PTS screening value, RT PTS, and to develop new operating pressure–temperature curves. The NRC has approved this application using a shift-based methodology and higher safety margins than those proposed by the utility and its contractors. Beaver Valley Unit 1, a First Energy nuclear plant, has performed similar fracture toughness testing, but none of the testing has been conducted at EOL or EOLE at this time. Therefore, extrapolation of the life-limiting plate data to higher fluences is necessary, and the projections will be checked in the next decade by Master Curve fracture toughness testing of all of the Beaver Valley Unit 1 beltline materials (three plates and three welds) at fluences near or greater than EOLE. A supplemental surveillance capsule has been installed in the sister plant, Beaver Valley Unit 2, which has the capability of achieving a higher lead factor while operating under essentially the same environment. The Beaver Valley Unit 1 evaluation has been submitted to the NRC. This paper reviews the shift-based approach taken for the Beaver Valley Unit 1 RPV and presents the use of the RT T 0 methodology (which evolved out of the Master Curve testing and endorsed through two ASME Code Cases). The applied margin accounts for uncertainties in the various material parameters. Discussion of a direct measurement of RT T 0 approach, as originally submitted for the Kewaunee case, is also presented.

The effect of non-metallic inclusions on the fracture toughness master curve in high copper reactor pressure vessel welds

The fracture toughness of two high copper reactor pressure vessel welds having low upper shelf energy was evaluated in accordance with the master curve method of ASTM E1921. The resultant data were correlated to the metallurgical factors involved in the brittle fracture initiation to provide a metallurgical-based understanding of the master curve. The tests were performed using pre-cracked Charpy V-notched specimens and the master curve was made with an average of T 0 values determined at different temperatures. In all specimens, the cleavage fracture initiated at non-metallic inclusion ranging from 0.7 to 3.5 μm in diameter showing a scatter with the specimens and testing temperatures. Temperature dependency of the triggering particle size was not found. The fracture toughness ( K J C ) was inversely proportional to the square root of the triggering inclusion diameter ( d i ) at respective temperatures. From this relationship, we determined median K J C values which correspond to the average value of triggering inclusion diameter of all tested specimens and defined them as a modified median K J C ( K ′ J C(med) ). The obtained K ′ J C(med) values showed quite smaller deviation from the master curve at different temperatures than the experimental median K J C values. This suggests that the master curve is on the premise of a constant dimension of key microstructural factor in a material regardless of the testing temperature. But the inclusion size at trigger point played an important role in the absolute position of the master curve with temperature and the consequent T 0 value.

Application of the Master Curve in the ASME Code

The ASME Code reference toughness curves are based upon an approach that utilizes a material normalizing and indexing parameter, RT NDT. This parameter is based upon the results from Charpy V-notch and drop weight nil-ductility transition temperature tests as defined in the Code. In many cases, this indexing parameter is overly conservative relative to the real toughness of ferritic pressure vessel steels, and a more direct index and measure of the true fracture toughness is needed. The American Society for Testing and Materials (ASTM) recently documented Standard E1921-97 for the determination of a directly measured toughness temperature index, T 0, through use of the Master Curve approach. Efforts are underway in the ASME Code to utilize the Master Curve approach, and the determination of T 0, to provide an alternative definition to the existing reference temperature, RT NDT, and statistically defined fracture toughness curve(s) for unirradiated and irradiated ferritic pressure vessel steels. In the technical basis document, published by EPRI, the Master Curve fracture toughness method is profiled as a better indicator of fracture toughness behavior in terms of a directly measured toughness temperature index. The document also provides technical arguments in support of using the Master Curve fracture toughness approach for defining a new fracture toughness-based reference temperature, RT T0 . This paper discusses the ASME Code efforts to utilize the Master Curve approach and outlines the technical basis for determination of an alternative reference temperature, RT T0 , for use in reactor pressure vessel integrity assessment.

Effect Of Orientation And Loading Rate On The Toughness Transition CurveOf A Ship Steel

Toughness measurements are needed to assess the risk of brittle fracture in warship hulls under operational conditions. This demands laboratory testing at minimum service temperatures and loading rates equivalent to hull impact events. However plate thickness is insufficient to permit valid Kjc determinations at the required strength and toughness levels. The master curve approach specified in ASTM Standard El921 provides an alternative means of quantifying the cleavage fracture resistance in the brittle-to-ductile transition temperature regime. Limited replicate testing yields a reference temperature T<,. It also defines a material-specific fracture toughness master curve, and a lower bound failure probability, which can be applied to material selection and structural integrity analyses. The determination of TO and master curves for a 400 MPa yield strength steel plate is described. Variations in specimen orientation and test loading rate have been investigated, and comparisons are made between the reference temperature results and those from other standardized fracture tests, ft is concluded that the master curve approach is a useful way of quantifying hull toughness for warship fracture control.