Validity of the Master Curve Temperature Dependence Assumption for Highly Embrittled RPV Materials: Results From the IAEA Coordinated Research Project (CRP-8)

2009 ◽  
Author(s):  
Tapio Planman ◽  
Kunio Onizawa ◽  
William Server
Keyword(s):  
Fracture Toughness ◽  
Temperature Dependence ◽  
Master Curve ◽  
Shape Change ◽  
Transition Curve ◽  
Curve Shape ◽  
Topic Area ◽  
Material Inhomogeneity ◽  
Realistic Description ◽  
Definition Of

The fracture toughness temperature dependence (transition curve shape) has been discussed almost since the original empirical definition of the curve in 1991. The data sets showing anomalous fracture behaviour of highly irradiated VVER-1000 pressure vessel steels presented in 2000’s have further enhanced this discussion and even a special model has been proposed for highly irradiated steels, including a mathematical definition of the curve shape change. Although in most cases the standard Master Curve (MC) approach, assuming a constant transition curve shape, has proven to give a realistic description for also highly irradiated ferritic steels, there are grades which show for example abnormally weak temperature dependence. In these cases, however, an obvious reason for the behaviour may be that the material does not fail by the mechanism assumed in the MC model. The fracture toughness data collected and analysed in the CRP-8 Topic Area 3 supports the validity of the curve shape assumption of ASTM E1921 also in case of irradiated steels and gives no rise to change the present definition. The Master Curve C-parameter (the shape parameter) estimation is proposed as an appropriate analysis method when there is need to estimate also the temperature dependence, whereas the SINTAP procedure is recommended for ensuring conservative lower bound estimates when material inhomogeneity is suspected. The results show that irradiation may slightly lower the fracture toughness in the upper transition region in relation to that predicted by ASTM E1921, but the effect after moderate T0 shift values (up to about 100°C) seems to be negligible. The investigated steels exhibit no or very weak correlation between the C-parameter and T0.

IAEA Coordinated Research Project on Master Curve Approach to Monitor Fracture Toughness of RPV Steels: Applicability for Highly Embrittled Materials

2007 ◽  
Author(s):  
Tapio Planman ◽  
Kunio Onizawa ◽  
William Server ◽  
Stan Rosinski
Keyword(s):  
Fracture Toughness ◽  
Temperature Dependence ◽  
Fracture Mode ◽  
Master Curve ◽  
Material Behavior ◽  
Curve Shape ◽  
Inhomogeneous Materials ◽  
Research Project ◽  
Energy Agency ◽  
Transition Range

In the Master Curve (MC) fracture model, a universal temperature dependence is assumed for ferritic steels, including those used in reactor pressure vessel (RPV) applications. The assumed curve shape also has been observed to be generally valid for highly irradiated or thermally aged materials that exhibit a high value of reference transition temperature, To. Lower than predicted fracture toughness behavior occasionally has been observed, however, in the upper transition range. It has been suggested that this behavior possibly may be associated with a lowered upper shelf toughness due to high irradiation doses. One objective of the present International Atomic Energy Agency (IAEA) Coordinated Research Project 8 (CRP-8) is to clarify the MC shape issue by collecting and analyzing relevant fracture toughness data measured on irradiated (or thermally aged) RPV and corresponding steels. For thermally aged or highly irradiated materials the fracture mode typically tends to gradually change from cleavage to one of intergranular fracture (IGF) which, if the IGF proportion is high, may significantly affect both the scatter and temperature dependence of fracture toughness. The data reviewed to date in this CRP show, in general, a very consistent fracture behavior with the basic Master Curve model that further confirms the applicability of the assumed curve shape. In cases where the basic homogeneity or fracture mode assumptions of the MC model were not satisfied due to high proportions of IGF, correspondence with the measured and predicted behavior could be markedly improved by applying available models developed to address inhomogeneous materials (e.g., SINTAP or the multi-modal model). The onset of upper shelf (TUS) and its correlation with To is presented as a possible approach for characterizing material behavior in the upper transition region when sufficient upper shelf fracture toughness data are available.

Remarks to the Upper Shelf Master Curve Concept

2008 ◽  
Author(s):  
Jiri Novak
Keyword(s):  
Fracture Toughness ◽  
Temperature Dependence ◽  
Ductile Fracture ◽  
Master Curve ◽  
Critical Strain ◽  
Peierls Stress ◽  
Transition Curve ◽  
Dislocation Slip ◽  
Ferritic Steels ◽  
Deformation Work

Recently, several works appeared in which temperature dependence of ductile fracture toughness of ferritic steels on the upper shelf of brittle-ductile transition curve was analyzed and Upper Shelf Master Curve concept was formulated. Generally, fracture toughness at different temperatures characterized by JIc or dJ/da should be proportional to the deformation work of unit volume evaluated from zero to the critical strain for ductile fracture. As in many other cases, critical strain for ductile fracture initiation may be identified with critical strain for initiation of shear bands, calculated for hyperelastic material with the corresponding stress-strain curve. This concept is successful, among others, in determination of the temperature dependence of fracture toughness of ferritic steels on the upper shelf. Two most important physical mechanisms controlling temperature dependence of constitutive behaviour of ferritic steels in the corresponding temperature range, hence the temperature dependence of both deformation work to initiation of ductile fracture and fracture toughness, are friction resistance to dislocation slip (Peierls stress) and dynamic recovery (dislocation annihilation). Predicted Upper Shelf Master Curve shape based on temperature dependence of constitutive parameters of different ferritic steels corresponds well to the published data.

Prediction of Fracture Toughness for WWER RPV Integrity Assessment on the Basis of the Unified Curve Approach and Surveillance Specimens Testing

2009 ◽  
Author(s):  
Boris Margolin ◽  
Victoria Shvetsova ◽  
Alexander Gulenko ◽  
Valentin Fomenko
Keyword(s):  
Fracture Toughness ◽  
Master Curve ◽  
Present Report ◽  
Temperature Shift ◽  
Curve Shape ◽  
Integrity Assessment ◽  
Master Curve Method ◽  
Curve Method ◽  
Unified Curve ◽  
Surveillance Specimens

For construction of the fracture toughness temperature curve that may be used for WWER RPV integrity assessment on the basis of tests of cracked surveillance specimens, the issues have to be solved as follows. First of all, it is important to determine how fracture toughness varies as a function of temperature, and how the fracture toughness vs. temperature dependence, KJC(T), changes with in-service material degradation due to neutron irradiation. These variations of KJC(T) curve are known to be the shift of KJC(T) curve to higher temperature range and change in the KJC(T) curve shape. At present, two advanced engineering methods are known that allow the prediction of KJC(T) curve on the basis of small-size fracture toughness specimens (for example, pre-cracked Charpy specimens), namely, the Master Curve and the Unified Curve methods. Procedures of test result treatment for the Master Curve and the Unified Curve are very similar. The Master Curve method uses the lateral temperature shift condition and, therefore, does not describe possible change in the KJC(T) curve shape. The Unified Curve method has an advantage as compared with the Master Curve as the Unified Curve describes a variation of the KJC(T) curve shape when degree of embrittlement increases. This advantage becomes important for RPV integrity assessment when the reference KJC(T) curve is recalculated to the crack front length of the postulated flaw that is considerable larger than thickness of surveillance specimens. Application of the KJC(T) curve determined from test results of cracked surveillance specimens to RPV integrity assessment requires also to introduce some margins. These margins have to take into account the type and number of tested specimens and the uncertainty connected with spatial non-homogeneity of RPV materials. Indeed, there is sufficient number of experimental data showing variability in fracture toughness for various parts of RPV. Therefore, situation is possible when the material properties near the postulated flaw will be worse than the properties of surveillance specimens. In the present report, advanced approaches are considered for prediction of fracture toughness for WWER RPV integrity assessment that allow one: • to construct the KJC(T) curve for irradiated RPV steels with any degree of embrittlement; • to provide transferability of fracture toughness data from cracked surveillance specimens to calculation of resistance to brittle fracture of RPV with a postulated flaw.

Evaluation of Fracture Toughness of a SA580 Gr. 3 Reactor Pressure Vessel Using a Bimodal Master Curve Approach

2019 ◽  
Vol 141 (6) ◽  
Author(s):  
Jong-Min Kim ◽  
Seok-Min Hong ◽  
Min-Chul Kim ◽  
Bong-Sang Lee
Keyword(s):  
Fracture Toughness ◽  
Cooling Rate ◽  
Pressure Vessel ◽  
Master Curve ◽  
Random Parameter ◽  
Random Inhomogeneity ◽  
Reactor Pressure Vessel ◽  
Inhomogeneous Materials ◽  
Material Inhomogeneity ◽  
Reactor Pressure

Abstract The standard master curve (MC) approach has a major limitation in that it is only applicable to homogeneous datasets. In nature, steels are macroscopically inhomogeneous. Reactor pressure vessel (RPV) steel has different fracture toughness with varying distance from the inner surface of the wall due to the higher cooling rate at the surface (deterministic material inhomogeneity). On the other hand, the T0 value itself behaves like a random parameter when the datasets have large scatter because the datasets are for several different materials (random inhomogeneity). In this paper, four regions, the surface, 1/8 T, 1/4 T, and 1/2 T, were considered for fracture toughness specimens of Korean Standard Nuclear Plant (KSNP) SA508 Gr. 3 steel to provide information on deterministic material inhomogeneity and random inhomogeneity effects. Fracture toughness tests were carried out for the four regions at three test temperatures in the transition region and the microstructure of each region was analyzed. The amount of upper bainite increased toward the center, which has a lower cooling rate; therefore, the center has lower fracture toughness than the surface so reference temperature (T0) is higher. The fracture toughness was evaluated using the bimodal master curve (BMC) approach. The results of the BMC analyses were compared with those obtained via a conventional master curve analyses. The results indicate that the bimodal master approach considering inhomogeneous materials provides a better description of scatter in the fracture toughness data than a conventional master curve analysis does.

Assessment of the Temperature Dependence of Ferritic Steel Fracture Toughness on or Near the Lower Shelf

2015 ◽  
Author(s):  
Mark Kirk ◽  
Marjorie Erickson
Keyword(s):  
Fracture Toughness ◽  
Temperature Dependence ◽  
Cleavage Fracture ◽  
Master Curve ◽  
Companion Paper ◽  
Structural Factors ◽  
Temperature Trends ◽  
Current Guidance ◽  
American Society ◽  
Current Code

Within the American Society of Mechanical Engineers (ASME) the Section XI Working Group on Flaw Evaluation (WGFE) is currently working to develop a revision to Code Case N-830. This revision incorporates a complete and self-consistent suite of models that describe completely the temperature dependence, scatter, and interdependencies between all the fracture metrics (i.e., KJc, KIa, JIc, J0.1, and J-R) from the lower shelf through the upper shelf. A paper presented at the 2014 ASME Pressure Vessel and Piping Conference described most of these models; a companion paper at this conference describes the J-R model. This paper also supports the WGFE effort by performing an assessment of the appropriateness of Wallin’s Master Curve model to represent toughness data on the lower shelf, and by comparing the Master Curve with the current Code KIc curve on the lower shelf. The work presented in this paper supports the following conclusions: 1. The Master Curve provides a reasonable representation of cleavage fracture toughness (KJc) data at lower shelf temperatures. A statistical evaluation of a large database demonstrates that the Master Curve works well to temperatures approximately 140 °C below To or, equivalently, approximately 160 °C below RTTo. 2. The percentile of cleavage fracture toughness data falling below a KIc curve indexed to RTTo varies considerably with temperature. At lower shelf temperatures as much as half of the data lie below the KIc curve, while at temperatures close to RTTo this percentage falls to approximately ≈ 1.5%. The current guidance of Nonmandatory Appendix A to Section XI to use structural factors of √10 or √2 is one means of addressing this inconsistency. 3. The inconsistent degree to which the KIc curve, with or without structural factors, bounds fracture toughness data cannot be fixed within the current Code framework for two reasons: the KIc curve does not reflect the actual temperature dependence shown by the fracture toughness of ferritic RPV steels, and the ratio of a mean or median toughness curve to a fixed percentile bound is not a constant value. It is for these reasons that in the next revision of Code Case N-830 the ASME WGFE is moving away from use of the KIc curve coupled with structural factors and, instead, is adopting models of fracture toughness that represent both the temperature trends and the scatter in the data with high accuracy.

IAEA Coordinated Research Project on Master Curve Approach to Monitor Fracture Toughness of RPV Steels: Final Results of the Experimental Exercise to Support Constraint Effects

2009 ◽  
Author(s):  
Randy K. Nanstad ◽  
Milan Brumovsky ◽  
Rogelio Herna´ndez Callejas ◽  
Ferenc Gillemot ◽  
Mikhail Korshunov ◽  
...  
Keyword(s):  
Fracture Toughness ◽  
Master Curve ◽  
Reference Temperature ◽  
Compact Specimen ◽  
Bias Effect ◽  
Research Project ◽  
Topic Area ◽  
Energy Agency ◽  
Experimental Part

The precracked Charpy single-edge notched bend, SE(B), specimen (PCC) is the most likely specimen type to be used for determination of the reference temperature, T0, with reactor pressure vessel (RPV) surveillance specimens. Unfortunately, for many RPV steels, significant differences have been observed between the T0 temperature for the PCC specimen and that obtained from the 25-mm thick compact specimen [1TC(T)], generally considered the standard reference specimen for T0. This difference in T0 has often been designated a specimen bias effect, and the primary focus for explaining this effect is loss of constraint in the PCC specimen. The International Atomic Energy Agency (IAEA) has developed a coordinated research project (CRP) to evaluate various issues associated with the fracture toughness Master Curve for application to light-water RPVs. Topic Area 1 of the CRP is focused on the issue of test specimen geometry effects, with emphasis on determination of T0 with the PCC specimen and the bias effect. Topic Area 1 has an experimental part and an analytical part. Participating organizations for the experimental part of the CRP performed fracture toughness testing of various steels, including the reference steel JRQ (A533-B-1) often used for IAEA studies, with various types of specimens under various conditions. Additionally, many of the participants took part in a round robin exercise on finite element modeling of the PCVN specimen, discussed in a separate paper. Results from fracture toughness tests are compared with regard to effects of specimen size and type on the reference temperature T0. It is apparent from the results presented that the bias observed between the PCC specimen and larger specimens for Plate JRQ is not nearly as large as that obtained for Plate 13B (−11°C vs −37°C) and for some of the results in the literature (bias values as much as −45°C). This observation is consistent with observations in the literature that show significant variations in the bias that are dependent on the specific materials being tested. There are various methods for constraint adjustments and two methods were used that reduced the bias for Plate 13B from −37°C to −13°C in one case and to − 11°C in the second case. Unfortunately, there is not a consensus methodology available that accounts for the differences observed with different materials. Increasing the Mlim value in the ASTM E-1921 to ensure no loss of constraint for the PCC specimen is not a practicable solution because the PCC specimen is derived from CVN specimens in RPV surveillance capsules and larger specimens are normally not available. Resolution of these differences are needed for application of the master curve procedure to operating RPVs, but the research needed for such resolution is beyond the scope of this CRP.

Application of Master Curve Approach to Surveillance Test Data for WWER-1000 Reactor Pressure Vessels

2017 ◽  
Author(s):  
Volodymyr M. Revka ◽  
Liudmyla I. Chyrko
Keyword(s):  
Fracture Toughness ◽  
Weld Metal ◽  
Test Data ◽  
Master Curve ◽  
Pressure Vessels ◽  
Reference Temperature ◽  
Transition Curve ◽  
Surveillance Program ◽  
Data Set ◽  
Reactor Pressure

An important issue in the safety operation of WWER-1000 type reactor is a decrease in fracture toughness for reactor pressure vessel steels due to neutron irradiation. This effect for RPV metal is known as radiation embrittlement. The radiation induced temperature shift of the fracture toughness transition curve is considered as a measure of the embrittlement rate. The Charpy impact and fracture toughness specimens are included in the surveillance program for an assessment of changes in fracture toughness of RPV materials. The present analysis is based on a large data set which includes mostly experimental results for pre-cracked Charpy specimens from a WWER-1000 RPV surveillance program. A Master curve approach is applied to analyze the surveillance test data with respect to a shape of the fracture toughness transition curve and a scatter of KJC values. The RPV base and weld metal in unirradiated, thermally aged and irradiated conditions are considered in this study. The maximum shift in a reference temperature T0 due to irradiation is 107 degree Celsius. It is shown that the Master curve, 5 % and 95 % tolerance bounds describe adequately the temperature dependence and the statistical scatter of KJC values for WWER-1000 RPV steels both in unirradiated condition and after irradiation up to design as well as long term operation neutron fluence. Furthermore, a development of the Weibull plots for considered data sets is shown that the Weibull slope is close to the expected one of 4 on average. Finally, a comparison of the reference temperature T0 and a scatter of KJC values derived from the pre-cracked Charpy and 0,5T C(T) specimens of base and weld metal in unirradiated condition is done. The analysis has shown a significant discrepancy between the T0 values derived from the two different types of specimens for both RPV metals.

Solution of a Sample Problem Related to Revision 1 of Code Case N-830

2017 ◽  
Author(s):  
Mark Kirk ◽  
Steven Xu ◽  
Cheng Lui ◽  
Marjorie Erickson ◽  
Yil Kim ◽  
...  
Keyword(s):  
Fracture Toughness ◽  
Temperature Dependence ◽  
Master Curve ◽  
Nuclear Reactor ◽  
Sample Problem ◽  
Asme Code ◽  
American Society ◽  
Technical Basis ◽  
Direct Use ◽  
Current Code

Within the American Society of Mechanical Engineers (ASME) the Section XI Working Group on Flaw Evaluation (WGFE) is currently working to develop a revision to Code Case (CC) N-830. CC N-830 permits the direct use of fracture toughness in flaw evaluations as an alternative to the indirect/correlative approaches (RTNDT-based) traditionally used in the ASME Code. The current version of N-830 estimates allowable fracture toughness values in the transition regime as the 5th percentile Master Curve (MC) indexed to the transition temperature T0. The proposed CC N-830 revision expands on this capability by incorporating a complete and self-consistent suite of models that describe completely the temperature dependence, scatter, and interdependencies between all fracture metrics (i.e., KJc, KIa, JIc, J0.1, and J–R) used currently, or useful in, a flaw evaluation for conditions ranging from the lower shelf through the upper shelf. Papers presented in previous ASME Pressure Vessel and Piping (PVP) Conferences since 2014 provide the technical basis for these various toughness models. This paper contributes to this overall CC N-830 documentation suite by presenting the results of a sample problem run to assess the proposed revision of the CC. The objective of the sample problem was (1) to determine if the revised CC was written with adequate clarity to permit different engineers to accurately and consistently calculate the various allowable toughness values described by the equations in the CC, (2) to assess how these allowable toughness values would be used to calculate allowable flaw depths using standard ASME SC-XI approaches, and (3) to compare allowable flaw depths calculated using established Code practices (RTNDT-based) to those calculated using proposed CC practices (T0-based). The sample problem demonstrated that (1) the CC was written with sufficient clarity to allow different engineers to arrive at the same estimated value of allowable toughness, (2) the latitude associated with the provisions of the ASME Code pertinent to estimation of allowable flaw depth are responsible for some differences in the allowable flaw depth values reported by different participants, and (3) current Code estimates of allowable flaw depth are far more conservative (that is: smaller) than values estimated by the candidate CC methods based on the MC, this mostly due to the generally-conservative bias of the Code’s RTNDT & KIc approach. The candidate CC methods provide much more consistent conservatism than current Code approaches for all conditions in the operating nuclear reactor fleet via their use of an index temperature (T0) defined by actual fracture toughness data and a temperature dependence defined by those data. The WGFE is continuing to evaluate candidate approaches to estimate allowable toughness values for CC N-830 using a T0-indexed Master Curve. Associated work is addressed by two companion papers presented at this conference.

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