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.

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.

Direct Use of the Fracture Toughness Master Curve in ASME Code, Section XI, Applications

2013 ◽  
Author(s):  
William Server ◽  
Russ Cipolla
Keyword(s):  
Fracture Toughness ◽  
Master Curve ◽  
Potential Problem ◽  
Standard Test ◽  
Test Method ◽  
Alternative Approach ◽  
Alternative Index ◽  
Asme Code ◽  
Technical Basis ◽  
Direct Use

The ASME Code, Section XI, has adopted the indirect use of the fracture toughness Master Curve to define an alternative index (RTT0) rather than RTNDT for using the Code KIC and KIa curves in Appendices A and G. RTT0 is defined as T0 + 19.7°C (T0 + 35°F), where T0 is the Master Curve reference temperature as defined in ASTM Standard Test Method E 1921. This alternative approach was first approved in ASME Code Case N-629 for Section XI and Code Case N-631 for Section III. Most recently this approach has been integrated directly into the Code, Section XI, and will be published in the 2013 Edition. When this alternative indexing approach was developed, it was recognized that the direct use of the Master Curve itself also could be used as an alternative to the Code KIC curve. A Code Case for the direct use of the fracture toughness Master Curve has been developed and has been presented to Section XI for approval. This paper provides the technical basis for using the fracture toughness Master Curve as an alternative to the Section XI KIC curve. An adjustment to the Master Curve at very low temperatures is included which alleviates a potential problem for low temperature overpressure (LTOP) protection setpoints as would be determined using the existing Code KIC curve.

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

2013 ◽  
Vol 135 (6) ◽  
Author(s):  
Naoki Miura ◽  
Naoki Soneda ◽  
Shu Sawai ◽  
Shinsuke Sakai
Keyword(s):  
Fracture Toughness ◽  
Lower Bound ◽  
Master Curve ◽  
Surveillance Program ◽  
Data Sets ◽  
Ductile Brittle Transition Temperature ◽  
Asme Code ◽  
American Society ◽  
Reactor Pressure

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.

A Fracture-Toughness Based Transition Reference Temperature for Use in the ASME Code With the Crack Arrest (KIA) Curve

2014 ◽  
Author(s):  
Mark Kirk ◽  
Hieronymus Hein ◽  
Marjorie Erickson ◽  
William Server ◽  
Gary Stevens
Keyword(s):  
Fracture Toughness ◽  
Master Curve ◽  
Functional Form ◽  
Neutron Radiation ◽  
Crack Arrest ◽  
Reference Temperature ◽  
Radiation Embrittlement ◽  
Temperature Separation ◽  
Asme Code ◽  
Current Code

In the early 2000s, ASME adopted Code Cases N-629 and N-631 [1–2], both of which permit the use of the Master Curve reference temperature (To) to define an reference temperature RTTo, as follows (in SI units, as are used throughout the paper):RTTo=To+19.4℃The Code Cases state that “this reference temperature … may be used as an alternative to [the] indexing reference temperature RTNDTfor the KIcand KIatoughness curves, as applicable, in Appendix A and Appendix G [of Section XI of the ASME Code].” KIa is now only used in Appendix A. The functional form of the ASME KIc and KIa curves dictate that the temperature separation between them remains constant irrespective of the degree of neutron radiation embrittlement, as quantified by ΔRTNDT or ΔRTTo. However, data collected from the literature and new data reported by Hein et al. show that radiation embrittlement brings the KIc and KIa curves closer together as embrittlement increases. As a result, current Code guidance will not produce a bounding KIa curve in all situations when RTTo is used as an reference temperature. To reconcile this issue, this paper summarizes available data and, on that basis, concludes that use of the following reference temperature will ensure that the ASME KIa curve bounds currently available KIa data:RTKIa=RTTo-19.4+44.97×exp⁡−0.00613×RTTo-19.4

Estimation of Master Curve Based RTTo Reference Temperature From CVN Data

2005 ◽  
Author(s):  
Kim R. W. Wallin ◽  
Gerhard Nagel ◽  
Elisabeth Keim ◽  
Dieter Siegele
Keyword(s):  
Fracture Toughness ◽  
Pressure Vessel ◽  
Brittle Failure ◽  
Master Curve ◽  
Simple Relation ◽  
Systematic Evaluation ◽  
Data Sets ◽  
Irradiation Embrittlement ◽  
Pressure Vessel Steels ◽  
Asme Code

The ASME code cases N-629 and N-631 permits the use of a Master Curve-based index temperature (RTTo ≡ T0 + 19.4°C) as an alternative to traditional RTNDT-based methods of positioning the ASME KIc, and KIR curves. This approach was adopted to enable use of Master Curve technology without requiring the wholesale changes to the structure of the ASME Code that would be needed to use all aspects of Master Curve technology. For the brittle failure analysis considering irradiation embrittlement additionally a procedure to predict the adjustment of fracture toughness for EOL from irradiation surveillance results must be available as by NRC R.G. 1.99 Rev. 2 e.g.: ART = Initial RTNDT + ΔRTNDT + Margin. The conservatism of this procedure when RTNDT is replaced by RTTo is investigated for western nuclear grade pressure vessel steels and their welds. Based on a systematic evaluation of nearly 100 different irradiated material data sets, a simple relation between RTToirr, RTToref and ΔT41JRG is proposed. The relation makes use of the R.G. 1.99 Rev. 2 and enables the minimizing of margins, necessary for conventional correlations based on temperature shifts. As an example, the method is used to assess the RTTo as a function of fluence for several German pressure vessel steels and corresponding welds. It is shown that the method is robust and well suited for codification.

A Combined Model Approach for Estimating T0

2019 ◽  
Author(s):  
Marjorie Erickson
Keyword(s):  
Fracture Toughness ◽  
Master Curve ◽  
Reference Temperature ◽  
Initiation Toughness ◽  
Correlation Model ◽  
Combined Model ◽  
Best Estimate ◽  
Curve Model ◽  
Asme Code ◽  
Model Approach

Abstract The current best-estimate model describing the fracture toughness of ferritic steels is the Master Curve methodology standardized in ASTM E1921. Shortly following standardization by ASTM, efforts were undertaken to incorporate this best-estimate model into the framework of the ASME Code to reduce the conservatisms resulting from use of a reference temperature based on the nil-ductility temperature (RTNDT) to index the plane strain fracture initiation toughness (KIc). The reference temperature RTT0, which is based on the ASTM E1921-defined T0 value, was introduced in ASME Code Cases N-629 (replaced by Code Case N-851) and N-631 to replace RTNDT for indexing the ASME KIc curve. Efforts are continuing within the ASME Code to implement direct use of the Master Curve model; using the T0 reference temperature to index an elastic-plastic, KJc fracture toughness curve. Transitioning to a direct T0-based fracture toughness assessment methodology requires the availability of T0 estimates for all materials to be assessed. The historical Charpy and NDT-based regulatory approach to characterizing toughness for reactor pressure vessel (RPV) steels results in a lack of T0 values for a large population of the US nuclear fleet. The expense of the fracture toughness testing required to estimate a valid T0 value makes it unlikely that T0 will ever be widely available. Since direct implementation of best-estimate, fracture toughness models in codes and regulatory actions requires an estimate of T0 for all materials of interest it is necessary to develop an alternative means of estimating T0. A project has been undertaken to develop a combined model approach to estimating T0 from data that may include limited elastic-plastic fracture toughness KJc, Charpy, tensile, ductile initiation toughness, arrest toughness, and/or nil-ductility temperature data. Using correlations between these properties and T0 a methodology for combining estimates of T0 from several sources of data was developed. T0 estimates obtained independently from the Master Curve model, the Simple T28J correlation model, and a more complex Charpy correlation model were combined using the Mixture Probability Density Function (PDF) method to provide a single estimate for T0. Using this method, the individual T0 estimates were combined using weighting factors that accounted for sample size and individual model accuracy to optimize the accuracy and precision of the combined T0 estimate. Combining weighted estimates of T0 from several sources of data was found to provide a more refined estimate of T0 than could be obtained from any of the models alone.

Statistical Analysis of the ASME KIc Database

1998 ◽  
Vol 120 (1) ◽  
pp. 24-28 ◽  
Author(s):  
M. A. Sokolov
Keyword(s):  
Fracture Toughness ◽  
Distribution Function ◽  
Lower Bound ◽  
Weibull Distribution ◽  
Master Curve ◽  
Lower Boundary ◽  
Transition Range ◽  
Weibull Distribution Function ◽  
American Society ◽  
Function Models

The American Society of Mechanical Engineers (ASME) KIc curve is a function of test temperature (T) normalized to a reference nil-ductility temperature, RTNDT, namely, T – RTNDT. It was constructed as the lower boundary to the available KIc database. Being a lower bound to the unique but limited database, the ASME KIc curve concept does not discuss probability matters. However, a continuing evolution of fracture mechanics advances has led to employment of the Weibull distribution function to model the scatter of fracture toughness values in the transition range. The Weibull statistic/master curve approach was applied to analyze the current ASME KIc database. It is shown that the Weibull distribution function models the scatter in KIc data from different materials very well, while the temperature dependence is described by the master curve. Probabilistic-based tolerance-bound curves are suggested to describe lower-bound KIc values.

Collapse of Nuclear Reactor SG Tubes Pressurized From Outside: The Influence of Imperfections

2010 ◽  
Vol 133 (1) ◽  
Author(s):  
Lelio Luzzi ◽  
Valentino Di Marcello
Keyword(s):  
Numerical Investigation ◽  
Nuclear Power ◽  
Nuclear Reactor ◽  
External Pressure ◽  
Initial Stresses ◽  
Asme Code ◽  
Conservative Attitude ◽  
Computer Codes ◽  
American Society ◽  
Collapse Behavior

Some innovative nuclear power plant proposals consider for the design tubes of considerable thickness subjected to external pressure (e.g., steam generators tubes). The collapse of thick tubes is expected to be dominated by yielding but, because of the decreasing nature of the postcollapse evolution, interaction with buckling is likely to be significant enough to demand consideration. At the present, few studies have been carried out both experimentally and numerically, as witnessed by the really conservative attitude that codes assume for thick tubes. A numerical investigation has been performed in this context at the Politecnico di Milano, which was originally intended as a support for requesting a relaxation of American Society of Mechanical Engineers (ASME) regulations. Actually, in 2007, ASME code case N-759 was approved, permitting significant thickness saving in the tube design. Nevertheless, the numerical investigation was pursued to assess the influence of different parameters, such as eccentricity, initial stresses, and material hardening, on the collapse of tubes with diameter to thickness ratios D/t<20. Results are thought to be useful under at least two respects: first, providing some understanding on the collapse behavior in a thickness range so far unexplored; second, giving an indication on the assumptions on which computer codes ought to be based when numerical analyses are required.

Direct Use of Fracture Toughness for Flaw Evaluations of Pressure Boundary Materials in Section XI, Division 1, Class 1 Ferritic Steel Components

2017 ◽  
Author(s):  
Marjorie Erickson ◽  
Mark Kirk
Keyword(s):  
Fracture Toughness ◽  
Crack Initiation ◽  
Crack Growth ◽  
Pressure Vessel ◽  
Round Robin ◽  
Statistical Characterization ◽  
Best Estimate ◽  
Asme Code ◽  
Technical Basis

The ASME Boiler and Pressure Vessel Code; Section XI provides Rules for inspection and fracture safety assessment of nuclear plant pressure boundary components. This Code provides methods for assessing the stresses and moments contributing to the forces available to drive crack growth in a component as described by stress intensity factors as well as the measures of material resistance to crack extension, measured by fracture toughness. Much of the current Code is based on linear elastic fracture mechanics methodologies developed 40 years ago [1], or more, at a time when drop weight tear tests [2] and Charpy V-notch impact tests [3] were the accepted standards used for characterizing a material’s resistance to brittle fracture. Ensuing research produced experimental methods to directly measure a material’s resistance to both brittle and ductile fracture. Data from such experiments provided the evidence supporting a suite of best estimate models describing fracture toughness behavior across a range of temperatures and strain rates. These models include cleavage crack initiation and crack arrest fracture toughness (KJc and KIa behavior, respectively) on the lower shelf and through transition, and also ductile crack initiation and crack growth resistance (JIc, J0.1, and J–R behavior) on the upper shelf. Best-estimate models provide a more accurate means of assessing a material’s expected behavior under all loading and temperature conditions; they also enable an explicit characterization of uncertainties. For these reasons, there is a growing advocacy within ASME Code groups for incorporating these best estimate toughness models into Sections III and XI of the Boiler and Pressure Vessel Code. The first direct implementation of the KJc best-estimate model in the ASME Code was in Code Case (CC) N-830, which was adopted by the ASME Code in 2014. N-830 states that the 5th percentile lower bound of the KJc Master Curve [4], indexed by T0, can be used as an alternative to the ASME RTNDT-indexed KIc curve in a flaw evaluation performed using Non-Mandatory Appendix A to Section XI. Since that time, work has progressed within the Working Group on Flaw Evaluation (WGFE) to further improve the CC. The proposed Revision 1 of CC N-830 incorporates a complete and self-consistent suite of models that completely describe the temperature dependence, scatter, and interdependencies (such as those resulting from irradiation or other hardening mechanisms) between all fracture toughness metrics (i.e., KJc, KIa, JIc, J0.1, and J–R) from the lower shelf through the upper shelf. By incorporating both a statistical characterization of fracture toughness as well as the ability to estimate a bounding curve at any percentile, the revised CC provides a consistent basis for the conduct of both conventional deterministic flaw evaluations as well as probabilistic evaluations that may be pursued in certain circumstances. Additionally, for the first time within ASME Section XI, both transition and upper shelf toughness properties are provided in a consistent manner in the same document, which provides the analyst an easy means to determine what fracture behavior (i.e., transition or upper shelf) can be expected for a particular set of conditions. The WGFE conducted round-robin assessments of the proposed CC N-830-R1 equations and their use in flaw evaluations, and is supporting documentation of the technical basis supporting the development and implementation of N-830-R1. This paper summarizes that technical basis report. A companion paper presented at this meeting describes the round-robin assessments.

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