Master curve analysis of the “Euro” fracture toughness dataset

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Abstract

Brittle fracture in the ductile to brittle transition regime is connected with specimen size effects and – more importantly – tremendous scatter of fracture toughness, which the technical community is currently becoming increasingly aware of. The size effects have the consequence that fracture toughness data obtained from small laboratory specimens do not directly describe the fracture behavior of real flawed structures. Intensive research has been conducted in the last decade in order to overcome these problems. Different approaches have been developed and proposed, one of the most promising being the master curve method, developed at VTT Manufacturing Technology.

For validation purposes, a large nuclear grade pressure vessel forging 22NiMoCr37 (A508 Cl.2) has been extensively characterized with fracture toughness testing. The tests have been performed on standard geometry CT-specimens having thickness 12.5, 25, 50 and 100 mm. The a/W ratio is close to 0.6 for all specimens. One set of specimens had 20% side-grooves. The obtained data consists of a total of 757 results fulfilling the ESIS-P2 test method validity requirements with respect to pre-fatigue crack shape and the ASTM E-1921 pre-fatigue load. The master curve statistical analysis method is meticulously applied on the data, in order to verify the validity of the method. Based on the analysis it can be concluded that the validity of all the assumptions in the master curve method is confirmed for this material.

Introduction

The European Structural Integrity Society (ESIS) and the International Standardisation Organisation (ISO) are currently drafting test standards for characterizing the fracture toughness of metals which are supposed to be basis for CEN standards. The drafts include also the characterization of the transition from ductile to brittle fracture of steels. However, this transition region is connected with specimen size effects and – more importantly – tremendous scatter of fracture toughness, which the technical community is currently becoming increasingly aware of.

The size effects have the consequence that fracture toughness data obtained from small laboratory specimens do not directly describe the fracture behavior of real flawed structures. Intensive research has been conducted in the last decade in order to overcome these problems. Different approaches have been developed and proposed, one of the most promising being the master curve method, developed at VTT Manufacturing Technology. The master curve method, provides a description for the fracture toughness scatter, size effect and temperature dependence both for the transition region as well as the lower shelf. The master curve method has already led to an American testing and analysis standard ASTM E1921-97 and it is an obvious candidate for European standardization. In order to verify the validity of the master curve method and other proposed methods, a sufficiently large data set which shows continuously the scatter and the size effects from the lower shelf to the upper shelf of a single material is needed. Such a data has not existed until now.

In 1995 a project entitled “Fracture toughness of steel in the ductile to brittle transition regime” was launched under the “Measurement and Testing Programme of the European Community”. In the project, more than 750, specimens of four different sizes have been tested at different temperatures to produce a sufficiently large data set for the validation of the different statistical methods. In this report, the data from the project is meticulously analyzed with the, VTT based, master curve method and the validity of the different aspects of the method is verified, both separately and in combination.

Section snippets

Master curve distribution

The master curve cumulative failure probability distribution has the form (Eq. (1)) [1].Pf=1−expKJC−KminK0−Kmin4where KJC is the fracture toughness corresponding to Pf, K0 is the fracture toughness corresponding to 63.2% cumulative failure probability and Kmin is the lower bound fracture toughness.

Failure probability diagram

The single temperature data is presented in the “original” master curve failure probability diagram which produces a linear presentation of the master curve cumulative failure probability

Material

All specimens were extracted from a single segment of a large nuclear grade pressure vessel forging 22NiMoCr37 (A508 Cl.2) so that the crack front was located in the region 1/4T−1/2T which had been found to be “homogeneous” in the preliminary investigations performed by GKSS [12]. The sectioning diagram is presented in Fig. 5.

The measured yield stress values were fitted by an exponential temperature dependence expression, the result of which is presented in Fig. 6.

Due to available time and

Master curve analysis

The master curve analysis is first discussed separately for each test temperature and subsequently an overall synthesis is made (Section 5). The statistical size adjustment (Eq. (8)) is not performed for the analysis of the different test temperatures, but only for the synthesis analysis. 4.1 , 4.2 , 4.3 , 4.4 , 4.5 , 4.6 , 4.7 , 4.8 , 4.9 contain the detailed analysis of all the different test temperatures. Each data set is analyzed by the standard master curve expression using a fixed Kmin (

Synthesis analysis

The synthesis analysis will make an assessment, based on the master curve predictions, of the lower shelf behavior of the initiation sites, the size effect of K0, the temperature dependence of the master curve and the validity of a fixed Kmin=20MPam assumption. Finally, an overall multi-temperature master curve analysis is made and recommendations for the applicability of the master curve are given.

Summary and conclusion

A large nuclear grade ferritic steel pressure vessel forging 22NiMoCr37 (A508 Cl.2) has undergone extensive fracture toughness testing. The tests were performed on standard geometry CT-specimens having thickness 12.5, 25, 50 and 100 mm. The a/W-ratio was close to 0.6 for all specimens. Specimens in one of the sets were 20% side-grooved. A total of 757 results fulfilling the ESIS-P2 test method validity requirements with respect to pre-fatigue crack shape and the ASTM E-1921 pre-fatigue load,

Acknowledgements

This work is a part of the Structural Integrity Project (STIN), belonging to the Finnish Research Programme on Nuclear Power Plant Safety (FINNUS), performed at VTT Manufacturing Technology and financed by the Ministry of Trade and Industry, the Technical Research Centre of Finland (VTT) and the Radiation and Nuclear Safety Authority (STUK).

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