Tapio Planman

Applicability of miniature size bend specimens to determine the master curve reference temperature T0

Abstract The master curve method enables characterisation of the brittle fracture toughness based on a few relatively small specimens. Presently the general view is that pre-cracked Charpy-V specimens constitute, effectively, the smallest specimens that can be used with the master curve. However, even though the method includes a specific measuring capacity limit for the specimen, it does not specify a minimum specimen size to be used. In this work, the applicability of miniature specimens, smaller than the normal Charpy size bend specimen, are investigated by comparing the test results of miniature and normal Charpy size specimens. Furthermore, the possible differences in estimates from CT- and 3PB-specimen tests are examined.

Evolution of the Charpy-V test from a quality control test to a materials evaluation tool for structural integrity assessment

Abstract Originally, the Charpy-V test was used mainly as a quality control test. However, after World War II, with the development of the transition temperature philosophy, the Charpy-V test evolved into a tool for material selection and toughness evaluation. With the development of fracture mechanics, further evolution of the interpretation of the Charpy-V test has made it a quantitative materials evaluation tool for fracture mechanics based structural integrity assessment. This presentation will give an outline of the evolution of the Charpy-V test, focussing on the latest developments regarding its use in structural integrity assessment.

Consistency of fracture assessment criteria for the NESC-1 thermal shock test

Abstract In this report, a best estimate analysis of the NESC Spinning Cylinder test is performed and the consistency of the state of the art application of the different fracture parameters and assessments criteria is examined. Direct fracture toughness measurements best describes the fracture behavior of the NESC Spinning Cylinder in the simulated pressurized thermal shock loading. The concept based on the ASME RT NDT reference temperature (either Pellini or Cv test) does not provide consistent description of the NESC1 fracture assessment. In general the fracture mode, i.e. ductile initiation and tearing followed by cleavage event, was successfully estimated by 3D FE based fracture assessment, combined with Master Curve description of fracture toughness.

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

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.Copyright

Application of the Master Curve Approach for Abnormal Material Conditions

The range of applicability of Master Curve testing Standard ASTM E 1921 is limited to macroscopically homogeneous steels with “uniform tensile and toughness properties”. A majority of structural steels appear to satisfy this requirement by exhibiting fracture toughness data which comply with the assumed KJc vs. temperature dependence and scatter within the specified validity area. As indicated in ASTM E 1921 a criterion for material macroscopic inhomogeneity is often applied using the 2% lower bound (possibly also the 98% upper bound). Data falling below this 2% lower-limit curve may be an indication of material inhomogeneity or susceptibility to grain boundary fracture. When this situation occurs, it is recommended to analyze the material with the so-called SINTAP procedure, which is intended for randomly inhomogeneous materials to assure a conservative lower-bound estimate. When a data set distinctly consists of two or more different data populations instead of one (due to variation of irradiation dose or specimen extraction depth, for instance) adoption of a bimodal (or a multimodal) Master Curve model is generally appropriate. These modal models provide information if the deviation of distributions is statistically significant or if different distributions truly exist for values of reference transition temperature, T0 , characteristic of separate data populations. In the case of data sets representing thick-walled structures (i.e., reactor pressure vessels), indications of abnormal fracture toughness data can be encountered such that material inhomogeneity or fracture modes other than pure cleavage should be suspected. A state-of-the-art review for extended, non-standard Master Curve data and techniques highlights limits of applicability in situations where the basic ASTM E 1921 procedure is not appropriate for material homogeneity or different fracture modes.© 2007 ASME

Fracture mechanics based scaling criteria for miniature and sub-size Charpy-V specimens

Abstract Besides the normal-size (ISO-V) Charpy specimen (10 * 10 * 55 mm 3 ), various types of sub-size specimens have been introduced. One standardised sub-size specimen is the so-called KLST specimen, which size is 3 * 4 * 27 mm 3 and the center notch is 1 mm (DIN 50 115). However, the test data published for the KLST specimen, as well as sub-size specimens in general, is still very limited, though they can provide an overwhelmingly effective use of test material. The results from small specimen testing are typically used to evaluate the fracture behaviour of the ISO-V Charpy specimen. If there are no test results available for the correlation, as there usually is not, a general correlation has to be applied to evaluate the fracture behaviour of the ISO-V specimen. The applicability of a sub-size specimen depends therefore significantly on how reliably this relationship has been established. Here, the Charpy-V test is given a fracture mechanical interpretation and, based on this, new generally applicable scaling criteria are proposed both for miniature as well as sub-size Charpy-V specimens.

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

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 .Copyright

PERFECT—Prediction of Irradiation Damage Effects in Reactor Components: Update of Progress in RPV Mechanics Sub-Project

The EURATOM 6th Framework Integrated Project PERFECT (Prediction of Irradiation Damage Effects in Reactor Components) addresses irradiation damage in RPV materials and components by multi-scale modeling. This approach offers many potential advantages over the conventional empirical methods used in current practice of nuclear plant lifetime management. Launched in January 2004, PERFECT is a 48-month project focusing on two main components of nuclear power plants which are subject to irradiation damage: the ferritic steel reactor pressure vessel (RPV), and the austenitic steel internals. It is the purpose of the present paper to provide an update of progress of work being carried out in the Mechanics Sub-project of PERFECT to predict the fracture behavior of RPVs in PWR and WWER systems.Copyright