Comparison of the Stress Intensity Factor Influence Coefficients for Axial ID Surface Cracks in Cylinders of RSE-M and API 579-1

Analytical evaluation procedures for determining the acceptability of flaw detected during in-service inspection of nuclear power plant components are provided in Appendix 5.4 of the French RSE-M Code. Linear elastic fracture mechanics based evaluation procedures require calculation of the stress intensity factor (SIF). In Appendix 5.4 of the RSE-M Code, influence coefficients needed to compute the SIF are provided for a wide range of surface axial or circumferential flaws in cylinders, the through-wall stress field being represented by a cubic equation. On the other hand, Appendix C of API 579-1 FFS procedure provides also a very complete set of influence coefficients. The paper presents the comparison of the influence coefficients from both documents, focused on axial ID semi-elliptical surface flaws in cylinders. The cylinder and crack geometries are represented by three ratios: Ri/t, a/t, and a/c, where Ri, t, a, and c are respectively the inner radius, the wall thickness, the crack depth and one-half of the crack length. The solutions for the coefficients G0 and G1 at the deepest point and at the surface point are investigated. At the deepest point, the agreement between the solutions is good, the relative difference being lower than 2 %, except for the plate (Ri/t = ∞) at a/c = 0.125 and 0.0625 and a/t = 0.8 (around 5 %). At the surface point, the agreement between both solutions is not so good. At this point, the relative differences depend strongly on the a/c ratio, being larger for elongated cracks (with low a/c ratios). However, it must be recalled that the absolute values of the coefficients are low at the surface point for elongated cracks, and that for these cracks the critical point regarding the stress intensity factor is the deepest point.

The Prediction of Size Effect on J-R Curve for Eurofer97 Steel by Simplified Mechanical Model

The possibilities to derive fracture toughness from small specimens are naturally limited due to constraint requirements which are especially restrictive in toughness testing. The loss of constraint at the crack tip is more likely to occur as specimen size decreases. Application of miniature specimens in fracture toughness testing thus requires a suitable methodology or correction procedure to deal with phenomenon of the constraint loss. Schindler et al. have proposed a simplified mechanical model that can be used to scale-up the key test characteristics from miniature specimen to the larger one. The model is applied to the miniature bending specimens to describe size effect on J-R curve of the Eurofer97 steel. The examined steel exhibits quite high toughness values at upper shelf region of fracture toughness. As a result, experimentally determined J-R curves of three different sizes of pre-cracked bending specimens showed high values of J-integral, which were significantly different each other. Using semi-empirical definition of the exponent of the power law function of J-R curve the performance of the Schindler’s model was quite successful. It was shown that the model is able to handle with size effect of tested pre-cracked three-point-bend specimens.

Development of Fitness-for-Service Standard for Pressure Equipment With Metal Loss Based on Reliability

Concern about probabilistic approach for Fitness-For-Service (FFS) assessment has been growing over the last several years. The FFS assessment based on reliability helps to make a rational decision as to whether to run or repair the equipment. High Pressure Institute of Japan (HPI) formed a committee to develop a HPI FFS standard that can be used for pressure equipment with metal loss. This new standard provides an assessment procedure to evaluate structural integrity of components with metal loss based on reliability. This paper introduces the assessment procedure which is standardized and under preparation for publication, and the technical backgrounds. The standard provides information about limit state of pressure equipment, probabilistic properties of basic variables and target reliability. Probabilistic approach can be applied easily to metal loss assessment by using the standard.

EDF/NRC High-Cycle Fatigue Database Proposal

Revised fatigue curves for austenitic stainless steels are currently being considered by several organizations in various countries, including Japan, South Korea, and France. The data available from laboratory tests indicate that the mean air curve considering all available austenitic material fatigue data may be overly conservative compared to a mean curve constructed from only those data representative of a particular type of material. In other words, developing separate fatigue curves for each of the different types of austenitic materials may prove useful in terms of removing excess conservatism in the estimation of fatigue lives. In practice, the fatigue curves of interest are documented in the various international design codes. For example, in the 2009 Addenda of Section III of the ASME Boiler and Pressure Vessel (BPV) Code [1], a revised design air fatigue curve for austenitic materials was implemented that was based on NRC research models [2]. More recently, in Japan, various industrial groups have joined their efforts to create the Design Fatigue Curve Sub-committee (DCFS) with the objective to reassess the fatigue curves [3]. In France, EDF/AREVA and CEA are developing a new fatigue curve for austenitic stainless steels [4]. More specifically, in 2014, EDF presented a paper on high-cycle fatigue analysis which demonstrated that the factor on the strain amplitude could be reduced from 2 to 1.4 for the RCC-M austenitic stainless steel grades [5]. Recently, discussions between EDF and the U.S. Nuclear Regulatory Commission (NRC) have led both parties to recognize that there is a need to exchange worldwide research data from fatigue testing to promote a common, vetted database available to all researchers. These discussions have led EDF and NRC to pursue a collaborative agreement and associated fatigue data exchange, with the intent to assemble all available fatigue data for austenitic materials into a standardized format. The longer term objective is to perform common analyses on the consolidated set of data. This paper summarizes the intent and of the preliminary results of this cooperation and also provides insights from both organizations on possible future activities and participation in the global exchange of fatigue research data.

Study on a New Design Fatigue Evaluation Method

In order to develop new design fatigue curves for austenitic stainless steels, carbon steels and low alloy steels and a new design fatigue evaluation method that is rational and has a clear design basis, the Design Fatigue Curve (DFC) subcommittee was established in the Atomic Energy Research Committee in the Japan Welding Engineering Society. Tentative design fatigue curves were developed and studies on the effects of mean stress and design factors are ongoing. Design fatigue curves, including the effects of mean stress and design factors, are needed to establish a new fatigue design evaluation method. This paper describes the study on the new fatigue design evaluation method.

Safety Factor for Flaw Evaluation of a Pipe Having a Circumferential Surface Flaw

This paper describes the safety factors used for fracture assessments of pipes having circumferential surface flaws. The “Fitness-For-Services Codes (the FFS Codes)” of the Japan Society of Mechanical Engineers (JSME) restrict the depths of flaws according to their angles to prevent pipes with flaws from being fracture. Past restrictions were determined based on deterministic evaluations. In fracture assessments of pipes having flaws, however, the effects of measurement errors in flaw dimensions and of variations on material strength must be taken into account. Thus, we evaluated the effects of such variations on fracture assessments of pipes having flaws, and examined safety factors for giving failure probability (or reliability) equal, irrespective of the cracking angles. We found out that failure probability is heavily dependent on the measurement accuracy of flaw depths and material strength (flow stress). In view of this finding, we examined and proposed a simple approach which meets the target reliability without conducting complex evaluations by the Monte Carlo method and reliability evaluation methods (e.g., first-order second-moment method (FOSM)).

Fatigue Evaluation for Section III, Class 1 Components Using Flaw Tolerance Method to Consider Environmental Effects

An ASME Section III Task Group (TG) was formed in 2012 to develop alternate rules for the design assessment of Section III Class 1 nuclear components subject to fatigue service with environmental effects. A Section III Code Case has been proposed with the purpose of providing a method for performing fatigue evaluations of Class 1 components when the effects of a light water reactor environment on fatigue life are judged to be significant and cumulative usage factor (CUF) limits may not be satisfied. The Code Case implements a flaw tolerance approach by postulating that a fatigue crack initiates at the beginning of life and is subjected to fatigue crack growth under the specified design cycles. It must be demonstrated that the crack would remain stable with set margin throughout the design life of the component or part under consideration, and would remain confined to an acceptable fraction of the wall thickness. At this time, the application is limited to type 304/304L and 316/316L austenitic steel. This paper discusses the methodology and technical background of the proposed Code Case.

Utilization of SPT Techniques for Evaluation of Changes in the Properties of the VVER-440 Reactor Pressure Vessel Steels After Their Irradiation in the Research and Power Reactors

The paper describes the testing procedures and the basic results of the evaluation of the Small Punch Test (SPT) specimens after their irradiation in the Halden reactor in Norway. The SPT technique was used for estimation of basic mechanical properties as ultimate tensile strength and yield stress of the tested materials as well as the Fracture Appearance Transition Temperature (FATT). The main aim of the work was to compare the SPT results obtained from the surveillance specimen programs implemented in the Slovak power reactors with the SPT results from the specimens irradiated in the research reactor in Halden. For the project there were chosen 3 types of steels used for construction of the reactor VVER 440/213 type in Bohunice NPPs in the Slovak Republic. The experimental materials were two bainitic steels — base metal and weld metal of the reactor pressure vessel wall and austenitic cladding of the reactor wall. Two sets of SPT specimens together with mini-tensile specimens prepared from the experimental materials were irradiated in the Halden reactor. The samples were irradiated at 275°C to two fluence values which are equivalent to approximately of 4 and 6 campaigns in the power reactor. Obtained results are compared to up-to-date SPT results from the surveillance specimen program applied at Bohunice NPP in the Slovak Republic.

Study on LTA Measurement for FFS Assessment

FFS assessment technologies for pressure equipment have been studied and standardized in recent 15 years in Japan. FFS assessment of local thin area, LTA, is the most frequently used in process industries. However reliability of thickness measurement of LTA and influence to FFS assessment has not been studied much in the past. Uncertainty of thickness measurements and Remaining Strength Factor, RSF, were investigated on Round Robin Testing using manual UT and additional new technology such as flexi-alley UT and 3D LED system for LTA in pipes, It is recommended to use suitable combination of Manual or mechanized UT and 3D LTA measurement system in case of assessment of critical flaws and decision making for repair and replacement of pressure equipment.