U. Eisele

A procedure for the experimental assessment of the J-integral by means of specimens of different geometries

Abstract It is necessary to determine the elastic-plastic fracture mechanics parameter J on specimens of different loading mode and size in order to further clarify the dependence of the J- integral on these parameters. Using the fundamental equations of linear-elastic fracture mechanics and fully plastic material behaviour, the factors η el and η pl for the experimental evaluation of the J- integral for different specimen geometries are determined. The basic equations used are summarised, the relations obtained for η el and η pl are plotted. The application of these relations in the testing of large scale specimens of different specimen type and size shows the variation of the J- integral over the stable crack growth.

Evaluation of different fracture-mechanical J-integral initiation values with regard to their usability in the safety assessment of components

Abstract Determining fracture-mechanical material characteristic values on the basis of the J -integral is described and stipulated in a variety of standards and guidelines. The individual specifications differ in terms of procedure when determining the characteristic values and, therefore, also in terms of the meaningfulness of the results. This paper presents the different procedures, suggested in the course of the development of test methods in the field of elastic—plastic fracture mechanics, used to characterize crack initiation behaviour with regard to their features as material characteristic values and their usability in the safety assessment of components.

Fracture mechanics evaluation of cracked components with consideration of multiaxiality of stress state

Abstract The fracture-mechanics evaluation of cracked components is an essential part of safety analyses. This evaluation is usually based on one-parametric evaluation procedures without taking into account the multiaxiality of the stress state. Considering the multiaxiality of the stress state across the flawed component cross-section, it is possible to recognize and extend the limits of application of fracture mechanics, which among others, are given by the limited transferability of fracture-mechanics material laws. Within the scope of the research project “Phenomenological Vessel Burst Tests-Phase IV”, T-branches and elbows with dimensions like the primary coolant lines of PWR plants were investigated. In addition to the experimental investigations, extensive numerical calculations were performed by means of the finite element method (FEM). To determine the stress and the gradient of multiaxiality across the ligament of the component, 3-D finite-element analyses were carried out concerning elastic-plastic material behaviour. The evaluation with regard to crack initiation has been proven by experimental results as well as the qualitative assessment of the fracture behaviour on the basis of the multiaxiality analyses.

The influence of the material toughness and the state of stress on fracture of large scale specimens

Abstract The influence of the material toughness on fracture behaviour of large scale tensile specimens is being described in this report. Selected large scale specimens show that the amount of stable crack growth prior to instability is affected by the constraint in the specimen. The constraint depends on the specimen geometry, the specimen size, and in the case of elastic plastic material behaviour on the ductility of the material. A possibility of estimating these influences is presented by quantifying the constraint with the multi-axiality quotient q .

Application and evaluation of different numerical methods for determining crack resistance curves

Abstract The engineering assessment of precracked components is frequently carried out with the help of crack resistance curves on the basis of the J- integral . Nevertheless, there are severe uncertainties regarding the validity of the J- integral in the case of large plastic deformations and unloading processes due to crack growth. Numerical and theoretical derivations are used to examine the influence of large plastic deformations and stable crack growth on the calculation of the J- integral . Numerical investigations were carried out on the example of a CT 25-specimen made of 20 MnMoNi 5 5 by means of the finite-element method. The following methods of calculations were selected: • - Calculation of the specimen behaviour without consideration of the stable crack growth. • - Calculation of the specimen behaviour using a J-Δ a -curve as crack crack growth law. • - Calculation of the specimen behaviour using a damage model (“ocal approach”) to compute the crack growth. The results of the calculation carried out on the basis of the damage model makes it possible to carry out an assessment of the various methods for the experimental determination of the J- value .

Classification and characterization of materials by means of fracture mechanics parameters

Abstract Within the scope of the “Integrity of Components” research project a large number of heats of reactor pressure vessel steels representing a broad quality spectrum have been investigated. In this paper the conventional and fracture mechanics parameters of four typical materials are presented. The difficulties in determining the reference temperature for nil ductility transition and the fracture mechanics parameters in the transition and upper shelf region of the Charpy energy are discussed. The technique developed at MPA for the evaluation of a physically meaningful crack initiation parameter based on the size of the stretched zone ahead of the crack tip is described, and values are reported.

Planning and use of condition-dependent NDT for the interaction between fracture mechanics and quantitative NDT

The proof of safety for reactor components is generally based on non-destructive testing results, which much be evaluated in reference to their safety relevance. The use of fracture mechanics concepts in flaw evaluations requires the quantification of the dependencies between flaw geometry, material characteristics, and load condition. The solution consists of the combination of optimal test preparation using computer-aided design software modules and modeling of the inspection, a quantitative flaw testing using qualified reconstruction methods and qualification procedures of the material parameters like fracture toughness. Besides the discussion of the condition-dependent strategies of lifetime management many results from large-scale round-robin testing concerning crack initiation parameters versus fracture toughness or J-integral or ray tracing in connection with defect sizing in a CAD-environment are shown.

Der Kurzrisseffekt bei der bruchmechanischen Prüfung: A unified method to treat fatigue damage

Kurzfassung Unter dem ausgeprägten Kurzrisseffekt versteht man den Anstieg von zähbruchmechanischen Initiierungskennwerten bei reduzierter Ermüdungsrisslänge. Für Bruchmechanikproben mit niedrigem a/W-Verhältnis, also geringer Ermüdungsrisslänge, werden bei Vorliegen eines ausgeprägten Kurzrisseffekts demnach höhere Initiierungskennwerte ermittelt als für Proben mit größerem a/W-Verhältnis. In mehreren Forschungsarbeiten der Vergangenheit wurden teilweise unterschiedliche Ergebnisse zum Kurzrisseffekt bei Stahl erzielt. Ein im vergangenen Jahr durchgeführtes gemeinsames Forschungsprojekt der Materialprüfungsanstalt Uni Stuttgart (MPA) und des Instituts für Eisenhüttenkunde der RWTH Aachen (IEHK) sollte die Ursachen für die unterschiedlichen Ergebnisse klären.

Transferability of results of PTS experiments to the integrity assessment of reactor pressure vessels

Abstract The integrity assessment of the reactor pressure vessel (RPV) is based on the fracture mechanics concept as provided in the code. However, this concept covers only the linear-elastic fracture mechanics regime on the basis of the reference temperature RT NDT as derived from Charpy impact and drop-weight test. The conservatism of this concept was demonstrated for a variety of different materials covering optimized and lower bound material states with regard to unirradiated and irradiated conditions. For the elastic-plastic regime, methodologies have been developed to describe ductile crack initiation and stable crack growth. The transferability of both, the linear-elastic and elastic-plastic fracture mechanics concept was investigated with the help of large scale specimens focusing on complex loading situations as they result from postulated thermal shock events for the RPV. A series of pressurized thermal shock (PTS) experiments were performed in which the applicability of the fracture mechanics parameters derived from small scale specimen testing could be demonstrated. This includes brittle (static and dynamic) crack initiation and crack arrest in the low Charpy energy regime as well as stable crack initiation, stable crack growth and crack arrest in the upper shelf toughness regime. The paper provides the basic material data, the load paths, representative for large complex components as well as experimental and theoretical results of PTS experiments. From these data it can be concluded that the available fracture mechanics concepts can be used to describe the component behavior under transient loading conditions.

Extension of fracture mechanics evaluation methods by consideration of multiaxiality of stress state for piping components

Abstract Experimental investigations and numerical calculations by means of the finite element method concerning linear elastic as well as elastic-plastic material behaviour were performed to develop a methodology for the fracture mechanics evaluation taking into account the multiaxiality of stress state. A description of this fracture mechanics evaluation methodology and its application on degraded piping components (T-branches and elbows with dimensions like the primary coolant lines of PWR-plants) is provided and discussed.