Päivi Karjalainen-Roikonen

Effect of neutron irradiation on fracture toughness behaviour of copper alloys

One of the most important factors in deciding about the applicability of materials in the structural components of ITER, is the effect of neutron irradiation on the fracture toughness behaviour of these materials. In the present work, the fracture toughness properties of two candidate materials for the first wall and divertor components of ITER, namely precipitation hardened CuCrZr and dispersion hardened CuAl25 alloys, have been studied in the unirradiated and irradiated conditions. In parallel, tensile properties of these alloys have been also investigated in the unirradiated and irradiated conditions.

Comparison of approaches for estimating pipe rupture frequencies for risk-informed in-service inspections

Abstract This paper describes the comparative study of two approaches to estimate pipe leak and rupture frequencies for piping. One method is based on a probabilistic fracture mechanistic model while the other one is based on statistical estimation of rupture frequencies from a large database. In order to be able to compare the approaches and their results, the rupture frequencies of some selected welds have been estimated using both of these methods. This paper highlights the differences both in methods, input data, need and use of plant specific information and need of expert judgement. The study focuses on one specific degradation mechanism, namely the intergranular stress corrosion cracking. This is the major degradation mechanism in old stainless steel piping in BWR environment, and its growth is influenced by material properties, stresses and water chemistry.

Long-life multiaxial fatigue of a nodular graphite cast iron

Abstract Nodular graphite cast iron is one example of a material that fails in fatigue primarily by the initiation and growth of Mode I cracks. With this in mind, a tensile critical plane damage parameter would be expected to correlate life for different stress states. Long life fatigue tests have been performed on cast nodular graphite iron using uniaxial tension, torsion and equi-biaxial loading. Results are compared to a Goodman-type fatigue limit criterion previously developed for this material that successfully correlates data for a variety of stress states: uniaxial tension with and without mean stresses, fully reversed torsion, cyclic torsion with mean shear stress, and cyclic torsion with static normal stresses. The criterion includes a multiaxial correction physically based on the growth of small cracks from notches and small defects, and is able to account for the detrimental effect of the negative second principal stress in torsion. The expected benefit of a positive second principal stress in biaxial tension was not observed in experiments. The equi-biaxial stress state greatly increased the observed tortuosity of fatigue cracking as compared to the uniaxial or torsion stress states. It is assumed that this produced a lower fatigue limit because the crack driving force was nearly equal in all directions thus allowing cracks to link up weaker regions of the complex cast iron microstructure.

Applicability of the Master Curve Method to Ultra High Strength Steels

Although Ultra High Strength Steels (UHSS) with nominal strengths up to 1500 MPa have been available on the market for many years, the use of these steels in the civil engineering industry is still rather uncommon. One critical point limiting the use of UHSS steels lies in their rather poorly documented fracture properties in relation to more conventional steels covered by the codes. The major concept governing the assessment of steels is the Master Curve (MC) methodology. It provides a description for the fracture toughness scatter, size effect and temperature dependence in the ductile to brittle transition region. It enables a complete characterization of brittle fracture toughness of a material based on only a few small size specimens. The method combines a theoretical description of the scatter, a statistical size effect and an empirically found temperature dependence of fracture toughness. The fracture toughness in the brittle fracture regime is thus described with only one parameter, the transition temperature T0. At this temperature the mean fracture toughness for a 25.4 mm thick specimen is 100 MPa√m. The Master Curve method as defined in ASTM E1921-13a is applicable to ferritic structural steels with yield strength between 275 MPa and 825 MPa. Very few studies have been made with respect to the applicability of the Master Curve to Ultra High Strength Steels with yield strengths in the excess of 900 MPa. This is the topic of this work. Focusing on novel directly quenched high performance steels, the applicability of the Master Curve methodology with special emphasis on the temperature dependence will be investigated. Possible improvements to the Master Curve will be proposed for further consideration.Copyright

Overview of the nuclear power plant structural integrity research in Finland

Abstract An overview of the Finnish programmes concerning safety related research in pressure boundary components of nuclear power plants is presented. The focus is on recent results in the areas of fracture mechanics, large scale component tests, irradiation damage, environmental influence on materials as well as nondestructive testing (NDT).

Results of EC FP7 Structural Performance of MULTI-METAL Component Project: Dissimilar Metal Welds Fracture Resistance Investigation

The purpose of this paper is to disseminate the results of an EURATOM project MULTI-METAL focusing on the structural integrity assessment of dissimilar metal welds. The project started in February 2012 and ended in February 2015. The project is coordinated by VTT with 10 partner organizations from Europe : Technical Research Centre of Finland, Finland (VTT) – Coordinator, AREVA NP, France and Germany (ANP), Commissariat a l’Energie Atomique et aux energies alternatives, France (CEA), Joint Research Centre of the European Commission, Belgium (JRC), EdF-Energy, United Kingdom (BE), Bay Zoltan Foundation for Applied Research, Hungary (BZF), Electricite de France, France (EDF), TECNATOM, Spain (TEC), Jožef Stefan Institute, Slovenia (JSI), Studsvik Nuclear AB, Sweden (STU).The underlying aim of the project is to provide recommendations for a good practice approach for the integrity assessment (especially testing) of tough dissimilar metal welds as part of overall ductile integrity analyses; this has been presented in the project overview [1].Experience on typical DMWs concerning manufacturing, residual stresses, flaw assessment and testing have been reviewed. The specimens were taken from mock-ups of welded plates. Three DMWs design variants have been covered: narrow gap DMW with Ni-52, DMW with austenitic steel buttering and a DMW with Nienriched austenitic steel buttering. Mechanical characterization and fracture mechanics testing (CT, SEN(B) and SEN(T) specimens) have been performed. Interpretation of the test has required numerical analysis since the standard ASTM E1820 [2] (CT, SEN(B)) and guidelines dealing with SEN(T) [3][4] are not directly intended to cover DMW.The motivation of the project and its results are generally presented and discussed.Copyright

Effect of specimen type and size on fracture resistance curve determination for CuCrZr alloy

The precipitation hardened copper alloy CuCrZr is one of the candidate materials for the first wall and divertor components of ITER. In order to validate the applicability of copper alloys in the structural components of ITER, the effect of neutron irradiation on the fracture toughness behaviour has to be studied. Fracture toughness testing of irradiated materials requires the use of miniaturised specimens and the verification of specimen size effects on fracture toughness test results. In the present study the effects of specimen type, size as well as temperature on the ductile tearing fracture resistance behaviour of CuCrZr alloy will be shown. The applicability of miniaturised specimens for fracture resistance curve determination is considered by studying the allowable crack extension with which the miniaturised specimens still have similar fracture resistance curves as full-size specimens. The crack length and J-integral values will be compared to ASTM standard requirements for the maximum allowable crack extension and J-integral values in the fracture resistance test.