Some components of the main primary circuit of PWR nuclear power plants contain nickel-base alloy 600 parts (steam generator (SG) tubes, steam generator partition plates, lower internal radial supports). It is well known that this alloy is prone to stress corrosion cracking in the primary water environment. In 2002, surface cracks were discovered for the first time in SG partition plates of EDF 900 MWe NPP. The integrity of the SG containing these cracks must be demonstrated for all operating conditions, including accidental conditions. Due to the high tensile consolidation rate and the high fracture toughness of alloy 600, this was proved using limit load analysis. However, for a thorough demonstration, an experimental program was launched at EDF/R&D to better understand the behaviour of cracks in this high fracture toughness material. Centre Cracked Tensile (CCT) specimens were selected for this experimental program, being closer to the industrial case than conventional CT specimens. Two tests have been conducted at room temperature on large CCT specimens containing a semi-elliptical crack. The paper presents the design of the CCT tests, the material characterisation, the main results of the tests and their numerical interpretation.

The ASTM E 1820 standard provides procedures and guidelines for the determination of fracture toughness of metallic materials, characterized by the J-integral. The recommended specimens are single-edge bend, SEN(B), compact, C(T), or disk-shaped, DC(T). Two alternative procedures for measuring crack extension are provided in the standard, the basic procedure and the resistance curve procedure. The basic procedure involves physical marking of the crack advance and multiple specimens are used to develop an R-curve (or J-Δa curve). The resistance curve procedure is an elastic-compliance method where multiple points are determined from a single specimen. Other procedures for measuring crack extension are allowed, typically the electric potential drop method. To use the elastic-compliance method, the displacement transducer (clip-gage) must have a very high resolution and stability, and a very low noise. For temperature ranging from −200°C to 100°C, a clip-gage with conventional strain gages (i.e. plastic resistance strain gages) gives generally good results. However, at higher temperatures, one must use either high temperature inductive or capacitive clip-gages, or conventional clip-gages placed outside of the oven and connected to the specimen by ceramic or quartz rods. In both cases, the results are not very satisfactory. So, for the measurement of the load-line displacement, a new method using the digital image correlation technique (DIC) was developed at EDF R&D. The CT specimens have integral-machined knife edges and a thermally resistant paint is sputtered on these edges, in order to have irregular patterns. During the test, a high resolution camera placed outside of the oven takes pictures of the knife edges at regular time intervals. These pictures are real time processed to calculate the relative displacements of the dots, and deduce the load-line displacement. The paper presents the technique and the results obtained on various materials.

Some components (elbows, pump casings and lateral connections) of the primary loop of French PWRs are made of static cast duplex stainless steels. This kind of steel may age even at relatively low temperatures (in the temperature range of PWR service conditions), depending on the material composition. An important consequence of this ageing process is the decrease in the ductility and fracture toughness of the material. It is feared that an embrittlement, associated with the occurrence of casting defects, may increase the risk of failure. In order to build the primary loops, these components are welded and the behaviour of the weld heat affected zone (HAZ) is not well known. So a specific program was launched on this topic, involving metallurgical studies, fracture mechanics tests, medium-scale experiments, and finite element analyses. This paper presents the main characteristics and results of an experiment conducted on a 6″ aged cast pipe. This pipe contained a machined notch in the heat affected zone of a butt-weld and was tested under four-point bending at 300°C. The chemical composition of the steel was chosen to obtain a fast thermal ageing and low fracture toughness properties. During the test, the defect initiated and grew subsequently by ductile tearing. The test showed that it was possible to obtain a significant amount of stable crack growth (about 4 mm) despite the low toughness properties of the aged material. A detailed fracture mechanics analysis, based on finite element calculations, was performed. These calculations fairly simulated the overall behaviour of the tested structure, gave a conservative prediction of the crack initiation pressure and well predicted the crack size associated with the maximum applied bending moment. These tests and their detailed analyses contribute to validate and justify the methodology used in the integrity assessment of in-service cast duplex stainless steel components.