Matti Valo

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.

On irradiation embrittlement and recovery annealing mechanisms of Cr-Mo-V type pressure vessel steels

Abstract Surveillance specimens of 15Kh2MFA reactor pressure vessel (RPV) steel and Sv-10KhMFT weld metal were investigated using internal friction, electrical resistivity and Mossbauer spectroscopy measurements in order to obtain information on their irradiation embrittlement and recovery annealing mechanisms. Internal friction measurements showed that carbon becomes trapped by irradiation-induced defects (fluence=4.1×10 19 n/cm 2 , E >1 MeV, T =265 °C). During the recovery annealing (450 and 475 °C/1 h) carbon does not redissolve into the solid solution. Although irradiation usually increases the electrical resistivity of the RPV steels, in this study irradiation decreased the electrical resistivity of both the base and the weld metals. This can be due to a decrease in the dislocation density or precipitation under irradiation. The annealing temperature range for recovery was 300–600 °C for the base and the weld metals according to the electrical resistivity measurements. Some evidence of irradiation-induced carbide formation was achieved by the Mossbauer spectroscopy measurements.

Towards a uniform precracking procedure for fracture toughness testing

The precracking requirements for fracture toughness testing of metallic materials are investigated in this study. In addition, some experiments were performed in the transition region to quantify the consequences of violating these requirements. It is found that the different standards are not consistent. For most of the requirements some rationale can be found. Some theoretical developments were performed to justify the use of a fixed maximum stress intensity factor. This limit is tightly related to the fracture mechanism and to the fracture process zone. From an experimental point of view, a relatively limited effect of precracking on the fracture toughness is found.

Comparison of Charpy-V and J-integral transition temperature shifts in CrMoV pressure vessel steels

Abstract Both Charpy-V and precracked Charpy-size specimens were included in the original surveillance programme of the Loviisa nuclear power plant pressure vessel made of CrMoV steel. With precracked specimens the J- value at the onset of brittle fracture was determined as a function of temperature and irradiation dose. The irradiation induced transition temperature shifts measured with Charpy-V and precracked three-point bend specimens are correlated by using indexed temperature values. The shapes of brittle fracture transition curves measured with the fracture mechanical specimens both for the irradiated and non-irradiated material state are compared using a statistical brittle fracture model.

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.

Descriptive characteristics of different types of test for irradiation embrittlement

Abstract Within the IAEA coordinated programme on optimizing of reactor pressure vessel surveillance programmes and their analysis, phase 3, a specially tailored radiation sensitive correlation monitor material, a Japanese steel plate with code designation JRQ, a French forging material (FFA) and a Japanese forging material (JFL) were selected for the investigations to be carried out in Finland. Based on the evaluation of the experimental results it was demonstrated that dynamic fracture toughness transition shift is equivalent to the Charpy-V shift, but the static fracture toughness transition shift may be considerably larger than the dyanamic shift. Thus, Charpy-V is not suitable for estimating the static fracture toughness transition shift. These findings have a strong impact upon the design of future surveillance programmes.

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).

Experience of irradiation embrittlement in the Loviisa reactor pressure vessels

Abstract The water gap between the wall and the core of the RPV (Reactor Pressure Vessel) in a VVER-440 plant is small compared with typical Western type LWR 5 . The neutron fluence on the RPV wall is, consequently, much higher in a VVER-440 plant. In older VVER-440 plants the material of the RPV, especially the horizontal core weld, contains so much impurities (P- and Cu-content) that the irradiation embrittlement has become a problem. On bases of fracture mechanics analyses in Loviisa, IVO has been forced to make several measures to ensure safe operation of the plants. According to IVOs current understanding, both plants may be in operation for the design life without annealing of the RPVs.

Application of the advanced instrumented impact testing facility for the irradiation damage assessment

Abstract A new test method suitable for the dynamic fracture toughness evaluation of irradiated specimens has been developed. The method is based on the application of the new pendulum type impact tester equipped with a self-adjusting IR-laser extensometer and fully computerized data acquisition and processing. The testing system contains several unique features, which make the impact testing more appropriate from the fracture mechanics point of view. These features include, for example, the inversion of the test geometry to reduce inertia and specimen oscillation effects, the utilization of larger than normal Charpy-size specimens, and the application of the IR-extensometer to evaluate ductile fracture initiation enabling the determination of the energy-based fracture toughness parameters. In this paper the testing procedure will be described and its application on the characterization of the unirradiated and irradiated IAEA phase III correlation monitor material for surveillance programmes will be presented.