Rauno Rintamaa

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.

Evaluating the NESC-I test and the integrated approach to structural integrity assessment

The NESC-I spinning cylinder test was designed to simulate selected conditions associated with an ageing reactor pressure vessel (RPV) subjected to severe pressurised thermal shock (PTS) loading and containing hypothetical flaws. It formed the focal point of the first project of the Network for Evaluation of Structural Components (NESC), with the objective of validating the combination of non-destructive inspection and structural mechanics assessment procedures for evaluating the integrity of such an aged structure containing postulated flaws. The huge amount of data generated over the seven-year project has been evaluated and is now available to the international community. The test demonstrated that, for the specific conditions considered, defects of up to 74 mm depth in material related to that of an ageing RPV would not propagate to cause catastrophic failure under a severe PTS-type thermal shock. This outcome was fully in line with the pre-test analysis forecasts, which combined the defect-size information supplied from blind inspections trials, a comprehensive materials data set, and a range of structural analysis tools.

Optimization of metallurgical variables in fracture prevention

Abstract The effect of metallurgical variables on the two most important crack growth mechanisms - stable crack growth by environmentally assisted cyclic crack growth and unstable crack growth by cleavage - in light water reactor pressure vessel steels is evaluated. The analyses are based on micromechanisms of fracture and sensitivity analysis, when applicable. Metallurgical variables considered are non-metallic inclusions and carbides as well as other parameters through their effects on yield strength and other mechanical properties.

Consistency of fracture assessment criteria for the NESC-1 thermal shock test

Abstract In this report, a best estimate analysis of the NESC Spinning Cylinder test is performed and the consistency of the state of the art application of the different fracture parameters and assessments criteria is examined. Direct fracture toughness measurements best describes the fracture behavior of the NESC Spinning Cylinder in the simulated pressurized thermal shock loading. The concept based on the ASME RT NDT reference temperature (either Pellini or Cv test) does not provide consistent description of the NESC1 fracture assessment. In general the fracture mode, i.e. ductile initiation and tearing followed by cleavage event, was successfully estimated by 3D FE based fracture assessment, combined with Master Curve description of fracture toughness.

Fracture behaviour of large scale pressure vessels in the hydrotest

Abstract Within the Nordic Countries a four-year research programme in the area of elastic-plastic fracture mechanics was initiated in 1985. This programme aims to assess the leak-before-break (LBB) criteria for pressure vessels and piping. The main experimental effort of the programme is to rupture large size pressure vessels, one having dimensions resembling those of a nuclear reactor pressure vessel, under internal pressure. Artificial flaws were made on the inner wall of the vessels. The dimensions of the flaws were defined by calculations so that the LBB condition was just anticipated during the test. For the time being two tests have been performed. The first test with a large pressure vessel was pressurized by water at 60°C, which was the lowest acceptable temperature for the hydrotest. In this paper experimental details including flaw preparation, instrumentation and material characterization are described. The fracture behaviour as well as experimental results of the tests are reported and compared to the analytical solutions of the analyses.

Crack initiation and arrest in a pressurized thermal shock test for a model pressure vessel made of VVER-440 reactor pressure vessel steel

Abstract A joint pressure vessel integrity research programme involving three partners is being carried out during 1990–1995. The partners are the Central Research Institute of Structural Materials “Prometey” from Russia, IVO International Ltd (IVO) from Finland, and the Technical Research Centre of Finland (VTT). The main objective of the research programme is to increase the reliability of the VVER-440 reactor pressure vessel safety analysis. This is achieved by providing material property data for the VVER-440 pressure vessel steel, and by producing experimental understanding of the crack behaviour in pressurized thermal shock loading for the validation of different fracture assessment methods. The programme is divided into four parts: pressure vessel tests, material characterization, computational fracture analyses, and evaluation of the analysis methods. The testing programme comprises tests on two model pressure vessels with artificial axial outer surface flaws. The first model vessel had circumferential weld seam at the mid-length of the vessel. A special embrittling heat treatment is applied to the vessels before tests to simulate the fracture toughness at the end-of-life condition of a real reactor pressure vessel. The sixth test on the first model led to crack initiation followed by arrest. After the testing phase, material characterization was performed. Comparison of calculated and experimental data generally led to a good correlation, although the work is being continued to resolve the discrepancies between the measured initiation and arrest properties of the material.

Cyclic crack growth evaluation of a 20MnMoNi55 piping steel in high-oxygen reactor water

Abstract Samples of a low alloy steel piping material taken from the full scale corrosion fatigue test loop of the Heissdampfreaktor (HDR) plant have been tested at 240°C in high oxygen reactor water. The small-scale specimens (CT25) were exposed to a similar loading spectrum to that which has been used in the full-scale corrosion fatigue tests at the HDR-plant. During the autoclave tests cyclic crack growth rates were determined. Fracture surface investigations were performed not only for the laboratory test specimens but also for the fracture surface of a sample taken from the HDR test loop piping after the full scale test. In this paper the autoclave testing results and fracture surface observations are presented and compared to those obtained in the HDR piping tests.

The feasibility of small size specimens for testing of environmentally assisted cracking of irradiated materials and of materials under irradiation in reactor core

Abstract Environmentally assisted cracking (EAC) or, in other words, stress corrosion cracking (SCC) of in-core materials has become an increasingly important reason for the downtime and maintenance costs of nuclear power plants (NPPs). Use of small size specimens for stress corrosion testing of irradiated materials is necessary because handling of high dose rate materials is difficult and the availability of these materials is limited. A drawback of using small size specimens is that they do not in some cases fulfil the requirements of the relevant testing standards and sometimes their limited load-bearing capacity prevents corrosion fatigue tests and tests with static loading at reasonable K I values. The test results show that the ductile fracture resistance curves of a Cu–Zr–Cr alloy are, to some extent, independent of the specimen geometry and size. However, the curves of small specimens deviate from the curves of larger specimens at high J values (large plastic zone relative to the remaining ligament) or when the crack growth exceeds about 30% of the remaining ligament. The size dependency of the tested Cu–Zr–Cr alloy seems to be a consequence of decreasing stress triaxiality as the size of the specimen is decreased. The results of the SCC tests of sensitized SIS 2333 stainless steel (equal to AISI 304) specimens in simulated boiling water reactor (BWR) water show that the plastic deformation of the remaining ligament of the specimen has no significant effect on the environmentally assisted crack growth rate. This indicates that stress corrosion testing is not limited by the specimen size. The size dependency in SCC tests should be further studied by conducting tests using various specimen sizes.

Validation of experimental and computational fracture assessment methods for flawed pressure components

Abstract Safety and integrity assessments of pressure boundary components require reliable knowledge of the material property values and the validated experimental and computational analysis methods. To improve the accuracy and validity of the experimental and computational fracture assessment methods, a four year Nordic research programme under the auspices of the Nordic Liaison Committee of Atomic Energy was initiated in 1985 and is now under completion. The main technical objective of the programme was to clarify how catastrophic failure can be prevented in pressure vessels and pipings. Experiments with small fracture mechanics specimens and pressure vessels were performed to validate the computational fracture assessment analysis. Two tests were conducted on a decommissioned full-scale chemical reactor pressure vessel from an oil refinery plant, and were extensively instrumented, e.g. by utilizing a 64-channel acoustic emission monitoring system. The scattering of their material property values were determined by numerous fracture mechanics samples. In addition, as a part of the experimental work, the reactor pressure vessel was repaired by welding after the first test. The repair was carried out without postweld heat treatment and welding was done by applying the temper-bead technique. Residual stresses were measured during and after welding. Different fracture assessment methods were developed and subsequently applied to the tested components. Inter-laboratory round robin programmes with the participation of several laboratories were arranged to examine elastic-plastic finite element calculations and fracture mechanics testing.

Fracture behaviour simulation of flawed full scale pressure vessel

Abstract In 1985 a four year research programme in elastic-plastic fracture mechanics was initiated within the Nordic countries. The aim of the programme was to verify the methods used for fracture analysis of real structures. A large cylindrical pressure vessel, having dimensions resembling those of a nuclear reactor pressure vessel, was tested in 1986. An artificial sharp axial surface flaw was made on the inner wall of the vessel. One of the circumferential welds intersected the crack at its midpoint. Failure of the vessel occurred as local rupture in the weld area. A maintenance deck, which was located around the midsection of the pressure vessel, was partially removed so as to prevent interference with the expected vessel deformations upon pressurization. After the test, two three-dimensional nonlinear finite element analyses were performed taking into account the existence of the circumferential weld in the ligament. In the first case, the original flawed structure was modelled allowing for different stress-strain curves of the base and weld material. In the second analysis, the model included a short through-the-wall crack in the weldment. Additionally, a simple two-dimensional analysis was made assuming the crack to be infinitely long in the axial direction. A three-dimensional analysis was repeated without considering the effect of the maintenance deck. For fracture mechanics evaluation, J- integrals along the crack front were calculated. In this paper, results of the three-dimensional analyses are reported and compared to experimental findings.