Fracture Toughness Tests on Western RPV Steels Using Small Scale Specimen Technique

In order to ensure continued safe operation of European nuclear reactors, it is necessary to solve specific issues that arise from irradiation induced reactor pressure vessel (RPV) embrittlement under long term operation. This requires an extension of the RPV surveillance programs to cover longer operation times than originally planned. The limited availability of surveillance materials poses a challenge for the feasibility of such programs. Among others, the use of the small specimen technology is a promising option to overcome the lack of materials. For example, a number of not less than 16 sub-sized 0.16 C(T) specimens (4 mm thickness) can be manufactured from two tested Charpy sized (10 mm × 10 mm × 55 mm) specimens, allowing a reliable determination of the reference temperature T0. Such Charpy sized fracture mechanics specimens are currently widely used in the RPV surveillance programs. To establish the methodology for fracture mechanics testing of irradiated and unirradiated RPV steels using sub-sized specimens, a joined R&D project was launched partly financed by the German Federal Ministry for Economic Affairs and Energy. Moreover the following points shall be addressed: • Manufacturing, pre-cracking procedure, measurement of the crack opening displacement and load line displacement under hot cell conditions • Demonstration of the transferability of fracture mechanics data The purpose is to demonstrate that the results measured on sub-sized specimens can safely be used in the safety assessment of RPVs. In addition, the results will establish a basis to assess results from international projects regarding sub-sized fracture mechanics specimens.

Evaluation of Deformation and Fracture Behaviors of Nuclear Components Using a Simulated Specimen Under Excessive Seismic Loads

This study designed a specimen that can simulate deformation and crack initiation in system, structure, and components (SSCs) of nuclear power plants (NPPs) under excessive seismic loads, and conducted ultimate strength tests using this specimen at room temperature (RT) and 316°C. The specimen designed was a compact tension (CT) type with a round notch, and both SA312 TP316 stainless steel (SS) and SA508 Gr.3 Cl.1 low-alloy steel (LAS) were used in the experiment. Displacement-controlled cyclic loads with constant and random amplitudes were applied as input loads for the test. One set of input loads consisted of 20 cycles, and the input amplitudes of load-line displacement (LLD) were determined to induce the maximum elastic stress of 6∼ 42Sm on the specimen, where Sm is allowable design stress intensity. The input LLD had a triangular waveform and was fully reversed for both types of amplitude. During the test, multiple sets of input cyclic loads, with increasing amplitude of input LLD, were applied to the specimen until a crack was initiated. The results demonstrated that the specimen used in this study adequately simulates the deformation and failure behaviors of SSCs under excessive seismic loads. In addition, the samples in both materials failed under cyclic load levels that were several times higher than those of design basis earthquake (DBE). The SA316 TP316 SS specimen had a greater safety margin under excessive seismic loading conditions than SA508 Gr.3 Cl.1 LAS specimen, regardless of test temperature.

Load Line Displacement Partitioning in Creep Crack Growth Analyses of 316H Stainless Steel

Accelerated creep crack growth tests in the laboratory can lead to greater levels of plasticity at the tip of a creep crack than would be experienced in service. This is problematic when trying to determine C* which is used to model the stress field ahead of a crack. Deflection partitioning methods must be used in order to determine the contribution to the load line displacement rate as a result of creep which in turn is used to calculate C*. This partitioning can lead to negative values of the creep load line displacement rate due to the high contribution from plasticity. The amount of assumed plasticity is likely to be erroneously high as it is currently assumed that the material behaviour fits a Ramberg-Osgood model, when in reality such a fit does not predict the behaviour well over a large range of stress. This work compares the load line displacement determined from solutions based on a Ramberg-Osgood model with those calculated from finite element simulations using uniaxial tensile data to model the plasticity. The simulations formulated crack growth by means of a crack length vs time criterion using experimental crack growth data. It is found that the theoretical solutions do over predict the amount of plastic deformation compared to the numerical results. It is also found that for the short term test considered, the load-line displacement due to creep deformation was small compared to that from crack growth.

Proposal of the hybrid solution to determining the selected fracture parameters for SEN(B) specimens dominated by plane strain

In the paper, new hybrid (numerical-analytical) methods to calculate the J-integral, the CTOD, and the load line displacement are presented. The proposed solutions are based on FEM calculations which were done for SEN(B) specimens dominated by plane strain condition. The paper includes the verification of the existing limit load solution for SEN(B) specimen with proposal of the new analytical formulae, which were used for building hybrid equations for determining three selected fracture mechanics parameters.

Improved Elastic Compliance Equation and its Inverse Solution for Compact Tension Specimens

ASTM E1820 is a well-developed fracture test standard and has been used worldwide for fracture toughness testing on ductile materials in terms of the J-integral or J-R curve. This standard recommends the elastic unloading compliance technique for measuring crack length in a single specimen test, and an accurate elastic compliance equation is needed to estimate physical crack length. Compact tension (CT) specimen is one of the most often used standard specimens with crack length ratios of 0.45≤a/W≤0.70 prescribed in E1820 for J-R curve testing. The stress intensity factor K of CT specimens used in E1820 was developed by Srawley (IJF, 1976) and has been commonly accepted as the most accurate solution. The compliance equation of CT specimens was developed by Saxena and Hudak (IJF, 1978) and has been used in ASTM E1820 for decades. However, recent results showed that the load-line displacement (LLD) compliance equation is not consistent with that determined from its K solution, and the maximum error of LLD compliance can be larger than 7% at a/W = 0.32 and ∼ 5% at a/W = 0.45 (E1820 standard crack size). The FEA results confirmed that the K solution in E1820 is indeed very accurate, but its compliance equation is less accurate. Thus, an improved compliance equation with high accuracy is developed from the accurate K solution using the numerical integration technique and shooting method.