A Statistical-Based Fatigue Crack Initiation Model for Axial Flaws in Zr-Nb Pressure Tubes

Flaws found during in-service inspection of CANDU Zr-2.5Nb pressure tubes include fuel bundle scratches, debris fretting flaws, fuel bundle bearing pad fretting flaws, mechanical damage flaws and crevice corrosion marks. The CSA Standard N285.8 contains procedures and acceptance criteria for evaluation of the structural integrity of CANDU Zr-2.5Nb pressure tubes containing flaws. One of the requirements is to evaluate the flaws for fatigue crack initiation. There was a need to develop a statistical-based model of fatigue crack initiation at flaws for use in deterministic and probabilistic assessments of Zr-2.5Nb pressure tubes. A number of fatigue crack initiation experiments have been performed on notched specimens from irradiated and unirradiated Zr-2.5Nb pressure tube material with a range of hydrogen equivalent concentrations. These experiments were performed in an air environment and included temperature and load rise time as test parameters. The test data has been used to develop a statistical-based model of fatigue crack initiation at flaws that covers the effects of flaw root radius, load rise time and irradiation. This paper describes the development of the statistical-based model.

Modeling and Simulation of Creep Crack Growth in High Chromium Steels

Safe and accurate methods to predict creep crack growth (CCG) are required in order to assess the reliability of power generation plants components. With advances in finite element (FE) methods, more complex models incorporating damage can be applied in the study of CCG where simple analytical solutions or approximate methods are no longer applicable. The possibility to accurately simulate CCG depends not only on the damage formulation but also on the creep model since stress relaxation, occurring in the near tip region, controls the resulting creep rate and, therefore, crack initiation and growth. In this perspective, primary and tertiary creep regimes, usually neglected in simplified creep models, plays a relevant role and need to be taken into account. In this paper, an advanced multiaxial creep model [1], which incorporates damage effects, has been used to predict CCG in P91 high chromium steel. The model parameters have been determined based on uniaxial and multiaxial (round notched bar) creep data over a wide range of stress and temperature. Successively, the creep crack growth in standard compact tension sample was predicted and compared with available experimental data.

Fracture Failure Research on Feculence Discharge Pipe of LPG Spherical Tanks

The rupturing of feculence discharge pipe happened during the gas-tightness test of LPG (Liquefied petroleum gas) spherical tanks. In order to find out the reasons of fracture failure of feculence discharge pipe, the Scanning electron microscopy (SEM) analysis, mechanical property testing, metallographic examination and energy spectrum analysis of corrosion products were used. The examination results show that both chemical composition and metallographic structure are normal, and the cracking is the brittle cleavage fracture. The energy spectrum analysis of corrosion products reveals that there are many sulfides in the corrosion products. Based on the loading conditions of spherical tanks, crack morphology and corrosion products, the primary reason of pipe cracking is stress corrosion cracking induced by H2S existed in liquefied petroleum gas. So it is suggested that the examination should be fulfilled for the feculence discharge pipe of such spherical tanks, the cracked pipes should be replaced, and the welding residual stress in the pipes should be relieved before replacing operation.

Comparisons of Monotonic, Low-Cycle and Ultra-Low-Cycle Fatigue Behaviours of X52, X60 and X65 Piping Steel Grades

Seismic actions, settlements and landslides, accidental loads, fluctuations in the layers of permafrost and pipelines reeling induce large plastic deformations, with widespread yielding in the pipelines which may lead to failure, either due to monotonic loading or due to cyclic plastic strain fluctuations with high amplitude and short duration (Nf<∼100 cycles). The damage mechanisms from the high intensity cyclic loading show distinct mechanisms from the monotonic and low-cycle fatigue (LCF) (∼100<Nf<∼10000cycles). This fatigue domain is often called ultra-low-cycle fatigue (ULCF) or extreme-low-cycle fatigue (ELCF), in order to distinguish it from LCF. Despite of monotonic ductile fracture and LCF have been subjected to significant research efforts and a satisfactory understanding of these damaging phenomena has been already established, ULCF regime is not sufficiently investigated nor understood. Consequently, further advances should be done since the data available in literature is scarce for this fatigue regime. In addition, the performance of ULCF tests is very challenging and there is no specific help from standards available in literature. In this work, the performance of X52, X60 and X65 API steel grades under monotonic, LCF and ULCF loading conditions are investigated. An experimental program was carried out to derive monotonic, LCF and ULCF data for three piping steel grades. Typical smooth geometries are susceptive to instability under ULCF tests. To overcome or minimize this shortcoming anti-buckling devices may be used in the ULCF tests. The use of notched specimens facilitates the deformation localization and therefore contributes to overcome the instability problems. However, the non-uniform stress/strain states raise difficulties concerning the analysis of the experimental data, requiring the use of multiaxial stress/strain parameters. Optical methods and non-linear finite element models were used to assess the strain and stress histories at critical locations, which are used to assess some damage models.

Numerical Investigation of Stress Concentration Factor at Irregular Corrosion Pit Under Tension-Torsion Loading

Finite element analysis is adopted to study the stress concentration of pit area under tension-torsion loading. The stress concentration factors under regular evolution and irregular evolution of pits are investigated by conducting a series of three-dimensional semi-elliptical pitted models. Based on the finite element analysis, it can be concluded that pit aspect ratio (a/2c) is a significant parameter affecting stress concentration factor (SCF) for regular evolution pits. Pits, having higher aspect ratio, are very dangerous form and can cause significant reduction in the load carrying capacity. When local dissolution occurs in the pitting area, SCF will have a sharp increase, it is more probable for a crack to initiate from these areas compared with pits for regular evolution. Furthermore, local dissolution coefficient is proposed to study effect of local dissolution within the pit on SCF.

STYLE: Modelling of the Extraction of Bend Specimens From Repair Weld and the Influence of Residual Stress on Fracture Testing

Within the EU 7th framework programme, STYLE, a number of large-scale tests have been performed. One of these tests, Mock-Up 2 (MU-2), was performed on a through wall crack located at a repair weld adjacent to a multi-pass narrow-gap weld. The aim of MU-2 was to investigate ductile crack growth under conditions with significant levels of residual stress. As part of the materials testing programme, low-constraint fracture specimens (three-point bend specimens with a/t=0.1) were extracted from the weld to test the weld materials fracture toughness. An overview of these tests is provided here. However, these low constraint tests demonstrated somewhat unusual fatigue crack growth on inserting the crack, leading to the crack depth being shorter in the centre of the specimens to the outside. Subsequently, although it has not been possible to use these specimens to determine the materials J-R curve, it does provide a features test for ductile modelling with the Gurson-Tvergaard-Needleman (GTN) local approach model for ductile crack growth. This paper provides an overview of the modelling associated to understand these observations, including an estimate of the retained residual stress, fatigue growth estimates and subsequent ductile modelling. An overview of the calibration of the GTN model is also provided using the weld material’s tensile tests, high constraint compact-tension tests and MU-2.

A Study on the Normalized Load Function for J-R Curve Testing Using Normalization Method

The purpose of this study is to suggest a more appropriate normalized load G-function for normalization method of J-R curve testing in the ASTM 1820 standard. For leak before break (LBB) design of reactor coolant piping system, J-R curve testing is required to verify the crack stability integration using J-T analysis. The normalization method of J-R curve testing is an excellent candidate testing method for dynamic J-R curve testing. In the normalization function, the load value is normalized by the G-function based on the plastic η factor. The normalized load function, G-function is important because the resultant J-R curve depends on the normalized function type used. However, for existing J-R curve calculation using the G-function in ASME standards, there exists a mismatch of estimated J integral values between the two different J-integral calculation approaches: the standard method in ASTM 1820, and the approach based on a J-integral physical concept with energy release rate. This problem is caused by the G-function type. To set up a more appropriate G-function, finite element (FE) analysis has been performed for compact tension specimens with various a/W and strain hardening index n. Also, application of the newly proposed G-function to J-R curve estimation is discussed.

Reversible Bending Fatigue Testing System Design Concepts for Spent Fuel Vibration Integrity Study

The dynamic analyses of a reversible bending fatigue testing system designed for the spent nuclear fuel (SNF) vibration integrity study was presented in this paper. Different design concepts were evaluated to assist the U-frame tester development. A finite element analysis (FEA) based on the vertical test setup was established to simulate the U-frame test system dynamic performances. The FEA results provided general guideline on predicting the dynamic behaviors of U-frame bending fatigue system for testing SNF. The initial vertical test setup design concepts were investigated in detail and the associated deficiencies were also discussed in the paper. The horizontal test setup was also evaluated using FEA. Based on dynamic analysis results, the horizontal test setup with symmetric loading profile was recommended for developing a reversible U-frame bending fatigue testing system.

Fracture Toughness in the Ductile-Brittle Transition and Thermal Ageing Behavior of Decarburized Heat Affected Zone of Alloy 52 Dissimilar Metal Welds of Nuclear Components

Dissimilar metal welds (DMW) are used in nuclear power plants between the nozzles of main components in low alloy steel and stainless steel pipes, or safe-ends connected to the main coolant line pipes. AREVA proposes for EPR™ an improved design of DMW involving narrow gap welding without buttering between the low alloy steel nozzles and the stainless steel safe-ends, and the use of a corrosion resistant weld filler metal (Alloy 52). AREVA performed a thorough characterization of this type of welds, which shows a particular microstructure close to the fusion line between the low alloy steel and the nickel base alloy, where the heat affected zone of the low alloy steel is decarburized. This paper presents results of fracture toughness tests performed with the crack tip located in this area, in the ductile to brittle transition in the as post-welded heat treated condition and after thermal ageing. The results show an excellent fracture toughness behavior of this particular area, compared to that of low alloy steel parent metal.

Thermal Aging Effects on Microstructures of Type-II Boundary in Dissimilar Metal Weld Joints

In order to investigate the long-term thermal aging effects on the type-II boundary region in Alloy 152 weld metal, a representative dissimilar weld mock-up made of Alloy 690–Alloy 152–A533 Gr. B has been fabricated and heat treated under accelerated temperature conditions. To simulate the thermal aging effects, the heat treatment was performed at 450°C for 15, 30 and 60-yr equivalent times (1,375, 2,750 and 5,500 h). The aging time was determined by the diffusion equation based on the activation energy for chromium diffusion. The microstructure characterization was primarily conducted in the type-II boundary region of the weld root, which is a boundary parallel to fusion boundary existing within 100um from the fusion boundary and is known to be less resistant to stress corrosion cracking than other regions in the weld. The investigations were performed by scanning electron microscope, electron backscatter diffraction, and nanoindentation test. In this study, the dilution zone of the chromium content was observed at the weld metal region within a 1.5-mm range from the fusion boundary. Ferrites and high angle grain boundaries are found at the type-II boundary region of weld metal. In the narrow zone between the type-II boundary and fusion boundary, the hardness is relatively higher than that of other regions. These results show that the chromium content in the dilution zone increases with heat treatment, but the stiff chemical gradient still exists in the weld region at the narrow zone between the type-II boundary and fusion boundary.