Time-dependent leak behavior of flawed Alloy 600 tube specimens at constant pressure

Leak rate testing has been performed using Alloy 600 tube specimens with throughwall flaws. Some specimens have shown time-dependent leak behavior at constant pressure conditions. Fractographic characterization was performed to identify the time-dependent crack growth mechanism. The fracture surface of the specimens showed the typical features of ductile fracture, as well as the distinct crystallographic facets, typical of fatigue crack growth at low ΔK level. Structural vibration appears to have been caused by the oscillation of pressure, induced by a high-pressure pump used in a test facility, and by the water jet/tube structure interaction. Analyses of the leak behaviors and crack growth indicated that both the high-pressure pump and the water jet could significantly contribute to fatigue crack growth. To determine whether the fatigue crack growth during the leak testing can occur solely by the water jet effect, leak rate tests at constant pressure without the high-pressure pump need to be performed.     

Assessment of a creep-fatigue crack initiation and crack propagation for a welded cylindrical shell

An assessment of the creep-fatigue crack bahaviour for a cylindrical structure with weldments has been carried out by using a structural test and an evaluation with an assessment procedure. The structural specimen with a diameter of 600 mm and thickness of 7 mm is a welded cylindrical shell made of 316L stainless steel (SS) for one half of the cylinder and 304 SS for the other half. Eight artificial defects were machined and the defect behaviours were examined. In the creep-fatigue test, the hold time was 1 h at 600 °C and the primary nominal stress was 45 MPa. The evaluation results for the creep-fatigue crack initiation and crack propagation were compared with those of the observation images from the structural test. The assessment results for the creep-fatigue crack behaviour by using the French A16 procedure showed that the A16 guide is reasonably conservative but overly conservative for the creep-fatigue crack propagation in the case of a short hold time. It was shown that the crack initiation and propagation were dominated by a creep.     

Notched double-curvature shells with cracks under pulsating internal pressure

In the present paper, a double-curvature thin-walled shell with a surface crack starting from the root of an external circular-arc notch under cyclic loading is examined. The stress-intensity factor (SIF) along the elliptical-arc crack front is computed through a three-dimensional finite element analysis, for several elementary polynomial stress distributions perpendicular to the crack faces. By using the power series expansion of the actual stress field and the superposition principle, an approximate expression of the SIF in the case of a generic complex loading can be determined. Then the effect of the notch stress concentration on the SIF (and on the fatigue) behaviour is analysed for both cylindrical and spherical shells under (pulsating) internal pressure.     

Deterministic assessment of reactor pressure vessel integrity under pressurised thermal shock

Numerical investigations were carried out to assess the integrity of reactor pressure vessels under pressurised thermal shock (PTS). The 4-loop reactor pressure vessel with cladding was subjected to thermo-mechanical loading owing to loss of coolant accident. The loss of coolant accident corresponding to small break as well as hot leg breaks were considered separately, which led to axisymmetric and asymmetric thermal loading conditions respectively. Three different crack configurations, 360° circumferential part through, circumferential semi-elliptical surface and circumferential semi-elliptical under-clad cracks, were postulated in the reactor pressure vessel. Finite element method was used as a tool for transient thermo-elastic analysis. The various fracture parameters such as crack mouth opening displacement (CMOD), stress intensity factor (SIF), nil ductility transition temperature (RT_NDT) etc. were computed for each crack configuration subjected to various type of loading conditions. Finally for each crack a fracture assessment was performed concerning crack initiation based on the fracture toughness curve. The required material RT_NDT was evaluated to avoid crack initiation.     

Fracture analysis on the metal-lined hoop-wrapped cylinders with internal axial cracks

The metal-lined (steel-lined and aluminum-lined) hoop-wrapped cylinders with internal axial semi-elliptical cracks in the cylindrical portion center of the metal-liner are modeled by a three-dimensional finite element method. Crack front regions are modeled using singular elements, whereas the rest of the cylinder is modeled using twenty-node hexahedron elements. Not only the cylindrical body, but also the neck and transition areas of the cylinder, are considered in the modeling. The stress intensity factor K_I and crack mouth opening displacement (C ) for the metal-lined hoop-wrapped cylinders are calculated. The influence of the hoop-wrapped materials, the internal pressure and the crack sizes on the fracture behavior of these cylinders are discussed and the different fracture behaviors of the steel-lined hoop-wrapped cylinder and the aluminum-lined hoop-wrapped cylinder are discussed.     

Modelling of elliptical cracks in an infinite body and in a pressurized cylinder by a hybrid weight function approach

In this work, a hybridization technique is proposed. It consists of using two weight functions to model elliptical cracks for computation of the stress intensity factor ‘SIF’ in mode I. The idea of hybridization consists of dividing the ellipse into two zones, then to use each weight function in the area where it is more efficient. The proportion between the two zones is determined by optimization of the ellipse axis ratio. A computer code is developed for the computation of SIF. The treatment of the numerical procedures including singularities are presented in detail. The approach is tested on several applications (elliptical crack in infinite body, semi-elliptical cracks in thin and thick cylinders), to demonstrate its accuracy by minimization of the error of SIF and its correlation with respect to other researchers.     

Effect of vibration loading on the fatigue life of part-through notched pipe

A systematic experimental and analytical study has been carried out to investigate the effect of vibration loading on the fatigue life of the piping components. Three Point bend (TPB) specimens machined from the actual pipe have been used for the evaluation of Paris constants by carrying out the experiments under vibration + cyclic and cyclic loading as per the ASTM Standard E647. These constants have been used for the prediction of the fatigue life of the pipe having part-through notch of a/t = 0.25 and aspect ratio (2c/a) of 10. Predicted results have shown the reduction in fatigue life of the notched pipe subjected to vibration + cyclic loading by 50% compared to that of cyclic loading. Predicted results have been validated by carrying out the full-scale pipe (with part-through notch) tests. Notched pipes were subjected to loading conditions such that the initial stress-intensity factor remains same as that of TPB specimen. Experimental results of the full-scale pipe tests under vibration + cyclic loading has shown the reduction in fatigue life by 70% compared to that of cyclic loading. Fractographic examination of the fracture surface of the tested specimens subjected to vibration + cyclic loading have shown higher presence of brittle phases such as martensite (in the form of isolated planar facets) and secondary micro cracks. This could be the reason for the reduction of fatigue life in pipe subjected to vibration + cyclic loading.     

Enhancement of leak rate estimation model for corroded cracked thin tubes

During the last couple of decades, lots of researches on structural integrity assessment and leak rate estimation have been carried out to prevent unanticipated catastrophic failures of pressure retaining nuclear components. However, from the standpoint of leakage integrity, there are still some arguments for predicting the leak rate of cracked components due primarily to uncertainties attached to various parameters in flow models. The purpose of present work is to suggest a leak rate estimation method for thin tubes with artificial cracks. In this context, 23 leak rate tests are carried out for laboratory generated stress corrosion cracked tube specimens subjected to internal pressure. Engineering equations to calculate crack opening displacements are developed from detailed three-dimensional elastic-plastic finite element analyses and then a simplified practical model is proposed based on the equations as well as test data. Verification of the proposed method is done through comparing leak rates and it will enable more reliable design and/or operation of thin tubes.     

Effect of intermittent unloading or reversed loading on high temperature crack growth behaviour of Grade 91 steel under constant tensile load

Abstract

As a part of the efforts for developing a reliable assessment procedure for crack growth in high temperature components, crack growth tests at various loading conditions were performed on Grade 91 steel. 1T compact tension specimens of 20 mm thickness were kept under constant tensile load at 600°C, but periodically unloaded or reversely loaded to compressive side to observe these effects on deformation behaviour as well as crack growth behaviour. It was found that periodical reversed loading accelerates crack growth due to re-acceleration of inelastic deformation during load holding, but its extent was not as large as predicted by creep J-integral in a conventional way. The predictions were improved by introducing an additional parameter to take account of creep damage recovery which was caused by the excursion to compressive load.     

The shape of a surface crack in a plate based on a given stress intensity factor distribution

A new method is developed for the evaluation of a crack shape based on a given stress intensity factor (SIF) distribution for a surface crack under Mode-I loading conditions. The SIF distribution along the crack front is investigated using a direct simulation technique, in which the effect of crack closure on fatigue crack growth is considered. Then a SIF distribution function is chosen based on the numerical results. Crack shape (and the SIF) is achieved based on the given SIF distribution function using a numerical iterative procedure. Empirical SIF equations for surface cracks in plates subjected to tension and pure bending fatigue load are determined by systematic curve fitting of the numerical results. The depth ratio and the aspect ratio are considered in the ranges of 0.1–0.9 and 0.2–1.2, respectively. The aspect-ratio variation of surface cracks under fatigue loading is predicted. The application of the new method to predict the shape of a surface crack in plates subjected to tension and pure bending and comparisons of the results obtained with the predictions of the empirical equations proposed by Newman and Raju are presented.