Circumferential Part-through Crack Research Articles

Allowable stresses related to crack growth stability and leak-before-break for circumferentially cracked pipes governed by limit loads: Part II, Pipes subjected to bending moment

If cracks are detected in power plant pipes and the cracks are allowable, the plants can continue operating without the need for repair/replacement. The allowability of cracks is determined by means of allowable stresses, based on failure stresses. Failure bending stress at crack penetration is estimated by applying the Local Approach of Limit Load Criteria, which is based on the net-section stress approach. Using the stress at crack penetration, the allowable bending stress can be calculated for a pipe with a circumferential part-through crack. The allowable bending stress decreases with increasing crack depth and angle, as expected. However, the allowable bending stress can have one of three characters, depending on crack size: i) leak-before-break (LBB) and crack growth stability, ii) non-LBB and crack growth stability, and iii) non-LBB and crack growth instability. Character iii) represents a severe crack, and special attention needs to be paid to such a crack, even if it is allowable. Frequent non-destructive inspection and precise calculation of subcritical crack growth are recommended in order to prevent unexpected failure, especially for small-diameter thin-walled pipes. The boundaries of leak-before-break and crack growth stability can be successfully expressed by appropriate equations.

Allowable stresses related to crack growth stability and leak-before-break for circumferentially cracked pipes governed by limit loads: Part I, Pipes subjected to tensile loading

When cracks are found in high toughness ductile pipes governed by limit loads at in-service inspections, and the applied stresses at the cracked locations are less than the allowable stresses, the cracks are allowable. The allowable stresses are generally determined in combination with failure stresses and safety factors. Failure of pipes with circumferential part-through cracks subjected to tensile loading occurs in a two-step sequence: first there is the onset of crack penetration, and then there is the break after the crack penetration. The Local Approach of Limit Load Criteria, which is based on the net-section stress approach, can estimate both crack penetration and break stresses for high-toughness ductile pipes. Using the Local Approach of Limit Load Criteria, allowable tensile stresses based on crack penetration stresses are determined by means of safety factors for plant service level conditions. The allowable stresses decrease with increasing crack angles and depths. The characters of the allowable tensile stresses are found to vary with changing sizes of crack angles and depths. These different characters involve the presence/absence of two parameters: leak-before-break (LBB) and crack growth stability. The crack size boundary on LBB vs non-LBB can be expressed by appropriate equations containing crack depths and angles. In addition, the boundaries of crack growth stability/instability are also expressed by crack depths and angles. These boundaries are taken into consideration during non-destructive inspection and crack growth evaluation.

Strain-based J-estimation scheme for fracture assessment of misaligned clad pipelines with an interface crack

A reference strain J-estimation formulation is developed to perform the fracture assessment of misaligned clad pipelines containing a circumferential part-through crack located at the interface of weld metal and outer pipe. An extensive parametric study including a total of 896 finite element (FE) models is carried out to investigate the effects of various parameters (pipe diameter, crack depth, crack length, centreline offset, strength mismatch level, and strain hardening exponent) on the J-integral. Then, the estimation formulation characterizing the linear relationship between the J-integral and the applied displacement loading is proposed using the standard least square fitting, and a goodness of fit for the J-integral values obtained from the formulation and the FE analysis is endorsed. Finally, the application range of the estimation formulation is mostly extended by conducting further comparative study.

CTOD estimation procedure for clad pipe girth welds subjected to bending load

This work describes a crack driving force estimation procedure applicable for structural integrity assessments of dissimilar girth welds in clad pipes subjected to high levels of bending load. The purpose of this study is to explore the application of the equivalent stress-strain relationship approach coupled with a weld bevel simplification scheme and the EPRI procedure to evaluate the CTOD for defects located at the weld centerline, including the effects of the weld strength mismatch, weld bevel geometry and the clad layer thickness. Parametric 3D finite element analyses of pipes with circumferential part-through cracks are performed to validate the weld joint simplification and to assess the accuracy of the crack driving forces predictions. Finally, the results show that the estimations using the proposed methodology are in close agreement to those obtained directly from finite element analyses

A New Solution Method for Vibration Analysis of Circular Cylindrical Thin Shells with a Circumferential Surface Crack

In this paper, a new solution method is proposed for determining the natural frequency of a given mode for a finite-length circular cylindrical thin shell with a circumferential part-through crack. The governing equation of the cracked cylindrical shell is derived by integrating the line-spring model with the classical thin shell theory. The proposed method calculates the natural frequency from an initial trial to satisfy both the governing equations and appropriate boundary conditions through an optimization process. The initial trial is proposed to satisfy the governing equations by using the beam modal function to determine the modal wavenumbers and mode shapes of cylindrical shells in the axial direction, assuming the flexural mode shapes of cylindrical shells in the axial direction to be of the same form as that of a flexural vibration beam with the same boundary conditions. Four representative sets of boundary conditions are considered: simply supported (SS-SS), clamped-clamped (C-C), clamped-simply supported (C-SS), and clamped-free (C-F). Compared with the finite element (FE) method, the proposed solution method is verified to provide an accurate and efficient way to calculate the dynamic characteristics of both intact and cracked cylindrical shells.

Dynamic Analysis of Finite-Length Circular Cylindrical Shells with a Circumferential Surface Crack

AbstractIn this paper, a new solution method is proposed for investigation of the vibration characteristics of finite-length circular cylindrical shells with a circumferential part-through crack. Four representative sets of boundary conditions are considered: simply supported, clamped-clamped, clamped-simply supported, and clamped-free. The governing equation of the cracked cylindrical shell is derived by integrating the line-spring model with the classical thin shell theory. A computationally efficient numerical solution method for determining the natural frequency of the system of a given mode is proposed. The algorithm calculates the natural frequency from an initial trial through a one-dimensional optimization process. Two initial trial estimation methods are considered: the beam-function method and Soedel’s expression method. On the basis of the case study results, a recommendation is made on the selection of a suitable initial trial calculation method. The verification of the proposed method is divi...

Development of a reference strain approach for assessment of fracture response of reeled pipelines

As a result of recent increase in exploitation of hydrocarbon resources in harsher environments and also installation techniques which utilize the materials plastic deformation capacity, accurate assessment of fracture response of pipelines subject to large plastic strains (e.g., typical of reeled pipes) has attracted particular interest nowadays. In this paper, an approach, based on the evaluation of the J-integral, is developed for assessing the integrity of such pipelines, manifested in a model of a pipeline with a circumferential part-through crack subjected to plastic bending. The proposed approach is an extension of the reference strain method developed earlier by other researchers, and takes advantage of the displacement controlled loading nature in such pipes (thus being suitable for Strain Based Design methodologies), and the resulting high strain levels, which often cause fracture response of the material in the plastic regime. The developed formulation relates the fracture response of the pipe (in terms of the non-dimensionalized J-integral) as a linear function of the axial strain in the pipe at its uncracked state. A series of 300 3D nonlinear finite element models using the ABAQUS software were analyzed in preparation of the equation that could assess the fracture response of such pipes with great accuracy. The resulting equation, calibrated by the finite element results, can predict the fracture response of pipes with a maximum error of 2% for a practical uncracked material strain range of 1.5% ⩽ εunc ⩽ 4%.

The influences of mesh subdivision on nonlinear fracture analysis for surface cracked structures

The leak-before-break (LBB) concept can be expected to be applied not only to safety assessment, but also to the rationalization of nuclear power plants. The development of the method to evaluate fracture characteristics is required to establish this concept. The finite element method (FEM) is one of the most useful tools for this evaluation. However, the influences of various factors on the solution are not well understood and reliability has not been verified enough. In this study, elastic-plastic 3D analyses are performed for two kinds of surface cracked structure, and the influences of mesh design are discussed. The first problem is for surface crack growth in a carbon steel plate subjected to tension loading. A crack extension analysis is performed under the generation phase simulation using the crack release technique. The numerical instability of the J- integral solution is observed when the number of elements in the thickness direction of the ligament is reduced to three. The influence of mesh design in the ligament on the solution is discussed. The second problem is for a circumferential part-through crack in a carbon steel pipe subjected to a bending moment. Two kinds of mesh design are employed, and a comparison between two sets of results shows that the number of elements on the crack surface also affects solution as well as the number of elements in the ligament.

Round-robin activities on finite element analyses of elastic-plastic fracture in Japan

The establishment of the leak-before-break (LBB) concept requires a method to evaluate the fracture characteristics. The finite element method can be used for this purpose but the solution is more or less influenced by the method employed. In this study, two round-robin analyses are performed for three-dimensional crack problems. The first problem is for surface crack growth in a carbon steel plate subjected to tension loading. Ten solutions are obtained by ten participants, and calculated results are compared with each other as to the applied load, displacement and J-integral. Though the relation between applied load and displacement is affected by modeling of the stress-strain curve, fairly good agreement is obtained between the solutions. The second problem is for a circumferential part-through crack in a carbon steel pipe subjected to a bending moment. Nine solutions are obtained by eight participants. The difference between the solutions is relatively significant as to the relation between J- integral and load-point displacement. A discussion is made about the sources of difference between each solution.

Circumferential Part-Through Cracks in Cylindrical Shells Under Tension

Circumferential Part-Through Cracks in Cylindrical Shells Under Tension

Evaluation of Load at Leak for Pressurized Pipes Containing Circumferential Part-through Cracks

Evaluation of Load at Leak for Pressurized Pipes Containing Circumferential Part-through Cracks

Ductile Fracture of Pipes and Cylindrical Containers With a Circumferential Flaw

The paper deals with the problem of ductile fracture of a pipe or cylindrical container having a relatively long and deep circumferential part-through crack or a through crack and subjected to a uniform axial membrane load in the crack region. After describing the evolution of the ductile fracture process, first the results of the elasticity solution for the circumferentially cracked cylindrical shell based on the Reissner’s transverse shear theory are presented. The elastic-plastic part-through crack problem is then considered. In the analysis the plastic deformations are approximated by a perfectly plastic layer similar to the conventional Dugdale model. The load carrying capacity of the cylinder is then estimated in various ways by using the crack opening stretch along the leading edge of the crack as the critical load factor.