Fatigue Modelling of Semi-Elliptical Surface Cracks in Welded Pipe Geometries

2016 ◽  
Author(s):  
Hsin Jen Hoh ◽  
John H. L. Pang ◽  
Kin Shun Tsang
Keyword(s):  
Stress Intensity Factor ◽  
Fatigue Life ◽  
Stress Intensity ◽  
Stress Concentration ◽  
Stress Intensity Factors ◽  
Surface Cracks ◽  
Intensity Factors ◽  
Magnification Factor ◽  
Weld Toe ◽  
Welded Pipe

Offshore pipelines and risers transfer oil and gas across long distances, from seabed to production facility to the surface. The long pipelines are formed by welding together pipe segments. The welded joints formed are a source of stress concentration and defects from which fatigue cracks can grow. Hence, it is imperative that the effect of the weld geometry on the stress concentration be understood so that appropriate measures can be taken to assess the potential remaining service life of the welded structure. The effects can be understood by the linear elastic fracture mechanics approach, where the stress intensity factors quantify the stress concentration. While the classical equations of Newman and Raju have been long available for semi-elliptical surface cracks in plates, no similarly elegant stress intensity factor solutions are available for pipes. There have been solutions in tabular form which can be cumbersome in practice. Moreover, solutions of welded pipe geometries have not been developed. The objectives of the current work are to develop closed-form solutions for stress intensity factors for external semi-elliptical surface cracks in plates. The welded pipe geometry will also be studied to develop solutions for the weld toe magnification factors of welded pipe geometries. The stress intensity factors can be used to determine the propagation rate of cracks in pipe or welded pipe geometries. The stress intensity factors are obtained by the J-integral output of the three-dimensional finite element method. First, a plate with a circular crack is modelled. The initial step transforms the model to a plate with a semi-elliptical crack with the appropriate crack aspect ratio and width. A second transformation follows to transform the geometry to pipe form. The main parameters studied are the relative crack depth to thickness, crack aspect ratio, radius and thickness. The developed stress intensity factor solutions can be reduced to the classical equations. The new solutions show good agreement compared to previous work. A similar approach is developed to study the welded pipe geometry to develop weld toe magnification factor solutions. The weld toe magnification factor solutions for certain geometries are presented as a function of the relative crack depth. The stress intensity factor solutions are then applied to predict the crack growth rates of cracks in pipe geometries. The prediction was conducted by a program written to assess the fatigue life of single and multiple cracks in pipes and welded pipes. The fatigue life assessment of welded pipes using the weld toe magnification factor solutions shows how significantly the weld geometry affects fatigue life.

Stress intensity factors for semi-elliptical internal surface cracks in autofrettaged thick-walled cylinders

1997 ◽  
Vol 32 (5) ◽  
pp. 351-363 ◽  
Author(s):  
X B Lin ◽  
R A Smith
Keyword(s):  
Stress Intensity Factor ◽  
Fatigue Life ◽  
Stress Intensity ◽  
Intensity Factor ◽  
Stress Intensity Factors ◽  
Inner Core ◽  
Yield Criterion ◽  
Interface Element ◽  
Surface Cracks ◽  
Intensity Factors

Stress intensity factors for internal semi-elliptical surface cracks in autofrettaged cylinders with and without internal pressures applied are presented. The three-dimensional finite element based displacement method with the crack tip square-root singularity of stresses and strains simulated is used to evaluate the stress intensity factors along the crack front. Both allowing and disallowing the overlapping of crack faces are considered in this investigation, the latter being simulated by considering crack surface contact through a kind of interface element introduced into the cracked area. The residual stress distribution assumed to act on the crack face is obtained according to Tresca's yield criterion with the material assumed to be elastic-perfectly plastic. Three different overstrains of autofrettage are chosen. The results show that the stress intensity factor is generally underestimated if the crack contact that has actually occurred is ignored, which may lead to a danger in the assessment of either fracture strength or fatigue life. Implications of the stress intensity factor results are also briefly discussed, particularly for the prediction of fatigue lives, and it is shown that the full autofrettage treatment might be the most beneficial for increasing the fatigue life of cracks initiated from the inner core.

Stress Intensity Factors for Partially Autofrettaged Pressurized Thick-Walled Cylinders Containing Closely and Densely Packed Cracks

2010 ◽  
Vol 132 (5) ◽  
Author(s):  
Q. Ma ◽  
C. Levy ◽  
M. Perl
Keyword(s):  
Residual Stress ◽  
Stress Intensity Factor ◽  
Fatigue Life ◽  
Stress Intensity ◽  
Stress Intensity Factors ◽  
Bauschinger Effect ◽  
Crack Depth ◽  
Internal Surface ◽  
Surface Cracks ◽  
Intensity Factors

Due to acute temperature gradients and repetitive high-pressure impulses, extremely dense internal surface cracks can be practically developed in highly pressurized thick-walled vessels, typically in gun barrels. In the authors’ previous studies, networks of typical radial and longitudinal-coplanar, semi-elliptical, internal surface cracks have been investigated assuming both ideal and realistic full autofrettage residual stress fields (ε=100%). The aim of the present work is to extend the analysis twofold: to include various levels of partially autofrettaged cylinders and to consider configurations of closely and densely packed radial crack arrays. To accurately assess the stress intensity factors (SIFs), significant computational efforts and strategies are necessary, especially for networks with closely and densely packed cracks. This study focuses on the determination of the distributions along the crack fronts of KIP, the stress intensity factor due to internal pressure KIA, the negative stress intensity factor resulting from the residual stress field due to ideal or realistic autofrettage, and KIN, the combined SIF KIN=KIP−|KIA|. The analysis is performed for over 1000 configurations of closely and densely packed semicircular and semi-elliptical networked cracks affected by pressure and partial-to-full autofrettage levels of ε=30–100%, which is of practical benefit in autofrettaged thick-walled pressure vessels. The 3-D analysis is performed via the finite element method and the submodeling technique employing singular elements along the crack front and the various symmetries of the problem. The network cracks will include up to 128 equally spaced cracks in the radial direction: with relative longitudinal crack spacing, 2c/d, from 0.1 to 0.99; autofrettage level of 30–100%; crack depth to wall thickness ratios, a/t, from 0.01 to 0.4; and, cracks with various ellipticities of crack depth to semicrack length, a/c, from 0.2 to 2. The results clearly indicate that the combined SIFs are considerably influenced by the three dimensionality of the problem and the Bauschinger effect (BE). The Bauschinger effect is found to have a dramatic effect on the prevailing combined stress intensity factors, resulting in a considerable reduction of the fatigue life of the pressure vessel. While the fatigue life can be finite for ideal autofrettage, it is normally finite for realistic autofrettage for the same crack network. Furthermore, it has been found that there are differences in the character of the SIFs between closely packed and densely packed crack networks, namely, more dramatic drop-offs in KIA and KIN at the crack-inner bore interface for densely packed cracks further influenced by crack depth.

Stress Intensity Factors for Circular-Arc Cracks in Plates

2018 ◽  
Author(s):  
D. J. Shim ◽  
S. Tang ◽  
T. J. Kim ◽  
N. S. Huh
Keyword(s):  
Stress Intensity Factor ◽  
Stress Intensity ◽  
Intensity Factor ◽  
Stress Intensity Factors ◽  
Crack Model ◽  
Surface Cracks ◽  
Intensity Factors ◽  
Element Analysis ◽  
Crack Face ◽  
Circular Arc

Stress intensity factor solutions are readily available for flaws found in pipe to pipe welds or shell to shell welds (i.e., circumferential/axial crack in cylinder). In some situations, flaws can be detected in locations where an appropriate crack model is not readily available. For instance, there are no practical stress intensity factor solutions for circular-arc cracks which can form in circular welds (e.g., nozzle to vessel shell welds and storage cask closure welds). In this paper, stress intensity factors for circular-arc cracks in finite plates were calculated using finite element analysis. As a first step, stress intensity factors for circular-arc through-wall crack under uniform tension and crack face pressure were calculated. These results were compared with the analytical solutions which showed reasonable agreement. Then, stress intensity factors were calculated for circular-arc semi-elliptical surface cracks under the lateral and crack face pressure loading conditions. Lastly, to investigate the applicability of straight crack solutions for circular-arc cracks, stress intensity factors for circular-arc and straight cracks (both through-wall and surface cracks) were compared.

Effect of Weld Residual Stress Fitting on Stress Intensity Factor for Circumferential Surface Cracks in Pipe

2012 ◽  
Author(s):  
Do-Jun Shim ◽  
Matthew Kerr ◽  
Steven Xu
Keyword(s):  
Residual Stress ◽  
Stress Intensity Factor ◽  
Stress Intensity ◽  
Intensity Factor ◽  
Wall Thickness ◽  
Stress Intensity Factors ◽  
Surface Cracks ◽  
Intensity Factors ◽  
Weld Residual Stress ◽  
Order Polynomial

Recent studies have shown that the crack growth of PWSCC is mainly driven by the weld residual stress (WRS) within the dissimilar metal weld. The existing stress intensity factor (K) solutions for surface cracks in pipe typically require a 4th order polynomial stress distribution through the pipe wall thickness. However, it is not always possible to accurately represent the through thickness WRS with a 4th order polynomial fit and it is necessary to investigate the effect of the WRS fitting on the calculated stress intensity factors. In this paper, two different methods were used to calculate the stress intensity factor for a semi-elliptical circumferential surface crack in a pipe under a given set of simulated WRS. The first method is the Universal Weight Function Method (UWFM) where the through thickness WRS distribution can be represented as a piece-wise cubic fit. In the second method, the through thickness WRS profiles are represented as a 4th order polynomial curve fit (both using the entire wall thickness data and only using data up to the crack-tip). In addition, three-dimensional finite element (FE) analyses (using the simulated weld residual stress) were conducted to serve as a reference solution. The results of this study demonstrate the potential sensitivity of stress intensity factors to 4th order polynomial fitting artifacts. The piece-wise WRS representations used in the UWFM was not sensitive to these fitting artifacts and the UWFM solutions were in good agreement with the FE results.

Stress Intensity Factor Interaction for Subsurface to Surface Flaw Transformations Under Stress Concentration Fields

2018 ◽  
Vol 140 (6) ◽  
Author(s):  
Pierre Dulieu ◽  
Valéry Lacroix ◽  
Kunio Hasegawa
Keyword(s):  
Stress Intensity Factor ◽  
Free Surface ◽  
Stress Intensity ◽  
Stress Concentration ◽  
Intensity Factor ◽  
Stress Intensity Factors ◽  
Concentration Field ◽  
Surface Flaw ◽  
Intensity Factors ◽  
Surface Proximity

If a single subsurface flaw is detected that is close to a component's free surface, a flaw-to-surface proximity rule is used to determine whether the flaw should be treated as a subsurface flaw, or transformed to a surface flaw. The transformation from subsurface to surface flaw is adopted as flaw-to-surface proximity rules in all fitness-for-service (FFS) codes. These proximity rules are applicable when the component's free surface is without a stress concentration. On the other hand, subsurface flaws have been found under notches, such as roots of bolts, toes in welded joints, or geometrical discontinuities of components. The stress intensity factors of the subsurface flaws are affected by the stress concentrations caused by the notches. The stress intensity factor of the subsurface flaw increases with increasing stress concentration factor of the notch and decreasing ligament distance between tip of the subsurface flaws and the notch, for a given notch width. Such subsurface flaws are transformed to surface flaws at a distance from the notch tip for conservative evaluations. This paper shows the interactions of stress intensity factors of subsurface flaws under stress concentration fields. Based on the interaction, a flaw-to-surface proximity criterion is proposed for a circular flaw under the stress concentration field induced by a notch.

Fatigue Algorithm for Modeling Many Weld Toe Cracks Propagating at Welded Joints

2016 ◽  
Author(s):  
Kin Shun Tsang ◽  
John H. L. Pang ◽  
Hsin Jen Hoh
Keyword(s):  
Stress Intensity Factor ◽  
Fatigue Crack ◽  
Fatigue Life ◽  
Stress Intensity ◽  
Fatigue Crack Growth ◽  
Welded Joints ◽  
Multiple Cracks ◽  
Surface Cracks ◽  
Paris Law ◽  
Weld Toe

Fatigue crack growth at welded joints often propagates from as many as tens to hundreds of small weld toe cracks along the weld toe line in offshore welded structures. This paper will present a fatigue algorithm for modeling many small weld toe cracks propagating from a welded joint. Cracks usually initiate at the weld toe region of the structures and propagate as surface cracks at the stress concentration regions of the weld-toe line. The presence of such weld defects or crack-like flaws can have a severe detrimental effect on their fatigue life and fracture resistance. Currently, there is a lack of studies that considers the effects of multiple cracks and their distribution density in welded joints. This work focuses on the fatigue analysis and modeling of multiple weld toe cracks, specifically in T-butt joints. Fatigue crack growth prediction is usually determined by the stress intensity factor range and crack propagation rate through Paris law. To predict the Stress Intensity Factor (SIF) of a weld toe crack, the magnification (Mk) factor was used. The Mk factor is influenced by the size of the welded attachment, as well as the size and depth of the weld toe crack. Simplified solutions for practical prediction of Mk factors were determined from 3D extended finite element method (XFEM) by modelling a semi-elliptical weld toe crack in a T-butt weld for cracks of different dimensions. The accuracy of the Mk factor solutions was verified by comparison to HSE fatigue data on 16 mm thick tubular joints. The Mk factor solutions were used to predict the growth of fatigue cracks using a model based on Paris Law and SIF solutions by Newman and Raju with plastic zone size corrections. Fatigue life was predicted for plates with and without attachments. It could be seen that the predicted life of a weld toe crack was severely reduced with the addition of a welded attachment. The model was extended to the multiple surface cracks commonly observed at the weld toe, where each crack is treated as independent, following established code procedures. The multiple cracks will coalesce as they propagate, until a single dominant crack emerges and fracture occurs. In this paper, the relationship between the fatigue life and the number and density distribution of the initial cracks was investigated. Fatigue life was predicted for plates with attachments with 1, 2, 10 and 100 cracks initially. The results show that as the number of cracks increases, the predicted fatigue life decreases.

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