scholarly journals Comparison of BS 7910 and API 579-1/ASME FFS-1 Solutions With Regards to Leak-Before-Break

2016 ◽  
Author(s):  
Renaud Bourga ◽  
Bin Wang ◽  
Philippa Moore ◽  
Yin Jin Janin
Keyword(s):  
Experimental Data ◽  
Stress Intensity Factor ◽  
Stress Intensity ◽  
Material Properties ◽  
Crack Opening ◽  
Surface Cracks ◽  
Not Given ◽  
Reference Stress ◽  
The Common ◽  
Leak Before Break

One of the ways to aid the decision whether or not to live with defects in pressurised components is through the demonstration of Leak-Before-Break (LBB). In this paper, three of the main solutions to carry out the LBB assessment, namely Stress Intensity Factor (SIF), Reference Stress (RS) and Crack Opening Area (COA) have been evaluated and compared for both BS 7910 and API 579/ASME FFS-1 standards. Differences with respect to the choice of solutions and boundary conditions are illustrated and discussed. The same applied loads and material properties have been used when applying each procedure. Different geometries for potential pressurised components which are of interest with regards to LBB have been considered for each solution. Focus is made on cylinders where axially and circumferentially oriented through-wall and surface cracks were analysed. While SIF solutions produce similar results for both standards, reference stress solutions show greater differences in the results. However, in LBB assessments it is the reference stress solution which is more relevant, since most LBB assessments pre-suppose the material to be ductile. In terms of COA, solutions are not given exactly equivalent, however they seem to agree well within the common range of applicability. Differences in the assessment route between the standards is also discussed. Experimental data from literature has also been compared to the different standard predictions, to illustrate the accuracy of the solutions for axially oriented surface cracks. The ability of solutions to predict the boundary between leak and break is discussed, in relation to how this shows the level of conservatism.

Influence of Nozzle Transition on Leak Before Break Parameters

2012 ◽  
Author(s):  
Peter Baas ◽  
Frederic Blom
Keyword(s):  
Stress Intensity Factor ◽  
Stress Intensity ◽  
Stress Concentration ◽  
Finite Elements ◽  
Intensity Factor ◽  
Wall Thickness ◽  
Crack Opening ◽  
Negative Influence ◽  
Inner Radius ◽  
Leak Before Break

In the framework of leak before break a circumferential through-wall crack in a pipe with nozzle is investigated by means of finite elements and compared to a pipe only geometry. A range of crack angles (3–180°) and inner radius to wall thickness ratios (Ri/t = 5, 10, 20, 50 and 100) have been analysed in order to investigate parameter sensitivity. The nozzle and pipe models are compared for the leak before break parameters stress intensity factor (KI) and Crack Opening Area (COA). The results of the comparisons are presented as KI nozzle factor (KI nozzle / KI pipe) and COA nozzle factor (COAnozzle / COApipe). At small crack angles the KI nozzle factor is larger than unity, caused by the stress concentration at the nozzle to pipe intersection. At larger crack angles the KI nozzle factor decreases below unity, caused by the so-called bulging effect, which is apparent in the pipe only model. The COA nozzle factor is smaller than unity for all crack angles, caused by the increased stiffness of the nozzle. As the ratio Ri/t increases, both the KI and COA nozzle factors decrease since the nozzle stiffness relative to the pipe increases. For demonstration of leak before break the nozzle has a negative influence regarding the COA as smaller leaks are more difficult to detect. However, since the KI nozzle factor decreases at larger cracks the influence of nozzles regarding leak before break is not so straight forward.

Crack Opening Areas Under Combined Primary and Secondary Loading

2020 ◽  
Author(s):  
W. J. Brayshaw ◽  
P. James ◽  
J. Sharples ◽  
C. Aird
Keyword(s):  
Stress Intensity Factor ◽  
Finite Element ◽  
Stress Intensity ◽  
Intensity Factor ◽  
Plastic Material ◽  
Crack Opening ◽  
Combined Loading ◽  
Element Analysis ◽  
Elastic Plastic ◽  
Leak Before Break

Abstract Leak-before-break is a methodology to assess whether a leak through a defect in a pressurised component can be detected prior to the defect attaining a critical size. Developing leak-before-break arguments in non-stress relieved piping components can be challenging, in part due to the lack of solutions available, including in R6, to predict the crack opening area (required to evaluate leak rate) for combined primary and secondary stresses under elastic-plastic conditions. This is because the nature of the secondary stresses is to relax with plasticity, which can be captured in the calculation of the crack driving force (elastic-plastic stress intensity factor), but methods to account for the additional crack-tip strain this induces and its influence on crack opening are not available. Here primary stresses are those resulting from an applied force, such as pressure, and secondary stresses are those which result from an internal mismatch and do not contribute to plastic collapse, such as thermal or residual stresses. There is, of course, potential for a higher accuracy of crack opening area evaluations from finite element analysis modelling approaches, which include elastic-plastic material properties, in the presence of combined loading scenarios. In this study, a series of finite element analyses have been conducted whereby crack opening area and stress intensity factor have been calculated from circumferential through-wall defects, under the influence of combined primary and secondary stresses, where the magnitude and order of the combined stress has also been varied. The crack opening areas have been compared to current elastic ‘handbook’ solutions, which are conservative for primary stresses, to better understand the effect of plasticity on crack opening area and to help inform assessments when accounting for the inclusion of plasticity with secondary stresses.

The Effect of Crack Shape Idealisation on Leak-Before-Break Assessment

2016 ◽  
Author(s):  
Renaud Bourga ◽  
Bin Wang ◽  
Philippa Moore ◽  
Yin Jin Janin
Keyword(s):  
Stress Intensity Factor ◽  
Stress Intensity ◽  
Intensity Factor ◽  
Crack Growth ◽  
Crack Front ◽  
Structural Integrity ◽  
Crack Opening ◽  
Integrity Assessment ◽  
Crack Shape ◽  
Leak Before Break

Based on detailed 3D finite element (FE) analyses, idealized and non-idealized axial through-wall flaws were evaluated in a cylinder under internal pressure. The key parameters (Stress Intensity Factor, Reference stress, and Crack Opening Area) from widely accepted structural integrity assessment procedures (BS 7910 and API 579-1/ASME FFS-1) were explored and compared between idealized (perpendicular straight-sided flaw) and non-idealized geometry. The effect of crack shape on the evolution of stress intensity factors and crack opening areas along the crack front were also investigated. Non-idealized crack shapes have been modelled assuming a straight crack front with different internal and external crack lengths. The influence of crack shape has been evaluated by varying the crack front location and lengths ratios. The current findings highlight the significance of assessing a more realistic crack shape and should be considered in a leak-before-break (LBB) analysis. A non-idealized crack has a significantly smaller crack opening area than the equivalent idealized through-wall crack. Therefore the leakage rate at this stage of crack growth will be lower than predicted by standard solutions. Stress intensity factor solutions should also take the crack shape variation into account with regards to fatigue crack growth as a surface flaw propagates through-thickness.

scholarly journals Numerical Calculation of Stress Intensity Factors for Semi-Elliptical Surface Cracks in Buried-Arc Welded Thick Plates

Metals ◽  
2021 ◽  
Vol 11 (11) ◽  
pp. 1809
Author(s):  
Krešimir Jukić ◽  
Mato Perić ◽  
Zdenko Tonković ◽  
Ivica Skozrit ◽  
Tomislav Jarak
Keyword(s):  
Stress Intensity ◽  
Arc Welding ◽  
Crack Opening ◽  
Weld Line ◽  
Welding Process ◽  
Surface Cracks ◽  
Mode Iii ◽  
Thick Plates ◽  
Crack Behavior ◽  
Good Agreement

The present study deals with the influence of residual stresses induced by the buried-arc welding on the crack behavior in two butt-welded 20 mm thick plates. The following steps were undertaken: the thermo-mechanical simulation of the welding process, the mapping of stress results from a finite element (FE) mesh used for the welding simulation to a new FE mesh with a crack, the stress balancing, and the stress intensity factor (SIF) calculation. The FE and weight function (WF) methods were used to investigate the SIFs at the deepest point of semi-elliptical surface cracks with different geometries, orientations, and positions in relation to the weld line. In the case of cracks perpendicular to the weld line, the FE and WF results showed a good agreement for smaller cracks, while deviation between the results increases with the size of the crack. Considering the SIF solutions for the cracks of arbitrary orientation, it was observed that for some cases, the SIF value for mode III of crack opening can be of significant influence.

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.

Reliability of Welded Structures Containing Cracks in Heat-Affected Zones

2000 ◽  
Vol 122 (4) ◽  
pp. 225-232 ◽  
Author(s):  
David B. Lanning ◽  
M.-H. Herman Shen
Keyword(s):  
Fracture Toughness ◽  
Stress Intensity Factor ◽  
Stress Intensity ◽  
Crack Front ◽  
Surface Crack ◽  
Material Properties ◽  
Coarse Grained ◽  
Welded Structures ◽  
Weakest Link ◽  
Link Model

This study investigates the reliability of a plate containing a semi-elliptical surface crack intersecting regions of dissimilar material properties. A weakest-link model is developed to express fracture toughness distributions in terms of effective crack lengths that account for the varying stress intensity factor along the crack front. The model is intended to aid in the development of fracture toughness distributions for cracks encountering local brittle zones (LBZ) in the heat-affected zones (HAZ) of welded joints, where lower-bound fracture toughness values have been measured in the laboratory when a significant portion of the crack front is intersecting the coarse-grained LBZs. An example reliability analysis is presented for a surface crack in a material containing alternating bands of two Weibull-distributed toughnesses. [S0892-7219(00)01203-6]

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