Bulging Factor for Axial Surface Cracks in Pipes

2014 ◽  
Author(s):  
Do-Jun Shim ◽  
Gery Wilkowski
Keyword(s):  
Surface Crack ◽  
Maximum Pressure ◽  
Surface Cracks ◽  
Correction Factors ◽  
Constant Depth ◽  
Dugdale Model ◽  
Crack Driving Force ◽  
Actual Surface ◽  
Secant Equation ◽  
Axial Surface

The bulging factor for an external constant-depth axial surface crack in a pipe was calculated by 3D FE simulations. This was done in a manner consistent with Folias’s original work for the axial through-wall-cracked pipe bulging factor (MT), but was evaluated in the elastic to full-plastic conditions. The results demonstrated that the actual surface-cracked pipe bulging factor is considerably lower than the bulging factor empirically derived by Maxey/Kiefner (Mp) back in the 1970s. Based on the results of the present study, it is suggested that Mp function in the Ln-Secant equation is not truly a bulging factor for axial surface crack. Rather it is an empirically developed equation with many correction factors embedded in it to apply the Dugdale model for prediction of maximum pressure of axial surface-cracked pipes. However, due to this empiricism, this method becomes invalid (or overly conservative) when it is applied in predicting the crack-driving force using the J-based Ln-Secant equation.

J-Estimation Schemes for Internal Circumferential and Axial Surface Cracks in Pipe Elbows

1998 ◽  
Vol 120 (4) ◽  
pp. 418-423 ◽  
Author(s):  
R. Mohan ◽  
A. Krishna ◽  
F. W. Brust ◽  
G. M. Wilkowski
Keyword(s):  
Shell Model ◽  
Driving Force ◽  
Three Dimensional ◽  
Surface Cracks ◽  
Dimensional Model ◽  
Three Dimensional Model ◽  
Crack Driving Force ◽  
Cracked Structures ◽  
Axial Surface ◽  
J Estimation

In the spirit of GE/EPRI fracture mechanics procedure, estimation schemes for the crack driving force for circumferentially and axially surface-cracked pressurized elbows subjected to bending are developed. These schemes are based on the results of line-spring/shell model. The line-spring/shell model offers an attractive and inexpensive alternative to performing a large number of analyses of surface-cracked structures. This model has been shown to provide accurate predictions in comparison with the more involved three-dimensional model by Mohan (1998). Using the results of this model and following the GE/EPRI procedure, the coefficient functions, F1 and h1, which provide the necessary information for predicting the crack driving force in cracked elbows, for several elbow and crack geometries are tabulated.

Constraint Effects of Surface Crack Depth on Toughness: Experimental and Numerical Assessments

2019 ◽  
Author(s):  
G. Wilkowski ◽  
S. Kalyanam ◽  
Y. Hioe ◽  
F. W. Brust ◽  
S. Pothana ◽  
...  
Keyword(s):  
Surface Crack ◽  
Crack Depth ◽  
Limit Load ◽  
Initiation Toughness ◽  
Surface Cracks ◽  
Flat Plates ◽  
Depth Effect ◽  
Ductile Tearing ◽  
Wide Range ◽  
Axial Surface

Abstract Work published for the first time at the ASME PVP 2017 conference showed that when on the upper-shelf, the toughness measured directly from surface-cracked pipe tests decreased as the flaw depth increased. A similar trend existed in SENT tests. Initially it was found that this flaw depth sensitivity of the toughness occurred for a very tough material like TP304 stainless steel. The significance of that result was that even for a material where limit-load was thought to exist, as the flaw depth increased the toughness dropped appreciably, and the failure analysis mode changed from limit-load to elastic-plastic fracture. Experimentally, this made sense because it explained the observed phenomena of load-controlled leak-versus-break behavior for circumferential surface-cracked pipes (as will be shown for several pipe tests), but that LBB behavior is not predictable from circumferential flaw limit-load analysis. Furthermore, the flaw depth effect on toughness also exists for axial surface cracks and even in flat plates with surface cracks. For axial surface cracks the implication was that the long-used empirical surface-crack bulging factor from Maxey/Kiefner (incorporated in many international codes and standards) actually incorporated both the bulging factor and the toughness changes with flaw depth. Because of the change in toughness with flaw depth, when using detailed finite-element fracture analyses for the crack-driving force it is possible to have more error in the failure stress predictions if a constant toughness is assumed for all surface-flaw depths. In fact, in another paper in the ASME 2019 PVP conference it will be shown that the toughness in a wrought TP304 elbow at crack initiation of a circumferential surface crack that was 68% of the thickness was about 1/3rd of the toughness from a standard 1T CT specimen made from the same material. Those results will also be reviewed. Similar results of toughness decreasing with flaw depth in surface-cracked pipes and SENT specimens for various materials over a large range of strain-hardening behavior will show the toughness decrease trend with flaw depth is consistent. To understand these trends more theoretically, 3D FE analyses were also conducted for one initial set of TP304 SENT specimens with a wide range of a/w values (0.3 < a/w < 0.9). The initiation toughness decreased by a factor of 5 to 6 as the crack depth increased; however, the Q value coinciding to the load at the start of ductile tearing was constant for the wide range of a/W values. Q at the start of ductile tearing in the SENT (Qi) was more consistent at normalized distances from the crack tip, rσo/J that were in the range from 0.25 to 1.5 rather than just the popularly considered rσo/J = 2. Hence, by having one SENT test result with a single a/W value, the Ji value for any other a/W can then be calculated. This is consistent with the experimental trends to date, but unfortunately Ji was found to be not proportional to the Q values as is conventionally assumed by many researchers at this time.

Fracture Toughness Variation With Flaw Depth in Various Specimen Geometries and Role of Constraint in Material Fracture Resistance

2017 ◽  
Author(s):  
Y. Hioe ◽  
S. Kalyanam ◽  
G. Wilkowski ◽  
S. Pothana ◽  
J. Martin
Keyword(s):  
Fracture Toughness ◽  
Crack Initiation ◽  
Crack Growth ◽  
Surface Crack ◽  
Crack Depth ◽  
Limit Load ◽  
Surface Cracks ◽  
Maximum Moment ◽  
Toughness Variation ◽  
Axial Surface

A series of pipe tests with circumferential surface cracks has been conducted along with fracture toughness tests using single-edge notch tension (SENT) specimens having similar crack depths and crack orientations as the surface-cracked pipes. This paper presents observation of measured fracture toughness variation due to the crack depth and discusses the effect of constraint on the material resistance to fracture. Crack-tip-opening displacement (CTOD) measurements were obtained with the use of a dual clip-gauge mounted on both the SENT specimens and center of the surface-cracks in the pipes. CTOD was obtained at both the crack initiation and during the crack growth through the ligament. CTOD is a direct measure of the material toughness in the pipe and SENT tests. CTOD at crack initiation and during crack growth can also be related to the material J-Resistance (J-R) curve. Commonly, the material resistance is assumed to be the same for all circumferential surface-crack geometries in a surface-cracked pipe fracture mechanics analyses. However, based on experimental observations on a series of recently conducted surface-cracked pipe tests, the CTOD at the center of the surface crack at the start of ductile tearing and maximum moment changed with the depth of the surface crack. This is believed to be a constraint effect on plasticity in the ligament which depends on crack depth. The CTOD values at crack initiation were decreasing linearly with crack depth. This linear decrease in CTOD trend with flaw depth was also observed in SENT tests. More importantly, the decrease in CTOD with surface crack depth was significant enough that the failure mode changed from being limit-load to elastic-plastic fracture even in relatively small-diameter TP304 stainless steel pipe tests. This toughness drop explains why the Net-Section-Collapse (limit-load) analysis overpredicted the maximum moment for some crack geometries, and why the deeper surface cracks tore through the pipe thickness at moments below that predicted by the NSC analysis for a through-wall crack of the same circumferential length. An “Apparent NSC Analysis” was developed in a companion paper to account for the changing toughness with crack depth [1]. Finally, this same trend in decreasing toughness with flaw depth is apparent in surface-cracked flat plates [2] and axial surface flaws in pipes [3]. The leak-before-break behavior for axial surface cracks is also not explained by numerical calculations of the crack-driving force when assuming the toughness is constant for all surface cracks and the through-wall cracks, but the change in toughness with surface flaw depth explains this behavior. Previously, axial flaw empirical limit-load solution was developed by Maxey and Kiefner [4], and is consistent with the observations from this paper.

Development of Burst Pressure Estimation Equations for Steam Generator Tubes With Multiple Axial Surface Crack

2018 ◽  
Author(s):  
Ji-Seok Kim ◽  
Myeong-Woo Lee ◽  
Jin-Weon Kim ◽  
Yun-Jae Kim
Keyword(s):  
Steam Generator ◽  
Surface Crack ◽  
Burst Pressure ◽  
Damage Analysis ◽  
Surface Cracks ◽  
Test Results ◽  
Estimation Equations ◽  
Pressure Estimation ◽  
Steam Generator Tubes ◽  
Axial Surface

In this paper, the burst pressure estimation equations for steam generator tubes with multiple axial surface cracks are proposed based on the local collapse load approach concept. The proposed equations are for a single axial surface crack, two collinear axial surface cracks and two non-aligned axial surface cracks. The proposed equations are validated against experimental tube burst test results and finite element damage analysis for twin cracks. Predicted burst pressures agree well with the experimental results and FE analysis results, suggesting validity of the proposed equations to estimate burst pressures for twin axial surface cracks.

Study on the Prediction of the Maximum Crack Extension Location for Surface Cracks

2016 ◽  
Vol 853 ◽  
pp. 3-7
Author(s):  
Jie Yang
Keyword(s):  
Crack Extension ◽  
Equivalent Plastic Strain ◽  
Simulation Method ◽  
Resistance Force ◽  
Surface Cracks ◽  
Constraint Effect ◽  
Crack Driving Force ◽  
Suitable Parameter ◽  
Maximum Crack ◽  
Finite Element Numerical Simulation

In this paper, the plate with different surface cracks (different constraints) was selected, the finite element numerical simulation method was used to mod el the J-integral and the equivalent plastic strain (εp) distributions ahead of crack front, after the unified constraint characterization parameter Ap was calculated, a new parameter which considered both J-integral and constraint effectwasdefined and a new methodology was provided to ensure the maximum crack extension location of surface crack. The results show that if the location of the maximum is defined as the maximum crack extension location, the prediction results is consistent with the measured results in experiments. The parameter which considered both crack driving force and material resistance force is a suitable parameter, and can be used to predict the maximum crack extension location.

J and CTOD Based Tensile Strain Capacity Prediction for Pipelines with a Surface Crack in Girth Weld

2021 ◽  
Author(s):  
Youn-Young Jang ◽  
Nam-Su Huh ◽  
Ik-Joong Kim ◽  
Young-Pyo Kim
Keyword(s):  
Crack Initiation ◽  
Internal Pressure ◽  
Surface Crack ◽  
Driving Force ◽  
Tensile Strain ◽  
Structural Integrity ◽  
Accurate Estimation ◽  
Long Distance ◽  
Crack Driving Force ◽  
Girth Welding

Abstract Long-distance pipelines for the transport of oil and natural gas to onshore facilities are mainly fabricated by girth welding, which has been considered as a weak location for cracking. Pipeline rupture due to crack initiation and propagation in girth welding is one of the main issues of structural integrity for a stable supply of energy resources. The crack assessment should be performed by comparing the crack driving force with fracture toughness to determine the critical point of fracture. For this reason, accurate estimation of the crack driving force for pipelines with a crack in girth weld is highly required. This paper gives the newly developed J-integral and crack-tip opening displacement (CTOD) estimation in a strain-based scheme for pipelines with an internal surface crack in girth weld under axial displacement and internal pressure. For this purpose, parametric finite element analyses have been systematically carried out for a set of pipe thicknesses, crack sizes, strain hardening, overmatch and internal pressure conditions. Using the proposed solutions, tensile strain capacities (TSCs) were quantified by performing crack assessment based on crack initiation and ductile instability and compared with TSCs from curved wide plate tests to confirm their validity.

The Effects of Classification of Misalignment-Induced Stresses in Engineering Critical Assessments of Welded Joints With Some Misalignment

2010 ◽  
Author(s):  
Liwu Wei
Keyword(s):  
Welded Joints ◽  
Driving Force ◽  
Limit Load ◽  
Surface Cracks ◽  
Plastic Limit ◽  
Secondary Stress ◽  
Butt Weld ◽  
Crack Driving Force ◽  
Plastic Limit Load

In the ECA of a structure or component such as a pipeline girth weld, the bending stress component arising from misalignment across the weld is often classified as primary, partly because standards such as BS 7910 and API 579-1/ASME FFS-1 do not give definitive guidance on this subject. This approach may be over-conservative as the σmis is localised. In order to obtain a more realistic assessment of the structural integrity of structures containing misalignment, it is necessary to understand the conservatism or non-conservatism in an ECA associated with the classification of σmis. To address the above concerns, systematic investigations were carried out of surface cracks in a plate butt-weld including some misalignment, external circumferential surface cracks and external fully circumferential cracks in a misaligned pipe connection. FEA of these cracked welded joints with some misalignment (typically from 1mm to 2mm) was performed to calculate crack driving force and plastic limit load. The results from FEA were compared with the existing solutions of KI and σref in BS 7910 generated by assuming three options of treating the σmis. The three options were: (1) classification of σmis wholly as primary stress; (2) 15% of σmis as primary and 85% of σmis as secondary stress; and (3) classification of σmis wholly as secondary stress. Variations in parameters (eg misalignment, crack size, loading, weld overmatch and base material properties) were taken into account in order to determine the effects of these parameters on plastic limit load and crack driving force. The implication of different classifications of σmis in terms of ECAs of misaligned welded joints was revealed by conducting BS 7910 Level 2B assessments with the use of a FAD. It was found in this work that for the cases examined, use of the σmis as entirely primary bending in an ECA was over-conservative, and even treatment of σmis as entirely secondary bending was generally shown to be still conservative, when compared with the assessments based on FEA solutions. Furthermore, caution should be exercised in using the solutions of KI and σref given in the existing BS 7910 for crack-containing structures subjected to a bi-axial or tri-axial stress state. A non-conservative estimate may result from the use of these solutions which have been derived based on a uniaxial stress condition.

Development of Z-Factor for Circumferential Part-Through Surface Cracks in Dissimilar Metal Welds

2008 ◽  
Author(s):  
D.-J. Shim ◽  
G. M. Wilkowski ◽  
D. L. Rudland ◽  
F. W. Brust ◽  
Kazuo Ogawa
Keyword(s):  
Correction Factor ◽  
Carrying Capacity ◽  
Limit Load ◽  
Maximum Load ◽  
Load Carrying Capacity ◽  
Surface Cracks ◽  
Crack Driving Force ◽  
Dissimilar Metal ◽  
Load Carrying ◽  
J Estimation

Section XI of the ASME Code allows the users to conduct flaw evaluation analyses by using limit-load equations with a simple correction factor to account elastic-plastic fracture conditions. This correction factor is called a Z-factor, and is simply the ratio of the limit-load to elastic-plastic fracture mechanics (EPFM) maximum-load predictions for a flaw in a pipe. The past ASME Section XI Z-factors were based on a circumferential through-wall crack in a pipe rather than a surface crack. Past analyses and pipe tests with circumferential through-wall cracks in monolithic welds showed that the simplified EPFM analyses (called J-estimation schemes) could give good predictions by using the toughness, i.e., J-R curve, of the weld metal and the strength of the base metal. The determination of the Z-factor for a dissimilar metal weld (DMW) is more complicated because of the different strength base metals on either side of the weld. This strength difference can affect the maximum load-carrying capacity of the flawed pipe by more than the weld toughness. Recent work by the authors for circumferential through-wall cracks in DMWs has shown that an equivalent stress-strain curve is needed in order for the typical J-estimation schemes to correctly predict the load carrying capacity in a cracked DMW. In this paper, the Z-factors for circumferential surface cracks in DMW were determined. For this purpose, a material property correction factor was determined by comparing the crack driving force calculated from the J-estimation schemes to detailed finite element (FE) analyses. The effect of crack size and pipe geometry on the material correction factor was investigated. Using the determined crack-driving force and the appropriate toughness of the weld metal, the Z-factors were calculated for various crack sizes and pipe geometries. In these calculations, a ‘reference’ limit-load was determined by using the lower strength base metal flow stress. Furthermore, the effect of J-R curve on the Z-factor was investigated. Finally, the Z-factors developed in the present work were compared to those developed earlier for through-wall cracks in DMWs.

Predicting the Brittle-to-Ductile Transition Temperatures for Surface Cracks in Pipeline Girth Welds: It’s Better Than You Thought

2008 ◽  
Author(s):  
Gery Wilkowski ◽  
David Rudland ◽  
Do-Jun Shim ◽  
David Horsley
Keyword(s):  
Transition Temperature ◽  
Surface Crack ◽  
Master Curve ◽  
Crack Depth ◽  
Temperature Shift ◽  
Transition Temperatures ◽  
Surface Cracks ◽  
Line Pipe ◽  
Brittle To Ductile Transition ◽  
Girth Welds

A methodology to predict the brittle-to-ductile transition temperature for sharp or blunt surface-breaking defects in base metals was developed and presented at IPC 2006. The method involved applying a series of transition temperature shifts due to loading rate, thickness, and constraint differences between bending versus tension loading, as well as a function of surface-crack depth. The result was a master curve of transition temperatures that could predict dynamic or static transition temperatures of through-wall cracks or surface cracks in pipes. The surface-crack brittle-to-ductile transition temperature could be predicted from either Charpy or CTOD bend-bar specimen transition temperature information. The surface crack in the pipe has much lower crack-tip constraint, and therefore a much lower brittle-to-ductile transition temperature than either the Charpy or CTOD bend-bar specimen transition temperature. This paper extends the prior work by presenting past and recent data on cracks in line-pipe girth welds. The data developed for one X100 weld metal shows that the same base-metal master curve for transition temperatures works well for line-pipe girth welds. The experimental results show that the transition temperature shift for the surface-crack constraint condition in the weld was about 30C lower than the transition temperature from standard CTOD bend-bar tests, and that transition temperature difference was predicted well. Hence surface cracks in girth welds may exhibit higher fracture resistance in full-scale behavior than might be predicted from CTOD bend-bar specimen testing. These limited tests show that with additional validation efforts the FITT Master Curve is appropriate for implementation to codes and standards for girth-weld defect stress-based criteria. For strain-based criteria or leak-before-break behavior, the pipeline would have to operate at some additional temperature above the FITT of the surface crack to ensure sufficient ductile fracture behavior.

Sign in / Sign up

Export Citation Format

Share Document