Improvements to J-Estimation Scheme for Predicting Moment-Rotation Response of Circumferential Through-Wall Cracks in Piping

2014 ◽  
Author(s):  
Jeong Soon Park ◽  
Richard Olson
Keyword(s):  
Thin Shell ◽  
Limit Load ◽  
Response Curves ◽  
Applicability Limit ◽  
Limited Applicability ◽  
Estimation Scheme ◽  
Suitable Form ◽  
Bending Loads ◽  
The Moment ◽  
J Estimation

This paper presents improvements to the LBB.ENG2 method for predicting the moment-rotation response of a circumferential through-wall cracked (TWC) pipe under combined tension and bending loads. The LBB.ENG2 method provides closed-form equations for J-integral (J) estimation using a load-displacement relationship, where the Mode I stress intensity factor (K) solution, beam differential equations, and limit load solutions are utilized for elastic and plastic displacements under a thin-shell assumption. Due to its good predictions and simplicity, LBB.ENG2 has been incorporated into the recent probabilistic fracture mechanics codes, PRO-LOCA and xLPR. The LBB.ENG2 method, however, has a limited applicability inherited from the thin-shell assumption and the K solution. That is, it might yield an unrealistic J for a thick pipe, or very short and long cracks. In this study, improvements are made to the method, and the thin-shell assumption is revisited. First, in order to extend the applicability limit of the K solution and, hence, the limit of the LBB.ENG2 method, newly developed and more accurate K solutions are implemented in a suitable form to derive equations explicitly for prediction of the crack instability point. Second, thin-shell and thick-shell assumptions are compared and technical justification for the use of the thin-shell theory is provided. In addition, based on the LBB.ENG2 method, moment-rotation response curves of circumferential through-wall cracked pipes are generated.

Improvement of the LBB.ENG2 Circumferential Through-Wall Crack J-Estimation Scheme

2016 ◽  
Author(s):  
Richard Olson ◽  
Sureshkumar Kalyanam ◽  
Jeong Soon Park ◽  
Frederick W. Brust
Keyword(s):  
Finite Element ◽  
Limit Load ◽  
Failure Surface ◽  
Crack Size ◽  
Finite Element Analyses ◽  
Critical Crack ◽  
Estimation Scheme ◽  
The Difference ◽  
Critical Crack Size ◽  
J Estimation

The LBB.ENG2[1] circumferential through-wall crack (TWC) J-estimation scheme forms the basis for the Extremely Low Probability of Rupture (xLPR)[2] probabilistic pipe fracture analysis for TWC elastic-plastic fracture mechanics (EPFM) stability assessment. The LBB.ENG2 methodology uses a reduced thickness pipe wall analogy to approximate the behavior of actual cracked pipe and sets the thickness of the reduced section by making the usual cracked pipe limit load assumption. Sometime during the original LBB.ENG2 development process, it was discovered that LBB.ENG2 was not as good as desired at predicting the maximum moment carrying capacity of pipe fracture experiments with longer cracks. Accordingly, the effective thickness equation was modified to be 1.0 at crack angles less than π/4, 4/π at angles greater than π/3, and linear between these values using a so-called ψ function. When LBB.ENG2 was coded for the TWC stability module for xLPR, TWC_fail, the behavior described above was implemented. Quite unexpectedly, with the new coding, exploration of TWC_fail’s bounds uncovered two discontinuities in the complete moment-pressure-critical crack size failure surface. Subsequently, it was found that these discontinuities were caused by the discontinuity in the derivative of the ψ function. This paper documents the approach used to smooth the TWC_fail moment-pressure-critical crack size surface by making a ψ function fit that minimizes the difference between J from LBB.ENG2 and J from finite element analyses results. The results of the finite element analyses and fitting methodology are described and the basic equations for the solution are presented. Following this, the new ψ function is applied to cases to evaluate the efficacy of the approach.

Determination of the Elastic-Plastic Fracture Mechanics Z-Factor for Alloy 182 Weld Metal Flaws for Use in the ASME Section XI Appendix C Flaw Evaluation Procedures

2007 ◽  
Author(s):  
G. Wilkowski ◽  
H. Xu ◽  
D.-J. Shim ◽  
D. Rudland
Keyword(s):  
Finite Element ◽  
Fracture Mechanics ◽  
Weld Metal ◽  
Alloy Steel ◽  
Limit Load ◽  
Full Scale ◽  
Low Alloy Steel ◽  
Evaluation Procedures ◽  
Estimation Scheme ◽  
J Estimation

One of the ways that the ASME Section XI code incorporates elastic-plastic fracture mechanics (EPFM) in the Section XI Appendix C flaw evaluation procedures for circumferential cracks is through a parameter called Z-factor. This parameter allows the simpler limit-load (or net-section-collapse) solutions to be used with a multiplier from EPFM analyses. Traditionally the EPFM solution was determined by using the GE-EPRI J-estimation scheme to determine the maximum load by EPFM, and Z = limit load / EPFM solution. The Z-factor is a function of the material toughness as well as the pipe diameter. With the advent of primary water stress-corrosion cracks (PWSCC) in pressurized water reactor (PWR) dissimilar metal welds (DMW), there is a need to develop Z-factors for Alloy 82/182 nickel-based alloy welds that are susceptible to such cracks. Although there have been Z-factor solutions for cracks in stainless and ferritic pipe butt welds, the DMW are somewhat different in that there is a much lower yield strength material on one side of the weld (typically forged or wrought 304 stainless steel) and on the other side of the weld the low alloy steel has a much higher strength than even the weld metal. This paper shows how 3D finite element analyses were used for a particular pipe size to determine the sensitivity of the crack location in the Alloy 182 weldment (crack in the center of weld, or closer to the stainless or low alloy steel sides), and how an appropriate stress-strain curve was determined for use in the J-estimation schemes. A Z-factor as a function of the pipe diameter was then calculated using the LBB.ENG2 J estimation scheme using the appropriate stress-strain curves from the finite element analysis. The LBB.ENG2 analysis was used rather than the GE-EPRI estimation scheme since it has been found that the LBB.ENG2 analysis is more accurate when compared with full-scale pipe tests. From past work, the GE-EPRI method was found to be the most conservative of the J-estimation schemes in predicting the maximum loads for circumferential flaws when compared to full-scale circumferentially cracked-pipe tests. The proposed Z-factor relationship should be restricted to normal operating temperatures (above 200C) with low H2 concentrations, where the Alloy 182 weld metal exhibits high toughness.

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.

The detachment of an inclined micro-pillar adhered to a dissimilar substrate

2021 ◽  
pp. 1-22
Author(s):  
Nitish Kumar ◽  
Syed Nizamuddin Khaderi
Keyword(s):  
Stress Intensity ◽  
Axial Load ◽  
Interfacial Crack ◽  
Axial Loading ◽  
Stress Fields ◽  
Singular Stress ◽  
Elastic Mismatch ◽  
Bending Loads ◽  
Singular Stress Fields ◽  
The Moment

Abstract We investigate the mechanics of the detachment of an inclined micro-pillar adhered to a dissimilar substrate when subjected to a combination of an axial load and end moment. When the micro-pillar has adhered to the substrate, singular stress fields exist at the bi-material corners. The order of singularity is estimated using asymptotic analysis. The first two terms in the asymptotic expansion lead to singular stress fields. The magnitude of the singularity is evaluated in terms of the elastic mismatch between the pillar and substrate and the micro-pillar inclination. The asymptotic stress due to the moment loading is more sensitive to the micro-pillar inclination when compared to that due to the axial loading. They are insensitive to the micro-pillar inclination when the micro-pillar is rigid when compared to the substrate. A short interfacial crack is further assumed to exist at the bi-material corner. This crack is embedded in the corner singularity region and is loaded by the singular fields due to axial and bending loads. A boundary layer analysis is performed on the singular zone to estimate the stress intensity factor when a short crack embedded in it is subjected to the singular fields. The stress intensity factors are also calculated for a long interfacial crack at the bi-material corner, which extends beyond the singular zone. Using the above results, we investigate the detachment of the inclined micro-pillar under the combination of an axial load and end moment.

Dissimilar Metal Weld Pipe Fracture Testing: Analysis of Results and Their Implications

2012 ◽  
Author(s):  
D. Rudland ◽  
R. Lukes ◽  
P. Scott ◽  
R. Olson ◽  
A. Cox ◽  
...  
Keyword(s):  
Experimental Data ◽  
Carrying Capacity ◽  
Nuclear Industry ◽  
Load Carrying Capacity ◽  
Base Metals ◽  
Dissimilar Metal ◽  
Load Carrying ◽  
Estimation Scheme ◽  
The Stability ◽  
J Estimation

Typically in flaw evaluation procedures, idealized crack shapes are assumed for both subcritical and critical crack analyses. Past NRC-sponsored research have developed estimation schemes for predicting the load-carrying capacity of idealized cracks in nuclear grade piping and similar metal welds at the operating conditions of nuclear power reactors. However, recent analyses have shown that growth of primary water stress corrosion cracks (PWSCC) in dissimilar metal (DM) welds is not ideal; in fact, very unusual complex crack shapes may form, i.e., a very long surface crack that has a finite length through-wall crack in the same plane. Even though some experimental data on base metals exists to demonstrate that complex shaped cracks in high toughness materials fail under limit load conditions, other experiments demonstrate that the tearing resistance is significantly reduced. At this point, no experimental data exists for complex cracks in DM welds. In addition, it is unclear whether the idealized estimation schemes developed can be used to predict the load-carrying capacity of these complex-shaped cracks, even though they have been used in past analyses by the nuclear industry. Finally, it is unclear what material strength data should be used to assess the stability of a crack in a DM weld. The NRC Office of Nuclear Regulatory Research, with their contractor Battelle Memorial Institute, has concluded an experimental program to confirm the stability behavior of complex shaped circumferential cracks in DM welds. A combination of full-scale pipe experiments and a variety of laboratory experiments were conducted. A description of the pipe test experimental results is given in a companion paper. This paper describes the ongoing analyses of those results, and the prediction of the load-carrying capacity of the circumferential cracked pipe using a variety of J-estimation scheme procedures. Discussions include the effects of constraint, appropriate base metal material properties, effects of crack location relative to the dissimilar base metals, and the limitations of the currently available J-estimation scheme procedures. This paper concludes with plans for further development of J-estimation scheme procedures for circumferential complex cracks in DM welds.

J-R Curves From Circumferentially Through-Wall-Cracked Pipe Tests Subjected to Combined Bending and Tension—Part II: Experimental and Analytical Validation

1998 ◽  
Vol 120 (4) ◽  
pp. 412-417 ◽  
Author(s):  
N. Miura ◽  
G. M. Wilkowski
Keyword(s):  
Internal Pressure ◽  
Three Dimensional ◽  
Combined Loading ◽  
Element Analysis ◽  
Dimensional Finite Element ◽  
Estimation Scheme ◽  
Materials Used ◽  
Three Dimensional Finite Element ◽  
Η Factor ◽  
J Estimation

In Part I (Miura and Wilkowski, 1998) of this paper, the theory of the two η-factor solutions for circumferentially through-wall-cracked pipes subjected to combined bending and tension due to internal pressure was presented. These solutions seemed to give reasonable predictions by comparing with the existing simplified J-estimation scheme. It was also ascertained that the J would be underestimated if the effect of the internal pressure was not properly considered. Consequently, this paper presents the application of these solutions to full-scale pipe tests. The tests were performed at 288°C (550°F) under combined bending and internal pressure. The materials used for the tests were both carbon steel and stainless steel. The effect of combined loading on the J-R curves was determined and compared to C(T) specimen J-R curves. The solutions were then verified by using three-dimensional finite element analysis.

Limit Load Solution and Crack Driving Force Estimation Scheme for Embedded Flaws in Pipeline Girth Welds

2015 ◽  
Author(s):  
Aurélien Pépin ◽  
Tomasz Tkaczyk ◽  
Noel O’Dowd ◽  
Kamran Nikbin
Keyword(s):  
Driving Force ◽  
Research Work ◽  
Limit Load ◽  
Embedment Depth ◽  
Material Behaviour ◽  
Force Estimation ◽  
Crack Driving Force ◽  
Estimation Scheme ◽  
Girth Welds ◽  
Subsea Pipelines

The acceptability of flaws in a subsea rigid pipeline is usually sanctioned based on the results of an engineering criticality assessment (ECA), carried out considering all loads seen by the pipeline from fabrication to the end of service life. Reel-lay is an efficient installation method, frequently used for installing subsea pipelines. Unlike surface breaking flaws, embedded flaws are not directly assessed in a reeling ECA because the available assessment solutions are too conservative. A work around approach is often used, whereby acceptable surface breaking flaw sizes are deemed acceptable beneath the surface, provided that the embedment depth is equal to or greater than half of the flaw height. However, the results of more recent research work suggest that this approach could be non-conservative in some cases. In this work, a parametric finite-element (FE) study was carried out to assess the effect of the embedment depth, the crack length and the crack height on the load required to cause collapse of the shorter ligament of an embedded flaw. Subsequently, a closed form limit load solution was developed, and compared against available solutions for pipes subjected to tension. A J-based crack driving force (CDF) estimation scheme was developed for a selected material behaviour. Finally, recommendations were made for the direct reeling ECA of subsea pipelines with embedded flaws.

Sign in / Sign up

Export Citation Format

Share Document