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.

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.

Analysis and Testing of a 13Cr Pipeline to Demonstrate “Leak Before Break”

2012 ◽  
Author(s):  
Robert M. Andrews ◽  
Neil Millwood ◽  
Sanjay Tiku ◽  
Nick Pussegoda ◽  
Menno Hoekstra ◽  
...  
Keyword(s):  
Fracture Mechanics ◽  
Weld Metal ◽  
Limit Load ◽  
Parent Material ◽  
Full Scale ◽  
Small Scale ◽  
Element Analysis ◽  
Girth Welds ◽  
Full Scale Tests ◽  
Leak Before Break

As part of a safety case for a subsea 13Cr pipeline, the operator wished to demonstrate that if a circumferential through wall crack developed, the crack would remain stable as a leak rather than growing to a full bore rupture. An initial fracture mechanics analysis had suggested that the margins on crack length were too small to make such a “leak before break” argument. This paper reports an integrated programme of small scale testing, numerical modelling and full scale testing which showed that a leak before break case could be made. 13Cr martensitic steel generally shows excellent toughness at the service temperature, as does the super duplex weld metal that was used for the girth welds. However, as the pipeline had been installed by reeling, there was some concern that the toughness may have been reduced. Hence a programme of fracture toughness testing was designed to generate tearing resistance curves for both as-received and pre-strained parent material and weld metal. Deep and shallow through thickness notched specimen geometries were tested to explore the effect of constraint on the toughness. Finite element analysis was used to predict the stress intensity for a range of crack lengths, including the effects of misalignment. Non-linear analyses were used to estimate the limit load for the cracked pipe. The test results were used as input to tearing analyses to Level 3 of BS 7910. These showed that the tolerable length of a through wall crack exceeded the length of anticipated defects by a factor of at least two. To confirm the fracture mechanics predictions, two full scale tests were carried out. These used pressure cycling to grow a through wall crack by fatigue. These cracks were stable under an internal pressure equal to the pipeline design pressure. The cracked specimens were then axially loaded to failure. Extensive tearing occurred before final failure at loads above those predicted by the fracture analysis, confirming the conservatism of the predictions.

To Analyze the Effects of Geometric Misalignment in a Cylindrical Pressure Vessel

2012 ◽  
Vol 152-154 ◽  
pp. 964-969 ◽  
Author(s):  
Musharaf Abbas ◽  
Asif Israr ◽  
Atiq Ur Rehman
Keyword(s):  
Finite Element ◽  
Alloy Steel ◽  
Pressure Vessel ◽  
Structural Integrity ◽  
High Strength ◽  
Fe Analysis ◽  
Low Alloy Steel ◽  
Affected Area ◽  
Cylindrical Pressure Vessel ◽  
Geometrical Effects

This particular work consider a pressurized vessel typically made of high strength low alloy steel and containing the geometric misalignment at the cylinder-to-cylinder junction. This misalignment produce in the vessel’s structure is because of girth weld that is evident in most of the fabrication of such type of structures apart from other factors which is beyond the scope of this study. This study evaluates the geometrical effects of mismatch on the structural integrity of the pressure vessel and prediction of stresses at the affected area of the cylinder. Analytical and Finite Element (FE) approaches are employed to analyze the configuration. FE analysis is performed by the use of ANSYS on one quarter of the structure due to symmetry. FE results are also compared with the analytical results of different authors. In addition, maximum allowable mismatch is also determined and is a part of this study.

scholarly journals Impact Toughness of Steel WMD After TIG Welding

2017 ◽  
Vol 62 (3) ◽  
pp. 1647-1650
Author(s):  
T. Węgrzyn ◽  
J. Piwnik ◽  
Z. Stanik ◽  
D. Węgrzyn ◽  
D. Sieteski
Keyword(s):  
Artificial Intelligence ◽  
Impact Toughness ◽  
Weld Metal ◽  
Alloy Steel ◽  
Gas Mixtures ◽  
Tig Welding ◽  
Low Alloy Steel ◽  
Jet Cooling ◽  
Metal Deposit ◽  
Alloy Weld

AbstractThe material selected for this investigation was low alloy weld metal deposit after TIG welding with various amount of oxygen in weld metal deposit (WMD). After TIG process it is difficult to get proper amount of oxygen in WMD on the level much lower than 350 ppm. The highest impact toughness of low alloy WMD corresponds with the amount of oxygen in WMD above 350 ppm. In the paper focuses on low alloy steel after innovate welding method with micro-jet cooling that could be treated as a chance on rising amount of oxygen in weld. Weld metal deposit (WMD) was carried out for TIG welding with micro-jet cooling with various amount of oxygen in WMD. In that paper various gas mixtures (gas mixtures Ar-O2and Ar-CO2) were tested for micro-jet cooling after TIG welding. An important role in the interpretation of the results can give methods of artificial intelligence.

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