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.

scholarly journals Damage Tolerance Analysis of Compressor Blade

1994 ◽  
Author(s):  
Jinxin Mao ◽  
Olin Hong ◽  
Rongqiao Wang ◽  
Mingda Hua
Keyword(s):  
Finite Element ◽  
Damage Tolerance ◽  
Tolerance Analysis ◽  
Crack Size ◽  
Compressor Blade ◽  
J Integral ◽  
Critical Position ◽  
Critical Crack ◽  
Damage Tolerance Analysis ◽  
Critical Crack Size

In this paper, we apply damage tolerance analysis to a certain compressor blade by finite element and fracture mechanics methods. The blade is analysed by using the isoparametric finite element with 20 nodes, its stress and displacement fields are determined, and the critical position where the crack probably initiates is located. Then the value of J–integral with different crack sizes in the critical position are computed using the 3-D J-integral program. On the basis of the fracture toughness of blade material, Je, the blade extension limit δe, and the deviation of natural frequency, Δfe, the failure criterion of the blade is discussed and the critical crack size, ae, is determined. Then according to the crack growth rate obtained from the testing, the residual life of the blade with different crack sizes is evaluated. The maximum allowable crack size, a0*, is determined using the condition that the crack would not grow to critical crack size during double overhaul periods. This value can be used to design the structure of compressor blade or to establish the criterion to judge whether a compressor blade with a crack can be used. In this way the safety and reliability of airplanes is maintained and the costs are reduced. So the damage tolerance analysis of compressor blades is practical and economical.

Benchmark Evaluations to Compare R6 Calculations to European Engineering Assessment and Leak-Before-Break Methods Based on STYLE Mock-Up Experiments

2014 ◽  
Author(s):  
Colin Madew ◽  
John Sharples ◽  
Peter Budden ◽  
Dana Lauerova
Keyword(s):  
Nuclear Power ◽  
Large Scale ◽  
Crack Opening ◽  
Limit Load ◽  
Crack Size ◽  
Opening Displacement ◽  
Critical Crack ◽  
Four Point Bending ◽  
Critical Crack Size ◽  
Leak Before Break

As part of the recent STYLE European project, three large scale mock-up experiments (Mock-up 1, Mock-up 2 and Mock-up 3) were performed. The Mock-ups were representative of nuclear power plant locations containing welds or cladding and contained an initial defect, either through-wall or surface breaking, and were loaded under four point bending conditions. Benchmarks based on each of these experiments were launched to investigate current Engineering Assessment Methods (EAMs) and Leak-before-Break (LbB) procedures used throughout Europe and how the application of these methods compare to each other and to the experimental data. Parameters that were specified for the benchmark participants to evaluate included crack opening displacement (COD), critical crack size, limit load, and load vs various amounts of ductile tearing. This paper presents and compares the results of the calculations performed by the participants in the benchmark. It focuses on the AMEC calculations using R6 and draws conclusions regarding its relative conservatism compared with each of the other methods and through comparison with the results of the Mock-up experiments.

Effect of Rotational Stiffness Evolution at Crack Part on Critical Crack Size in SFR

2012 ◽  
Author(s):  
Takashi Wakai ◽  
Hideo Machida ◽  
Shinji Yoshida
Keyword(s):  
Evaluation Method ◽  
Large Diameter ◽  
Crack Size ◽  
Evolution Model ◽  
Piping System ◽  
Rotational Stiffness ◽  
Critical Crack ◽  
Rotational Spring Model ◽  
Size Evaluation ◽  
Critical Crack Size

This paper describes the efficiency of the deployment of rotational stiffness evolution model in the critical crack size evaluation for Leak Before Break (LBB) assessment of Sodium cooled Fast Reactor (SFR) pipes. The authors have developed a critical crack size evaluation method for the thin-walled large diameter pipe made of modified 9Cr-1Mo steel. In this method, since the SFR pipe is mainly subjected to displacement controlled load caused by thermal expansion, the stress at the crack part is estimated taking stiffness evolution due to crack into account. The stiffness evolution is evaluated by using the rotational spring model. In this study, critical crack sizes for several pipes having some elbows were evaluated and discuss about the effect of the deployment of the stiffness evolution model at the crack part on critical crack size. If there were few elbows in pipe, thermal stress at the crack part was remarkably reduced by considering the stiffness evolution. In contrast, in the case where the compliance of the piping system was small, the critical crack size could be estimated under displacement controlled condition. As a result, the critical crack size increases by employing the model and LBB range may be expected to be enlarged.

Determination of Critical Crack Size Utilizing a Fracture Mechanics Test in Equipment Under Fatigue

2009 ◽  
Author(s):  
Irene Garcia Garcia ◽  
Radoslav Stefanovic
Keyword(s):  
Heat Treatment ◽  
Fracture Mechanics ◽  
Crack Size ◽  
Operational Experience ◽  
Element Analysis ◽  
Critical Crack ◽  
Circumferential Welds ◽  
Fea Model ◽  
Critical Crack Size

Equipment that is exposed to severe operational pressure and thermal cycling, like coke drums, usually suffer fatigue. As a result, equipment of this sort develop defects such as cracking in the circumferential welds. Operating companies are faced with the challenges of deciding what is the best way to prevent these defects, as well as determining how long they could operate if a defect is discovered. This paper discusses a methodology for fracture mechanics testing of coke drum welds, and calculations of the critical crack size. Representative samples are taken from production materials, and are welded employing production welding procedures. The material of construction is 1.25Cr-0.5Mo low alloy steel conforming to ASME SA-387 Gr 11 Class 2 in the normalized and tempered condition (N&T). Samples from three welding procedures (WPS) are tested: one for production, one for a repair with heat treatment, and one for repair without heat treatment. The position and orientation of test specimen are chosen based on previous surveys and operational experience on similar vessels that exhibited cracks during service. Fracture mechanics toughness testing is performed. Crack finite element analysis (FEA) model is used to determine the path-independed JI-integral driving force. Methodology for the determination of critical crack size is developed.

Critical crack size in the fracture of metals

2007 ◽  
Vol 52 (7) ◽  
pp. 937-939
Author(s):  
V. A. Ivanskoĭ
Keyword(s):  
Crack Size ◽  
Critical Crack ◽  
Critical Crack Size

Accelerated Limit Loads Using Repeated Elastic Finite Element Analyses

2003 ◽  
Author(s):  
Prasad Mangalaramanan
Keyword(s):  
Finite Element ◽  
Lower Bound ◽  
Limit Load ◽  
Finite Element Analyses ◽  
Limit Loads ◽  
Bound Theorem

This paper demonstrates the limitations of repeated elastic finite element analyses (REFEA) based limit load determination that uses the classical lower bound theorem. The r-node method is prescribed as an alternative for obtaining better limit load estimates. Lower bound aspects pertaining to r-nodes are also discussed.

scholarly journals On the fracture toughness of snow

2004 ◽  
Vol 38 ◽  
pp. 1-8 ◽  
Author(s):  
Jürg Schweizer ◽  
Gerard Michot ◽  
Helmut O.K. Kirchner
Keyword(s):  
Fracture Toughness ◽  
Crack Size ◽  
Laboratory Measurements ◽  
Critical Crack ◽  
Arrhenius Relation ◽  
Linear Fracture ◽  
Different Temperatures ◽  
Increasing Temperature ◽  
Critical Crack Size ◽  
Key Parameter

AbstractThe release of a dry-snow slab avalanche involves brittle fracture. It is therefore essentially a non-linear fracture mechanics problem. Traditional snow-stability evaluation has mainly focused on snow strength measurements. Fracture toughness describes how well a material can withstand failure. The fracture toughness of snow is therefore a key parameter to assess fracture propagation propensity, and hence snows lope stability. Fracture toughness in tension KIc and shear KIIc was determined with notched cantilever-beam experiments in a cold laboratory. Measurements were performed at different temperatures and with different snow types of density ρ = 100–300 kgm–3, corresponding to typical dry-snow slab properties. The fracture toughness in tension KIc was found to be larger (by about a factor of 1.4) than in shear KIIc. Typical values of the fracture toughness were 500–1000 Pam1/2 for the snow types tested. This suggests that snow is one of the most brittle materials known to man. A power-law relation of toughness KIc on relative density was found with an exponent of about 2. The fracture toughness in tension KIc decreased with increasing temperature following an Arrhenius relation below about –8°C with an apparent activation energy of about 0.16 eV. Above –6°C the fracture toughness increased with increasing temperature towards the melting point, i.e. the Arrhenius relation broke down. The key property in dry-snow slab avalanche release, the critical crack size under shear at failure, was estimated to be about 1 m.

A Benchmark Study of Bending Limit Load Finite Element Analysis for Locally Wall Thinned Pipes

2013 ◽  
Author(s):  
Phuong H. Hoang ◽  
Bostjan Bezensek ◽  
Howard J. Rathbun
Keyword(s):  
Finite Element ◽  
Limit Load ◽  
Benchmark Problems ◽  
Square Root ◽  
Bending Moments ◽  
Finite Element Analyses ◽  
Element Analysis ◽  
Benchmark Analysis ◽  
Benchmark Study ◽  
Computer Codes

Finite element analyses (FEA) have been used to study the effects of multi-axial loadings on bending limit load of local wall thinned pipes. It has been shown by investigators that torsion can be combined with bending moments using SRSS (Square Root of the Sum of the Squares) method for planar flaws with a limited axial extent. The treatment of torsion for non-planar flaws, which exceed the axial extent limit, will be a subject for future investigations. Since the reported FEA results are for various pipe sizes, flaw shapes with different mesh sizes, element types and computer codes, a set of benchmark problems was proposed and analyzed by participating investigators. The benchmark analysis results are presented in this paper.

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