Allowable Axial Flaw Sizes for Stainless Steel Pipes Derived From Limit Load Criteria and Failure Assessment Diagram Procedure

Piping containing flaws that exceed the Acceptance Standards of Section XI of the ASME Code is evaluated using analytical procedures described in Section XI to determine plant operability for the evaluated time period. Subarticle IWB-3640 of Section XI provides allowable axial and circumferential part-through-wall flaws determined from limit load criteria. ASME Section XI Code Case N-494-3 also provides evaluation procedures based on use of a failure assessment diagram to determine allowable flaw sizes. To understand the allowable flaw sizes determined by the limit load criteria and the failure assessment diagram procedure, anstenitic stainless steel pipes with axial part-through-wall flaws with a wide range of pipe diameters were analyzed. The allowable flaw depth based on limit load from Code Case N-494-3 was determined to be very close to that determined from IWB-3640 of Section XI, when the predicted failure mode is elastic-plastic fracture. It was found that the allowable flaw depths derived from the failure assessment diagram procedure of Code Case N-494-3, are lower, but are not significantly different, from those determined from the limit load criteria of IWB-3640. This is due to the relatively high fracture toughness that was used for the austenitic stainless steel.

Case Study on LBB Application for Field Fabrication Weld Metal of Nuclear Primary Piping Systems

In order to apply leak before break (LBB) design for nuclear primary piping systems, dynamic and static J-R tests of field fabrication weld metal were carried out to determine mechanical properties at 316°C. For the reactor coolant piping system made by SMAW (Shielded Metal Arc Welding) process of the SA508 Cl.1a, the variation of J-R fracture characteristics with the loading rate of 1mm/min and 1,000mm/min was examined to prevent the catastrophic break under seismic loading. In the J-R test results, the J-R curves at 1,000mm/min are about 60% higher than those at 1mm/min. It suggests that the welding joints of the reactor coolant piping may be susceptible to dynamic strain aging at 316°C. For the surge line piping made by GTAW (Gas Tungsten Arc Welding) process of the SA312 TP347, excellent static J-R properties are required because the nominal diameter of the pipe is relatively small size of 12 inch. In order to examine the effect of carbon content in the filler metal on the fracture toughness of its welded metal, weld metal specimens were made by using 3 kinds of filler metals whose carbon contents were 0.050, 0.030 and 0.025%, respectively. In the static J-R test results, weld metal made by one of three electrodes satisfied the LBB acceptance criteria. Much better J-R fracture characteristics with decreasing carbon content of filler metal can be shown.

C-Specimen Fracture Toughness Testing: Effect of Side Grooves and η Factor

Elastic-plastic crack front fields in arc-shaped tension specimens (C-specimens) were analyzed by a three-dimensional finite element method. The effect of side grooves on the ductile fracture behavior was investigated by studying the J-integral distribution, plane-strain constraint parameter, and development of plastic zones and comparing to experimental data. The applicability of the η factor (derived for use with compact tension specimens) for the calculation of J-integral values for the C-specimen was also investigated. The results show that side grooves promote and establish near plane strain conditions at the crack front in sub-size specimens. It was also found that a two-dimensional plane-strain analysis in conjunction with the standard American Society for Testing and Materials (ASTM) tests was sufficient to determine the fracture toughness values from side-grooved C-specimen. The results indicate the η factor for compact tension specimen as specified in the ASTM standards appears to produce reliable results for the calculation of J of C-specimens.

Fracture Toughness Transferability Study Between the Master Curve Method and a Pressure Vessel Nozzle Using Local Approach

Local approach methods are to greater extent used in structural integrity evaluation, in particular with respect to initiation of an unstable cleavage crack. However, local approach methods have had a tendency to be considered as methodologies with ‘qualitative’ potential, rather than quantitative usage in realistic analyses where lengthy and in some cases ambiguous calibration of local approach parameters is not feasible. As such, studies need to be conducted to illustrate the usability of local approach methods in structural integrity analyses and improve upon the transferability of their intrinsic, material like, constitutive parameters. Improvements of this kind can be attained by constructing improved models utilizing state of the art numerical simulation methods and presenting consistent calibration methodologies for the constitutive parameters. The current study investigates the performance of a modified Beremin model by comparing integrity evaluation results of the local approach model to those attained by using the constraint corrected Master Curve methodology. Current investigation applies the Master Curve method in conjunction with the T-stress correction of the reference temperature and a modified Beremin model to an assessment of a three-dimensional pressure vessel nozzle in a spherical vessel end. The material information for the study is extracted from the ‘Euro-Curve’ ductile to brittle transition region fracture toughness round robin test program. The experimental results are used to determine the Master Curve reference temperature and calibrate local approach parameters. The values are then used to determine the cumulative failure probability of cleavage crack initiation in the model structure. The results illustrate that the Master Curve results with the constraint correction are to some extent more conservative than the results attained using local approach. The used methodologies support each other and indicate that with the applied local approach and Master Curve procedures reliable estimates of structural integrity can be attained for complex material behavior and structural geometries.

Computational Modelling of High Temperature Steady State Crack Growth Using a Damage-Based Approach

In this work a computational study of creep crack growth in a carbon manganese steel is presented. The constitutive behaviour of the steel is described by a power law creep model and the accumulation of creep damage is accounted for through the use of a well-established model for void growth in creeping materials. Two dimensional finite element analyses have been performed for a compact tension specimen and it has been found that the predicted crack growth rate under plane strain conditions approaches that under plane stress conditions at high C* levels. Furthermore it has been shown, both experimentally and numerically, that an increase in test temperature causes the convergence of the cracking rate to occur at higher values of C*. This trend may be explained by the influence of crack-tip plasticity, which reduces the relative difference in constraint between plane stress and plane strain conditions. The constraint effect has been quantified through the use of a two-parameter characterisation of the crack tip fields under creep conditions.

Increased Temperature Margins Due to Constraint Loss

Enhanced levels of toughness associated with constraint loss are related to temperature shifts in the ductile-brittle transition curve. An argument to quantify the temperature shift is developed using self-similarity of near-tip stress fields under small-scale yielding combined with scaling techniques developed by Dodds and co-workers [1,2] for cleavage. The change in the yield stress and hence temperature that give the same stress field at failure in constrained and unconstrained fields has been determined. The procedure is illustrated using the data of Sherry et al [3] for A533B pressure vessel steel. The results are consistent with the empirical expressions proposed by Wallin [4], and enable a discussion of the physical implications for the micro-mechanics of cleavage.

The Application of Fracture Mechanics to Predict the Critical Initial Crack Length for Inspection

In this study, a method is developed based on fracture mechanics, for predicting the equivalent critical initial crack size, aic in a particulate composite material. The predicted aic is the crack size that should be used to develop an inspection criterion to determine the reliability of a structure made of the particulate composite material.

A Simplified Evaluation of Stress Intensity Factors for a Small Diameter Pipe Penetrating a Thick Plate

Recent trouble with stress corrosion cracking in the internal parts of boiling water reactors and similar issues found in pressurized water reactors has prompted interest in developing simplified methods to determine stress intensity factors for such cracks. Currently, there are many practical and accurate simplified methods to calculate stress intensity factors for a surface crack in plates and pipes. However, there are none that deal with the complex geometry that can arise within the reactors. The complex geometry found within the vessels often entails reentrant comers, welds, holes, and other stress amplifiers. This paper sets forth a means by which some commonly known and accepted simplified solutions to cracks in pipes and plates can be modified to improve the accuracy of stress intensity factors when applied to this complex geometry. The effort to do so included axisymmetric and fully three-dimensional numerical modeling of both the cracked and uncracked body with a variety of assumed surface flaws. It was confirmed that the simplified methods lead to exceedingly conservative estimates for the stress intensity factors of the complex geometry. Finally, a correction factor based on the axisymmetric analyses was applied to the three-dimensional results to improve the accuracy of the simplified solutions.

Statistical Model for Prediction of Plate-Specific Fracture Toughness Properties of ASTM A285 Steel

A materials test program was developed to measure mechanical properties of A285 carbon steel under conditions relevant to waste storage tanks at the Savannah River Site. Fracture toughness and tensile testing were performed on ASTM Type A285 steels that span tank plate compositions. Variables relevant to the material and load conditions for Type I and Type II tanks were defined and a statistical test matrix was designed for fracture toughness testing. The test matrix consisted of eight variables expected to influence mechanical properties. The independent variables were carbon content, manganese content, sulfur content, thickness, pearlite fraction, and grain size. The responses were the fracture toughness JIc, and a figure-of-merit at J3mm to allow for sub-critical ductile crack growth. A total of 85 J-R curves were compiled. The full data set was used to construct statistical models to predict fracture properties as a function of metallurgical properties and operating conditions. Eight independent models resulting from combinations of loading rate (quasi-static, dynamic) and orientation (T-L, L-T) were developed. The statistical significance of terms was determined for each of the models. Thickness and grain size were found to be of statistical significance in the models developed for the dynamic strain rate testing data. Compositional variables were found to be of statistical significance for the quasi-static loading rate fracture toughness data.

A New Approach to Evaluate Fracture Toughness of Structural Materials

A new method, designated as Spiral Notch Torsion Test (SNTT), is developed recently to measure the intrinsic fracture toughness (KIC) of structural materials. The SNTT overcomes many of the limitations inherent in traditional techniques and makes it possible to standardize fracture toughness testing. It is uniquely suitable for testing a wide variety of materials used extensively in pressure vessel and piping structural components and weldments, including others such as ceramics, their composites, and concrete. The SNTT system operates by applying pure torsion to uniform cylindrical specimens with a notch line that spirals around the specimen at a 45° pitch. The KIC values are obtained with the aid of a three-dimensional finite-element computer code, TOR3D-KIC, developed at ORNL.