Simplified Assessment of Studs Allowing for Low Constraint Conditions

2017 ◽  
Author(s):  
P. James ◽  
C. Madew ◽  
M. Jackson
Keyword(s):  
Fracture Toughness ◽  
Finite Element ◽  
Structural Integrity ◽  
High Stress ◽  
Pressure Vessels ◽  
Materials Testing ◽  
Screw Thread ◽  
Surface Point ◽  
Finite Element Analyses ◽  
Supporting Materials

Defect tolerance assessments are carried out to support the demonstration of structural integrity for high integrity components such as nuclear reactor pressure vessels. These assessments often consider surface-breaking defects and assess Stress Intensity Factors (SIFs) at both the surface and deepest points. This can be problematic when there is a high stress at the surface, for example due to the stress concentration at the root of a screw thread. In the past this has led to the development of complex and costly 3D finite element analyses to calculate more accurate SIFs, and still resulting in small apparent limiting defect sizes based on initiation at the surface point. Analysis has been carried out along with supporting materials testing, to demonstrate that the increased SIF at the surface point is offset by a reduction in crack-tip constraint, such that the material exhibits a higher apparent fracture toughness. This enables a more simplistic assessment which reduces the effective SIF at the surface such that only the SIF at the deepest point needs to be considered for many defects. This then leads to larger calculated limiting defect sizes. This in turn leads to a more robust demonstration of structural integrity, as the limiting defect sizes are consistent with the capability of non-destructive examination techniques. An overview of the supporting materials testing is provided in an accompanying paper. The accompanying paper details how it was not possible to demonstrate the required material response with conventional tests, such as those using shallow-notched bend specimens. Instead it was necessary to develop modified specimens in which semi-elliptical defects were introduced into a geometry which replicated the notch acuity at the root of a screw thread. These tests were used to quantify the stud materials sensitivity to constraint. Conventional three-point bend tests were also seen to confirm these values. A series of R6 constraint modified assessments have been considered to understand the benefit from including a loss of constraint, particularly when assessing the surface breaking SIF. This has necessitated a series of complex finite element analyses to define the elastic SIF as well as the elastic constraint parameter, T-Stress, T. Further verification analyses have also been performed to determine the equivalent elastic-plastic J and Q parameters. These have been used to provide guidance on how best to assess surface breaking defects within studs. This has shown that the increased perceived toughness at the surface location means that under the majority of conditions, the assessment can simply be based upon the SIF at the depth location using high constraint fracture toughness. This paper provides an overview of the process undertaken to provide simplified guidance on assessing defects within studs that allows benefit from constraint loss.

Features Testing of Stud Material Under Low Constraint Ductile Tearing

2017 ◽  
Author(s):  
P. James ◽  
M. Jackson ◽  
P. Birkett ◽  
C. Madew
Keyword(s):  
Structural Integrity ◽  
High Stress ◽  
Crack Tip ◽  
Pressure Vessels ◽  
Materials Testing ◽  
Screw Thread ◽  
Surface Point ◽  
Defect Tolerance ◽  
Material Response ◽  
Crack Tip Constraint

Defect tolerance assessments are carried out to support the demonstration of structural integrity for high integrity components such as nuclear reactor pressure vessels. These assessments often consider surface-breaking defects and assess Stress Intensity Factors (SIFs) at both the surface and deepest points. This can be problematic when there is a high stress at the surface, for example due to the stress concentration at the root of a screw thread. In the past this has led to the development of complex and costly 3D finite element analyses to calculate more accurate SIFs, and still resulting in small apparent limiting defect sizes based on initiation at the surface point. Analysis has been carried out along with supporting materials testing, to demonstrate that the increased SIF at the surface point is offset by a reduction in crack-tip constraint, such that the material exhibits a higher apparent fracture toughness. This enables a more simplistic assessment which reduces the effective SIF at the surface such that only the SIF at the deepest point needs to be considered. This then leads to larger calculated limiting defect sizes. This in turn leads to a more robust demonstration of structural integrity, as the limiting defect sizes are consistent with the capability of non-destructive examination techniques. The high SIF at the surface location, and the concomitant reduction in crack-tip constraint, meant that it was not possible to demonstrate the material response with conventional tests, such as those using shallow-notched bend specimens. Instead it was necessary to develop modified specimens in which semielliptical defects were introduced into a geometry which replicated the notch acuity at the root of a screw thread. These feature tests were used to demonstrate the principle, prior to testing with more conventional specimens to fit more accurately the parameters required to represent the material response in a defect tolerance assessment. Margins in defect tolerance assessments are usually measured against the initiation of tearing, even though the final failure for the material may occur at a higher load following stable crack extension. This work measured and assessed the benefit of reduced crack-tip constraint on both the point of initiation and on the development of the tearing resistance curve. This demonstrated that the effect of constraint was valid with tearing for this material and that there was additional margin available beyond the onset of tearing. The feature test geometry also provided evidence of the tearing behaviour at the surface and deepest points of a surrogate component under representative loading. This paper provides an overview of the range of tests performed and the post-test interpretation performed in order to provide the R6 α and k constraint parameters.

Fracture Analysis of Multiple Cracks in a Spinning Cylinder Experiment

1997 ◽  
Vol 119 (2) ◽  
pp. 232-235
Author(s):  
J. A. Keeney ◽  
B. R. Bass
Keyword(s):  
Finite Element ◽  
Structural Integrity ◽  
Plastic Material ◽  
Pressure Vessels ◽  
International Standards ◽  
Coolant Temperature ◽  
Multiple Cracks ◽  
Finite Element Analyses ◽  
Integrity Assessment ◽  
Spinning Cylinder

This paper presents finite-element analyses of the cylinder specimen being used in the international Network for Evaluating Steel Components (NESC) large-scale spinning-cylinder project (NESC-1). The NESC was organized as an international forum to exchange information on procedures for structural integrity assessment, to collaborate on specific projects, and to promote the harmonization of international standards. The objective of the NESC-1 project is to focus on a complete procedure for assessing the structural integrity of aged reactor pressure vessels. Current plans for the testing program call for two large cracks to be installed in the NESC-1 cylinder separated by 90 deg. Three-dimensional finite-element analyses were carried out to determine: 1) the extent of interaction between multiple cracks in the cylinder; and 2) the predicted effects of using an initial cylinder temperature of 295°C and coolant temperature of 5°C in the experiment. The cylinder was modeled with innersurface through-clad cracks having a depth of 74 mm and aspect ratio of 2:1. The cylinder specimen was subjected to centrifugal loading followed by a thermal shock and analyzed with a thermo-elastic-plastic material model. The analytical results indicate that the stress-intensity factor changes less than 0.2 percent between a model with one crack and a model with four cracks evenly spaced around the circumference. Cleavage initiation is likely to be achieved for initial and coolant temperatures of 295 and 5°C, respectively.

ORNL Evaluation of Safety Cases for Two Belgian Reactor Pressure Vessels Containing Quasi-Laminar Defects

2017 ◽  
Author(s):  
Hilda B. Klasky ◽  
B. Richard Bass ◽  
Terry L. Dickson ◽  
Sarma B. Gorti ◽  
Randy K. Nanstad ◽  
...  
Keyword(s):  
New York ◽  
Finite Element ◽  
Structural Integrity ◽  
National Laboratory ◽  
Relevant Literature ◽  
Pressure Vessels ◽  
Oak Ridge ◽  
Safety Margins ◽  
Safety Cases ◽  
Reactor Pressure

The Oak Ridge National Laboratory (ORNL) performed a detailed technical review of the 2015 Electrabel (EBL) Safety Cases prepared for the Belgium reactor pressure vessels (RPVs) at Doel 3 and Tihange 2 (D3/T2). The Federal Agency for Nuclear Control (FANC) in Belgium commissioned ORNL to provide a thorough assessment of the existing safety margins against cracking of the RPVs due to the presence of almost laminar flaws found in each RPV. Initial efforts focused on surveying relevant literature that provided necessary background knowledge on the issues related to the quasi-laminar flaws observed in D3/T2 reactors. Next, ORNL proceeded to develop an independent quantitative assessment of the entire flaw population in the two Belgian reactors according to the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code, Section XI, Appendix G, “Fracture Toughness Criteria for Protection Against Failure,” New York (both 1992 and 2004 versions). That screening assessment of the EBL-characterized flaws in D3/T2 used ORNL tools, methodologies, and the ASME Code Case N-848, “Alternative Characterization Rules for Quasi-Laminar Flaws”. Results and conclusions derived from comparisons of the ORNL flaw acceptance assessments of D3/T2 with those from the 2015 EBL Safety Cases are presented in the paper. The ORNL screening analyses identified fewer flaws than EBL that were not compliant with the ASME Section XI (1992) criterion; the EBL criterion imposed additional conservatisms not included in ASME Section XI. Furthermore, ORNL’s application of the updated ASME Section XI (2004) criterion produced only four non-compliant flaws, all due to design-basis loss-of-coolant loading transients. Among the latter, only one flaw remained non-compliant when analyzed using the warm-prestress (WPS) cleavage fracture model typically applied in USA flaw assessments. ORNL’s independent refined analysis of that flaw (#1660, which was also non-compliant in the EBL screening assessments) rendered it compliant when modeled as a more realistic individual quasi-laminar flaw using a 3-dimensional XFEM (eXtended Finite Element Method) approach available in the ABAQUS© finite element code. Taken as a whole, the ORNL-specific results and conclusions confirmed the structural integrity of Doel 3 and Tihange 2 under all design transients with ample margin in the presence of the 16,196 detected flaws.

Finite Element Analysis of Residual Stresses in Threaded End Closures

1991 ◽  
Vol 113 (3) ◽  
pp. 398-401 ◽  
Author(s):  
A. Chaaban ◽  
U. Muzzo
Keyword(s):  
Finite Element ◽  
Residual Stresses ◽  
Bauschinger Effect ◽  
High Stress ◽  
Empirical Method ◽  
Pressure Vessels ◽  
Element Analysis ◽  
High Stress Concentration ◽  
Proof Testing ◽  
Pressure Cycle

Due to the high stress concentration at the root of the first active thread in threaded end closures of high pressure vessels, yielding may occur in this region during the application of the first pressure cycle or proof testing. This overstraining introduces residual stresses that influence the fatigue performance of the vessel. This paper presents a parametric analysis of threaded end closures using elastic and elasto-plastic finite element solutions. The results are used to discuss the influence of these residuals on the estimated fatigue life when the vessel is subjected to repeated internal pressure. A simple empirical method to allow for the Bauschinger effect of the material is also proposed.

Estimation of Through-Wall Cracking Frequency of RPV Under PTS Events Using PFM Analysis Method for Identifying Conservatism Included in Current Japanese Code

2014 ◽  
Author(s):  
Kazuya Osakabe ◽  
Koichi Masaki ◽  
Jinya Katsuyama ◽  
Genshichiro Katsumata ◽  
Kunio Onizawa
Keyword(s):  
Fracture Toughness ◽  
Fracture Mechanics ◽  
Crack Initiation ◽  
Structural Integrity ◽  
Probabilistic Approach ◽  
Pressure Vessels ◽  
Assessment Procedure ◽  
Analysis Method ◽  
Integrity Assessment ◽  
The U.S

To assess the structural integrity of reactor pressure vessels (RPVs) during pressurized thermal shock (PTS) events, the deterministic fracture mechanics approach prescribed in Japanese code JEAC 4206-2007 [1] has been used in Japan. The structural integrity is judged to be maintained if the stress intensity factor (SIF) at the crack tip during PTS events is smaller than fracture toughness KIc. On the other hand, the application of a probabilistic fracture mechanics (PFM) analysis method for the structural reliability assessment of pressure components has become attractive recently because uncertainties related to influence parameters can be incorporated rationally. A probabilistic approach has already been adopted as the regulation on fracture toughness requirements against PTS events in the U.S. According to the PFM analysis method in the U.S., through-wall cracking frequencies (TWCFs) are estimated taking frequencies of event occurrence and crack arrest after crack initiation into consideration. In this study, in order to identify the conservatism in the current RPV integrity assessment procedure in the code, probabilistic analyses on TWCF have been performed for certain model of RPVs. The result shows that the current assumption in JEAC 4206-2007, that a semi-elliptic axial crack is postulated on the inside surface of RPV wall, is conservative as compared with realistic conditions. Effects of variation of PTS transients on crack initiation frequency and TWCF have been also discussed.

Numerical Study of an Impusively Loaded Vessel Containing Double Versus Single Closure Bolt Patterns

2017 ◽  
Author(s):  
Joseph E. D. Hess
Keyword(s):  
Finite Element ◽  
Structural Integrity ◽  
Numerical Study ◽  
Axial Loading ◽  
Element Model ◽  
Pressure Vessels ◽  
Bolted Joint ◽  
Single Row ◽  
Asme Code ◽  
Calculation Results

Impulsively loaded pressure vessels are often closed using a bolted joint configured in a double staggered row pattern. The bolted joint design must maintain the placement of the vessel opening covers to support the structural integrity of the shell and also provide the necessary preload of sealing surfaces for leak prevention. Good design practice suggests configuring tensile loaded bolted joints with a double rows pattern in order to minimize prying against the bolt head induced by localized moments. Double bolt row patterns allow moments induced by load offsets to be reacted through contact of the faying surfaces of the bolted members and if separation occurs by differential axial loading of the two bolt rows. This acts to reduce direct prying of the mated members against the bolt heads. Material cost and operational time savings could be realized if a single bolt row design with acceptable performance was implemented. In this paper a detailed finite element model is described and calculation results are presented for two vessel configurations subjected to an impulsive load; a double staggered 64 bolt pattern and a single row 32 bolt pattern. Finite element results are compared to each other and to the rules of ASME Code Case 2564 in Section VIII, Division 3. Special attention is given to the loading induced in the bolts and to the relative deflection of faying surfaces containing seals. It will be shown that reducing the bolt count per opening from 64 to 32 results in increased peak response of the bolts, seal opening gaps, and shell. Nonetheless a single row bolt pattern does appear feasible and within the bounds of the Code Case.

Weight-Saving Plastic Design of Pressure Vessels

1997 ◽  
Vol 119 (2) ◽  
pp. 161-166
Author(s):  
J. S. Porowski ◽  
W. J. O’Donnell ◽  
R. H. Reid
Keyword(s):  
Finite Element ◽  
Three Dimensional ◽  
Pressure Vessels ◽  
Engineering Practice ◽  
Finite Element Analyses ◽  
Equilibrium Models ◽  
Plastic Design ◽  
Primary Stress ◽  
Asme Code ◽  
Analytical Approaches

Within the last two decades, the use of elastic finite element analyses to demonstrate design compliance with the rules of the ASME Code has become a generally accepted engineering practice. Linearized stresses from these analyses are commonly used to evaluate primary stresses. For redundant structures or complex structural details, the use of such analyses, instead of simple equilibrium models, often results in significant overconservatism. Direct use of finite element results is often preferred because equilibrium solutions are not unique and effective equilibrium models are not easily constructed for complex three-dimensional structures. However, finite element analyses include secondary stresses, even for pressure, mechanical, and shock loading. For primary stress evaluation, the ASME Code allows the use of inelastic methods based on lower-bound solutions and plastic analysis. For primary stresses, the Code requires equilibrium to be satisfied without violating the yield strength of the material. The use of finite element inelastic analysis to partition mechanically induced stresses into the primary and secondary categories was introduced by Porowski et al. (1993). The latter provides a detailed discussion of the technical approach and the results for the axisymmetric junction between the plate and shell in a pressure vessel. This example was selected by the Session Organizer as a benchmark case to compare the efficiency of various analytical approaches presented at the Session. The authors have since used this approach to design more efficient structures. The practical application of this method to reduce the weight of complex redundant structures designed to meet primary stress limits is described herein for a more complex three-dimensional case. Plastic design utilizes the ability of actual materials to find the most efficient load distribution. A heat exchanger subjected to pressure, accelerations, and nozzle external loads is evaluated as a practical example. The results of elastic analyses are compared with those obtained by inelastic analyses. It is shown that inelastic analyses can be used effectively to reduce the weight of structures using only modern PCs for the engineering computations, as illustrated in this paper.

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