Constraint in the Failure Assessment Diagram Approach for Fracture Assessment

1995 ◽  
Vol 117 (3) ◽  
pp. 260-267 ◽  
Author(s):  
R. A. Ainsworth ◽  
N. P. O’Dowd
Keyword(s):  
Crack Size ◽  
Structural Constraint ◽  
Failure Assessment Diagram ◽  
Failure Assessment ◽  
T Stress ◽  
Fracture Assessment ◽  
The Subject ◽  
Constraint Effects ◽  
Two Parameters ◽  
Diagram Approach

This paper presents a framework for including constraint effects in the failure assessment diagram approach for fracture assessment. As parameters for describing constraint are still the subject of development, the framework is illustrated using both the elastic T-stress and the hydrostatic Q-stress. It is shown that constraint effects can be treated by modifying the shape of the failure assessment curve. In their simplest form, the modifications involve only two parameters: one quantifying the magnitude of structural constraint which depends on geometry and crack size; and the second quantifying the influence of constraint on fracture toughness.

Revision of ISO 27306 for CTOD Toughness Correction for Constraint Loss

2016 ◽  
Vol 879 ◽  
pp. 54-59
Author(s):  
Fumiyoshi Minami ◽  
Mitsuru Ohata ◽  
Yasuhito Takashima
Keyword(s):  
Fracture Toughness ◽  
Fracture Mechanics ◽  
Stress Criterion ◽  
Failure Assessment Diagram ◽  
Failure Assessment ◽  
Fracture Assessment ◽  
International Standardization ◽  
Diagram Approach ◽  
Weibull Stress

As the result of the international standardization work in Japanese IST project, ISO 27306 were published in 2009 for correction of CTOD fracture toughness for constraint loss in steel components. ISO 27306 employs an equivalent CTOD ratio based on the Weibull stress criterion, which leads to more accurate fracture assessment than the conventional fracture mechanics assessment. On the occasion of the 1st periodical review, the revision of ISO 27306 has been proposed from Japan. This paper describes the key contents of the new ISO 27306. A case study is included on the fracture assessment of a wide plate component according to FAD (failure assessment diagram) approach specified in BS 7910:2013.

Numerical Validation of a Strain-Based Failure Assessment Diagram Approach to Fracture

2009 ◽  
Author(s):  
Peter J. Budden ◽  
Michael C. Smith
Keyword(s):  
Finite Element ◽  
Plastic Strain ◽  
Present Author ◽  
Plastic Relaxation ◽  
Failure Assessment Diagram ◽  
Defect Assessment ◽  
Failure Assessment ◽  
Basic Approaches ◽  
Assessment Procedures ◽  
Diagram Approach

The basic approaches in defect assessment procedures such as R6 consider the stresses on the section containing the flaw. Such approaches can be overly conservative and lead to unacceptably small estimates of limiting defect sizes for cases where the applied loads are due to displacements or strains well in excess of yield, when significant plastic relaxation of stress occurs. The potential for over-conservative assessments has led to a renewed interest in recent years in strain-based assessment methods, in both the power and pipeline industries. Significant levels of plastic strain can be imposed across the flawed section in some cases. Recently, the present author has published a general approach to strain-based fracture that uses a strain-based failure assessment diagram (SB-FAD). This includes a range of Options similar to that of the basic R6 approach. The present paper describes some validation of the SB-FAD approach based on elastic-plastic cracked-body finite element data for plates and cylinders.

Failure Assessment Diagrams of Semi-Elliptical Surface Crack with Constraint Effect

2007 ◽  
Vol 353-358 ◽  
pp. 1952-1955
Author(s):  
Hyung Yil Lee ◽  
Jin Haeng Lee ◽  
Tae Hyung Kim
Keyword(s):  
Finite Element ◽  
Finite Element Modeling ◽  
Surface Crack ◽  
J Integral ◽  
Finite Element Analyses ◽  
Constraint Effect ◽  
Failure Assessment ◽  
T Stress ◽  
Cracked Structures ◽  
Constraint Effects

For accurate failure assessment, a second parameter like T-stress describing the constraint is needed in addition to the single parameter J-integral. In this work, selecting the structures of surface-cracked plate and pipe, we perform line-spring finite element modeling, and accompanying elastic-plastic finite element analyses. We then present a framework, which includes the constraint effects in the R6 FAD approach for failure assessment of cracked-structures.

Technical Basis for the Extension of ASME Code Case N-494 for Assessment of Austenitic Piping

1996 ◽  
Vol 118 (4) ◽  
pp. 513-516 ◽  
Author(s):  
J. M. Bloom
Keyword(s):  
Lower Bound ◽  
Pressure Vessel ◽  
Strain Curve ◽  
Stress Strain Curve ◽  
Stress Strain ◽  
Failure Assessment Diagram ◽  
Strain Behavior ◽  
Failure Assessment ◽  
Asme Code ◽  
Constraint Effects

In 1990, the ASME Boiler and Pressure Vessel Code for Nuclear Components approved Code Case N-494 as an alternative procedure for evaluating flaws in light water reactor (LWR) ferritic piping. The approach is an alternate to Appendix H of the ASME Code and allows the user to remove some unnecessary conservatism in the existing procedure by allowing the use of pipe specific material properties. The Code case is an implementation of the methodology of the deformation plasticity failure assessment diagram (DPFAD). The key ingredient in the application of DPFAD is that the material stress-strain curve must be in the format of a simple power law hardening stress-strain curve such as the Ramberg-Osgood (R-O) model. Ferritic materials can be accurately fit by the R-O model and, therefore, it was natural to use the DPFAD methodology for the assessment of LWR ferritic piping. An extension of Code Case N-494 to austenitic piping required a modification of the existing DPFAD methodology. Such an extension was made and presented at the ASME Pressure Vessel and Piping (PVP) Conference in Minneapolis (1994). The modified DPFAD approach, coined piecewise failure assessment diagram (PWFAD), extended an approximate engineering approach proposed by Ainsworth in order to consider materials whose stress-strain behavior cannot be fit to the R-O model. The Code Case N-494 approach was revised using the PWFAD procedure in the same manner as in the development of the original N-494 approach for ferritic materials. A lower-bound stress-strain curve (with yield stress comparable to ASME Code specified minimum) was used to generate a PWFAD curve for the geometry of a part-through wall circumferential flaw in a cylinder under tension and bending. Earlier work demonstrated that a cylinder under axial tension with a 50-percent flaw depth, 90 deg in circumference, and radius to thickness of 10, produced a lower-bound FAD curve. Validation of the new proposed Code case procedure for austenitic piping was performed using actual pipe test data. Using the lower-bound PWFAD curve, pipe test results were conservatively predicted (failure stresses were predicted to be 31.5 percent lower than actual on the average). The conservative predictions were attributed to constraint effects where the toughness values used in the predictions were obtained from highly constrained compact test specimens. The resultant development of the PWFAD curve for austenitic piping led to a revision of Code Case N-494 to include a procedure for assessment of flaws in austenitic piping.

Constraint-based failure assessment diagrams

1995 ◽  
Vol 451 (1943) ◽  
pp. 757-777 ◽  
Keyword(s):  
Crack Tip ◽  
Single Parameter ◽  
Elastic Plastic ◽  
The Past ◽  
Crack Tip Fields ◽  
Failure Assessment ◽  
Constraint Effects ◽  
Two Parameters

Constraint-dependent toughness has been addressed by characterizing elastic-plastic crack tip fields and the associated toughness in terms of two parameters: J , and a parameter that indexes the level of constraint ( T/Q ). In the past, failure assessment diagrams have been developed on the basis of a single-parameter characterization of toughness. The present work modifies these diagrams to incorporate constraint effects and indicates the loadings where advantage can be taken of constraint enhanced toughness.

Failure Assessment Diagrams of Semi-Elliptical Surface Crack with Constraint Effect

2007 ◽  
Vol 353-358 ◽  
pp. 98-101
Author(s):  
Hyung Yil Lee ◽  
Jin Haeng Lee ◽  
Tae Hyung Kim
Keyword(s):  
Finite Element ◽  
Finite Element Modeling ◽  
Surface Crack ◽  
J Integral ◽  
Finite Element Analyses ◽  
Constraint Effect ◽  
Failure Assessment ◽  
T Stress ◽  
Cracked Structures ◽  
Constraint Effects

For accurate failure assessment, a second parameter like T-stress describing the constraint is needed in addition to the single parameter J-integral. In this work, selecting the structures of surface-cracked plate and pipe, we perform line-spring finite element modeling, and accompanying elastic-plastic finite element analyses. We then present a framework, which includes the constraint effects in the R6 FAD approach for failure assessment of cracked-structures.

Extensions of the Failure Assessment Diagram Approach Semi-Elliptical Flaw in Pressurized Cylinder

1985 ◽  
Vol 107 (1) ◽  
pp. 25-29 ◽  
Author(s):  
J. M. Bloom
Keyword(s):  
Safety Assessment ◽  
Pressure Vessels ◽  
Failure Assessment Diagram ◽  
The United Kingdom ◽  
Safety Margins ◽  
Failure Assessment ◽  
Assessment Approach ◽  
Model Versus ◽  
Pressurized Cylinder ◽  
Diagram Approach

A simple, viable engineering method for assessing the integrity of nuclear pressure vessels has been developed at Babcock & Wilcox. The method uses results given in a plastic fracture handbook developed by General Electric and which are in the format of the Central Electricity Generation Board of the United Kingdom R-6 failure assessment diagram. The method is currently limited to two-dimensional/axisymmetric structural models with continuous flaws. Failure assessment of nuclear pressure vessels with assumed continuous flaws result in the calculation of overly conservative safety margins. This paper presents the extension of the existing failure assessment approach to include semi-elliptical flaw models, as well as example problems which demonstrate increased safety margins over the continuous flaw assumptions. In particular, failure assessment diagram curves and the corresponding failure assessment point expressions for an axially cracked pressurized cylinder with an ASME Section III, Appendix G semi-elliptical flaw are presented. The results of the example problems considering the less conservative semi-elliptical flaw model versus the continuous flaw model dramatically illustrate increased safety margins of 50 percent when more realistic semi-elliptical flaws are postulated. The results given in this paper are particularly valuable in the safety assessment of PWR vessels which have low toughness welds in their beltline regions.

Reliability Assessment of Failure Assessment Diagram Based Fitness for Service Procedure Including the Effect of Bias in Modeling

2017 ◽  
Vol 139 (5) ◽  
Author(s):  
Rohit Rastogi ◽  
Siddhartha Ghosh ◽  
A. K. Ghosh ◽  
K. K. Vaze
Keyword(s):  
Factor Of Safety ◽  
Structural Reliability ◽  
Common Factor ◽  
Crack Size ◽  
Acceptance Criteria ◽  
Acceptance Criterion ◽  
Failure Assessment Diagram ◽  
Applied Loading ◽  
Load Combination ◽  
Failure Assessment

This paper presents the estimation of the reliability levels associated with a cracked pipe found acceptable as per the failure assessment diagram (FAD) based acceptance criteria of ASME Section XI, Appendix H. This acceptance criterion is built on the concepts of fracture mechanics. The parameters which enter the acceptance criteria are piping geometry, applied loading, crack size, and the material properties (tensile and fracture). Most of these parameters are known to exhibit uncertainty in their values. The FAD used also has an associated modeling bias. The code addresses these uncertainties by providing a factor of safety on the applied load. The use of a common factor of safety for a variety of pipe sizes, crack configuration, load combination, and materials may not ensure consistent level of safety associated with the piping component being evaluated. This level of safety can be evaluated by using structural reliability concepts. This paper analyzes the reliability level which is achieved if a cracked pipe passes the acceptance criteria prescribed by the code. The reliability is evaluated for a range of pipe and crack geometry, different load combination, and different materials using Monte Carlo method. The realistic assessment of reliability also requires the assessment of modeling bias associated with the FAD. This bias is also evaluated using the results from the published fracture experiments.

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