Extensions of the Failure Assessment Diagram Approach Semi-Elliptical Flaw in Pressurized Cylinders—Part II

1986 ◽  
Vol 108 (4) ◽  
pp. 485-489 ◽  
Author(s):  
J. M. Bloom
Keyword(s):  
Closed Form ◽  
Crack Depth ◽  
Strain Hardening Exponent ◽  
J Integral ◽  
Failure Assessment Diagram ◽  
Aspect Ratios ◽  
Failure Assessment ◽  
Deformation Plasticity ◽  
Estimation Scheme ◽  
Pressurized Cylinder

Approximate closed-form J-integral expressions based on the estimation scheme for use in the Deformation Plasticity Failure Assessment Diagram (DPFAD) approach are presented for an axially oriented semi-elliptical flaw in a pressurized cylinder for crack depth to wall thickness ratios, a/t, from 1/4 to 3/4 and aspect ratios, a/l, from 0 to 1/2. The DPFAD approach was used to derive closed-form J-integral expressions from limited elastic-plastic finite element solutions. Results are also presented in terms of the DPFAD curves as functions of a/t and a/l for the strain-hardening exponent of n = 8.6. Curves are given for the calibration constant h1 as a function of a/t and a/l for ease of interpolation. Lastly, discussion is provided as to the applicability of the solutions and a possible interpolation scheme for obtaining h1 values for n other than 8.6.

Validation of the Deformation Plasticity Failure Assessment Diagram (DPFAD) Approach—The Case of an Axial Flaw in a Pressurized Cylinder

1990 ◽  
Vol 112 (3) ◽  
pp. 213-217 ◽  
Author(s):  
J. M. Bloom
Keyword(s):  
Crack Depth ◽  
Pressure Vessels ◽  
Inside Surface ◽  
Failure Assessment Diagram ◽  
Steel Technology ◽  
Failure Assessment ◽  
Deformation Plasticity ◽  
Structural Tests ◽  
Structural Configurations ◽  
Pressurized Cylinder

The validation of the deformation plasticity failure assessment diagram (DPFAD) approach for application to the prediction of failure pressures for pipes or pressure vessels with axial flaws is addressed in this paper. The DPFAD approach has been extensively documented with regard to its validity in open literature for various configurations of test specimens. For actual structural configurations, however, no such comparisons appear in open literature. In particular, the model of a part-through wall axial flaw in a pressurized cylinder has not been validated through comparisons with actual structural tests results. Two sources of test data from structural tests of axially flawed pressurized cylinders were evaluated. • Heavy-Section Steel Technology (HSST) intermediate test vessels. • Eiber/Battelle Columbus Laboratories (BCL) axially cracked pipes. The DPFAD axial flaw model was developed using finite-element results to generate calibration constants as functions of crack depth to wall thickness and crack depth to crack length for an axially oriented semi-elliptical flaw on the inside surface of a pressurized cylinder. The calibration constants were then used to generate failure assessment curves that can be used to assess or predict failure of pipes or vessels with axial flaws under pressure loading. A key assumption in the analysis was the use of the failure assessment curve for the inside surface flaw in the prediction of outside-surface-flawed cylinder failures. Based on the excellent results from the comparisons with predicted failures to actual vessel and pipe failures, this assumption was found to be reasonable. Furthermore, based on predicted test results of the HSST vessel tests and the Eiber/BCL pipe tests, it was concluded that the DPFAD semi-elliptical axial flaw model can be used reliably in assessing part-through flaws in pressurized vessels and pipes.

Deformation Plasticity Failure Assessment Diagram (DPFAD) for Materials With Non-Ramberg-Osgood Stress-Strain Curves

1995 ◽  
Vol 117 (4) ◽  
pp. 346-356 ◽  
Author(s):  
J. M. Bloom
Keyword(s):  
Tensile Stress ◽  
Manganese Steel ◽  
Uniaxial Tensile ◽  
J Integral ◽  
Stress Strain ◽  
Failure Assessment Diagram ◽  
Failure Assessment ◽  
Strain Data ◽  
Deformation Plasticity ◽  
Uniaxial Tensile Stress

This paper presents a brief history of the evolution of the Central Electricity Generating Board’s (CEGB) R-6 failure assessment diagram (FAD) procedure used in assessing defects in structural components. The reader is taken from the original CEGB R-6 FAD strip yield model to the deformation plastic failure assessment diagram (DPFAD), which is dependent on Ramberg-Osgood (R-O) materials to general stress-strain curves. An extension of the DPFAD approach is given which allows the use of material stress-strain data which do not follow the R-O equation such as stainless steel or carbon manganese steel. The validity of the new approach coined piecewise failure assessment diagram (PWFAD) is demonstrated through comparisons with the J-integral responses (expressed in terms of failure assessment diagram curves) for several cracked configurations of non-R-O materials. The examples were taken from both finite element and experimental results. The comparisons with these test cases demonstrate the accuracy of PWFAD. The use of PWFAD requires the availability of deformation plasticity J-integral solutions for several values of the strain-hardening exponent as well as uniaxial tensile stress-strain data at the temperature of interest. Lacking this information, the original R-O DPFAD approach using known engineering yield and ultimate strengths would give the best available approximation. However, it is strongly recommended that actual uniaxial tensile stress-strain data be used when available.

scholarly journals Fully Plastic J-Integrals for Mixed Mode Fracture Induced by Inclined Surface Cracks in Pressurized Ductile Pipes

2020 ◽  
Author(s):  
Weigang Wang ◽  
Wei Yang ◽  
Chun-Qing Li
Keyword(s):  
Inclination Angle ◽  
Mixed Mode ◽  
Strain Hardening Exponent ◽  
Mixed Mode Fracture ◽  
J Integral ◽  
Surface Cracks ◽  
Mode Fracture ◽  
Aspect Ratios ◽  
Predictive Formulas ◽  
Inclined Surface

Surface cracks have been recognized as major causes for fracture failures of ductile pipes. This paper intends to derive a normalized fully plastic J-integral solution to mixed-mode fracture caused by inclined surface cracks in pressurized ductile pipes. A combined J-integral and finite element method is developed to evaluate the J-integral for inclined surface cracks. A set of predictive formulas for normalized fully plastic J-integrals are developed. It is found in this paper that the normalized fully plastic J-integral increases with the decrease of crack inclination angle and aspect ratios, and the increase of strain hardening exponent. It is also found that the critical locations of crack propagation occur between the surface point and the deepest point of cracks when the inclination angle is relatively small. The paper concludes that the developed formulas can accurately predict the normalized fully plastic J-integrals along the front of inclined surface cracks. The results presented in the paper can enable researchers and practitioners to accurately predict the mixed-mode fracture failure of pressurized pipes subject to inclined surface cracks.

Thick-Wall Cylinder Reference Stress Crack Solutions for API 579-1/ASME FFS-1 Annex 9C

2021 ◽  
Author(s):  
Greg Thorwald ◽  
Lucie Parietti
Keyword(s):  
Crack Depth ◽  
Strain Curve ◽  
Thick Wall ◽  
J Integral ◽  
Strength Ratio ◽  
Stress Strain Curve ◽  
Fracture Failure ◽  
Reference Stress ◽  
Typical Material ◽  
Failure Assessment

Abstract A new set of reference stress solutions for cracks in thick-wall cylinders were computed for addition in the next edition of the API 579-1/ASME FFS-1 standard, and are described in this paper. The geometry cases used ratios for the cylinder radius, wall thickness, crack depth, and crack length. The crack locations included axial, circumferential, internal, and external cracks. 3-D crack meshes were generated for each case to compute J-integral versus pressure result trends, which were used to determine the reference stress. The Failure Assessment Diagram (FAD) method uses reference stress solutions to compute the Lr ratio on the FAD x–axis to evaluate cracks for plastic collapse; the FAD y-axis Kr ratio evaluates fracture failure. The elastic-plastic J-integral reference stress method will be briefly reviewed using results from this project. A stress-strain curve was selected to represent typical material used for high-pressure components. The computed reference stress was shown to depend on the yield strength to tensile strength ratio, and a ratio of 90% was selected for use in this project. Some shallow internal cracks in the thicker cylinder cases showed unexpected behavior in the J-integral versus pressure results, which prevented the reference stress from being computed. An alternative method was developed to use the maximum converged pressure as the nominal load to obtain reference stress solutions for those cases.

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.

Development of J-Integral Prediction Model for Surface Cracks in Round Bars under Combined Loadings

2013 ◽  
Vol 315 ◽  
pp. 665-669 ◽  
Author(s):  
Al Emran Ismail
Keyword(s):  
Mathematical Model ◽  
Crack Depth ◽  
Mode I ◽  
J Integral ◽  
Surface Cracks ◽  
Limit Loads ◽  
Aspect Ratios ◽  
Reference Stress ◽  
Design Defects ◽  
Combined Mode

Solid cylindrical bars have tremendous applications in mechanical engineering. However due to several factors such as corrosion, material and design defects, mechanical failure is unavoidable. In the present of mechanical loadings, the crack initiated at such defects and growth. Therefore, this work focuses on a numerical simulation to analyze the behavior of the surface cracks under combined mode-I loadings. Different crack aspect ratios, a/b and relative crack depth, a/D are used and the plastic stress/strain is assumed to follow the Ramberg-Osgood relation. Then, the limit loads were combined with a reference stress method to develop a mathematical model to predict J-integral along the crack front. Referring to the results, the J-integral can be predicted along the crack fronts. However, the capabilities to predict J-integral are reduced when a/b and a/D is increased.

A Procedure for the Assessment of the Structural Integrity of Nuclear Pressure Vessels

1983 ◽  
Vol 105 (1) ◽  
pp. 28-34 ◽  
Author(s):  
J. M. Bloom
Keyword(s):  
Structural Integrity ◽  
Pressure Vessels ◽  
Assessment Procedure ◽  
Plastic Collapse ◽  
Failure Assessment Diagram ◽  
Linear Elastic ◽  
Failure Assessment ◽  
Deformation Plasticity ◽  
Margin Of Safety ◽  
Two Parameters

This paper presents a simple engineering procedure that the utility industry can use to assess the integrity of typical nuclear-grade pressure vessels. The procedure recognizes both brittle fracture and plastic collapse and is based on a set of proposed failure assessment curves which make up a safety/failure plane. The plane is defined by the stress intensity factor/fracture toughness ratio as the ordinate and the applied stress/reference plastic collapse stress ratio as the abscissa. The failure assessment procedure is based in part on the British Central Electricity Generating Board’s R-6 failure assessment diagram and the deformation plasticity solutions of the General Electric Company. Two parameters, a plastic collapse parameter (Sr′) and linear elastic fracture mechanics parameter (Kr′) are calculated by the user. The point (Sr′, Kr′) is plotted on the appropriate failure assessment diagram. If the point lies inside the respective curve, the structure is safe from failure. Moreover, for a given pressure and a postulated or actual flaw size, the margin of safety of the structure can be simply determined. Consistent with Appendix A of Section XI, (Division 1) of the ASME Boiler and Pressure Vessel Code the procedure presented in this paper is limited to ferritic materials 4 in. (102 mm) and greater in thickness. Details of the derivation of the proposed set of failure assessment curves are provided along with a sample problem illustrating the use of these curves.

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