Failure Assessment Diagram Curve Research Articles

Applicability investigation on failure assessment diagram of ASME code section XI - Appendix H for ferritic nuclear piping

The failure assessment diagram (FAD) is a widely used tool for evaluating the fracture acceptance of structural components. In addition to linear elastic fracture mechanics, elastic-plastic fracture mechanics, and limit load analysis, the ASME Code incorporated FAD as an alternative safety evaluation method for flaws in nuclear pipes starting from 2004 edition. This paper conducts a series of finite element elastic-plastic fracture mechanics analyses to establish the specific J–based FAD curves considering various loading and geometry conditions for ferritic pipes containing various circumferential flaws. By comparing with the suggested FAD of ASME Code Section XI - Appendix H, it is found that the radius-to-thickness ratio of pipes and the flaw depth influence the applicability of FAD, but with minor effects of crack length. In addition, the effects of material properties between base metal and weld materials on FAD are also discussed. The result of this work could be a reference when applying the FAD to evaluate the flawed piping for nuclear power plants.

Application of Failure Assessment Diagram (FAD) for Steel Welded Connection Based on BS7910 and DNV-RP-108

The failure assessment diagram (FAD) method has been widely accepted to evaluate the extent to which cracks may affect structural safety. The usage of this FAD method has been validated and included in [1]-[3]. The structure under investigation, described in four fully welded T-joint (BCC5) specimens, where these welded joints are a source of stress concentration and defects from which fatigue cracks can grow. The four specimens were modeled under different displacement loading using a finite element analysis program Ansys and SolidWorks software. In this work, the application of a FAD (Lr, Kr) using maximum stress, cumulative stress ranges, and the last half-cycle stress range was investigated. The results are showing that all the points were lying outside the FAD curve except for the BCC5D specimen point was inside FAD when using maximum stress.
 Conclusions made that the cumulative stress gives Lr and Kr are extremely large and hence predict failure too early. With the Crack Tip Opening Displacement (CTOD) of the test specimen assumed to be about 1mm rather than 0.1mm it was found that, if a FAD is to be used to indicate failure, then both Lr and Kr should be based on the maximum stress. It appears that the FAD methodology does help to predict the final failure (which is the usual application in such cases). This represents more effectively the structural behavior and would be more easily used by designers.

Application of Failure Assessment Diagram (FAD) for Steel Welded Connection Based on BS7910 and DNV-RP-108

The failure assessment diagram (FAD) method has been widely accepted to evaluate the extent to which cracks may affect structural safety. The usage of this FAD method has been validated and included in [1]-[3]. The structure under investigation, described in four fully welded T-joint (BCC5) specimens, where these welded joints are a source of stress concentration and defects from which fatigue cracks can grow. The four specimens were modeled under different displacement loading using a finite element analysis program Ansys and SolidWorks software. In this work, the application of a FAD (Lr, Kr) using maximum stress, cumulative stress ranges, and the last half-cycle stress range was investigated. The results are showing that all the points were lying outside the FAD curve except for the BCC5D specimen point was inside FAD when using maximum stress.
 Conclusions made that the cumulative stress gives Lr and Kr are extremely large and hence predict failure too early. With the Crack Tip Opening Displacement (CTOD) of the test specimen assumed to be about 1mm rather than 0.1mm it was found that, if a FAD is to be used to indicate failure, then both Lr and Kr should be based on the maximum stress. It appears that the FAD methodology does help to predict the final failure (which is the usual application in such cases). This represents more effectively the structural behavior and would be more easily used by designers.

Analysis of Crack Behaviour in Pipeline System Using FAD Diagram Based on Numerical Simulation under XFEM

For a long time, cracked structures have triggered various researchers to develop a structural integrity approach and design models to address the fracture problems. In the present study, a pipeline with an axial semi-elliptical surface defect was examined in detail. Recent works have highlighted the use of the classical finite element method (CFEM) as numerical tools to solve the fracture mechanics; however, this approach comes with a few difficulties in the modelling aspects. To overcome this issue, we proposed the use of the extended finite element method (XFEM), which was implemented in the commercial version of Abaqus software. Moreover, we have used the results based on this technique in the volumetric method to estimate the stress intensity factors (SIFs). Then, this parameter was employed to build the failure assessment diagram (FAD). The FAD curve was used in the current investigation because it is one of the conventional methods for the evaluation of flaws in steel pipes. The XFEM simulations enable us to draw an FAD curve that can be used as a practical reference for defect evaluation in pipeline systems in the industrial world.

A comparison of failure assessment diagram options for Inconel 690 and Incoloy 800 nuclear steam generators tubes

In the context of structural integrity assessment of cracked Steam Generator Tubes (SGTs), the Failure Assessment Diagram (FAD) has been proposed as a modern tool able to reduce the conservativeness of traditional plastic limit load analyses. As different FAD failure curves are available in the literature, this paper presents a comparison between the so-called Options 1, 2 and 3 FAD curves derived for Inconel 690 and Incoloy 800 nuclear SGTs. Detailed Option 3 curves are obtained from elastoplastic finite element analyses and experimental data from fracture tests. Material-specific Option 2 curves are drawn from stress vs. strain data of uniaxial tests of SGTs. It is shown that the simplest generic Option 1 curve is conservative but appropriate for structural integrity assessments of cracked nuclear SGTs in operation. Alternatively, Option 3 FAD curves must be used when an excessive conservatism shall be avoided.

Study on FAD curves of steel pipes based on stress-strain curves

The failure assessment diagram (FAD) method is one of the mainstream methods for assessment of defects in steel pipes and girth welds. In this paper, the FAD curves based on Ramberg-Osgood model and tensile test requirements for steel pipelines from X52 to X100 are established with an accuracy of less than 3%. The results show that the generalized FAD curve is more conservative in evaluating high-grade defective pipelines than in evaluating low-grade pipelines. Based on the tensile test data of DIC technology, the FAD evaluation models of the weld, base metal and the HAZ are established, and the critical crack lengths are evaluated respectively combining with CVN impact test data. R6 Option 1 and BS 7910 Option 1 are found to be conservative in the same regional assessment, ranging from 20.83% to 31.09%. This study can provide convenience and reference for defect assessment of pipeline and girth weld.

Safety Assessment of Damaged Multi-planar Square Hollow Section Welded Joints Using the New BS 910:2013+A1:2015

The safety assessment procedure in the new BS 7910:2013+A1:2015 guide is based on the failure assessment diagram (FAD) method. This paper aims to validate the above procedure for complex geometries such as damaged multi-planar square hollow section (SHS) welded joints and to recommend optimal solutions if necessary. FAD curves are constructed for cracked multi-planar SHS TT-, YT- and KT-joints for the first time and are compared with the Option 1 curve of the BS guide. A robust novel automatic finite element (FE) mesh generator, which is validated using the full-scale experimental test results, is used in this study. The new FE mesh generator addresses the issue of non-convergence by using a key-hole for the modelling the crack tip in elastic-plastic analyses. The new FE mesh generator is capable to model cracks and geometries of arbitrary dimensions and is able to achieve convergence of solutions even at a high plastic deformation. It is shown to be aiding in speedy generation of cracked FE mesh models which is otherwise time consuming to generate using commercial software packages. The results show that the Option 1 curve does not always guarantee safe solutions for multi-planar SHS welded joints. Hence, a penalty factor of 1.1 is recommended to be used to calculate the plastic collapse load. The use of proposed penalty factor gives optimal solutions for cracked multi-planar SHS TT-, YT- and KT-joints.

The effect of alloying elements on fracture resistance of E7010-P1 cellulosic welding electrodes

In this paper, Assessment for fracture resistance of three different cellulosic welding electrodes (type E7010-P1) with various chemical compositions is made. This assessment is done by using Failure Assessment Diagram (FAD) method and based on the principles of fracture mechanics in accordance with BS 7910-2015 standard. The mechanical and fracture toughness properties of weld materials have been determined carrying out tensile test as well as CTOD fracture toughness test. The results are then used for finite element (FE) simulations in ABAQUS commercial software to compute stress intensity factor (SIF) along the crack front in the weld material. These results are then used to plot FAD curves and assessment points for each welding electrode to evaluate the existing crack in simulation. The analysis reveals that the presence of different amount of Mn in the weld metal influences directly on the resistance to fracture in weld metal at moderate temperature applications.

Failure assessment of cracked uni-planar square hollow section T-, Y- and K-joints using the new BS 7910:2013+A1:2015

This paper covers the validation of standard safety assessment procedure in the new BS 7910:2013+A1:2015 for cracked uni-planar square hollow section (SHS) T-, Y- and K-joints using the finite element analyses. The procedure is based on the failure assessment diagram (FAD) method. A completely new and robust finite element mesh generator is developed, and it is validated using the full-scale experimental test results. FAD curves are constructed using the elastic J-integral (J_{mathrm{e}}), the elastic-plastic J-integral (J_{mathrm{ep}}) and the plastic collapse load (P_{mathrm{c}}) values calculated using the 3D cracked models of the joints. The results reveal that the standard Option 1 FAD curve of the new BS code is not always safe in assessing the safety and integrity of cracked uni-planar SHS joints. Therefore, a penalty factor of 1.2 for plastic collapse load is recommended to move all the constructed Option 3 FAD curves above the standard Option 1 curve. The new Option 3 FAD curves are found to generate optimal solutions for cracked uni-planar SHS T-, Y- and K-joints.

Failure pressure prediction of a cracked compressed natural gas (CNG) cylinder using failure assessment diagram

This paper describes two different levels of failure assessment diagram (FAD) to predict the failure pressure of a cracked Type 1 full metal compressed natural gas (CNG) cylinder containing an internal longitudinal surface crack. The fracture toughness and stress–strain curve of the metal are determined by carrying out crack tip opening displacement (CTOD), Charpy impact and tensile coupon tests, respectively. Combining the stress intensity factor (KI) and reference stress (σref), the Level 2A FAD curve is used to predict the failure pressures of 12 cracked CNG cylinders. After then, finite element (FE) mesh models are generated to calculate the elastic and elastic–plastic J-integral as well as the plastic collapse load used to construct the Level 3C FAD curve. From the detailed study, it is found that the failure pressures predicted from the Level 2A FAD curve tends to be conservatively safe, and the level of conservatism ranges from 17.7% to 42.2% as compared to the one predicted from the Level 3C FAD curve. In practice, it is safe to use Level 2A FAD curve to predict the failure pressure of a Type 1 full metal cracked CNG cylinder. For convenient purposes, a regressed equation based on Level 3C FAD analysis is proposed to determine the failure pressures of the cracked CNG cylinders.

Effects of Negative Biaxial Loadings and Notch on Failure Assessment Diagrams

The failure assessment diagram (FAD) is a simplified and robust flaw assessment methodology, which simultaneously connects two dominant failure criteria: linear elastic fracture mechanics on one end and plastic collapse on the other end. This interaction is in the realm of elastic-plastic fracture mechanics. It is popularly known as the R6 approach, which graphically characterizes the impact of plasticity on crack driving force. In recent years, there has been continuous interest in using FADs to assess the failure of cracked structures subjected to biaxial loadings. Biaxiality is defined as the ratio of stress applied parallel and normal to the crack. Some pressure loaded aircraft components operate under negative biaxial ratios up to −0.5. In this paper, a detailed study on FAD was conducted using finite element analysis computed J-integral methods to investigate the effect of biaxial loading using different FAD approaches for geometries with notches. Geometries with a crack that emanates at a fillet region were simulated with various biaxial loading ratios from −0.5 to +0.5 using 2014-T6 material. FAD curves were numerically generated for cracks at notched regions subjected to various biaxial loadings using J-integral values from finite element analyses. These results were compared with standard FAD approaches. All comparison studies were made between uniaxial and biaxial loading cases with FAD curves created using four different crack sizes. Under small scale yielding, this study clearly shows that FAD curves are not influenced by negative biaxial loading at low load (up to 40% of yield strength). It was clearly confirmed that the majority of previously developed analytical FAD curves do not effectively account for notch and plasticity effects due to negative biaxiality. Based on this study, tension normal to the crack and compression parallel to the crack is the worst combination, and it has a very pronounced effect on FAD curve shapes. The standard analytical FAD curves are nonconservative compared with the approach recommended here, particularly under the worst case condition. FAD curves developed are shown to predict lower failure loads as compared with the currently accepted analytical FAD approaches defined in existing standards, e.g., R6 and API 579. The impact of negative biaxial loading can be investigated directly using a J-integral FAD approach but can be compared with ease by plotting both approaches in a FAD format.

Validation of BS7910:2005 failure assessment diagrams for cracked square hollow section T-, Y- and K-joints

This paper describes the usage of finite element (FE) analyses results to validate the standard BS7910 assessment procedure for the safe design of cracked square hollow section (SHS) T-, Y- and K-joints. In the study, the actual 3D surface cracks obtained from previous fatigue tests have been included in the FE models. An automatic mesh generation program is then developed and used to produce the failure assessment diagram (FAD) through the J-integral method. The ultimate strength of uncracked SHS joints with reduced load bearing areas have been referenced to derive the plastic collapse loads of cracked SHS joints for the development of FAD. These loads have been validated against the previous experimental results. In comparison with the existing standard BS7910 Level 2A/3A FAD curve and the proposed assessment procedure for circular hollow section joints, it is found that a plastic collapse load with a penalty factor of 1.05 will be sufficient for the safe assessment of cracked SHS T, Y, and K-joints under brace end axial loading.

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

Abstract 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.

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

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.