Failure Assessment Diagram Method Research Articles

Thermal stresses of large-dimension lithium hydride ceramics during the sintering process and strength analysis based on process-based failure assessment diagram

Lithium hydride (LiH) is an important hydrogen storage, heat storage and neutron shielding material. When used in neutron shielding structure, its size and strength have certain requirements, which would be met by LiH ceramics. However, it fails easily during sintering due to severe thermal stress and high brittleness. In this study, the influences of the dimension, furnace temperature cooling rate, and convection condition on the mechanical behavior were systematically examined. The results indicated that larger dimensions and faster furnace temperature cooling rates resulted in higher maximum stresses. Furthermore, convection was beneficial for reducing the thermal stress. Accordingly, a process-based failure assessment diagram method was proposed to evaluate the safety of LiH ceramics, and a bilinear precise temperature control approach was proposed. Based on this method, the tube thickness was increased by 25 % and the cooling time was reduced by 32 % compared to those using linear temperature control in an example. The conclusion and method could be applied to optimize the temperature control for more practical large-dimension sintering.

Effect of hydrogen embrittlement on the safety assessment of low-strength hydrogen transmission pipeline

Currently, no systematic approach exists for the safety assessment method of hydrogen transmission low-strength pipeline, it is necessary to study the effect of hydrogen embrittlement (HE) on the method of failure assessment diagram (FAD) for the defective pipe. The hydrogen charging experiments of tensile and Charpy impact specimens for the L245 pipe steel were carried out with charging time of 0, 30, 90, and 120 h, and then the tensile and Charpy impact tests were immediately conducted. The HE indexes of tensile and Charpy impact properties were presented to quantitatively analyze the effect of HE under tensile and impact deformation processes. Subsequently, the FAD method coupled with the effect of HE was investigated to analyze the difference in the safety assessment method between hydrogen and non-hydrogen environments. Finally, the fracture morphology of tensile and Charpy impact specimens was observed to analyze the responses of microscopic fracture modes between hydrogen and non-hydrogen environments. The results show that the effect of HE on the tensile properties of the base material is more sensitive than that of the weld joint. The FAC with the effect of HE for safety assessment of defective pipes shrinks by reducing the safe zone. The decrease ratio of safety factors affected by HE presents an increasing tendency as the increase of crack depth while showing a decreasing tendency as the increase of internal pressure.

An interval failure assessment diagram method for analyzing the reliability of structure containing crack defect

The failure assessment diagram (FAD) is the most widely used method for analyzing the reliability of structure containing crack defect. In practical engineering, the failure assessment point (FAP) and failure assessment curve (FAC) are impossible to be determined accurately. Therefore, an interval failure assessment diagram (IFAD) method is proposed in this paper to analyze the reliability of structure containing crack defect when the FAP and FAC cannot be obtained accurately. Firstly, based on the interval method and FAD method, an interval failure assessment diagram (IFAD) model is established by considering the FAP and the failure critical point (FCP) as interval variables. And then the FAP and FCP intervals are converted into the normalized region to design the IFAD reliability index [Formula: see text] according to the relationship between the limit state function and the normalized region. The structural arbitrary state can be described by the IFAD reliability index [Formula: see text]. In addition, 10 cases are given as examples to clearly expound the IFAD method, and the analysis results are compared with the FAD method, which confirms that the IFAD method is feasible and valid to analyze the structural reliability with the bounds of FAP and FCP. Finally, the IFAD reliability analysis of a certain propellant storage tank ellipsoidal bottom containing surface crack is taken as an application example to verify the practicability of the proposed method. And the results show that the IFAD reliability index can be obtained without the exact value of parameters.

J Predictions for Defective Pipe Elbows Via the Reference Stress Method

Abstract In R6, the J-based failure assessment diagram (FAD) method is used in the fracture assessment and is underpinned by the reference stress J scheme. Therefore, an assessment using the R6 FAD method is equivalent to a J prediction using the reference stress method. In this paper, the effect of global and local limit load solutions for defective elbows on the reference stress and hence the J predictions is investigated using published three-dimensional elastic-plastic finite element (FE) J results, in order to create guidance for users to follow when performing structural integrity assessments of defective elbows using the R6 procedure. The results show that using the global limit load solutions recommended in this paper can lead to good and reasonably conservative J predictions. However, the availability of global limit load solutions is very limited. The results also show that using the local limit load evaluated from the local limit load model recommended in this paper can lead to conservative J predictions for most of the cases considered.

Ultimate Axial Load Prediction Model for X65 Pipeline with Cracked Welding Joint Based on the Failure Assessment Diagram Method

Crack defects in the girth welds of pipelines have become an important factor affecting the safe operation of in-service oil pipelines. Therefore, it is necessary to analyze the factors affecting the safe operation of pipelines and determine the ultimate load during pipeline operation. Based on the failure assessment diagram (FAD) method described in the BS 7910 standard, the key factors affecting the evaluation results of the suitability of X65 pipeline girth welds are analyzed, and the effects of crack size, pipe geometry, and material properties on the evaluation results are investigated. The results indicate that the crack depth is more crucial to the safe operation of the pipeline than the crack length. While the effect of wall thickness is not significant, the misalignment can seriously aggravate the stress concentration. In general, the higher the yield ratio and tensile strength of the pipe material, the more dangerous the condition at the weld. The ultimate axial load that a crack-containing girth weld can withstand under different combinations of the above factors was determined. Furthermore, a data driven model via the optimized support vector regression method for the ultimate axial load of the X65 pipe was developed for engineering application, and the comparison results between the FEM results and the predicted results proved its accuracy and reliability.

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.

3-D FE analyses of WRS and simplified elastic-plastic fracture mechanics assessment of a repaired weld

Abstract An effective excavation and re-welding process is often needed to correct the installation slope and geometric misalignment in the piping, where significant local plastic strain damage and complex welding residual stresses (WRS) often occur during the repair process. At the same time, it is complex to perform elastic-plastic fracture assessment for combined loads such as the WRS, bending moment and internal pressure. In this paper, three-dimensional (3-D) finite element (FE) analyses of WRS of the repair welding are performed, and the influence of weld bead sequencing as well as the displacement caused by external loads is simulated. In the study, the plastic strain damage is also assessed to prevent void formation. To avoid the complex elastic-plastic fracture assessment, a simplified elastic-plastic fracture study is carried out based on the failure assessment diagram (FAD) method. The results show that the repair process is effective, and the strain damage caused by the weld repair is acceptable. There are clear differences between WRS caused by initial weld and that of the re-weld, and there is palpable influence of the repair process on the WRS for the unrepaired/unexcavated part of the pipe. Based on the simplified elastic-plastic FAD method, the repair process has obviously influence on the margin of safety. The repair process described in this paper is generic and thus can be applied in similar pipe deflection correction applications.

Admissibility of External Cracks in a Pipeline API X60 Using the SINTAP Procedure

In this paper we tried to apply the failure assessment diagram method on an API X60 pipeline under two pressures 70 and 90 bar, this work will be divided into two parts; the first part will be devoted to modeling and simulation of a pipeline under pressure 70/90 bar. With abaqus software to determine the stress intensity factor of several ratios, The second part will focus on the exploitation of these results in order to draw the diagram of evaluation of the failure (FAD), once finished, We can pronounce on the vulnerability of the cracks which can cause the ruin of the pipeline to study, on mode of ruin and proposed safety factors.

Application of failure assessment diagram methods to cracked straight pipes and elbows

This paper reports fracture assessments of large-scale straight pipes and elbows of various pipe diameters and crack sizes. The assessments estimate the load for ductile fracture initiation using the failure assessment diagram method. Recent solutions in the literature for stress intensity factor and limit load provide the analysis inputs. An assessment of constraint effects is also performed using recent solutions for elastic T-stress. It is found that predictions of initiation load are close to the experimental values for straight pipes under pure bending. For elbows, there is generally increased conservatism in the sense that the experimental loads are greater than those predicted. The effects of constraint are found not to be a major contributor to the initiation fracture assessments but may have some influence on the ductile crack extension.

A modified reference strain method for engineering critical assessment of reeled pipelines

Reel-lay is a highly efficient method employed to install small or medium diameter offshore pipelines that play a key role in transportation of crude oil and natural gas. However, pipelines are subjected to 1–3% plastic straining during the process of reel-lay; thus engineering critical assessment (ECA) must be carried out to obtain the crack-like defect acceptance criteria. In our work, the effects of pipe geometry, crack geometries, and material properties on the reference strain are investigated. Furthermore, an empirical formula is proposed to estimate the reference strain. By incorporating the reference strain, the application range of the strain-based failure assessment diagram method is extended to ECA of severe circumferential surface cracks in reeled pipes.

A Novel Engineering Evaluation Method for Pressure Piping Containing Circumferential Defects

Safety evaluation for in-service pressure piping containing defects plays an important role in ensuring safety production. Based on Central Electricity Generating Board (CEGB) Assessment of the Integrity of Structures Containing Defects (R6) revision 4 failure assessment diagram (FAD), a novel safety assessment method, named Q factor method, is presented for pressure piping containing circumferential surface defects through analysis and simplified in this paper. The Q factor method is simple and efficient, established and given in a more acceptable form in engineering application, and suitable for the safety assessment of defective piping of different materials, without complex fracture or limit load analysis. Besides, the nature of the Q factor approach is consistent with the FAD method, so its validation isn’t mentioned in this paper.

Ductile and brittle failure assessment of containment vessels subjected to internal blast loading

Several major international design methods of explosion containment vessels (ECVs) refer to the items of pressure vessel codes and standards, where the fracture mechanics analysis of pressurized components should be performed to prevent the occurrence of brittle fracture. However a ductile damage mode in the form of adiabatic shear band (ASB) is frequently found as a failure mode for the containment vessels subjected to internal blast loading. A rate-dependent failure criterion was proposed to account for ASB propagation, and a finite element analysis of a cylindrical containment vessel with different size of cracks is performed, where the overpressure caused by detonation was calculated and the propagation of cracks and the final fracture profile are obtained, which shows a good agreement with experimental result. The Failure assessments based on ASB mode and failure assessment diagram (FAD) method were conducted, respectively. It was found that the final fracture mode primarily depends upon the intensity of explosive load as well as loading rate in ASB mode. The assessment result based on FAD method resembles that of ASB at low or intermediate loading rate. However with increment of loading rate, the difference of assessment results based on two methods became obvious, the comparison indicates that for the structures under high strain-rate loading the ASB assessment provides better estimation of crack growth than FAD method does.

Proposal of reference stress for a surface flaw on a cylindrical component from a review-with-comparison of the local metal loss assessment rule between API 579-1 and the p-M diagram method

The Remaining Strength Factor (RSF) approach in Part 5 of API 579-1/ASME FFS-1 is an assessment method for a cylindrical component with a local metal loss based on surface correction factors. Also, reference stress solutions that are applied in the Failure Assessment Diagram (FAD) method for a cylindrical component with a crack-like flaw are provided in Annex D using surface correction factors. In the recently-developed p-M diagram method, the reference stress solution for local metal loss evaluation in a cylindrical component is derived using bulging factors, which are similar but not identical to the surface correction factors used in API 579-1/ASME FFS-1. This paper describes the results of a comparative study among the RSF approach, reference stress solutions for the FAD method, and the p- M diagram method, in terms of plastic collapse evaluation of a cylindrical component. These results were compared with the FEA and experimental results to confirm how these estimated stresses could be validated. This study also involves recommended reference stress solutions for a cylindrical component with a crack-like flaw or a local metal loss, which should be adopted as fitness-for-service rules, and a discussion on the influence of the design margin of the construction code on allowable flaw depth.

구형 산소용기 내 표면균열에 대한 수치파괴역학 평가

During the last decade, possibility of flaw occurrences has been rapidly increased world-widely as the increase of operating times of petro-chemical facilities. For instance, from a recent in-service inspection, three different sized surface cracks were detected in welding parts of a spherical oxygen holder in Korea. While API579 code provides corresponding engineering assessment procedures to determine crack driving forces, in the present work, numerical analyses are carried out for the cracked oxygen holder to investigate effects of complex geometry, analysis model and residual stress. With regard to the detailed finite element analysis, stress intensity factors are determined from both the full three-dimensional model and equivalent plate model. Also, as an alternative, stress intensity factors are calculated for equivalent plate model by employing the noted influence stress function technique. Finally, parametric structural integrity evaluation of the cracked oxygen holder is conducted in use of failure assessment diagram method, J/T method and DPFAD method. Effects of the geometry and so forth are examined and key findings from the simulations are fully discussed, which enables to determine practical safety margins of spherical components containing a defect.

Failure assessment diagram methods for strain-based fracture

Stress-based fracture assessment procedures contain conservatism when assessing strain- or displacement-controlled loads in excess of yield. Alternative strain-based methods are discussed in this paper and compared with an existing approach in the R6 defect assessment procedure that allows reduction in conservatism for the case of fixed displacement or rotation loading. Published finite element results for semi-elliptical cracks in plates are re-interpreted as values of J against remote strain and used to assess recent strain-based estimates of J. An assessment procedure based on a strain-based failure assessment diagram is outlined.

Fatigue Crack Growth Analyses of Offshore Structural Tubular Joint

Fracture mechanics fatigue life estimation procedures have been developed for offshore structural tubular joints through analyzing a K-joint under the North Sea environment. The objective of this study was to establish reliable procedures for estimating the remaining fatigue life of a tubular joint with cracklike defects. The analysis approach was the utilization of fracture mechanics methods for fatigue crack growth and failure analyses. In this study, the fully mixed mode stress intensity factors of weld toe surface flaws of the K-joint, which were calculated through detailed three-dimensional finite element analyses, were used for fatigue crack growth simulation. For the failure analyses, the failure assessment diagram method was used to predict the conditions for brittle fracture during fatigue crack propagation. The loading conditions considered in the analyses are the brace axial force, in and out-of-plane bending, and torsion.