Failure Assessment Curve Research Articles

Guidance for structural integrity assessment of smooth pipe elbows containing defects using R6

Recently, guidance for fracture assessment of defective elbows using the R6 failure assessment diagram (FAD) method has been included in R6 Revision 4 Amendment 13 (2023). This paper provides an overview of the new guidance and supporting validation. The guidance is validated using 75 cases of 3-D finite element (FE) elastic-plastic J results of smooth pipe elbows of various geometries with axial/circumferential internal/external semi-elliptical/extended/fully-circumferential surface cracks under internal pressure, in-plane/out-of-plane bending moment, torsion and various load combinations, by comparing the FE-based Option 3 failure assessment curves (FAC) with the R6 Option 2 FAC. The results show that, for 74 out of 75 cases, the FE-based Option 3 data points are above the R6 Option 2 FAC with reasonable conservatism when the guidance for evaluating the elbow stress intensity factor and limit load is followed. In other words, following the new guidance can lead to reasonably conservative assessment results.

Constraint-corrected failure assessment diagrams based on crack tip plastic zone size

Failure assessment diagram (FAD) is affected by constraints, and it has been corrected by in-plane T-stress (T11) in the R6 defect assessment procedure, but the influence of out-of-plane constraint is usually neglected. The combined effects of in-plane and out-of-plane constraints on FAD are investigated in this work. Crack tip plasticity can characterize the in-plane and out-of-plane constraints comprehensively. Hence, by using plastic zone size (rp0), a new correction model of FAD is proposed theoretically in small scale yielding condition. Then the influences of constraints on failure assessment curves and allowable loads are analyzed in central-cracked unstiffened and stiffened plates, and the failure mechanisms are studied too. The results reveal that, compared with traditional FAD and T11-stress corrected FAD, the new correction model of FAD based on rp0 is more accurate and reasonable; and both in-plane and out-of-plane constraints play important roles in failure assessment curves and allowable loads. Additionally, the new correction model of FAD is an effective approach to analyze the failure mechanisms, and the failure modes depend on crack length and type of structures.

Data-driven probabilistic failure assessment curve based on similitude principle

In the study, based on the similarity in crack-tip fields between pipeline structure and standardized single-edge notched tension (SE(T)) test specimen, a methodology using a data-driven machine learning technique is proposed to determine the specific failure assessment curves for full-scale pipeline girth welds. By considering constraint similitude and ductile tearing, the probabilistic failure assessment line obtained from SE(T) resistance curves with a 50% survival rate can provide the most accurate failure assessment, as validated using the experimental full-scale pipeline data in the literature, particularly for the zone dominated by ductile fracture. Moreover, Option 1 (in R6 terminology) fracture assessment curve of British Energy R6 approach, which corresponds to a 15% survival rate, is proven to be overly-conservative.

Strain-based fracture analysis for internal surface cracks of X80 pipe girth welds

The girth weld is the weakest region of the entire pipeline. When the pipeline crosses areas with complex terrain or where natural disasters are frequent, the pipeline may be subjected to large displacement effects and large plastic deformations. In these scenarios, the traditional stress-based assessment methods will no longer be applicable. Therefore, it is of great importance to develop strain-based assessment methods for pipelines. This paper investigates the effects of crack size, strength mismatch, heat affected zone (HAZ) width and HAZ softening/hardening coefficient on the crack driving force (CDF) of weld center crack (WCC) and HAZ center crack (HAZCC) on the inner surface of X80 pipes using finite element method. The results show that the CDF is significantly affected by crack size, crack location, strength mismatch, HAZ softening coefficient but slightly affected by HAZ width when HAZ width is greater than 2 mm. The CDF of WCC is greater than that of HAZCC under large strain conditions with HAZ width of 2 mm. The distribution of the J-integral at the crack tip is then investigated and the results show that the maximum value of the J-integral does not necessarily occur at the deepest point of the crack tip. Finally, the factors influencing the failure assessment curve (FAC) of WCC are investigated and the lower bound strain-based failure assessment diagram (SB-FAD) considering strength mismatch and HAZ softening coefficient is developed with varying crack depths. This study can provide a reference for the safety assessment of crack-like defects on the inner surface of X80 pipeline girth welds.

Crack Growth Assessment Investigation of Seamless Stainless Steel Tube

In this paper, a crack propagation assessment on a seamless stainless steel tube was carried out by comparing the crack growth driving force and the resistance. Crack growth driving force was given by finite-element analysis (FEA), the stress–strain relationship was given by digital image correlation (DIC) tensile testing, and the crack growth resistance was given by single-edge notched tensile (SENT) testing with the single-specimen flexibility method. Then, the proposed approach was compared with the failure assessment curve and validated by the full-size hydrotest of tube with an inner wall notch. The result of full-size test showed the assessment method of FEA was more accurate and the result of failure assessment curve was more conservative.

Study of Strain Capacity for High-Strain Marine Pipe.

In this paper, a strain capacity assessment on high-strain marine pipe was carried out by comparing the crack growth driving force and the crack growth resistance. The crack growth driving force was given by FEA, the stress-stain relationship was given by a DIC tensile test, and the crack growth resistance was given by a single-edge notched tensile (SENT) test using a single-specimen flexibility method. The proposed approach was compared with the failure assessment curve and validated against full-scale tests with a girth weld notch. The results of the full-scale tests showed that the assessment method using FEA was more accurate and the result of the failure assessment curve assessment was more conservative.

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.

Structural Integrity Assessment of Plates Containing Semi-elliptical Surface Cracks: Finite Element Fracture Analyses

This paper summarizes results of limit load solutions for plates containing semi-elliptical surface cracks subject to complex loadings, to include biaxial tension and positive cross-thickness bending (crack opening moment). The elastic-plastic J-integral data have been determined from a series of cracked body finite element analyses (FEA) incorporating various plate and crack geometries, loading combinations, and elastic and elastic-plastic material models.The FEA results are used to validate the Lei and Budden limit load solutions, principally via production of R6 Option 3 failure assessment curves (FACs) and comparison with the R6 Option 1 and Option 2 FACs. Comparison with other available limit load solutions in literature has also been carried out. The paper aims to provide some improved guidelines for the safe life assessment margins regarding to structural integrity of aged components in energy industries.

Constraint-based failure assessment for pipelines with coplanar circumferential cracks under biaxial loading

The presence of coplanar or multiple cracks emerges as a common problem in many engineering structures, e.g., at the toe of circumferential welds of pipelines subjected to cyclic actions and in other piping systems for the nuclear power plants. The failure assessment diagram provides a convenient means to examine the competing failure between the fracture failure and plastic collapse failure, often for structural components with a single crack. The existing failure assessment diagram relies on the leading term characterizing the near-tip stress solutions, and ignores the geometry or plasticity-induced constraint variations. This paper aims to integrate the crack interaction effect between two coplanar cracks, measured by a constraint-based crack interaction factor, into the option 3 failure assessment curve in the engineering standard. This study examines three types of circumferential coplanar cracks in a pipe: two coplanar embedded cracks; two coplanar surface cracks; and a surface crack interacting with an embedded crack. Using a modified J solution based on a proposed crack interaction factor, this investigation quantifies the plasticity-driven increase in the near-tip opening stress for coplanar cracks. The stress field estimated based on this modified J solution agrees closely with the numerically computed stress field near the coplanar crack tips at large deformation levels. This work subsequently integrates the modified J solution into the failure assessment diagram. The failure assessment diagram based on the equivalent crack size recommended in BS7910 appears to be un-conservative for a number of coplanar cracks considered in this study.

Failure assessment curve for austenitic stainless steel pipes of nuclear power plants

This paper presents a failure assessment curve (FAC), which predicts a conservative failure load of austenitic stainless steel pipes with a circumferential surface crack. Changes in material deformation properties due to cold working, neutron irradiation and thermal aging were taken into account. First, stress-strain curves for stainless steels were assumed for welded, irradiated, cold worked, thermal aged materials as well as undamaged materials using the Swift-type constitutive equation, whose material constants were estimated from the yield and ultimate strengths. Second, the FACs were analyzed for various material properties and crack and pipe geometries. It was shown that the FAC depended on the crack and pipe geometries, and the lower bound FAC were obtained. Finally, the universal FAC, applicable to stainless steel pipes regardless of crack geometries and material properties, was proposed.

Engineering critical assessment of RPV with nozzle corner cracks under pressurized thermal shocks

Nozzle corner cracks present at the intersection of reactor pressure vessels (RPVs) and inlet or outlet nozzles have been a persistent problem for a number of years. The fracture analysis of such nozzle corner cracks is very important and critical for the efficient design and assessment of the structural integrity of RPVs. This paper aims to perform an engineering critical assessment of RPVs with nozzle corner cracks subjected to several transients accompanied by pressurized thermal shocks. The critical crack size of the RPV model with nozzle corner cracks under transient loading is evaluated on failure assessment curve. In particular, the influence of cladding on the crack initiation of nozzle corner crack under thermal transients is studied. The influence of primary internal pressure and secondary thermal stress on the stress field at nozzle corner and SIF at crack front is analyzed. Finally, the influence of different crack size and crack shape on the final critical crack size is analyzed.

A modification of reference strain approach for thin-walled submarine pipelines under large-scale plastic strain and internal pressure

Thin-walled pipelines are widely used in submarine oil and gas transportation. During the process of reel-lay and service, pipelines are subjected to large-scale plastic strain and internal pressure. Reference strain approach takes advantage of natural strain-controlled boundary condition directly. However, it is only reliable for small cracks by the assumption of equality between the reference strain and nominal strain. The effect of internal pressure is not considered in traditional failure analysis diagram (FAD). In this study, a theoretical solution of reference strain considering the influence of internal pressure is proposed. A corresponding failure assessment curve (FAC-New) considering the influence of internal pressure is also constructed based on the derived nominal strain. The proposed FAC-New is in contrast with FAC-R6. It is proved that FAC-New presents a more explicit classification of the influence of internal pressure in Engineering Critical Assessment (ECA).

Fracture Toughness and Application of X80 Pipeline Steel

The fracture toughness of X80 pipeline steel, which has been widely used in China, plays a key role in pipeline safety insurance. In order to know the accurate fracture toughness of X80, the methods of acoustic emission and high K ratio have been carried out to judge the initiation of crack propagation and improve flatness of fatigue pre-crack front. By this method, fracture toughness of base metal, weld and heat affect zone of X80 pipeline steel have been tested, and failure assessment curves of X80 have been established and fitting equations has been provided. The results suggested that different failure assessment curves should be established according to different crack sizes and material parameters.

Reference Stress Approach for Fracture Assessment of Extrados Circumferential Through-Wall Crack at the Interface between Elbow and Pipe Subjected to Internal Pressure

The fracture assessment of a cracked component can provide a technical support for an effective maintenance and a prevention of catastrophic accident. The present study applies the reference stress method to extend the J-integral solution from the finite element analysis for the Ramberg-Osgood stress-strain relationship to other types of stress-strain relationship. The problem in this study is an extrados circumferential through-wall crack at the interface between elbow and pipe subjected to internal pressure. The finite element results from the literature were analyzed to determine the optimized reference pressure. The reference-stress-based failure assessment curve which incorporated with the obtained reference pressure can be used to estimate the finite-element-based failure assessment curve with a sufficient accuracy in fracture assessment. The failure load predicted from the reference-stress-based failure assessment curve deviated from that predicted from the finite-element-based failure assessment curve within the range of -7% to 11%.

Consideration of a proposed SBAD method for BS7910

Concerns have been published about the cases of non-conservatism when the BS 7910 option 2 Failure Assessment Curve (FAC) is used in the Strain Based Analysis and Design (SBAD) regime. An assessment of these concerns has been made using an analytical solution for the Crack Driving Force (CDF) associated with an infinite notched strip. It was shown that the option 2 FAC could underpredict the CDF for this case and that the level of error was bigger for lower levels of strain hardening. A modification to the option 2 FAC is proposed. The modification has been studied in other publications and has been shown to provide conservatism in all cases investigated to-date.

Elastic-plastic fracture analyses for misaligned clad pipeline containing a canoe shape surface crack subjected to large plastic deformation

Offshore pipelines are subjected to large plastic deformation during the reeling stage or in-service operation. These pipelines are joined together by a welding process, and defects and weld misalignment are frequently introduced into the pipelines, posing tremendous challenges to the integrity of these pipelines. In this study, the fracture responses of misaligned clad pipeline containing a canoe shape surface crack located in weld centre line (WCL) and fusion line (FL) were investigated using 3-D elastic-plastic finite element (FE) analyses. The influences of crack depth ratio, crack length to perimeter diameter ratio and centerline offset ratio on the crack tip opening displacement (CTOD) were analyzed in detail. The relationship between the CTOD and the global strain (εg) was built up, and a linear relationship was observed for εg ranging from 0.6% to 2%. The fracture assessment equation of British Standard (BS) 7910 predicted an over-conservative result in comparison with that obtained by FE analyses. Therefore, a new strain-based failure assessment curve was developed to assess the fracture behaviour of misaligned clad pipeline subjected to large plastic deformation.

Improvement of the failure-assessment diagrams used to check the harmfulness of pipe defects

In this paper, the principle of the failure-assessment diagram (FAD) is presented including the definitions of the failure-assessment curve and the assessment point. Classical FAD allows computing a safety factor associated with defect geometry and loading conditions. Extension to a probabilistic FAD (PFAD) is used to compute the failure probability. In order to reduce the conservatism associated with the ‘crack-like defect’ procedure, the use of notch-failure-assessment diagram (NFAD) takes advantage of the increase of fracture toughness by reducing the defect acuity. The domain-failure-assessment diagram (DFAD) are pertinent to associate the right fracture-mechanics’ tool with the failure mechanism: brittle, ductile, or plastic collapse. The fatigue-assessment diagram (fAD) use the Wohler curve as failure-assessment curve and is used to develop a maintenance policy by determining loading conditions to guarantee a life duration with a given probability.

Probabilistic fatigue initiation assessment diagram pipe steel ×52: influence of hydrogen

A probabilistic fatigue assessment diagram is proposed as a tool to guarantee a prescribed number of applied fatigue cycles with a conventional low probability of failure. The fatigue failure assessment curve is given by a power law described by two parameters: the fatigue initiation resistance and the fatigue initiation exponent. From the targeted applied stress range, it is possible to find the probability of failure and its associated safety factor. An example of fatigue life duration of a pipe steel submitted to hydrogen embrittlement is given.

A probabilistic fatigue assessment diagram to get a guaranteed lifetime with a low probability of failure

Abstract In order to guarantee a fatigue lifetime with a conventional and low probability of failure, a probabilistic fatigue assessment diagram is proposed as a tool. The safe domain is limited by the fatigue failure assessment curve, which depends on material properties through Basquin's exponent, the endurance limit and low cycle fatigue domain. From the double-truncated distribution of the targeted maximum applied stress, it is possible to find the probability of failure and its associated safety factor. An example of an aeronautical component made of TA6V titanium alloy welded by laser and exhibiting an undercut at the weld toe is given.

基于极限载荷与断裂强度的5A06铝合金焊接缺陷安全性评定的数值方法研究

本文建立了一种针对5A06铝合金带焊接缺陷平板的安全性评定的数值方法。首先采用极限载荷分析法建立了极限应力和裂纹长度的定量关系，从而获得失效评估曲线(FAC)。对130 mm厚的5A06铝合金对接焊接平板，采用SINTAP标准获得焊接残余应力分布和残余应力强度因子,并用虚拟裂纹闭合技术(VCCT法)计算工作载荷作用下的应力强度因子，建立了5A06铝合金失效评估图(FAD)。论文提出的数值方法能够对给定缺陷条件下的承载能力进行安全性评定；相反，在给定外载荷条件下，对缺陷的容纳限值进行确定。 In this paper, a numerical method for assessment the safety of 5A06 aluminum alloy strip with welded defects is established. First, the quantitative relationship between the limit stress and the length of the crack is established by the limit load analysis method, and then the failure assessment curve (FAC) can be obtained. For the 130 mm thick 5A06 Aluminum Alloy butt welding plate, welding residual stress distribution and residual stress intensity factor are obtained by SINTAP, and the stress intensity factor of the working load is calculated by the virtual crack closure tech-nique (VCCT method), then the 5A06 Aluminum Alloy failure assessment diagram (FAD) is estab-lished. The numerical method proposed in this paper can be used to evaluate the safety of the bearing capacity under the condition of a given defect. On the contrary, under the given external load conditions, the limitation of the defect can be determined.