Failure Assessment Diagram Approach Research Articles

Crack assessment in spiral-welded pipelines repaired by composite patch: a SMART and failure assessment diagram approach

Crack assessment in spiral-welded pipelines repaired by composite patch: a SMART and failure assessment diagram approach

Leak before break fracture assessment of pressurized unplasticized PVC pipes with axial surface crack: Experimental and analytical analysis

This paper compares Failure Assessment Diagram-based and Stress Intensity Factor-based analytical approaches with experimental results for assessing leak before break in axial finite semi-elliptical cracked Polyvinyl Chloride pipes subjected to internal pressure. Surface crack length of 25, 40 and 60 mm and depth to thickness ratio of 0.25, 0.5 and 0.75 were investigated. Burst pressures are experimentally evaluated for all cracked and uncracked samples. Results illustrated that the Failure Assessment Diagram approach is more reliable than Stress Intensity Factor approach for fracture assessment of unplasticized PVC pipes.

Application of the Failure Assessment Diagram approach for contact fatigue damage evaluation in railway wheel steels

AbstractA Failure Assessment Diagram (FAD), developed for rolling contact, was specifically formulated for railway wheel steels. The FAD allows combining a multiaxial fatigue criterion with the linear elastic fracture mechanics concept; in particular, it was used to assess the fatigue failure in wet contact, related to the pressurization of the fluid entrapped in surface cracks. The approach was applied to the results of previous bi‐disc tests carried out on three railway wheel steels, subjected first to a dry rolling‐sliding contact step, then to a wet contact step. The crack size distribution after the dry contact step was evaluated by a statistical approach; subsequently, the stress intensity factor during the wet contact step was estimated by finite elements. The results of the FAD agreed with the damage observed on the cross section of the specimens at the end of the tests. Furthermore, the FAD was used to determine the maximum allowable crack to prevent rolling contact fatigue, this way showing its potential as a damage tolerant approach.

Unified constraint-based FAD assessments for ductile fracture in cracked pipes

In this work, the unified constraint-based FAD (failure assessment diagram) assessment methodology has been investigated and developed for ductile fracture in cracked pipes. Two unified constraint parameters Ap and Ad are utilized to incorporate both in-plane and out-of-plane constraints. The results show that the FAD approaches based on two-parameter K-Ap and K-Ad can predict similar fracture initiation loads. With decreasing crack length and depth, the constraint level decreases and fracture toughness increases, thus the predicted fracture load increases. For some shallow and short pipe cracks with lower constraints, the unified constraint-based FAD assessment can reduce conservatism degree of the conventional FAD assessment. For some deep and long cracks, their constraint levels may be higher than the standard plane strain specimen, and the application of the unified constraint-based FAD assessment approaches may eliminate non-conservative assessment in the conventional FAD approach. The unified constraint-based FAD approaches and CDF (crack driving farce) approaches using parameters Ap and Ad can produce similar fracture prediction results, thus they are basically equivalent.

Damage tolerant analysis of corner cracks in small nozzle process penetration of the CAREM-25 reactor pressure vessel

This paper presents a damage tolerant approach assessment of corner cracks in small nozzle process penetrations in the pressure vessel of the small modular reactor CAREM-25 subjected to mechanical and thermal shock loadings during certain operation events. Fracture mechanics analyses were performed following ASME XI guidelines considering a representative geometry and specific loading conditions. Stress distributions in the uncracked component were calculated by finite element techniques, while fracture driving forces were estimated using closed form solutions. Maximum allowable corner crack sizes were calculated using linear elastic fracture mechanics and failure assessment diagram (FAD) approaches. The onset of stable crack growth and crack instability were considered as failure conditions. Additionally, fatigue crack growth estimations during the service life were performed integrating a Paris law up to the maximum allowable crack sizes. The results indicate that it seems unlikely that pre-existing cracks reach allowable sizes during the reactor's service life.

Methods for Complex Cracked Body Finite Element Assessments

This paper presents approaches developed to allow consistent best practice assessments for complex 2/3D defects in engineering structures subjected to primary loading and secondary thermal stresses. Methods are outlined and discussed with reference to recent two- and three-dimensional cracked body analyses of components which have been undertaken to determine tolerable defect sizes and inform sub-critical crack growth calculations. The analyses considered a range of postulated semi-elliptical, through-wall and fully circumferential defects in pressure systems. Mixed element type meshing strategies with tied contact in combination with multiple node transformation techniques around the defect front were employed during mesh generation. This facilitated highly refined meshes along the defect front which were required to accurately model extensive plastic deformation in the region of interest. Displacement driven thermal loads required detailed assessment with multiple elasto-plastic material models, since lower bound properties do not necessarily provide the most conservative results. The elasto-plastic J-Integral analyses were shown to provide significant benefit over application of the more conservative Failure Assessment Diagram (FAD) approach. The undertaken assessments were validated against analytical solutions and historic inspection evidence and showed good agreement. In summary the adopted modelling techniques released conservatisms and permitted detailed assessment of complex geometries and load cases.

Revision of ISO 27306 for CTOD Toughness Correction for Constraint Loss

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.

A Combined Experimental and Modelling Approach to Elastic–Plastic Crack Driving Force Calculation in the Presence of Residual Stresses

Since all residual stress measurement methods have inherent limitations, it is normally impractical to completely characterise a three-dimensional residual stress field by experimental means. This lack of complete information makes it difficult to incorporate measured residual stress data into the analysis of elastic–plastic fracture without resorting to simplified methods such as the Failure Assessment Diagram (FAD) approach. We propose a technique in which the complete residual stress field is reconstructed from measurements and used in finite element analysis of the fracture process. Residual elastic strains and stresses in three-point bend fracture specimens were measured using neutron diffraction and an iterative method was used to generate a self-consistent estimate of the complete residual stress field. This enabled calculation of the J contour integral for a specimen acted on by both residual stress and an externally-applied load, allowing the interaction between residual and applied stress to be observed in detail.

Application of the strain-based FAD to failure assessment of surface cracked components

Purpose – The purpose of this paper is to validate the strain-based failure assessment diagram (SB-FAD) approach for surface cracks in components subjected to displacement controlled boundary conditions. Design/methodology/approach – Numerical analyses are performed for several crack geometries and materials representative for aerospace applications. The performance of the SB-FAD is judged by comparing numerically calculated J-integrals to respective analytical estimates, using both Options 1 and 2 approximations. Findings – In the most cases, both Options 1 and 2 SB-FAD method results in reasonably conservative J-estimates. Exceptions are for surface cracks in a pressurized vessel made of a material with low-strain hardening, for which Option 2 assessment produces non-conservative results. In contrast, Option 1 assessment is conservative for all geometries considered. In general, Option 1 results in a considerable overestimation of the crack driving force, whereas Option 2 produces rather accurate results in many cases. Originality/value – The results demonstrate both the potential of the SB-FAD method and needs for its further improvements.

The Strain-Based Failure Assessment Diagram for Commercially Pure Titanium TA2

Strain-based failure assessment methods have been usually used in recent years to propose the fitness-for-service flaws. In this paper, the stress-based and strain-based failure assessment diagram (SB-FAD) approaches were reviewed firstly. Then, SB-FAD approach was used to the material commercially pure titanium TA2 with high strain-hardening coefficient. Several failure assessment curves obtained from strain-based method for four typical specimens, i.e., CCP (center-cracked plate), DECP (double edge cracked plate), SECP (single edge cracked plate) and CT (compact tension) were proposed. It was found that the geometries and crack sizes had obvious effects on the failure assessment curves. By comparing with the stress-based failure assessment curves, the traditional strain-based failure assessment curves could lead to more conservatism in the stage after yielding. However, the SB-FAD modified by Ainsworth can be used.

Fracture mechanics defect assessment diagram on pipe from steel P264GH with a notch

The design for a pipeline component is considered a tough challenge once defects in the base material develop. To address this issue, we present a gouge defect located in a pipe submitted to internal pressure. This problem is often encountered in the field of pipeline network and boiler components because of structural imperfection. To simulate the activity of such pressurized pipeline, a longitudinal cylindrical shell under pressure from a material of steel P264GH were involved in the research. Secondly, by this paper we propose a safety methodology to detect the boundary of failure assessment diagram (FAD), as a new solution to predict the service life of pipeline products. The methodology is connected to the modified FAD approach, whereas the specific parameters were settled using the Volumetric Method calculation, based on the notch Stress Intensity Factor, determined in the elasto-plastic field. This new modified FAD methodology entails an accurate solution, suitable to be used as expert tool for the assessment of workability and reliability of pipes network, in the case of corrosion defects presence.

Approximate J estimates for combined primary and secondary stresses with large elastic follow-up

This paper proposes the V-factor for approximate J estimates under combined primary and secondary stresses with large elastic follow-up within the failure assessment diagram approach. The elastic follow-up is characterized by three parameters. The first two are the relative magnitudes of the primary load and of the secondary stress, β. The third parameter is the initial slope of the Vo–β curve where Vo is the value of V for zero primary load. Elastic follow-up effects are more pronounced for contained yielding, smaller β and a larger initial slope of the Vo–β curve. An approximate expression for V/Vo is proposed, showing good agreements with finite element solutions.

Elastic-Plastic Analysis and Failure Assessment of the Pipeline Steel with Surface Crack

A detailed elastic-plastic analysis on different semi-elliptical surface crack geometries under uniaxial and biaxial load was conducted using 3D finite element analysis for pipeline steel X100. After quantitating fracture driving force J-integral and plastic yielding load, the effects of biaxial load and crack geometry were investigated by using the Option 3 of R6 failure assessment diagram approach for the surface cracks with the different crack sizes ( aspect ratios c/a = 1, 1.5 and 3 ) and the various biaxial loading ratios λ from -1 to 2. And three option curves of the R6 assessment were compared to evaluate the applicability of R6 assessment method for the X100 steel.

Improved reliability analysis method based on the failure assessment diagram

With the uncertainties related to operating conditions, in-service non-destructive testing (NDT) measurements and material properties considered in the structural integrity assessment, probabilistic analysis based on the failure assessment diagram (FAD) approach has recently become an important concern. However, the point density revealing the probabilistic distribution characteristics of the assessment points is usually ignored. To obtain more detailed and direct knowledge from the reliability analysis, an improved probabilistic fracture mechanics (PFM) assessment method is proposed. By integrating 2D kernel density estimation (KDE) technology into the traditional probabilistic assessment, the probabilistic density of the randomly distributed assessment points is visualized in the assessment diagram. Moreover, a modified interval sensitivity analysis is implemented and compared with probabilistic sensitivity analysis. The improved reliability analysis method is applied to the assessment of a high pressure pipe containing an axial internal semi-elliptical surface crack. The results indicate that these two methods can give consistent sensitivities of input parameters, but the interval sensitivity analysis is computationally more efficient. Meanwhile, the point density distribution and its contour are plotted in the FAD, thereby better revealing the characteristics of PFM assessment. This study provides a powerful tool for the reliability analysis of critical structures.

An engineering assessment methodology for non-sharp defects in steel structures – Part I: Procedure development

This Part I paper describes a new engineering assessment methodology for ferritic steel structures containing non-sharp defects within the context of a Failure Assessment Diagram (FAD) approach. Although the modification of the FAD for non-sharp defects can be applied whether the initiating failure mechanism is cleavage or ductile tearing, this paper focuses on cleavage fracture. The parameters describing the sensitivity of the material toughness to the notch effect can either be measured by testing notched specimens of the same thickness as the structure, or for cleavage fracture they can be obtained using look-up tables generated using the Weibull stress toughness scaling model. The other parameters in the procedure can either be conservatively estimated using simple equations or they can be determined more accurately using finite element analysis. Validation of the new method is presented in the companion Part II paper: this shows that assessments of U-notched SE(B) specimens have significantly reduced conservatism when using the new assessment methodology compared to the standard FAD approach for sharp cracks.

Determination of the fracture toughness by applying a structural integrity approach to pre-cracked Small Punch Test specimens

Abstract The Failure Assessment Diagram (FAD) is a procedure for evaluating the structural integrity of cracked components. The component’s failure conditions (load and crack size) are based on the material fracture properties (Kmat) considering its plastic behavior. In this paper, a new methodology that combines the FAD approach and the load–displacement curve obtained from pre-cracked Small Punch Test (SPT) specimens is presented, in order to estimate the fracture toughness of 15.5PH stainless steel. This research is based on Finite Element Modeling (FEM) of the pre-cracked specimen to determine both the plastic collapse load and the stress intensity factors during the loading process. A set of interrupted tests on pre-cracked SPT specimens are also conducted in order to identify the initiation point of the crack propagation. This set of numerical and experimental values allows the toughness ratio (Kr) and load ratio (Lr), at the instant the cracked specimen fails, to be determined. The only unknown variable in this process is the value of the material toughness, Kmat. The parameters in question are then combined with the FAD of the material generated from the different options available on the ASME-API 579 procedure, thus yielding an estimate for the value Kmat. Finally, to evaluate the accuracy of this methodology, predicted toughness values are compared to those from normalized tests of the material being analyzed.

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.

Approximate J estimates for circumferential cracked pipes under primary and secondary stresses

This paper presents finite element solutions for elastic–plastic J for circumferentially cracked pipes under combined mechanical and thermal loads in terms of the V/ V o factor used within the failure assessment diagram approach. Systematic analyses suggest that the two major variables affecting V/ V o are the relative magnitude of the secondary stress and the primary load magnitude. It has been found that the variations with these parameters obtained from the FE results agree well with current R6 predictions for modest thermal loads and low primary loads. For larger thermal stresses, the R6 predictions are overly conservative and more accurate predictions are suggested.

Influence of Fatigue Loading on the Engineering Critical Assessment of Steel Catenary Risers in Sour Deepwater Oil and Gas Developments

Steel catenary risers (SCR) are used in deepwater oil and gas developments to transfer produced fluids from the seabed to surface facilities. SCRs can be subject to fatigue loading from a variety of sources including wave and tidal motion, vortex induced vibration (VIV) and operating loads. When the produced fluids are sour (ie contain water and H2S) higher fatigue crack growth rates (FCGR) are expected, and this can have a significant effect on defect tolerance. The aim of this paper is to provide guidance on the current best practice methods for performing engineering critical assessments (ECA) on internal surface breaking defects in SCRs operating in a sour environment and subject to VIV fatigue loads. Example ECA calculations are presented for circumferential girth weld flaws, based on the failure assessment diagram (FAD) approach within the framework of BS 7910 [1]. The influence of certain key input variables is demonstrated, including the FCGR, determined from recent sour test data generated as part of this research.

Full-Scale Testing of Beam-to-RHS Column Connections with Partial Joint Penetration Groove Welded Joints and Assessment of Safety from Brittle Fracture

This paper concerns the applicability of partial joint preparation (PJP) groove welds to beam-to-column connections. Two full-sized beam-to-column connections, with rectangular hollow section columns and PJP groove welds at the ends of the beam flanges, were tested under cyclic loads. All the specimens were made with one-sided connections. When the unfused regions created by PJP groove welds were reinforced by fillet welds so that the welded joints have a sufficient cross-sectional area, the connections showed sufficient deformation capacity, although ductile cracks initiated at the weld toes and grew stably. Test results were well reproduced by non-linear FE analyses. Strains sustained at points around internal discontinuities were found to be low because of greater cross-sectional areas of welded joints compared with the cross-sectional area of the beam flanges. Fracture toughness properties of numerically modelled connections were evaluated by using a failure assessment diagram approach, which was modified by considering the effect of enhanced apparent toughness of material due to the loss of crack tip constraint. Both the test results and the fracture mechanics-based assessment demonstrated that it is unlikely to initiate brittle fracture at these discontinuities when materials used in the connections have sufficient fracture toughness.