Structural Integrity Assessment Procedure Research Articles

Plastic Limit Pressure and Stress Intensity Factor for Cracked Elbow Containing Axial Semi-Elliptical Part-Through Crack

The aim of this study is to provide a solution for the plastic limit pressure and stress intensity factor of the elbows containing a part-through axial semi-elliptical crack by considering various crack sizes. The supporting system and loading conditions of the pipeline are described. The critical part of the observed pipeline was isolated for analysis and subjected to various sizes of semi-elliptical cracks. By performing numerical analysis, results were obtained for crack dimension ratios of c/a, and depth/thickness ratios of a/t. The obtained results include plastic limit pressure and stress intensity factor. The results were analyzed with a symbolic regression algorithm, and closed-form solutions for the limit pressure and stress intensity factor were proposed. To validate pipeline integrity, the Structural Integrity Assessment Procedure (SINTAP) was applied, and the FAD (Failure Assessment Diagram) was generated for cracks below the FAD function. The failure pressure was calculated by determining the points where the loading paths intersect the FAD function.

Analysis of additively manufactured notched PLA plates using failure assessment diagrams

This paper provides a methodology for the estimation of the load-bearing capacity of additively manufactured (AM) PLA plates containing different types of notches (U-notches, V-notches and holes). The methodology is based on the use of Failure Assessment Diagrams (FADs), which are the main fracture-plastic collapse assessment tool provided by structural integrity assessment procedures, such as BS7910 and API 579‐1/ASME FFS‐1. When analyzing notch-type defects, the FAD methodology requires the application of a notch correction which, in this work, is based on the Theory of Critical Distances (TCD) and the Creager-Paris stress distribution ahead of the crack-tip. The results show that the FAD methodology can be efficaciously applied in this AM polymer, providing safe conservative estimations of critical loads in U-notched and V-notched plates, and accurate slightly unsafe estimations in plates with central hole. The cracking behavior in the different tested plates is a complex procedure generated by a combination of filament failures and debonding processes.

Evaluation on Nonirradiated Thermo-mechanical Response of WCCB TBM Following RCC-MRx

Since the submodules as the most important structural component in Water-Cooled Ceramic Breeder (WCCB) Test Blanket Module (TBM) contributes to tritium breeding and extracting, structural integrity assessment is necessary to keep normal operation and function of WCCB TBM-set. Recently, the nonirradiated material properties of F82H which is considered as the structural material of submodule has been summarized following the requirement of RCC-MRx, combined with EU directive, French order, and pressure vessel construction code (RCC-MRx), structural integrity assessment is investigated under the ITER relevant thermo-mechanical load in this study. Meanwhile, cooling water conditions of WCCB TBM-set with its operation scenario, relationship, and detailed values (temperature and pressure) are constituted. Based on the determined structural integrity assessment procedure from RCC-MRx and numerical analysis results, structural integrity of current design of submodule in WCCB TBM-set could be validated. According to the current achievements and established structural integrity assessment procedure, further evaluation on structural integrity combined with electromagnetic load shall be investigated.

Structural integrity assessment procedure of PCSG unit block using homogenization method

In this paper, a procedure for evaluating the structural integrity of the PCSG (Printed Circuit Steam Generator) unit block is presented with a simplified FE (finite element) analysis technique by applying the homogenization method. The homogenization method converts an inhomogeneous elastic body into a homogeneous elastic body with same mechanical behaviour. This method is effective when the inhomogeneous elastic body has repetitive microstructures, and thus the method was applied to the sheet assembly among the PCSG unit block components. From the method, the homogenized equivalent elastic constants of the sheet assembly were derived. The validity of the determined material properties was verified by comparing the mechanical behaviour with the reference model. Thermo-mechanical analysis was then performed to evaluate the structural integrity of the PCSG unit block, and it was found that the contact region between the steam header and the sheet assembly is a critical point where large bending stress occurs due to the temperature difference.

Correlation of Charpy-V notch and different Charpy-U notch impact specimen energies for structural metals

The most common mechanical test to evaluate the fracture resistance of a structural metal is the Charpy-V impact test. Nearly all structural integrity assessment procedures use it. The preferred notch geometry is the V-notch, but other geometries are or have been sometimes used. The U-notch has normally a root radius of 1 mm, compared to the V-notch which has a root radius of 0.25 mm. Even though the V-notch specimen is today the main geometry, many different specimen versions are still being used to some extent. More importantly, vintage data corresponding to something else than V-notch data may be the only information available for older structures, still in use. For such cases, it is imperative to have a correlation between different notch geometries. Here simple correlations for the upper shelf energy and the 35 J/cm2 transition temperatures are developed for four different U-notch geometries.

Sizing limitations of ultrasonic array images for non-sharp defects and their impact on structural integrity assessments

Existing structural integrity assessment procedures typically assume flaws to be infinitely sharp when they cannot be considered as local thinned areas. This assumption is often over-conservative, resulting in a pessimistic assessment of structural components and a significant underestimation of their margins of safety against fracture. One of the main challenges while adopting non-destructive evaluation techniques is distinguishing between sharp cracks (e.g., fatigue) and non-sharp defects and identifying the more severe ones. Towards this broader challenge, the present work aims to examine the sizing limitation and accuracy of ultrasonic array image-based techniques for non-sharp defects (surface breaking u-notches) and investigate how these measurements would affect the structural integrity assessment of components. Parametric numerical simulations and experimental measurements are performed to generate full-matrix capture datasets, which are then processed using the total focusing method to form an image. The image-based sizing approach is shown to perform efficiently for notch depths higher than the inspection wavelength (λL), i.e. as small as 0.2 mm, and semi-notch widths as small as 0.1 mm. The influence of ultrasonic measurements on structural integrity assessments is highlighted using different case studies in the context of non-sharp defects of fatigue and fracture strength estimates. For the cases under analysis, resolving non-sharp defects led up to 5× and 3× values of effective fracture toughness and fatigue strength, respectively. We have also seen that a 30 % uncertainty in semi-notch width sizing would result in a 30 % and 20 % error in fatigue strength and fracture toughness estimations, respectively.

Probabilistic risk assessment for the piping of a nuclear power plant: Uncertainty and sensitivity analysis by using SINTAP procedure

For the purpose of probabilistic fracture mechanics estimation in the field of reliability analysis of pressure vessels and piping, a probabilistic method by using SINTAP (Structural INTegrity Assessment Procedures for European Industry) is developed. Monte Carlo simulation (MCS) is applied to investigate the influence of both the uncertainties of random parameters and inspection strategies on the failure probability (FP). A sensitivity analysis with regard to the above-mentioned aspects is conducted to identify the most significant parameters and efficient countermeasures to reduce failure risk in specially-assumed cases. The results of this study point out that the thickness and the inner radius of the pipe have the strongest impact on the FP of the pipe, followed by the bending moment, weld residual stress (WRS), crack length, initial crack depth, yield strength (YS), ultimate tensile strength (UTS), flow strength, fracture toughness, inner pressure and axial force. Compared with the detection quality and the inspection start time, selecting an appropriate inspection interval is a more effective method to reduce the failure risk. All these results provide a reference for the design of piping systems and guide the selection of in-service inspection (ISI) strategies with suitable flaw-detection capabilities and inspection frequencies.

Structural integrity assessment of additively manufactured ABS, PLA and graphene reinforced PLA notched specimens combining Failure Assessment Diagrams and the Theory of Critical Distances

Failure Assessment Diagrams (FADs) are a widely used engineering tool for the analysis of structural components containing cracks, and are included in recognised structural integrity assessment procedures such as BS7910 and API 579 1/ASME FFS 1. Their consistency and reliability has been demonstrated over the years through numerous laboratory validation tests and industrial applications. Nevertheless, both their theoretical definition and their subsequent validation have been performed in metallic materials and, therefore, their use in other types of materials still requires theoretical support and experimental validation. Moreover, FADs have been initially defined for the analysis of crack-like defects, whereas there are many situations where the defects that are, or might be, responsible for structural failure are not necessarily cracks. This is the case of (non-sharp) defects with a finite radius on their tip, which here will be referred to as notches.Simultaneously, additive manufacturing (AM) is an emergent technology that allows practically any type of geometry to be fabricated through a relatively simple process. One of the main AM techniques is fused deposition modelling (FDM), which consists in the extrusion of heated feedstock plastic filaments through a nozzle tip. The resultant printed materials have rather particular properties that are very dependent on the printing parameters and on the final state of internal defects. Concerning AM polymers and polymer-matrix composites, their use as structural materials, beyond their main current use in prototyping, requires the development of specific structural integrity assessment procedures.This paper provides FAD analyses for three additively manufactured (FDM) materials containing U-shaped notches: ABS, PLA and graphene reinforced PLA. The results show that the FAD methodology may be applied for the estimation of fracture loads in these particular materials, as long as linear-elastic fracture toughness values are used.

Analysis of additively manufactured PLA containing notches using Failure Assessment Diagrams

This paper provides a methodology for the estimation of the load-bearing capacity of additively manufactured (AM) PLA specimens that may be applied to both cracked and notched conditions. The methodology is based on the use of Failure Assessment Diagrams (FADs), which are, in practice, the main fracture-plastic collapse assessment tool provided by structural integrity assessment procedures. When dealing with notch-type defects, the methodology requires, additionally, the application of a notch correction that it is based on the Theory of Critical Distances (TCD) and the Creager-Paris stress distribution ahead of the crack-tip. The results show that the FAD methodology (alone, in cracked conditions, or in combination with the TCD in notched conditions) can be successfully applied in this AM polymer.

Prediction of distortions and residual stresses in narrow gap weld joints prepared by hot wire GTAW and its validation with experiments

The present paper aims to predict distortions and residual stresses in thick plate weld joints of austenitic stainless steel material prepared by automatic hot wire gas tungsten arc welding (GTAW) process and narrow gap technique using finite element analysis (FEA). The prediction has been validated by carrying out experiments. The variables considered in welding of plate joints are narrow groove configuration, high deposition rate leading to lower heat input (hot wire GTAW), and stress relieving treatment. A suitable stress-relieving treatment has been proposed for relaxing residual stresses and preventing sensitization of the heat-affected zone (HAZ) of the ASS weld joint. Predicted and experimental results of temperature distribution, distortions, and residual stresses were found to be in good agreement. The applicability of prediction of residual stresses for higher weld thicknesses has also been verified. The nature of residual stresses across thickness has also been compared with that given in Structural Integrity Assessment Procedures for European Industry (SINTAP) and BS 7910 standards. Details of the above finding are discussed in this paper.

Sub-sized and miniature CVN specimen conversion methodology

Presently most structural integrity assessment procedures still allow the use of Charpy-V notch impact tests as a measure of fracture toughness. The use is generally made through some more or less reliable correlations between standard Charpy-V notch energy and fracture toughness or tearing resistance. A problem arises if the structure has such a geometry that standard size Charpy-V notch specimens cannot be used. Application standards contain some guides on how to convert sub-sized specimen data to correspond to full size specimens, but these are often inaccurate and limited in their application range. Procedures like ASTM A370, BS7910 and API 579 give some advice on the use of sub-sized Charpy-V specimens but none of them cover the whole Charpy-V transition curve, nor all possible specimen dimensions. Here, a new simple procedure is presented. It is shown that it is applicable over the whole transition curve, thus enabling a point-wise conversion of sub-sized and miniature Charpy-V data to correspond to full size specimens. The new procedure is applicable for steel strengths ranging from 200 MPa to 1400 MPa.

Studying the interaction of crack-like flaws using the MATLAB toolbox int_defects

int_defects is a free and open-source toolbox for MATLAB which uses the Abaqus finite element analysis suite to automate the analysis of interacting crack-like flaws in structures. It is designed to aid the development of flaw interaction criteria for structural integrity assessment procedures and to simplify sensitivity analysis of flaw proximity. It can also perform automatic 3D elastic, elastic-plastic or limit load analysis of elliptical embedded flaws and semi-elliptical surface flaws in plates and thick-walled pipes under a wide range of loading conditions. This technical note outlines the capabilities of int_defects and gives some examples of its use.

Structural Analysis and Evaluation Method for Nuclear Class 1 Piping Systems at Elevated Temperature

To solve the problems encountered in the design of the piping system at elevated temperature, the present paper demonstrates a methodology of structural analysis and safety evaluation procedures for high temperature piping. A bridge between piping analysis software and ASME-NH is established. Firstly, the detailed analytical solutions of stress components for piping components (straight pipe and bent pipe or elbow) under multiple loading combinations are proposed. Then, the analytical predictions of the stresses are compared with the solutions given by finite element analysis (FEA). The results show that the proposed analytical results agree with FEA results very well. And finally, the piping structural analysis and integrity assessment procedures are given by coupling the pipe line model and the newly proposed analytical solutions. And thus a method to implement the piping design criteria of ASME-NH-3600 for the analysis of nuclear class 1 piping components is provided.

Effects of residual stresses and localised strain-hardening on the fracture of ductile materials

The strain history of a ductile material can affect its apparent initiation fracture toughness as well as its resistance to subsequent crack growth. Using neutron diffraction or synchrotron X-ray diffraction in conjunction with digital image correlation, it is possible to map both the total strain around the tip of a propagating crack, and the elastic strain component. Separating the elastic and irreversible strain components allows us to experimentally investigate the effects that residual stresses and prior strain-hardening have on crack propagation. This article presents the results of two experiments using these techniques. In the first experiment, it was shown that residual stresses in a 7xxx-series aluminium alloy affect not only the initiation of fracture but also fracture propagation. During crack propagation, residual stress relaxation occurs due to extension of the crack and plastic deformation in the region ahead of the crack tip. This affects the material’s apparent crack growth resistance in a non-linear manner and hence changes its fracture stability. In a second experiment, it was shown that the plastic zone ahead of a crack in a ductile material (in this case a ductile ferritic steel) can be modified by localised strain-hardening prior to fracture. Localised strain-hardening using an indentation tool was observed to cause small increases in the specimens’ peak load capacity and its energy absorption prior to the onset of unstable tearing. Using synchrotron diffraction, it was shown that is because hardening a region ahead of the crack tip causes stress redistribution to this region and away from the crack tip itself during fracture loading. These findings can inform how ductile and semi-ductile fracture is treated in structural integrity assessment procedures, particularly in the presence of residual stress. They also suggest that new localised peening treatments to improve the fracture resistance of structures in certain areas may be possible.

Relocation of existing jacket platform in western offshore of india ‎

ONGC (Oil & Natural Gas Corporation Limited) started its hunt for “Black Gold” in the Indian western offshore region in 1976 and is ‎currently operating more than 285 fixed steel platforms in water depths ranging between 25-‎‎90m. Many of the existing wells on the platforms have matured over a period of time, resulting ‎in decline of productivity, even after employing usage of Improved Oil Recovery ‎‎(IOR)/Enhanced Oil Recovery (EOR) techniques. Considering the techno-commercial ‎constraints for offshore development, IEOT-ONGC has taken up a study for re-location of ‎existing non-producing offshore platform to a newer location. The structural adequacy of the ‎existing structures at new locations has been ascertained for ‎‎100 year return period extreme ‎storm conditions for 8 different directions and other design loads as per API RP ‎‎2A ‎‎(WSD) (Ref. 1), considering in-situ met-ocean and soil conditions in the new field. The structural integrity ‎assessment ‎procedure explores the need ‎for ‎strengthening/mitigation measures in case the inherent capacity of the structure to ‎withstand ‎the incoming loading is not sufficient at the new site of installation. For relocation of ‎existing jacket platforms to new locations at different water depths in ‎western ‎offshore of India, ‎a general philosophy for foundation design has been developed considering installation of ‎plinth platform for relocation to field where water depth is higher than existing one and ‎installation of insert piles and extra clamp-on skirt piles, if required, where water ‎depth at ‎relocation is similar to existing water depth for the platform.‎ In this paper, relocation ‎methodology of existing well head platforms for use at new locations in Western offshore of ‎India at similar or different water depths has been discussed ‎covering foundation philosophy, well plugging and abandonment (P&A), ‎decommissioning and refurbishment aspects.

A strain-based approach to study interaction between non-coplanar through-thickness edge notches

Structural integrity assessment procedures to assess the effect of interaction between multiple adjacent flaws normally consist of two stages. First, alignment rules categorize non-coplanar flaws as aligned or non-aligned. Second, combination rules classify aligned flaws as interacting or non-interacting. Although these criteria are applied to different failure modes like brittle fracture, elastic–plastic fracture and plastic collapse, most of them were developed based on linear elastic fracture mechanics for the sake of simplicity. However, there are very limited references available for the technical background of these criteria. This lack of justifying backgrounds becomes more critical when applying these procedures to any other failure modes other than brittle fracture. This article studies the interaction between non-coplanar edge notches in scenarios of large deformation. Hereto, strain patterns are studied through full-field deformation measurements utilizing both experimental and numerical tools. Digital image correlation is used to measure strain during experiments and to verify the finite element analyses. The results show that in addition to the crack driving force, which represents a local response of notches, the global strain distribution within the specimen in terms of strain patterns can be used to probe the interaction between non-coplanar flaws. The authors suggest a novel criterion based on the trajectory of maximum equivalent strain to distinguish between aligned and non-aligned flaws. This study is based on double-edge notched tension specimens and gives a fundamental insight into flaw interaction in failure modes other than brittle fracture.

Recent development in low-constraint fracture toughness testing for structural integrity assessment of pipelines

Fracture toughness measurement is an integral part of structural integrity assessment of pipelines. Traditionally, a single-edge-notched bend (SE(B)) specimen with a deep crack is recommended in many existing pipeline structural integrity assessment procedures. Such a test provides high constraint and therefore conservative fracture toughness results. However, for girth welds in service, defects are usually subjected to primarily tensile loading where the constraint is usually much lower than in the three-point bend case. Moreover, there is increasing use of strain-based design of pipelines that allows applied strains above yield. Low-constraint toughness tests represent more realistic loading conditions for girth weld defects, and the corresponding increased toughness can minimize unnecessary conservatism in assessments. In this review, we present recent developments in low-constraint fracture toughness testing, specifically using single-edge-notched tension specimens, SENT or SE(T). We focus our review on the test procedure development and automation, round-robin test results and some common concerns such as the effect of crack tip, crack size monitoring techniques, and testing at low temperatures. Examples are also given of the integration of fracture toughness data from SE(T) tests into structural integrity assessment.

Application of structural integrity assessment procedure on an axle pin of a wind turbine

AbstractThe axle pin is one of the most critical components of a wind turbine. Because 2 bearings are mounted on the axle pin, it is not subjected to torque and is designed only to carry bending stresses. The axle is usually made by a casting process, which can induce defects. After casting of the axle pin, radial surface cracks were detected outside by ultrasonic and magnetic particle inspection. The first goal of this paper is to establish defect tolerance in the design of the axle pin, ie, at what critical crack size can we expect failure of the axle pin? The second aim is assessing the remaining lifetime of the defected axle pin used in operation of a wind turbine. Both issues are important to prevent unexpected failure and establish a system for on‐line structural health monitoring. This paper presents a procedure for the structural integrity assessment of axle pins by considering their material properties and fatigue crack growth parameters and comparing them with their fatigue design curve.

Sub-sized CVN specimen conversion methodology

Presently most structural integrity assessment procedures still allow the use of Charpy-V notch impact tests as a measure of fracture toughness. The use is generally made through some more or less reliable correlations between standard Charpy-V notch energy and fracture toughness or tearing resistance. A problem arises if the structure has such a geometry that standard size Charpy-V notch specimens cannot be used. Application standards contain some guides on how to convert sub-sized specimen data to correspond to full size specimens, but these are often inaccurate and limited in their application range. Procedures like ASTM A370, BS7910 and API 579 give some advice on the use of sub-sized Charpy-V specimens but none of them cover the whole Charpy-V transition curve. Here, a new simple procedure, that is in line with BS7910, is presented. It is shown that it is applicable over the whole transition curve, thus enabling a point-wise conversion of sub-sized Charpy-V data to correspond to full size specimens. The new procedure is applicable for steel strengths ranging from 200 MPa to 1400 MPa.

A review on structural integrity assessment procedures

Purpose– The purpose of this paper is to compare different existing assessment procedures for their limitations and applicable areas.Design/methodology/approach– Procedures have been studied in-depth along with their criterion for applications.Findings– The study shows applicability of different procedures along with their limitations and future scope.Originality/value– The paper provides starting point for performing damage assessment based on relevant procedures.