Integrity Assessment Of Components Research Articles

Application of ANN concepts for prediction of crack growth and remaining life of circumferentially cracked piping components under different loading scenarios

This paper presents the details of Artificial Neural Network (ANN) based models to predict crack depth and remaining life of circumferentially part-through cracked piping components used in the nuclear industry. Crack growth data from experimental studies on (i) straight pipes made up of SA 312 Type 304 LN stainless steel having part-through circumferential notch under combined bending and torsion and (ii) dissimilar metal pipe weld joints having circumferential through-wall crack in the weld were used. The dissimilar metal pipe weld joint is made up of SA312 Type 304 LN austenitic stainless steel and SA508 Gr. 3 Cl. 1 low alloy carbon steel and joined by Nickel-rich Inconel alloy weld. About 75 % of the mixed experimental data has been used for development of model and the remaining data for validation. For the development of ANN model, (i) back propagation technique (ii) sigmoidal transfer functions and (iii) Levenberg-Marquardt algorithm are used. The maximum percentage difference observed between the predicted and the experimental results for crack depth and remaining life was 19.59 % and 15.78 % respectively. The efficiency of the developed model was verified using several statistical parameters. The developed models are useful for the structural integrity assessment of piping components under different loading scenarios.

Fatigue assessment of structural components through the Effective Critical Plane factor

The integrity assessment of structural components under complex loading conditions relies on the evaluation of the fatigue damage typically arising from stress concentrations, such as geometric irregularities, notches, weld beads, grooves etc.. Various methodologies, including the Notch Stress Approach (NSA), the Theory of Critical Distances (TCD), the Strain Energy Density (SED), and the Critical Plane (CP) concept, have been pivotal in assessing fatigue strength for notched and welded components. Recent works combine some of the above mentioned methodologies, while other works propose to vary the embedded parameters accounting for the loading type or the fatigue lives, trying to improve the accuracy of the fatigue assessment process. This paper introduces a novel approach, the Effective Critical Plane (ECP), which is founded on the critical plane concept. The CP factor is, however, calculated starting from an averaged, over a small volume, stress–strain field. The size of the averaging volume is assumed to be a material parameter and is determined by a best fitting procedure over different experimental data sets. The novel approach is illustrated by means of the Fatemi-Socie and the Smith-Watson-Topper CP damage factors. Its potential application to other CP formulations is straightforward, as well. Literature experimental data for low carbon steel specimens possessing different notches and loading conditions are used to validate the method’s capability in accurately determining the fatigue life and to set the radius of the averaging volume for the given material and CP parameter. A spherical volume or circular area are used in case of fully 3D or 2D numerical models, respectively. Results are compared to those of some of already existing methods, namely SED, TCD and the Modified Wöhler Curve Method.

Prediction of stress corrosion crack growth rate of cold worked stainless steels in BWR conditions by the slip oxidation model

Stress corrosion cracking (SCC) is a critical issue for boiling water reactor (BWR) components, and their cold working is considered to be an accelerating factor for SCC crack growth rates (CGRs). However, no method has been proposed to quantitatively evaluate the SCC CGRs of cold worked stainless steels. In this study, the changes in material properties due to cold working were reflected in the input parameters of the Hashimoto-Koshiishi model, a mechanistic model of SCC growth based on the slip oxidation mechanism, to enable a quantitative evaluation of the SCC crack growth rate (CGR) of cold worked SSs. In addition, a simplified equation as a function of the yield strength of cold worked 316LSSs was developed for ease of practical use in the integrity assessment of components. The CGRs calculated by the model and/or the simplified equation were in good agreement with the experimentally obtained CGRs for cold worked SSs and the heat affected zone (HAZ) of Type 316L SSs.

Evaluation of the Multiaxial Fatigue Life of Electro-Mechanical Actuator for Aircraft Blade Pitch Control Based on Certification Standards

To achieve the commercialization of electric vertical takeoff and landing (eVTOL) aircrafts, which have recently garnered attention as the next-generation means of transportation, objective certification based on rigorous procedures is essential. With the advancement of structural analysis technology, aircraft airworthiness standards recommend a combination of testing and analytical methods to demonstrate structural integrity. In this study, we propose analytical techniques for demonstrating the structural integrity of components for eVTOL aircrafts in accordance with airworthiness standards. We evaluated the static structural integrity and fatigue safety of an electro-mechanical actuator. Multibody dynamics analysis was performed to calculate the loads for application in finite element analysis. Subsequently, static analysis and fatigue analysis based on finite element analysis were conducted to calculate the safety margin and fatigue life of all key components. Therefore, we have confirmed the feasibility of utilizing analytical methods for the structural integrity assessment of aircraft components. We propose the utilization of the technique introduced in this study as one of the approaches for demonstrating compliance with airworthiness standards for eVTOL aircrafts through the application of analytical methods.

Effect of a circular hole on the SIF of a plate with a center crack subjected to uniaxial loading

BACKGROUND: Structural integrity assessment of components containing cracks is of paramount importance when they are part of critical structures. In this context, the fracture behavior of components containing cracks is crucial. OBJECTIVE: To investigate the effect of the diameter, number and position of holes drilled in the vicinity of the centre crack in a finite plate on the mode I stress intensity factor (SIF). METHODS: A 2D model with central horizontal crack was created in ABAQUS® simulation software and validated with the analytical results after performing mesh convergence. Circular holes of different diameters and at different positions were then introduced in the vicinity of the crack. RESULTS: By increasing the diameter of the hole and its number, the SIF reduced significantly. An optimum amount of distance between the crack and the hole was found at which the SIF was minimum, beyond it increased and asymptotically approached that of the plate without holes. CONCLUSION: The diameter of the hole and the spcacing between the hole-centre and the crack axis has a major influence on the mode I SIF. By introducing multiple holes, the mode I SIF significantly reduced.

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.

Scaled Fracture Toughness Based on the Weibull Stress for the Ferritic Steel Used in Nuclear Power Plants

As an important material property in structural integrity assessment of nuclear power components, fracture toughness can be measured by single edge-notched bend (SENB) specimens or compact tension (CT) specimens. However, the tested values may be inhomogeneous with the crack size and specimen thickness. Some toughness scaling models (TSMs) were proposed to transfer the tested value to fracture toughness under small-scale yielding. Combined with the tested data by SENB specimens with different crack sizes and CT specimens with different thicknesses, the scaled fracture toughness is investigated on the global and local approaches to fracture. Using the scaled toughness values, the Weibull scale parameter K0 is estimated and compared for the ferritic steel DIN 22NiMoCr37 widely used in nuclear power plants. The results show that the estimated scale parameter K0 using the scaled toughness obtained by the global approach on the J-Q theory is reasonable in agreement with that of the local approach on the Weibull stress with a relative error of about 10%.

Acceptability assessment of casting defects under transient thermal loading

Weld repairs of castings during manufacture is a well-known and acceptable practice if conducted in accordance with approved standards and procedures. However, either original casting induced cracks or cracking of these weld repairs in high temperature and aging plant is not uncommon. Structural integrity assessments of such components must not only consider reduced material toughness, due to temper embrittlement, but also the stress intensities generated during transient thermal events such as start/stops and quenching incidents. In the first of two case studies the acceptability of weld repair defects in a high pressure turbine outer casing is presented. Metallurgical assessments concluded significant temper embrittlement had occurred which was taken into account in a finite element based structural integrity assessment that considered design operating conditions as well as a hypothetical quenching event. Low pressure turbine bypass valves are considered in the second case study. In this case the material temper embrittlement was found not to be as severe but the criticality of pre-warming to reduce transient thermal stress and by extension crack stress intensities during trips or shutdowns was clearly demonstrated. Remaining life assessments concluded the defects to be acceptable for operation to the next planned outage.

The influence of corrosion pit–pit spacing on the pit‐to‐crack transition and fatigue lifetime

AbstractThe influence of separation distance between pre‐induced corrosion pits on both crack initiation and fatigue lifetimes is studied in an air environment by testing specimens containing pit pairs in the low‐stress, high cycle fatigue regime. The results of these tests are compared with equivalent tests with samples containing similar sized single pit. Within the scope of pit sizes and separation distances investigated, when the separation distance is below a threshold, decreasing the separation distance between pits leads to earlier onset of crack initiation and decreased fatigue lifetimes; this threshold being dependent on the applied stress and relative pit sizes. When the separation distance is above this threshold, multiple pits tend to behave independently of each other. The implication of the results from the viewpoint of structural integrity assessment of components containing flaws such as corrosion pits in close proximity is discussed.

Non-destructive integrity assessment of aging steel components

This contribution addresses the integrity assessment of steel components in operation by introducing a simple non-destructive investigation technique. The proposed methodology is based on superficial (according to ASTM E18 Standards classification) hardness tests, complemented by the mapping of the geometry of the residual imprint. The diagnostic analysis can be performed in situ, potentially in a fully automated way. The informative content of the collected data is verified on the results of previous laboratory studies. The proposed material characterization procedure is applied to determine the evolution over the years of the mechanical properties of an exercised gas pipeline.

A predictive model unifying hydrogen enhanced plasticity and decohesion

The detrimental effect of hydrogen on metals which manifests itself as a transition from a ductile to a brittle failure mode is, for the first time, incorporated into a unified continuum-scale predictive framework. The complete Gurson model, designed to predict ductile failure by voiding, is extended to include failure by decohesion. Hydrogen enhanced plasticity is accounted for through acceleration of the voiding process while hydrogen induced decohesion is realized by a degradation of the decohesion threshold. The interplay between these two failure modes driven by hydrogen concentration are well captured. This model can predict a realistic level of embrittlement as well as the suppression of dimples in a hydrogen induced fracture surface. Being generic, versatile, and easy to implement, the model may serve as a basis for interpretation of laboratory experiments and enable the transferability of the laboratory results to the integrity assessment of engineering components in hydrogen environment.

Overview of effect of residual stress, elastic follow-up & stress redistribution using whole structure approach

Structural integrity assessments of components require accurate predication of failure of components. One of the major challenges while doing structural integrity assessment is to understand whether the presence of residual stresses in the component acts a major part in the failure of complete structure or not. Residual stresses may arise during manufacturing, fabrication or can also get induced during service and with addition of applied load promote failure at much below safe design loads. In general, residual stresses are considered as secondary stresses but in cases where they doesn’t self-equilibrate in the structure, leads to plastic collapse, they should be treated as primary stresses. Whether residual stresses contribute to failure or not depend upon the overall elastic follow-up of the structure which will affect the residual force distribution during the process of crack extension and plastic deformation and applied load the structure is subjected to. In this paper, an overview of importance of elastic follow-up in the structure’s integrity analysis and the benchmark models are presented to study the outcome of stress redistribution, elastic follow-up and residual stresses.

Quick Evaluation Method for Defect Exceeding the Allowable Flaw Size in Pressure Vessel of Nuclear Reactor for Power Plant and Space Heating

Nuclear power can be used for power generation, space heating, and other fields, producing a limited level of greenhouse gases and no atmospheric pollutants. However, the safety of nuclear reactors is always a public concern. The reactor pressure vessels (RPVs) play an important role in the safe operation of a nuclear power plant. When a defect is inspected in the RPV, complex analytical evaluation procedures, including fatigue analysis and fracture assessment, are necessary to ensure the structural integrity of the defective component. Based on the RSE-M, a quick evaluation approach for RPVs with defects exceeding acceptance standards is proposed in this work to reduce the computational complexity and analysis time. The flaw evaluation is simplified by adjusting the inspection period based on the analysis of fatigue crack growth. The new method was applied to the RPVs with embedded defects and underclad semi-elliptical defects, respectively. The proposed evaluation approach was verified by the case of a typical RPV cylinder containing an embedded crack, where all possible transients during the operation of nuclear power plants are considered. During the allowable residual life obtained of 5-years, failure would not occur in the defective component via the conventional method, which gives confidence to the availability of the new approach. Consequently, the proposed method can be a valid reference for the structural integrity assessment of nuclear reactor components with defects exceeding acceptance standards.

Limit loads of dissimilar metal welded joints for joining safe end to pipe-nozzle of nuclear pressure vessel

Limit load is an important parameter in structural integrity assessment of cracked components. In this work, the limit loads of an actual dissimilar metal welded joint (DMWJ) for joining safe end to pipe-nozzle of a reactor pressure vessel have been investigated based on extensive three-dimensional finite element analyses. The effects of a wide range of crack sizes and locations on limit loads have been analyzed. The results show that for shorter and shallower cracks, the limit loads are mainly determined by global plastic yielding of safe end material with the lowest yield strength, and the crack sizes have nearly no effect on them. For longer and deeper cracks, the limit loads are mainly determined by general yielding of the ligament area ahead of a crack. With increasing crack length or depth, the limit load decreases. With moving crack locations from the pipe-nozzle through buttering and weld metal to safe end, the limit load decreases. The normalized limit load solutions for the DMWJ are given.

Quickly analyze the limit load of thinning defect elbows with elastic modulus adjustment method

In the process of production and service of pressure pipeline, local thinning defects may occur due to dielectric corrosion, manufacturing defects and other reasons. These defects reduce the carrying capacity of the pipeline and lead to leakage and rupture of the pipeline. Therefore, the integrity assessment of components and structures is required. Traditionally, the inelastic finite element method (FEM) is highly dependent on the specification of adequate mesh density and an assurance of numerically stable solutions. However, in engineering design, independent validation methods, such as Elastic Modulus Adjustment Procedures (EMAP), provide fast and stable solutions at relatively low cost. On the basis of elastic finite element analysis (FEA), the upper and lower bounds of limit load can be obtained by specifying the variations of elastic modulus in EMAP method. In this paper, an elastic modulus adjustment parameter k, which can improve the convergence of EMAP, is used to achieve stable convergence in the application of elbow with double defects. The EMAP method is also proved to be more efficient than the inelastic finite element analysis (FEA). Furthermore, the effect of defect size and distribution on the limit load of the elbow is studied.

Incorporation of Obstacle Hardening into Local Approach to Cleavage Fracture to Predict Temperature Effects in the Ductile to Brittle Transition Regime.

Ductile-to-brittle-transition refers to observable change in fracture mode with decreasing temperature—from slow ductile crack growth to rapid cleavage. It is exhibited by body-centred cubic metals and presents a challenge for integrity assessment of structural components made of such metals. Local approaches to cleavage fracture, based on Weibull stress as a cleavage crack-driving force, have been shown to predict fracture toughness at very low temperatures. However, they are ineffective in the transition regime without the recalibration of Weibull stress parameters, which requires further testing and thus diminishes their predictive capability. We propose new Weibull stress formulation with thinning function based on obstacle hardening model, which modifies the number of cleavage-initiating features with temperature. Our model is implemented as a post-processor of finite element analysis results. It is applied to analyses of standard compact tension specimens of typical reactor pressure vessel steel, for which deformation and fracture toughness properties in the transition regime are available. It is shown that the new Weibull stress is independent of temperature, and of Weibull shape parameter, within the experimental error. It accurately predicts the fracture toughness at any temperature in the transition regime without relying upon empirical fits for the first time.

Verification of Probabilistic Fracture Mechanics Analysis Code for Reactor Pressure Vessel

Abstract Nowadays, it has been recognized that probabilistic fracture mechanics (PFM) is a promising methodology in structural integrity assessments of aged pressure boundary components of nuclear power plants, because it can rationally represent the influencing parameters in their inherent probabilistic distributions without over conservativeness. A PFM analysis code PFM analysis of structural components in aging light water reactor (PASCAL) has been developed by the Japan Atomic Energy Agency to evaluate the through-wall cracking frequencies of domestic reactor pressure vessels (RPVs) considering neutron irradiation embrittlement and pressurized thermal shock (PTS) transients. In addition, efforts have been made to strengthen the applicability of PASCAL to structural integrity assessments of domestic RPVs against nonductile fracture. A series of activities has been performed to verify the applicability of PASCAL. As a part of the verification activities, a working group was established with seven organizations from industry, universities, and institutes voluntarily participating as members. Through one-year activities, the applicability of PASCAL for structural integrity assessments of domestic RPVs was confirmed with great confidence. This paper presents the details of the verification activities of the working group, including the verification plan, approaches, and results.

Structural integrity assessment for ITER lower VS coil feedthrough

The ITER vertical stabilization coil system is designed to provide fast plasma stabilization in all scenarios with elongated plasma cross-section. It consists of two water-cooled coils located in upper and lower portions of the vacuum vessel (VV), with feedthroughs responsible for the transfer of electricity and cooling water across the vacuum boundary of VV. Depending on location, the feedthroughs can be divided into upper port feedthroughs (UPR feedthroughs) and lower port feedthroughs (LWR feedthroughs), each family sharing the same design. Both designs are subject to demanding operation scheme, a severe working environment and the presence of interfacing components, which impose important thermal, electromagnetic and seismic loads on the components. In particular, the LWR feedthrough, object of present paper, is more impacted by seismic loads due to the more flexible structure compared to the UPR feedthrough. During the worst seismic event, it is subjected to a vertical acceleration of 28.8 m s−2, acting on an effective mass of 249.9 kg at its first order modal mode. Furthermore, the current circulating in the conductor of the LWR feedthrough, impose significant linear forces, up to 27.7 kN m−1 during normal operation and 30.3 kN m−1 during categories I & II abnormal plasma events, respectively. Finally, the effect of differential thermal expansions of interfacing structures imposes a relative displacement between the supports up to 32.9 mm in X (radial) direction and up to 14.67 mm in Z (vertical) direction, during baking. Being part of the vacuum boundary, the feedthrough must assure confinement function under all normal and accidental loading conditions, with proper structural integrity assessment by applying analysis approach and design criteria foreseen by the applicable nuclear codes. The paper details the load types and categories, the analysis methodologies, the structural design criteria, and the assessment results in views of static stress and fatigue for the LWR feedthrough. This study could be a comprehensive reference for the structural integrity assessment of other similar components in fusion devices.

A review of probabilistic creep assessment reporting relating to volume 2/3 of the R5 procedure

Probabilistic assessments allow the uncertainties present in reactor design and operation to be understood and reflected in the quantification of reliability. Increased awareness of the limitations of deterministic structural integrity assessments have highlighted that the unquantified margins do not provide an adequate measure of component risk.The R5 procedure is a “UK nuclear power industry standard, frequently used in safety cases for structural integrity assessments of Advanced Gas-Cooled Reactor components operating in the creep range” [Assessment Procedure R5, EDF Energy Nuclear Generation Ltd.]. Within this procedure, Volume 2/3 covers creep-fatigue crack initiation in defect-free structures and a drafted appendix provides guidance on the application of probabilistic approaches.A review of probabilistic approaches using the R5 procedure has been conducted, including a wider review of probabilistic assessments to ascertain general good practice. The review identifies a lack of consistency in the reporting of probabilistic assessments, particularly for correlations and convergence of outputs. Providing a structured method for reporting probabilistic assessments would be valuable to ensure consistent application and enable reproducibility of results.

Stress intensity factor solutions for surface cracks with large aspect ratios in cylinders and plates

In recent years, a large number of surface cracks caused by stress corrosion cracking (SCC) have been reported in dissimilar metal welds of light water reactors. For some of these cracks, the depth (a) is greater than the half-length (l/2). Upon the detection of cracks, the integrity of cracked components should be assessed in accordance with the fitness-for-service (FFS) codes such as the ASME Boiler and Pressure Vessel Code Section XI or JSME code of Rules on Fitness-for-Service for Nuclear Power Plants. Current FFS codes provide SIF solutions of surface cracks with small aspect ratios (i.e. a/l ≤ 0.5) only. For the integrity assessment of components with surface cracks of large aspect ratios (i.e. a/l > 0.5), it is necessary to develop the SIF solutions for those cracks. This study calculates the SIF solutions of surface cracks with aspect ratios of 0.5 ≤ a/l ≤ 4 in both cylinders and plates by characterizing the cracks as rectangular shaped ones. Finite element simulations are performed to develop the database of SIF solutions for rectangular shaped surface cracks subjected to a 4th order polynomial stress distribution. Additionally, the universal weight function method (UWFM) in calculating the SIF solutions of rectangular shaped surface cracks with large aspect ratios is investigated. Example SIF calculations for rectangular shaped surface cracks subject to residual stress were conducted using the UWFM. The SIF solutions calculated by the UWFM are compared with those from the finite element simulations to show the effectiveness of the UWFM.