Circumferential Through-wall Crack 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 304LN 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 304LN 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.

Development of an improved limit pressure solution for shape-distorted 90o pipe bends with through-wall circumferential cracks

PurposeThis paper presents the approximate limit pressure solution for shape-imperfect and through-wall circumferential cracked (TWCC) 90° pipe bends at the intrados region. Finite element (FE) limit analysis was used to estimate the limit pressure by considering the small geometrical change effects.Design/methodology/approachThree-dimensional (3D) geometric linear FE methodology was implemented to investigate the limit pressure of structurally deformed TWCC 90° pipe bends. The material considered in the analysis is elastic perfectly plastic (EPP). The limit pressure of TWCC shape-distorted pipe bends was predicted from the corresponding internal pressure when von-Mises stress was equal to or just exceeded the material’s yield strength for all the models. The theoretical solution which was published in the literature was used to evaluate the current FE approach.FindingsOvality Co and TWCC at the intrados region caused a considerable impact on pipe bends, while the thinning? Ct produced a negligible effect and hence was not included in the analysis. With the combined effect, the bend portion of pipe bend experiences substantial influence, and the TWCC effect consequently increases with 45o, 60o and 90o crack angles and decreases the limit pressure of pipe bends. An improved closed-form empirical limit pressure solution was proposed for TWCC shape-distorted pipe bends at the intrados region.Originality/valueIn the limit pressure analysis of 90° pipe bends, the implications of structural irregularities (ovality and thinning) and TWCC have not been examined and reported.

Limit load evaluation for healthy elbows and elbows with notch at intrados under in-plane bending moment

Elbows are critical parts of any piping system used in nuclear power plant industry. These elbows experience internal pressure and also undergoes in-plane bending due to external loading. In the present study, limit load for two types of elbows, viz., healthy elbow and elbow having circumferential through-wall notch at intrados, with and without internal pressure subjected to in-plane bending moment is determined. Limit load is dependent on the parameters, namely, thickness, mean radius, bent radius, internal pressure, yield stress, notch size and deformed diameter of the elbow. Based on the experimental and numerical studies, the expressions for limit load evaluation were proposed by various researchers. In the present study, the limit load evaluation expressions proposed by Chattopadhyay and Tomar (2006), Chang-Sik and Kim (2006) and Hong et al. (2010) are considered. The general approach followed is to arrive at a weakening factor for considering the effects of bend geometry, internal pressure and presence of notch. Limit load obtained by using the above expressions have been compared with the twice elastic slope (TES) method and experimental measured maximum load (EMML) values. For elbows without any defect, limit load of TES method are about one-third approximately compared to EMML values. But for elbows with defect, it is lesser than EMML values by 20%. Predicted limit load for healthy elbows with and without internal pressure using the different expressions, found to vary less than 10% compared to TES method. For the elbows with circumferential through-wall crack at intrados without internal pressure, limit load obtained using TES method is around 15% more compared to the values obtained using the expressions.

Development of an improved structural integrity assessment correlation for circumferential through-wall cracked 90° shape-distorted pipe bends subjected to in-plane opening bending moment

A detailed and systematic assessment of the plastic collapse load of 90° shape imperfect pipe bends subjected to in-plane opening bending was conducted by implementing three-dimensional non-linear finite element analyses. The material considered in the analysis is the elastic-perfectly plastic type with a large change geometry option. Shape distortions in the pipe's cross-section and through-wall circumferential crack at the intrados region were the defects analyzed in this study. Ovality [Formula: see text] and thinning [Formula: see text] were varied from 0% to 20% with a 5% incrementation, and crack angles of 45°, 60°, and 90° were considered at the intrados. The twice-elastic slope method effectively evaluates the plastic collapse moment as mandated by the ASME B&PV code NB-3213.25, section III. The reaction moment and angular rotation curves were generated for all pipe bend models. The outcomes of the analysis revealed that the plastic collapse moment load of pipe bends was affected by through-wall circumferential crack and [Formula: see text], while thinning produced negligible effect, which was subsequently excluded from the analysis. The combined effect of through-wall circumferential crack and [Formula: see text] produces a consequential influence on the pipe bend at 45° angle, which is severe at 60°, and even more critical at 90°. In the analysis, the plastic collapse moment for short bend radius was reduced, while that of thin-walled pipe bends increased with an increase in thickness and bend radius. The plastic collapse moment of the present analysis was validated using experimental data acquired from open literature. An enhanced structural correlation assessment was proposed for through-wall circumferential crack shape-distorted 90° pipe bends.

Role of welding residual stress in stainless steel piping with application of the leak-before-break technology

The predicted leakage rate of piping circumferential through-wall cracks (CTWC) under various loading levels is a critical factor for the application of leak-before-break (LBB) technology. In current engineering approaches, the effect of welding residual stress has not been carefully taken into account. In this paper, both numerical analyses and comparative verification are adopted to examine the influence of typical welding residual stress field on the crack opening displacement for austenitic stainless steel piping with representative geometric dimensions and in situ measured material performance curves. An in-depth investigation is carried out to reveal the effect of the residual stress on morphological parameters of the CTWC flow channel. In the case where the residual internal stress of welding has the greatest effect, the Henry-Fauske model is employed to analyze the flow medium passing through the CTWC and the corresponding leak rate for a typical nuclear power plant. The results indicate that the welding residual stress leads to a substantial change in the crack opening displacement and crack morphology parameters. Both the current GE/EPRI method and the NUREG/CR-6837 modification recommended by the American Electric Power Research Institute underestimate the effect of this phenomenon, resulting in a higher prediction of the medium leakage rate. A similar situation is most likely to occur for the cases of short cracks in thin-walled piping and long cracks in thick-walled piping. Additionally, the obtained results reveal that the welding residual stress causes the whole crack surface to open negatively, and the axial line of the long crack on the thick-walled piping is close to a conic curve under specific conditions.

Study on crack identification for pipe with through-wall crack by cuckoo optimization algorithm

In recent years, the aging of power plants and other infrastructure facilities has become an issue. Therefore, a health-monitoring technology has attracted attention in order to ensure the integrity of structures. An inverse analysis is conducted to estimate the size and position of cracks in this technology. The accuracy of the inverse analysis is dependent on the form of the objective function and the optimization calculation method. Generally, optimization calculation methods using the gradient of the objective function is adopted. However, it is known to have the drawback that the search solution strongly depends on the initial value setting. The biomimetics-based cuckoo optimization algorithm (COA) is a method that can overcome this drawback. However, there are few examples of estimating the crack size and position in structures using the COA. In this study, an inverse analysis method is developed to estimate the crack angle and position of circumferential through-wall crack in pipes. In this study, transfer matrix method (TMM) was utilized to numerically calculate the deformation and rotation angle of the pipe, which are required in the inverse analysis. For this purpose, the fundamental theory of the TMM was extended to the element with a through-wall crack. Consequently, it was confirmed that the crack angle, position and magnitude of the applied shear load can be accurately estimated by the developed method.

The Huge Missing Factor in Leak-Before-Break Analysis: How a Circumferential Through-Wall-Crack in a Pipe System Changes the Flexibility and Reduces the Applied Moments

Abstract In typical leak-before-break (LBB) analyses in the nuclear industry, the uncracked piping normal operating forces and moments are applied in a cracked-pipe analytical procedure to determine normal leakage, and the combined forces and moments under normal operating condition and safe shutdown earthquake seismic loading are used in a fracture analysis to predict margins on “failure.” The International Piping Integrity Research Program (IPIRG) performed in 1990–1998 provided some insights to typical LBB behaviors where pipe system tests were conducted with simulated seismic loadings. The test results showed a large margin on LBB, which was also recognized in 2011 when the Argentinian Atucha II plant was analyzed using a robust full FE model. It was found that when circumferential through-wall cracks were put in the highest stressed locations, the applied moment dropped for both normal operating and N + safe shutdown earthquake (SSE) loading as the crack length increased. The through-wall crack size for causing a double-ended guillotine break (DEGB) was greater than 90% of the circumference. Similar results were also found for a petrochemical pipe system where thermal expansion stresses are much higher than the primary stresses. Even with very low toughness materials, the critical crack size leading to DEGB was greater than 80% of the circumference. The implication of this work is that pragmatically there is much higher margin for DEGB failure in nuclear plant operation, and efforts would be better focused on the potential for a small-break loss-of-coolant accident (SB-LOCA).

Failure behavior of SA508 Gr.1a LAS and SA312 TP316 SS pipes with a circumferential through-wall crack under large amplitude cyclic loads

This paper presents cyclic fracture test results using small-scale through-wall cracked pipe specimens to investigate the effects of large-amplitude cyclic loads on failure behaviour for different pipe materials. SA508 Gr.1a low-alloy steel (LAS) and SA312 TP316 stainless steel (SS) pipes were used in the experiment. The tests were conducted under quasi-static monotonic, displacement-controlled, and load-controlled cyclic loadings at room temperature (RT). Regardless of the pipe material, displacement-controlled reversed cyclic loading significantly reduced deformation-ability and fracture resistance, promoting crack growth. These effects were more pronounced in the SA312 TP316 SS than in the SA508 Gr.1a LAS specimen. The material-dependence on fracture behaviour was considerable under monotonic load, but was near-negligible under displacement-controlled reversed cyclic loading. Under load-controlled cyclic loading, the number of cycles to failure, the maximum allowable crack extension, and deformation-ability of pipe specimens decreased as the amplitude and compressive level increased. For all load-controlled cyclic loads, the SA312 TP316 SS specimen was less resistant to failure than the SA508 Gr.1a LAS specimen. Regardless of the loading type, a circumferential through-wall crack grew out of the initial crack plane for the SA508 Gr.1a LAS specimen, but propagated in a straight line from the initial crack plane for the SA312 TP316 SS specimen.

The role of a plain dent on the failure mode of a crude oil pipeline

A failure of a subsea crude oil API 5L X52 welded steel pipeline with an 18-inch outside diameter has caused oil leakage and this event was reported after 20 years in service. Based on the post-failure investigation, it was found that there was a circumferential through-wall crack in the center of the plain dent at the leak point of the pipeline. This study investigates the role performed by the dent in the failure mode of the pipeline. Macroscopic and microscopic observation revealed that the internal surface of the pipeline contains a local erosion around the fracture zone while the outer surface contains a lot of surface cracks. The results of the investigation showed that crack was started on the outer surface at the bottom of the severely deformed (concave) zone. The concavity of the deformed pipe outer surface could result in sufficient stress cycles and residual stresses, the condition necessary for the initiation of a corrosion fatigue crack. Through thickness propagation of fatigue crack continued toward the pipe inner surface until operating stresses exceeded the yield strength of the material. As the cracks progressed, the stresses exceeded the strength of the remaining intact pipe, and a failure occurred.

Application of cohesive zone model to large scale circumferential through-wall and 360° surface cracked pipes under static and dynamic loadings

This paper presents ductile fracture simulation of full-scale cracked pipe for nuclear piping materials using the cohesive zone model (CZM). The main objective of this study is to investigate the applicability of CZM to predict ductile fracture of cracked pipes with various crack shapes and under quasi-static/dynamic loadings. The transferability of the traction-separation (T-S) curve from a small-scale specimen to a full-scale pipe is demonstrated by simulating small- and full-scale tests. T-S curves are calibrated by comparing experimental data of compact tension specimens with finite element analysis results. The calibrated T-S curves are utilized to predict the fracture behavior of cracked pipes. Three types of full-scale pipe tests are considered: pipe with circumferential through-wall crack under quasi-static/dynamic loadings, and with 360° internal surface crack under quasi-static loading. Computational results using the calibrated T-S curves show a good agreement with experimental data, demonstrating the transferability of the T-S curves from small-scale specimen.

Plastic Limit Pressure Solutions for Elbows With Slant Through-Wall Cracks

Abstract For leak-before-break (LBB) assessment, an idealized through-wall crack (TWC) is typically postulated to determine the critical crack length of cracked piping. However, such an idealization in terms of crack shape can lead to underestimations of plastic limit pressure. Although many studies have been performed to obtain accurate limit load solutions for cracked straight pipes by considering realistic crack geometries, there is still a lack of information regarding slant TWC at elbow. Therefore, three-dimensional finite element (FE) models of an elbow considering the effects of slant TWC on plastic limit pressure are developed. The proposed FE model and analysis procedure were verified through comparisons to the existing solutions for idealized TWCs in elbow. On this basis, the effect of slant TWC on the plastic limit pressure is analyzed, and a closed-form solution of the plastic limit pressure is proposed, for an elbow containing a longitudinal or a circumferential through-wall crack.

Fracture studies on carbon steel straight pipes having off-centred circumferential through-wall crack under finite compliance

The level-3 Leak-Before-Break (LBB) assessment of nuclear piping system is based on simplified fracture mechanics calculations. A postulated through-wall circumferential crack is shown to be stable even under extreme loading conditions. An important aspect of this fracture assessment, which is generally not considered, is the reduction in load at the cracked section that occurs due to the local and global compliance of the piping system. In this study, the effect of piping system compliance, crack size and crack location on the load carrying capacity of SA-333 Gr. 6 carbon steel pipes was analysed. Experimental investigations were performed on 8״ NB Schedule 100 pipes (Outer diameter 219 mm and wall thickness of approx. 12.6 mm) under quasi-static four point bending loads. Out of the six fracture tests, one specimen had a through-wall circumferential crack at the centre (along the length of pipe) and the remaining five tests had off-centred cracks. While the initial crack size showed a weak influence on the load carrying capacity of a stiff piping system, the effect of crack location was, however, quite significant. Calculations based on twice-elastic slope (TES) method and limit analysis of uncracked piping system (in some cases) for evaluation of load carrying capacity of pipes under finite compliance may lead to large safety margins.

Stress triaxiality based transferability of cohesive zone parameters

Present study is to investigate the dependency of cohesive zone parameters on crack tip stress triaxiality for SA333 Grade 6 steel. An exponential cohesive law is used to simulate ductile fracture behavior of 14 three-point bend specimens made of SA333 Gr. 6 steel. Cohesive parameters are determined by varying peak stress to match experimental results with the computed data. To extend the validity of parameter peak stress as a function of triaxiality, experimental results of six piping components with through-wall circumferential crack made up of SA333 Grade 6 steel are used. A confidence interval band is then plotted to study the transferability of cohesive parameters. The variation in peak stress for a single value of multiaxiality quotient is attributed to the micro structural variation in material during manufacturing near the crack tip, which is statistical in nature. A normal variation of peak stress is assumed for a given value of multiaxiality quotient. To test the accuracy of the cohesive parameters with such normal variation, experimental results of six piping components are again used. Significant match of computed results with the measured values shows transferability of the present material parameters. The methodology presented in this work can be applied to other structural steels to find out cohesive zone parameters. Such material parameters will be useful to the designers to carry out safety analysis of piping components of nuclear and other installations.

Crack Growth Evaluation for Cracked Stainless and Carbon Steel Pipes Under Large Seismic Cyclic Loading

Abstract Some Japanese nuclear power plants have experienced several large earthquakes beyond the design basis ground motion. In addition, cracks resulting from long-term operation have been detected in piping systems. Therefore, to assess the structure integrity and to evaluate the fragility of cracked pipes taking the occurrence of large earthquakes into account, it is very important to establish a crack growth evaluation method for cracked pipes that are subjected to large seismic cyclic response loading. In our previous study, we proposed an evaluation method for crack growth during large earthquakes through experimental study using small size specimens and investigation using finite element analyses. In the present study, to confirm applicability of the proposed method to pipe, crack growth tests were conducted on both stainless and carbon steel pipe specimens with a circumferential through-wall crack, considering large seismic cyclic response loading with complex waveforms. The predicted crack growth values are in good agreement with the experimental results and the applicability of the proposed method was confirmed.

Evaluation of fracture resistance of AISI type 316LN stainless steel base and welded pipes with circumferential through-wall crack

Fracture resistance of 316LN SS base and welded straight pipes with circumferential through-wall cracks has been evaluated. The pipe J-R curves have been determined using existing experimental methods and analytical expressions available in literature. The J-R curves of 88.9 mm OD pipe and pipe welds have been compared with the J-R curves obtained from C(T) specimen tests. The results indicate that the C(T) specimen data is not suitable for fracture assessment of pipes due to difference in constraint level and the fracture resistance of the base pipe is inferior to welded pipe. Further, the limit load analyses based on expressions available in literature reveals that the initiation and crack propagation are observed prior to the maximum bending moment.

Limit load solutions for elbows with circumferential through-wall crack under the pressure-induced bending restraint effect

A typical pipe finite element model is made by assuming there is internal pressure with a single crack located in the middle of the pipe where both ends are constraint-free. In reality, however, both ends are constrained by their connections to components such as the pressure vessel or pump. Furthermore, due to the complexity of the pipe system, it also experiences a self-constraint effect. This is called the pressure-induced bending restraint effect, and is known to affect the fracture mechanics parameters at the crack tip. Some studies exist on the pressure-induced bending restraint effect of straight pipes, but none have focused on elbow pipe and junction between straight pipe and elbow.This study suggests limit load solutions for elbow and their jointing regions with circumferential through-wall cracks under the pressure-induced bending restraint effect. The relevant equations can be used to estimate the maximum withstand load evaluation of a highly ductile pipe, and can also be used to estimate the elasto-plastic fracture mechanics parameters using the reference stress method.

New CMOD based η-factor equations considering R-O strain hardening for circumferential through-wall cracked pipe under bending moment

The fracture study of circumferential through-wall cracked (CTWC) pipe under bending load is of interest for integrity assessment of conventional piping system and especially important for Leak-Before-Break (LBB) analysis of primary heat transport piping system of nuclear power plants (NPP). The fracture study in the elastic-plastic regime needs to obtain material J-resistance curve where J-integral is usually calculated from test data using the η-factor methodology. The evaluation of J-R curve for CTWC pipe is also important for study of transferability of material fracture resistance data from small specimen to full scale components. Further, η-factor to evaluate J-integral may be based on either load-lone-displacement (LLD) or crack mouth opening displacement (CMOD). The conventionalη-factor equations for CTWC pipe are LLD based and derived based on the assumption that the material is elastic perfectly plastic. However, use of LLD based η-factor needs to subtract no-crack displacement which poses additional difficulty and real materials always show some form of strain hardening. Therefore, an effort has been made in this paper to obtain the CMOD based η-factor for CTWC pipe considering material strain hardening as per the Ramberg-Osgood (R-O) relation. In the present study, plastic η-factor (ηpl) has been evaluated by elastic-plastic finite element analyses (FEA) over a wide range of crack angles (2θ = 30°–150°) and R-O strain hardening exponent (n = 2–7) for three R/t ratios (5, 10 and 20). To capture the ovalization of pipe cross section during deformation, geometric non-linearity is also considered in FEA. Based on these FEA, new ηpl equations are proposed by 2-D surface curve fitting for different R/t ratios. For validation of FE model, a separate set of geometrically linear FEA was also carried out for comparison of FE J-integral with EPRI Handbook solutions. Further, semi-analytical η-factor derived from EPRI relations of J-integral and CMOD were also compared with the present (geometric-linear) finite element results. In both the cases, the matching was very good.

Investigating the Effectiveness of a Composite Patch on Repairing Pipes Subjected to Circumferential Cracks under Combined Loadings

The purpose of this study is to investigate bending moment and the axial load capacity of a pressurized pipe suffering from a through-wall circumferential crack repaired by a composite sleeve. The three-dimensional finite element method (FEM) was adopted to compute the results, and the failure assessment diagram (FAD) was employed to investigate the failure behavior of the repaired pipe. The findings revealed that, for the investigated range of applied loads and angles of the crack, the interaction of brittle and ductile failure modes is negligible. Additionally, the yield strength of the cracked pipe was considered as reference stress to achieve a conservative design. Two cases of the combined loading state consisting of internal pressure/bending moment and internal pressure/axial tensile force were investigated. Repairing the crack under combined loadings using carbon-epoxy composites was studied where the influences of various parameters, including internal pressure, crack angle, and the composite patch thickness on the capacity of the cracked pipe to withstand bending moment and axial load were included. The results indicated that the bending moment and axial load capacities of the cracked pipe depend on internal pressure, crack angle, and the composite patch thickness; nevertheless, the crack angle is the main parameter. A composite sleeve can increase both bending moment and axial load capacity of the cracked pipe, but bending moment can be increased further than axial load. Using the composite patch to repair the cracked pipe caused the bending moment capacity to improve from 14.28% to 120%. On the other hand, the composite patch raised the axial load capacity from 5.1% to 93.5%. Additionally, an increase in the composite patch thickness caused the axial load capacity to extend more than bending load capacity.

Failure Analyses of Propagation of Cracks in Repaired Pipe Under Internal Pressure

In this study, the analysis of the behavior of circumferential through-wall cracks in repaired elbow with bonded composite patch subjected to internal pressure is performed using three-dimensional finite element analyses. The effect of length cracks, the wall thickness (Rm/t), angle of the elbow (ψ) and properties of the patch is presented for calculating the stress intensity factors. The obtained results show that the bonded composite repair significantly reduces the stress intensity factors at the tip of repaired cracks. The Monte Carlo method is used to predict the distribution function of the mechanical response. According to the obtained results, we note that the crack length variations are important factors influencing the distribution function of KI. The uncertainty in this parameter has a significant effect on increasing the probability of failure of pipe and reducing the durability of structure.

Determination and verification of triaxiality dependent cohesive zone parameters of SA333 Grade 6 steel

Cohesive zone models are employed to simulate crack propagation in fracture process zone. It has been demonstrated in the present study that the cohesive zone parameters depend not only on the material, but also on stress triaxiality. To determine cohesive zone parameters, experimental results of 14 three-point bend specimens (TPBB) made of SA333 Grade 6 steel were analyzed using 3D finite element model using WARP3D. Cohesive parameters were determined by carrying out parametric studies of these specimens by varying peak stress ‘T’ to match experimental load-displacement data with the computed data. An exponential traction separation law was used for this purpose. In addition, elastic-plastic finite element analyses (FEA) are conducted to determine the multi-axiality quotient ‘q’ at the crack tip for TPBB specimens and pipe components. This helped to get a relation between peak stress ‘T’ and ‘q’. To extend the range of validity of parameter ‘T’ as a function of ‘q’, experimental results of six piping components with through-wall circumferential crack made of SA333 Grade 6 steel tested earlier were also used similarly. A confidence interval band between q and T is then plotted to study the transferability of cohesive parameters. The variation in ‘T’ for the same value of ‘q’ is attributed to the microstructural changes in the material near crack tip.To test the accuracy of the cohesive parameters determined above, experimental results of six piping components were again used. Using the ‘q’ parameters determined from elastic-plastic FEA on pipes, the upper and lower limits of peak stress ‘T’ from the confidence band determined above were found out. This range of ‘T’ was then used to carry out cohesive zone analyses (CZA) of these piping components to determine load-displacement curves. For each piping component, a set of load-displacement curves were determined by assuming a normal variation of parameter ‘T’ over the maximum and minimum limits. The simulation results were compared with the experimental results and it has been observed that experimental results lie reasonably well within the computed results.