Circumferential Surface Cracks Research Articles

Dynamic failure assessment analysis of 15MnTi steel supporting structure with a circumferential surface crack under impact loads

Supporting structures, whose strength have great influence on the safe operation of the whole equipment, are widely used in various ship equipment. In this paper, dynamic failure assessment analysis is carried out to discuss the structural integrity of the cracked supporting structure under impact loads. Considering the dynamic response characteristics of structures, the dynamic fracture toughness determined by the loading rate of dynamic J-integrals is used to calculate the failure assessment point (FAP). Due to the widespread uncertainties in practical engineering, such as crack size and mechanical properties of materials, the failure assessment results will also have uncertainty. To analyze the structural integrity affected by uncertain variables, the polygonal convex set (PCS) model is introduced to the failure assessment diagram (FAD) to propose a non-probabilistic failure assessment method. The FAP is extended to a failure assessment region, and structural integrity can be analyzed by the non-probabilistic reliability degree. Compared with the traditional interval model, the non-probabilistic reliability degree calculated by PCS model is closer to the probabilistic reliability degree, which avoids premature judgement of structural failure. For the 15MnTi supporting structure with a circumferential semi-elliptical surface crack under impact loads, the critical depth obtained by non-probabilistic methods decrease by 10% compared with deterministic results, and the failure mechanism is summarized as plastic collapse resulting from the excessive net section stress. In the absence of statistical data on crack size or mechanical properties, the non-probabilistic failure assessment method can be used as an effective alternative to the probabilistic method.

Fracture mechanics analysis of multipurpose canister for spent nuclear fuels under horizontal/oblique drop accidents

In this paper, elastic-plastic fracture mechanics analysis is performed to determine the critical crack sizes of the multipurpose canister (MPC) manufactured using austenitic stainless steel under dynamic loading conditions that simulate drop accidents. Firstly, dynamic finite element (FE) analysis is performed using Abaqus v.2018 with the KORAD (Korea Radioactive Waste Agency)-21 model under two drop accident conditions. Through the FE analysis, critical locations and through-thickness stress distributions in the MPC are identified, where the maximum plastic strain occurs during impact loadings. Then, the evaluation using the failure assessment diagram (FAD) is performed by postulating an external surface crack at the critical location to determine the critical crack depth. It is found that, for the drop cases considered in this paper, the principal failure mechanism for the circumferential surface crack is found to be the plastic collapse due to dominant high bending axial stress in the thickness. For axial cracks, the plastic collapse is also the dominant failure mechanism due to high membrane hoop stress, followed by the ductile tearing analysis. When incorporating the strain rate effect on yield strength and fracture toughness, the critical crack depth increases from 10 to 20%.

Magnetic field and thermal environment effects on sound transmission loss of double-walled cracked piezoelectric cylindrical nanoshells

Based on a non-local strain gradient theory (NSGT), this article analytically studies the sound transmission loss (STL) performance of a special type of double-walled partially cracked piezoelectric nanoshell structures of cylindrical shape undergoing initial electric potential and longitudinal magnetic field. The structure, which includes a specific number of axial and circumferential surface cracks, is struck by a plane wave in a thermal setting. Such cracks have minimal size compared to the nano-shell structure as a whole, minimizing the effects of curvature for this problem. The first-order shear deformation theory (FSDT) is utilized to explain the displacement components, and Hamilton’s concept is applied to obtain the coupled vibroacoustic equations that are used to generate the governing equations of the system for the two crack types, giving us an idea of the resulting structural stiffness in the presence of cracks using coefficients of crack compliance that are derived from the line spring model (LSM). The mechanical and acoustic properties are then checked to see their effect on the system response followed by a series of validation case studies. This evaluation also allows one to optimize a similar structure by selecting appropriate external parameters such as electric voltage, magnetic field intensity, different crack types, nonlocal and strain gradient parameters, variations of temperature, and incident angle of sound waves.

A numerical study on the correlation of ductile crack extension in Ni-based clad pipe girth welds with circumferential surface cracks and clamped SE(T) specimens

This work describes a numerical investigation on the relationships between ductile crack extension in Ni-based clad pipe girth welds and an API X80 pipeline steel with circumferential surface cracks and SE(T) specimens under fixed grip conditions. A primary objective is to gain further insight and understanding of the effects of constraint on ductile tearing behavior in piping components with dissimilar girth welds and how well crack growth resistance curves measured using single edge notched tension fracture specimens correlate with ductile fracture behavior in circumferentially surface cracked pipes. The numerical description of CTOD−R curves employs a computational cell approach incorporating the Gurson–Tvergaard (GT) constitutive model to simulate ductile crack extension in the crack configurations analyzed. The numerical analyses consider predictions of ductile crack growth in 3-D models of circumferentially cracked pipes with varying crack depth over pipe thickness ratio, a/t, from measured J-resistance curves derived from fracture resistance testing of clamped SE(T) specimens. The analyses also focus on comparing predictions of ductile crack extension using two different definitions for the CTOD.

Closed-Form Stress Intensity Factor Solutions for Circumferential and Axial Surface Cracks With Large Aspect Ratios in Pipes

Abstract The ASME Boiler and Pressure Vessel Code Section XI prescribes the stress intensity factor solutions at the surface and deepest points for a semi-elliptical crack. The ASME Code Section XI, however, provides no solutions for a crack with a large aspect ratio, that is a crack in which the crack depth a is larger than the half-length c. The difficulty in treating the crack with a large aspect ratio relates to the position of the maximum stress intensity factor, which appears at neither the surface point nor the deepest point. In this paper, we investigate the influence of the stress intensity factor at the maximum point for circumferential and axial inside surface cracks with a large aspect ratio in a cylinder. First, we obtain the influence coefficients Gi for the stress intensity factor at the surface point, the deepest point, and the maximum point by finite element analysis, and developed a series of closed-form Gi solutions. Three geometrical factors are considered as parameters affecting the influence coefficients Gi: aspect ratio (a/ℓ = 0.5, 1.0, 2.0, and 4.0), crack depth ratio (a/t = 0.01, 0.1, 0.2, 0.2, 0.4, 0.6, and 0.8), and radius to thickness ratio (Ri/t = 2, 5, 10, 20, 40, and 80). Finally, we propose methods for evaluating the stress intensity factor for a crack with a large aspect ratio in a manner that characterizes the influence of the solutions at the maximum point.

Assessing the Secondary Stress in a Pipe System With Circumferential Surface Crack Using Finite Element Modeling

Abstract In flaw evaluation criteria, the design limits for the secondary stresses are frequently different than the primary stresses. The evaluation procedure for primary stresses is load-controlled based which is independent of pipe deformation. The evaluation procedures for secondary stresses are displacement controlled which are dependent on pipe deformation. Certain stress components such as thermal expansion, thermal stripping, welding residual stress, misalignment/cold-springing, dynamic anchor motion are historically considered secondary stresses and their design limits are based on elastic stress analysis. In reality, there can be much more rotation/displacement of the pipe with nonlinear fracture behavior due to nonlinear material behavior and plasticity at the crack plane providing extra margins on the elastically calculated rotation values that come from uncracked-pipe design analyses. In assessing secondary stress margin, a secondary stress reduction factor is defined as the ratio of elastic-plastic moment to elastic moment. This is equivalent to another concept using a plastic reduction factor (PRF) as well as the inverse of structural (or safety) factor (SF) in ASME Section XI flaw evaluation criteria for various service levels. In this work, the secondary stress reduction factor was determined for a representative pipe system with multiple crack sizes, crack locations, and loading conditions. Nonlinear finite element (FE) analyses of a whole uncracked-pipe system were performed using abaqus® under various loading combinations to determine the critical locations for cracks in the pipe system. Next, FE analyses of the cracked-pipe system were carried out using cracked-pipe element—a methodology developed by the authors. Cracked-pipe system analyses were conducted for two loading conditions—one producing contained plasticity or single-hinge system and the other producing larger plasticity in the pipe system. Several analyses were conducted for each loading condition with a combination of two crack sizes at two key locations. Secondary stress reduction factors were then calculated for both loading conditions in the pipe system. Finally, the margin in secondary stress was assessed for the pipe system by comparing the secondary stress reduction factors with that for straight pipe sections (determined for experimental bend tests) as well as with the recommended equivalent PRF and the equivalent ASME secondary stress correction factors.

VIV Fracture Investigation into 3D Marine Riser with a Circumferential Outside Surface Crack

Vortex-induced vibration (VIV) is one of the most common dynamic mechanisms that cause damage to marine risers. Hamilton’s variational principle is used to establish a vortex-induced vibration (VIV) model of a flexible riser in which the wake oscillator model is used to simulate cross-flow (CF) and inline flow (IL) vortex-induced forces and their coupling, taking into account the effect of the top tension and internal flow in the riser. The VIV model is solved by combining the Newmark-β and Runge–Kutta methods and verified with experimental data from the literature. Combining Option 1 and Option 2 failure assessment diagrams (FADs) in the BS7910 standard, a fracture failure assessment model for a marine riser with circumferential semielliptical outside surface cracks is established. Using the VIV model and FAD failure assessment chart, the effects of riser length, inside/outside flows, and top tension on the VIV response and safety assessment of marine risers with outside surface cracks are investigated. It is shown that increasing the top tension can inhibit the lateral displacement amplitude and bending stress in a riser, but excessive top tension can increase the axial stress in the riser, which counteracts the decrease in the bending stress, so that the effect of top tension on crack safety is not significant. The increasing outside flow velocity significantly increases the lateral vibration amplitude and bending stress in the riser and reduces the crack safety. When other parameters remain unchanged, increasing riser length has no significant effect on the vibration amplitude of the lower part of the riser.

Further development of the gamma exponent model for assessment of flaws in oil and gas pipelines

The Gamma Exponent Model (“GEM”) is a recently developed methodology for failure pressure prediction of flaws in oil and gas pipelines. The model has previously demonstrated applicability for the assessment of axial surface breaking (or “part wall”) cracks and localized metal loss/corrosion. This work extends the applicability of the model to circumferential surface cracks. In addition, the GEM is shown to be consistent and complementary to the log-secant model for assessment of axial through wall cracks, developed by the NG18 Committee of the Battelle Institute in the early 1970s. The ongoing development of the GEM raises some important theoretical considerations related to understanding of the stress state at the flaw tip during the failure process. The current industry models are not explicit regarding the plane stress or plane strain conditions at the flaw tip, though this has a significant effect on model development. Analysts often assume plane stress conditions, due to oil and gas pipelines typically being considered “thin wall” pipe. However, this work confirms the theory that surface cracks are subject to plane strain conditions, and the selection of fracture toughness correlations from impact energy data becomes very important. The model is validated against available laboratory, hydrotest and in-service failure data. Statistical analyses demonstrate the mathematical form of the new model is valid. Failure pressure predictions are shown to be improved over current industry models. This can have a significant effect on the reliability pipeline integrity programs and improved efficiency in the form of cost savings.

Flaw Evaluation Procedure for Cast Austenitic Stainless-Steel Materials Using a Newly Developed Statistical Thermal Aging Model

Abstract Thermal embrittlement of some cast austenitic stainless steels (CASS) occurs at reactor operating temperatures leading to very low fracture toughness. Because of their low aged toughness with high variability, flaw evaluations of CASS material need to be established with an understanding of the materials aged condition, especially since most U.S. pressurized water reactor (PWR) nuclear plants have been given plant life extensions for 60-year operation. A flaw evaluation procedure for CASS materials is presented here using a new statistical model developed to predict the toughness of fully aged CASS using the materials' chemical compositions. In this procedure, the dimensionless-plastic-zone-parameter (DPZP) analysis is used to determine when limit-load is applicable and also approximate the elastic-plastic correction factor (Z-factor) to predict the failure stress for CASS pipe/fittings with a circumferential surface crack. The procedure was validated against several CF8m pipe test results which include various pipe diameters, crack sizes, ferrite contents, failure modes. The as-developed flaw evaluation procedure was also used to determine the Z-factors for four different pipe diameters for a database of 274 pipe/elbows in U.S. PWR plants –solving 1096 sample problems to understand what range of Z-factors in U.S. PWR plants (for CF8m CASS materials). Finally, the applicability of the CF8m-based statistical model for use with CF3 and CF8 CASS materials was also verified with available test results. The outcome of this work has been implemented in an ASME boiler and pressure vessel (BPV) code case and has been approved in 2020 as Code Case N-906.

Appropriate number of inspections for pipes with circumferential surface cracks

Appropriate number of inspections for pipes with circumferential surface cracks

J-integral Estimation by Reference Plastic Slope Method

It is difficult to obtain J-integral values (J-values) without performing numerical analyses for materials of which stress-strain relation is given by a poly-linear curve. The current author has proposed the reference plastic slope (RPS) method, which can estimate J-values for arbitrary stress-strain curves. The RPS method derives the J-values for bi-linear stress-strain curves whose plastic slope is determined using the reference stress and original stress-strain curve. This study aimed at showing applicability of the RPS method to straight pipes with a circumferential surface crack subjected to a tensile or bending load. Four poly-linear stress-strain curves were assumed for validation. It was shown that well-approximated J-values were obtained by assuming bi-linear stress-strain curves. Second, it was demonstrated that the J-values for bi-linear stress-strain curves could be estimated by the equations proposed in author’s previous study.

Effect of Cracks on the Natural Frequency of Cylindrical Shell Structures

Shell structures are liable to different kinds of defects and damage like cracking and corrosion which may destroy their structural safety and affect the service life. The cracks' effects are significant considerations in the design of cylindrical shell structures as they influence the vibration characteristics and safety. This present work is an experimental study on the free vibration analysis of a cylindrical shell involving circumferential surface crack. The influence of the ratio of shell’s radius to a shell’s thickness (R/h)of the shell structure, crack length in the shell, crack depth in the shell, crack location of the shell, and crack orientation in the shell are investigated under a clamped - clamped and simply supported boundary conditions at each end in the shell. Results showed that the minimum impact of the crack is at the angle of crack 75, and the circumferential fissure has more effect than a longitudinal fissure, In addition to this, under SS-SS, C-C the natural frequency will decrease if the fissure is located in the middle of the shell is greater than other locations. but when crack animated across in the ends of the limits the decrease in the natural frequency under C-C only. Results were compared with the literature there was a close agreement.

Finite Element Analysis of Fatigue in Offshore Pipelines with Internal and External Circumferential Cracks

Fatigue lifetime of offshore pipelines with semi-elliptical circumferential surface cracks is often underestimated. An accurate prediction of the pipeline structural integrity is nevertheless important in order to prevent unnecessary and expensive downtime, failures leading to leakage or spillage of pipeline contents to the surrounding environment, and ultimately improve the reliability of the pipeline. The estimation of crack growth in pipelines under varying loads is highly dependent on the calculation of crack driving parameters, such as the stress intensity factor and the crack tip opening displacement (CTOD) using the 3D J-integral or its equivalent. This paper presents a numerical study to predict the fatigue lifetime of cracks in pipes, determining the J-integral that includes first and second derivatives of the displacement field for pipes containing a range of circumferential surface cracks. A pipe segment is structurally loaded and stress intensity factors (SIF) evaluated using the finite element method (FEM). Based on the results, a number-of-cycles to failure curve shows a longer lifetime than previously predicted by about 5% for a pipe with semi-elliptical external surface cracks. In addition, they indicate that the external short cracks are more dangerous than the internal long surface crack hereby requiring earlier assessment.

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

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

Reference stress based J estimation formula for cracked cylinders with a wide range of radius-to thickness ratios: Part II- circumferential surface cracks

In this paper, reference stress based J estimation equations are presented for circumferential constant-depth surface and through-wall cracked cylinders with the radius to thickness ratio ranging from three up to seventy. The equations are developed based on extensive finite element (FE) analysis results. Firstly it is shown that application of existing J estimation equations is limited to thick-walled cylinder problems and can give non-conservative J estimates for a cylinder with a large radius-thickness ratio. To accommodate the effects of the crack depth, length and the radius-to-thickness ratio, correction factors for the reference stress are proposed based on FE results. By comparing with FE results, it shows that the proposed solutions can be applied to both an external and internal crack; and to combined axial tension and global bending.

Creep fracture parameter C* solutions for circumferential surface cracks in pressurized cylinders

The creep fracture mechanics parameter C* is an important input parameter in creep life assessment of cracked components, and it should be accurately calculated. In the author's previous work, the C* solutions for axial surface cracks in pressurized cylinders have been investigated and given. In present work, the C* solutions for circumferential surface cracks have further been investigated and developed. Based on extensive finite element analyses, the creep influence function Hc values for calculating C* have been calculated, tabulated and fitted into equations for circumferential internal and external surface cracks in pressurized cylinders. The equations for estimating C* for materials with different creep exponent n are also investigated and given. The accuracy of all equations has been verified by finite element analyses. The effects of the crack sizes, orientation (axial and circumferential), location (internal and external) and cylinder geometries on the C* distributions along the crack fronts have been analyzed.

Analysis of the constraint levels and the creep crack initiation times for pressurized pipes with long surface cracks

Abstract The constraint levels and the creep crack initiation (CCI) times for pressurized pipes with four types of long surface cracks and different geometrical sizes were investigated by using 3D finite element (FE) method. The C*–Q* two-parameter approach was also applied to estimate the CCI times for different conditions. For each type of long surface cracks, it could be found that the larger crack depth (a/t) was associated with the larger constraint and shorter the CCI time. Furthermore, for the same crack depth (a/t) and pressure, the order of constraint levels was axial internal crack > circumferential internal crack > axial external crack > circumferential external crack, while the CCI times were in contrary sequence. Generally, long external surface cracks were less dangerous than long internal surface cracks no matter they were axial or circumferential surface cracks. Additionally, the fitting formulae between CCI times and crack depth ratio a/t and pressure (or stress intensity factor) were developed and verified to be effective. The power law relation between the CCI times and constraint parameter Q* was also established and validated. Finally, the suitability of the transient prediction models in C*–Q* two-parameter approach was verified by comparing results of the two-parameter theoretical analysis and simulation. What's more, it was known that for long internal surface cracks (axial or circumferential), the K-RR solutions were more accurate than the HRR-RR solutions when pressure is below 10 MPa, while the HRR-RR solutions would be better for higher pressure. As to the long external surface cracks, the critical pressure between these two solutions was about 12 MPa.

A reversed η approach to estimate load-deformation curves for fracture specimens and surface-cracked pipes

This paper presents a reversed η approach to estimate the load-deformation curve for fracture specimens subjected to monotonic and reversed loadings based on the measured J-R curve and extends the same procedure to surface-cracked pipes under monotonic actions. The proposed method predicates on the original η approach to uncouple the load-deformation data from the potential energy at different crack sizes, through a load separation method. This approach estimates closely the load-deformation curve for SE(B), C(T) and clamped SE(T) specimens under monotonic loadings. Besides, the proposed procedure predicts closely the envelope of the load-deformation curve for C(T) and SE(T) specimens under low-cycle actions. For pipes with a circumferential surface crack, the reversed η approach also leads to a reasonably accurate estimation of the load-deformation response. The proposed method provides a basic approach to estimate the fracture dependent load-deformation curve for large scale structural components, which allows upscaling of the material fracture resistance into the load-deformation characteristics required in the integrity assessment of large-scale engineering structures.

Fracture assessments with elastic follow-up for combined primary and secondary loadings: Part 2: Numerical validation

This paper presents comparisons of finite element (FE) results with the J estimation equations proposed in Part 1 for treating fracture under combined primary and secondary loadings. The FE analyses are performed for a pipe with a circumferential internal surface crack subjected to axial tension combined with thermal loading. The crack depth and length, the relative magnitude of axial tension and thermal loadings, loading sequence and material properties are systematically varied. Based on the FE results, the applicability of the proposed equations in Part 1 is discussed and new estimation equations obtained by modifying the existing equations are proposed and validated.

An investigation on the circumferential surface crack growth in steel pipes subjected to fatigue bending

In the present paper, we propose an analytical method to calculate the Stress Intensity Factor (SIF) of circumferential surface cracks in steel pipes subjected to bending. In light of pipe geometry and bending load case, the analytical formula is raised by introducing new bending correction factors and new geometry correction factors on the basis of the Newman-Raju’s method. The bending correction factors are deduced based on the bending stress gradient, while the geometry correction factors are determined by parametric studies for internal surface cracks and external surface cracks respectively. Owing to a large data set requirement by the parametric studies, three-dimensional finite element (FE) models of evaluating SIFs of circumferential surface cracks are developed. The FE method is validated to ensure that it could provide accurate SIF estimations. Analytical verification is conducted which shows that the SIF evaluated by the proposed analytical method match well with the results evaluated by the recommended analytical method. Then experimental investigations of external surface crack growth in offshore steel pipe subjected to fatigue bending are implemented to further validate the analytical method of predicting surface crack growth rate. The analytical results match well with the test results and the available experimental data from literature, indicating that the analytical method can be used for practical purposes and facilitate the crack growth evaluation and residual fatigue life prediction of cracked steel pipes.