Circumferential Crack Research Articles

Ultimate bearing capacity of plate on Winkler foundation subjected to a circular uniform load

The present study investigated the problem of a large slab on a Winkler-type foundation subjected to a circular load. The slab is modeled as a ductile Kirchhoff plate and obeys Rankine’s yield criterion with associative flow rule. A circular uniform load is applied onto the top of the plate. As the load increases, the radial cracks occur and spread out at the bottom of the slab. The slab’s load-carrying capacity is defined by the onset of a negative yield line, namely a circumferential crack at the upper side of the slab. Depending on whether the circumferential bending curvature of the plate reaches its elastic limit, the slab was divided into two regions: the inner rigid-plastic region and the outer elastic region. A closed-form solution of the governing equations for each region is found. The influence of the material and geometrical parameters, size of the loaded area, and temperature gradient on the plate’s load-carrying capacity is then investigated. The load-carrying capacities found in previous investigations are compared with the predictions of the present analysis. Based on the analytical results, a regression formula for calculating the plate’s ultimate bearing capacity resting on a Winkler foundation is proposed.

Finite-discrete element modelling of sea ice sheet fracture

A rate-independent, de-cohesive damage model for the fracture modelling of large, cellular, plate-like, quasi-brittle structures is proposed. A hybrid, three-dimensional finite-discrete element method to investigate sea ice sheet fracture is then introduced, followed by three applications. The uniaxial tensile fracture of an ice sheet of varying physical sizes is examined first. The effects of both the size of an ice sheet and the loading rate applied on the effective tensile strength are investigated. The vertical penetration fracture of an ice sheet loaded by a rigid, flat-ended, cylindrical indenter is examined next. The breakthrough loads and strengths of an ice sheet of varying physical sizes are computed, applicable scaling rules as regards to the vertical breakthrough strength searched for. To conclude, the breaking of an ice sheet containing a circular hole by a surfacing, rigid, truncated cone is studied (an axisymmetric contact problem). The loads on the cone are computed and then compared with loads that can be obtained analytically for a case in which a structure is stationary, a sheet moves, and the contact is unilateral. While computing the tensile and the breakthrough strengths, a set of self-similar sheet samples with an in-plane size range of 1:16 is examined. The samples are square; have a side length of either L=10, 20, 40, 80, or 160 m; and a thickness of either h=0.5, 1.0, or 1.5 m. With the sheets containing holes, only the largest samples (L=160 m) are investigated. The results indicate that i) both the tensile and the breakthrough strengths are strong functions of both L and h; ii) the tensile strength is a strong function of the applied loading rate; iii) the failure mode as regards to the vertical penetration fracture changes drastically as a function of L; iv) the model is able to demonstrate both radial and circumferential cracking; and that v) the proposed (in-direct) approach to compute ice loads on a conical offshore structure provides realistic results.

Fatigue damage assessment from cyclic spherical contact of borided and nitrided H13 steel

In this study, boride and nitride layers formed on H13 steel were assessed by cyclic spherical contacts. Boriding was conducted at 800 °C for 1 and 5 h, and nitriding at 580 °C for 1 h. The hardness and strain hardening exponents n of the layers were estimated by spherical indentations (5 µm radius). The standing contact fatigue (SCF) performance was investigated by cyclic spherical contacts (Al2O3, 3 mm diameter), first applying monotonic load tests from 300 to 1000 N to establish the critical loads considering the onset of certain cracks for each layer; later, between 102 and 105 cycles applying different subcritical loads. The results revealed strain hardening exponents of nB1h = 0.226, nB5h = 0.230 and nN1h = 0.241. In SCF, a critical load of 600 N in both layers was found. In cyclic tests, the thicker boride layer presented larger cracking and moderate spallations, attributed to its mechanical properties; the thinner layer exhibited a better SCF resistance. Regarding nitriding, well-defined circumferential cracks without spallations were observed.

Development of a biaxial grid-coil-type electromagnetic acoustic transducer

This study proposes a grid-coil-type transducer to achieve the simultaneous detection of axial and circumferential cracks in the pipeline, reduce the number of permanent magnets in dynamic detection, and improve crack detection efficiency. The proposed system can detect biaxial cracks in the pipeline. Based on Biot–Savart’s Law, a mathematical model is established between the magnetic induction intensity and the coil of the electromagnetic acoustic transducer to determine the influence on the conductor. Experiments verify the relationship between the guided wave propagation characteristics of the grid-coil-type electromagnetic acoustic transducer and the ability to detect biaxial cracks. Results show that the grid-coil-type electromagnetic acoustic transducer can generate the biaxial ultrasonic guided wave propagating along the axial and circumferential pipeline, in which cracks are thereby simultaneously detected. Thus, the grid-coil-type electromagnetic acoustic transducer increases the detection scope, reduces the number of permanent magnets in dynamic detection, and improves crack detection efficiency. This study provides a theoretical and experimental basis for the research and engineering application of biaxial crack detection in the pipeline.

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.

Mode-I stress intensity factors for cracked fin-shaped shell under bending

Determining the stress intensity factor (SIF) for cracked engineering structures under different loading conditions is one of the core issues of fracture analysis. A slender fin-shaped shell possesses both shell and beam characteristics, in the cross section of which some complex cracks, such as the circumferential and T-shaped cracks, may arise. There will be different singular stress fields next to crack tips for a cracked fin-shaped shell under bending. Based on the conservation law and elementary mechanics, a technique to determine SIFs is proposed in this article, which is simple and easy to understand. The SIFs calculated using the present method agree well with FEM results even if elementary mechanics is used.

Interaction of circumferential SH0 guided wave with circumferential cracks in pipelines

ABSTRACT This paper aims to provide a method for detecting the circumferential crack by analysing the two reflected SH0 guided wave from the front-edge and back-edge of the circumferential crack. We established a finite element model for the propagation of SH0 guided waves in the pipe, and the waves were generated by a periodic-permanent-magnet electromagnetic acoustic transducer (PPM EMAT). The influences of the geometric parameters of the circumferential crack on the two edge-reflected waves were investigated. Subsequently, the accuracy of crack’s circumferential extent estimation by the time-shift of the two edge-reflected waves was provided. Finally, the simulated results were verified by experiments. The results show that the peak of the crack’s front-edge wave shows a linearly increasing trend with the increasing cross-section area of the crack in the circumferential direction. The peak of the crack’s back-edge wave shows a roughly periodical variation when the circumferential extent of the through-thickness crack is increased. When the crack’s circumferential extent is no less than 10 mm, the relative deviation between the estimated circumferential extent and the actual extent is less than 5.1%. The relevant information of the two edge-reflected waves can provide an accurate and quantitative pipeline defect characterisations.

Diagnostic Damage Control of Shaft Trains in Turbine Units According to Torsional Vibration Parameters

An analysis of existing methods for determining the natural frequencies of torsional vibrations of the shaft trains in power turbine units is carried out in order to assess the potential for using changes in natural frequencies to diagnose shaft damage. A computational modeling of torsional vibrations of a shaft train having a circumferential crack allowed informative natural frequencies to be determined for the purposes of detecting damages and the locations of natural frequency measurements to be established to a satisfactory degree of accuracy. An assessment of changes in natural frequencies from depth and characteristic locations of a circumferential crack is carried out. An alternative method for detecting violations of the shaft train integrity, based on monitoring changes in the angles of static twisting, is also considered.

Exploring Strain Characteristics and Bearing Capacity of a Carbon Filament-Wound Composite Cylindrical Shell under Hydrostatic Pressure

In order to study the strain characteristics and bearing capacity of a filament-wound composite cylindrical shell and its different dome structures under hydrostatic pressure, experiments were carried out. Firstly, static tests were conducted to study the axial and circumferential strain of the composite cylindrical shell on its different positions. The bearing capacity of the ellipsoid dome was compared with that of the hemisphere dome. The blasting test and the nonlinear analysis of the strain were conducted. The relationship between the strain trend and the crack propagation path was studied, and the structural failure mode was explored. The study shows that as the hydrostatic pressure increases, the strain increases and that the strain amplitudes of measuring points gradually appear different and show varying degrees of nonlinearity. Along the circumferential direction of the circumferential crack, the axial strain amplitude gradually decreases by 20%. But the circumferential strain amplitude gradually increases by 94%. As the load of the composite cylindrical shell increases to a certain extent, its final failure mode is strength failure, but its instability is not obvious.

Numerical Analysis of the Effect of External Circumferential Cracks in Transition Thickness Zone of Pressurized Pipes Using XFEM – Elastic-Plastic Behavior

The elastic-plastic behavior of the material is considered to analyze the effect of an external circumferential crack on a pipe with thickness transition and double slopes. Using the extended finite element method (XFEM), the J-integral of 3D cracks were investigated and compared between straight pipes and pipes with thickness transition and different slopes. Considering internal pressure, this work highlighted the investigation of a 3D crack problem in a thickness transition pipe with a double slope, In the extended finite element method (XFEM), the level sets and the enrichment zone were defined. A crack is easily modeled by enrichment functions. The comparison between the values of the J-integral showed that the pipe containing thickness transition with double slopes is more sensitive to the considered cracks, more precisely, the parameters of the first thickness transition have more influence on the variation of J-integral than the parameters of the second thickness transition. The decreasing of the angle of the slopes and the increase of the ratio of the thicknesses is one effective method of reducing the J-integral.

Minor aortic injury may be at risk of progression from uncontrolled shear stress: An in-vitro model demonstrates aortic lesion expansion

Background Non-operative management is considered appropriate treatment for minor aortic injury, while blood pressure and anti-impulse therapy are routinely utilized to prevent higher grade aortic injury progression. However, a universal medical regimen for low grade intimal injuries has not been adopted and risks of low-grade injury progression not well described. The purpose of this study is to determine the fracture response of minimally damaged aortic tissue to the various applied forces. Our hypothesis is that internal circumferential shear within the aortic wall is a primary fracture mode. This knowledge may help guide clinical management to minimize risk of injury progression, including instituting standard medical regimens with anti-impulse therapy and β-blockade for such minor injuries. Methods Human ascending aortic tissue was obtained after aneurysm repair or heart transplant, stored at 4°C and tested within 48 hours. Minor injury was modeled with a small radial notch on the luminal aspect of aortic rings, circumferentially expanded under video acquisition and analyzed to determine lesion propagation. Results 15 rings were obtained from 8 aneurysmal and 4 healthy aortas. All specimens demonstrated circumferential crack propagation. Propagation was longer (8.02 ± 5.92 mm vs 2.70 ± 1.23 mm) and initiation of crack propagation earlier in aneurysmal tissue (1.54 ± 0.17 versus 1.90 ± 0.17 times initial diameter). Conclusions Dilation of minimally injured aortic rings is associated with lesion expansion and injury progression in all specimens including healthy and aneurysmal tissue. This propagation illustrates the mechanical response to increased levels of internal shear, compromising structural integrity and increasing risk of aortic rupture in all injured aortas. Shear forces are routinely generated through normal circumferential aortic expansion with each pulsation, the magnitude of these forces determined by pulse and blood pressure. This suggests minor aortic injuries are not trivial and strategies to reduce shear stress be implemented in all such patients without contraindications to β- blockers.

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.

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.

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.

Water leakage susceptible areas in loess multi-arch tunnel operation under the lateral recharge conditions

In this study, the water leakage susceptible areas in a loess multi-arch tunnel were examined, providing a basis for the prevention and control of water leakage disease during the operation periods of multi-arch tunnels in loess areas. Through the field investigation of the leakage in loess multi-arch tunnel, the leakage mainly focuses on three joints: construction joints, expansion joints, and settlement joints. The spatial distribution of the leaking part in the tunnel is summarized and abstractly into three types: annular cracks, longitudinal cracks and full longitudinal cracks + circumferential cracks. By setting up different number of annular and longitudinal cracks, a total of 20 combinations were designed. This paper sets three kinds of working conditions, including 40 and 60 m of reinforcement water head and 60 m of no reinforcement water head, considering groundwater lateral recharge conditions. Using the midas/GTS geo-technical analysis system software, 3D numerical simulations were conducted to determine the distributions of the total water head and water pressure in cracks for all the combinations of water leakage positions under different working conditions and to analyze the variations in the seepage field of the surrounding rock. Based on the areas where the total water head and water pressure in the cracks sharply changed, the water leakage susceptible areas were determined. The rationality and reliability of the method were verified by a comparison with the results of a field investigation.

Development and evaluation of sub-element testing of SiC/SiC ceramic matrix composites at elevated temperatures

SiC/SiC ceramic matrix composites (CMCs) are being developed for use in aero-engines to replace nickel superalloy components. Sub-element testing acts as the key stepping stone in bridging understanding derived from basic coupon testing and more complex component testing. This study presents the development of high temperature C-shape sub-element testing with the use of digital image correlation to study damage progression. The specimen is designed with a bias towards a mixed mode-stress state more similar to what a CMC component may see in service. Both monotonic and fatigue tests were completed on C specimens and compared with predicted behaviour from modelling. Test data from both test types suggested that specimens were failing once they reached a critical radial stress level. However evidence from fractography of specimens showed that in both monotonic and fatigue tests radial cracks (driven by hoop stresses) are initiating prior to circumferential cracks.

Crack growth analysis and remaining life prediction of dissimilar metal pipe weld joint with circumferential crack under cyclic loading

Fatigue crack growth model has been developed for dissimilar metal weld joints of a piping component under cyclic loading, where in the crack is located at the center of the weld in the circumferential direction. The fracture parameter, Stress Intensity Factor (SIF) has been computed by using principle of superposition as KH + KM. KH is evaluated by assuming that, the complete specimen is made of the material containing the notch location. In second stage, the stress field ahead of the crack tip, accounting for the strength mismatch, the applied load and geometry has been characterized to evaluate SIF (KM). For each incremental crack depth, stress field ahead of the crack tip has been quantified by using J-integral (elastic), mismatch ratio, plastic interaction factor and stress parallel to the crack surface. The associated constants for evaluation of KM have been computed by using the quantified stress field with respect to the distance from the crack tip. Net SIF (KH + KM) computed, has been used for the crack growth analysis and remaining life prediction by Paris crack growth model. To validate the model, SIF and remaining life has been predicted for a pipe made up of (i) SA312 Type 304LN austenitic stainless steel and SA508 Gr. 3 Cl. 1. Low alloy carbon steel (ii) welded SA312 Type 304LN austenitic stainless-steel pipe. From the studies, it is observed that the model could predict the remaining life of DMWJ piping components with a maximum difference of 15% compared to experimental observations.

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.

Failure bending stresses for pressurized pipes with circumferentially part-through cracks

Stress corrosion cracks detected in austenitic stainless pipes were circumferential cracks at inside of the pipes. Failure stresses for the cracked pipes were developed by net-section stress concept. Limit Load Criterion provided by the ASME Code based on the net-section stress concept can estimate fully plastic collapse stress, which is called as failure stress. Local Approach of Limit Load Criteria for estimating stress at crack penetration had developed for a part-through cracked pipe based on the Limit Load Criteria. Advanced Local Approach of Limit Load Criteria for estimating stress at crack penetration is newly developed based on the Local Approach in this paper. In comparing with experimental stresses of pipe crack penetrations, the Limit Load Criteria give un-conservative estimation, and the Local and the Advanced Local Approaches are close to the experimental stresses. In addition, the Advanced Local Approach always gives higher stresses than the Local Approach for internal cracks, and it is shown that the Advanced Local Approach is beneficial for thick wall pipes with large angle cracks.

Parametric characterization of underwater laser ablation vis-a-vis laser-assisted standard defect simulation in sintered UO2 pellets

We report here on the generation of simulated defects on sintered uranium dioxide fuel pellets by taking advantage of the underwater laser ablation procedure and their subsequent characterization. This work, we believe, can play a role towards the validation of the performance of fuel pellet inspection machines. A repetitive fiber laser, capable of delivering pulses of nanosecond duration, in conjunction with a galvo-scanner served as the machining tool in the experiment. The study of the dependence of mass ablation rate on laser fluence, water column height, repetition rate, and beam scanning speed formed the bulk of this work. A water column of height ∼3 mm above the pellet surface in combination with a laser fluence of lying within 6–7 J/cm2 yielded the maximum ablation rate. The generated defects were analyzed using optical and electron microscopy. Clean defects, e.g. longitudinal cracks, circumferential cracks, pits, and end caps of different sizes and depths were created on the pellet surface by varying the laser parameters, beam travel trajectory, and beam scanning parameters. The mechanical pressure arising out of shock wave generated as a result of the confinement of the laser-produced plasma and the collapse of cavitation bubbles together with the microfluid jet formed due to the implosion of the bubbles pushed the molten ablated products from the interaction zone thereby facilitating the generation of cleaned machined surfaces. An insight into the physical processes’ operative in the underwater laser machining process has been offered, albeit qualitatively.