Circumferential Crack Research Articles

Composite coating effect on stress intensity factors of aluminum pressure vessels with inner circumferential crack by X-FEM

In this paper, the fracture behavior of a pressure vessel involving a full circumferential wall crack at the inner surface is investigated by applying the axisymmetric model. By using the extended finite element method (X-FEM), the stress intensity factor is calculated and compared for the case of a crack in the Aluminum monolayer cylinder as well as when the composite coating is mounted on its outer surface. Also, the stress intensity factor is obtained for different crack lengths, materials, and fiber orientations in the composite laminate. The materials of composite laminate are Glass-epoxy and Carbon-epoxy. An axisymmetric extended finite element formulation is developed for the pressure vessel, and a MATLAB programming code is implemented to solve the problem. To validate the developed code, two examples are solved and compared with the available results of two-level finite element method (2LFEM) and exact solutions. The results obtained from the axisymmetric X-FEM model is in good agreement with the existing results. The results show that the most reduction of SIFs occurred in pressure vessels with the Carbon-epoxy coating with 0° fiber orientation. • An axisymmetric X-FEM model is developed for calculation of stress intensity factors (SIFs) in cracked pressure vessel. • An interaction integral is used for the computation of mixed-mode SIFs. • The effect of Glass-epoxy and Carbon-epoxy coating of pressure vessel on SIFs is investigated. • The effect of different fiber orientations of composite coating on SIFs is investigated. • The most reduction of SIFs occurred in pressure vessels with the Carbon-epoxy coating with hoop (0°) fiber orientation.

Evaluation of Crack resistance and Adhesive Energy of AlCrN and Ag doped a-C Films deposited on Chrome Nitrided 316 LVM Stainless Steel

ABSTRACT The present study characterises AlCrN and Ag doped a-C (a-C:Ag) biocompatible coatings for their nanomechanical, scratch, cracking behaviour, fracture toughness, and adhesion energy on chrome nitrided (CrN) medical grade 316 LVM stainless steel. The mechanical properties of the coatings are correlated with their structural make-up using XRD analysis. The results demonstrated superior behaviour of AlCrN compared to a-C:Ag coating in terms of high hardness and elastic modulus of the former. Owing to the inability of Ag to form carbide with carbon in a-C network contributed to discontinuity in the carbon structure resulting in compromised hardness and elastic modulus of the a-C:Ag coating. Further, a lower Hc 3/Ec 2 ratio, relatively higher plasticity, and higher compressive stress of the AlCrN coating facilitated absorption of indentation load by virtue of plastic deformation, thus demonstrating less severe radial cracks and more pile-up induced circumferential cracks. The a-C:Ag coating predominantly suffered severe radial cracks attributed to easy debonding of immiscible Ag-C network. Thanks to the superior surface quality of a-C:Ag coating, it registered higher L c1 in the microscratch test; nonetheless, owing to the improved compressive stress, mechanical properties, and fracture toughness, AlCrN coating registered higher critical stress of coating failure and adhesion energy.

Femtosecond Laser Trimming with Simultaneous Nanostructuring to Fine Piercing Punch to Electrical Amorphous Steel Sheets.

A CVD (Chemical Vapor Deposition) diamond coated tungsten carbide (WC) and cobalt (Co) sintered alloy punch was trimmed by the femtosecond laser machining to sharpen its edge with about 2 μm and to simultaneously make nanostructuring to its side surface. In addition to the sharpened edge, its edge profile was formed to be homogeneous enough to reduce the damage layer width by piercing the electrical amorphous steel sheet stack. Each brittle sheet in the stacked work was damaged to have three kinds of defects by piercing; e.g., the droop-like cracking in the thickness and at the vicinity of hole, the wrinkling in peak-to-valley with partial cracking on the peaks, and the circumferential cracking. When using the WC (Co) punch with the inhomogeneous edge profile in the sharpened edge width, these three damages were induced into each sheet and the maximum damage width exceeded 80 μm. When using the punch with the sharpened edge and homogeneous edge profile, the wrinkling mode was saved and the total affected layer width was significantly reduced to less than 20 μm. Through the precise embossing experiments, this effect of punch edge profile condition to the induced damages was discussed with a statement on the nanostructuring effect on the reduction of damaged width in electrical amorphous steel sheets. The developed tool with the sharpened edge and homogenous edge condition contributes to the realization of a low iron loss motor with a reduced affected layer width.

Destruction of the hydraulic unit shaft: Why it is possible?

Hydraulic units produce around 20% from the total world energy and almost 75% from total renewable energy as well as play a vital role in providing energy system stability. The shaft is one of the key lifetime-determining elements for hydraulic units. Its durability, lifetime, fatigue and fracture resistance define reliability and safety of a hydraulic unit in general. Despite the presence of functioning continuous and periodic hydraulic unit technical condition control systems, there are several known cases of shaft destruction leading to their replacement, long downtimes and significant financial losses for energy industries. The goal of this research is determining main reasons for shaft destruction in hydraulic power plant operating conditions. Using computational modeling for a large vertical high-head hydraulic unit as an example shows that a growing crack doesn't significantly influence shaft dynamic properties. As a result, even a long through crack may escape detection by present vibration diagnostics systems. At the same time, non-destructive metal state control methods are also not always effective due to structural design features and large size of hydraulic unit shafts. To prevent serious incidents in the future there is a suggestion to improve vibration diagnostics data-processing algorithms, introducing an additional diagnostic sign – a ratio of registered double rotation frequency runout values to runouts at rotation frequency, which allows to identify a hydraulic unit with a growing shaft circumferential crack even with vibration state meeting compliance norms and requirements. Additionally, it is necessary to estimate residual HU shaft lifetime through calculation taking into account possible initial cracks not revealed previously by non-destructive control.

Ballistic impact response of resistance-spot-welded (RSW) double-layered plates for Q&P980 steel

Ballistic impact response of resistance-spot-welded (RSW) double-layered (2 × 1.6 mm) plates (190 mm × 150 mm) for Q&P980 steel impacted by a round-nosed steel bullet (12 mm diameter and 30 mm length) was investigated by using gas gun and high-speed camera system. The RSW specimens were spot welded using a 6 mm diameter electrode face producing a 7.2 mm diameter fusion zone of the spot weld. The ballistic curve and energy balance for the tests of the spot weld of the RSW specimens at different velocity were analyzed to characterize the ballistic behavior of the RSW specimens under bullet impact. The fracture mechanisms of the RSW specimens under bullet impact were presented. For the tests below the ballistic limit, the cracks initiated from the notch-tip and propagated along the faying surface or obliquely through the thickness depending on the impact velocity. For the tests above the ballistic limit, the plug fracture in the front plate of the RSW specimen could be caused by the thinning-induced necking in the BM near the HAZ, while the plug fracture in the rear plate of the RSW specimens may be consist of the circumferential cracking from the rear surface and the bending fracture of the hinged part of material. The effects of the electrode indentation and the weld interfaces on deformation and fracture of the RSW specimens under bullet impact were revealed. For the tests above the ballistic limit, the circumferential fracture from the rear surface of the RSW specimens was always initiated along the interior periphery of the electrode indentation and the crack paths were along the FZ/CGHAZ or CGHAZ/FGHAZ interface. When the circumferential crack also formed outside the electrode indentation, the fracture on the BM/HAZ interface could be found. On the front plate of the RSW specimens, the shear/bending induced cracking from the notch-tip were observed and the crack paths were along the FZ/CGHAZ or CGHAZ/FGHAZ interface.

Study of the effects of stress wave behavior in laser-induced exit-surface damage of fused silica using time-resolved multi-polariscopic imaging

During laser-induced exit-surface damage of fused silica components, strain and stress are generated and accompanied by cracks in the substrate, further deteriorating the component. The authors studied the behaviors of surface and shear waves quantitatively using a time-resolved multipolariscopic imaging system, showing their potential to affect the development of circumferential and longitudinal cracks. When the material received a cumulative shot, a greater stress intensity was observed, and the stress wave had a greater impact on the laser-induced damage. The results indicate that the two stress waves from the cumulated shots dominated the developmental characteristics of the damage morphology.

Detection of circumferential cracks in heat exchanger tubes using pulsed eddy current testing

Heat exchanger tubes in industrial pressure vessels typically require periodic inspection, and detection of circumferential cracks is one of the foremost issues. In this study, the pulsed eddy current method was used for the detection, and new types of probes were designed. Transverse probes were designed to render the eddy currents perpendicular to circumferential cracks to improve detection sensitivity, and the corresponding numerical simulations showed that the eddy currents were approximately perpendicular to the circumferential cracks along more than half of the tube circumference. Thus, two probes could cover the entire circumference for more economical inspection to be achieved. The experiments were performed on copper and steel tubes. The results show that when compared to longitudinal probes, the designed transverse probes significantly improved the detection sensitivity towards circumferential cracks. Particularly, the effectiveness of the proposed method is shown regarding detection of deep defects with good sensitivity in steel tubes without saturation magnetization.

Fracture Mechanical Analysis of Thin-Walled Cylindrical Shells with Cracks

The fracture mechanical behaviour of thin-walled structures with cracks is highly significant for structural strength design, safety and reliability analysis, and defect evaluation. In this study, the effects of various factors on the fracture parameters, crack initiation angles and plastic zones of thin-walled cylindrical shells with cracks are investigated. First, based on the J-integral and displacement extrapolation methods, the stress intensity factors of thin-walled cylindrical shells with circumferential cracks and compound cracks are studied using linear elastic fracture mechanics, respectively. Second, based on the theory of maximum circumferential tensile stress of compound cracks, the number of singular elements at a crack tip is varied to determine the node of the element corresponding to the maximum circumferential tensile stress, and the initiation angle for a compound crack is predicted. Third, based on the J-integral theory, the size of the plastic zone and J-integral of a thin-walled cylindrical shell with a circumferential crack are analysed, using elastic-plastic fracture mechanics. The results show that the stress in front of a crack tip does not increase after reaching the yield strength and enters the stage of plastic development, and the predicted initiation angle of an oblique crack mainly depends on its original inclination angle. The conclusions have theoretical and engineering significance for the selection of the fracture criteria and determination of the failure modes of thin-walled structures with cracks.

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.

Experimental investigation on ice-breaking performance of a novel plasma striker

Aircraft icing has long been a plague to aviation for its serious threat to flight safety. Recently, researches about a newly proposed deicing method based on Plasma Synthetic Jet Actuator (PSJA) have just started. To meet the requirements of in-flight deicing, structure of PSJA needs to be adjusted. This paper completed the detailed design and experimental validation of a novel plasma striker, which was a modified version of PSJA. Influences of mass of the moving part and rod shapes on the ice-breaking performances were also studied. Besides, a “conical-nosed rod configuration” was proposed. Its purpose was to ensure a good ice-breaking performance of the plasma striker on long ice, by generating a splitting failure. Results show that, though mass of the moving part was just several grams, ice-breaking performance was better when the mass was lighter. The rectangular rod could generate an elliptical circumferential crack, whose major axis was parallel to the direction of the long side of the rectangular rod. And the “conical-nosed rod” concept was verified to be able to generate a splitting crack which can spread completely to the far end of long ice, and the crack direction was parallel to edge line of the cone. In general, the plasma striker has the advantages of simple structure, low energy consumption, little harm to the flow field and the aircraft skin. Simulations will be carried out in future works to study in detail the working process of the plasma striker.

J-integral estimation by reference plastic slope method for poly-linear stress-strain curves

Abstract It is difficult to obtain J-integral values (J-values) for materials using various types of constitutive equations without performing numerical analyses. In this study, the reference plastic slope (RPS) method was applied to obtain J-values for engineering materials, for which the deformation property was expressed by a poly-linear stress-strain curve. The RPS method derives the J-value using bi-linear stress-strain curves obtained from the original stress-strain curve. First, the basic concept and characteristics of the RPS method is explained. It is shown that well-approximated J-values were obtained by assuming bi-linear stress-strain curves whose plastic slope were determined using the reference stress for representative materials (three carbon steels and two stainless steels). Second, it is demonstrated that the J-values for bi-linear stress-strain curves could be estimated by relatively simple equations for a center cracked plate subjected to a tensile load. Third, to apply the RPS method for practical problems, the equations are developed to estimate the J-values for bi-linear stress-strain curves for a straight pipe with a circumferential crack subjected to a tensile or bending load. Finally, it is confirmed that the developed equations could predict the failure load of which error was within 10% at the most unconservative case.

Evaluation of Cracks on the Welding of Austenitic Stainless Steel Using Experimental and Numerical Techniques

This paper deals with investigation and characterization of weld circumferential thin cracks in austenitic stainless steel (AISI 304) pipe with eddy current nondestructive testing technique (EC-NDT). During welding process, the heat source applied to the AISI 304 was not uniform, accompanied by a change of the physical property. To take into consideration this change, the relative magnetic permeability was considered as a gradiently changed variable in the weld and the heat affected zone (HAZ), which was generated by the Monte Carlo Method based on pseudo random number generation (PRNG). Numerical simulations were performed by means of MATLAB software using 2D finite element method to solve the problem. To verify, results from the modeling works were conducted and contrasted with findings from experimental ones. Indeed, the results of comparison agreed well. In addition, they show that considering this changing of this magnetic property allows distinguishing the thin cracks in the weld area.

Hydrogen assisted cracking driven by cathodic protection operated at near −1200 mV CSE – an onshore natural gas pipeline failure

A 22-inch onshore natural gas transmission pipeline in operation since 1960 experienced an in-service rupture in January of 2018. The rupture was caused by an axial crack located 150 km (93 miles) downstream of the closest compression station. While the root cause analysis of the rupture was being carried out, excavations of indications reported by a Hard Spot In-Line Inspection approximately 42 km (26 miles) downstream of the rupture found leaks from predominantly circumferential crack colonies mixed with spiderweb ones not associated with hard spots. The conclusion from this investigation is that the cracking mechanism for the in-service rupture and leaks is mainly hydrogen environment assisted cracking (HAC), with the hydrogen being generated by the cathodic protection. The root cause for the failures is the combination of an aged and defective coating and the cathodic protection (CP) system operated for decades at near the pipe-to-soil potential limit of −1200 mV CSE. An extensive literature search indicates that this investigation is the first fully documented case of an onshore transmission pipeline failure due to HAC driven by CP, with the cracks occurring in the pipe body of X52 steel, away from welds and not related with hard spots. The paper summarizes the approach used for the root cause analysis and the findings, where the implications in the management of the pipeline integrity of the pipeline system are discussed. The pipeline operator implemented a series of preventive and corrective measures, which included pressure testing, cathodic protection improvement, coating rehabilitation programs and In-Line Inspection programs.

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. • Failure behaviour of pipes with a circumferential through-wall crack was experimentally investigated under large amplitude cyclic loadings. • The material-dependence of failure resistance under large amplitude cyclic loading were different from that under monotonic loading. • The failure resistance of a cracked pipe under load-controlled cyclic loading were more dependent on the material strength than the fracture toughness. • The amplitude and ratio of load-controlled cyclic loading affected the allowable crack extension and crack opening as well as crack growth rate. • The present results will contribute to the development of crack assessment procedure for excessive seismic loadings.

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

• Plain dent increases fatigue stress concentration factor, and reduce fatigue life. • Plain dent causes a turbulent flow of crude oil inside the pipeline. • High corrosive agents with oil promote localized attack. • Pipeline failure could be attributed mainly to environmentally fatigue cracking. 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.

Constraint-based failure assessment for pipelines with coplanar circumferential cracks under biaxial loading

The presence of coplanar or multiple cracks emerges as a common problem in many engineering structures, e.g., at the toe of circumferential welds of pipelines subjected to cyclic actions and in other piping systems for the nuclear power plants. The failure assessment diagram provides a convenient means to examine the competing failure between the fracture failure and plastic collapse failure, often for structural components with a single crack. The existing failure assessment diagram relies on the leading term characterizing the near-tip stress solutions, and ignores the geometry or plasticity-induced constraint variations. This paper aims to integrate the crack interaction effect between two coplanar cracks, measured by a constraint-based crack interaction factor, into the option 3 failure assessment curve in the engineering standard. This study examines three types of circumferential coplanar cracks in a pipe: two coplanar embedded cracks; two coplanar surface cracks; and a surface crack interacting with an embedded crack. Using a modified J solution based on a proposed crack interaction factor, this investigation quantifies the plasticity-driven increase in the near-tip opening stress for coplanar cracks. The stress field estimated based on this modified J solution agrees closely with the numerically computed stress field near the coplanar crack tips at large deformation levels. This work subsequently integrates the modified J solution into the failure assessment diagram. The failure assessment diagram based on the equivalent crack size recommended in BS7910 appears to be un-conservative for a number of coplanar cracks considered in this study.

On radial, circumferential, and spiral cracks in fractured glass plates

AbstractThis is a fractographic case study of a mechanically loaded glass plate that formed a remarkable spiral crack when the plate fractured. Common radial and circumferential cracking in disks and plates in general are first reviewed. The exceptional spiral crack is then analyzed. The origin of failure was an unusual flaw on the nominal compression side of the plate. The complex spiral fracture pattern has been observed in other large plate glass fractures that break under blunt impact conditions. Spiral cracks can form and propagate from flaws on the front side of a flexed plate that is rigidly supported at its rim or, alternatively, is supported from underneath and has a large overhang.

An Insight into the Excavation-Induced Stress Paths on Mechanical Response of Weak Interlayer Zone in Underground Cavern Under High Geostress

A weak interlayer zone (WIZ) is a poor zonal geotechnical system with loose structure, weak mechanical properties, variable thickness, random distribution, and strong extension, that occurs between different rock strata (e.g., tuff and basalt), due to the intense tectonic movement, representing a potential threat to the overall stability of rock masses with WIZs in large underground cavern excavations. Focusing on the excavation-induced hazards in the weak interlayer zone (WIZ) occurring in underground cavern under high geostress, the mechanical response of WIZ under different loading and unloading stress paths has been well investigated and unearthed, by a series of automatic stress path controlled triaxial tests, as well as through scanning electron microscope (SEM) analysis. Results show that the mechanical characteristics of WIZ are closely related to the initial confining pressure and stress paths. Sudden increases in the circumferential strain and strong dilations of WIZ occur from the start of unloading, among which the unloading total strains are most strongly promoted by stress path II (i.e., axial pressure loading and confining pressure unloading), whereas the unloading stress path IV (i.e., axial pressure and confining pressure unloading) has the greatest promotion on enhancing the plastic volumetric dilatation. The deformation modulus and the Poisson’s ratio follow a deterioration law under the unloading process of confining pressure, and it is the unloading stress path II that most influences the damage of elastic parameters. The ultimate bearing strength, the internal friction angle, and the cohesion of WIZ appear extremely obvious degradation in course of unloading, among which the damage effect of stress path IV is the most remarkable. The macroscopic and mesoscopic analyses reveal that the failure mechanism of WIZ under complicated unloading stress paths lays in the accumulation and propagation of axial and circumferential cracks and fractures, as well as pervasive particle breakage, with the intergranular fractures, transcrystalline fractures, and the shear scratches on a meso level. The research could provide an effective basis and good lessons for the mechanical response and failure mechanism of WIZ under unloading stress paths in underground cavern under high geostress.

Internal Pressure Creep Rupture Test and Finite Element Analysis of a Tube with a Circumferential Crack on the Outer Surface

Internal Pressure Creep Rupture Test and Finite Element Analysis of a Tube with a Circumferential Crack on the Outer Surface

Analysis of Crack Stress Intensity Factor of Coke Drum under Thermal Stress by Weight Function Method

The coke drum undergoes severe cyclic temperature changes during service, and it is prone to ring cracks. Based on linear elastic fracture mechanics, this paper uses the weight function method to solve the stress intensity factor of the coke drum with circumferential cracks in a two-dimensional transient temperature field. Based on the edge crack stress intensity factor of the finite width slab, the weight function of the circumferential crack stress intensity factor of the cylindrical shell is deduced according to the self-consistent condition of the crack stress intensity factor and the curvature of the crack mouth opening. Using the weight function obtained, the stress intensity factors of different depth cracks in the coke drum are analyzed, and compared with the results of the literature, the agreement is good. Then, the influence of different liquid rise rates and different thermal conductivity coefficients in the coke drum on the crack stress intensity is analyzed. The influence of factors. The research in this paper lays a theoretical foundation for the prediction of crack propagation and remaining life of the coke drum.