Axial Cracks Research Articles

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%.

Mechanical behavior of round bamboo under transverse local compression

Bamboo is a material with low transverse mechanical properties. To study the mechanical behavior of round bamboo under transverse local compression, 60 round bamboo culms were subjected to three-point bending tests. The effects of different parameters on the transverse local compressive behavior were investigated experimentally. Finally, the transverse local compressive ultimate bearing capacity of round bamboo was estimated using prediction equations. The results indicated that under three load boundary conditions, axial cracks appeared on both sides of the specimens in the midspan region, dents of different sizes appeared at the loading points, and additional fractures occurred in the inner diaphragms of the specimens with the loading point at the node. The specimens subjected to point and line loads underwent brittle failure. Under the space-curve load, the specimens with a loading point at the internode exhibited ductile failure, whereas the specimens with a loading point at the node exhibited brittle failure. To obtain a favorable transverse local compressive ultimate bearing capacity, it was optimal to consider the space-curve load as the load boundary condition and position the transverse local compression at the bamboo node. The results of the prediction equations agreed well with the test results.

Combined effect of rays and vessels to achieve high strength and toughness in balsa wood

Balsa wood, as one of the lightest woods available, presents superior mechanical properties, and thus it is one of the most appealing core materials particularly for marine and aerospace applications. However, the strengthening and toughening mechanisms in balsa wood are still poorly understood. The present work indicates that the samples exhibit higher strength and toughness concurrently, provided the loading direction is perpendicular to rays; in this case, vessels with strong walls can provide mechanical support, blunt cracking and stop horizontal cracking, and rays can arrest axial cracking. The present research findings are expected to provide fundamental insight into the design of high-performance bionic materials with a highly fibrous and porous structure.

Experimental study on the channeling leakage properties of compact-size casing-cement sheath-stratum assembly in deep and ultra-deep wells

Within deep and ultra-deep wells, the casing-cement sheath-stratum rock assembly(CCS) bears thermal, mechanical, and chemical multi-field coupling, causing some damage or creating micro-annular gaps at the interface between the casing and the cement sheath as well as the interface between the cement sheath and the stratum. Radial and axial cracks in the cement sheath occur, and even channeling-leakage channels form due to shear failure. These are critical reasons for sustained annular pressure and the risk of wellbore sealing integrity. This paper establishes an evaluation device for channeling-leakage properties of a compact-size CCS assembly. It observes the microstructure and morphology characteristics of cement sheath integrity, tests the channeling-leakage pressure and paths of two cement slurries, and explores the relationships between the channeling-leakage properties of the assembly and the damage to the cementing interfaces as well as the cracking characteristics of cement sheath. Results showed that the proposed method was suitable for evaluating unpredictable security risks of CCS assembly caused by a complex load in deep and ultra-deep wells, such as overload-induced cracking, cementing interface unsealing, and channeling-leakage of the cement sheath. The findings revealed the evolution law from deformation points to cracks and channeling-leakage channels inside the cement sheath under the action of load and proposed technical measures to avoid channeling-leakage of the cement sheath. This research provides a theoretical basis and testing method for evaluating the wellbore sealing integrity and formulating technical safety measures for deep and ultra-deep wells.

Integrity assessment considering unified constraint for reactor pressure vessel under pressurized thermal shock

The integrity assessment methodology based on a new unified constraint parameter Ad* and the Master Curve has been investigated for a typical Chinese reactor pressure vessel (RPV) containing axial surface cracks subjected to pressurized thermal shock (PTS). The thermal–mechanical analysis was carried out under two different PTS transients using finite element method. The integrity assessment has been conducted by comparing the KJ-T curve and KJc-T curve (KJ is elastic–plastic stress integrity, KJc is fracture toughness and T is the crack-tip temperature), and the safety margin was quantitatively analyzed. The results show that the Ad* value does not change with temperature and loads (thermal load and internal pressure load) during PTS due to its load and material independences, and which facilitates the constraint analysis for a RPV under PTS. For smaller size surface cracks usually occurring in the RPV, the integrity assessment considering unified constraint under PTS can reduce excessive conservatism of traditional assessment method. The safety margin of integrity assessment is mainly influenced by constraint levels, PTS transients and fracture toughness of irradiation materials, and methods for increasing safety margin are discussed.

Progressive Initiation Phenomenon of Radial and Axial Cracks of the Borehole During Rock Hydraulic Fracturing

Progressive Initiation Phenomenon of Radial and Axial Cracks of the Borehole During Rock Hydraulic Fracturing

Pipe geometry effect on fatigue cracking location of 90° elbows under in-plane bending

In this technical note, the criterion for fatigue cracking location for 90° elbows under in-plane bending is presented. The criterion is found using published elastic solutions and elastic-plastic FE analysis results. It is found that the most important parameter affecting fatigue crack location is the mean radius-to-pipe thickness ratio, rm/t. The circumferential crack at outer intrados is predicted for rm/t <4–5, and the axial crack at inner crown for rm/t >4–5. The exact value of rm/t depends slightly on the bend radius-to-mean pipe radius ratio. This conclusion is found to be appropriate from high cycle fatigue to very low cycle fatigue loading, and to be independent on the material.

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.

Magnetic Permeability Perturbation Testing for Internal Axial Cracks in Small-Diameter Thick-Walled Steel Pipes

Special geometric features and complex working environments render the internal defects of small-diameter thick-walled steel pipes “easy to expand, difficult to detect”. In this paper, a magnetic permeability perturbation testing (MPPT) method is proposed to assess the internal axial cracks of small-diameter thick-walled steel pipes. The mechanism of the MPPT method and its corresponding probe and magnetizer are introduced, and its feasibility is verified through a series of simulations and experiments. Experiments conducted using different sizes of small-diameter thick-walled pipes show that this method offers good performance with respect to the detection of internal axial cracks. Additionally, both diameter and wall thickness significantly affect the MPPT signal. To a certain extent, a greater wall thickness or a smaller diameter brings about a weaker signal. This method does not benefit from the lift-off effect, nor is it limited by the skin effect, which has great practical value as a supplement to the evaluation of thick-walled steel pipes.

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

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

Mitigation of transient torque reversals in indirect drive wind turbine drivetrains

AbstractBearing failure in wind turbine gearboxes is one of the significant sources of downtime. While it is well‐known that bearing failures cause the largest downtime, the failure cause(s) is often elusive. The bearings are designed to satisfy their rolling contact fatigue (RCF) life. However, they often undergo sudden and rapid failure within a few years of operation. It is well‐known that these premature failures are attributed to surface damages such as white surface flaking (WSF), white etching cracks (WECs) and axial cracks. In that regard, transient torque reversals (TTRs) in the drivetrain have emerged as one of the primary triggers of surface damage, as explained in this paper. The risk associated with TTRs motivates the need to mitigate TTRs arising in the drivetrain due to various transient events. This paper investigates three TTR mitigation methods. First, two existing devices, namely, the torsional tuned mass damper and the asymmetric torque limiter, are studied to demonstrate their TTR mitigation capabilities. Then, a novel idea of open‐loop high‐speed shaft mechanical brake control is proposed. The results presented here show that while the torsional tuned mass damper and the asymmetric torque limiter can improve the torsional vibration characteristics of the drivetrain, they cannot mitigate TTRs in terms of eliminating the bearing slip risk associated with TTRs. However, the novel approach proposed here can mitigate TTRs both in terms of improving the torque characteristic in the high‐speed shaft and reducing the risk of bearing slip by actuating the high‐speed shaft brake at the onset of the transient event. Furthermore, the control method is capable of mitigating TTRs with the mechanical limitations of a pneumatic actuator in terms of bandwidth and initial dead time applied to it. This novel approach allows the wind turbines to protect the gearbox bearings from TTRs using the existing hardware on the turbine.

Effect of different hole shapes on the stress intensity factor of the finite plate with edge crack

Generally, flaws or defects are formed while manufacturing the components either internally or externally. Not all such flaws are unstable under service conditions. Cracks are often developed in the structural member due to high-stress concentration factors. Fracture mechanics help us in evaluating the life and safety of structures in the presence of cracks. This is accomplished by analyzing the intensity of the crack condition by estimating certain fracture parameters. This study aims at evaluating the mode I stress intensity factor (SIF) of a finite plate with edge crack in the presence of different hole shapes. The holes are placed along and perpendicular to the crack axis. The objective is to study the effect of the size, number, and shape of holes on the mode I SIF through finite element simulation. A 2D finite plate with an edge crack submitted to tensile loading was validated against analytical results. It was found from the simulation the existence of a range of distance from the crack tip within which when the holes are placed, results in the decrease of SIF. Beyond a certain distance that is far from the crack tip, the introduction of holes does not affect the SIF. When the elliptical holes (either single or multiple) was placed along the crack axis, it reduces the SIF effectively better than the circular and square hole. On the other hand, a single square hole when placed about the vertical axis, was found to diminish the SIF effectively relative to circular and elliptical holes. Similarly, the multiple circular holes reduce the SIF significantly more than the square and elliptical holes about the vertical axis.

The effect law of shell surface crack propagation under implosion loading

The fracture process of the metal cylindrical shell under the action of implosion is numerically simulated by AUTODYN finite element software for the expansion of cracks on the surface of the cases under the action of implosion loading. Numerical computations were used to determine the effect law of engineering factors such as the energy density of explosives, shell thickness, and shell material on crack-related parameters. The results show that the shell fracture radius along the axial direction remains nearly constant, and its value is less influenced by the energy density of explosives. The axial crack propagation velocity of the shell from the location of crack initiation to the non-exploding end of the shell shows a trend of change from high to low, fluctuating down. The axial crack propagation velocity and crack circumferential density both increase with the explosive energy density. The fracture density fluctuation range is between 0.4~0.73/mm. The law can be used to study the fracture damage of metal cylindrical shell structures under high impact.

Tension Stiffening and Cracking Behavior of Axially Loaded Alkali-Activated Concrete.

Alkali-activated concrete is an eco-friendly construction material that is used to preserve natural resources and promote sustainability in the construction industry. This emerging concrete consists of fine and coarse aggregates and fly ash that constitute the binder when mixed with alkaline activators, such as sodium hydroxide (NaOH) and sodium silicate (Na2SiO3). However, understanding its tension stiffening and crack spacing and width is of critical importance in fulfilling serviceability requirements. Therefore, this research aims to evaluate the tension stiffening and cracking performance of alkali-activated (AA) concrete. The variables considered in this study were compressive strength (fc) and concrete cover-to-bar diameter (Cc/db) ratios. After casting the specimen, they were cured before testing at ambient curing conditions for 180 days to reduce the effects of concrete shrinkage and obtain more realistic cracking results. The results showed that both AA and OPC concrete prisms develop slightly similar axial cracking force and corresponding cracking strain, but OPC concrete prisms exhibited a brittle behavior, resulting in a sudden drop in the load-strain curves at the crack location. In contrast, AA concrete prisms developed more than one crack simultaneously, suggesting a more uniform tensile strength compared to OPC specimens. The tension-stiffening factor (β) of AA concrete exhibited better ductile behavior than OPC concrete due to the strain compatibility between concrete and steel even after crack ignition. It was also observed that increasing the confinement (Cc/db ratio) around the steel bar delays internal crack formation and enhances tension stiffening in AAC. Comparing the experimental crack spacing and width with the values predicted using OPC codes of practice, such as EC2 and ACI 224R, revealed that EC2 tends to underestimate the maximum crack width, while ACI 224R provided better predictions. Thus, models to predict crack spacing and width have been proposed accordingly.

Numerical study on damage response and failure mechanism of gas-containing coal-rock combination under confining pressure effect

The research on mechanics properties and destroy behavior of coal-rock combination is extremely crucial for deep coal mining. However, there are few studies on the gas-containing coal-rock combination under the confining pressure effect, resulting in insufficient comprehension of the failure mechanism of the combination with different confining pressure. In this paper, a discrete element method (DEM) model of combination with stress-gas coupling is established. The related mechanics properties of single coal samples and combination samples under stress-gas coupling are compared and studied, such as axial stress, crack generation and failure mode. The results demonstrate that the coal mass in the coal-rock combination is continuously damaged and compacted during the loading process with different confining pressure. The interfacial effect caused by the circumferential expansion or irregular deformation of coal mass, and the improvement of compression intensity of the compacting coal mass jointly induce damage of the rock mass. The combination with confining pressure 4, 7 and 10 MPa exhibit the macroscopic failure characteristics of coal mass dominant, coal and rock mass together, and rock mass dominant, respectively. Meanwhile, the confining pressure effect also greatly determines the formation of the final failure pattern of combination samples after macroscopic failure.

EXPERIMENTAL INVESTIGATION OF THE EFFECT OF STRAIN RATE ON THE CRUSHING STRENGTH OF A CELLULAR CONCRETE

The strain rate dependent compressive strength of an autoclaved aerated concrete (AAC) having a density of 600 kg m-3 was experimentally investigated between quasi-static and high strain rates (2x10-3-~4150 s-1) through quasi-static and dynamic compression, confined compression and indentation tests. High strain rate equilibrium and direct impact non-equilibrium compression tests in conjunction with the high strain rate confined compression and indentation tests were conducted in a compression Split Hopkinson Pressure Bar. The experimental results showed two different regions of the compressive strength-dependency on the strain rate: a low-strain rate-dependent region from quasi-static to ~18 s-1 and a high-strain ratedependent region from ~18 s-1 to ~1000 s-1. The switch of the failure mode from the single axial cracking at quasi-static strain rates to the extensive axial and circumferential cracking at increasing strain rates was ascribed to both the axial and radial inertia. The dynamic increased factor (DIF=dynamic strength/static strength) showed an abrupt increase after ~18 s-1 as similar with the compressive strength. The mean confined and indentation strength values also increased as the velocity increased, while the mean confined compression strength values were shown to be comparable with the dynamic compressive strength values. The inertia and strain rate contributions to the enhancement of DIF until about 1000 s-1 were predicted by taking the quasistatic indentation strength as the full confinement strength.

Mechanical Behavior of Cross-Winding Steel-Wire–Reinforced Thermoplastics Pipe under Internal Pressure

Cross-winding steel-wire–reinforced thermoplastic pipes (CSWRTPs) have been widely applied in oilfield gathering environments in China because of their excellent physical properties. In this study, the failure mode, short-term burst pressure, and strain for a CSWRTP with an external diameter of 75 mm and a wall thickness of 10 mm under internal pressure are investigated through experimental measurements conducted with a burst test system and simulations completed with ANSYS software. A new three-dimensional finite-element burst failure model of CSWRTPs is proposed. Simulation results are in good agreement with the measurements. An axial ductile crack was found on the tested pipe with an uneven circumferential expansion deformation. The simulated short-term burst pressure is in a good agreement with the measured value (19.7 MPa) with an error of +8.7%. The growth of the strain is initially zero, followed by a linear growth, rapid growth, and a clifflike drop when the crack is generated. The effective plastic strain within the inner polyethylene liner is greater than that within the outer polyethylene sheath, and the effective stress within the inner steel wire is greater than that within the outer steel wire. The ultimate pressure–bearing capacity decreases linearly as the strength of the steel wire decreases. Therefore, reduction of the ultimate pressure–bearing capacity of CSWRTP results from the decrease of steel wire strength from corrosion when applied in oilfield gathering systems. This finite-element analysis method can provide a reference to optimize the structural design of CSWRTP.

Evaluation of the applicability of microwave nondestructive testing to crack detection on a pipe inner surface

This study explores the applicability of microwave nondestructive testing to detect a metal pipe’s inner crack. Three-dimensional finite element simulations were conducted to study the inspectability of cracks using microwaves in different modes and the dependency of the reflection characteristics of microwaves on crack size. The simulation results showed that microwaves in the TM01 and TE01 modes can detect circumferential and axial cracks, respectively. The positive relationship between crack size and intensity corresponding to the reflected microwaves was obtained in simulations and then verified by experiments. In both simulations and experiments, the axial crack length showed a small influence on the results, especially for shallow crack detection. In the experiments, circumferential and axial cracks with a width of 0.3 mm were detected using microwaves. The experimental results revealed that signal amplitudes decreased when a slit penetrated a pipe wall, probably due to a microwave leakage.

Characteristics of allowable axial cracks for pressurized pipes governed by limit load criteria

When axial cracks were detected in pressurized pipes, failure stresses for high-toughness ductile pipes are estimated using the Limit Load Criteria. The allowable stresses for the cracked pipes are derived from the combination of the failure stresses and safety factors. Then, the allowable sizes of crack depths and lengths are determined from the allowable stresses. From the comparison of the allowable and failure stresses for through-wall cracks, the allowable cracks are not uniform. The allowable crack sizes can be separated into different characteristics of leak-before-break (LBB) and crack growth stability. Inspectors and users should pay special attention to allowable cracks with non-LBB and crack growth instability characteristics to prevent unexpected failure, particularly for thin-wall pipes. The boundaries of allowable crack depths and lengths that require special attention can be expressed by appropriate equations.

Stress intensity factor of through-wall-cracked steel pipe wrapped with prestressed CFRP composites

Externally wrapped prestressed carbon fiber reinforced polymer (CFRP) composites are effective in cracked steel pipe repairs. In this study, the effects of pipe and crack geometry, CFRP properties, and prestress levels on the Mode Ⅰ stress intensity factor (SIF) were addressed quantitatively for a prestressed CFRP-wrapped steel pipe with a through-walled crack through dimensional and finite element (FE) analysis. Five hundred and ten FE models were established to analyze the effects of the parameters. The effects of radius and wall thickness on the SIF were interactional. The SIF was negatively correlated with the CFRP stiffness and decreased linearly with the increasing prestress. The ratio between the SIFs of a CFRP-wrapped cracked pipe and a cracked pipe was determined by the dimensionless CFRP stiffness. The SIF decreases with the increasing crack angle following the cosine function mode. The ratio between the SIFs of an inclined crack and an axial crack was determined only by the crack angle. The influences of pipe geometry, crack angle, and CFRP properties on SIF were independent of each other. An empirical equation for calculating the Mode Ⅰ SIF was proposed for the CFRP-wrapped thin-walled pipe with a through-walled crack. An accurate mechanical method to calculate the decrement of pipe substrate bearing internal pressure caused by prestress was proposed.