Axial Cracks Research Articles

Creep crack energy dissipation and instability prediction in micritic bioclastic limestone

Time-dependent deformation and long-term stability of rocks are important issues in water conservancy and geotechnical engineering. Currently, there are no well-accepted theoretical criteria with which to predict stability and damage considering time-dependent deformation. In-depth research is still needed. Multilevel creep experiments were performed on micritic bioclastic limestone obtained from a continuously deforming tunnel in Xinjiang, China. Based on crack strain theory, the axial crack strain evolution characteristics during loading and creep processes were investigated. The evolutionary characteristics of the crack dissipation energy density, which was obtained via integration of the crack strain during loading and creep, were revealed. The energy dissipation leading to rock fracture in each multilevel creep experiment was identified and used to calculate the total energy dissipated at the point at which the rock loses strength. A rock instability index based on the creep crack dissipation energy density was proposed. An instability index evolution model was proposed, expressed as a surface for the change in instability index with the bearing state (Rd) and time. Based on the instability index surface, the stress threshold creep-sensitive stress σcs was defined. The time-dependent instability index surface was divided into a long-term stability zone (Z1), an initial time-dependent instability zone (Z2), a time-dependent instability stable evolution zone (Z3) and a time-dependent instability sensitive zone (Z4) by the crack initial stress, crack damage stress, and creep sensitive stress. The instability index model was established as an energy instability evolution criterion to evaluate and predict the rock instability and lifespan under specific stress states.

Automatic identification and location method for shaft-hole interference fit crack based on deep learning

Cracking on the shaft of the interference fit joint surface is a common failure form for the shaft-hole interference fit. Because the crack is small and is located at the shaft edge, the crack signal is affected by echo and reflection signals and noise when traditional ultrasonic guided wave methods are used to detect the crack. When the crack is small-sized, the crack signal may even be submerged completely in the echo and reflection signals. Therefore, the traditional detection method is unable to obtain accurate information about the crack locations easily and thus cannot evaluate the overall safety performance of the interference fit accurately. To solve these problems, this paper presents an on-axis crack monitoring system for interference fit joints using laser ultrasound and an improved YOLOv5 network that integrates early warning, detection, positioning, and analysis functions. The system detects axial cracks at the shaft-hole interface via laser ultrasonic guided waves on the workpiece, and captures the crack information by deep learning; then, the captured crack information is analyzed to extract the essential crack location information. Finally, the method is verified experimentally. The experimental results show that this method can not only identify axial cracks at the shaft-hole interference fit surface automatically, but also can locate these cracks accurately and obtain both the number of cracks and the distance between pixels.

Post-irradiation examination of low burnup U3Si5 and UN-U3Si5 composite fuels

This work presents post-irradiation examination data on UN-U3Si5 and U3Si5 fuels at low burnup (i.e., <10–15 GWd/tHM) with Kanthal AF® cladding. The results suggest good irradiation performance for both the silicide and nitride-silicide composite pellets. Optical microscopy revealed that the pellet-cladding gap is still open, and limited axial cracking was observed only in UN-U3Si5 pellets. Microcracking was isolated to the U3Si5 phase in all cases and was observed in pre-irradiation and depleted pellets, indicating that it was not irradiation induced. The fission gas release was minimal for the calculated fission density achieved (2.6 – 3.15 × 1020 fiss/cm3). No fission gas bubbles were observed in the optical metallography. These results suggest acceptable swelling and fission gas behavior for both the single phase and composite compositions.

Influences of the number of non-consecutive joints on the dynamic mechanical properties and failure characteristics of a rock-like material

The dynamic mechanical properties and failure characteristics of jointed rocks are the focus of research in rock engineering. In this study, cement mortar specimens with 0–4 parallel non-consecutive joints were prepared. The split Hopkinson pressure bar (SHPB) was used for dynamic impact testing of prepared specimens. Nuclear magnetic resonance (NMR) was used to detect the pore structure deterioration of the specimens after impact. Finally, the failure characteristics of the specimens were studied by extracting the surface cracks of the specimens. The results show that the number of non-consecutive joints have a significant effect on the dynamic mechanical properties of cement mortar specimens. The increased number of joints will change the stress–strain curve from a single-peak shape to a multi-peak shape and enhance the ductility. The dynamic peak strengths and the proportion of dissipated energy of the specimens decrease with the increase of number of joints, but the magnitude of change decreases gradually. NMR analysis reveals the pore structure alterations of the specimens. After being impacted, the internal pores of the specimens increase significantly, especially the macro-pores and micro-pores. Some micro-pores become meso-pores and macro-pores, and some pores are connected. These changes of microscopic pore structure lead to macroscopic failure of specimens. A quantitative analysis of the porosity of the specimens shows that the rate of change in porosity is in direct proportion to the number of joints, which means that the greater the number of joints, the larger the damage degree of the specimens. The failure characteristics of the specimens indicate that tensile stress mainly dominates the fracture behavior of the specimens, while shear stress has limited damage to the specimens. After the tensile wing cracks start from both tips of the joint, they extend along the rock bridge to the adjacent joint or the end of the specimen. After the wing cracks coalesce, they form a rectangular failure zone. In addition, axial tensile cracks will start near the middle of the first joint and the middle of the last joint, extending to the top and bottom of the specimen, respectively. Slight shear cracks only develop at the joint ends.

In-line inspection technology of oil and gas long-distance pipeline based on alternating current field measurement

Abstract Alternating current field measurement (ACFM) has advantages of fast inspection speed, high efficiency, sensitive inspection, and easy quantification. In this study, a probe based on ACFM has been carried out, trying to inspect cracks on the inner wall of pipeline with high precision. An inspection tool which is working according to ACFM is developed in order to determine the stress corrosion cracking. The lift-off distance and excitation frequency parameters are optimized, and the influence of uneven magnetic permeability of the pipeline on the test results is analyzed. In addition, experiments are carried out to test the practicality of the tool. The minimum axial crack length that the probe can detect is 15 mm and the minimal depth is 1 mm. The research results show that the probe has the ability to quantitatively evaluate the axial cracks on the inner surface of the pipeline, which has certain guiding significance for the design and development of the AC electromagnetic field inspection probe.

Mechanical properties, damage evolution and energy dissipation of cemented tailings backfill under impact loading

In order to explore the influence of average strain rate(ASR) on mechanical properties, damage evolution and energy dissipation of cemented tailings backfill(CTB), the dynamic test of CTB was carried out in the laboratory. This study shows that dynamic peak stress and initial crack stress of CTB increase exponentially with the increase of ASR, indicating that the characteristic stress of CTB has a significant strain rate effect. The dynamic peak strain and initial crack strain of CTB also increase exponentially with the increase of ASR, and the CTB with lower cement content is more likely to produce deformation and failure. The increase of ASR can improve the energy indexes of CTB at the peak stress point, and the increase of cement content can also improve the energy parameters of CTB. The energy storage limit of CTB increases exponentially with the increase of ASR. In addition, a dynamic statistical damage constitutive model of CTB was established considering ASR, and the accuracy and reliability were verified. There are three stages for the damage failure of CTB: rapid growth stage of damage, slow growth stage of damage and stable growth stage of damage. In addition, the high strain rate has a certain retarding effect on the development of cracks inside the CTB, thus inhibiting the rapid growth of damage value. Under impact loading, the CTB samples are mainly axial macroscopic main crack tensile failure and crushing failure. When the average strain rate is higher than the critical strain rate, the failure pattern of CTB is mainly crushing failure. When the average strain rate is basically the same, the damage degree of CTB with higher cement content is smaller.

Influence of curvature radius on the axial crack signal of the magnetic flux leakage detection for tapered roller bearing rings

Tapered roller bearings are widely used in heavy machinery, railway transportation, aviation, and other fields. Their quality and reliability are related to the operational safety of mechanical equipment. In the axial crack magnetic flux leakage (MFL) detection of the bearing ring, the MFL signals obtained by the sensor from different curvature radius of the surface are inconsistent, affecting the detection accuracy of cracks and subsequent quantitative analysis. In order to address the above problems, the finite element simulation is performed to analyze the influence of the surface curvature radius of the bearing ring on the magnetic field distribution inside the workpiece and the MFL signal in the circumferential magnetization. Through the parallel magnetic circuit, the difference in curvature radius is identified as the basic reason for non-uniform magnetization. On this basis, the compensation method based on the normalization of surface magnetization is proposed. Furthermore, the effectiveness of the compensation method is verified by experiments. The relative change in the amplitude of the crack MFL signal is reduced from 30% to 5%.

MODELLING OF TENSILE STRAIN DISTRIBUTION ALONG THE AXIAL DIRECTION ON SIMPLIFIED DAMAGE ESTIMATION OF REINFORCED CONCRETE STRUCTURES

In recent years, studies on crack propagation analysis and evaluation methods for RC structural members have been carried out for the sake of reparability performance. The purpose of this study is to develop a simple method for estimating crack propagation without using advanced and complicated analytical techniques. This paper is divided into two main chapters: The first focuses on the relationship between axial strain distribution and crack length obtained from an experimental result to improve the existing damage estimation method. The second is a study of the applicability of the proposed theory to structural damage estimation by combining it with static incremental analysis.

Study on the Fracturing of Rock by High-Speed Water Jet Impact

Due to the opaqueness of rock and the limitation of detection technology, it is impossible to accurately describe the crack growth process and determine the law of rock breakage. Based on smoothed particle hydrodynamics and the finite element method (SPH-FEM), a numerical model for high-speed water jet breaking was established in this work to simulate the fragmentation process of rock impacted by a high-speed water jet, and to study the effects of different jet angles on the propagation of microscopic cracks inside the rock. Additionally, we further analyzed the jet impact angle on the microscopic crack propagation trend of the rock by applying confining pressure to the rock. Theoretical and experimental analyses showed that the inclination angle of the jet determined the direction of axial crack propagation in the tension-type center. When the inclination angle of the jet exceeded 20°, the ability of water jet erosion was insufficient, and the efficiency of rock fragmentation was low. However, in the range of 15° to 20°, the capacity of erosion was strong, lamellar crack propagation was obvious, and rock chip block spalling was easily produced. The impact of the water jet on the rock at varying angles under rock confining pressure will make the crack propagation direction deviate from the direction without confining pressure and gradually become parallel to the rock plane, thereby promoting unilateral crack propagation in the direction of water jet impact, making the rock more likely to produce unilateral rock chip spalling.

A unified method to estimate J-resistance curve of thin-walled metallic tube containing axial cracks

Fracture toughness of thin-walled metallic tubes is an important mechanical property index to ensure their safety in service. Because of the particularity of geometric structure, the fracture toughness test of thin-walled metallic tubes cannot be carried out using traditional standard specimens. In this work, the expressions of load–displacement relationship and J-integral of thin-walled tube specimens containing bilateral axial cracks are proposed based on the energy density equivalence principle, in which the effect of material and specimen geometry has been considered. Through three-dimensional finite element simulation analysis, the relevant parameters of the expression are obtained and validated, and then a fracture toughness test method for thin-walled metallic tubes is established. The method is applicable to the fracture toughness measurement of thin-walled metallic tubes with different sizes and materials. Finally, the fracture toughness tests of two thin-walled alloy tubes were completed by the new method, and the J-resistance curves were obtained.

A Creep Constitutive Model, Based on Deformation Mechanisms and Its Application to Creep Crack Growth

In this paper, a constitutive model, based on the creep deformation mechanism in P91 steel, under a wide range of stress levels, was established and embedded into finite element software. The accuracy and reliability of the model was verified by comparing the simulation of uniaxial creep tensile test results and the experimental data under different stress levels for P91 steel at 600 °C. The creep crack growth behavior of P91 steel, under a wide range of stress levels was simulated using a ductility-exhaustion-based damage model, combined with the stress-dependent creep ductility model, and the predicted creep crack growth (CCG) rates were compared with the experimental data. Finally, the established model was used to predict the CCG behavior for the pressurized pipes with axial surface cracks. The results show that the constitutive model, established on the basis of the creep deformation mechanism, agrees better with the experimental data than other constitutive models. The CCG rate varies at different direction angles θ for the axial surface cracks. The direction angle θ corresponding to the maximum creep crack length is about 33°, when the internal pressure exceeds 10 MPa. The initial crack shape (a0/c0) = 1, and it does not change with different initial crack depth ratios (a0/t). The established constitutive model can be well used in CCG life analyses and designs of high-temperature structures.

Transient Force-Magnetic Coupling Simulation Research on Pipeline Stress Concentration

The stress concentration of oil and gas pipelines will produce corrosion cracks and axial fatigue cracks under the action of long-term and repeated internal pressure, and will rapidly expand under fluctuating internal pressure, resulting in serious pipeline accidents. In order to effectively detect the stress concentration of oil and gas pipelines, a stress concentration detection technology of unsaturated local magnetization pipeline is proposed. This paper introduces the theoretical basis of local pipeline stress concentration detection, establishes a two-dimensional finite element model of pipeline stress concentration transient force-magnetic coupling, and analyzes the influence of excitation source position, sensor lift-off value, and stress concentration value on the response signal. The simulation results show that the corresponding signal B x of the stress concentration changes more obviously than B y and the signal is regular, Bx has a good two-stage linear positive correlation with the stress concentration value. The above research provides technical support for the development of pipeline on-line stress concentration detection probe.

Mechanisms of the Cladding Tubes Axial Cracking in Power Ramp Tests

Mechanisms of the Cladding Tubes Axial Cracking in Power Ramp Tests

Study on mesoscopic failure mechanism of grout-infilled sandstone under uniaxial compression using an improved AE localization technique

In this paper, to study the influence of grouting on meso-failure mechanism, acoustic emission (AE) localization technique was used to monitor the micro-cracking during the uniaxial compression tests on single flawed sandstone specimens with or without grouting. Due to the inhomogeneous geometrical structure of the flawed specimens, the mesoscopic damage evolution is also inhomogeneous during the loading process. As a result, the wave velocity of the specimens varied temporally and spatially, which greatly increased the difficulty in accurate AE localization. To solve this problem, the wave velocities versus the axial load for the studied sandstone were first measured, and the stress concentration effect near the flaw was also theoretically analyzed. Accordingly, the localization arithmetic was also improved to match the complex wave velocity field. Results show that compared to the specimens without grouting, more microcracks (particularly the shear microcracks) are induced in the grout-infilled specimens prior to the final failure of the specimens. Moreover, the effect of grouting on microcrack initiation also varies with the oblique angle of the flaw. For θ (flaw inclination) ≤30°, grouting significantly increases the shear microcrack at macrocrack propagation stage which states that the axial tensile cracking was limited by the supporting effect of the grout material and made the failure mode trend to shear mode. For θ = 45° and 60°, grouting improved the ratio of shear microcrack during the whole failure process, indicating that the direction of maximum shear stress is close to the flaw direction. Hence, failure develops in stable shear mode. For θ = 75° and 90°, the primary microcrack generated during the first macrocrack initiation changed to shear mode, but the grouting has little influence on the subsequent crack propagation. So that, the reinforcement effect is poor in this set of specimens.

Experimental study of deep-burial underground structures subjected to multiple 45° side–top far-field explosions

A deeply buried straight-walled round-arch structure in a rock medium is taken as the research object in this study. Model tests of the underground structure subjected to multiple 45° side–top far-field explosions, with a scale of 1:20 and a proportional explosion distance of 1.68, are designed and carried out to study the dynamic responses, failure modes and damage mechanisms of the deep-burial underground structure after ground shocks induced by explosions. The macroscopic phenomena and measured data of the tests are reported and analyzed. The results show that after the first explosion and the second explosion, the surface of the surrounding rock model and the reserved charging port are seriously damaged. In contrast, the damage to the underground structure is relatively slight. The overall structure produces downward-bending deformation, and an axial crack occurs on the inner surface of the vault. The peak accelerations of the underground structure after the explosions are significant, with values reaching 2400 g, and the corresponding prototype value can exceed 120 g. After the first explosion, the damage to the upper part of the surrounding rock and the axial crack at the vault lead to a decrease in local stiffness. The crushed areas and cracks of the surrounding rock formed by the first explosion intensify the attenuation and leakage of shock wave energy during the second explosion, resulting in reductions in the ground shock pressure on the structure, the structural strain and the acceleration response. It is also found that on the floor of the prototype structure, the maximum spectral acceleration in the range of the natural vibration period of the human body exceeds 10 g, which poses a safety threat to the usage of underground structures. The research results in this study provide an experimental reference for the anti-explosion design of deeply buried underground structures and provide a basis for the establishment of performance and vulnerability indicators for straight wall round arch underground structures subjected to far-field explosions.

Investigation of Transverse Vibration Characteristics of Cracked Axially Moving Functionally Graded Beam Under Thermal Load

This article presents a new analysis of the vibration characteristics of open-edge cracks for graded moving beams under thermal load. The material property gradient is based on the distribution of the power law in the direction of the beam thickness. The vibration equation is obtained depending on the precept of Hamilton and resolved by the extension of Galerkin’s approach. A rotational spring is used to represent the cracking in the beam. The effects of the axial velocity, gradient index, thermal load, and cracking parameters on vibration characteristics are observed. Furthermore, the mode shapes of the simple support cracked moving graded beam are determined. The results show that the rise in the axial velocity, the crack depth, and the material property index led to a decrease in the natural frequencies, which indicates the results obtained. HIGHLIGHTS Stability and vibrations analysis of the cracked gradient beams with an axial motion under a uniform temperature rise The effect of axial velocity, gradient index, thermal load, and cracks parameter was studied Galerkin’s technique is developed for vibrational analysis of cracked graded beams with longitudinal motion The Euler-Bernoulli beam model was used taking into account the effect of rotatory inertia GRAPHICAL ABSTRACT

The use of circumferential guided waves to monitor axial cracks in pipes

Guided wave testing is in routine use to detect corrosion, particularly in the oil and gas sector, but the detection of axial cracking is difficult using the axially propagating waves commonly used for corrosion detection. This paper presents a novel guided wave monitoring technique for the detection of axial cracking in piping. Circumferential SH0 waves, travelling around the circumference were used in a monitoring configuration to detect axial defects in a 6 inch schedule 80 steel pipe. Finite Element (FE) analysis showed that both the defect reflection and the decline in the transmitted SH0 wave can be used for defect detection. Baseline subtraction was utilised to produce residual signals that can be better analysed for small defect reflections. The Root-Mean-Square (RMS) of the residual signal after baseline subtraction was found to be the most satisfactory means of monitoring defect progression. An amplification effect was identified where the residual signal is compounded by continued interaction with the defect on each revolution of the incident wave. The FE predictions were validated by an experiment in which an Electrical Discharge Machining (EDM) notch was grown in four stages. Temperature compensation using Location Specific Temperature Compensation (LSTC) was applied to the experimental data, allowing residuals to be compared for a ∼10 °C temperature swing over a monitoring period of over 2 months. It was determined that a 10 mm long (∼23% of wavelength), 5 mm deep (∼45% of the wall thickness) defect at an axial offset from the line of the transducers of 1.5 wavelengths (65 mm) would be readily detectable with a very high Probability of Detection (POD) and virtually no chance of a false call. Therefore, this novel guided wave monitoring system for axial crack detection is likely to be attractive for applications in a range of industries for its sensitivity to axial cracking combined with large coverage from a single transducer location.

State-Based peridynamics for thermomechanical modeling of fracture mechanisms in nuclear fuel pellets

This study presents a coupled thermomechanical ordinary state-based peridynamic (OSB-PD) model, including the effects of randomly varying strength parameter and crack gap heat transfer, to investigate the fracture mechanisms in nuclear fuel pellets. The three-dimensional (3D) models of fuel pellets are constructed through irregular non-uniform discretization. The model includes the random variability of the critical stretch of each bond based on normal distribution. The accuracy of the OSB-PD model is validated against the FEM results for the thermomechanical behavior of a fuel pellet without damage. Subsequently, the crack propagation and crack patterns of fuel pellet are studied at different power levels (10 ∼ 40 kW/m). The results show that radial and axial cracks mainly develop during the power rise stage while the circumferential cracks develop during power down stage. These three crack types are verified by comparing with the experimental observations. The number of cracks and fragments of the fuel pellet increases with the increase in power level. Moreover, the fuel pellet can be completely separated at the mid-height cross section when the power level is equal or greater than 25 kW/m.

Axial compressive behavior of concrete-filled steel tubes with GFRP-confined UHPC cores

The steel–concrete-GFRP-UHPC (SCGU) composite column is a new type of column consisting of steel, concrete, ultra-high performance concrete (UHPC) and glass fiber reinforced polymer (GFRP). The sectional form of the SCGU column is a circular steel tube as the outer layer, a GFRP-wrapped UHPC column as the core, and concrete between the core column and the steel tube. In this paper, axial compression tests were conducted on twenty-four SCGU composite columns, and their damage morphology, load–strain curves and influencing factors were analyzed. The results show that the damage modes of the specimens present multiple convex curves of the external steel tube, corresponding to the fragmentation of its plain internal concrete, the fracture of GFRP and the large axial crack of the UHPC core column; that the load–displacement curves of the composite columns can be roughly simplified to elastic, elastoplastic and plastic flow stages; that the composite columns still have high residual bearing capacity after compression damage; and that effect of tube thickness, number of GFRP layers and UHPC core column diameter of the specimen on the load bearing capacity and ductility of composite columns. Based on the mechanical analysis of each component of the SCGU composite column, a formula of axial compression bearing capacity of the SCGU composite column was presented, and agreement between the formula predicted results and the experimental results was obtained. In addition, finite element (FE) models that can simulate the SCGU composite column are developed.

A safety assessment approach to pressure vessels based on machine learning

The safety assessment of a pressure vessel with a surface crack is an important part of the safety assessment of engineering equipment. However, the existing methods are mostly based on the assumption of plane specimens and the K criterion applicable to brittle fracture, which may lead to unacceptable errors when applied to a fracture problem in an elastoplastic pressure vessel. In this article, based on the finite element method (FEM) and artificial neural network (ANN), the elastic-plastic three-dimensional J-integral of a crack tip in a pressure vessel with an axial semi-elliptic crack on the surface under the loading of internal pressure is studied. First, the influence of the vessel geometry, the crack size, and internal pressure on the three-dimensional J-integral is analyzed. Second, the machine learning dataset is constructed based on the results of 1,200 cases of FEM calculation; then ANNs are used to discover the potential relationship between multiple parameters and the three-dimensional J-integral. The results show that the neural network constructed in this article can well predict the elastoplastic three-dimensional J-integral of a pressure vessel surface crack.