Axial Cracks Research Articles

GPR laboratory tests and numerical models to characterize cracks in cement concrete specimens, exemplifying damage in rigid pavement

Cracks in road pavements are usual damages because of heavy and dense traffic. Cracks usually are hidden to visual inspections in the case of rigid pavements. In these cases, cracked cement concrete layers are located below the asphalt layer. Therefore, detection becomes a difficult task, being only possible in the case of major damage or even failure. As consequence, rehabilitation is complex due to the technical difficulty in finding those cracks.Non-destructive surveys are in many cases applied to detect those damages. However, the difficulty in data interpretation limits their use forcing the combination of different technologies and core samples. One of the most useful method, commonly applied to define the thickness of layers, is Ground Penetrating Radar (GPR). In order to understand better the GPR images obtained as consequence of damage in the case of rigid pavement, several tests were designed to determine the ability of GPR to non-destructively detect and quantify cracks in concrete samples covered by an asphalt layer. The laboratory experiment consisted of three testing configurations: (1) one concrete sample without cracks; (2) a model of a rigid pavement with the asphalt layer without cracks; (3) the same model with different crack widths. The test was performed with a 1.6 GHz centre frequency GPR antenna in two relative positions with respect the cracks, being always the radar line perpendicular to the crack axis. These measurements allowed also evaluate the effect of orientation of the crack with respect the antenna dipoles in the GPR images. In addition, numerical models were compared to the laboratory results. The results showed that the crack width was detected in most cases, even the thinnest ones (2 mm), being the higher amplitude anomaly corresponding to the case of the data acquired with the antenna parallel to the radar line.

Mechanical analysis and modeling of porous thermal barrier coatings

Thermal barrier coatings (TBCs) are applied to the surface of components that are used in gas turbines and aircraft engines in order to increase the heat load. In this study, the cross-sectional microstructures of TBCs were investigated via scanning electron microscopy. After calculating the porosity and the pore size distribution, a two-dimensional finite element model with four different porosities (0, 1, 3, and 5%) were established. A stochastic method was used to generate a randomly distributed porous structure. Based on the results of FE analysis and nanoindentation tests, It was found that, at a high porosity (>3%), radial cracks are affected by the interaction between neighboring pores and occur at the top/bond coating interface. After the cooling stage, radial cracks continue their propagating trends. However, axial cracks are commonly generated from the ZrO2–8 wt.%Y2O3 surface as the tensile stress caused by a single micro-pore can be as much as three times higher than that of its surrounding area (porosity < 3%) at 1473 K. The addition of a 20 μm sealing layer can effectively eliminate the stress effects of the micro-pores on the YSZ coating surface.

A Novel ACFM Probe With Flexible Sensor Array for Pipe Cracks Inspection

Pipes are important components for well drilling, production, and transportation in petroleum industry. However, due to factors such as vibration, corrosion, fatigue damage, etc., pipes are prone to cracks. This paper presents a novel ACFM probe with flexible sensor array for the inspection of inner and outer cracks in pipes with different diameters. The simulation model of the ACFM probe with flexible sensor array is established by the finite element method. The influences of the crack length and the lift-off on the distance between the Bz peaks and troughs are analyzed respectively. The relationship between the offset distance of the crack from the pickup coil center and the detection sensitivity is studied. The probe made by flexible PCB and the testing system are developed. The experiments are carried out. The results of experiments and simulations show that the probe with flexible sensor array can detect inner and outer surface cracks for pipes with different diameters, and both circumferential and axial cracks can be identified. The distance between the Bz peaks and troughs can measure the crack length. When the crack is offset from the center of the pickup coil, the adjacent pickup coil can compensate for the detection sensitivity.

A flexural mode guided wave transducer for pipes based on magnetostrictive effect

The flexural mode guided waves of pipes which are sensitive the axial crack and suitable for wave focused gain more attention recently. In this paper, a non-contact flexural mode guided wave transducer based on magnetostrictive effect is provided for pipes. Based on the magnetostrictive transduction principle and the wave structure of the flexural mode guided wave, the sensing method for generating and receiving the flexural mode guided waves based on magnetostrictive effect is obtained. According to the theoretical analysis, a non-contact magnetostrictive transducer for F (3, m) mode guided waves is given. Six permanent magnets which are evenly distributed in the circumferential direction of the pipe and arranged in opposite polarities are employed to provide the bias magnetic field in the circumferential direction. A solenoid coil is employed to induce the axial alternating magnetic field. The bias magnetic field distribution of the flexural mode guided wave in the pipeline is analyzed by the finite element simulation. The mode of the transduction guided wave in the pipe is verified by experiments based on the dispersion curves.

Finite Element Analysis to Investigate the Effect of Loading Modes on the CTOA of DWTT Specimens

To replace the traditional Charpy-test-based fracture arrest methodology, the crack tip opening angle (CTOA) has been proposed as a fracture propagation parameter for prediction of fast ductile fracture propagation and arrest of axial cracks in pipelines. Transferability of CTOA from the DWTT specimens determined according to ASTM E3039 to other specimens and especially pipes is critical to the applications of CTOA. As part of the research effort to investigate the transferability of crack tip opening angle (CTOA) from the drop weight tear test (DWTT) specimens to pipelines, this work focuses on the effect of loading modes (bending vs. tension) of DWTT specimens on CTOA based on finite element analysis. The damage-mechanics model (Modified Mohr-Coulomb Model) and element deletion technique to simulate crack initiation and propagation is found to be accurate and agrees well with test data. This model accounted for variations in stress triaxiality and Lode angle at the crack front. For the DWTT model under bending, the surface CTOA is considerably larger than the interior CTOA consistent with the experimental observations. However, for the DWTT specimen under tension, the CTOA on surface and interior are almost the same. The interior CTOA is found to be very similar in all cases. The CTOA distribution as a function of distance behind the crack tip is found to be very consistent up to one half-thickness behind the crack tip, where CTOA starts to increase gradually. For both the bending and tension DWTT simulations, steady-state crack growth was achieved at a reasonably low crack extension. These results support the transferability of CTOA from standard DWTT tests according to ASTM E3039 to other loading cases.

Multiaxial Fatigue and Cracking Orientation of Forged AZ80 Magnesium Alloy

The effect of the multiaxiality and proportionality of loading on the cyclic behaviour and early cracking behaviour was studied for closed die forged AZ80 Mg alloy. Several different loading paths were presented, uniaxial and biaxial with varying levels of non-proportionality. In multiaxial loading the effect of proportionality is only present on the shear response of the material and increasing levels of non-proportionality is detrimental to the fatigue life. When compared with proportional loading, a 50% reduction in life was observed under 90° out of phase non-proportional loading. The pure axial cracking behaviour is dominated by transverse cracks along the plane of maximum normal stress, whereas in pure shear the material exhibited longitudinal cracking (shear dominated) behaviour in LCF regime and helical cracking (normal stress dominated) in the HCF regime. The synergistic effects of each individual uniaxial cracking modes contribution towards the combined macroscopic cracking behaviour in multiaxial loading was investigated, and was dominated by transverse cracking when the shear strain energy density (SED) contribution to the total SED was low, and mixed cracking when the shear SED contribution was high.

Experimental verification of a TE01 mode converter to locate a crack in a metal pipe

In this study, a TE01 mode converter was developed to locate an axial crack on the inner surface of a metal pipe. Three-dimensional finite element simulation was conducted to evaluate the effect of inserted coaxial cables on the transmission characteristic of the mode converter. The result showed that the energy of TE mode microwaves leaned to transmit to one side when the cables penetrated with inclination. To demonstrate the effectiveness of the mode converter, experimental verification was conducted. The mode converter was fabricated based on the simulation result. Microwaves were emitted through the pipe wall of the converter to propagate on both sides (‘right side’ and ‘left side’) of a pipe with an artificial slit. Compared with the signals from each side, the reflection from the slit on the right side was more significant than that on the left side. This result is consistent with the numerical simulation result.

The effect of aluminosilicate modifying additive on the deformation properties of adhesive dry building mixtures

The article provides information about the possibility of using amorphous aluminosilicates as a modifying additive for dry building mixtures. The data on the synthesis of the additive, its chemical and physical properties are given. A recipe for adhesive dry building mixtures with the use of amorphous aluminosilicates is proposed synthesized by the proposed method. The technical and physical properties of tile adhesives made using amorphous aluminosilicates as a modifying additive are considered. The deformative properties of tile adhesive made on the basis of the proposed recipe are considered. The elastic modulus of the coating, shrinkage deformation, ultimate tensile strength, axial tensile strength and crack resistance coefficient are calculated. According to the results obtained, appropriate conclusions were made.

Non-destructive damage analysis in Kariye (Chora) Museum as a cultural heritage building

Non-destructive testing methods are being increasingly used in the evaluation of cultural heritage buildings. Combined geophysical methods especially can be applied to structural evaluations of these kinds of buildings. Ultrasonic Pulse Velocity (UPV) Test and Ground Penetrating Radar (GPR) were used in a complementary way as non-destructive testing techniques in this study. The main aim of the study is to analyze the internal structural configuration as well as the quality of the stone, internal geometry and physical properties of some structural elements. It was determined that the main cracks in all walls within the building are on an axis. The cracks on this crack axis were determined to have been filled during previous repairs on the GPR profiles. However, the filling material is different from the building materials that constitute the building walls and at the same time the presence of void spaces was determined.

Crystal-pit-cracking formation on composite modified slag fiber in long-term water immersion

Composite modified slag fiber (CMSF) has been widely used as insulation material for enclosures in the field of construction; however, its corrosion characteristics and crack formation are unclear. Therefore, the damage behavior of CMSFs with two acidity coefficients (ACs) immersed in water for 365 days was studied; the fiber structure and composition were measured. The crystalline phase on the fiber surface was identified as CaCO3 deposits that dissolved after long-term immersion, resulting in pits. Owing to continuous replacement of H+, Ca2+ continuously migrates from the inside to the outside of the fiber, resulting in a stepwise increase in the AC of the fiber cross-section; the surface pits significantly increased and joined together to form an axial crack. However, the Si-O-Si bond has no significant fracture during long-term immersion. These findings support the theory on the crack behavior of CMSFs during long-term immersion for practical applications.

Experimental Study on Mechanical Properties, Failure Behavior and Energy Evolution of Different Coal-Rock Combined Specimens

To investigate the effect of the pure coal/rock strength on the mechanical behavior, failure behavior, and energy evolution of coal-rock combined (CRC) specimens, an AG-X250 Shimadzu Precision Universal Test was used to conduct uniaxial compressive loading, uniaxial cyclic loading, and unloading compression experiments on pure coal, pure rock, and different CRC specimens. The results show that the uniaxial compressive strength, Young’s modulus, and peak strain of the CRC specimen mainly depend on the coal specimen instead of the rock strength. The major failure modes of CRC were the shearing fracture and axial splitting failure, and for the CRC specimen with the same hard rock, the CRC specimen severely failed due to axial splitting cracks. In addition, the released elastic energy Ue, dissipated energy Ud, and kinetic energy Ur increase with increasing rock mass/coal strength, and for CRC specimen with the same coal, the greater the difference in strength between the rock and coal is, the greater the kinetic energy is.

Research on internal detection technology for axial crack of long-distance oil and gas pipeline based on micromagnetic method

Due to the lack of the efficient and accurate detection method, the axial crack of pipeline seriously threatens the safe operation of oil pipeline, and therefore the application of micromagnetic nondestructive testing technology in the field of pipeline axial crack detection is of practical significance. To study the characterization of axial crack by micromagnetic signal, a numerical model of micromagnetic signal to detect the axial crack of pipeline is established based on the micromagnetic theory in this article. The characteristics of micromagnetic detection signals of cracks with different sizes and directions are calculated. The propagation law of micromagnetic detection signal is analyzed, and the experimental study of X70 pipeline is carried out to verify the correctness of theoretical model and simulation. The results show that at each tip of the axial crack, the axial component of the micromagnetic detection signal has a peak and a valley, and the radial component has an extreme value. The amplitude of the micromagnetic internal detection signal at the axial crack linearly increases as the depth or length of the crack increases. The propagation law of micromagnetic signal conforms to the falling exponential function, specifically, the smaller the crack depth, the faster the signal decays and eventually stabilizes. The micromagnetic detection signal of the probes to detect the crack having a certain angle with the axial direction is arranged with equal displacement spacing.

Crack Closure Effect and Energy Dissipation Model for Rocks under Uniaxial Compression

Through the uniaxial compression tests of sandstone, the crack closure effect and the relationship between the crack parameters and the height to diameter ratio (H/D) were analyzed. Then, a nonlinear model for characterizing energy dissipation based on the crack closure effect was established. The results show that the peak strength and peak axial strain of sandstone decrease gradually with the increase of H/D, but the elastic modulus has a small change. The maximum crack axial closure strain and crack axial closure stress of sandstone increase with the H/D. While the peak crack axial strain and axial crack initiation stress decrease. The dissipative energy firstly increases with the axial stress, and the increase rate decreases gradually. Then, the dissipative energy nearly remains as a constant. The axial crack closure model was established to describe the crack closure effect of sandstone, and the equivalent elastic modulus of crack closure of sandstone calculated by axial crack closure model decreases nonlinearly with the H/D. A rock energy dissipation model based on crack closure under uniaxial compression was established. The experimental results agree well with the theoretical values, which indicates that the proposed model can describe the energy dissipation of rocks under uniaxial compression. The research conclusion provides a theoretical reference for the stability evaluation of underground rock masses with different sizes.

Estimation of residual strength of a crack-containing welded pressure vessel exposed to a subzero operating temperature

This paper presents a method for estimating the residual-strength of welded pressure vessel made of HSLA steel which contains a surface axial crack. In order to analyze the behavior of the structure, a set of specimens were extracted from the welded plates using the same welding procedure as used for welding of the vessel. Before the fatigue precracking, the notches were positioned in parent material, heat-affected zone and weld metal of standard bend specimens.The J-integral resistance curve method for residual strength prediction comprises the following steps:1. Obtaining of JRresistance curves for the pressure vessel material using convenient specimens;2. Obtaining Jcrack driving force curves for the investigated structure for different crack sizes and different levels of applied stresses by applying the appropriate model of plasticity (Dugdale model);3. Determination of the instability point as the tangency point between the crack driving force curve and the resistance curve.J-resistance curve of welded joint constituents were determined at −40 °C using a single specimen compliance procedure.

Shear Creep Tests on Fissured Mudstone and an Improved Time-Dependent Model

All rocks undergo creep when under a long-term load, and shear sliding is a major failure mode of natural slopes. However, the mechanical properties of fractured mudstone differ from those of intact rock. To investigate these properties and the influence of normal stress and pre-cut crack length on mudstone shear creep characteristics, we performed shear creep tests on fractured mudstone. The experimental results show that axial load magnitude and crack length have a marked influence on the rock’s shear creep behavior. The larger the axial compressive stress, the smaller the shear creep deformation. The longer the crack, the more significant the shear creep deformation. When stress is low, existing creep models can reflect the creep properties of mudstone well. However, most published creep models cannot accurately portray the nonlinear creep behavior of mudstone in a tertiary creep stage. We propose an improved time-dependent creep model that overcomes the shortcomings of the traditional models in describing nonlinear creep. The new model has fewer parameters and can be applied to practical engineering problems involving soft rock. During the shear creep tests on soft argillaceous samples under different loads, shear failure zones in fractured mudstone exhibited both gradual and sudden failure. The shear time-dependent model for mudstone developed in this study can effectively explain mudstone instability.

Code development and characteristics analysis for Leak Before Break in the pipelines of Sodium-cooled Fast Reactor

The Leak Before Break (LBB) technology plays a vital role in the safety evaluation of Sodium-cooled Fast Reactor (SFR). In this study, the leak rate considered in LBB assessment of SFR was predicted and analyzed. Based on the models of fluid resistance in cracks, two-dimensional numerical simulation for leakage process of uniform cross-section axial cracks were conducted with Computational Fluid Dynamics (CFD) to obtain recommended values of crack empirical parameters, including entrance resistance coefficient, effective parameters and friction factors. With consideration of the complex crack morphologies, the leak rate prediction models with a wide range of parameters were established for liquid sodium, and corresponding computation code (LAPIS) was developed. The proposed models were verified and validated by seepage model, numerical results and experimental data with deviations within 1%, 35% and 10%, respectively. The effects of the crack morphology and thermal-hydraulic parameters on the leak rate for tight and wide cracks were studied with the LAPIS. The leak rate increases with Crack Opening Displacement (COD), crack length, ratio of entrance to exit area, entrance resistance coefficient, stagnation pressure and decreases with temperature. This work could provide theoretical and technical support for the study of leak rate considered in LBB assessment of SFR.

Fiber type effect on strength, toughness and microstructure of early age cemented tailings backfill

In this paper, an experimental investigation was carried out to study strength, toughness and microstructural properties of early age cemented tailings backfill (CTB) reinforced with three different types of fiber. Polypropylene, polyacrylonitrile and glass fibers were used in CTB to better understand the effect of fiber addition on the backfill’s strength and toughness properties. Different fiber contents (0 wt%, 0.3 wt%, 0.6 wt% and 0.9 wt%) and curing times (3 and 7 days) were designed for the preparation of CTB samples with two cement-to-tailings ratios (c/t = 1:4 and 1:6). The results indicated that the addition of different type and content of fibers caused a significant change in CTB’s toughness. The fiber content influenced the backfill’s strength performance. The strength gain of fiber-reinforced CTB samples showed an increasing trend as the fiber content rises. Linear relationships could be used to express relations between strength gain and fiber content. The peak strain factor K was defined during the toughness tests. K showed a monotonous increasing trend by the increase of fiber content. The polypropylene fiber was larger than polyacrylonitrile, but less than glass fiber when fiber content was 0.3 wt%. The unreinforced CTB samples were mainly parallel to the axial tensile cracks and a small amount of shear cracks, while the deformation of fiber-reinforced CTB was relatively large, it did not break into small pieces. The crack resistance of fiber-reinforced specimens was notably better than that of unreinforced ones. Failure mechanism and modes of fiber addition in CTB samples were also studied. As a result, this study could provide a theoretical/application basis for mines to reduce cement usage within the CTB matrix, achieve safe mine production, increase ore extraction and reduce ore losses and dilution.

Determination of Holmquist–Johnson–Cook Constitutive Parameters of Coal: Laboratory Study and Numerical Simulation

The main sensitivity parameters of the Holmquist–Johnson–Cook constitutive model for coal were obtained from a variety of tests such as uniaxial compression, uniaxial cyclic loading, splitting and triaxial compression tests, as well as the indirect derivation equation of a briquette. The mechanical properties of briquettes under dynamic impact were investigated using a split Hopkinson pressure bar experiment. Based on the experimental measurement of the Holmquist–Johnson–Cook constitutive model, the numerical simulation of briquette was performed using ANSYS/LS-DYNA software. A comparison between experimental and simulation results verified the correctness of simulation parameters. This research concluded that the failure of briquette at different impact velocities started from an axial crack in the middle of the coal body, and the sample was swollen to some extent. By the increase of impact velocity, the severity of damage in the coal body was increased, while the size of the coal block was decreased. Moreover, there was good compliance between experimental and simulated stress wave curves in terms of coal sample failure and fracture morphology at different speeds. Finally, the parameters of the validated Holmquist–Johnson–Cook constitutive model were applied to the numerical simulation model of the impact damage of heading face and the process of coal seam damage in the roadway was visually displayed. The obtained results showed that the Holmquist–Johnson–Cook constitutive model parameters suitable for the prominent coal body were of great significance for the improvement and exploration of the occurrence mechanism of coal and rock dynamic disasters.

An experimental investigation of subcooled choked flow in actual steam generator tube cracks

The work presented here is an experimental investigation of the two-phase choked flow of initially subcooled water through a stress corrosion crack on the steam generator tube. The study was designed to provide an extensive experimental data set for predicting leak flow rates of nuclear reactor coolant using actual tube steam generator tubes sample. Five samples of steam generator tubes with stress corrosion axial cracks taken from steam generators of the US pressurized water reactor and Canadian pressurized heavy water reactor (CANDU) were used in this study. Wall thicknesses of the samples vary from 1.1 mm to 1.3 mm with the crack channel length to hydraulic dimeter ratios from 3 to 15. The experiment was carried out for various stagnation pressures up to 6.89 MPa and liquid subcoolings ranging from 5 °C to 70 °C. Parametric dependence of choked flow rate on stagnation pressure, crack channel length to hydraulic diameter ratio, and degree of liquid subcooling was demonstrated for different crack geometries. All the parameters are shown to have significant effects on the choking mass flux. The experimental results indicate that the channel length strongly affects the choking mass flux as well as the correlation between this parameter and the stagnation pressure. These data are unique as they were obtained with actual steam generator tube cracks and hence are advantageous in understanding the choked flow in the very short channel length channel and similar flow geometries.

Detection of cracks with arbitrary orientations in a metal pipe using linearly-polarized circular TE11 mode microwaves

This study proposed a long-range non-destructive pipe inspection approach using linearly-polarized circular TE11 mode microwaves, enabling rapid detection of both axial and circumferential cracks on the inner surface of a metallic pipe. The applicability of the TE11 mode microwaves for crack detection was discussed in the light of the currents induced on the inner surface of a pipe due to the propagation of the TE11 mode microwaves. A dual-bend structured mode converter was designed to propagate linearly-polarized TE11 mode microwaves inside a pipe with a certain inner diameter, based on the theoretical and numerical analysis of mode conversion due to a bend. A 3.5 GHz working bandwidth of the mode converter was achieved with a conversion efficiency of the TE11 mode of over 90%. Experimental verification was conducted using stainless-steel pipes with an inner diameter of 23 mm and a total length of 7 m. An axial and a circumferential slit were introduced to simulate cracks. The experimental results confirmed reflections from both the axial and circumferential slits, indicating that the TE11 mode microwaves are applicable to crack detection. The results also confirmed the correlation between TE11 mode polarization and its detection sensitivity toward slits with different orientations along the circumferential direction. Furthermore, the axial and circumferential slits situated at different longitudinal and circumferential locations in the pipe were successfully detected by orthogonally deploying the mode converter to change the polarization of the TE11 mode by 90°.