Crack Direction Research Articles

Damage identification of CFRP laminate based on finite element analysis and FBG sensor network

Carbon fiber reinforced polymer (CFRP) is broadly used in the defense and aerospace industries due to its superior performance, and the real-time monitoring of its structural damage can enhance the reliability and safety of utilization. This paper is aimed at collecting the surface strain of CFRP laminates in real-time using FBG sensor network and constructing a structural damage identification model based on strain characteristics to identify the location of hole damage and the length and direction of crack damage. To investigate the impact of damage on the surface strain of CFRP laminates, this research conducted an extensive finite element analysis of laminates without damage and with various degrees of damage, combined with the genetic algorithm and linear regression to identify and optimize the parameters in the damage identification model. Then two CFRP laminates with comparable properties were fabricated, and the FBG sensor network was arranged on the surface to verify the efficiency of the damage identification model through experiments. Results of experiments show that the damage identification model proposed in this paper can accomplish accurate identification of hole damage location and crack damage length and direction within the error range, also proving the feasibility and effectiveness of combining finite element analysis, FBG sensor network, and intelligent algorithm to establish CFRP structural damage identification system proposed in this paper.

An Experimental Study on Mechanical Properties and Fracture Characteristics of Saturated Concrete under Coupling Effect of Low Temperature and Dynamic Load

Concrete is widely used in bridge foundation, water supply, and drainage engineering. On the one hand, the saturated concrete is always in the saturated state. In the cold winter, northeast China and the alpine region suffer from freezing disaster. On the other hand, it has to continue to bear the dynamic load action of vehicles and running water, which makes the stress state of saturated concrete more complicated under the coupling action of low temperature and dynamic load. In order to study the mechanical properties and fracture characteristics of saturated concrete under the coupling effect of low temperature and dynamic load, the impact compression tests of concrete under normal temperature 20°C, -5°C, -10°C, and -15°C were carried out with a diameter of 74 mm split Hopkinson pressure bar (SHPB). The stress-strain characteristics, energy dissipation, and failure modes of specimens under different low temperatures were studied. From a detailed point of view, the failure mechanism of low-temperature water-saturated concrete is expounded. The results show that under the same dynamic load, the dynamic stress-strain curve of saturated concrete changes obviously with the change of low temperature. The dynamic compressive strength of the natural specimen at room temperature is high while that of the water-saturated specimen is low, and the dynamic compressive strength is opposite at low temperature. At the same temperature, the energy time-history curves of concrete in the saturated state are different from those in the natural state, mainly in the plastic section. The energy time-history curves of saturated concrete are different at different temperatures, and the energy dissipation rate of saturated concrete increases linearly with the decrease of temperature. Under the experimental conditions, the dynamic strength of saturated concrete increases linearly with the decrease of temperature, and the peak strain of saturated concrete decreases linearly with the increase of temperature. With the decrease of temperature, the fragmentation of saturated concrete under the impact of the same air pressure gradually increases, and the integrity of the specimen gradually improves. Low temperature can improve the impact resistance of saturated concrete, which is consistent with the failure law of natural state concrete. The water-saturated low-temperature state of the concrete void is filled with ice crystal particles; for low-temperature water-saturated concrete in the impact of the dynamic load, the microstructure is affected by the ice crystal structure which is not easy to change; the specimen along the axial force direction microdefect development produces a crack, the crack along the parallel to the pressure direction of cracking, through the two ends of the specimen, and finally produces axial splitting tensile damage. The research results have important theoretical significance for the safety design of low temperature saturated concrete structures.

Fine Characterization Method of Concrete Internal Cracks Based on Borehole Optical Imaging

The internal cracks of concrete are very important in the safety evaluation of structures, but there is a lack of fine characterization methods at present. Borehole cameras are a piece of in situ borehole detection technology which can measure the structural elements of a borehole wall with high precision. In this paper, borehole camera technology is used to measure the concrete cracks of a tunnel floor, and the morphological characteristics (depth, width, and orientation) of the cracks are analyzed. The results show that the average extension depth of the crack extending from the orifice exceeds 1.195 m, and the width decreases with the increase in depth. The crack orientation is basically stable, with the maximum deviation of 19° at the orifice of different boreholes and 30° at different depths of the same borehole. The crack inside the concrete (not extending to the orifice) usually has a small extension depth and a relatively stable width, but the crack orientation changes greatly. The coarse aggregate and concrete interface have different effects on the extension direction of cracks. This paper also conducted a second measurement on two of the boreholes after an interval of 15 days, and found the difference in crack development in the two measurements. The work of this paper provides a new attempt for the detection and monitoring of concrete crack morphology.

A Direction-Sensitive Microwave Sensor for Metal Crack Detection

For metal crack nondestructive detection, most conventional crack sensors are unable to realize the crack direction detection. In this work, a direction-sensitive microwave sensor is proposed for metal crack detection. The proposed sensor consists of a rectangular patch resonator and two perpendicular coupled feeding ports, which improves the current distribution on patch surface and the sensitivity for crack direction detection. The performances of the proposed sensor are verified by simulation and measurement experiments. The results show that the width sensitivities of two feeding ports are 100 MHz/mm and 63.3 MHz/mm, respectively, and the sensitivity of the sensor for crack direction detection are 6.10 MHz/5 degrees and 1.93 MHz/5 degrees, respectively. Due to the advantages of a simple structure, low profile, large coverage area and high sensitivity, the proposed sensor has a great application potential in nondestructive detection fields.

Effect of intermetallic compounds distribution on bonding strength of steel–aluminum laminates

In this study, steel–Al laminates are prepared via hot-roll bonding and annealed for various durations to generate intermetallic compounds (IMCs) with different distributions at the interface. The fracture forms of the laminates with different IMCs distributions are analyzed via in-situ tensile–shear experiments, and the shear stress distribution in the regions adjacent to the bonding interface during the tensile–shear process is investigated via finite element modeling using Abaqus. Results show that granular IMCs can change the direction of cracks and limit crack expansion during the tensile–shear process. The location of the maximum shear stress shifts from the interface to the aluminum matrix, causing fracture to be generated in the Al matrix. When an approximate IMC layer is formed at the bonding interface, the maximum shear stress occurs at the IMC–Al interface. Additionally, Kirkendall voids, which are the source of cracks, are observed at the IMCs–Al interface. They expand along the IMC–Al interface during the tensile–shear process, thus reducing the bonding strength. This study shows that granular IMCs are beneficial for improving the bonding strength, and that the bonding strength begins to deteriorate when the IMCs accumulate to form a layer.

Estimation of Depth Direction of Crack by measuring the Residual Magnetic Vector Flux Density around Grains

We suggests an algorithm to estimate the depth direction of fatigue cracks. This algorithm uses the distribution of residual magnetic vector flux density (RMVFD) on the specimen. Magnetic domains are the region with a magnetic moment in the same direction. A grain has several magnetic domains. Domain wall is the boundary of the magnetic domain. Magnetic energy stabilized when no external magnetic field applied. That is, the sum of the total magnetic moments is close to zero. It is because that the magnetic flux densities are canceling each other at both ends of the domain wall. The domain wall moves when an external magnetic field applied. As a result, the magnetic domain, which has the same direction with the external magnetic field, enlarged. This phenomenon is magnetization. The domain wall stops moving when it meets with factors that limit the domain wall movement (FLWM). Here FLWM are dislocations, defects, and crystal boundaries. The magnetic energy is unstable at the end of FLWM in the magnetic hysteresis phenomenon. Other end of FLWM has to have the opposite direction of RMVFD to stabilize the magnetic energy. Thus, there are opposite direction of RMVFD each other around crack or grain boundaries. Three dimensional RMVFD (3-RMVFD) can indicates crack existence and depth direction of crack. To verify this theorem, we prepared fatigue crack specimens with different depth direction. The three dimensional shapes of grains reconstructs at the crack. The RMVFD measured by scanning a 3-axis magnetic sensor. And the 3-RMVFD indicated in each grains around crack. The depth directions of crack and grains could estimated by using the 3-RMVFD.

Study on the Mechanical and Toughness Behavior of Epoxy Nano-Composites with Zero-Dimensional and Two-Dimensional Nano-Fillers.

The mechanical properties of epoxy resin can be enhanced by adding nanofillers into its matrix. This study researches and compares the impacts of adding nanofillers with different dimensions, including two-dimensional boron nitride and zero-dimensional silica, on the mechanical and toughness properties of epoxy resin. At low fractions (0–2.0 wt%), 2DBN/epoxy composites have a higher Young’s modulus, fracture toughness and critical strain energy release rate compared to SiO2/epoxy composites. However, the workability deteriorated drastically for BN/epoxy composites above a specific nanofiller concentration (2.0–3.0 wt%). BN prevents crack growth by drawing and bridging. SiO2 enhances performance by deflecting the crack direction and forming voids. Additionally, the dimension and content of nanofiller also influence glass transition temperature and storage modulus significantly.

A mechanism-based simulation algorithm for crack propagation in non-uniform geomaterials

Cracks widely exist in various geomaterials. They can weaken the overall mechanical performance and increase the permeability, and therefore remarkably affect the safety of a practical project. A novel algorithm to simulate crack propagation was proposed in this study. The location and propagation direction of cracks were determined by the fracture criterion and strength parameters of geomaterials. The randomness of the crack network was assumed to result from the spatial variability of the strength parameter; the latter is described as a random field. In addition, Minkowski densities were adopted to quantify the morphology of the simulated crack networks, including their area, length and connectivity. The proposed simulation algorithm was compatible with the finite element method; as a result, it can be readily imported into finite element method software for numerical modelling, which was exemplified by an elastic–plastic problem. The proposed simulation algorithm shall be of practical value in dealing with geomaterials with cracks in practice.

Multiaxial fatigue of forged Inconel 718 at room and high temperature

This work investigates the effects of high temperature on the fatigue behavior of forged Inconel 718 under axial/torsional loading conditions. To this end, strain-controlled tension, torsion, and proportional fatigue tests were performed at 20 °C and 450 °C on thin-walled tubular specimens extracted from an as-forged turbine disk. All tests were conducted with axial and shear strain ratios equal to zero, and the applied equivalent von Mises strain amplitude was such to result in fatigue lives within the range of 104 to 106 cycles, which is the typical fatigue life of the component of interest in this study. It was observed that for a given loading condition and prescribed equivalent strain amplitude, the fatigue lives of tests at 450 °C were longer than the room temperature tests. Moreover, the longest lives were observed in the torsion tests, followed by the proportional and tension tests in all temperature and prescribed equivalent strain amplitude tested. The capability of the cyclic plasticity model proposed by Chaboche in describing the cyclic response of Inconel 718 was also addressed. This model successfully simulated the stress–strain behavior of this material under all loading conditions, and for both temperatures investigated. In addition, the cracking behavior of the material was examined. For both temperatures, the tension tests failed by Mode I tensile cracks, while Mode II shear cracks were observed in the torsion and proportional tests. After a scanning electron microscope analysis, it was concluded that the transgranular propagation mode was the predominant failure mechanism of all tested conditions. The fatigue life of Inconel 718 was predicted by the Fatemi–Socie (FS) and the multiaxial version of the Smith–Watson–Topper (SWT) fatigue models, resulting in estimations within a factor-of-two and a factor-of-four, respectively. Moreover, crack directions were also determined by the critical plane models and compared with the orientation of the observed cracks. The SWT model correctly predicted the crack direction of the tension tests, while the FS model was more accurate in determining the orientation of the cracks seen in the torsion tests. None of the two models was able to correctly predict the angles of the critical plane in all tested loading conditions.

Hydraulic fracture initiation for perforated wellbore coupled with the effect of fluid seepage

Hydraulic fracturing technology is one of the most effective well stimulation techniques for low-permeability reservoirs, however, the mechanism of the fracture initiation and propagation is not fully understood. Based on Biot’s consolidation theory and linear elasticity theory, this paper develops a general analytical model to investigate the hydraulic fracture initiation from arbitrarily spiral perforation or oriented perforation coupled with the effect of fluid seepage. The analytical solution can be used to determine formation breakdown pressure, the location and direction of an initial crack, and to investigate the factors that affect the formation breakdown pressure and the fracture initiation position. The calculation results show that the direction of fracture initiation does not always align with the direction of the maximum horizontal principal stress and may deviate from the 0° direction of the perforation circumferential angle to the 90° direction. The seepage effect of fracturing fluid can significantly reduce the formation breakdown pressure and increase the possibility of fracture initiation from the tip of perforation. The smaller the formation depth and transfer factor (TF) are, the more favorable the fracture initiation at the tip of the perforation is. With the increase of the Biot coefficient, the formation breakdown pressure decreases significantly. To avoid the failure of cement sheath induced by the fracture initiation from the bottom of perforation, the low-viscosity fracturing fluid with a stronger seepage effect and the casings with harder materials and lower transfer coefficient are recommended.

Can we obtain the internal information of a surface crack from Rayleigh waves?

In this study, the Rayleigh wave responses of surface cracks with irregular planes are investigated utilizing novel numerical approaches. The effects of the equivalent width, filling situation and geometrical properties of the crack on the wave propagation are analyzed based on the scattered surface wave signals. The scattering process of the source due to an irregular surface crack is implemented by the staggered grid finite difference method (SG-FDM). The complex boundary conditions on the crack surface are classified by a modified stress image method (SIM) with a series of cells. A well comparison result is presented between SG-FDM and the finite element method. Three typical models are designed to analyze the effects of the crack width and filling material on Rayleigh waves. Furthermore, we focus on the influences of the effective width spatial variation characteristics of an irregular crack on Rayleigh wave propagation with different crack depth-wavelength ratios. The energy distribution of the scattered Rayleigh waves depends on the crack average width and the integrity of the filling material after damage repair. Our study reveals the specific wave phenomena (e.g., crack width-dependent and filling condition-dependent scattered waves) influenced by the crack interior structures, and this provides a method for detecting the subsurface structures (e.g., variation of crack width along the depth direction) of a breaking crack and evaluating the repair quality regarding local defects.

Micromechanical simulations and experimental characteristics of randomly distributed carbon nanotubes reinforced Mg matrix composites

In this study, finite element analysis was performed using representative volume elements (RVE). The cohesive zone model was implemented to govern the failure behavior of the CNTs/matrix interface. The RVE to understand the micromechanical response and damage behavior of randomly distributed carbon nanotubes (CNTs)/Mg matrix composites (MMCs). The cohesive zone model was implemented to govern the failure behavior of the CNTs/Mg interface. The RVE for macroscopic structural analysis was strained under assumed boundary conditions, where it provided a stress-strain curve for the composite. The simulation data can achieve excellent agreement with the experimental results. The simulations and experimental results indicate that the initial damage in the composite occurs in the vicinity of CNTs. The stress concentration at the CNT end increases with the increasing tensile load, resulting in interfacial debonding, the main form of crack growth of the CNTs/Mg composite. The CNTs can passivate the crack tip and change the propagation direction of the main crack, causing a delay in fracture. CNTs played an important role in governing the fracture pattern of composite. The pullout and bridging of CNTs were the main strengthening mechanisms of the composites, which was beneficial for improving strength and toughness.

Simulation of crack growth for metal plates with holes based on cohesive zone model

Hole defects are very common in metal plates. The propagation of cracks in engineering structural members will be affected by the holes of the members, which will not only change the propagation direction of cracks, but also have the ability to limit the growth of cracks. Therefore, the study of material fracture crack propagation caused by the distribution of holes and other factors under loading conditions has become a problem that needs to be considered in structural design. The cohesive zone model (CZM) can effectively avoid the singularity problem of crack tip stress when simulating crack growth, and at the same time can clearly show the crack growth path. Based on exponential CZM, the finite element analysis of tensile test for metallic materials with hole defects was carried out. The crack propagation law under constant load was obtained. It was proved that CZM could simulate the crack propagation in metallic materials with holes more accurately. The influence of the pore size, shape, distribution on the crack propagation is discussed. The results show that the smaller the distance difference between holes, the greater the supporting reaction force. The smaller the radius of the hole, the higher the fracture toughness and the maximum supporting reaction force of the metal plate. And it also can be seen that as the number of holes increases, the maximum supporting reaction force and fracture toughness of the metal plate decrease. The use of staggered holes distribution has greater supporting force and better toughness. The research results of this paper provide a theoretical basis and reference for investigating the propagation and propagation of cracks in the process of damage and failure of materials and for structural design and performance evaluation of engineering materials.

Longitudinal graded porous titanium fabricated by spark plasma sintering: Design of layers, direction of crack path

The design of layers and fabrication methods strongly affect the stress bearing capacity and crack propagation in functionally graded porous titanium (FGPT). Thus, in this study, uniform and functionally graded porous titanium (in three and five-layer structures) samples were fabricated using spark plasma sintering at 440 °C and 50%vol. space holder (NaCl). Compression results proved that selecting the highest NaCl in the center of five-layered samples, besides the smoothest porosity gradient, increases energy absorption up to ≅360 % compared with the uniform foam. The architectural design, along with the energy absorption, affects the crack direction and fracture pattern remarkably. Obviously, two types of cracks path, i.e., vertical and oblique, were detected in uniform and different graded samples, which the latter direction highlighted the Griffith rupture mechanism. Meanwhile, although trumpet-like and crushed/disc fracture patterns perceived in layered structures, promptly fragmented patterns were dominated in the uniform porous sample. This work provides an appropriate method with a suitable layers arrangement to fabricate FGPTs with incredible energy absorption due to crack arresting/deflecting and homogenizing strut thickness.

Directions of high cycle fatigue cracks emanating from circular notches studied by optical profilometry

Current models for predicting the fatigue endurance of notched solids use the stresses along a straight line, beginning at the notch root, as a simplification of the real crack propagation path. In this work, the experimental crack paths for hollow notched samples were analysed through different microscopy techniques, with the objective of establishing high cycle fatigue crack growth directions in a mild steel. Fully reversed tension–compression fatigue tests (R=−1) of thin-walled tube specimens with a passing-through hole were carried out. The crack paths observed in the outer cylindrical surface were studied in each case, with special attention to the crack initiation point and the crack direction along the first grains. Moreover, the analysis of the fracture surfaces allowed the same analysis to be performed to determine the internal crack paths. It was observed that the crack initiation point was close to the maximum principal stress point at the hole contour as obtained from linear elastic finite element analysis, and the crack direction in its initiation was generally close to Mode I direction, contrary to the conventionally accepted 45∘ crack growth direction.

Experimental Study on the Dynamic Response and Pore Structure Evolution of Coal under High-Pressure Air Blasting.

High-pressure air blasting (HPAB) is one of the main feasible technologies to improve the extraction efficiency of unconventional gases. At present, there are few visual studies on the evolution characteristics of pore structure in coal under HPAB, resulting in an unclear understanding of the mesoscopic damage evolution mechanism of coal under HPAB. To study the dynamic response and mesoporous structure evolution characteristics of coal under HPAB, simulated coal specimens were used in HPAB experiments. The pore structure characteristics of coal at different locations away from the blasthole after HPAB were analyzed by using computed tomography scanning and 3D reconstruction technology. The maximum sphere algorithm was used to study the law of pore connectivity and reveal the mesoscopic damage evolution mechanism of coal under HPAB. The results indicate that the stress wave and attenuation and the crack propagation direction are greatly affected by the confining pressure. Compared without confining stress, the radial strain attenuation index decreases by 11.97% and the lateral strain attenuation index increases by 15.36% under confining pressure. Without confining pressure, the crack direction is disordered. On the contrary, the crack expands along the σ1 and σ2 directions with confining pressure, while the expansion along other directions is inhibited. The stress wave has a great influence on the pore structure in the nearby zone. Compared with before HPAB, at 25 mm distance from the blasthole, the number of pores increased by 24.80%, the number of throats increased by 12.96 times, the maximum equivalent radius of throats increased by 52.15%, and the maximum channel length of the throat increased by 56.06%. With the increase of the distance, the stress wave has little influence on the pore structure in the middle and far zones. The porosity of representative elementary volume and the distance from the blasthole decay in a power function trend. The maximum disturbance distance under HPAB can reach nearly 110 times of the blasthole radius. The study results provide a theoretical basis for enhancing the coal seam permeability and gas drainage of low-permeability coal seam by HPAB.

Study the internal cracks effect on vibration of laminated composite square plates

The study of cracks behaviour in a composite plate is of significant importance in the dynamics of the Mechanical parts in order to avoid design failures due to resonance or high amplitude vibrations. In this paper, a square glass-epoxy composite plate is adopted. The plate has four layers with symmetric and asymmetric lamination. Assuming the cracks are profound as defects. The results were obtained by using a numerical solution of mechanical APDL from ANSYS. It has been found for different boundary conditions that the rank of natural frequencies is decreased by increasing the crack ratio due to the reduction of the plate’s stiffness, whereas the crack direction has no mentioned effect for a small angle of rotation. The accuracy of results is verified by comparing a single case of the current work with other previous investigations. Evaluate the influence of the crack length ratio, angle of the crack rotation, boundary conditions and lamination angles on the natural frequencies of the square composite plate with glass-epoxy materials.

A Study on the Freeze–Thaw Damage and Deterioration Mechanism of Slope Rock Mass Based on Model Testing and Numerical Simulation

Engineering rock mass in cold regions has obvious freeze–thaw damage as a result of extreme differences in temperature, rainfall, snow, and other factors, which is one of the main causes of frequent geological disasters. Therefore, it is important to investigate the deterioration mechanism and evolution laws of rock mass freeze–thaw damage. Considering a hydropower station’s left bank slope in a cold region, model testing and numerical testing of slope rock mass failure under freeze–thaw conditions are here carried out by developing a generalized model. The results reveal that heat is transmitted from the outside to the inside of the slope and that the rate of temperature change varies with depth; the frost-heave force causes tensile cracks in the rock mass, with crack propagation taking the form of a circular arc; the presence of an original structural plane influences the propagation direction of the frost-heave crack, whereas the freezing rate of the fissure water influences the amplitude and growth rate of the frost-heave force. Additionally, a novel method of measuring frost-heave force is proposed. The largest frost-heave force caused by water–ice phase change is 19.3 kPa, which is equivalent to 3.17 MPa of the actual slope.

Multiscale Pore Structure Characteristics and Crack Propagation Behavior of Coal Samples from High Gas Seam.

Investigation on the pore-fracture features and crack propagation behavior of coal is necessary to prevent coal mine disasters. The pore structure features of coal samples taken from high gas seam were obtained by mercury injection porosimetry (MIP) and gas adsorption methods. The process of deformation and failure for coal samples under three-point bending conditions were obtained. The results demonstrate that the adsorption pores with diameter less than 100 nm are the most developed and their surfaces are the roughest (the average surface fractal dimension Ds is 2.933). The surface of micro-cracks is smoother (Ds is 2.481), which is conducive to gas seepage. It may be the explanation for that 14-3# coal seam is a high gas seam, while there was almost no gas outburst accident so far. At the initial stage of crack propagation, the main crack on the coal sample expanded along the direction of the natural cracks. In the process of crack propagation, the surface fractal dimension of the main crack increased, suggesting that the bending degree of the main crack enhanced. The brittle characteristics of coal samples can be reflected by the ratio of the dissipated energy to the accumulated energy.

Study on Freezing-Thawing Failure Process of Natural Damage Sandstone Based on Cellular Automata

Natural damage such as internal cracks, pores, and discontinuous surfaces of rocks induces the formation, expansion, and interaction of mesofractures until macroscopic failure under the action of freeze-thaw and load. The failure process is the evolution of self-organization and has localization characteristics. The CT scanning test of sandstone with different freezing and thawing cycles was carried out. According to the theory of cellular automata analysis, combined with CT image processing technology, the numerical model of the mesostructure of the sandstone constructed with natural damage, and the numerical simulation test of the Brazilian splitting failure of natural damage sandstone was completed under freezing and thawing. The results show that the combination of CT image processing technology and CASRock numerical simulation software can dynamically describe the whole process of initial damage propagation, new crack generation, and penetration of rock containing natural damage under tension in a freeze-thaw environment. Due to random and disordered natural damage inside the rock, gradually changing to an orderly state under freezing and load, the rock shows discontinuous deformation after the plastic deformation reaches a certain degree and gathers at the initial damage expansion. The fracture process of natural damaged rock under freezing and load has continuous-discontinuous deformation characteristics. The existence of initial rock damage causes the rock’s localized deformation and failure characteristics. The form and size of the initial damage determine the direction of development of the surface of the sandstone fracture and the coalescence of the crack. The initial damage area of sandstone is expanded due to the freeze-thaw cycle. After loading, the local deformation and failure characteristics caused by initial damage have delay characteristics for penetration of the main cracks in the sandstone, which determines the formation and development direction of secondary cracks in the rock failure process.