Crack Shape Research Articles

Predicting characteristics of cracks in concrete structure using convolutional neural network and image processing

The degradation of infrastructures such as bridges, highways, buildings, and dams has been accelerated due to environmental and loading consequences. The most popular method for inspecting existing concrete structures has been visual inspection. Inspectors assess defects visually based on their engineering expertise, competence, and experience. This method, however, is subjective, tiresome, inefficient, and constrained by the requirement for access to multiple components of complex structures. The angle, width, and length of the crack allow us to figure out the cause of the propagation and extent of the damage, and rehabilitation can be suggested based on them. This research proposes an algorithm based on a pre-trained convolutional neural network (CNN) and image processing (IP) to obtain the crack angle, width, endpoint length, and actual path length in a concrete structure. The results show low relative errors of 2.19%, 14.88%, and 1.11%, respectively for the crack angle, width, and endpoint length from the CNN and IP methods developed in this research. The actual path length is found to be 14.69% greater than the crack endpoint length. When calculating the crack length, it is crucial to consider its irregular shape and the likelihood that its actual path length will be greater than the direct distance between the endpoints. This study suggests measurement methods that precisely consider the crack shape to estimate its actual path length.

Effects of angles and shapes of a corner crack on the driving force at a set-in nozzle-cylinder in a PWR pressure vessel

Corner cracks are one of the common forms of flaws in the reactor pressure vessels (RPVs) of nuclear power plants (NPPs). Accurate evaluation of the driving force of such corner cracks is essential and critical for the efficient design and integrity assessment of the RPVs. In this study, the combined effect of the depth, length and angle of a given corner crack under the service loading was analyzed using elastic–plastic finite element analysis (EPFEA). Based on the consideration of the distribution profiles of stress intensity factor (SIF) and J integral as the crack growth along corner crack front, the crack growth stability with various crack shapes and angles was analyzed. Finite element analysis (FEA) results showed that the crack growth driving force is affected by the shape and angle of the initial crack. It is also found that the sliding mode (mode II) crack or tearing mode (mode III) crack should be taken into account in the structural integrity analysis of RPVs.

Stress intensity factor by combined perturbation method and universal weight functions

The stress intensity factors (SIF) of three-dimensional cracks are known exactly only for a very limited number of shapes and loading scenarios. In this work, the perturbation method is combined to the universal weight function approach to estimate the mode I, stress intensity factor of cracks with crack fronts obtained as perturbations of any known, reference crack shape. This work is limited to cracks and loading scenarios with two planes of symmetry for which the universal weight function could be established. The novel approach is illustrated for a quasi-square crack and long, flexural fatigue cracks in brittle plates. The accuracy and limitations of the proposed method are also discussed. The proposed approach provides the means to approximate the SIF for a variety of shapes and loads without relying on FEM. Furthermore, the proposed approximation is the only method that can accurately approximate the SIF’s for cracks with large aspect ratios.

Portray rail head surface crack 3D contour through image analysis and morphology reconstruction

Rail head spalling and fractures are strongly correlated with fatigue crack propagation. Fatigue cracks typically form along evolving surfaces, but their propagation paths are not accurately described by idealized semi-elliptical crack morphology. This study aims to capture the true three-dimensional shape of fatigue cracks during initiation and propagation. New mathematical and physical models, based on reconstructed 3D cracks, are proposed to predict rail crack propagation at various service stages. The analysis reveals that the crack propagation path in the 2D lateral rail section approximates a quadratic parabola or circular curve, and the 3D crack morphology is similar to a partial spherical surface with a curvature radius of approximately 18.5 mm. The three-dimensional crack morphology remains consistent within a small rail range. The reconstruction model proposed in this study can predict internal crack propagation on the rail head surface, assisting in track inspection, maintenance, and repair.

An Inverse Perspective Mapping-Based Approach for Generating Panoramic Images of Pipe Inner Surfaces

We propose an algorithm for generating a panoramic image of a pipe’s inner surface based on inverse perspective mapping (IPM). The objective of this study is to generate a panoramic image of the entire inner surface of a pipe for efficient crack detection, without relying on high-performance capturing equipment. Frontal images taken while passing through the pipe were converted to images of the inner surface of the pipe using IPM. We derived a generalized IPM formula that considers the slope of the image plane to correct the image distortion caused by the tilt of the plane; this IPM formula was derived based on the vanishing point of the perspective image, which was detected using optical flow techniques. Finally, the multiple transformed images with overlapping areas were combined via image stitching to create a panoramic image of the inner pipe surface. To validate our proposed algorithm, we restored images of pipe inner surfaces using a 3D pipe model and used these images for crack detection. The resulting panoramic image of the internal pipe surface accurately demonstrated the positions and shapes of cracks, highlighting its potential utility for crack detection using visual inspection or image-processing techniques.

Crack Identification by Digital Image Correlation Method Using Crack Shape as Prior Information

Abstract A new crack identification method that estimates the cracks in invisible locations based on the surface deformation measured by digital image correlation (DIC) is developed. An inverse problem is setup to estimate such invisible cracks from surface deformations. The inverse problem has an ill-condition because of noise contained in surface deformations. Our proposed regularization method uses prior information and Expectation a Posteriori (EAP) estimation. Prior information includes candidate crack shapes and surface deformations due to cracks. The candidate crack shapes are created by determining a crack's starting point and propagating it based on the force at its perimeter (ligament). A prior distribution is the surface deformations due to the candidate crack shapes. The likelihood distribution is a surface deformation measured by the DIC method. A posterior distribution is defined from the prior and likelihood distributions. In this study, the estimated result is the expected value of the posterior distribution. The validation test was performed, and the result shows that the proposed method superior to the conventional L1-norm regularization method.

Temporal and spatial distribution characteristics of hydraulic crack propagation under true triaxial loading using the direct current method

Hydraulic fracturing (HF) has been widely used in coal and rock dynamic disasters prevention and control and coalbed methane production in underground coal mines. However, how to effectively and accurately monitor and evaluate the hydraulic crack propagation process and impact range has not been resolved. We developed a novel experimental system for HF-direct current (DC) monitoring under true triaxial loading conditions. With the use of this system, we conducted HF-DC monitoring experiments under true triaxial loading conditions. The response characteristics of the emission current, apparent resistivity, and apparent resistivity difference contour map throughout the entire experimental process were studied. The results showed that the DC monitoring data contain a wealth of effective information on hydraulic cracks propagation. The emission current and apparent resistivity reflected the overall electrical conductivity of the rock. The contour map of the apparent resistivity difference between two measuring lines could accurately characterize the propagation process and spatial distribution of hydraulic cracks. The evolution process of the difference contour map showed that hydraulic cracks were more likely to spread into areas with low homogeneity. The change range and degree of the apparent resistivity were mainly affected by the spatial distribution, water-bearing state of the hydraulic cracks and wet area surrounding the hydraulic cracks. As the hydraulic crack network expanded, a highly conductive channels were formed inside the rock, which could affect the rock conductive properties and patterns. By comprehensively considering the temporal and spatial evolution characteristics of the apparent resistivity difference contour map and the regional apparent resistivity change curve, the dynamic expansion and spatial shape of hydraulic cracks in the rock could be accurately characterized via the DC monitoring method.

Experimental Study on Mechanical Properties of Different Resins Used in Oral Environments.

Background and Objectives: Acrylic resins remain the materials of choice for removable prosthesis due to their indisputable qualities. The continuous evolution in the field of dental materials offers practitioners today a multitude of therapeutic options. With the development of digital technologies, including both subtractive and additive methods, workflow has been considerably reduced and the precision of prosthetic devices has increased. The superiority of prostheses made by digital methods compared to conventional prostheses is much debated in the literature. Our study's objective was to compare the mechanical and surface properties of three types of resins used in conventional, subtractive, and additive technologies and to determine the optimal material and the most appropriate technology to obtain removable dentures with the highest mechanical longevity over time. Materials and Methods: For the mechanical tests, 90 samples were fabricated using the conventional method (heat curing), CAD/CAM milling, and 3D printing technology. The samples were analyzed for hardness, roughness, and tensile tests, and the data were statistically compared using Stata 16.1 software (StataCorp, College Station, TX, USA). A finite element method was used to show the behavior of the experimental samples in terms of the crack shape and its direction of propagation. For this assessment the materials had to be designed inside simulation software that has similar mechanical properties to those used for obtaining specimens for tensile tests. Results: The results of this study suggested that CAD/CAM milled samples showed superior surface characteristics and mechanical properties, comparable with conventional heat-cured resin samples. The propagation direction predicted by the finite element analysis (FEA) software was similar to that observed in a real-life specimen subjected to a tensile test. Conclusions: Removable dentures made from heat-cured resins remain a clinically acceptable option due to their surface quality, mechanical properties, and affordability. Three-dimensional printing technology can be successfully used as a provisional or emergency therapeutic solution. CAD/CAM milled resins exhibit the best mechanical properties with great surface finishes compared to the other two processing methods.

Numerical study on the effect of microporous layer crack changes on water management in gas diffusion layer of proton exchange membrane fuel cell

The existence of the microporous layer (MPL) enhances the transport of reactive gas and liquid water in the gas diffusion layer (GDL), which is a crucial part of the proton exchange membrane fuel cell (PEMFC). However, it is still unclear how the shape and size of MPL surface cracks affect the transmission of liquid water in GDL. Therefore, a three-dimensional volume of fluid (VOF) model is developed to study the relationship between MPL crack and GDL liquid water transport. First, the random direction method is used to reconstruct the carbon fiber layer of GDL, which is in good agreement with the experimental data. Secondly, the morphological changes of MPL are analyzed, and two kinds of cracks with different topological shapes are proposed. The parameters of crack area, width and depth of the numerical model are designed according to the MPL porosity. Finally, the relationship between MPL crack variation and liquid water saturation/variation in GDL is investigated by numerical modeling. The results show that the shape, area, width and depth of MPL crack affect the water fraction in GDL. The impact of crack on the transfer path of liquid water in GDL is mainly in the early stage of water inflow. With the increase of water inflow, this impact is mainly related to the structure of GDL.

Experimental investigation of the shear behavior of steel fiber reinforced concrete slender beams without stirrups through crack kinematics and shear transfer mechanisms

AbstractThis paper aims to contribute to a more rational understanding of the shear strength behavior of steel fiber‐reinforced concrete beams without stirrups. For this purpose, an experimental program comprising eight slender beams varying the fiber content and type, tested in three‐point bending configuration with a complete monitoring of the displacement fields of the failure span, was carried out. A detailed analysis of the shape and kinematics of the critical crack, associated with the investigation of the shear transfer mechanisms during the test given by mechanical models available in the literature, was performed. The theoretical–experimental analyzes proved to be a promising approach for quantifying the contribution of each relevant parameter to the shear capacity, such as the fiber concrete residual tensile response. The study, therefore, presents a contribution toward a rational and comprehensive methodology for the shear design of concrete members. The influence of shape and kinematics of the critical crack on the shear response is also reported and a theoretical explanation is provided to justify the fact that beams with the same characteristics have different cracking patterns but similar shear strength.

Stability of multi-cracked FG plate on elastic foundations

Recently, the stability calculation of the functionally graded (FG) plate has attracted many scientists, especially when considering the FG plate with many cracks. In this work, the plate is made from a new generation composite material consisting of two components, ceramic and metal, with the law of continuous exponential material distribution. The plate is placed on a Winkler - Pasternak elastic foundation with two background parameters. Then, we used the third-order shear deformation plate theory to establish the dynamical equations. After applying Phase-Field theory to simulate the crack state, we applied the finite element method to solve the equations to find the critical force causing instability of the plate. Next, we investigated the influence of material index, number of cracks, crack length, crack shape as well as elastic foundation parameters on the plate stability. The results show that the crack length and elastic foundation parameter have the great influence on the stability of the FG plate. Especially, the elastic foundation with large shear coefficient, creating high stability for the plate. That is very meaningful in exploiting and using plate structure when the cracks appear.

Effect of repair welding on the fatigue behavior of AA6082-T6 T-joints in marine structures based on FFS and experiments

In this study, the effect of repair welding on the fatigue behavior of AA6082-T6 welded T-joints in marine structures was investigated based on experiments and the fitness-for-service (FFS) assessment in BS7910. Repair welding led to an increase in residual stress and a decline in fatigue life according to fatigue tests and FEA. Meanwhile, repair welding changed the crack initiation site (CIS) and crack shape factor (CSF, the ratio of crack depth a to length 2c; i.e., a/2c), as indicated by fatigue fractography. In addition, when FFS assessment was performed, two methods for characterizing the CSF under different repair welding lengths during fatigue crack growth were presented: fixed and non-fixed a/2c. The results showed that repair welding led to a transition from ductile fracture to brittle fracture. For a fixed CSF, the critical crack depth and crack opening area of T-joints with repair welding lengths of 30 mm were greatly reduced by 63–66% and 86–89%, respectively, compared to T-joints without repair welding. However, the corresponding reductions for the non-fixed a/2c method were approximately 40% and 90%. The comparison indicated the results obtained with a non-fixed CSF were more consistent with the experiments, with a deviation below 15%.

Fabricated of the SCC Reference Specimen for Increasing of In-Service Inspection Accuracy in Nuclear Power Plant

To achieve the safety assurance of the nuclear power plants, in-service inspection have were enforced periodic for nondestructive test in weldment. Among them, the ultrasonic evaluation is widely used for the measurement of the defects. Generally, non-destructive inspectors have calibrated a UT machine using the reference specimen in the artificial cracks such as EDM, mechanical notch. As the reflected ultrasonic signal of real crack was detected amplitude value 50% lower than artificial cracks. Therefore, domestic ultrasonic inspectors have a little experience in measuring actual cracks generated in the nuclear power plant field, such as SCC(Stress corrosion cracks, below SCC) and thermal fatigue cracking. Accordingly, in order to improve the inspection accuracy of non-destructive inspectors during operation inspection of nuclear power plant structures, it is necessary to have a lot of experience in inspecting actual cracks.In this study, non-penetration SCC for using in educational specimen of the NDT inspector were fabricated by using the simulated nuclear environment system in base metal of STS 304 pipe. The size of the used pipe were 8 inch(80 sch., thickness 12.7mm). The end of the pipe was used a flange to simulating a fixed environment of the nuclear power plant structure. To accelerate the fabrication of SCC, a corrosive environment was composed by used a blending corrosion solution of the NaOH 1M and Na2S9-H2O 0.1M inside the pipe. The potential of hydrogen of the caustic solution was 14. The fabricated SCC was confirmed for crack size and shape by used UT and RT.

Observation of Transition Area Origin Cracks on Fracture Surfaces of Carburized JIS SCM415 Bars

Under high cycle and very high cycle fatigue, high strength steels break as a result of internal fracture from inclusions. In order to understand this fracture, “Fisheye” crack has been investigated. In our previous work we found that cracks grew from the boundary between the hard surface and soft core of case-hardened S45C, SUJ2 and SCM415 steel bars under rotating bending fatigue. These cracks were called “Transition area origin (TRO)” cracks. In this study, we closely observed the fracture surface of TRO crack areas in carburized JIS SCM415 specimens (under 734, 776 and 865 MPa). We found three features of the TRO cracks: outside of the TRO cracks had asperities; the shapes of TRO cracks were almost circular, and were different from those in S45C and SUJ2 steels; and the HAZ-TRO area which was located at hardened layer had some ridges, and the Core-TRO area at unheat-treated layer was smooth and flat.

Three-Dimension Crack Propagation Behavior of Conical-Cylindrical Shell

The conical-cylindrical shell is prone to stress concentration in the convex cone position under the action of deep-sea pressures. This results in unidirectional or bidirectional positive tensile stresses on the surfaces of the shell. The conical-cylindrical shell is a large, welded structure. Welding residual stress was generated at the cone-column joint position, resulting in high-stress concentration at this location. Under both the residual stress of welding and seawater pressure, cracks easily form and propagate on the shell weld toe, leading to fatigue damage and even structural failure. In this paper, based on the seawater’s alternating load and the residual stress of welding, the three-dimensional crack propagation process was studied for the submarine conical-cylindrical shell. The effects of crack depth and shape ratio on crack propagation trend and fatigue life were analyzed. The results can provide references for predicting the crack propagation trend, assessing the remaining life and evaluating the structural safety of the submarine conical-cylindrical shell.

Exploring Exact Effects of Various Factors on Chloride Diffusion in Cracked Concrete: ABAQUS-Based Mesoscale Simulations

Chloride ion attack is a major cause of concrete durability problems, and existing studies have rarely addressed the effects of damage zones. In this paper, an improved mesoscale model including five phases was constructed using the finite element software ABAQUS to study the diffusivity of chloride ions in cracked concrete. It was found that the damage zone is negligible when the crack width is less than 50 μm, while the width and depth of the damage zone are about 15 times the crack width and 15% of the crack depth when the crack is greater than 50 μm. The results show that the diffusion of chloride is greatly influenced by the crack width, while it is little-influenced by the crack shape. Low water-cement ratio and adequate hydration of the concrete are also key factors affecting chloride diffusion. In contrast, regular rounded aggregates have a positive effect on reducing chloride diffusion compared to irregularly shaped aggregates, and this effect becomes weaker with increasing service time. In addition, the protective layer can effectively prevent the diffusion of chloride in concrete. Therefore, when designing marine concrete, efforts should be made to ensure that the concrete has a low water-cement ratio, adequate hydration, less cracking and a protective layer.

The role of intralaminar damages on the delamination evolution in laminated composite structures

Delamination is a major failure mode affecting laminated composite structures. This failure mechanism, if not properly monitored, can lead to uncontrolled cracks growth and premature structural collapse. Thus, predicting delamination propagation is mandatory to determine the structural integrity. At present, delamination has been extensively investigated in laminated composite structures but only a few studies have been performed on how the intralaminar damages influence the interfacial release energy and, consequently, the delamination evolution. In this paper, the well-established SMart-Time XB delamination simulation tool, has been coupled with a Hashin Criteria based User-Material Subroutine (UserMat) to study the role of the intralaminar damages in delamination propagation. A benchmark case based on the mixed-mode I/II Single Leg Bending (SLB) specimen for delamination evolution assessment under quasi-static loading has been investigated. Subsequently, a composite material plate, characterized by an artificial circular delamination, under compression has been considered. This study proved to be valuable by underlining the influence of fibre and matrix breakage on the interlaminar damages evolution. Interestingly, taking into account the effect of intralaminar flaws changes the local energy release rates values on the delamination front and, consequently, the crack shape and evolution.

Effects of Crack Formation on the Mechanical Properties of Bilayer Graphene: A Comparative Analysis

We present a molecular dynamics simulation study on the effects of crack formation on the mechanical properties of bilayer graphene. Bilayer graphene possesses unique electronic properties that can be modified by applying a voltage, making it an attractive material for various applications. We examined how the mechanical properties of bilayer graphene vary under various crack configurations and temperatures, measuring Young’s modulus, fracture toughness, fracture strain, and fracture stress. We compared the effect of crack presence on single and both layers and found the appearance of double peaks in the stress–strain curves in the case of a monolayer crack, indicating a subsequent fracture of the cracked layer and the uncracked layer. We also examined the effect of crack shape, size, and orientation on mechanical properties, including circular, hexagonal, and rectangular cracks along two axes. We found that both circular and hexagonal cracks had a smaller Young’s modulus and toughness than rectangular cracks, and the orientation of the crack had a significant impact on the mechanical properties, with a 2.5-times higher toughness for cracks with a length of 15Å. Additionally, we found that Young’s modulus decreases with increasing temperature in bilayer graphene with cracks on both layers. Our findings provide valuable insights into the potential applications of bilayer graphene in the design of advanced nanoscale electronic devices.

Stress corrosion of copper in sulfide solutions: Variations in pH‐buffer, strain rate, and temperature

AbstractCopper is the intended canister material for the disposal of spent nuclear fuel in Sweden. At repository depth the groundwater may contain dissolved sulfide. The main goal for this work is to study the tendency for stress corrosion of copper in sulfide solutions and examine the influence of various experimental parameters on stress corrosion. Slow strain rate testing was performed on copper test rods in solutions with 1.0 mM sulfide. The pH was kept near neutral with phosphate or borate buffer. The test matrix included variations in temperature, strain rate, and duration of the tests as well as salt and buffer concentrations. Cross‐sections of the specimens after testing were investigated using scanning electron microscope/energy dispersive X‐ray spectroscopy detector. Stress–strain curves do not reveal any signs of stress corrosion. However, intergranular corrosion in the shape of crack and pit‐like features developed in all tests with 1.0 mM sulfide. The length of the deepest features in all these tests was of the same order of magnitude (10–20 µm). The suggested mechanism proposes that crack‐like features originate at the surface of the copper metal from the oxidation of grain boundaries that behave as slightly less noble.

Three-dimensional characterization of cracks in undisturbed Mile expansive soil using X-ray computed tomography

The distribution of cracks strongly affects the hydro-mechanical behavior and strength of soil. In order to characterize the distribution of cracks in undisturbed expansive soil, X-ray computed tomography (X-ray CT) tests were carried out for one type of undisturbed expansive soil from Mile City, China. Seven undisturbed specimens retrieved from depths between 1 and 4.7 m were used to study the evolution of cracks along the drying path. The characteristics of the cracks were quantitatively studied by 3D parameters extracted from CT images by commercial software. The test results suggested that, with an increase in suction, the total volume of the cracks gradually increased, and one large crack formed due to the merging of sub-cracks before the shrinkage limit. When the suction was larger than the suction corresponding to the shrinkage limit, degradation of the cracks caused by the collapse of the soil structure was observed. Meanwhile, due to the development of sub-cracks, the crack anisotropy caused by the typically oriented primary cracks decreased along the drying path. Finally, a conceptual model of crack development for undisturbed Mile clay was proposed based on the evolution of crack volume, number, shape, and orientation.