Crack Boundary Research Articles

An attention-based progressive fusion network for pixelwise pavement crack detection

Detecting pavement cracks is a complex task in pavement maintenance. Convolutional neural network (CNN)-based approaches have been increasingly used for this purpose in recent years, but these approaches still have limitations, such as insufficient processing of local features, poor boundary detection ability and information loss issue. In this paper, a novel CNN-based model, named APF-Net, is presented. Specifically, to extract richer and more feature information, proposed model utilizes a progressive fusion module (PF) that enables effective feature enhancement by feature learning from cross-layer features, which operates progressively across adjacent network layers, enabling the crack segmentation model to extract more global and detail features from the crack images. Additionally, a hybrid multiple attention (HMA) module that incorporates both a spatial attention mechanism from two directions and a channel attention mechanism, is proposed, which could effectively capture the long-term dependence of crack features and enhance crack boundary detection. Moreover, proposed model employs a lightweight feature extraction block, named mobile inverted bottleneck convolution (MBC) block, which significantly reduces the model parameters and enables more efficient processing of the crack images. This approach results in a reduction of the computational load and faster detection of pavement cracks. The effectiveness and generalization of the proposed APF-Net have been evaluated through experiments on three publicly available crack datasets. The results demonstrate that the proposed model outperforms other state-of-the-art segmentation models in detecting pavement cracks while with 7.6M parameters and 5.0G FLOPs. Our proposed model can contribute to the development of automatic pavement crack detection systems, which can aid in the maintenance and preservation of roads and highways. The source code for proposed APF-Net is available at https://github.com/MMYZZU/APFNet.git.

Laser powder bed fusion additive manufacturing of molybdenum using a nitrogen build atmosphere

A method was developed to suppress the crack formation during laser powder bed fusion (LPBF) of molybdenum (Mo). The method uses a nitrogen (N2) atmosphere to fabricate crack-free samples without any additional processing. To compare with the effect of the N2 atmosphere, samples were prepared under an argon (Ar) atmosphere as well with the same process parameters. The microstructure analyses by optical and high-resolution electron microscopy revealed that crack-free samples were produced under this condition, whereas grain boundary cracks were observed throughout the sample fabricated in Ar atmosphere. To understand the mechanism of crack prevention by nitrogen (N), the energetics of solute components in the BCC-Mo structure were studied by Density Functional Theory (DFT). The theoretical calculations showed that the introduction of N into the BCC-Mo hinders oxygen (O) diffusion, causing O entrapment in the lattice at interstitial positions. As a result, reduced oxides within grains and at grain boundaries (GB) were observed. The results are consistent with the experimental observation made from the fracture surface investigation. The reduced embrittlement of GB from the reduced GB oxide formation is believed to suppress GB cracking.

Optimization of large cast Haynes 282 based on thermal induced cracks: formation and elimination

Haynes 282, a wrought material, attracts the attention from casting application due to its excellent mechanical properties. Fracture of large cast Haynes 282 were found before yielding during tensile test in previous researches, but the reasons have not been fully understood. In this study, various heat treatments were designed and applied for large cast Haynes 282 to reveal the fracture before yielding. Thermodynamic calculation, residual stress calculation combined with microstructural examination were utilized to analyze the reasons of fracture. It has been found that there are grain boundary cracking and grain boundary precipitate cracking in the tensile sample. The grain boundary cracking is mostly attributed to the melting of the low melting point phases such as some segregated matrix and μ phases. The grain boundary precipitate cracking is associated with the blocky Ti, Mo rich MC carbides, and it is more likely to be caused by the quenching after the second homogenization treatment. Optimized heat treatment has been proposed and proven to be workable for the large cast Haynes 282.

Semi-automatic crack width measurement using an OrthoBoundary algorithm

Evaluation of pavements' crack severity levels currently relies heavily on width measurement, which necessitates the development of a rapid, and high-accurate, automatic measurement approach for complex pavement cracks. This paper presents an OrthoBoundary algorithm that leverages the crack boundary and skeleton directions to determine crack propagation. Comparative analysis has been conducted between OrthoBoundary and Area-Length, Skeleton Shortest Distance (SSD), Edge Shortest Distance (ESD), and Orthogonal Projection (OP) methods. Results indicate that the OrthoBoundary algorithm achieves an average accuracy of 90.10%, outperforming the Area-Length (86.60%), SSD (76.01%), ESD (87.24%), and OP (88.07%) methods. Notably, the OrthoBoundary algorithm also exhibits processing speeds approximately 120 times faster than other considered methods while demonstrating improved robustness and user-friendliness. It has significant potential to quantify and assess the severity of pavement cracks, as well as to facilitate maintenance decision-making processes in road infrastructure management systems.

In-field electro-magnetic-force characteristics of high-temperature superconducting films containing cracks

Superconducting materials have so far been applied in the fields of superconducting motors, flux pumps and nuclear magnetic resonance, but they are prone to mechanical damage due to their material properties. In this paper, the influences of different crack types (including position, angle, size, quantity and spacing) on the characteristics of current-carrying and excited superconducting thin films are analyzed by numerical simulations, as well as their electromagnetic and mechanical characteristics. Several novel and important conclusions are found. In both states, the boundary crack presents the largest current density concentration, while the central crack produces the largest stress concentration. With the increase of crack spacing, the current density on the multi-crack film decreases and eventually tends to be stable. In the current-carrying state, compared with 3 mm cracked film, the current density of 1 mm cracked film is reduced by up to 62.5 %. The von Mises stress of high-temperature superconducting film with 0.1 mm single-spacing cracks is 22.5 % higher than that for 0.3 mm double-spacing cracks, which indicates that close spacing and a high number of crack types are most detrimental to the mechanical properties of high-temperature superconducting films. In the excited state, when the crack angle θ is 90°, the current density streamlines cannot be segmented effectively, which results in the current distribution trend closer to the defect-free film. A large amount of induced current flows into the low-flux region at the 3 mm crack, leading to an increased risk of film quench. The films with 1 mm single-spacing and double-spacing cracks form a low-flux region between the cracks, which makes the magnetic field more difficult to penetrate.

Resistance of Heterogeneous Metal Compositions to Fracture under Dynamic and Cyclic Loads

This paper presents the results of experimental data analysis, which indicate an increased resistance of heterogeneous multilayer clad composites to dynamic loading destruction compared with homogeneous materials. The reason for this is the crack retardation caused by lamination at the boundary of the layers. The destruction of heterogeneous compact composite samples by cyclic off-center stretching also occurs with crack retardation, with the fractogram clearly demonstrating the transverse tightening of the sample section. We argue that crack nucleation plays a decisive role in the process of dynamic destruction of heterogeneous composites obtained by both multilayer cladding and explosion welding. This study presents generalized calculated data confirming the influence of the sign and magnitude of residual stresses (the appearance of a stress discontinuity) on the conditions of fatigue surface crack nucleation and propagation. Unlike homogeneous materials obtained by casting, forging (rolling), or cladding, which are characterized by a linear dependence of the crack propagation velocity on the dynamic stress intensity coefficient, for multilayer composites consisting of strong and viscous layers, a sharp crack deceleration is observed. This is due to the transition of the crack boundary between the strong and viscous layers. This paper presents studies of the corresponding properties of adjacent layers on the integral characteristics of the deposited composite.

M23C6-assisted formation mechanism of grain boundary cellular structure and its role on the brittle fracture in an austenitic Fe-Mn-Al-C lightweight alloy

The austenitic Fe-Mn-Al-C lightweight alloy is limited using its full-hardened state due to its brittleness after long-term aging treatment. This study investigates the formation mechanism of grain boundary precipitates and their effect on crack initiation and propagation. The alloy showed typical age-hardening behavior but exhibited intergranular fracture after a critical aging time. TEM study revealed that the amount of Mn segregation increased and Al decreased at the grain boundary at the early aging stage. M23C6 is first formed at the grain boundary, and α-ferrite is formed at the coherent M23C6/γ interface. Finally, the grain boundary cellular structure of α-ferrite+κ-carbide is formed by the continuous formation of α + κ and growing into their incoherent interfaces with γ. As this cellular structure decorates the grain boundaries, the fracture type changed from ductile to intergranular brittle fracture. The grain boundary cracks initiated at the coarse grain boundary κ-carbide and propagated through the {100} planes of α and κ. The lowest theoretical cohesive strengths in the {100} planes of both phases are suggested as the origin of the crack initiation and propagation.

Stability analysis of unsaturated soil slopes with cracks under rainfall infiltration conditions

Pre-existing (open) cracks at slope crests, especially tension cracks, are a significant factor in the transition from shallow to deep-seated landslides in unsaturated soil slopes induced by rainfall. This paper presents an analytical framework to assess the stability of unsaturated soil slopes for deep-seated rotational failure considering arbitrary crack depths and unsteady infiltration, and its effectiveness is verified by previous literature and finite element analysis. A closed-form formula is proposed for the maximum depth of cracks in unsaturated soils by the stability of the vertical crack boundary. A parametric study is performed to investigate the impact of some key parameters on the stability of unsaturated soil slopes with cracks. Dimensionless stability charts are developed, which can calculate the factor of safety for the slopes with cracksof any depth and can reflect that the potential slip surfaces through or without passing through the crack tip. In addition, given the existence of a critical crack depth and maximum crack depth, with a ratio ranging from 0.5 to 0.6 based on the stability charts, deep-seated rotational failure is more likely to occur than failure caused by a crack boundary.

In-situ study of damage mechanisms in Mg–6Li dual-phase alloy

Interfaces play a crucial role in influencing the mechanical properties of Mg alloys. For Mg–Li dual-phase alloy, the type of interfaces is complex, which includes both grain boundary and phase boundary, and the influence of such interfaces on the damage nucleation is yet to be explored. In this paper, in-situ scanning electron microscopy (SEM) based measurements were carried out to investigate the meso‑scale damage nucleation mechanisms of the Mg–6Li dual-phase alloy. Results show that 94.8% of cracks are nucleated at the α-Mg grain boundary in the post-uniform elongation stage, while 5.2% are at phase boundary and almost no crack at the β-Li grain boundary. The initiation of α-Mg grain boundary cracks is attributed to strain incompatibility, which induces micro-strain localization, and then causes grain boundary sliding (GBS) and crack nucleation. Deformation compatibility analysis reveals that the geometric compatibility factor (Mk) can be used to predict the nucleation of α-Mg grain boundary crack. When Mk is lower than 0.075, α-Mg grain boundary cracks tend to form. Few cracks are generated at the phase boundary is due to the mild strain partitioning between α-Mg phase and β-Li phase and may also be partly attributed to multiple slip systems in body-centered cubic (BCC)-structured β-Li phase, which can accommodate well with the deformation of adjacent α-Mg phase.

Study of the damage and cracking of heterogeneous rocks based on the FFT method - A comparison of computer generation and image reconstruction models

Studying the influence of microstructure on the damage and cracking process of rocks is of great practical engineering importance for evaluating the mechanical properties of surrounding rocks in complex geological engineering environments. To incorporate rock microstructures into numerical models, two main methods, i.e. computer generation methods (e.g. molecular dynamics, MD) and image reconstruction methods (e.g. digital image processing, DIP), are frequently adopted. However, the strengths and weaknesses, the differences and applicability regarding the two major methods have not been systematically discussed and analyzed in simulating the damage and cracking in heterogeneous rocks. In this study, the MD model was firstly optimized based on the previous work, and the “Realistic grain size based microstructure model generation program” was developed by the observed mineral grain sizes from microscopic images. The DIP model was developed by directly using digital images of rock materials via the image reconstruction methods. On this basis, the damage and cracking process of rocks was investigated and compared with the above two models using a technique based on the Fast Fourier transforms (FFT). To this end, a series of macro and micro experiments were conducted to obtain the basic mechanical properties of the Dingziwan Granite (DG) and several examples are generated by the MD and DIP model with calibrated macro and micro parameters by the laboratory data. The two models combined with the FFT method are applied to predict the macro and micro mechanical behavior of the DG under different confining pressures and their differences and applicability are quantitatively analyzed and compared. The results indicate that, in general, the two models are both capable of predicting the macro and micro mechanical behaviors of the heterogeneous rocks, but each has its own strength. The improved MD model can efficiently study the influence of the rock heterogeneity on its macroscopic behavior, while the DIP model can highly reproduce the microcracking process such as intragranular, transgranular and grain boundary cracking.

Decomposition Mechanism of Prior Austenite in Low‐Temperature Zone and Its Relationship to Fracture Behavior of High‐Strength Medium‐Mn Steel

Herein, the isothermal and continuous cooling transformation mechanisms of prior austenite and its relationship to fracture behavior of high‐strength medium‐Mn steel are studied. Furthermore, the precipitation behavior of cementite is also analyzed. The results indicate that the short rod‐like cementite forms in the martensite matrix during the air‐cooling process, the solubility difference of supersaturated elements provides a driving force for the element diffusion. Accordingly, it is proved that no isothermal transformation of prior austenite occurs during the 360 °C × 7 days isothermal treatment. The lath‐like microstructure is the martensite, which is produced during the subsequent continuous cooling process. The prior austenite is “divided” by the preferential martensite (transformed via heterogeneous nucleation), and the interfaces between preferential martensite and untransformed austenite provide nucleation sites for the subsequent martensite. The introduction of initial martensite before the isothermal treatment leads to the reversible brittleness of medium‐Mn steel. The fracture strength decreases with the increase of the initial martensitic fraction and isothermal time. The grain boundary binding force of prior austenite grains (PAGs) increases in the experimental steels subjected to annealing, and the plastic deformation of the martensite matrix is prior to grain boundary cracking, which inhibits the intergranular brittle fracture.

Modeling of environmentally assisted intergranular crack propagation in polycrystals

AbstractPolycrystalline nickel‐based superalloys experience accelerated intergranular crack growth when exposed to dwell times in oxygen‐rich environments and a combination of high temperature and tensile mechanical loading. Increasing crack growth rates are observed for increasing amounts of environmental oxygen in a certain oxygen concentration range, while below and above that range crack growth rates remain approximately constant. A fully coupled chemo‐mechanical modeling framework accounting for the degradation of grain boundaries by oxygen has been presented by the authors. In this work, we expand the framework by a moving boundary condition to capture a realistic oxygen flux in grain boundary cracks for both edge cracks connected to the environment and interior cracks. In numerical simulation results, the behavior of the moving boundary condition is shown for intergranular crack propagation through a polycrystal subjected to cyclic loading. Finally, the capabilities of the modeling framework to qualitatively predict the dependence of the average crack growth rate on the environmental oxygen content, load level, and dwell time are evaluated and it is shown that predictions qualitatively agree with experimental observations for intergranular fracture.

New insights on the breakage mechanism of magnetite quartzite via high voltage pulse discharge: liberation, property difference, micro morphology, and numerical simulation

Herein, new insights on the breakage mechanism of magnetite quartzite via high voltage pulse discharge were proposed. The liberation of magnetite quartzite was significantly enhanced by high voltage pulse discharge. The ground products liberations of pulse numbers 300 were improved by 20.93% at grinding time 3 min. The iron distribution rate of high voltage pulse breakage in −0.074 mm were improved by 29.04 percentage points at pulse numbers 300. The linear thermal expansion coefficient of magnetite and quartz were 7.52 × 10−6 °C and 5.70 × 10−7 °C, thus different expansion deformation and thermal stress of grain boundary expansion generated to promote the grain boundary separation of quartz and magnetite. The impact wave force destroyed the intergranular force rather than the intragranular force preferentially during the high voltage pulse breakage. A distorted electric field was generated at the mineral interface to produce grain boundary cracks and realized the macro fracture of magnetite quartzite.

Transient thermoelastic fracture analysis of functionally graded materials using the scaled boundary finite element method

To model fracture in functionally graded materials (FGMs), the scaled boundary finite element method (SBFEM) is extended to examine the effects of fully coupled transient thermoelasticity. Previously developed SBFEM supplementary shape functions are utilized to model thermal stresses. The spatial variation of thermal and mechanical properties of FGMs are approximated by polynomial functions facilitating the semi-analytical evaluation of coefficient matrices. The dynamic stress intensity factors (SIFs) are also evaluated semi-analytically from their definitions without the need for additional post-processing. Scaled boundary polygon elements are employed to facilitate the meshing of complex crack geometries. Both isotropic and orthotropic materials with different material gradation functions are considered. To study the transient effects of thermoelasticity on fracture parameters, several numerical examples with different crack configurations and boundary conditions are considered. The current approach is validated by comparing the results of dynamic SIFs with available reference solutions.

Grain boundary microstructure-property relationships in the cast & wrought Ni-based superalloy René 41 with boron and carbon additions

Cast & wrought Ni-based superalloys are required for manufacturing high-performance components in gas turbine engines. Their excellent high-temperature properties and corrosion resistance are brought about by their complex microstructure of a γ-matrix, intra- and intergranular γ' precipitates, carbides, and borides. The most highly-alloyed superalloy grades provide the high-temperature strength required for the design of next generation gas turbine engines but their processability remains challenging due to grain boundary (GB) cracking. A common mitigation approach is the addition of B and C to improve GB cohesion and strength. However, their detailed roles in GB segregation and precipitation and the link to GB properties during processing remains largely unexplored.We provide a systematic study on decorated GBs in the cast & wrought Ni-based superalloy René 41 with B and C additions. Tensile testing reveals that B additions reduce its ductility due to GB cracking but slow down grain growth during heat treatments more effectively, than C additions. GB-M2B, M6C, M23C6, and GB-γ' precipitates decorating GBs are sequentially precipitated. Their structures and compositions are shown to be unaffected by B and C additions. However, significant differences in the interfacial segregation of solutes are revealed. B is captured by GB-M2B and therefore unavailable to improve GB cohesion. C depletion and enrichment of solutes at interphase boundaries are identified as origin for the reduced ductility and thus, GB cracking. The findings are summarized as microstructural model and advance the detailed understanding of the processing – microstructure – properties relationships of cast & wrought Ni-based superalloys.

Deformation behaviors and the related high-temperature mechanical properties of Mg–11Gd–5Y–2Zn–0.7Zr via regulating extrusion temperatures

The effects of extrusion temperatures (430, 450, 470 and 490 °C) on the microstructures and mechanical properties of Mg–11Gd–5Y–2Zn–0.7Zr (GW) alloys at room temperature (RT), 250, 300, 350 °C were systematically investigated. The dynamic precipitation of blocky and lamellar long period stacking ordered (LPSO) phases was taken place during hot extrusion. The dominant texture changed from <10 1¯0 >//ED to <0001>//ED with increasing extrusion temperature. Tensile results indicated that the GW alloy extruded at 430 °C (GW430) exhibited the highest mechanical properties at both RT and elevated temperatures, which was ascribed to the fine dynamic recrystallized (DRXed) grains, un-DRXed grains with an orientation of <10 1¯0 >//ED, and the LPSO phase strengthening. When tensile tested below 300 °C, slips on the blocky LPSO phase and kinks of lamellar LPSO phase were the primary deformation modes, followed by grain boundary crack. At 350 °C, grain boundaries became the vulnerable sites to be activated. For GW alloys extruded at 450, 470 and 490 °C, an abnormal increase of mechanical properties was observed at 300 °C. This was due to the occurrence of multiple-slip, which showed the intersection and entanglement of dislocations and thus improving the strength of Mg matrix.

Research on tool stick-slip erosion wear mechanism in cutting nickel-based alloy GH4169

Nickel-based superalloys GH4169 are typical hard-to-machine materials. It can still maintain high strength and hardness at high temperatures, so the tools for cutting nickel-based superalloys are worn seriously. In this paper, the tool wear mechanism when cutting nickel-based superalloy using YG8 (K-type cemented carbide tool) is studied. The stick-slip erosion wear mechanism of cutting tool is put forward in this paper. At high temperature, micro plastic deformation and macro sliding occurs when the chip slides against the tool surface, resulting in wear similar to that of a viscous liquid when it erodes a solid. Erosion wear has a latent period. The faster the cutting speed, the worse the wear is. When chip stick-slip erodes the tool, two adiabatic zones are generated. One is the adiabatic impact zone I generated on the front surface of the tool. The other is the adiabatic shear band II formed under chip compression. The adiabatic impact zone I causes the temperature of tool surface to rise sharply. After high-temperature erosion, the surface of the cutting tool exhibits wear patterns such as lips, pits, layers, and furrows. Among them, the lips are a typical morphology of erosion wear, but the quantity is relatively small. Pits and layers are the main morphological features after erosion wear. The main reason for the above characteristics is that GH4169 has a small deformation coefficient and weak chip deformation ability. Therefore, most lips are impacted and sheared by micro-convex bodies, ultimately leaving pits on the tool surface. In addition, when a portion of the lip has not yet formed, the fracture of the cell boundary cracks in the adiabatic shear zone leads to the occurrence of lamellar detachment, leaving a lamellar damaged surface.

Pavement Crack Detection Based on 3D Edge Representation and Data Communication With Digital Twins

With digital information applied in intelligent transportation system, pavement crack detection with digital twins has drawn widely attention since the past several years. However, it is still a challenge task to accomplish crack detection satisfactory results due to complex pavement background. This paper presents a pavement crack detection method based on 3D edge representation and data communication with digital twins. Firstly, pavement data is achieved by multiple sensors including video data and 3D scanning technology, and the digital twins based model is constructed based on data communication. Secondly, to distinguish between crack feature and pavement background in depth level, the point cloud data enhancement method based on fractional differential is proposed to strengthen crack boundary and maintain detail simultaneously. Thirdly, a 3D crack segmentation algorithm is proposed based on edge feature representation. After improved filter function with plane proximity and depth similarity, the gradient magnitude values of every point are estimated by the neighborhood and direction. Two thresholds are computed automatically using the iteration method to extract the crack from pavement. Finally, the dataset is established by 2600 crack data, and extensive experiments verify the proposed method could conduct better result compared with other existing methods using F-measure evaluation system.

A peridynamic model based on generalized thermoelastic theory in a plate with oblique cracks

In this paper, a peridynamic model based on generalized thermoelastic theory is derived by irreversible thermodynamics. This model avoids the spatial derivative and can be easily applied to generalized thermoelastic problems with discontinuities. The transient temperature response is captured using the non-Fourier CattaneoVernotte (C-V) model. The interaction integral is employed to calculate the transient thermal stress intensity factors (TSIFs). Numerical examples are presented for analyzing the effects of crack orientation angle and multi-crack distribution on transient thermoelastic response. And the crack propagation behavior under non-Fourier thermal shock is further discussed in detail. It is found that with the increase of the angle between inner crack and heating boundary, the sensitivity of mode-I TSIFs to the non-Fourier effect increases, while the sensitivity of mode-II TSIFs to the non-Fourier effect decreases. This peridynamic model may be useful for understanding the crack behaviors of thermal protection materials under thermal shock.

A self-supported agZIF-UC-4 glass membrane for gas separation

Metal-organic framework (MOF) membranes have become promising candidates for gas separation due to their adjustable structure, high separation performance, and environmental friendliness. However, the emergence of grain boundary defects, and interface defects between the support layer and separation layer significantly decrease the gas separation performance of MOF membranes. Herein, a self-supported ZIF (zeolitic imidazolate framework) glass membrane is fabricated, eliminating grain boundary defects and cracks through the molten-liquid phase. The fluorine-based linkers with high electronegativity and polarizability are introduced to enhance agZIF-62(a and g denote amorphous and glass, respectively) interaction of CO2 and CH4. The CO2/N2 and CH4/N2 ideal selectivity of agZIF-UC-4 membrane (fluorine-based, UC-4 named by Cambridge University) reach 36 and 6.2, respectively. In addition, the CO2 and CH4 permeance of the agZIF-UC-4 membrane with an independent separation layer and the non-grain boundary has improved to 4752 and 818 GPU, respectively. Under the mixed gas condition, agZIF-UC-4 membrane exhibits good separation performance, exceeding the 2019 Mckeown upper bond of CO2/N2. Moreover, agZIF-UC-4 membrane has good stability for CO2/N2 and CH4/N2 under continuous operation and variable pressure testing. Hence, the self-supported agZIF-UC-4 membrane is a potential candidate for preparing MOF glass membranes with good gas separation performance.