Cracks In Materials Research Articles

Molecular dynamics simulation and experimental study of the material machinability characteristics and crack propagation mechanisms for fused silica double nanoscratches

Fused silica is a high-purity, hard and brittle, difficult-to-machine amorphous material that is widely used in the aerospace, optical instruments, semiconductors, microelectronics and other important fields. However, the influence of adjacent scratches on material deformation and subsurface damage during ultraprecision machining by abrasive particles is not well understood. This study aims to investigate the influence of micron-spaced double scratches on the deformation and damage for fused silica through molecular dynamics simulations and double nanoscratch experiments. An amorphous model of fused silica is created using a two-step simulated annealing method with three-directional periodic boundary conditions. Variable loaded double scratches with different nanometer distances are simulated on the surface of the model, and variable loaded double scratch experiments with varying micron-sized distances between scratches are conducted for fused silica wafers. The simulation and experimental results reveal that the deformation of fused silica during nanoscale double scratching involves both elastic and plastic deformation, and the surface and subsurface damage primarily consists of radial, lateral, Hertzian, and median cracks. The distance between the double scratches affects the load variation, surface and subsurface crack propagation, extent of material damage and removal rate, and surface roughness. Examination of the material subsurfaces using the FIB-TEM sample preparation and testing technique shows that the distance between the double scratches also influences the subsurface damage to the material.

Numerical design of composite material crack arrestors for long-distance gas pipelines

Long-distance gas pipelines are the most common means of transporting natural gas resources. However, the propagation of longitudinal ductile fractures in gas pipelines can result in catastrophic accidents, which must be avoided as much as possible. Crack arrestors can prevent or at least limit the rapid spread of longitudinal cracks by constraining the outwall of pipelines. Carbon fiber-reinforced composite material is one of the best candidates for crack arrestors because of its excellent strength and toughness. However, due to the significant variations in composite material performance, clear design criteria for determining the size of crack arrestors have not yet been developed. This paper presents a comprehensive design procedure for determining the geometrical parameters of composite material crack arrestors based on a finite element model of dynamic pipeline fractures under crack arrestor constraints. Pipeline crack propagation is accomplished using the cohesive zone model, and composite material failure is defined by Hashin criteria. A series of numerical simulations are conducted to assess the ability of crack arrestors to stop a growing crack in a gas pipeline. Ultimately, the minimum wall thickness and axial length of the effective crack arrestor are determined. The proposed design procedure enables the numerical design of composite material crack arrestors and supports safe pipeline operation.

ICDW-YOLO: An Efficient Timber Construction Crack Detection Algorithm.

A robust wood material crack detection algorithm, sensitive to small targets, is indispensable for production and building protection. However, the precise identification and localization of cracks in wooden materials present challenges owing to significant scale variations among cracks and the irregular quality of existing data. In response, we propose a crack detection algorithm tailored to wooden materials, leveraging advancements in the YOLOv8 model, named ICDW-YOLO (improved crack detection for wooden material-YOLO). The ICDW-YOLO model introduces novel designs for the neck network and layer structure, along with an anchor algorithm, which features a dual-layer attention mechanism and dynamic gradient gain characteristics to optimize and enhance the original model. Initially, a new layer structure was crafted using GSConv and GS bottleneck, improving the model's recognition accuracy by maximizing the preservation of hidden channel connections. Subsequently, enhancements to the network are achieved through the gather-distribute mechanism, aimed at augmenting the fusion capability of multi-scale features and introducing a higher-resolution input layer to enhance small target recognition. Empirical results obtained from a customized wooden material crack detection dataset demonstrate the efficacy of the proposed ICDW-YOLO algorithm in effectively detecting targets. Without significant augmentation in model complexity, the mAP50-95 metric attains 79.018%, marking a 1.869% improvement over YOLOv8. Further validation of our algorithm's effectiveness is conducted through experiments on fire and smoke detection datasets, aerial remote sensing image datasets, and the coco128 dataset. The results showcase that ICDW-YOLO achieves a mAP50 of 69.226% and a mAP50-95 of 44.210%, indicating robust generalization and competitiveness vis-à-vis state-of-the-art detectors.

Rock crack initiation triggered by energy digestion

The critical value of rock failure is determined by irreversible deformation (inelastic deformation, damage, and other internal dissipation) processes and external conditions before rock failure. Nevertheless, a thorough explanation of the mechanism causing cracks in rock material has not yet been provided. The strain energy theory is applied in this work to assess the initiation of rock cracks and investigate the relationship between energy digestion and rock strength. Firstly, the uniaxial compression test was conducted on sandstone samples under quasi-static loading conditions and the results of energy evolution, non-linear cumulative digestion, and stored ultimate energy were obtained. Then, a novel algorithm for assessing the initiation of rock cracks has been put forth. The concept of energy digestion index (EDI), which is the ratio of digested energy over the external loading energy, has been developed to characterize the energy absorption capacity of rock material. The result shows a relationship between the maximum growth rate of energy digestion and the increasing rate of variable elasticity modulus and crack initiation. The mechanical characteristics and peak strength of the rock material are negatively correlated with the EDI. By monitoring the digested energy status, an evaluation of the residual strength is introduced based on the relationships, which will initiate further research into in-situ monitoring and failure prediction.

A lift-off suppression algorithm to improve the accuracy of ACFM in crack detection of ferromagnetic materials

Ferromagnetic pressurized components are widely used in petroleum, aerospace and nuclear industries, and their safety and stability are the key to the reliable operation of pressure equipment. In the process of crack detection of ferromagnetic materials by ACFM technology, the changes in lift-off distance caused by coating corrosion and irregularity of the surface will greatly affect the extraction of useful detection signals and reduce the detection accuracy. To overcome the drawback, a lift-off suppression algorithm for crack detection of ACFM is proposed. First, a differential strategy is proposed to discover the time-domain lift-off point of intersection (LOI) from the raw detection signals of ACFM for ferromagnetic materials. Then, the corresponding Bx and Bz signals are demodulated based on the amplitude of the LOI. The simulation and experimental results of the surface cracking of ferromagnetic materials by ACFM show that the novel Bx signals obtained according to the proposed method vary in a small range for different lift-off distances. The lift-off invariant property of the LOI enables the method to improve the interference of the lift-off effect on the conventional Bx signals and to enhance the detection accuracy of crack depth of ferromagnetic materials. The novel Bz signals perform consistently with the conventional Bz signals, and are virtually unaffected by the changes in lift-off distance.

Micro-surface crack inspection with arc-shaped bi-grating laser ultrasound spectroscopy

This paper introduces a novel method for detecting and characterizing initial micro-surface crack in metal structures and materials, with a particular focus on initial fatigue cracks. The proposed technique utilizes an arc-shaped double-spaced grating mask to spatially modulate laser beams, forming an arc-shaped bi-grating laser source on the surface of the specimen. This configuration excites two focused surface waves that have the same focus point but differ in frequency and propagate in opposite directions. By monitoring surface wave signals as they pass through defects and analyzing the relative changes in the amplitude of the frequency components in the spectrum of the reflected and transmitted waves, the initial defects can be identified. Both simulation and experimental results validate the effectiveness of this method in detecting tiny artificial surface cracks, including those shorter than 0.3 mm. In addition, this method has been successfully applied to the detection of actual initial fatigue cracks. Experimental findings demonstrate that focused surface waves enable highly sensitive detection of sub-millimeter fatigue cracks.

A generalized Rivlin–Thomas theory for nucleation of cracks in viscoelastic materials

Recent fracture experiments using the Rivlin–Thomas pure-shear geometry in rubbery viscoelastic materials have suggested nucleation of cracks at a critical stretch, independent on loading rate. For linear material using the rate-independent cohesive model theory, the critical elongation between grips (in a quite general testing geometry) at nucleation is instead monotonically decreasing with rate of up to the square root of the ratio between the instantaneous and relaxed elastic modulus of the material. Shrimali & Lopez-Pamies have made the assumption of a constant critical stretch to develop a theory which contrasts therefore with classical models. However, we further generalize the Rivlin–Thomas theory by assuming an arbitrary relation between nucleation stretch and loading rate, which therefore includes both models as limit cases. We present a simple case of a Double Cantilever Beam (DCB) geometry with linear standard materials, to show an analytical example.

Numerical modeling of shrinkage induced cracking in cementitious materials with three-dimensional simulation and phase-field method

This study investigates the damage and cracking of concrete induced by drying shrinkage. While previous studies have primarily focused on two-dimensional configurations, this research employs three-dimensional phase-field simulations of concrete samples undergoing the drying process. The distribution of inclusions is derived from tomographic images of real samples. The analysis includes the evolution of the damage field and the propagation of cracked zones. The study introduces the formulation and implementation of the numerical method and applies it to elucidate the cracking process resulting from drying shrinkage in concrete composites with various types of inclusions. Numerous numerical simulations were conducted to accurately depict the cracking process induced by drying shrinkage in the samples, evaluating the effects of different inclusions with varying stiffness on the concrete’s cracking phenomenon and comparing the impact of various energy degradation methods on crack simulation. Moreover, the study analyzes the influence of the spatial distribution of the inclusions in the three-dimensional simulation and compares the numerical results with experimental observations.

Influence of GH4169 addition on cracking control, microstructure and mechanical properties of GH4169/K465 composite material manufactured by selective laser melting

In the process of additive manufacturing, K465 nickel-based superalloy tends to form a eutectic structure with a low melting point. Subsequently, rapid cooling leads to the formation of significant residual tensile stress. This tensile stress makes the prepared sample susceptible to cracking. The research explored the susceptibility to cracking of the alloy material and analyzed the microstructure and properties of GH4169/K465 alloy obtained. The generation of cracks can be managed by decreasing the concentration of Al and Ti elements. The microstructure of GH4169/K465 alloy consists of a matrix phase γ, a white Laves precipitated phase, and spherical particle precipitated phase NbTi4. The mechanical properties of samples 1GH1K, 7GH5K, and 3GH1K were analyzed. The results indicated an elongation exceeding 20 %, a tensile strength exceeding 1000 MPa, and a reduction in hardness from 329 HV to 285 HV. The powder mixing process has been demonstrated to effectively address the shortcomings of individual materials by combining the strengths of different alloy materials to produce a hybrid material with excellent comprehensive performance.

Conduction thermography for non-destructive assessment of fatigue cracks in metallic materials

Metallic materials are widely used in many applications of mechanical and aerospace engineering. Due to severe environmental and loading conditions, components and structures can experience cracking phenomena. In this regard, different non-destructive techniques were developed in the last years to detect and monitor cracks that can propagate during the actual loading conditions.This work deals with the application of the conduction thermography as a non-destructive technique for the offline control and inspection of different notched metallic specimens characterized by short fatigue cracks. The potentials of the technique have been demonstrated considering a low-cost set-up consisting of a power supply of low current and limited voltage, and a microbolometer sensor. Moreover, to further simplify the inspection, the specimens were not black coated. Quantitative results, in terms of crack lengths, have been obtained using a new procedure of data analysis and then compared to those provided by thermoelastic stress analysis, a well-established technique adopted for the detection and monitoring of cracks.

Effect of quadratically varying electric potential on the multiple cracks in piezoelectric materials

A basic solution to the problem of three collinear equal cracks in a transversely isotropic piezoelectric material plate is obtained under the influence of electromechanical forces. Stroh formalism and complex variable methods are used to derive the mathematical expressions for SIF (stress intensity factor), COD (crack opening displacement), and COP (crack opening potential). The length of the saturation zones is investigated at each crack tip under the influence of quadratically varying electric displacement. Furthermore, a numerical study is carried out to discuss the effect of quadratically varying electrical potential on the saturation zone, COD, and COP. The results obtained numerically are reported graphically.

Conglomerate rock interface debonding characteristics and damage evolution based on cohesive phase-field method

Rock failure often occurs due to uneven mechanical strength and discontinuous strain at the interface. However, due to the complex interaction mechanism between different phases of conglomerate, it is necessary to improve the crack growth model at the conglomerates' interface. In view of the physical structure and mechanical properties of conglomerate materials, this paper innovatively adopts the cohesive phase field model to study the material deformation and crack growth of conglomerate materials. Different from the traditional phase field model, this model characterises the real structure of the interface by adding an interface phase field, which is used to simulate the debonding of the interface during crack propagation and damage evolution. The regularization of the phase field is carried out to ensure the smoothness of the interface. The transition state of the interface region is determined by the element and the additional phase field. The energy equation is coupled with the crack propagation process using the crack surface density function. The evolution of the damaged phase field and displacement field is driven by the principle of energy minimization. The innovative assumption is that there is a virtual crack at the crack tip, and the interface related elements show a certain cohesive softening form during the damage evolution, which is the main way to realize the particularity of interface mechanics. By controlling the crack dispersion state and stiffness degradation of the material, several ideal softening models are innovatively assumed according to the cohesive force change law to satisfy the mechanical properties and stress function constraints. When cutting blocks n=2.5×104, the errors of the linear model, exponential model, and compound inverse model are 1.76%, 7.48%, and 5.19%, respectively. The results of the exponential model agree well with the stress strength (the error is 2%) and interface crack kinking angle (68.89°). The simulation results indicate that the deformation process of the conglomerate can be divided into three stages: elastic deformation stage, crack propagation stage, and stable load stage. The tensile strength of conglomerate is positively correlated with interface's fracture toughness and proportion of gravel area, respectively.

Polytypes and twins in new boron-rich chalcogenides B6S and B6Se synthesized at high pressure

Microstructure of new boron-rich chalcogenides, orthorhombic B6X (X = S, Se) synthesized at high pressures and high temperatures, has been studied by high-resolution transmission electron microscopy. Two twins systems have been found along planes {101} and {002}. Apart from numerous twins on the {101} plane, two types of polytypes were found: ABAB in both chalcogenides and AB1CD1AB1CD1 in B6Se. It has been shown that shear bands in B6S and cracks in both materials lie in planes {011}.

Understanding mechanical stresses upon solid-state battery electrode cycling using discrete element method

Solid-state batteries (SSBs) often fall behind conventional lithium-ion batteries (LIBs) in performance. Electrochemical cycling protocols, in particular under isostatic compression, present ample opportunities for improvement to mitigate issues like contact loss and aging among numerous other challenges. This study introduces a novel Discrete Element Method (DEM) workflow to assess the effectiveness of uniaxial and isostatic compression as proceeding electrode preparation on the conductivity and structural integrity of SSBs. Isostatic compression achieves conductivity levels comparable to uniaxial compression with significantly reduced pressure requirements. In contrast, uniaxial compression at elevated loads, while resulting in higher conductivity, subjects the particles to significant stress, increasing the risk of cracking and deformation of the SSB materials. This study shows the impact of mechanical stress during different stages of electrochemical cycling on the microstructure's integrity to provide valuable insights into electrochemistry-mechanics couplings, which can be challenging to evaluate experimentally.

Reinforcement induced microcracking during the conversion of polymer-derived ceramics

This study investigates the role of discontinuous reinforcement on the microcracking of a preceramic polymer matrix during polymer to ceramic conversion. The material system comprised a carbosilane-based, preceramic polymer reinforced with mullite particles. The preceramic resin slurry was formulated for photopolymerization on a digital light projection 3D printer. Micro X-ray computed tomography, using a synchrotron light source, monitored a printed composite part during pyrolysis. The conversion of the carbosilane matrix into Si(O)C generated microcracks in the matrix that radially extended from particles. The likelihood of microcrack formation positively correlated with particle size. The largest particles (>104 µm3 volume or >60 µm side length) always abutted microcracks, while the matrix surrounding smaller particles (<5 µm) was free of microcracks. Numerical calculations showed the particles resist the shrinking matrix during its conversion. This created tensile stresses within the matrix. The driving force for microcrack growth was also analyzed with regard to the influence of matrix material parameters, particle form-factor and crack length. These results reveal the need to consider the form factor of discontinuous reinforcements, with regard to the material properties of the polymer-derived ceramic, in order to reduce or eliminate defects in the final part.

Dynamic propagation of moving cracks in brittle materials by field-enriched finite element method

We propose a field-enriched finite element model to simulate the dynamic growth of moving cracks in brittle solids. The paper introduces the dynamic model and verifies its accuracy by analyzing dynamic stress intensity factors during the propagation of a mode-I moving crack. We also simulate crack bifurcation using a specific criterion. Our numerical method successfully reproduces experiments, including the Kalthoff-Winkler experiment, three-point bending test under impact load, and compact compression specimen (CCS) experiment. Finally, we investigate how the location of a circular hole influences the propagation paths of a pre-existing straight crack using a numerical model including both the hole and the crack (COSSCC). Several numerical examples demonstrate the ability of dynamic field-enriched finite element method in simulating dynamic moving cracks.

Design of a hydrophobic nano-SiO2-modified ER@EC microcapsule: Improving rheology, regulating hydration while preserving self-healing in cementitious materials

Microcapsule-based self-healing concrete chronically suffers from rheology deterioration and hydration stagnation due to the hydrophilic and hygroscopic nature of microcapsules, limiting its efficacy and broader application. To address these issues, a novel hydrophobic nano-SiO2-modified epoxy resin (ER)@ethyl cellulose (EC) microcapsule was developed and characterized. Examinations revealed that both 5 % and 7 % SiO2-modified ER@EC microcapsules displayed favorable fundamental properties for cement-based materials, featuring high core contents (53.08 % and 48.04 %) and water contact angles over 90°. Compared to the unmodified capsules, the incorporation of 7 % SiO2-modified capsules prominently enhanced the rheology and hydration exotherm of fresh cement pastes, attributed to floc disassembly caused by electrostatic repulsion and increased water volume induced by replacement and ball-bearing effects. Moreover, incorporating a 4 % dosage of 5 % SiO2-modified capsules exhibited prominent healing efficiencies of 6.14 % for compressive strength and 35.11 % for chloride diffusion resistance. The hydrophobic nano-SiO2-modified ER@EC microcapsules actively improved rheology and hydration exotherm while preserving the ability to self-heal cracks in cement-based materials.

Study on the impact of ultrasonic vibration-assisted grinding of glass-ceramics on surface/subsurface damage mechanism

PurposeAn investigation was conducted into the impact of various process parameters on the surface and subsurface quality of glass-ceramic materials, as well as the mechanism of material removal and crack formation, through the use of ultrasonic-assisted grinding.Design/methodology/approachA mathematical model of crack propagation in ultrasonic-assisted grinding was established, and the mechanism of crack formation was described through the model. A series of simulations and experiments were conducted to investigate the impact of process parameters on crack depth, surface roughness, and surface topography during ultrasonic-assisted surface and axial grinding. Additionally, the mechanism of crack formation was explored.FindingsDuring ultrasonic-assisted grinding, the average grinding forces are between 0.4–1.0 N, which is much smaller than that of ordinary grinding (1.0–3.5 N). In surface grinding, the maximum surface stresses between the workpiece and the tool gradually decrease with the tool speed. The surface stresses of the workpiece increase with the grinding depth, and the depth of subsurface cracks increases with the grinding depth. With the increase of the axial grinding speed, the subsurface damage depth increases. The roughness increases from 0.780um/1.433um.Originality/valueA mathematical model of crack propagation in ultrasonic-assisted grinding was established, and the mechanism of crack formation was described through the model. The deformation involved in the grinding process is large, and the FEM-SPH modeling method is used to solve the problem that the results of the traditional finite element method are not convergent and the calculation efficiency is low.

Computational study of the influence of microstructure on fracture behavior near crack-tip field in nacre-like materials

The small-scale failure process around a plane strain mode I crack in nacre-like materials is analyzed. The displacement increments of boundary nodes are specified by the analytical solutions based on the orthotropic linear elasticity to conduct an incremental finite element analysis. We set up a structural model composed of staggered arranged brittle aragonite tablets and cohesive zones within a process window surrounding the crack tip to investigate the toughness as well as the deformation mechanism in the fracture process zone. The results obtained by the parametric studies of different geometries and mechanical properties in the microstructure of the materials show that the aspect of failure in the near-tip fields can be categorized into four distinct fracture mechanisms. These mechanisms significantly affect the fracture toughness with differences of hundreds of times. We show a failure-mechanism map in which these fracture mechanisms can be divided into regions of geometry and mechanical properties in microstructure. Findings obtained in this study about the toughening mechanisms give us the useful knowledge to design novel materials with specific microstructures to control fracture mechanisms.

Study on the evolutionary mechanism of shrinkage stress-strain behavior of EVA-modified cement mortar at an early age

Understanding the early deformation characteristics is important to predict early cracks in cementitious materials and to serve structural engineering. Incorporation of polyethylene-vinyl acetate (EVA) dispersible polymer powder can improve the early cracking of cementitious materials. In this paper, the homogeneity, shrinkage stress-strain behavior and the evolutionary mechanism of EVA-modified cement mortar (EMCM) with different polymer-to-cement ratios (p/c) were investigated at an early age. The homogeneity and shrinkage stress-strain behavior of EMCM were characterized by a self-designed test setup. Isothermal calorimetric analysis, scanning electron microscopy and X-ray diffraction were used for the evolutionary mechanism analysis. The results show that the homogeneity of shrinkage of EMCM can be improved by adding EVA and prolonging the mixing time. The shrinkage stress of EMCM can be reduced by 83 % when p/c is 10 % and 15 %. Meanwhile, the elastic modulus of mortar can be reduced by 45 % when the EVA content is 15 %. Isothermal calorimetric analysis and characterization of microscopic properties show that EVA can inhibit both the generation and conversion of C4AH13, as well as the generation of AFt and CH, and retard the hydration of cement mortar. Within 3 h of hydration, the change in shrinkage stress of EMCM is only affected by the amount of AFt generated, while the change in shrinkage strain is affected by both CH and AFt generation, and the correlation with CH is greater. Finally, the interpenetrating network structure between the EVA and the cementitious matrix increases the interaction and bond between them, giving good deformability and thus reducing shrinkage. Therefore, the reduction in shrinkage stress-strain of EMCM is a result of the combined effect of the polymer on delaying cement hydration and increasing internal interaction and bonding.