Continuous Crack Research Articles

Observation and modeling of dynamic fracture behaviors of battery cell under impact loading using enhanced representative volume element concept

The burgeoning electric automobile industry has increased interest in battery safety. Battery cells experience significant mechanical stress during operation, including the impact of accidents and vibrations from driving. The potential for thermal runaway reactions in battery cells raises safety concerns. Although numerous researchers have defined the dynamic behavior of battery cells and proposed numerical models to describe it, few studies have focused on the high-strain rate mechanical impact phase correlated with the onset of fracture. In this study, we describe the dynamic behavior of pouch battery cells and propose a modeling method to study their mechanical failure under impact situations. Impact tests are conducted at various velocities and heights. To overcome numerical issues commonly encountered under rapid deformation scenarios, a new finite element model is developed based on the representative volume element model. The proposed approach efficiently simulates continuous crack propagation and brittleness behavior during impact by permitting the individual behavior of the cell components. Therefore, engineers can reliably design safer electric vehicle battery cells by measuring the properties of the cell components.

A fully recrystallized heterogeneous grain structure with dispersed second phases for enhancing strength-ductility synergy in an Al-Cu-Mn alloy

Introducing heterogeneous grain structures has been demonstrated as an effective strategy to overcome the strength-ductility trade-off in metallic materials. Here, we designed a fully recrystallized heterogeneous microstructure via controlling local recrystallization around the grain boundaries coupled with the second phase. The representative feature of this microstructure is the topology of the nearest grain neighbors, where a fully recovered large grain is surrounded by multiple recrystallized small grains. Meanwhile, the second phase distribution around the grain boundary could be remarkably optimized during the processing. Within a commercial Al-Cu-Mn alloy, a remarkable ultimate tensile strength of 365 MPa with uniform elongation of 30 % was achieved through this heterogeneous structure, corresponding to an 18 % increase in ultimate tensile strength and a 29 % improvement in ductility compared with the uniform-grained counterpart. Characterized by quasi-in-situ EBSD, it was found that the multiple small grain neighbors coordinated local strain incompatibility near the hetero-interface by dislocation slips and local reorientations, which can better sustain the strain hardening. In situ tensile tests unveiled the micro-crack blunting by ductile large grains, endowing superior crack-arrest capability. Moreover, tailored dispersed Al2Cu particles inhibited inter-cluster void coalescence and interface-debonding by eliminating continuous crack growth paths, promoting stable damage evolution. These results give a new insight into the controllable design of heterogeneous-grained alloys with second phases for achieving superior combinations of mechanical properties.

Machine Learning-Enabled Acoustic Emission Signal Processing for real-time crack growth monitoring in heterogeneous structures

Heterogeneous structures pose unique challenges for structural health monitoring, demanding advanced techniques for early detection of crack propagation. Acoustic Emission (AE) is a non-destructive testing technique that relies on the detection and analysis of stress-induced high-frequency acoustic waves. It is commonly used for identifying and monitoring the development of cracks, defects, or other structural anomalies. However, raw AE data is often noisy and contains background noise and interference, which make them not suitable for reliable real-time crack monitoring applications. This study integrates AE data with machine learning algorithms to provide a proactive and accurate system for continuous crack propagation monitoring. The performance of the proposed approach for features extraction to derive relevant information from the AE signals, data labeling for training machine learning models, and the deployment of these models for real-time monitoring, are discussed.

The role of microstructure on the wear and rolling contact fatigue of railway steels: The performance of bainite

This study aims to describe the wear and rolling contact fatigue (RCF) behavior of microalloyed forged railway wheel steel Class D (7NbMo), with microstructures of upper bainite (UB - 350 HV0.5), lower bainite (LB - 450 HV0.5), and pearlite (PE - 350 HV0.5), against 7C steel with a tempered martensite microstructure (660 HV0.5) in a twin-disc tribometer. The results showed that bainite (LB and UB) exhibited greater wear resistance compared to pearlite (PE). The primary wear mechanism for all three microstructures was the fatigue wear. An analysis of the PE disc revealed continuous (single-layer) RCF cracks, which were relatively large and deep. In contrast, UB and LB cracks were discontinuous (multiple layers), thin, and smaller but more numerous (higher crack density). These characteristics indicated that bainitic microstructures also had greater resistance to RCF than pearlite did. Therefore, the combination of mechanical properties and the morphological arrangement of the microstructure led bainite to develop a shallower depth of deformed layer with a higher capacity for plastic deformation per unit volume. The greater wear and RCF performance of bainite compared to those of pearlite may be associated with how they release the accumulated deformation energy after reaching saturation in relation to volume deformation absorption: regarding bainite, the material releases energy by opening crack surfaces; conversely, regarding pearlite, it is through the detachment of large and wide cracks.

Formation of transverse cracks in cross ply laminates in an environment of nonuniformly distributed fibers

This work is concerned with extracting characteristic features of transverse crack formation in cross ply laminates under axial tension considering nonuniform distribution of fibers in the ply cross section. The fiber-matrix debond crack initiation sites are determined first by the dilatation induced cavitation criterion for an epoxy matrix. The growth of the debond cracks is analyzed by calculating the energy release rates using the virtual crack closure technique. The kink-out of the debond cracks into the matrix is determined in short incremental steps by the maximum energy release rate criterion. Different scenarios are considered for the linking up of the kinked-out cracks to form continuous transverse cracks. By studying two different degrees of fiber distribution nonuniformity, the interactive effects due to the fiber distribution on the transverse crack formation are clarified.

Self-healing efficiency of later-age cracked microbial mortar embedded with unencapsulated microorganisms under long-term water and ambient exposure conditions

To ensure the long-term self-healing ability of concrete structures throughout their lifespan is a crucial requirement for Microbially Induced Carbonate Precipitation (MICP) technology. In this study, unencapsulated microbial cells were directly mixed into the fresh mortar as a component to investigate their efficiency in repairing later-age cracks, respectively exposed to long-term water environment and ambient conditions. The crack width repair ratio for samples with cracks initially created by splitting tests was calculated at healing intervals of 6 and 12 days, revealing a sixfold higher repair ratio in 150-day samples with initial widths below 200 μm than those cases with 500 μm-wide cracks. The 26.3 % reduction in water absorption content after a 12-day healing period reflected an effective healing result for cracks and capillary pores. The microbially induced calcite precipitations were confirmed through SEM, XRD, and EDS analysis, with complete crack closure observed for 150-day samples with widths less than 200 μm. However, it was also found that the self-healing efficiency decreased with increasing crack width due to the limited availability of healing agents, with the maximum completely healed crack width of 507.8 μm. Despite prolonged exposure, no further reduction in crack width and sorptivity was observed, highlighting the challenges in achieving a complete healing effect. Surface images obtained from microscopes revealed the enduring presence of unencapsulated microorganisms, corroborating their role in continuous calcite precipitation and crack filling.

The tensile strength of brittle diamond lattice structure with material dispersion

Abstract This work investigates the effect of material dispersion on the tensile strength of brittle diamond lattice structure. In actual lattice structures fabricated by additive manufacturing, the dispersion of strength comes from microscale defect, geometric deviation and manufacture-induced anisotropy. The weakening of ultimate failure strength due to material dispersion cannot be predicted by most existing theoretical models, because they assume homogeneous and determinate mechanical properties of the lattice structure. In this paper, we employ diamond lattice structure made from brittle material as a typical example, and its tensile behavior is numerically investigated by incorporating the Gaussian distribution of strut strength. Inspired by the simulation results, a stochastic theoretical model is developed to predict the deformation and failure of diamond lattice structure with material dispersion. This model captures the fact that weaker struts break first even if the whole structure can still bear load. With the continuous increase of stress, these broken struts accumulate into continuous cracks, and ultimate failure occurs when the energy release rate of the initiated crack surpasses the intrinsic fracture toughness of the lattice structure. This research supplements stochastic feature into classical theories, and improves the understanding of potential strengthening and toughening designs for lattice structures.

The Crack Diffusion Model: An Innovative Diffusion-Based Method for Pavement Crack Detection

Pavement crack detection is of significant importance in ensuring road safety and smooth traffic flow. However, pavement cracks come in various shapes and forms which exhibit spatial continuity, and algorithms need to adapt to different types of cracks while preserving their continuity. To address these challenges, an innovative crack detection framework, CrackDiff, based on the generative diffusion model, is proposed. It leverages the learning capabilities of the generative diffusion model for the data distribution and latent spatial relationships of cracks across different sample timesteps and generates more accurate and continuous crack segmentation results. CrackDiff uses crack images as guidance for the diffusion model and employs a multi-task UNet architecture to predict mask and noise simultaneously at each sampling step, enhancing the robustness of generations. Compared to other models, CrackDiff generates more accurate and stable results. Through experiments on the Crack500 and DeepCrack pavement datasets, CrackDiff achieves the best performance (F1 = 0.818 and mIoU = 0.841 on Crack500, and F1 = 0.841 and mIoU = 0.862 on DeepCrack).

Martensite content effect on fatigue crack growth and fracture energy in dual‐phase steels

AbstractThe effect of different microstructural factors on crack growth and fatigue fracture mechanisms in dual‐phase (DP) steels has yet to be fully understood. The present research examines the relationship between crack growth, microstructure, and fracture mechanisms. The samples were intercritically annealed at different temperatures to produce three different martensite volume fractions (MVFs). The results show that the mechanical incompatibility of ferrite and martensite promotes continuous crack tip deflection. MVF increases are associated with elevated fracture tortuosity, more significant fracture energy surface formation, and higher Paris law exponent m values. The interaction of the microstructure with the crack tip, the strain energy density, and the softening caused by secondary microcrack propagation are all illustrated by Electron backscatter diffraction (EBSD) maps. Increasing MVF promotes slow crack growth and a fracture energy increase of 22.9% between the as‐received and heat‐treated steels.

Solid electrolyte cracking due to lithium filament growth and concept of mechanical reinforcement – An operando study

Lithium metal solid state batteries (LMSSB) attract great interest in academia and industry due to their projected high energy density and safety. However, hard short circuits due to the penetration of lithium filaments through the solid electrolyte impede their development for practical application. Here, multi-characterization methods ranging from operando and in situ scanning electron microscopy to ex situ focused ion beam scanning electron microscopy are employed for comprehensive understanding of the cell failure. The rapid failure is attributed to coupled crack propagation and subsequent lithium filament growth during the plating process which is demonstrated in a Li6PS5Cl (lithium argyrodite) based LMSSB. Cracking initiates at current constriction spots and voids, where inhomogeneous lithium plating/stripping causes high local stress fields that trigger continuous crack propagation. Lithium filament growth finally leads to a hard short circuit. Considering the low fracture toughness of ceramic electrolytes, strengthening with Al2O3 fibers is shown to be effective in significantly improving the critical current density and cycling stability. Our results clearly show that cracks can have a detrimental effect on LMSSB and we suggest focusing on strategies to improve the fracture toughness of thiophosphate electrolytes in order to suppress lithium filament growth.

A prediction model of the tensile properties of engineered cementitious composites (ECC) based on the continuous crack tracking algorithm

Considering the extensive application of the engineered cementitious composites (ECC) in aseismatic structures due to its high ductility, it is necessary to know the variation characteristics of its tensile property. This paper proposed a simulation model so as to capture the multiple cracking behavior and predict the stress-strain curve of ECC based on its mechanical information. By introducing the fiber bridging distribution model (FBDM) into the one-by-one crack tracking algorithm, the influence of randomly distributed short fibers on matrix cracking can be considered. The effectiveness of the model is confirmed by the good consistency between the simulation and result data from existing experiments. Therefore the three-stage cracking mechanism observed from the results can be deduced as the actual failure process of the component. In addition, the effects of the interface parameters and matrix strength distribution on ECC's tensile properties are analyzed, which can provide theoretical support for the prediction of the mechanical properties and material toughness improvement design of ECC.

Experimental and numerical investigation on meso-fracture behavior of Beishan granite subjected to grain size heterogeneity

Research on crack evolution mechanism and failure pattern of Beishan granite is vital for evaluation of the engineering stability of disposal repository. The mechanical properties, especially the strong heterogeneity, significantly affect the cracking behavior of granite. To investigate the fracture behavior of heterogeneous granite, mesoscopic three-point bending tests were conducted on Beishan granite using an in situ scanning electron microscope (SEM) with a loading device, and the entire cracking process was observed in real time. Additionally, the effect of grain size heterogeneity on strength and deformation behavior of granite was quantitatively investigated by introducing a statistically defined heterogeneity index into the grain-based model in two-dimensional Particle Flow Code (PFC-GBM), and the crack development was characterized through fractal theory. The results demonstrated that fracture mechanism of granite is mainly tensile and shear failure, and the fracture surface of tensile failure is rough and irregular, which is mainly caused by the tearing between various mineral crystals, whereas the fracture surface of shear failure is relatively smooth, which is mainly caused by the dislocation and slip between minerals. The crack edge can be regarded as the damage zone, and the development of the main crack is promoted by the continuous generation and combination of micro-damage at the crack tip. The crack propagation path from a mesoscale perspective is a process of continuous crack orientation, which depends on the competition mechanism between mineral properties and maximum tensile stress. Generally, the granite strength tends to increase with the increase of heterogeneity.

Lane Crack Detection Based on Saliency

Lane cracks are one of the biggest threats to pavement conditions. The automatic detection of lane cracks can not only assist the evaluation of road quality and quantity but can also be used to develop the best crack repair plan, so as to keep the road level and ensure driving safety. Although cracks can be extracted from pavement images because the gray intensity of crack pixels is lower than the background gray intensity, it is still a challenge to extract continuous and complete cracks from the three-lane images with complex texture, high noise, and uneven illumination. Different from threshold segmentation and edge detection, this study designed a crack detection algorithm with dual positioning. An image-enhancement method based on crack saliency is proposed for the first time. Based on Bayesian probability, the saliency of each pixel judged as a crack is calculated. Then, the Fréchet distance improvement triangle relationship is introduced to determine whether the key point extracted is the fracture endpoint and whether the fast-moving method should be terminated. In addition, a complete remote-sensing process was developed to calculate the length and width of cracks by inverting the squint images collected by mobile phones. A large number of images with different types, noise, illumination, and interference conditions were tested. The average crack extraction accuracy of 89.3%, recall rate of 87.1%, and F1 value of 88.2% showed that the method could detect cracks in pavement well.

Monotonical mechanical behavior of thermo-pressure-activated geopolymeric composites in the indirect tensile test using digital image correlation

There is an increasing interest in alternatives to Ordinary Portland Cement (OPC) for plug and abandonment of oil wells, aiming to obtain more reliable and durable well barriers, with reduced CO2 footprint. Geopolymers can have lower permeability and higher chemical resistance to downhole fluids, compared to OPC-based systems. However, there are concerns about their mechanical behavior, in particular their brittle cracking in tension, which may lead to continuous flow paths for escape of fluids. Fibers can improve this behavior by arresting the growth of cracks, reducing their width, and distributing the strain over a larger area. This work aims to study the tensile behavior of a neat metakaolin-based geopolymer system formulated for well applications, before and after addition of 2%v./v. polypropylene fiber or 2%v./v. wollastonite. The indirect tensile test (ITS) was performed in 11.3 mm diameter cylindrical samples cured under high pressure to minimize macroscopic defects. We used digital image correlation (DIC) to monitor the samples at the millimeter scale, aiming to quantify the efficiency of fibers as crack arrestors. We demonstrated a procedure to obtain reliable and quantitative strain and stress fields for the ITS using DIC, enabling the investigation of local stress heterogeneity. First, a penalized spline-based smoothing scheme is used to compensate random errors and to regularize the strain field. Then, two different approaches are used to determine the average elastic properties and calculate the stress field. A three-dimensional finite element model was used to compare the theoretical strain distribution in the sample to the observable strains at the surface. The experimental results were closer to the simulation than to the analytical solution, which is more representative of the behavior inside the specimen. Hot spots of stress concentration were analyzed to obtain the monotonic stress-strain behavior of the material until failure, showing that local stresses were 33% larger that the macroscopic strength measured in the ITS. The neat geopolymer had higher stiffness and tensile strength, but the samples displayed surface delamination in compression, near the force application points. Wollastonite prevented this superficial phenomenon, but the samples had lower tensile strength. Polypropylene, on the other hand, was very effective in arresting crack opening, allowing significant post-peak deformation. These polypropylene-reinforced geopolymer systems are more suitable for downhole applications, since they can withstand higher strains before the formation of continuous cracks and subsequent loss of hydraulic isolation. The methodology proposed in this paper enabled us to observe and quantify the performance of the reinforcement.

Diffusion and Thermal Shock Behaviour of Silicate Dental Ceramics

The purpose of this study is to examine the diffusion and thermal shock behaviour of the alternative dental silicates. The samples derived from BaO-SiO2 system have been produced by melting process and followed by the heat treatment. X-ray examinations have indicated that the amount of orthorhombic sanbornite phase rised due to treatment temperature as revealed by the peak intensities. The alteration of the morphology by treatment temperature was evident from scanning electron microscopy SEM images. Crystallization at low temperature produced small crystallites and coarsening was not observed with a rise in temperature. The failure of the dental samples after the thermal shock test was examined using SEM. A continuous crack geometry and very few crack branching were observed. Diffusivity constant (D) and diffusion rate were examined vs treatment temperature for the present system. Diffusion depth (L) was doubled when the crystallization treatment was applied. Based on the obtained results, these silicate samples might take part in the further characterization studies to be used as alternative dentistry materials.

Various HFCVD diamond coatings synergistically tuned using CH4 gas flow and working pressure and key merit evaluation of their coated tools

Diamond-coated tools can meet the service requirements of high precision, high efficiency and long life with multi-element coupling, but the milling characteristics of graphite materials with high spindle speed, feed speed and high force may require tools with a high number of micro-edges and good adhesion strength. In the present study, microcrystalline diamond (MCD), nanocrystalline diamond (NCD) and gradient composite diamond (GCD) coatings were deposited on commercial end mills subjected to distinct CH4 gas flows and working pressure by hot filament chemical vapor deposition. The adhesion strength, erosion resistance and milling performance of the diamond coatings in milling graphite materials were evaluated. Besides, the effect of CH4 gas flows and working pressure on the growth model of diamond coatings and the mechanism of crack propagation when subjected to stress concentration were studied. The results show that the combination of high CH4 gas flows and low working pressure achieves the re-nucleation of diamond particles, the smaller grain, more graphite/amorphous carbon in the boundaries and larger residual stresses lead to continuous crack propagation, causing poor adhesion strength, erosion resistance and milling performance in NCD and GCD coatings. Conversely, the low CH4 gas flows to ensure the epitaxial growth required for MCD coating, the larger grain, higher diamond purity, columnar structure and smaller residual stresses allow the pinning effect between coating and substrate, altering and increasing the path of crack propagation, making it suitable for milling conditions with high speed and high force. This study provides significant insights into the links between diamond morphology, structure, grain size, boundaries, and phase composition and the mechanical properties of adhesion strength, erosion resistance, and milling properties by synergistically tuning or alternating the CH4 gas flows and working pressure.

Fatigue failure mechanism analysis of 1Cr17Ni2 stainless steel blades ground by an abrasive belt

Fatigue failure, as the main failure form of aero-engine blades, has a direct impact on the reliability and service life of aviation equipment. In order to improve the service performance of machined blades, it is necessary to understand the failure process and failure mechanism of blades and then optimize the grinding process. This paper takes abrasive belt grinding of an 1Cr17Ni2 stainless steel blade as the research object and analyzes the fatigue failure mechanism by characterizing the surface morphology, cross-sectional microstructure, and cross-sectional characteristics of the fatigue failure blade. The results show that cracks are prone to propagate in carbon-rich areas with poor mechanical properties inside the material, and the accumulation of large-size carbon-rich areas leads to continuous cracks easily and accelerates crack growth. The grinding process promotes the migration and consumption of surface carbon elements and forms a carbon consumption layer on the surface of the material, which can inhibit the initiation of fatigue cracks. The point-like pits on the ground surface have an adverse effect on the fatigue life and play a role in the initiation of fatigue crack enhancement. The direction of material research and development to homogenize the structure of the material and the direction of anti-fatigue grinding to increase the thickness of the carbon consumption layer on the ground surface and avoid the damage of micro-pits are proposed. The research has important guiding significance for anti-fatigue machining of key components.

Enhanced crack buffering of additively manufactured Ti–6Al–4V alloy using calcium fluoride particles

Ti-based alloys fabricated by wire and arc additive manufacturing (WAAM) exhibit continuous grain boundaries of α phase (GB-α), which easily initiate cracks and provide a continuous crack propagation path. Herein, an activating flux of CaF2 particles was introduced into WAAM-fabricated Ti–6Al–4V alloy to tailor the microstructure for crack buffering (i.e., inhibit smooth propagation of cracks). The results showed that the addition of CaF2 particles disrupted the continuous GB-α through the dowel-like lamellar α phase (DL-α), which was primary α lamella deeply embedded in the adjacent prior-β grains. This microstructure imparted an optimal combination of strain and strength even in the presence of pores in the WAAM-fabricated Ti–6Al–4V alloy. The enhanced mechanical properties can be attributed to uniform plastic deformations and tendency to transgranular fracture of tensile specimens. DL-α relieved stress concentration at the grain boundary and guided cracks into the prior-β grain interior (i.e., avoided inter-crystalline fracture), thereby promoting the crack buffering effect. The study can provide theory guidance and data support for improving the mechanical performance of WAAM-fabricated Ti–6Al–4V alloy.

Continuous crack detection using the combination of dynamic mode decomposition and connected component-based filtering method

Concrete surface cracks are one of the earliest indicators used to assess structural damage, serviceability and durability. This study proposes a novel method to detect concrete cracks and evaluate the crack features. The proposed approach introduces the dynamic mode decomposition (DMD) algorithm to identify individual cracks in each frame of a video based on the analysis of the DMD spectrum. Since cracks contain different types of noise, a connected component-based filtering method is proposed to discard the noise of the bubbles and blobs. The inspected cracks can be used to evaluate crack features, including area, orientation and perimeter. The proposed crack detection approach is then applied to inspect the crack development during a cubic concrete sample test under axial loading. The detected results demonstrate that the proposed method can correctly identify most cracks. Because video frames generate enormous data volumes that are not conducive to quick detection, dimensionality reduction based on normalized singular values is also discussed and compared with the proposed method. After the cracks are identified, the crack features of the area, orientation and perimeter are determined over time, which helps to assess crack properties and to understand structural performance. The accumulated database of crack characteristics will provide useful information of lifetime predictions.

A step toward bio-inspired dental composites

This feasibility study aimed to develop a new composite material of aligned glass flakes in a polymer resin matrix inspired by the biological composite nacre. The experimental composite was processed by an adapted method of pressing a glass flake and resin monomer system. By pressing and allowing the excess monomer to flow out, the long axis of the flakes was aligned. The resultant anisotropic composite with silanized and non-silanized glass flakes were subjected to fracture toughness tests. We observed increasing fracture toughness with increasing crack extension (Δa) known as resistance curve (R-curve) behavior. Silanized specimens had higher stress intensity KR-Δa over non-silanized specimens, whereas non-silanized specimens had a much lower Young’s modulus, and higher nonlinear plastic-elastic JR-Δa R-curve. In comparison with conventional composites, flake-reinforced composites can sustain continued crack growth for more significant extensions. The primary toughening mechanism seen in flake-reinforced composites was crack deviation and crack branching. We produced an anisotropic model of glass flake-reinforced composite showing elevated toughening potential and a prominent R-curve effect. The feasibility of flake reinforcement of dental composites has been shown using a relatively efficient method. The use of a biomimetic, nacre-inspired reinforcement concept might guide further research toward improvement of dental restorative materials.