Initial Microcracks Research Articles

Graph neural networks for simulating crack coalescence and propagation in brittle materials

High-fidelity fracture mechanics simulations of multiple microcracks interaction via physics-based models can become computationally demanding as the number of microcracks increases. This work develops a Graph Neural Network (GNN) based framework to simulate fracture and stress evolution in brittle materials due to multiple microcracks’ interaction. The GNN framework is trained on the dataset generated by XFEM-based fracture simulator. Our framework achieves high prediction accuracy on the test set (compared to an XFEM-based fracture simulator) by engineering a sequence of GNN-based predictions. The first prediction stage determines Mode-I and Mode-II stress intensity factors (which can be used to compute the stress evolution by LEFM), the second prediction stage determines which microcracks will propagate, and the final stage actually propagates crack-tip positions for the selected microcracks to the next time instant. The trained GNN framework is capable of simulating crack propagation, coalescence and corresponding stress distribution for a wide range of initial microcrack configurations (from 5 to 19 microcracks) without any additional modification. Lastly, the framework’s simulation time shows speed-ups 6x–25x faster compared to an XFEM-based simulator. These characteristics, make our GNN framework an attractive approach for simulating microcrack propagation and stress evolution in brittle materials with multiple initial microcracks.

Micromechanisms and modelling of crack initiation and growth in tool steels: role of primary carbides

AbstractMicromechanisms of damage initiation and crack growth in high speed and cold work steels are investigated using scanning electron microscopyin situexperiments. The role of primary carbides in initiation and growth of cracks in tool steels is clarified. It is shown that initial microcracks in the steels are formed in primary carbides and then join together. A hierarchical finite element model of damage initiation, which included a macroscopic model of the deformation of the specimen under real experimental conditions and a mesomechanical model of damage in real microstructures of steels, was developed. Using the hierarchical model, the conditions of local failure in the steels have been obtained. In order to study the effect of carbide arrangement on fracture, numerical simulations of fracture in steels with different ideal carbide arrangements were carried out and compared with each other. It is found that the heterogeneous arrangements of primary carbides can lead to strong deviations of a crack from the mode I path and, therefore, to a significant increase of the fracture energy of the steels.

A nanoconcrete welding strategy for constructing high-performance wound dressing

Engineering biomaterials to meet specific biomedical applications raises high requirements of mechanical performances, and simultaneous strengthening and toughening of polymer are frequently necessary but very challenging in many cases. In this work, we propose a new concept of nanoconcrete welding polymer chains, where mesoporous CaCO3 (mCaCO3) nanoconcretes which are composed of amorphous and nanocrystalline phases are developed to powerfully weld polymer chains through siphoning-induced occlusion, hydration-driven crystallization and dehydration-driven compression of nanoconcretes. The mCaCO3 nanoconcrete welding technology is verified to be able to remarkably augment strength, toughness and anti-fatigue performances of a model polymer poly(3-hydroxybutyrate-co-3-hydroxyvalerate)-based porous membrane. Mechanistically, we have revealed polymer-occluded nanocrystal structure and welding-derived microstress which is much stronger than interfacial Van der Waals force, thus efficiently preventing the generation of microcracks and repairing initial microcracks by microcracks-induced hydration, crystallization and polymer welding of mCaCO3 nanoconcretes. Constructed porous membrane is used as wound dressing, exhibiting a special nanoplates-constructed surface topography as well as a porous structure with plentiful oriented, aligned and opened pore channels, improved hydrophilicity, water vapor permeability, anti-bacterial and cell adherence, in support of wound healing and skin structural/functional repairing. The proposed nanoconcrete-welding-polymer strategy breaks a new pathway for improving the mechanical performances of polymers.

Simulation-Based Stochastic Method to Model Microcrack Coalescence in Asphalt Pavements: Concept Paper

Fatigue cracking is a major distress in hot mix asphalt (HMA) pavements. The coalescence of microcracks into macrocracks has been widely recognized as a prevalent cause of fatigue failure in HMA. This technical note presents and demonstrates the concept of using agent-based modeling (ABM) and simulations to stochastically evaluate the effect of the initial number of microcracks on the number of macrocracks that are developed as a result of their growth and coalescence. The basic principles of this concept are that the macrocracks emerge from the randomly located and randomly growing microcracks, and this can be simulated by modeling the pavement as a complex system with many interacting parts. Ten thousand simulations were run, each with a different number of initial microcracks. The model was successful in simulating the growth of complex patterns of macrocracks from microcracks, which were assigned relatively simple rules at the start of the simulations. The results indicate a threshold value of approximately 100, below which none or only a few macrocracks can be expected. The variation in the number of macrocracks also starts decreasing from the threshold.

Anomalous effect of grain size on the room-temperature bendability of Mg–Gd alloy sheet

In this work, grain size impacted on the bendability of the Mg-2Gd (wt. %) alloy sheets was investigated. The microstructures and textures evolution during the bending test were analyzed by the electron back-scattered diffraction (EBSD) and the scanning electron microscopy (SEM) technique. The results showed an anomalous grain size-dependent bendability in this alloy. The coarser grain size sample corresponded to a poorer bendability in the Mg–Gd alloys, which was dramatically different from the traditional Mg alloys (such as AZ31B alloys). This opposite mode between the Mg–Gd and traditional rare earth (RE) free Mg alloys was mainly attributed to the difference of microcracks nucleation mechanism during the bending test. The nucleation sites of microcracks were almost in the grain boundaries for the Mg–Gd alloys, while in the intragranular for the traditional Mg alloys. Furthermore, the larger size of initial microcracks and the more heterogeneous strain distributions in the coarser grain Mg–Gd samples led to a premature failure in the bending process and gave rise to this anomalous grain size-dependent bendability.

A novel 3D-FDEM method using finite-thickness cohesive elements to simulate the nonlinear mechanical behaviors of rocks

The crack closure stage and a high ratio of unconfined compressive strength (UCS) to tensile strength (TS) are frequently observed in laboratory tests of highly fractured rocks. However, few numerical methods can capture the crack closure behavior and high UCS/TS ratio in three dimensions. Hence, in this study, a novel three-dimensional finite-discrete element method (3D-FDEM) using finite-thickness cohesive elements was proposed, and random initial microcracks were introduced to the method. The crack closure stage and high UCS/TS ratio of highly fractured rock (Lac du Bonnet granite) are well matched through the present method. The simulated results show that the crack intensity determines the occurrence of the crack closure stage and the crack width significantly influences the evolution process of tangent modulus and crack closure strain. Due to the combination of stiffness degradation and failure of cohesive elements and initial microcrack closure, this method can capture the evolution trends of tangent modulus of various rock materials. The decreases in the simulated UCS, TS and elastic modulus with an increase in crack intensity are consistent with the experimental results of rocks after thermal treatment. A calibration procedure, which can ease the calibration of the parameters of this method, was proposed.

The influences of cooling regimes on fire resistance of ultra-high performance concrete under static-dynamic coupled loads

Cooling regimes have a significant effect on the mechanical properties of ultra-high performance concrete (UHPC) after exposure to elevated temperatures. Load-bearing structures are easily subjected to the coupling action of static and dynamic loads in the fire. This study investigated the dynamic behavior and failure characteristics of UHPC after exposure to heating-cooling treatments. The variations in the P-wave velocities, compressive strengths, elastic modulus and dynamic increase factor (DIF) of the specimens in terms of the different cooling regimes (natural cooling, water cooling) were analyzed. The experimental results indicated that as compared to natural cooling, water cooling caused significant losses in the P-wave velocities, quasi-static and dynamic compressive strength, and dynamic elastic modulus due to the thermal shock induced by water cooling. As the target temperature increased from 250 to above 500 °C, the difference of mechanical property losses varied from small to large, indicating that the thermal shock induced by water cooling caused insignificant damage to the specimen at 250 °C, whereas considerable damage was induced to the specimen above 500 °C. Water cooling enhanced the DIF values of the specimens below 500 °C, but reduced the DIF values at 750 °C. Axial static-pressure loading improved the mechanical properties of the specimen when UHPC was in the elastic phase, which could be attributed to the initial microcracks and pores closed and compacted by the axial static-pressure. However, when the axial static-pressure exceeded the elastic limit of UHPC, microcracks were prone to expansion under impact loads and the mechanical properties of the specimens deteriorated rapidly. Likewise, the failure pattern of the specimen under a given impact load was mainly determined by the heating temperature and cooling regime, whereas the axial static-pressure loading could only affect the fragment size.

Evolution of Damage in All‐Oxide Ceramic Matrix Composite After Cyclic Loading

While structural ceramics usually display a brittle mechanical behavior, their composites may show nonlinearities, mostly due to microcracking. Herein, the stiffness evolution of a sandwich‐like laminate of an Al2O3−15%vol. ZrO2 matrix reinforced with Nextel 610 fibers is studied as a function of number of cycles N in tension. The stiffness of the composite degrades with increasing N, indicating microcracking. However, synchrotron X‐ray refraction radiography shows that the internal specific surface of such cracks varies differently. A modeling strategy is developed for the calculation of the equivalent stiffness of mixtures (first the matrix and then the sandwich), based on the Voigt and Reuß schemes. The Bruno–Kachanov model is then used to estimate the initial microcrack density in the matrix (due to the thermal expansion mismatch) and the amount of microcracking increase upon cyclic loading. The stiffness in the composite degrades dramatically already after 20 000 cycles but then remains nearly constant. The combination of mechanical testing, quantitative imaging analysis, and modeling provides insights into the damage mechanisms acting: microcrack propagation is more active than microcrack initiation upon cyclic loading, but the second also occurs. This scenario is similar but not equal to previous results on porous and microcracked ceramics.

Effect of inherent microcrack populations on rock tensile fracture behaviour: numerical study based on embedded discontinuity finite elements

Inherent microcrack populations have a significant effect on the fracture behaviour of natural rocks. The present study addresses this topic in numerical simulations of uniaxial tension and three-point bending tests. For this end, a rock fracture model based on multiple intersecting embedded discontinuity finite elements is developed. The inherent (pre-existing) microcrack populations are represented by pre-embedded randomly oriented discontinuity populations. Crack shielding (through spurious locking) is prevented by allowing a new crack to be introduced, upon violation of the Rankine criterion, in an element with an initial crack unfavourably oriented to the loading direction. Rock heterogeneity is accounted for by random clusters of triangular finite elements representing different minerals of granitic numerical rock. Numerical simulations demonstrate the strength lowering effect of initial microcrack populations. This effect is substantially stronger under uniaxial tension, due to the uniform stress state, than in semicircular three-point bending having a non-uniform stress state with a clear local maximum of tensile stress.

Fiber-reinforced mortar with 100% recycled fine aggregates: A cleaner perspective on 3D printing

The huge labour-consumption, construction and demolition (C&D) waste pollution, and shortage of river sand have become increasingly serious problems facing the construction industry. Therefore, the possibility of fully replacing natural fine aggregates (NFA) with recycled fine aggregates (RFA) for fiber-reinforced 3D mortar printing (3DMP) was carefully evaluated by Digital Image Correlation (DIC) technique, mechanical testing, and microscopic analysis in this study. The results show that the 100% replacement of RFA had very limited impact on the failure pattern of the 3D printed (3DP) specimen, while the addition of 1% polyethylene (PE) fibers would change the failure pattern of specimens from brittle to ductile. This study also defines the parameter of anisotropic degree to reflect the anisotropy. It is found that the anisotropic degree of 3DP specimens was different under various loading conditions and obviously affected by both RFA and fibers. The microscopic analysis indicated that the effect of RFA on the 3DP specimens was mainly due to the porous structure and the existence of initial micro-cracks, while the effect of fibers was mainly due to the interfacial bond between PE fibers and matrix. This study also found that after reinforcing with appropriate fibers, the 3DP mortar mixed with 100% RFA has higher mechanical properties and better deformation ability than the fiber-free mortar with 100% NFA. It is believed that the 100% replacement of RFA is applicable in 3DMP, which will bring significant benefits to the cleaner production and sustainable development of the construction industry.

The role of tension–compression asymmetrical microcrack evolution in the ignition of polymer-bonded explosives under low-velocity impact

The influence of microcrack on the ignition of polymer-bonded explosives is missing a comprehensive description of the cracking microstructure characteristics. In this article, the mechanical–thermal–chemical response of octahydro-1,3,5,7-tetranitro-tetrazocine (HMX)-based polymer-bonded explosives (PBXs) under low-velocity impact is investigated by a dynamic damage viscoelastic model with tension–compression asymmetrical microcrack evolution and a friction-based hotspot formation mechanism. The microcrack evolution fully considers cracking nucleation and growth, where the nucleation depends on tension or compression condition and the growth follows the law of the energy-release rate. The simulation concerns the mechanical and ignition response of HMX-based PBX in the Steven test. The cracking formation from tension or compression is captured, and the damage morphology is well simulated. The simulation on the ignition response reveals the effect of the cracking microstructure characteristics, including the initial microcrack density number and the tension–compression asymmetrical nucleation rate. In addition, heterogeneous microcrack density number is constructed by unimodal normal distribution and bimodal normal distribution, corresponding to pristine and damaged scenarios, respectively. The uncertainties in the microcrack density number are propagated and quantities in the mechanical–thermal–chemical-coupled model and further, the effect on the ignition, are obtained.

Microporous formation and evolution mechanism of PTFE fibers/isotactic polypropylene membranes by interface separation

Studying microporous formation and evolution mechanism plays a crucial role in fabricating high-performance microporous membranes. A formation mechanism of PP microporous membranes, different from those reported previously, was proposed based on the interface separation of PTFE fibers and PP matrix during sequential biaxial stretching. The experimental results indicated that the initial microcracks, which were filled by microfibrillated PP, were originated from PTFE fibers-PP matrix interfaces perpendicular to the longitudinal stretching direction. As the strain increased, microfibrillated PP was further elongated, ultimately forming microfibrillated PP membranes at a relatively low strain (200%). The subsequent transverse stretching caused the separation of fibrillated PP to form an excellent microporous structure. As a result, PTFE/PP membranes after biaxial stretching exhibited the narrower pore size distribution, higher porosity, higher N2 flux and pure water flux than β-PP membranes.

Modeling damage evolution of graphene/aluminum composites considering crystal cracking and interface failure

Graphene is a significant reinforcement in metal matrix composites by virtue of its superior mechanical properties. The cracking of metal crystal and the failure of graphene-metal interface are the main reasons for the decrease of mechanical properties of graphene/metal composites, but the damage mechanism of that is not clear. In this paper, a novel two-dimensional microstructure model of graphene/polycrystalline metal composites is established by a self-developed structure modeling procedure. According to the actual structure of graphene/metal composites, the numbers, sizes, orientations, arrangements of graphene and grain can be controlled respectively. Moreover, by employing the method of combining the crystal plasticity finite element method (CPFEM) and the cohesive zone model (CZM), the damage mechanism of graphene/aluminum (Al) composites on polycrystalline Al matrix, graphene-reinforcement and graphene-Al interface under tensile load is revealed from the mesoscale for the first time, then the effects of graphene morphology and initial microcracks on the failure behavior and overall mechanical properties of graphene/Al composites are fully captured. This study provides a strong theoretical support and inspiration for the construction of graphene/Al composites with excellent properties.

Evolution of Damage in All-Oxide Ceramic Matrix Composite After Cyclic Loading

While structural ceramics usually display a brittle mechanical behavior, their composites may show non-linearities, mostly due to microcracking. In this work, we studied the evolution of the stiffness of a sandwich-like laminate consisting of an Al2O3-15%vol.ZrO2 matrix reinforced with Nextel™ 610 long fibers as a function of number of cycles N in uniaxial testing mode. We found that the stiffness of the composite degrades with increasing N, indicating increasing microcracking. However, synchrotron X-ray refraction radiography showed that the internal specific surface of inhomogeneities (mainly cracks and pores) decreases with N. A modeling strategy was developed, based on the combination of Voigt and Reus schemes for the calculation of equivalent stiffness of mixtures with the Bruno-Kachanov model for the calculation of equivalent stiffness in microcracked and porous materials. Through modeling we could estimate the initial microcrack density in the matrix (due to the simple thermal expansion mismatch between the two constituents) and the amount of microcracking increase upon cyclic load. We found that the stiffness in such a composite degrades dramatically already after as few as 20000 cycles, but then remains nearly constant. The combination of the mechanical testing, the quantitative imaging analysis, and the modeling provided insights into the damage mechanisms acting: microcrack propagation is far more active than microcrack initiation upon cyclic loading. This scenario corresponds to previous results obtained on porous and microcracked ceramics.

Research results of triaxial geomechanical tests of a coal sample in the light of tomographic tests

The study aimed to check the possibility of using computed tomography to observe fractures in coal samples before and after a triaxial compression test (TRX). The paper presents an innovative use of two specialized laboratory techniques: computed tomography and geomechanical triaxial compression test. Geomechanical tests were performed on a TRX-070, GCTS (USA) measuring triaxial system. A full stress-strain characteristic was recorded. Also, during the triaxial compression tests, the P and S wave velocities were measured and dynamic elastic moduli were determined. The results were compared. The sample was analyzed in two stages: CT measurement performed on raw coal sample (stage 1) and after the TRX test which destroyed the sample structure (stage 2). First, the sample was scanned reflecting the initial pores and microcracks system. The final stage represents the sample after the geomechanical test. A detailed pore structure analysis was performed for each of the stages. As a result of the proposed above-mentioned method, images of the pore structure were obtained after each step. The sample was analyzed for crack propagation and changes in the pore structure. The qualitative and quantitative interpretation was based on the determination of the number of voids, crack diameters, collapse angles, and comparison of the pore structure between the initial state of the sample and after the triaxial test. In the first stage (before the test), the largest object (identified as a microcrack) had a volume of approximately 16 mm3, after the sample was stress-tested (after the sample was destroyed), the largest identified object was about 15 times larger (240 mm3). It appeared that the coal sample was prone to fracturing in defined directions. Comprehensive research has demonstrated the legitimacy of using the proposed methodology to evaluate changes in the sample structure during the triaxial test.

Crystal cracking of grain-gradient aluminum by a combined CPFEM-CZM method

Gradient nanostructured metals attract extensive attention due to their excellent mechanical properties, while they are prone to microcracks during forming and servicing process and eventually develop into a sudden fracture. Actually, experimental studies show that microcracks occurred in both the grain boundary (GB) and the intragrain, but most of the current reports only focus on the GB cracking, very limited work has studied the intragranular cracking. In this paper, the crystal plasticity finite element method (CPFEM) and cohesive zone model (CZM) are combined to study the cracking mechanism of polycrystalline aluminum (Al) with different grain-gradient structures under tensile load, where molecular dynamics (MD) method is used to determine the cohesive parameters of the intragrain and GB. The results not only show the crack initiation and propagation process of polycrystalline Al with different grain-gradient structures, but also reveal the mechanism of intragrain fracture, GB fracture and crack transgranular. The grain-gradient distribution with optimal comprehensive performance is obtained. Moreover, it is also found that the initial microcracks with different positions, numbers and angles have a great influence on the cracking mechanism and effective properties of the whole material. This study provides a solid theoretical basis for improving the quality and operation life of metal parts.

Acoustic Emission Characteristics and Damage Mechanisms Investigation of Basalt Fiber Concrete with Recycled Aggregate.

This paper presents the compression failure process of basalt fiber concrete with recycled aggregate and analyzes the main factors of basalt fiber and recycled aggregate affecting the compressive strength of recycled concrete. The damage mechanism of recycled aggregate concrete is analyzed by the acoustic emission technique. With the method of acoustic emission (AE) b-value analysis, the evolution and failure process of recycled concrete from the initial defect microcrack formation to the macroscopic crack is studied. Based on the AE clustering analysis method, the damage state of recycled concrete under load grade is investigated. Finally, the failure mode of recycled concrete is explored according to the RA-AF correlation method. The results show that when the concrete reaches the curing age, the strength grade of basalt fiber regenerated coarse aggregate concrete is the highest. The basalt fiber increases the strength of regenerated fine concrete by 4.5% and the strength of coarse concrete by 5%, and reduces the strength of fully recycled aggregate concrete by 6.7%. The b-value divides concrete into three stages: initial damage, stable development of internal damage, and internal damage. The variation of AE energy, count, and event number is related to AE activity and crack growth rate. Matrix cracking is the main damage state of concrete, which is greatly affected by the strength of cement mortar. The load grade of fiber cracking in fully recycled aggregate, recycled fine aggregate, and recycled coarse aggregate concrete is 65, 90, and 85%, respectively. Basalt fiber increases the tensile failure event point of recycled concrete and delays the cracking of recycled concrete under compression. When the load grades of fully recycled fiber, recycled fine aggregate fiber, and recycled coarse aggregate fiber concrete are 65–95, 90–100, and 85–100%, respectively, the tensile failure activity increases.

Impact of the initial crack volume on the intact behavior of a bonded particle model

The flat-jointed (FJ) contact model implemented in the particle flow code resolves the long-standing issue of the code producing unrealistically low ratios of uniaxial compressive strength (UCS) to direct tensile strength. Despite the improvements introduced with the FJ model, an FJ sample calibrated to the UCS still produces a crack initiation stress much lower than the expected value of approximately 0.4 UCS, as commonly observed in laboratory compression tests for most rocks. In this study, we investigated the role of initial crack volume on the crack initiation stress of a rock. We found that the microcracks among the mineral grains and their associated void space played a major role in controlling the stress magnitude associated with crack initiation. We also observed that the closure of these cracks between mineral grains was the primary driver of the crack closure phase in the compression test, and the elastic bimodularity was due to the crack density. We developed a methodology to incorporate the initial microcracks using the FJ contact model to match the crack closure stress and bimodularity of a laboratory rock sample. By introducing microcracks and voids among the mineral grains, we automatically capture the crack initiation stress of Lac du Bonnet granite measured at approximately 40% of the UCS.

Artificial microcracking of granites subjected to salt crystallization aging test

Salt crystallization-induced decay of Vardavard granodiorite and Shirkouh monzogranite, two Iranian building stones, were assessed with two non-destructive methods: saturation-buoyancy technique and P- and S-wave velocity measurement. Moreover, polarized and fluorescence microscopy studies were used to evaluate the behavior of the studied stones at microscopic scale against a salt crystallization aging test. The aging test extended pre-existing microcracks and generated new ones. Intracrystalline microcracking was the most predominant microcrack type for both samples. Fine-grained Vardavard granodiorite experienced higher intercrystalline microcracking than coarse-grained Shirkouh monzogranite. The microcracking mechanism of feldspars substantially depends on their alteration degree and microstructural precursors. When a growing microcrack reaches a biotite, it propagates within the crystal if the growing microcrack coincides with the cleavage plane; otherwise, it propagates as an intercrystalline one. The increase in maximum microcrack length of the samples was higher than the increase in their mean microcrack length. Low-strength Vardavard granodiorite showed higher microcrack width after the aging test. Dry weight loss in low-strength Vardavard granodiorite was more pronounced than in high-strength Shirkouh monzogranite. Dry unit weight decreased at a higher rate than saturated unit weight with the increase of effective porosity. The reduction in ultrasonic wave velocities and the increment in effective porosity and water absorption were more pronounced for Vardavard granodiorite, indicating a higher degree of decay, i.e., higher microcrack generation, enlargement, and widening. Shirkouh monzogranite, which has large-sized crystals and pores, wider initial microcracks, high tensile strength, and low effective porosity and microcrack density, was more durable than Vardavard granodiorite.

Numerical estimation of fretting fatigue life of bolted joints using continuum damage mechanics and SEAM Tool

For fretting fatigue, micro-slipping often causes initial micro-cracks in the contact surfaces, which gradually propagate and eventually result in fracture failure. However, it is difficult to directly observe and measure the crack initiation and propagation processes of single-lap bolted joints due to the obstacle of testing technique. Therefore, this article presents an elastic analysis–based method, in which the total fretting fatigue lifetime is divided into initiation life predicted by continuum damage mechanics and propagation life calculated by SEAM Tool in combination with Paris’s law. Since the initiation life model implicitly reflects the result of damage process, one can directly calculate the initiation life based on the elastic stress analysis. The predicted fretting fatigue life, initial crack location, and propagation path are in reasonable agreement with the fretting fatigue test and scanning electron microscope observation results. In addition, it is found that among the main factors affecting fretting fatigue of steel single-lap bolted joints, the initial life is affected by cyclic stress, friction coefficient, and tightening torque, and the propagation path is mainly influenced by tightening torque.