Fracture Process Research Articles

Experimental and simulation-based engineering of calcium alginate self-healing asphalt capsules

An emerging technology to mitigate the cracking of asphalt pavements is the use of self-healing capsules embedded in asphalt mixtures. In this study, self-healing capsules were fabricated by encapsulating asphalt rejuvenators with calcium alginate shells. While past studies have used a “brute force” design process for such capsules, the design and formulation of these can be optimized through careful consideration of chemical interactions and crack healing mechanisms. In this study, experiments and molecular dynamics (MD) simulation were used to engineer capsule–rejuvenator formulations to mitigate premature failure of capsules and improve self-healing efficiency. To explore the thermodynamic process, the inter-diffusion coefficient and blending degree between materials in the capsule system were evaluated at different temperatures and capsule designs to determine the ratio of capsules which survive mixing and compaction through molecular scale studies. MD provided an understanding of healing behavior by simulating and quantifying the fracture–healing–fracture process of the binder–capsule system. The experiments verified the MD model by quantifying oil release percentage from micro-extraction and recovery and fine aggregate matrix (FAM) healing efficiency. The research revealed that the interaction between asphalt binder, rejuvenator, and capsules highly depended on their chemical compositions and that the dose of capsule content influences the penetration degree, and structural failure process. Polymeric capsules experienced significant premature failure and content release at high temperatures (160 °C) compared to intermediate temperatures (25 °C) due to enhanced thermal movement. To optimize capsule performance, rejuvenators C and E, which had higher viscosities, required 30%–40% calcium alginate, while rejuvenator A with lower viscosity needed 40%–60% calcium alginate. Rejuvenators with more asphaltenes were more sensitive to the capsule shell protection effect, and strengthened healing efficiency, while rejuvenators with more aromatics resulted in better wetting and diffusion processes during healing. The healing index, derived from FAM experimental healing test and validated by simulations, indicated that approximately 40% shell material effectively prevented capsule breakage and resulted in a 50% reduction in healing capacity. This research therefore established a framework for engineering a combination of capsules for use at pavement scale based on an understanding of chemical interactions, mechanical properties, and experimental verification.

Reactive molecular dynamics of the fracture behavior in geopolymer: Crack angle effect

Reactive molecular dynamics was applied in this study to construct the sodium aluminosilicate hydrate (N-A-S-H) and tensile fracture models with various crack angles. The impact of crack angle on the behavior of N-A-S-H fractures was explored while considering structural mechanical properties and energy evolution. Furthermore, the fracture toughness and brittleness index for various crack angle models were calculated. The findings indicated that the ultimate strength and elastic modulus of the fracture models grew linearly with the increase in crack angle. The fracture toughness value progressively grew while the model’s elastic energy efficiency and new surface energy efficiency decreased simultaneously. The 45° crack model possessed the largest oblique crack development surface in the fracture process due to the coupling effect of tensile and shear stress. Its elastic energy efficiency decreased as well the most, while the new surface energy efficiency increased and the fracture toughness value dropped sharply. It is crucial to place a stronger emphasis on spotting cracks both in the in-service structures or structures being demolished. This ensures optimal performance and safety by enabling more effective adjustments to the direction of external forces and energy input.

Mechanism based four-linear cohesive zone model for mode I fracture of different stacking sequence CFRP laminates

Delamination, a prevalent failure mode observed in laminated composites, exerts a significant impact on structural integrity and performance. The occurrence of fiber bridging during the fracture process adds complexity and elevates the research challenges associated with this phenomenon. Existing models exhibit limitations in accurately capturing bridging behavior and discerning its underlying mechanical mechanisms. This study addresses these limitations by analyzing experimental results, employing the J-integral, and analyzing R-curve behavior, proposing a mechanism-based four-linear cohesive zone model along with a new finite element implementation method. Comprising three overlapping bi-linear CZMs, this model effectively simulates mode I fracture behavior in laminates with different stacking sequences. Moreover, it intuitively illustrates the mechanical mechanisms during crack propagation and offers simplicity in implementation. This research contributes to a deeper understanding of composite fracture mechanics and provides a practical model for predicting delamination behavior in laminated structures.

Numerical and experimental crack-tip cohesive zone laws with physics-informed neural networks

The cohesive zone law represents the constitutive traction versus separation response along the crack-tip process zone of a material, which bridges the microscopic fracture process to the macroscopic failure behavior. Elucidating the exact functional form of the cohesive zone law is a challenging inverse problem since it can only be inferred indirectly from the far-field in experiments. Here, we construct the full functional form of the cohesive traction and separation relationship along the fracture process zone from far-field stresses and displacements using a physics-informed neural network (PINN), which is constrained to satisfy the Maxwell-Betti's reciprocal theorem with a reciprocity gap to account for the plastically deforming background material. Our numerical studies simulating crack growth under small-scale yielding, mode I loading, show that the PINN is robust in inversely extracting the cohesive traction and separation distributions across a wide range of simulated cohesive zone shapes, even for those with sharp transitions in the traction-separation relationships. Using the far-field elastic strain and residual elastic strain measurements associated with a fatigue crack for a ZK60 magnesium alloy specimen from synchrotron X-ray diffraction experiments, we reconstruct the cohesive traction-separation relationship and observe distinct regimes corresponding to transitions in the micromechanical damage mechanisms.

Fracture propagation characteristics driven by liquid CO2 BLEVE for coalbed methane recovery

Fracture propagation induced by liquid CO2 phase transition blasting (LCO2-PB) is the major reason for the spotty fracturing performance of LCO2-PB for enhancing coalbed methane (ECBM). Measures to effectively control the fracture propagation are of significant importance for ECBM recovery. Many researches to date have conducted the responses of the fracture propagation upon the CO2 fracturing under various injection conditions, and some have found that the thermodynamic state of CO2 can dominate the cracks distribution to some extent. This study therefore investigates the effect of various operated parameters of LCO2-PB on the fracture propagation alteration of coal seam subjected to the thermodynamic state of CO2 with different reservoir conditions. A series of fracture process simulations were performed based on the pressure–temperature-phase coupling model, and results indicate that high temperature of CO2 flow in the fracture process greater fracture expansion; the fracture width decreased around by 66.8 % from smaller smash district to larger one, especially for low CO2 flow rate; high elastic modulus and the temperature of coal seam favors greater length reduction and extensive crack connection, and as temperature of CO2 induces the interlaced fractures earlier formation in the higher reservoir temperature.

Contrasting fracturing and cementation timings during shale gas accumulation and preservation: An example from the Wufeng-Longmaxi shales in the Fuling shale gas field, Sichuan Basin, China

The marine shales in southern China are characterized by high thermal evolution and intensive deformation during multistage burial and uplift. Fracturing and cementation, associated with tectonic activity, diagenesis, hydrocarbon generation and migration processes, are widely and variably distributed in shale reservoirs experiencing different tectonic evolution. In this study, we utilized cross-cutting relationships, fluid inclusion microthermometry, and laser Raman spectroscopy of fluid inclusions, along with U-Pb dating of fracture-filling calcite veins, integrated with burial history modeling, to elucidate the timing and mechanism of fracturing in the Paleozoic Wufeng-Longmaxi shale reservoir, as well as regional variations in the Fuling shale gas field. Bedding-parallel fractures (BFs) in shale reservoirs are filled with calcite and quartz veins. The timing of fracturing, as determined by minimum homogenization temperatures and U-Pb dating, is estimated to have occurred during ca. 191 -122 Ma, during which some short vertical (or high-angle) fractures (VFs) opened around 163.4 Ma and 137 Ma, subsequently sealed by calcite veins, respectively. All calcite and quartz veins within these fractures contain high-density methane inclusions, while shale reservoirs were in a high-over-mature evolutionary stage. This suggests that fracturing was primarily driven by gas generation overpressure during deep burial. In shale reservoirs near fault belt folds, multistage fractures developed, including three stages of intersecting fractures (IFs) and one-stage of long VFs. The timing of fracturing and cementation corresponds to the Yanshanian tectonic uplift (ca. 83 - 69 Ma) with intense tectonic movement likely being the direct cause of fracture opening. The presence of abundant gas inclusions in veins recorded shale gas expulsion during rapid tectonic uplift, reflecting the destruction of shale gas preservation conditions. In contrast, no significant fracturing or cementation processes have been observed in the tectonically gentle zones. The different fracturing and cementation processes indicated that preservation conditions declined with increased fracture openings during uplift, correlating with the gas enrichment.

On the Formation of Auroral Spirals

AbstractThe paper contains a detailed analysis of the formation of an auroral spiral based on hitherto not published observations by the all‐sky camera in Kilpisjärvi, Northern Finland. We conclude that spirals appearing during a substorm form by a modification of the interface between tail and magnetosphere, the location of the generator current of the westward electrojet. Driven by the arriving flow bursts, this current is subject to ruptures by the appearance of a sequence of hook‐like structures. These structures can move eastward with speeds up to 3 km/s. The propagation is attributed to a constructive magnetic fracture process driven from behind by the power of the arriving flow bursts. Poleward bending and extension of a hook‐like structure, followed by a turning to the west and then equatorward, is the first step in spiral formation. It becomes the primary spiral arm, if a poleward arm grows out of weaker auroral structures, poleward and eastward of it. We suggest that the upward field‐aligned currents related to the bright spiral arms are largely balanced by adjacent downward currents. The electric fields associated with the connecting Pedersen currents are consistent with the counter‐clockwise motion. An important additional ingredient in the observed configuration is an eastward directed flow field, which is the generator of an additional upward current and possibly crucial for the spiral formation. Electric field data from literature throw confusing light on the propagation of a spiral, whether like a vessel in the ocean or by incorporating the magnetic flux ahead of it.

Experimental investigation on the fracture process and infrared radiation characteristics of structure rockburst under gradient loading

To investigate the macroscopic failure characteristics and infrared thermal imaging evolution of structure rockburst under gradient stress, gradient stress loading simulation tests were conducted using a true triaxial rockburst testing apparatus with combined gradient and hydraulic-pneumatic loading. Tests included three stress gradient coefficients and four structural surface angles. Macroscopic failure observations and infrared thermal imaging of the unloading surfaces were analyzed to understand the characteristics of structure rockburst and the influence of structural surfaces. Two infrared thermal imaging evolution parameters, the relative temperature mean (HRT) and the coefficient of variation (COV), were introduced to explore precursor indicators of structure rockburst. The results indicated that: (1) The dip angle of the structural surface (θ) and the stress gradient coefficient (k) both affect the peak stress during rockburst. (2) The structural surface angle significantly influences rockburst characteristics: θ = 30° or 60° results in shear slip-type rockbursts along or exposed on the structural surface, while θ = 0° or 90° manifests as buckling and tensile cracking-type rockbursts. (3) Infrared thermal imaging reveals that from initial loading to rockburst, temperature distribution transitions from uniform to normal, and then to non-normal. Accumulation of high-temperature points near rockburst indicates failure locations, with increased k intensifying non-normal distribution. (4) Peak values of HRT and COV positively correlate with k. (5) Fluctuations or sharp increases in HRT and COV values serve as precursors for predicting debris spalling and rockburst events.

Disturbance shear fracturing process and failure precursors of intermittent structural planes with different connectivity under true triaxial condition

The mechanisms, processes, and precursors of shear failure in rock structural planes under three-dimensional geostress and excavation disturbances are unclear, which poses challenges for disaster prediction and warning in underground rock engineering. Therefore, this study self-developed a new separation rolling-sliding shear device for rock structural plane under true triaxial condition, and proposed a true triaxial shear testing method to simulate the damage of rock structural plane induced by the three-dimensional geostress redistribution during engineering excavation and subsequent dynamic disturbance further triggering failure. True triaxial disturbance shear tests were conducted on intermittent structural planes with different connectivity. With connectivity increasing, disturbance shear strength and failure surface roughness of structural planes gradually decreases. An improved simplex acoustic emission spatial localization method was proposed to reveal disturbance shear fracturing evolution. During disturbance shear fracturing process, micro-tensile cracks dominate, and the proportion of micro-shear cracks continues to increase, showing a mixed tensile-shear fracture mechanism. Approaching to disturbance shear failure, the acoustic emission fractal dimension decreases rapidly then increases to the maximum value, the b-value decreases rapidly to less than 1, the rapid increase in LgN/b exceeds 3, and the low-frequency and high-amplitude dense fracture signal appears abundantly, which can be used as disturbance failure precursors of intermittent structural planes.

Dynamic analysis and experimental study of Down-the-Hole hammer based on DEM-MBD method

Pneumatic DTH(Down-The-Hole) hammer impact-rotary-compaction drilling is a well-established technology widely used in foundation engineering. This technique combines the advantages of impact rotary drilling and compaction drilling, making its working mechanism and fracturing process more complex. First, the dynamic differential equations for the DTH hammer piston and the coupled longitudinal vibration partial differential control equations for the DTH hammer and the soil stratum were established. The dynamic working characteristic curve of the piston was then obtained using the finite difference method(FDM).Subsequently, the DEM-MBD method was employed to compare the drilling efficiency of three types of drill bits—single-helix, multi-helix, and stepped bits—across three diameter specifications (Φ108, Φ135, Φ216) in three distinct strata: clay, pebble, and silty sand. The drilling efficiency of the multi-spiral bit was on average 11% and 21% higher than that of the single-spiral and stepped bits. From a micromechanical perspective, the shear failure effect of the drill bit on the soil was analyzed. The axial force of the drill bit (Z-axis) remained essentially consistent. Using the Y-axis as an example, 6% of the shear force for the single-helix bit falls within the 300–500N range, while 31% of the shear force for the multi-helix bit lies within the same range. Field tests demonstrated that the drilling efficiency and penetration effects simulated using the DEM-MBD method aligned with the actual test results, confirming the accuracy of this method.

How does heat generation affect the cut and chip wear of rubber?

AbstractTire wear is a fracture process that has a decisive impact on tire life and on the environment. When a tire rolls, a heating process occurs due to friction caused by the viscoelastic rubber sliding over uneven road. This process occurs globally in the contact patch area and locally around the asperity tips, heating the tread and transferring heat to the surrounding material. On roads with a good quality pavement, the stress, and therefore the heat, is evenly distributed throughout the rubber material of the tire, which has a direct effect on fatigue wear. In contrast, unevenly distributed stress, and therefore heat, is generated in the tread when the tire rolls and slides over sharp asperities in rough terrain. This leads to very pronounced, unstable fracture processes that cause unevenly distributed wear, known as cut and chip (CC). The extent of heat generation or temperature evolution and its effect on CC wear have not yet been investigated and described in scientific publications. Therefore, this study firstly presents the detailed characterization of the influence of temperature development on the CC wear of a styrene–butadiene rubber, which commonly is used in treads of consumer tires. The investigations were carried out using the unique instrumented cut and chip technique in combination with a high-speed thermography.

Accurate prediction of discontinuous crack paths in random porous media via a generative deep learning model

Pore structures provide extra freedoms for the design of porous media, leading to desirable properties, such as high catalytic rate, energy storage efficiency, and specific strength. This unfortunately makes the porous media susceptible to failure. Deep understanding of the failure mechanism in microstructures is a key to customizing high-performance crack-resistant porous media. However, solving the fracture problem of the porous materials is computationally intractable due to the highly complicated configurations of microstructures. To bridge the structural configurations and fracture responses of random porous media, a unique generative deep learning model is developed. A two-step strategy is proposed to deconstruct the fracture process, which sequentially corresponds to elastic deformation and crack propagation. The geometry of microstructure is translated into a scalar of elastic field as an intermediate variable, and then, the crack path is predicted. The neural network precisely characterizes the strong interactions among pore structures, the multiscale behaviors of fracture, and the discontinuous essence of crack propagation. Crack paths in random porous media are accurately predicted by simply inputting the images of targets, without inputting any additional input physical information. The prediction model enjoys an outstanding performance with a prediction accuracy of 90.25% and possesses a robust generalization capability. The accuracy of the present model is a record so far, and the prediction is accomplished within a second. This study opens an avenue to high-throughput evaluation of the fracture behaviors of heterogeneous materials with complex geometries.

Experimental Study on Acoustic Emission Features and Energy Dissipation Properties during Rock Shear-Slip Process.

The features of rock shear-slip fracturing are closely related to the stability of rock mass engineering. Granite, white sandstone, red sandstone, and yellow sandstone specimens were selected in this study. The loading phase of "shear failure > slow slip > fast slip" was set up to explore the correlation between fracture type, acoustic emission (AE) features, and energy dissipation during the rock fracturing process. The results show that there is a strong correlation between fracture type, energy dissipation, and AE features. The energy dissipation ratio of tension-shear (T-S) composite, shear, and tensile types is 10:100:1. The fracture types in the shear failure phase are mainly tensile and TS composite types. The differential mechanism of energy dissipation of different rocks during the shear-slip process is revealed from the physical property perspectives of mineral composition, particle size, and diagenetic mode. These results provide a necessary research basis for energy dissipation research in rock failure and offer an important scientific foundation for analyzing the fracture propagation problem in the shear-slip process. They also provide a research basis for further understanding the acoustic emission characteristics and crack type evolution during rock shear and slip processes, which helps to better understand the shear failure mechanism of natural joints and provides a reference for the identification of precursors of shear disasters in geotechnical engineering.

In-situ acoustic emission investigation of asphalt fracture process and damage quantification for different asphalt mixtures

Abstract The identification of cracking damage and the dynamic diagnosis of damage evolution are of great importance to prolong the service life of asphalt mixture materials. Acoustic emission (AE) can identify the formation and propagation of cracks by detecting the released energy of the damage; therefore, it provides a viable real-time technique to estimate the damage state in the context of structural health monitoring. In this study, the crack formation and propagation of three different asphalt mixtures were investigated using in-situ AE monitoring and microscopy imaging. A three-point bending test was performed using specimens fabricated from three different types of asphalt mixtures. The variation of AE parameters such as the cumulative AE energy and the AE count was obtained and correlated with the crack sizes to characterize the damage process of the three mixtures. Based on the AE parameters, the whole damage process can be divided into three distinct phases, namely, the elastic deformation, damage accumulation, and crack propagation. The AE parameters in the three different mixtures show unique features, and they can be used to capture the transition between phases and identify the damage state. A universal power law model is proposed to correlate the AE energy and the crack density for different types of asphalt mixtures, providing a viable means for damage quantification and predictive maintenance.

Influence of FSW process on dissimilar T-joint of aluminum alloy to pure copper

ABSTRACT This work investigated the influence of friction stir welding (FSW) parameter on forming the dissimilar T-joint of aluminum alloy 6061 (AA6061) to pure copper (C1100). The experimental outcomes indicated that the hook formation trended to move toward the center of welding line at low welding and/or high rotational speeds. In all cases, the kissing bond (KB) defects were significantly observed in advancing (AS) compared to retreating sides (RS). The thickness of diffusion and intermetallic compound (IMC) layers decreased with increasing welding and/or decreasing rotational speeds. A highest joint efficiency was attained with approximately 74% at 500/125 rev/mm of the skin test and 34% at 500/100 rev/mm of the stringer tests. The fracture process under the stringer test initiated from the AS to the RS along the interface while that of the skin test cracked in the heat affected zone. Improving dissimilar T-joint of AA6061/C1100 was impacted not only by controlling the KB defect but also the IMC layer thickness.

Analysis of spectra of external impacts, reactions to them and limit states under the non-stationary modes of loading

It is noted that under real operating conditions of technical systems the so-called non-stationary loading in which external loadings increase or decrease arbitrarily is realized most often. At the same time final ratios between strains and stresses are of practical interest to engineering calculations. It is shown that for simple cyclic loadings the state equations in final ratios which are based on the theory of small elastic-plastic strains can be used. In this case the existence of the uniform generalized deformation diagram assuming a final relation between the corresponding components of stresses and strains both for initial and cyclic stages of deformation is essential for the analysis of considered conditions of a non-stationary cyclic loading. The analysis of conditions of achieving limit states at all stages of the life cycle the criteria base and the system of the computation equations become more complicated and more saturated by the factors taken into account and among them first external and internal impacts of natural, technogenic, anthropogenous origin, load from these impacts, and the reactions of installations to such impacts and loads as well. At the same time a set of criteria defining a limit state is presented in the form of a functional dependence characterizing a set of force and deformation parameters of the state of technical system on the one hand, and, on the other hand, a complex of criterion characteristics of constructional materials taking into account their change during operation. The most important step of determining conditions for achievement of the limit state causing fracture is establishing the critical parts that determine risks of emergency and catastrophic situations in which processes of local and main fractures are initiated. In this case transition of the operated installation from design area to the area of initiation of refusals, accidents and disasters occurs when the parameters of damage, defectiveness and risk reach their critical values.

Study on the fracture behavior and toughening mechanisms of continuous fiber reinforced Wf/Y2O3/W composites fabricated via powder metallurgy

Tungsten (W) is a promising candidate material for the plasma facing components in fusion reactors. However, it has issues regarding the intrinsic brittleness. Tungsten fiber reinforced tungsten composites (Wf/W) have been developed based on the concept of extrinsic toughening mechanisms and they show a pseudo-ductile behavior during the fracture process. In the present work, continuous fiber reinforced Wf/Y2O3/W composites were fabricated via a powder metallurgy (PM) process, and the microstructure and mechanical properties were characterized. The fracture behavior and toughening mechanisms were analyzed in detail combining the results of experiments and numerical simulation. The Wf/Y2O3/W composites is toughened by multiple mechanisms such as fiber bridging, crack bending and deflection, interface de-bonding and plastic deformation of fiber. The energy dissipation by interface de-bonding can be neglected. However, it is a necessary factor to ensure any extrinsic toughening mechanisms. The main contribution of the energy dissipation while composite failure is the plastic deformation of fibers.

Numerical experimental study on the fracture process of shale containing internal prefabricated cracks based on CT scanning with different quartz contents

Numerical experimental study on the fracture process of shale containing internal prefabricated cracks based on CT scanning with different quartz contents

Influence of bedding on fracture toughness and failure patterns of anisotropic shale

The initiation and propagation of hydraulic fractures are closely related to the fracture ability of rocks. Such processes in shale reservoirs are, to a certain extent, controlled by bedding. However, the control mechanism of bedding on the anisotropy of fracture toughness and fracturing behavior remains unclear. In this study, a series of numerical notched semi-circular bend (NSCB) tests are conducted using the discrete element method (DEM) to investigate the influence of bedding properties on the anisotropy of fracture toughness and fracture patterns. Based on the DEM framework, a novel simulation method is proposed to accurately identify two key fracture indicators, the fracture process zone (FPZ) and crack tip opening displacement (CTOD), to reveal the fracture driving mechanism. The results show that the fracture toughness of shale is negatively correlated with bedding angles β but positively correlated with bedding spacing and bedding strength. Both the bedding strength and spacing significantly influence the fracture pattern of the specimens with β = 0°–60°, whereas the specimen with β = 90° is scarcely affected by the bedding planes. The evolution of the CTOD and FPZ in shale exhibits distinct phased characteristics. Due to the strong suppression effect of low-angle bedding planes on pre-peak crack deflection, the CTOD and FPZ exhibit opposite trends with respect to bedding angles before and after the peak load. This study facilitates the understanding of the fracture propagation process of anisotropic shale and could provide guidance for hydraulic fracturing design in shale reservoirs.

A Stress‐Driven Double‐Phase–Field Framework for Tensile Fracturing Processes in Transversely Isotropic Rocks

ABSTRACTWe present a double‐phase–field framework for tensile fracturing processes in transversely isotropic rocks. Two distinct phase‐field variables are introduced to represent smeared approximations of tensile fractures along the weak bedding planes and through the anisotropic rock matrix, respectively. Driving forces that control fracture propagation in the phase‐field framework are constructed as a stress‐based formula with a recently developed tensile failure criterion that distinguishes the two failure modes in transversely isotropic rocks. For numerical implementation, we adopt a staggered integration scheme and decouple the governing equations so that the displacement field and phase‐field variables can be updated in sequence for a given loading step. The finite element formulation of the proposed framework is introduced in detail in this paper and is implemented in an in‐house finite element code. The numerical implementation is then validated by reproducing the uniaxial tension test results of Lyons sandstone. After that, we conduct simulations on a pre‐notched square plate loaded in tension to demonstrate the features of the proposed framework. Finally, we conduct simulations of three‐point bending tests of Pengshui shale and show that the proposed model can reproduce the force–displacement curves and failure patterns of specimens with different bedding plane orientations observed in laboratory experiments.