Propagation Of Microcracks Research Articles

A New Symmetry-Enhanced Simulation Approach Considering Poromechanical Effects and Its Application in the Hydraulic Fracturing of a Carbonate Reservoir

The exploration of fractured-vuggy carbonate reservoirs usually involves hydraulic fracturing to maximize recovery. At present, effectively communicating natural discontinuities is a technical challenge. In this article, we investigated the origin and propagation of cracks in fractured-vuggy reservoirs using discrete element hydraulic fracturing simulations that included poromechanical effects. A particular focus on the microscopic force-displacement symmetry of adjacent pore pressures is introduced. Our results demonstrate that the poromechanical effect significantly increases the strength of overpressurized reservoir formations. Moreover, the effect of injected fluid viscosity on the hydraulic fracturing effectiveness was studied through two simulation tests. The outcomes highlight the critical influence of fluid viscosity on the propagation of micro-cracks in overpressure fractured-vuggy reservoirs.

Research on Acoustic Emission Characteristics and Crack Evolution during Rock Failure under Tensile and Tensile- and Compressive-Shear Stress States

Tensile, compressive-shear, and tensile-shear failure are three typical failure modes of rock- and lining-support structural materials in underground engineering. Comparative studies of the acoustic emission (AE) evolution characteristics of specimens of the same shape and size under different stress states are of great significance in determining universal disaster warning guidelines. Based on a self-developed multi-functional test system, direct tensile, compressive-shear, and tensile-shear tests were conducted on intact and jointed rock-like specimens, comparing AE amplitude and peak frequency parameters under different failure modes from the perspectives of crack scale and crack type evolution. All failure tests were monitored with microcrack propagation at an early stage until ultimate rupture occurred at the peak-load moment. As a result, the “quiet-period” could only be observed from the compressive-shear test. The AE signals distributed in three bands can be used as an indicator of failure identifications. Tensile and shear cracks can be identified by strong and weak amplitudes of low- and high-frequency signals. These results enhance the knowledge of the failure modes of rock mechanics for more applications in monitoring disasters in rock engineering.

Durability Properties of Macro-Polypropylene Fiber Reinforced Self-Compacting Concrete.

Concrete is one of the most commonly used construction materials; however, its durability plays a pivotal role in areas where the concrete is exposed to severe environmental conditions, which initiate cracks inside and disintegrate it. Randomly distributed short fibers arrest the initiation and propagation of micro-cracks in the concrete and maintain its integrity. Traditional polypropylene fibers are thin and encounter the problem of balling effects during concrete mixing, leading to uneven fiber distribution. Thus, a new polypropylene fiber is developed by gluing thin ones together, forming macro-polypropylene fibers. Thus, different amounts of fibers, 0-1.5% v/f with an increment of 0.5% v/f, are used in different grades of concrete to study their impact on durability properties, including resistance to freezing and thawing cycles, sulfate, and acid attacks. A total of 432 cube samples were tested at 28, 56, and 92 days. The results reveal that the maximum durability, in terms of compressive strength loss, is noted with a fiber content of 1% with improved resistance of 72%, 54%, and 24% against freeze-thaw cycles, sulfate attack, and hydrochloric acid attack, respectively, at 92 days. Thus, the resulting fiber-reinforced concrete may be effective in areas where these extreme exposure conditions are expected.

Mechanical behavior of cermets with different morphology of ceramic grains

It is challenging to describe quantitatively the effects of ceramic grain morphology on the stress and strain distributions in the cermet materials under different loading conditions. Here the dependence of the distribution of residual thermal stress (RTS) in the as-prepared cermets and its mechanical behavior under external stress on the morphology of ceramic grains were quantified and resolved by modeling and experimental studies, using WCCo cermets as a typical example. A model was proposed to visualize the strain localization and reveal the mechanisms of the initiation and propagation of micro-cracks in the microstructure of cermets under varying loads, and the calculation results were verified by experimental measurements. It was discovered that the effects of WC grain morphology on the mechanical behavior and failure mode of cermets are distinctly different under tension or compression loading conditions, whose mechanisms were disclosed based on detailed analysis of micro-scale strain responses. The results indicate that it is feasible to modulate the strain distribution by tailoring the grain shape character for adapting to external loading, hence optimizing the material mechanical behavior.

3D mesoscale study on the effect of ITZ and aggregate properties on the fracture behaviors of concrete based on discrete element method

The discrete element method (DEM) was utilized to study the effect of the interface transition zone (ITZ) and coarse aggregate properties on the compressive fracture behaviors of concrete at a micromechanical level, in which the ITZ was represented by the contact bond and the coarse aggregate was crushable. The linear parallel bonded model was introduced to represent the constitutive model of concrete, and the meso-scale parameters were determined by laboratory mechanical tests. And then, a series of numerical tests were conducted under compressive loading, and the research objects contain ITZ strength, and aggregate properties (i.e., strength, content, size, and shape). The simulation results show that as the ITZ strength increases, the compressive strength shows a nonlinear increase trend. Moreover, as the aggregate strength and content increase, both compressive strength and strain to damage consequently increase, the aggregate shape is independent of the compressive strength. The increase in the aggregate size can reduce the total surface area of the aggregate, leading to a small compressive strength. The compressive strength appears to increase first and then decrease in trend as aggregate surface roughness increases. In addition, the increase in aggregate strength and content induces the failure pattern changes from local microcrack propagation to global failure. Overall, this work comprehensively evaluates the effect of ITZ and coarse aggregate properties on the damage of concrete at a micromechanical level.

Characteristic stress variation and microcrack evolution of granite subjected to uniaxial compression using acoustic emission methods

The size of mineral grain has a significant impact on the initiation and propagation of microcracks within rocks. In this study, fine-, medium-, and coarse-grained granites were used to investigate microcrack evolution and characteristic stress under uniaxial compression using the acoustic emission (AE), digital image correlation (DIC), and nuclear magnetic resonance (NMR) measurements. The experimental results show that the characteristic stress of each granite decreased considerably with increasing grain sizes. The inflection points of the b-value occurred earlier with an increase in grain sizes, indicating that the larger grains promote the generation and propagation of microcracks. The distribution characteristics of the average frequency (AF) and the ratio of rise time to amplitude (RA) indicate that the proportion of shear microcracks increases with increasing grain size. The NMR results indicate that the porosity and the proportion of large pores increased with increasing grain size, which may intensify the microcrack evolution. Moreover, analysis of the DIC and AE event rates suggests that the high-displacement regions could serve as a criterion for the degree of microcrack propagation. The study found that granites with larger grains had a higher proportion of high-displacement regions, which can lead to larger-scale cracking or even spalling. These findings are not only beneficial to understand the pattern of microcrack evolution with different grain sizes, but also provide guidance for rock monitoring and instability assessment.

Influence of Different Glide Path Techniques on Microcrack Formation after Two Different Root Canal Preparation Treatments: Micro Computed Tomography Analysis.

This study aimed to evaluate the effect of WaveOne Glider (WOGG) with Waveone Gold (WOG) primary reciprocation systems on root dentine microcrack formation and to know the effect of TruNatomy Glider (TRNG) with TruNatomy (TRN) prime rotary systems on root dentine microcrack formation. In this study, 40 extracted mandibular first molar roots were selected and divided randomly into four groups (n=10). Group MWOG: a manual glide path was performed + WOG primary. Group MTRN: manual glide path performed+TRN prime. Additionally, group WOGG: glide path preparation with WOGG+WOG primary. Finally, for group TRNG, the glide path preparation was performed with TRNG+TRN prime. Micro-CT was used for pre and post-instrumentation image analysis. Statistical analysis was performed using the Kruskal- Wallis test (p<0.05) with Two-way ANOVA. The Kruskal-Wallis test showed no significant differences among all groups in all thirds for pre and post-instrumentation regarding the crack formation. The Two-way ANOVA showed no significant difference or interaction between the ways of glide path preparation, whether manual or reciprocal WOGG, or between the rotary TRNG and the motion used in root canal preparation (rotary TRN or reciprocal WOG) regarding the crack formation and propagation. Microcrack formation and propagation occurred independently of using different glide path techniques (manual, rotary, or reciprocal).

The biomechanical implications of lacunar and perilacunar microarchitecture on microdamage accumulation in cortical Bone

Purpose This study aims to explore how the microarchitectural features of lacunae and perilacunar zones impact the biomechanics of microdamage accumulation in cortical bone, crucial for understanding bone disorders' pathogenesis and developing preventive measures. Methods Utilizing the phase-field finite element method, the study analyzed three bone unit models with varying microarchitecture: one without lacunae, one with lacunae, and one including perilacunar zones, to assess their effects on cortical bone's biomechanical properties. Results The presence of lacunae was found to increase microcrack initiation risk, acting as nucleation points and accelerating microcrack propagation. Proximity to Haversian canals exacerbated stress concentration, speeding microdamage progression. Conversely, perilacunar zones mitigated both initiation and propagation. An elevated critical energy release rate correlated with slower crack growth and reduced damage severity. Conclusions The research sheds light on the intricate mechanisms governing microcrack behavior in compact bone, highlighting the significant role of bone's microarchitectural features in its biomechanical response to microdamage. These insights are valuable for the development of strategies to prevent and treat bone-related disorders.

Deterioration of equivalent thermal conductivity of granite subjected to heating-cooling treatment

Understanding the thermal conductivity of granite is critical for many geological and deep engineering applications. The heated granite was subjected to air-, water-, and liquid nitrogen (LN2-) coolings in this context. The transient hot-wire technique was used to determine the equivalent thermal conductivity (ETC) of the granite before and after treatment. The deterioration mechanism of ETC is analyzed from the meso-perspective. Finally, the numerical model is used to quantitatively study the impact of cooling rate on the microcrack propagation and heat conduction characteristics of granite. The results show that the ETC of granite is not only related to the heating temperature, but also affected by the cooling rate. The ETC of granite decreases nonlinearly with increasing heating temperature. A faster cooling rate causes a greater decrease in ETC at the same heating temperature. The higher the heating temperature, the stronger the influence of cooling rate on ETC. The main explanation for the decrease in ETC of granite is the increase in porosity and microcrack density produced by the formation and propagation of pore structure and microcracks during heating and cooling. Further analysis displays that the damage of granite at the heating stage is induced by the difference in thermal expansion and elastic properties of mineral particles. At the cooling stage, the faster cooling rate causes a higher temperature gradient, which in turn produces greater thermal stress. As a result, it not only causes new cracks in the granite, but also aggravates the damage at the heating stage, which induces a further decrease in the heat conduction performance of granite, and this scenario is more obvious at higher temperatures.

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.

In-Situ Fatigue Lifetime Modeling of a Reinforced Membrane by Projecting Critical Accumulated Plastic Dissipation Energy from Pressure Differential-Accelerated Mechanical Stress Tests

Polymer electrolyte fuel cells (PEFCs) have become increasingly appealing over internal combustion engines because of their high efficiency, low operating temperature, and zero CO2 emissions. Nevertheless, the transportation sector necessitates high durability and reliability, which may be difficult to predict for emerging technologies. A crucial aspect is the ability of the thin membranes that conduct ions in PEFCs to endure the chemical and mechanical stresses that arise during dynamic operations. For mechanical fatigue, temperature and relative humidity (RH) fluctuations induce dynamic stresses that lead to the formation and propagation of microcracks in the membrane. As the membrane is confined by other components in the membrane-electrode assembly (MEA), changes in temperature and humidity can generate thermal and swelling strains in the membrane, leading to dynamic and residual stresses [1].The US Department of Energy sets a passing criterion of 20,000 RH cycles for mechanical fatigue assessment. However, many modern reinforced membranes have already passed this threshold without failing [2]. Therefore, Ref [3] introduced a pressure differential between the cathode and anode sides of the membrane at 80°C to speed up the testing. In this research, the pressure differential-accelerated mechanical stress test (ΔP-AMST) method was applied to a reinforced membrane at two different temperatures and with four times faster humidity cycles in a wider range of ΔPs. This objective is to project the mechanical fatigue lifetime from the ΔP-AMST to the membrane under its in-situ conditions by using the critical accumulated plastic dissipation energy (CAPDE) in ΔP-AMSTs and the plastic dissipation energy (PDE) during a single cycle of humidity that the membrane experiences under complete fuel cell settings. The first step involves performing a series of ∆P-AMST, and in the second step, a finite element model (FEM) for ∆P-AMST based on the developed constitutive model for the tensile tests that covers temperature, humidity, and swelling strain impacts is built, and therefore its S-N curve is extracted. Next, a FEM model for a complete fuel cell is created, and the mechanical fatigue life is estimated by dividing the CAPDE in ∆P-AMST by the PDE in one cycle of the in-situ modeled membrane by considering amplitude stress as a link between the full fuel cell model and ∆P-AMST, as illustrated in Figure 1 and verified by previous studies [4,5]. In the final step, we will also discuss opportunities to integrate the present mechanical fatigue model with a chemical degradation module to simulate its impacts on fatigue lifetime. This integration includes the two main effects of chemical degradation, which are reflected in the thickness of the reinforced membrane [6] and its updated CAPDE [7]. Acknowledgments The authors gratefully acknowledge AVL Fuel Cell Canada and Mitacs for supporting this project. The authors also thank Roger Penn and Amy Nelson for technical advice.

Heterostructural nanolamellar oxide-dispersion-strengthened ferritic alloy with exceptional strength and ductility

Nanostructured ferritic alloys (NFAs) have demonstrated exceptional mechanical properties (strength and creep resistance) and radiation tolerance. However, the high strength (typically above 1.5 GPa) always comes with a significantly limited ductility. Here, we report a novel heterostructural nanolamellar ferritic alloy (NLFA) with microcrystalline (MC) domains (∼ 35 % of volume fraction) dispersed in an ultrafine-grained (UFG) matrix (∼ 500 nm grain size), enabling the alloy to achieve ultrahigh yield strength of 2 GPa and an excellent tensile elongation of ∼ 13 %. The nanolamellar-shaped grains remarkably strengthen the alloy via grain-boundary strengthening. The soft MC domains can effectively suppress the microcrack initiation and propagation along the oxide-rich preliminary powder boundaries (PPBs) via local plastic deformation. Moreover, linear oxide bands can induce numerous interlayer cracking and trigger delamination toughening, collaboratively ensuring the alloy a large ductility. The present study opens a new frontier toward high tensile ductility for the most cutting-edge ultra-strong NFAs.

Supershear rupture with a two-scale damage model

An investigation of intersonic rupture is performed in a two-scale framework, with a damage model obtained by homogenization from microstructures with dynamically evolving mode II microcracks. The macroscopic fracture is analyzed in connection with the microcrack propagation. It is found that the supershear transition and the subsequent intersonic failure are achieved by collective effects of propagation and coalescence of sub-Rayleigh microcracks ahead the macroscopic rupture front. This is similar to the behavior experimentally observed for mode I dynamic brittle fracture (Guerra et al. 2012). Numerical simulations of a pre-cracked plate under shear loading are performed and characteristic features like the formation of shear shock waves, the mother-daughter or direct transition mechanisms, the specific evolutions of the rupture speed or the symmetry breaking near the crack tip are retrieved. Parametric studies allow to determine the influence of the size of the microstructure, the applied loading and the fracture properties of the solid. For the asymmetric impact of a Homalite plate problem (Rosakis et al., 1999), the numerical results obtained with the damage model are successfully confronted with the experimental observations.

Acoustic emission characteristics of micro-discharges in initial pulsed PEO process on 60% SiCp/Al composite

The initial discharge process of pulsed plasma electrolytic oxidation (PEO) on the 60% SiCp/2009 aluminum metal matrix composite (Al MMC) in silicate solution was monitored by acoustic emission (AE) technique. Parameters and correlations of AE signals on the Al MMC sample and under water were analyzed, and their generation mechanism was discussed. It was found that the peak amplitudes of AE signals and AE hits during the pulse time quickly increased with the increase of micro-discharge intensity, and the absolute energy of AE signals improved several orders of magnitude. Moreover, different from the peak amplitude, duration and rise time, the duration and count had a strong correlation. Elastic stress waves resulted from the microjet of plasma bubble collapse, the inner-surface friction inside discharge channel, the expansion-shrinkage process of plasma bubbles and micro-crack propagation during rapid solidification of melt are sources of AE signals on the Al MMC sample during the pulse time. However, the expansion-shrinkage process of plasma bubbles plays a key role in the generation of underwater AE signals. In the pause time of one pulse period, the bursting and moving of vapor bubbles result in weak AE signals. It is demonstrated that the AE technique can effectively characterize the features of micro-discharges within a pulse period.

Hot Tensile Deformation Mechanism and Fracture Behavior of the ZW31/PMMC Laminate

In this work, a Mg-Zn-Y (ZW31) alloy with good plasticity was introduced into 10 μm 10 vol% SiCp/AZ91 composite materials (PMMCs) via the extrusion compound method, and then the ZW31/PMMC laminate was prepared via multi-pass hot rolling. The hot deformation mechanism and elevated temperature tensile fracture mechanism of ZW31/PMMC laminates were studied using the elevated temperature tensile test. The elevated temperature deformation mechanism is influenced by the strain rate. At low strain rates, grain boundary slip is the primary elevated temperature deformation mechanism of the ZW31/PMMC laminate. However, at high strain rates, the activation of pipeline diffusion is facilitated by the particle deformation zone (PDZ) in the PMMC layer with a high dislocation density, leading to the dominance of dislocation climbing as the main mechanism for elevated temperature deformation of the laminate. Additionally, the implementation of a ZW31/PMMC laminate structure effectively inhibits the initiation and propagation of cavities and microcracks within the laminate layer along the normal direction (ND) while simultaneously blunting crack tips via lattice dislocation emission toward the ZW31 layer. Upon cracking of the PMMC layer, stress concentration occurs in the fracture area of the ZW31 layer, ultimately resulting in necking-induced detachment.

Investigation on cracking resistance mechanism of basalt-polypropylene fiber reinforced concrete based on SEM test

In order to study the anti-crack mechanism of basalt-polypropylene fiber reinforced concrete (BPFRC), based on the restrained shrinkage test of BPFRC, the anti-crack enhancement mechanism of basalt fiber and polypropylene fiber was discussed through SEM microscopic test, and the sensitivity index of BPFRC's anti-crack performance was studied by combining grey relational theory and entropy weight method. The results show that basalt fiber has a significant inhibitory effect on the initiation and propagation of micro-cracks, and polypropylene fiber has a more obvious inhibitory effect on the propagation of macro-cracks. Mixing basalt fiber and polypropylene fiber into concrete can improve the pore structure of concrete matrix, reduce the initial defects of concrete matrix, enhance the density of the interface transition zone between aggregate and matrix, make concrete play a bridging role in different stages of stress deformation, and enhance the crack resistance of concrete more significantly. Grey relational theory combined with entropy weight method shows that the splitting tensile strength has the most significant influence on the crack resistance of BPFRC. This study can provide a new method for the sensitivity index analysis and crack resistance evaluation of concrete.

Linear Strain Sensors via a Spatial Heteromodulus Tricontinuous Structure Design for High-Resolution Recording of Snoring Breath.

Porous conductive elastomer composites are very attractive for designing flexible and air-permeable mechanical sensors for healthcare, while it is challenging to achieve a linear and sensitive electromechanical response over a wide strain range for high-resolution recording of physiological activities and body motions. Here, a scalable strategy is developed to construct porous elastomer composites with a bamboo-shaped heteromodulus microstructure in the pores for the fabrication of linear stretchable strain sensors. Such a spatial heteromodulus microstructure is fabricated via phase separation and selective location of high-modulus phase during melt compounding of elastomers and thermoplastics, together with green etching of the water-soluble plastic in the tricontinuous elastomer composites. The bamboo-shaped heteromodulus microstructure is constructed on the pore struts via the fracture of a high-modulus polymer self-assembled on the pore surface and relaxation recovery of the elastomer matrix after prestretching, which blocks the propagation of cut-through microcracks upon stretching. The composites with super low resistance after in situ growth of silver nanoparticles sustain up to 110% tensile strain with a linear and sensitive electromechanical response, demonstrating potential applications in discriminating respiration status and monitoring snoring breath. This work unveils a new approach to fabricate high-performance air-permeable strain sensors in a simple and scalable way.

Mechanical properties and fracture behavior in copper-brass heterostructured fibers

Inspired by natural structures like bamboo and bone, the incorporation of heterogeneous fibrous structure plays a significant role in skillfully optimizing their mechanical properties. A set of copper-brass heterostructured fibers has been successfully designed and fabricated by a novel strategy, named DRA (diffusion welding, rotary swaging and annealing). This leads to a significant improvement in the synergy between strength and ductility, which can be attributed to the HDI (hetero-deformation induced) hardening effect caused by the high density of hetero-interfaces. The experimental results found that once the GND (geometrically necessary dislocations) density around the hetero-interface reaches the saturation, some GNDs can spread into the adjacent copper and brass zones to alleviate partial stress concentration. The presence of four-sides interfaces in the fibrous composite restricts the dislocation storage capacity of individual fiber, thereby abundant GNDs accumulating near the hetero-interface to produce significant stress concentration and HDI stress. Simultaneously, the severe stress concentration promotes the initiation and propagation of micro-cracks at the hetero-interfaces. These results lead to a more significant HDI hardening while a faster decrease in the work hardening rate of the hetero-fiber during the later stage of tension. Our findings have significantly enhanced our understanding of the deformation mechanism of heterostructured materials. Additionally, this innovative structural design strategy has proven to be highly valuable in the development of heterostructured materials with substantial potential for various applications.

Humidity evolution in cement-based materials is monitored using fluorescent molecular humidity-sensitive responses

In situ detection technology for cement-based materials typically produces unclear physical meanings, which limits research on durability issues such as heat-moisture transfer, dry-wet cycles and microcrack propagation. Taking the external thermal insulation system of building exterior walls as an example, this paper explores in-situ fluorescence sensing techniques for moisture evolution of cementitious materials using the quenching effect caused by the aggregation of fluorescent molecules. This study includes the monitoring of fluorescence intensity under humidity evolution and a stability study of fluorescence intensity under different temperatures and humidity cycles. Results showed that the gradual transition of the polymeric state of fluorescent molecules led to the first increase and then decrease of fluorescence intensity with increasing humidity (0 % RH-95 % RH). The fluorescence intensity peaked at 65 % RH, decreases at high humidity due to aggregated caused quenching effects, and maintained good stability after 8 humidity cycles. With regard to ambient temperature, the fluorescence intensity decreased with increasing temperature (5 °C – 40 °C), and the fluorescence intensity peaked at 5 °C. The temperature sensitivity of the fluorescence is weakened by the methyl effect of the rhodamine 6G fluorescent molecules and the thermostatic properties of the insulation boards. Fluorescence sensing technology for humidity evolution of cement-based materials has moisture sensitivity and fluorescence stability, and thus has a wide range of applications in the sensing of humidity evolution of civil engineering materials.

Microstructural Controls on the Uniaxial Compressive Strength of Porous Rocks Through the Granular to Non‐Granular Transition

AbstractUnder uniaxial compression, a porous rock fails by coalescence of stress‐induced microcracks. The micromechanical models developed to analyze uniaxial compressive strength data consider a single mechanism for the initiation and propagation of microcracks and a fixed starting microstructure. Because the microstructure of clastic porous rock transitions from granular to non‐granular as porosity decreases during diagenesis, their strength cannot be captured by a single model. Using synthetic samples with independently controlled porosity and initial grain radius we show that high‐porosity granular samples, where microcracks grow at grain‐to‐grain contacts, are best described by a grain‐based model. Low‐porosity non‐granular samples, where microcracks grow from pores, are best described by a pore‐based model. The switch from one model to the other depends on porosity and grain radius. We propose a regime plot that indicates which micromechanical model may be more suitable to predict strength for a given porosity and grain radius.