Coalescence Of Microcracks Research Articles

Failure Process of Modeled Recycled Aggregate Concrete under Uniaxial Compression.

This paper investigates the failure processes of recycled aggregate concrete by a model test and numerical simulations. A micromechanical numerical modeling approach to simulate the progressive cracking behavior of the modeled recycled aggregate concrete, considering its actual meso-structures, is established based on the discrete element method (DEM). The determination procedure of contact microparameters is analyzed, and a series of microscopic contact parameters for different components of modeled recycled aggregate concrete (MRAC) is calibrated using nanoindentation test results. The complete stress–strain curves, cracking process, and failure pattern of the numerical model are verified by the experimental results, proving their accuracy and validation. The initiation, growth, interaction, coalescence of microcracks, and subsequent macroscopic failure of the MRAC specimen are captured through DEM numerical simulations and compared with digital image correlation (DIC) results. The typical cracking modes controlled by meso-structures of MRAC are concluded according to numerical observations. A parameter study indicates the dominant influence of the macroscopic mechanical behaviors from the shear strength of the interfacial transition zones (ITZs).

Experimental study on dynamic mechanical responses of coal specimens under the combined dynamic-static loading

Dynamic mechanical responses of the coal and rock mass under the combined dynamic-static loading are critical issues for exploring the disaster-causing mechanism of rock burst in coal mining. In this paper, the acoustic emission (AE) technique and high-speed camera imaging were employed, and the burst proneness of the coal specimen, the variation of AE parameters, the specimen failure mode, and the particle distribution of the fractured specimens were investigated by the combined dynamic-static loading experiments. The results indicate that the burst proneness of the coal specimen increases non-linearly as the dynamic loading rate increases. Both the energy input rate, the ringdown counts, and the hits undergo the slow increase, fast increase, and abrupt increase stages. The failure mode of the specimen is changed from shear failure to axial splitting and the final bursting failure. The relationships between the fractal dimension of the specimen fragmentation mass and the dynamic loading rate can be described by a quadratic function. When the specimen reaches the maximum damage degree, an extreme value of the dynamic loading rate (2.8 × 10−3 s−1) is obtained in experiments. The disaster-causing mechanism of rock burst induced by the combined dynamic-static load is discussed from the aspects of the burst proneness of coal specimens, dynamic variation of AE parameters, the change of failure modes of specimens, and the propagation and coalescence of microcracks.

Accelerating high-strain continuum-scale brittle fracture simulations with machine learning

Failure in brittle materials under dynamic loading conditions is a result of the propagation and coalescence of microcracks. Simulating this discrete crack evolution at the continuum level is computationally expensive or, in some cases, intractable, resulting in the need to make broad assumptions or neglect key physics. We have developed an approach using machine learning that overcomes the current inability to represent meso-scale physics at the macro-scale. Our approach leverages damage and stress data from a computationally expensive high-fidelity model that explicitly resolves microcrack behavior to build an inexpensive machine learning emulator. Once trained, the machine learning emulator is used to predict the evolution of crack length statistics, which then informs a continuum-scale constitutive model. This results in a significant speed-up of the workflow by four orders of magnitude. Both the machine learning emulator and the continuum-scale model are validated against the high-fidelity model and experimental data, respectively, showing excellent agreement. There are two key findings. The first is that we can reduce the dimensionality of the problem, establishing that the machine learning emulator only needs the length of the longest crack and one of the maximum stress components to capture the necessary physics. Another compelling finding is that the emulator can be trained in one experimental setting and transferred successfully to predict behavior in a different setting.

Hydrogen effects on electrochemically charged additive manufactured by electron beam melting (EBM) and wrought Ti–6Al–4V alloys

The hydrogen behavior in electrochemically charged electron beam melting (EBM) and wrought Ti–6Al–4V alloys with a similar β-phase content of ~6 wt% was compared. Hydrogenation resulted in the formation of microvoids, either adjacent to the surface or along interphases, their coalescence, and emanation of microcracks around them. The microstructure of the EBM alloy displayed a discontinuous arrangement of β-phase particles in the short-transverse direction, a smaller lattice constant of the β-phase, and more α/β interphase boundaries, making the EBM alloy more susceptible to hydrogen embrittlement. The hydrogenated alloys were composed of αH (hcp) and βH (bcc) solid solutions as well as δa (fcc) and δb (fcc) hydrides with lattice parameters that have not been reported before. These hydrides were transformed from the primary α-phase. No major microstrain difference was observed between the EBM and wrought alloys, either before or after hydrogenation. Microstrains in the α and β phases increased following hydrogenation; they were larger in the βH and δa phases compared to the αH and δb phases. A post-treatment that would increase the size of the β particles is suggested to improve the resistance of EBM Ti–6Al–4V to hydrogen-induced damage.

Catastrophic Failure: How and When? Insights From 4‐D In Situ X‐ray Microtomography

AbstractCatastrophic failure of brittle rocks is important in managing risk associated with system‐sized material failure. Such failure is caused by nucleation, growth, and coalescence of microcracks that spontaneously self‐organize along localized damage zones under compressive stress. Here we present X‐ray microtomography observations that elucidate the in situ micron‐scale processes, obtained from novel tri‐axial compression experiments conducted in a synchrotron. We examine the effect of microstructural heterogeneity in the starting material (Ailsa Craig microgranite; known for being virtually crack‐free) on crack network evolution and localization. To control for heterogeneity, we introduced a random nanoscale crack network into one sample by thermal stressing, leaving a second sample as‐received. By assessing the time‐dependent statistics of crack size and spatial distribution, we test the hypothesis that the degree of starting heterogeneity influences the order and predictability of the phase transition between intact and failed states. We show that this is indeed the case at the system‐scale. The initially more heterogeneous (heat‐treated) sample showed clear evidence for a second‐order transition: inverse power law acceleration in correlation length with a well‐defined singularity near failure and distinct changes in the scaling exponents. The more homogeneous (untreated) sample showed evidence for a first‐order transition: exponential increase in correlation length associated with distributed damage and unstable crack nucleation ahead of abrupt failure. In both cases, anisotropy in the initial porosity dictated the fault orientation, and system‐sized failure occurred when the correlation length approached the grain size. These results have significant implications for the predictability of catastrophic failure in different materials.

Fatigue Crack Initiation in the Iron-Based Shape Memory Alloy FeMnAlNiTi

The newly developed FeMnAlNiTi shape memory alloy (SMA) holds significant promise due to its desirable properties including ease of processing, room temperature superelasticity, a wide superelastic window of operation, and high transformation stress levels. In this study, we report single crystals with tensile axis near $$\left\langle {123} \right\rangle$$ exhibiting transformation strains of 9% with a high transformation stress of 700 MPa. The functional performance revealed excellent recovery of 98% of the applied strain in an incremental strain test for each of the 40 applied cycles. Concomitantly, the total residual strain increased after each cycle. Accumulation of residual martensite is observed possibly due to pinning of austenite/martensite (A/M) interface. Subsequently, under structural fatigue loading with a constant strain amplitude of 1%, the recoverable strains saturate around 1.15% in local residual martensite domains. Intermittent enhancement of recoverable strains is observed due to transformation triggered in previously untransformed domains. Eventually, fatigue failure occurred after 2046 cycles and the dominant mechanism for failure was microcrack initiation and coalescence along the A/M interface. Thus, it is concluded that interfacial dislocations, which play a crucial role in the superelastic (SE) functionality, invariably affect the structural fatigue performance by acting as the weakest link in the microstructure.

BI-AXIAL STRESS-STRAIN RELATIONS FOR CONCRETE WITH ISOTROPIC AND ANISOTROPIC DAMAGE

For the load paths with small or no confining pressure, the nucleation, growth, and coalescence of cracks and micro-cracks are the main sources of non-linearity in the observed behavior of concrete. The formations of cracks and micro-cracks destroy material bonds and render the material more compliant. These are typically irreversible internal changes and lead to strong directionality in concrete response. To model such nonlinear material behavior, a constitutive law for concrete utilizing damage mechanics is presented for small and isothermal deformations. The general theory is cast within the framework of the internal variable theory of thermodynamics where the dissipation inequality is used. A damage criterion is subsequently obtained using a damage function and the loading-unloading statement is provided. The decomposition of the compliance tensor into damaged and undamaged states is outlined and the flow rules for the inelastic strains are provided. Specific damage response tensors for isotropic and anisotropic modeling is proposed along with numerical simulations that are plotted for illustrating differences between isotropic and anisotropic formulations.

A STOCHASTIC MODEL FOR CRACK INITIATION LIFE PREDICTION OF AN AUSTENITIC STAINLESS STEEL UNDER CONSTANT AMPLITUDE LOADING

Predicting crack initiation life (CIL) of a mechanical component or a structure in service remains difficult since the crack formation process is of stochastic nature. To ensure a high level of safety and reliability, it is essential to have an appropriate probability distribution law of the CIL to ensure that cracks can be detected before reaching a critical length. In the present study, a stochastic model is used to predict the number of cycles corresponding to the formation of a crack 500 μm long resulted from the nucleation, growth, and coalescence of multiple microcracks. The model is applied in the case of a 316L austenitic stainless steel for different plastic strain ranges.

Embedded smart PZT-based sensor for internal damage detection in concrete under applied compression

Abstract An embedded PZT-based sensor for monitoring internal damage in concrete is presented. An experimental program for producing controlled increments in levels of stress and damage in the concrete is developed. Formation and coalescence of microcracks are evaluated using digital image correlation. The electrical impedance (EI) measurements are recorded from the embedded sensor for different levels of stress and damage in the concrete. Changes in the EI signature associated with the resonant conditions of the PZT patch produced by stress and damage in the concrete are identified. The EI measurements provide a sensitive indication of the applied stress. Applied stress and damage in the concrete produce two counteracting effects on the EI resonant frequency. The internal damage in the concrete is detected from the EI measurement significantly earlier than the occurrence of surface cracking. Embedded sensors are more sensitive to the internal cracking in concrete compared to surface mounted PZT patches.

Multi-scale crack propagation and damage acceleration during uniaxial compression of marble

The macroscale fracture of rocks is a progressive process consisting of the initiation, propagation, and coalescence of numerous microcracks. In general, problems regarding the microscopic and mesoscale characteristics of the crack propagation evolution process of the intact specimen and prefabricated crack specimen, the intrinsic physical control factors of the critical acceleration of the near-failure damage, and the rock damage prediction analysis are difficult to solve. This study investigated the micro- and meso-damage evolution characteristics based on the scanning electron microscopy (SEM) tests of marble specimens under compression. The propagation and evolution characteristics of microcracks at different locations in marble specimen under load were analyzed based on the SEM image. In addition, the microcrack and mesocrack propagation qualitative analyses of the marble specimen under different loadings were performed. Using a multi-scale damage mechanics model and a newly constructed geometric model that reflected the crack propagation evolution, the macroscopic strain function of the geometric parameters of the damage and crack under loading was derived. The factors influencing the acceleration of crack development were revealed, along with the basis of the relationship between the evolution of damage and fracture parameters and external loads over different scales. The SEM timely crack propagation experimental results of the intact and pre-formed crack samples verified and illustrated the correlation. Although the different samples presented different damage expansion characteristics and varying warning levels before instability, common features were observed at the critical locations. This study aimed to generate a warning for possible larger fractures under different engineering conditions to establish a fundamental research foundation for the safety and stability of engineering projects.

Damage mechanisms in CFRP/HNT laminates under flexural and in-plane shear loadings using experimental and numerical methods

This study is conducted to thoroughly scrutinize the role of nanotubes on the physics behind the deformation mechanisms and damage development in fiber reinforced polymer composites by using acoustic emission, digital image correlation, infrared thermography, fractography, and non-local meshless-numerical analysis, namely, Peridynamics. Carbon fiber laminates with and without Halloysite nanotubes (HNTs) are prepared and tested under flexural and in-plane shear loads. In depth analysis of the cumulative counts for acoustic emission data shows that the addition of HNTs mainly promotes the failure mechanisms associated with matrix cracking. Digital image correlation and infrared thermography analysis clearly prove that nanotubes prevent the coalescence of microcracks by blocking crack propagation or diverting its path. Fractography analysis shows that HNTs addition improves the interfacial strength despite promoting microcracks in the matrix. The hindrance of crack growth, crack tip splitting, and prevention of crack coalescence by HNTs clusters, are supported successfully by performing Peridynamic analysis.

An analytical model of multi-stress drops triggered by localized microcrack damage in brittle rocks during progressive failure

Stress drops in stress–strain constitutive curves of intact brittle rocks under high confining pressure have great significance for evaluating the earthquake mechanism and the safety of deep underground engineering. Microcrack growth in intact rock strongly influences the stress drops. However, the theoretical model of microcrack growth-dependent multi-stress drops rarely is proposed in stress–strain curves of intact rocks. In this study, a constitutive model depending on the damage variable relating to microcrack growth and strain increment is proposed to explain the multi-stress drops in stress–strain curves including strain hardening and softening phases of intact rocks. This model is formulated by combining the wing crack growth model, the suggested relationship between axial strain and wing crack growth, and the stepping function of damage relating to axial strain. This stepping function of damage relating to axial strain approximately is used to simulate the developing process of the small individual shear bands caused by the local microcrack accumulation and coalescence. The effects of parameters in the suggested stepping function of damage on the stress–strain curves containing stress drops are discussed. The theoretical model qualitatively explains the experimental phenomena of multi-stress drops in the stress–strain curves, which provides an important implication for evaluating the earthquake mechanism and the safety of deep underground engineering.

In-situ observation of microcrack evolution in a dual-phase steel during tensile straining

ABSTRACTIn this paper, the microcrack evolution in DP590 dual-phase steel was observed by in-situ straining in straining transmission electron microscopy. It was found that a void initiated ahead of a main crack. After the load was applied, a thinned area was nucleated ahead of the void tip, and it grew gradually into shallow nanovoid, penetrating nanovoid, and then new void. Meanwhile, the old void connected with the main crack. The repetitions of the procedures resulted in the continuous propagation of a crack. Interaction between microcracks was observed. The propagation direction of one microcrack may change, affected by other microcracks. Voids were observed between microcracks and have a significant effect on the coalescence of microcracks.

Inverse correlations between wear and mechanical properties in biphasic dental materials with ceramic constituents

The objective of this study is to elucidate the interdependence of competing mechanical degradation processes in biphasic dental materials with ceramic constituents in the region of high-pressure occlusal loading. It is hypothesized that wear resistance in this region correlates inversely with basic material parameters (modulus, hardness, toughness, strength) evaluated from 'standardized' test specimens. Ball-on-flat wear tests in simulation of oral function are used to quantify susceptibility to protracted sliding contact damage. Wear rates for this class of dental material tend to increase with quasistatic parameter values, so the latter do not provide a reliable guide to longevity. The generation of severe-wear facets involves cumulative quasiplastic deformation and microcrack coalescence at the grain level. It is implied that interplay between wear and fracture mechanisms should be an important consideration in future microstructural design of dental ceramics, especially in the quest to balance durability against esthetics.

The effects of intermediate principle stress on the mechanical behavior of transversely isotropic rocks: Insights from DEM simulations

SummaryThe discrete element method (DEM) is used to study the response of anisotropic rocks under true triaxial testing. Numerical samples of seven different bedding orientations (β = 0o, 15o, 30o, 45o, 60o, 75o, and 90o) are created. Six series of test simulations (σ3 = 0, 10, 30, 50, 70, and 100 MPa) are conducted on each sample, with five different σ2 values, varied from σ3 to σ1. The effects of anisotropy and intermediate stress on the peak strength, brittle‐ductile transition, and degree of anisotropy are subsequently explained through underlying micromechanics. Results show a “fan‐shaped” variation of the peak strength with σ2, displaying an ascending‐then‐descending trend. An increasing brittleness with σ2 is observed at lower confining pressures for all, but medium anisotropy angles. For higher confining pressures, increasing ductility with σ2 is seen for every anisotropy angle. A U‐shaped variation of peak strength with anisotropy angles is noted that flattens under high intermediate stress. Hence, for numerical models of Posidonia shale under normalized σ2 higher than 0.76, the anisotropy effect is found to be negligible. Micromechanical analyses reveal that the stress asymmetry, suppression of weak plane action as well as the localization and coalescence of microcracks in the intact rock matrix, due to σ2, are the contributors towards the obtained trends. Since existing failure criteria do not weigh in these features in geotechnical assessments, this paper helps future studies by providing a deeper understanding of these effects and a comprehensive data set for the analyses of anisotropic rocks under polyaxial stress conditions.

Effects of microstructural heterogeneity on fatigue properties of cast aluminum alloys

Cast Al alloys are widely employed for engine components, structural parts, gear box, chassis, etc. and subjected to mechanical cyclic load during operation. The accurate fatigue life prediction of these alloys is essential for normal operation as fatigue cracks initiated during operation induce the lubrication oil leak and serious safety hazard. Microstructural heterogeneity, including shrinkage/gaspores and secondary phase particles, is the most detrimental factor that affects fatigue life of cast Al alloys. The approximate fatigue life cycles could be estimated based on the size distribution and locations of shrinkage pores/defects. The relationship between crack population and stress was reported by statistical distributions and the cumulative probability for cast Al alloys fail at a certain stress could be predicted by combination of Paris law and pore size distribution. Pore depth was found to dominate the stress field around the pore on the surface and the maximum stress increases sharply when the pore intercepted with the surface at its top. The microstructure of cast Al alloys usually is composed of primary Al dendrites, eutectic silicon, Fe-rich particles and other intermetallic particles are dependent upon alloy composition and heat treatment. The coalescence of microcracks initiated from the fractured secondary phases was clearly found and can accelerate the initiation and propagation of the fatigue cracks. A link between defect features and the fatigue strength needs to be established through a good understanding of the fatigue damage mechanisms associated with the microstructural features under specific loading conditions. This paper reviews the influences of shrinkage/gaspores and secondary phase particles, formed during casting process, on the fatigue life of Al-Si-Mg cast Al alloys.

Numerical modeling on micro-to-macro evolution of crack network for concrete materials

Large quantities of microcracks existing in quasi-brittle materials like concrete can be graphically referred as crack network. Specifically, the network evolution includes microcrack growth and coalescence, macrocrack formation and propagation, and the final rupture. In this regard, the present work aims to formulate a numerical approach that is able to describe the whole process of crack network evolution for concrete materials. Particularly postulated is the stochastic distribution of microcracks in concrete, which coincides with the physical phenomenon in concrete. Highlighted are the growth and coalescence of microcracks, as well as the formation and propagation of macrocracks. A numerical case is carried out to study the concrete specimens that are subjected to tensile loading. The thorough process of crack network evolution is clearly simulated through the approach. Also investigated are the influence of aggregate volume fraction on concrete strength and ductility. The proposed approach provides a guide reference for concrete engineering practice, as well as modeling the complete process of concrete failure.

Multiscale analysis and experimental validation of crack initiation in quasi-isotropic laminates

A multiscale approach comprising laminate-scale finite-element (FE) analysis and fiber-diameter-scale periodic unit cell (PUC) analysis was developed to predict matrix microcracking in quasi-isotropic laminates; further, this method was validated through comparison of our predictions with experimental results. In the mesoscopic FE analysis, the nonlinear deformation in the unidirectional laminae was incorporated to reproduce the deformation behavior of the laminate and obtain deformation histories at the locations of expected crack initiation in the laminate. In the microscopic analysis, the nonlinear behavior and crack initiation in the matrix resin were simply modeled by an elasto-viscoplastic law and a stress-based failure criterion, respectively. To predict crack initiation considering both the macroscopic deformation fields and the microscopic heterogeneity of the material, the mesoscopic FE analysis was conducted first. Subsequently, the microscopic PUC analysis was undertaken based on the strain histories obtained from the mesoscopic analysis. Our multiscale approach was applied to quasi-isotropic laminates with several laminate configurations to predict the matrix cracks in the 90∘ ply of the laminates. In addition to referring to experimental data cited in literature, initial and transverse cracks were observed when conducting tensile tests of quasi-isotropic laminates using the in situ replication technique and ex situ X-ray computed tomography. Through comparison of the predicted values with experimental results quoted in literature and obtained in this work, we validated the prediction capability of our multiscale analysis and evaluated the process of crack formation from the mesoscopic and microscopic points of view. Moreover, we examined the sensitivity of the predicted results to fiber arrangement and the influence of constitutive and failure modeling of the two-scale analysis on the predicted cracking strains. The reported method can predict initial and transverse cracks on quasi-isotropic laminates; further, it depicts the damage progress wherein microcrack nucleation and coalescence occurring before the full-width transverse cracking in laminated composites are observed under tensile loading conditions.

Development of two-dimensional micromechanical, viscoelastic, and heterogeneous-based models for the study of block cracking in asphalt pavements

Block cracking is a thermally-induced damage occurring in asphalt pavements. The mechanism behind the formation of this type of cracking has not been fully investigated to date. The present work focuses on developing a series of two-dimensional viscoelastic and heterogeneous micromechanical-based discrete element models to investigate the formation of block cracking in asphalt pavements subjected to thermal loads. Several different block cracking patterns are explored by varying both material properties and thermal conditions. These properties include the dimensions of pavement segments, stress relaxation capacity and aging state of the asphalt materials, spatial aging and thermal properties gradients, cooling rates and pre-existing cracks. It is observed that formation of block cracking can be separated into two steps: (1) initiation of microcracks, followed by (2) the coalescence of microcracks and channeling of macrocracks. Results showed that when microcracks propagate in a symmetric or infinitely large pavement segment, they tend to develop in arbitrary directions to form a Y-junction pattern. When growing cracks intersect, they tend to join at right angles. In general, block cracking can be categorized by its reoccurring geometrical pattern. These cracks coalesce forming rectangular and hexagonal cracks. In this study, discrete element simulation results showed that both rectangular and hexagonal shaped cracking could occur under the same assumption. It was also observed that block cracking primarily occurs in the upper one-to-two centimeters of the top material layer, which is consistent with field observations.

Validation of Hot Corrosion and Fatigue Models in HOTPITS

Abstract HOTPITS is a set of physics-based modeling tools for treating Type II hot corrosion in Ni-based superalloys. The methodology includes modeling the nucleation, growth, and coalescence of pits and microcracks as a random process, as well as the transition of pits to micrcracks and the propagation of the resulting large crack to failure. In this investigation, critical experiments were performed on coupon and low-cycle fatigue (LCF) specimens in order to validate the hot corrosion and the fatigue models in HOTPITS. The pit nucleation, growth, and coalescence models in HOTPITS including the assumption of a random process are validated by the hot corrosion critical experiments performed at two salt contents. The LCF critical experiments, performed using a marker band protocol, validated the stress concentration factor-based models used to predict the pit-to-crack transition in the HOTPITS tool.