Mesoscopic Damage Research Articles

Effect of coarse aggregates on contact explosion resistance of concrete—A mesoscopic investigation

The intricate relationship between the mechanical properties of concrete and its internal microstructure underscores the importance of comprehending explosion performance and damage mechanisms at a mesoscopic level to effectively enhance blast resistance. This study employed three-dimensional (3D) mesoscale models to numerically investigate the dynamic behavior of concrete mixed with coarse aggregates under contact explosion. Rigorous validation of numerical models and simulation techniques was untaken through the contact explosion tests. The study explored mesoscopic damage mechanisms in heterogeneous concrete targets with randomly distributed coarse aggregates, drawing comparisons with a homogeneous concrete target. Critical mesoscopic parameters influencing the contact explosion resistance of concrete were thoroughly examined. Structural effects of coarse aggregates emerge as pivotal, shifting the damage mode from overall failure with spalling-dominated damage in homogeneous concrete to localized failure in mesoscopic concrete. The mesoscopic concrete experienced a distinct four-stage damage evolution—cratering, crack initiation, perforation, and dynamic fragmentation—diverging from homogeneous concrete with multi-layer spalling originating from the boundaries. The exponential attenuation of shock waves observed in homogeneous concrete was locally disrupted by coarse aggregates in mesoscopic concrete, attributed to wave impedance mismatch and aggregate extrusion effects. Mortar strength primarily contributed to concrete cracking, with minimal impact on damage modes. Failure modes were predominantly influenced by the content and particle size of coarse aggregates. Higher volumetric fractions significantly reduced concrete spalling, while increased coarse aggregate size exacerbated perforation failure. This comprehensive study advances our understanding of blast-resistant concrete design at a mesoscopic level, providing valuable insights for strategies aimed at enhancing structural resilience.

Modeling hard rock breakage behavior influenced by the tipped hob cutter's tooth structure using the 2D discrete element (DE) model

A deep understanding of the mesoscopic damage evolution mechanism of tipped hob cutter in hard rock, as influenced by tooth structure parameters and working parameters, can assist in optimizing the tooth of tipped hob cutter and uncovering the failure conditions and mechanism of rock. In this paper, a series of numerical simulations of rock breaking by tipped hob cutter's tooth were modelled, calibrated, and analyzed to investigate the failure conditions and mechanism of rock by tipped hob cutter, as well as the effects of tooth profiles and working parameters on the fracture evolution of hard rock. The results show that the most efficient combination of tooth profile and Dp differs depending on the rock types. D-type teeth with a penetration depth of Dp = 3 mm are optimal for crushing granite, while A-type teeth with a penetration depth of Dp = 4 mm are suitable for crushing sandstone.

Investigation on the evolution of concrete pore structure under freeze-thaw and fatigue loads

Hydraulic concrete structures in seasonal frozen regions serve under freeze-thaw cycles and cyclic loading conditions which cause notable alterations in the concrete pore structure and a significant reduction in the mechanical properties and durability of concrete. To understand the macroscopic and mesoscopic damage processes and mechanism of concrete subjected to freeze-thaw cycles and cyclic loading lead, the compressive experiment and computed tomography test of concrete after freeze-thaw and fatigue loading tests were conducted. And the influences of freeze-thaw cycle damage and cyclic loading damage on the pore structure of concrete were analyzed. The relationships between concrete pore fractal dimension, compressive strength, and elastic modulus were revealed. The concrete average pore size can be regarded to grows linearly under freeze-thaw and cyclic loading. The porosity, pore volume, and pore surface area increase with the increase in freeze-thaw cycles, and their relationship can be represented by a quadratic function. However, they first decrease and then increase with the increase of loading cycles after a specific freeze-thaw cycles number. Under the combined action of freeze-thaw cycles and cyclic loading, the non-uniformity degree of concrete pore distribution decreases, and the overall structure is more regular. The distribution of single pore morphology tends to be spherical and banded, respectively. The cyclic loading promotes the extension of the internal pores and cracks of freeze-thaw-damaged concrete which aggravates damage of concrete. Under freeze-thaw cyclic loading, the concrete compressive strength and elasticity modulus is linearly and positively correlated with fractal dimension. With the increase of loading cycles number after freezing and thawing, the fractal dimension and compressive strength decrease, and the elasticity modulus has a larger dispersion. The relationship model between concrete compressive strength and fractal dimension can well describe the macroscopic and mesoscopic evolution of concrete under freeze-thaw and cyclic loading. The results can provide a scientific basis for improving the design and maintenance of concrete and benefit extending the long-term service life of hydraulic concrete structures in cold climates.

Summary of Rock Damage Mechanics Classification

As an important means to study the mechanical properties and failure mechanism of rock materials and engineering mechanics, damage mechanics is often used to assist in the analysis of the failure process and mechanism of rock, metal, and other materials in the current research. In the use of damage mechanics to study the properties of rock materials and explore the deformation and failure law of rock, important research results have been achieved. The research methods of damage mechanics can be roughly divided into three types: statistical damage model, mesoscopic damage model, and continuous damage model. Based on the existing literature in hand, according to the division of damage mechanics research methods in the academic community, this paper collates and considers that the existing damage model construction can be summarized from the mechanical point of view. The pure theory is derived or fitted from the mathematical point of view. One of them is derived from the mechanism point of view, and the other is derived from the phenomenon point of view. The stress, strain, acoustic emission, and other data are analyzed and reconstructed, and both have advantages and disadvantages.

Mesoscopic damage mechanism of multiple freeze–thaw cycles of cement gravel based on particle flow theory

Mesoscopic damage mechanism of multiple freeze–thaw cycles of cement gravel based on particle flow theory

Experimental and Numerical Analysis of Flexural Properties and Mesoscopic Failure Mechanism of Single-Shell Lining Concrete

Despite ongoing research efforts aimed at understanding the structural response of steel fiber reinforced concrete (SFRC), there is very limited research on the failure characteristics and mesoscopic damage mechanism of SFRC, specifically when under flexure. In this study, a four-point bending test of plain concrete (PC) and SFRC with different fiber contents is carried out to investigate the flexural performance of SFRC. The crack propagation process, cracking load, ultimate load, and load-deflection curves of PC and SFRC beams are obtained. Additionally, the discrete element method (DEM), using PFC2D 6.0 software, is adopted to explore the mesoscopic properties of PC and SFRC. The test and simulation results of PC and SFRC beams are compared and analyzed, and some conclusions are drawn. The results show that steel fiber can efficiently improve the compressive strength of concrete when the fiber content is 30 kg/m3, and significantly improve the deformation resistance, crack resistance, and flexural capacity of concrete. The refined numerical models of PC and SFRC beams are established based on compressive strength and aggregate screening results. Through the numerical four-point bending test, the mesoscopic mechanical behaviors of models reveal the damage mechanism of SFRC. The horizontally distributed steel fibers bridge both sides of the cracks to resist crack development, and the vertically distributed steel fibers guide the cracks to the place with strong contact, thus resisting crack height development. The test results show that, for flexural properties, the optimal steel fiber content of SFRC is 31 kg/m3.

Study on the meso-damage characteristics of RC beam under shear failure based on acoustic emission and numerical analysis

Material meso-scopic damage is the fundamental cause of the degradation of the macroscopic mechanical properties of structures, so it is vital to appraise the meso-damage process of various material components. Acoustic emission (AE) technology as a nondestructive testing method can cover the shortage of traditional test to detect the meso-damage. Therefore, in this study, four-point shear tests were conducted on reinforced concrete (RC) beams and AE signals were collected during tests. Results indicated that the shear failure process of RC beam could be divided into three stages based on the correlation between the cumulative AE energy and load-time. Meanwhile, the finite element model (FEM) was established to intuitively obtain the meso-damage evolution process of RC beam in different failure stages. It was observed that the mortar damage, the ITZ damage, the aggregate damage and the reinforcement plasticity damage occurred inside the RC beam and the meso-damage all showed stage characteristics. Besides, the correlation between AE signals and different meso-damage were established by combining the wavelet packet and the cluster analysis. Based on these analysis, the method of assessing the damage severity of RC beam under shear failure was proposed eventually.

Mesoscopic Damage Mechanism of Red-bed Mudstone Based on Geometric and Mechanical Representative Elementary Areas

Mesoscopic Damage Mechanism of Red-bed Mudstone Based on Geometric and Mechanical Representative Elementary Areas

Study on mechanical properties and damage mechanism of alkali-activated slag concrete

The cement industry has become one of the important sources of greenhouse gas emissions, and the alkali-activated slag (AAS) has the same mechanical properties and lower carbon emissions as cement in the production process. Therefore, it can be used as an alternative binder for low-carbon buildings. The effect of alkali concentration (mass ratio of Na2O to binder) on the macro-mechanical properties and meso-damage mechanism of alkali-activated slag concrete (AASC) under uniaxial compression was investigated in this work. Six kinds of concrete with alkali concentration of 2 %, 4 %, 6 %, 8 %, 10 % and 12 % were prepared. The microscopic characteristics of AASC were characterized by nuclear magnetic resonance (NMR), scanning electron microscopy (SEM), x-ray diffraction (XRD) and x-ray energy dispersive spectroscopy (EDS); the meso-damage evolution mechanism of AASC was discussed by statistical damage theory. The results showed that the peak stress and elastic modulus of AASC specimens increased, and the microstructure became denser with the increase of alkali concentration. However, the mechanical properties of the specimen deteriorated to some extent when the alkali concentration exceeded 6 %; for example, the peak stress decreased by 24.00 MPa when the alkali concentration was increased from 6 % to 12 % at the age of 28d. Based on the statistical damage theory, the effective stress concept was introduced to quantitatively analyze the mesoscopic damage evolution of AASC. Both fracture and yielding damage modes were considered. By exploring the regular changes of the mesoscopic parameters, the connection between the macroscopic mechanical properties and the mesoscopic damage mechanism was established. Based on the above mechanical properties and mesoscopic damage results, the alkali concentration of 6 % can be used as the best mixture proportions of AASC.

The influence of blast furnace slag on the mechanical properties and mesoscopic damage mechanisms of recycled aggregate concrete compared to natural aggregate concrete at different ages

To improve the mechanical property defects caused by recycled aggregate (RCA) in recycled aggregate concrete (RAC) as well as the effect of blast furnace slag (BFS) addition on the mechanical properties and microscopic damage mechanisms of natural aggregate concrete (NAC) and RAC at long ages. In this paper, the effects of different curing ages (T = 7d, 14d, 28d, 56d, 90d, and 150d) and blast furnace slag contents on the mechanical properties and microscopic damage mechanisms under uniaxial compression of NAC and RAC were investigated, where the cement substitution rate of BFS was 0 % and 35 % (water-cement ratio of 0.46, water-reducing agent dosage of 0.25 %), respectively. Nuclear magnetic resonance (NMR) and scanning electron microscopy (SEM) were used to study the internal pore changes and the generation of hydration products in the specimens, and acoustic emission (AE) showed the microcrack development process. The results showed that the peak stress and modulus of elasticity of the specimens gradually increased with increasing age, the peak strain gradually decreased, and the internal porosity gradually decreased. It is worth noting that the addition of BFS caused the mechanical properties of RAC to decrease at 28 days but enhanced the mechanical properties of RAC at 56–120 days. This was attributed to the fact that the properties of BFS were not yet activated in the early stage, whereas it was activated in the later stage under an alkaline environment and reacted with CH to generate more C–S–H hydration products, which enhanced the interfacial transition zone and reduced the porosity. This highlights the potential of BFS to improve the performance of RCA over long periods of time. In contrast, BFS improved the mechanical properties of NAC throughout the age period and more significantly after 56 days. AE results showed that the peak ring counts lagged behind the peak stresses. Finally, combined with the statistical damage principal model, the fitting was performed by Matlab to analyze the microscopic damage mechanism of the specimen during uniaxial compression. It was found that the changes in the four characteristic parameters (εa, εb, εh and H) followed a certain regularity, reflecting the regular changes in fracture damage and yield damage. This revealed the intrinsic linkage between mesoscopic damage mechanisms and macroscopic mechanical properties. It was demonstrated that it was feasible to speculate on the law of the stress-strain curve at the macroscopic level by using the law of change of the four parameters (εa, εb, εh and H) at the microscopic level.

Study on multi-scale dynamic mechanical properties of metakaolin modified recycled aggregate concrete under coupling conditions of sulfate attack and dry-wet cycles

To explore the potential of modified recycled aggregate concrete for use in construction projects, the evolution of its properties in the combined effects of environment and loading was investigated. This paper considered various factors, including the replacement amount of metakaolin (0 %, 15 %), dry-wet cycles (0, 20, 60, and 120 cycles), and the dynamic strain rate (ε˙ = 10−5/s, 10−4/s, 10−3/s, and 10−2/s), and conducted uniaxial compression, scanning electron microscopy (SEM), and X-ray diffraction (XRD) tests. The experimental findings demonstrated that the peak stress and elastic modulus of recycled aggregate concrete (RAC) exhibited a pattern of initial increase followed by a decrease before and after 20 dry-wet cycles, a concurrent upward trend was observed in both parameters with an escalation in strain rate and the incorporation of metakaolin (MK), and no uniform trend was observed for the peak strain. The test results from SEM and XRD revealed that the persistent formation of sulfate crystals and expansive substances like gypsum and ettringite were the primary factors contributing to the deterioration of specimen damage as cycle counts escalated. A quantitative analysis was conducted to investigate the mesoscopic damage mechanism associated with the dynamic mechanical properties and durability evolution process of metakaolin-based RAC under sulfate erosion. The correlation between the macroscopic mechanical properties and the microscopic changes of RAC was explored using statistical damage theory.

Analysis of damage evolution and study on mesoscopic damage constitutive model of granite under freeze–thaw cycling

Analysis of damage evolution and study on mesoscopic damage constitutive model of granite under freeze–thaw cycling

Mesoscopic simulation of uniaxial compression fracture of concrete via the nonlocal macro–meso-scale consistent damage model

To capture the uniaxial compression fracture of concrete composite is still a great challenging problem. To this end, the newly proposed nonlocal macro–meso-scale consistent damage (NMMD) model is extended to simulate uniaxial compression fracture of concrete composites. In the NMMD model, a mesoscopic structure composed of material point pairs is attached to each material point. A decomposition scheme suitable for masonry-like materials is adopted to decompose the elongation quantity of point pairs. The topologic damage is defined as the weighted summation of mesoscopic damage in neighboring material point pairs driven by the decomposed elongation quantity. The physically-based energetic degradation function is employed to bridge the geometric discontinuity in solids and the energy dissipation, and thus the topologic damage is integrated into the framework of continuum damage mechanics. The Monte-Carlo method and two-phase random field model are employed to generate stochastic circular and irregular aggregates representing the meso structure of concrete, respectively. The interface transition zone is prescribed by reducing the critical value of interface point pairs. Numerical results indicate that both the crack patterns and stress–strain curves of the uniaxial compression experiments on concrete panels can be well captured by the proposed NMMD model without mesh size sensitivity. The influence of horizontal constraints on the top and bottom surfaces is investigated and shown to have an effect on the crack patterns, strength and deformation capability of the specimen. The necessity of decomposition is also illustrated.

Damage assessment of concrete via acoustic emission and mesoscopic damage constitutive model

This paper experimentally investigated the acoustic emission (AE) behavior of concrete specimens. By using the MTS810 servo hydraulic testing system and the multi-channel AE system, a series of specimens graded C40, C50, and C60 were loaded under uniaxial compression. The spatial distribution of the AE events, AE events rate, and AE energy rate were recorded. Additionally, a theoretical mesoscopic damage constitutive (MDC) model, which defines the theoretical mesoscopic damage variable as the ratio of the fractured micro-elements to the total micro-elements, was introduced. The results indicated that the spatial distribution of AE events resembled the shape of failure modes. Moreover, the theoretical mesoscopic damage calculated by the MDC model matched well with the observed experimental damage, represented by the AE events. This association not only related the theoretical mesoscopic damage with experimental macroscopic damage, but also revealed that the macroscopic damage of concrete stemmed from mesoscopic fracture. Additionally, the AE energy was related both to the energy of fractured micro-elements and its derivative in the MDC model.

Shear failure analysis of slip zone soil with different coarse particle shapes: Visualized shear test and PIV technology

Coarse particle shape in slip zone soil influences the mesoscopic structure of the soil, which in turn affects soil shear strength and failure behavior. In order to investigate the effect of particle shape on the shear characteristics of coarse–fine-grained mixed slip zone soil, three types of coarse particles (spheroidal, rounded, and angular) were selected for mixing and matching, and a total of 10 sets of medium-scale shear tests were designed for this paper. To quantify the shear deformation and failure process of slip zone soils, particle image velocimetry (PIV) technology and the hanging hammer method were used to obtain mesoscopic data of the soil (displacement vector data of soil particles and elevation data of the shear failure surface), which were used to calculate shear band thickness, shear dilatation, and roughness coefficient of the shear failure surface. The results indicate that coarse particle shape can considerably affect the macroscopic mechanical properties (internal friction angle and shear strength) and mesoscopic deformation characteristics (shear band thickness, shear dilatation, and shear surface morphology) of soils. Angular coarse particles have higher interlocking strength than spheroidal and rounded coarse particles, allowing angular coarse-grained slip zone soils to develop large shear band thickness and rough shear failure surfaces. In addition, mesoscopic damage analysis suggests that the damage rate of slip zone soils decreases with increasing coarse particle shape complexity. These findings enhance comprehension of the failure characteristics of soil-rock mixture slopes and serve as a good reference for the stability analysis of similar slopes.

NEPE Propellant Mesoscopic Modeling and Damage Mechanism Study Based on Inversion Algorithm.

To accurately characterize the mesoscopic properties of NEPE (Nitrate Ester Plasticized Polyether) propellant, the mechanical contraction method was used to construct a representative volume element (RVE) model. Based on this model, the macroscopic mechanical response of NEPE propellant at a strain rate of 0.0047575 s-1 was simulated and calculated, and the parameters of the cohesive zone model (CZM) were inversely optimized using the Hooke-Jeeves algorithm by comparing the simulation results with the results of the uniaxial tensile test of NEPE propellants. Additionally, the macroscopic mechanical behavior of NEPE composite solid propellants at strain rates of 0.00023776 s-1 and 0.023776 s-1 was also predicted. The mesoscopic damage evolution process of NEPE propellants was investigated by the established model. The study results indicate that the predicted curves are relatively consistent with the basic features and change trends of the test curves. Therefore, the established model can effectively simulate the mesoscopic damage process of NEPE composite solid propellants and their macroscopic mechanical properties.

Physically consistent nonlocal macro–meso-scale damage model for quasi-brittle materials: A unified multiscale perspective

Despite great efforts in capturing the damage evolution law in multiscale approaches in damage mechanics, much less attention has been paid to the conversion from geometric damage to energy dissipation. In the present paper, the newly proposed nonlocal macro–meso-scale damage (NMMD) model is physically consolidated and a multiscale point of view is consistently adopted in both the damage evolution and energy dissipation. In this model, for each macroscopic material point a mesoscopic structure is attached such that all the point pairs composed of this point and the points in its influence domain are connected to it when the material is intact. The deformation field under loading will result in deformation of point pairs, and thus mesoscopic damage related to a point pair will occur if the deformation index of this point pair exceeds the prescribed threshold. Such mesoscopic change among point pairs connected to a material point within its influence domain has two-fold consequences: geometrically the accumulation of mesoscopic damage will result in macroscopic discontinuity, and simultaneously the free energy will be dissipated, leading to degradation of mechanical behavior of quasi-brittle materials. In other words, the damage as a metric of the geometric discontinuity will lead to the damage in the sense of energy dissipation and degradation of mechanical behavior for quasi-brittle materials, which can be physically captured by the ratio of summation of all the dissipated mesoscopic free energy in point pairs to the free energy of intact material within the influence domain. This energy-based macroscopic damage could then be inserted into the framework of continuum damage mechanics. Therefore, the conversion from geometric damage to energetic damage is implemented on the mesoscale instead of the macroscale, and thus a macroscopic energetic degradation function is not needed. In addition, the structured strain which is suitable for quasi-brittle materials can be adopted to determine the deformation index of point pairs. Correspondingly, the macroscopic free energy can be split into dissipative and non-dissipative parts. In this way, several inconsistencies in the previous NMMD models are remedied. The influence of mesoscopic model parameters is investigated in depth, and several numerical examples including mode-I, mixed-mode and compressive splitting fracture problems of quasi-brittle materials are carried out. Numerical results indicate that the proposed model can not only well capture the cracking process with or without initial cracks but also quantitatively predict the load–deformation curves without mesh size sensitivity. Moreover, due to the inherent physical consistency, in mixed-mode cracking problems the proposed model performs better than the previous NMMD model. Problems to be further studied are also discussed.

Multi-Scale Analysis of the Damage Evolution of Coal Gangue Coarse Aggregate Concrete after Freeze-Thaw Cycle Based on CT Technology.

This study utilized X-ray computed tomography (CT) technology to analyze the meso-structure of concrete at different replacement rates, using a coal gangue coarse aggregate, after experiencing various freeze-thaw cycles (F-Ts). A predictive model for the degradation of the elastic modulus of Coal Gangue coarse aggregate Concrete (CGC), based on mesoscopic damage, was established to provide an interpretation of the macroscopic mechanical behavior of CGC after F-Ts damage at a mesoscopic scale. It was found that after F-Ts, the compressive strength of concrete, with coal gangue replacement rates of 30%, 60%, and 100%, respectively, decreased by 33.76%, 34.89%, and 42.05% compared with unfrozen specimens. The results indicate that an increase in the coal gangue replacement rate exacerbates the degradation of concrete performance during the F-Ts process. Furthermore, the established predictive formula for elastic modulus degradation closely matches the experimental data, offering a reliable theoretical basis for the durability design of CGC in F-Ts environments.

Mechanical properties and mesoscopic damage characteristics of basalt fibre-reinforced seawater sea-sand slag-based geopolymer concrete

This study aims to investigate the macroscopic mechanical properties and mesoscopic damage mechanisms of basalt fibre-reinforced seawater sea-sand slag-based geopolymer concrete (SS-SGC). At the macroscopic level, the influences of different volumes and lengths of basalt fibre (BF) on the workability, compressive strength, and splitting tensile strength of SS-SGC were analyzed. The optimum BF length was identified as 12 mm, with an optimal volume fraction of 0.6 %. This combination resulted in 9 % improvement in compressive strength and a 7 % enhancement in splitting tensile strength compared to the control group. Compared to Ordinary Portland Cement (OPC), SS-SGC exhibited a remarkable reduction of 79.6 %, 60.2 %, and 21 % in carbon emissions, energy consumption, and cost, respectively. At the microscopic level, scanning electron microscopy (SEM) was employed to analyze fibre distribution characteristics and the ITZ microstructure between the two aggregates and the matrix. The fibre bridging and aggregate synergistic mechanisms were proposed, highlighting the role of the high-density far-field ITZ of coral coarse aggregate (CCA) in altering the crack propagation trend and confining microscopic damage within the highly ductile matrix, thereby improving the mechanical properties of the concrete. At the mesoscopic level, a six-phase numerical model was established, considering different aggregates and their ITZ characteristics, to systematically analyze the mesoscopic damage evolution, as well as the macroscopic strength and stress-strain behavior of SS-SGC. The research outcomes serve as a reference for future multiscale studies on SS-SGC and its practical engineering applications.

Experimental Study on the Mechanical Stability and Mesoscopic Damage Characteristics of Coal Under Different Mining Disturbance Rates

Experimental Study on the Mechanical Stability and Mesoscopic Damage Characteristics of Coal Under Different Mining Disturbance Rates