Experimental investigation on the macro-mechanical behavior and micromechanical damage model of Xiyu conglomerate with pores and inclusions under triaxial compression

The complex and special mechanical properties of Xiyu conglomerate are of great significance to the construction of water conservancy and hydropower engineering.The crack characteristic stress,dilatancy behavior,and failure mechanism of Xiyu conglomerate collected from Momoke Water Control Project,southwestern China,were analyzed and discussed based on the experimental results of triaxial compression test and 3D X-ray computed tomography test.The results show that with increasing confining pressure,the deformation characteristics and all characteristic stresses increase monotonically,while the dilation angle and dilatancy index decrease,and exponential function model can accurately describe the evolution rule of dilatancy index with confining pressure.While the porosity is negatively correlated with confining pressure.The failure modes of Xiyu conglomerate include axial tensile cracks,shear cracks,local cross cracks and cracks around gravel.With increasing confining pressure,the failure modes transform from tension cracks to shear cracks.A non-associated micromechanical damage model considering pressure dependent matrix presenting tension-compression asymmetry is proposed and applied to Xiyu conglomerate with pores and a large number of gravels.By comparing numerical calculations and experimental results,the proposed micromechanical plastic damage model is able to describe the mechanical behavior of Xiyu conglomerate.

MICROMECHANICAL DAMAGE MODEL FOR ROCKS AND CONCRETES SUBJECTED TO COUPLED TENSILE AND SHEAR STRESSES

Based on the analysis of the deformation in an infinite isotropic elastic matrix with an embedded elliptic crack under far field coupled tensile and shear stresses, the energy release rate and a mixed fracture criterion are obtained using an energy balance approach. The additional compliance tensor induced by a single opening elliptic microcrack in a representative volume element is derived, and the effect of microcracks with random orientations is analyzed with the Taylor＇s scheme by introducing an appropriate probability density function. A micromechanical damage model for rocks and concretes is obtained and is verified with experimental results.

Finite Element Simulations on Failure Behaviors of Granular Materials with Microstructures Using a Micromechanics-Based Cosserat Elastoplastic Model

This paper presents a micromechanics-based Cosserat continuum model for microstructured granular materials.By utilizing this model,the macroscopic constitutive parameters of granular materials with different microstructures are expressed as sums of microstructural information.The microstructures under consideration can be classified into three categories:a medium-dense microstructure,a dense microstructure consisting of one-sized particles,and a dense microstructure consisting of two-sized particles.Subsequently,the Cosserat elastoplastic model,along with its finite element formulation,is derived using the extended Drucker-Prager yield criteria.To investigate failure behaviors,numerical simulations of granular materials with different microstructures are conducted using the ABAQUS User Element(UEL)interface.It demonstrates the capacity of the proposed model to simulate the phenomena of strain-softening and strain localization.The study investigates the influence of microscopic parameters,including contact stiffness parameters and characteristic length,on the failure behaviors of granularmaterials withmicrostructures.Additionally,the study examines themesh independence of the presented model and establishes its relationship with the characteristic length.A comparison is made between finite element simulations and discrete element simulations for a medium-dense microstructure,revealing a good agreement in results during the elastic stage.Somemacroscopic parameters describing plasticity are shown to be partially related to microscopic factors such as confining pressure and size of the representative volume element.

Modeling of anisotropic polydispersed composites by progressive micromechanical approach

A progressive micromechanical method is presented in order to predict the elastic constants of polydispersed composites including multi-directional or randomly ori- ented reinforcement particles. Heterogeneities of various types are introduced into the matrices in a gradual manner. At each step, the Mori-Tanaka method is used to ob- tain the stiffness tensor of the intermediate medium used as a matrix of the following step. The proposed method is capable of introducing any kind of heterogeneities based on their dimensions, orientations, mechanical properties, and volume fractions to the ma- trix. Furthermore, suitable probability density functions can be defined for physical and structural parameters of the composite, including the level of the filler-matrix interfacial bonding, the aspect ratio, and the orientation of reinforcement particles. The efficiency of the iterative approach and the convergence of the solution are studied by computing the stiffness tensors of unidirectional and bidirectional particulate composites. The results of the present study are also compared with the literature data for a randomly oriented particulate composite.

Analysis of mathematical model for micromechanical vibratory wheel gyroscope

By the sketch of structure of MVWG,the working laws of this kind of gyroscope we re explained.To the aid of Euler′s Dynamics Equation,a mathematical model of the gyroscope was constructed,and then by the basic working laws of MVWG the model was simplified.Under the conditions of the three axial direction rotations and general rotation,the mathematical model was resolved.And finally by the solutions, the working laws of the gyroscope, the working disparity among all sorts of gyrations and the influences from the gyrations in the axial directions were analysed.

A micromechanical model based on hypersingular integro-differential equations for analyzing micro-crazed interfaces between dissimilar elastic materials

The current work models a weak(soft) interface between two elastic materials as containing a periodic array of micro-crazes. The boundary conditions on the interfacial micro-crazes are formulated in terms of a system of hypersingular integro-differential equations with unknown functions given by the displacement jumps across opposite faces of the micro-crazes. Once the displacement jumps are obtained by approximately solving the integro-differential equations, the effective stiffness of the micro-crazed interface can be readily computed. The effective stiffness is an important quantity needed for expressing the interfacial conditions in the spring-like macro-model of soft interfaces. Specific case studies are conducted to gain physical insights into how the effective stiffness of the interface may be influenced by the details of the interfacial micro-crazes.

High-temperature Creep Behavior Characterization of Asphalt Mixture based on Micromechanical Modeling and Virtual Test

The high-temperature creep behavior of asphalt mixture was investigated based on micromechanical modeling and virtual test by using three-dimensional discrete element method（DEM）. A user-defined micromechanical model of asphalt mixture was established after analyzing the irregular shape and gradation of coarse aggregates, the viscoelastic property of asphalt mastic, and the random distribution of air voids within the asphalt mixture. Virtual uniaxial static creep test at 60 ℃ was conducted by using Particle Flow Code in three dimensions（PFC3D） and was validated by laboratory test. Based on virtual creep test, the micromechanical characteristics between aggregates, within asphalt mastic, and between aggregate and asphalt mastic were analyzed for the asphalt mixture. It is proved that the virtual test based on the micromechanical model can efficiently predict the creep deformation of asphalt mixture. And the high-temperature behavior of asphalt mixture was characterized from micromechanical perspective.

Micromechanical model for competition between intergranular and intragranular fracture in 7 X X X aluminum alloys

The 7XXX aluminum alloys having a microstructure with precipitate free zones (PFZ) nearby the grain boundary, have received a great deal of attention due to their high strength, light mass, and yet poor fracture toughness. Experimental investigation into the effect of microstructure on the ductility was well established and comprehensive in the literature. A micromechanical model using a unit cell including some voids and relevant microstructural features was created. The competition between intergranular and intragranular fracture was investigated by comparing the void growth velocity between PFZ and matrix. The effects of void aspect ratio, relative PFZ volume, orientation of PFZ on the ductility of 7XXX aluminum alloys were analyzed. The results show that the model can explain the effect of microstructure on the competition between intergranular and intragranular fracture.

A MICROMECHANICAL MODEL FOR γ-TiAl BASE PST CRYSTALS

An analytical micromechanical method is proposed to examine the dependence of plastic deformation on the microstructure for a PST crystal. The sub-domain microstructure of the γ phase and the effect of the α2 phase are taken into account by a proper micromechanical formulation, the dislocation slip and twinning deformation mechanisms are considered in the context of crystal plasticity. The model can well predict the dependence of stress-strain relations on loading angle with respect to the microstructure. The influence of the twinning and lamellar spacing on the deformation behavior and biaxial yield surfaces for PST crystals are also examined.

Permeability modeling of self-healing due to calcium carbonate precipitation in cement-based materials with mineral additives

The permeability modeling of self-healing due to calcium carbonate precipitation in cement-based materials with mineral additives was studied in this work. The parameters of calcium carbonate precipitation during self-healing were simulated. A permeability modeling of self-healing, combined with numerical simulation of calcium carbonate formation, was proposed based on the modified Poiseuille flow model. Moreover, the percentage of calcium carbonate in healing products was measured by TG-DTA. The simulated results show that self-healing can be dramatically promoted with the increase of pH and Ca2+ concentration. The calculated result of permeability is consistent with that measured for cracks appearing in middle or later stages of self-healing, it indicates that this model can be used to predict the self-healing rate to some extent. In addition, TG-DTA results show that the percentage of calcium carbonate in healing products is higher for mortar with only chemical expansion additives or cracks appearing in the later stage, which can more accurately predict the self-healing rate for the model.

Computational analysis of linear friction welding process and micromechanical modeling of deformation behavior for medium carbon steel

Finite element simulation of linear friction welding(LFW) medium carbon steel was carried out using the ABAQUS software. A two-dimensional(2D) coupled thermo-mechanical model was established. First, the temperature fields of medium carbon steel during LFW process were investigated. And then, the Mises stress and the 1st, 2nd and 3rd principal stresses fields' evolution of the steel during LFW process were studied. The deformation behavior of LFW carbon steel was analyzed by using micromechanics model based on ABAQUS with Python code. The Lode parameter was expressed using the Mohr stress circle and it was investigated in detail.

Nonlinear Micromechanical Modelling of Transverse Tensile Damage Behavior in Fiber-Reinforced Polymer Composites

The investigation focusing on the mechanical behaviors at the microstructural level in composite materials can provide valuable insight into the failure mechanisms at larger scales.A micromechanics damage model which comprises the coupling of the matrix constitutive model and the cohesive zone(CZM)model at fiber-matrix interfaces is presented to evaluate the transverse tensile damage behaviors of unidirectional(UD)fiber-reinforced polymer(FRP)composites.For the polymeric matrix that exhibits highly non-linear mechanical responses,special focus is paid on the formulation of the constitutive model,which characterizes a mixture of elasticity,plasticity as well as damage.The proposed constitutive model includes the numerical implementation of a fracture plane based ellipse-parabola criterion that is an extension of the classic Mohr-Coulomb criterion,corresponding post-yield flow rule and post-failure degradation rule in the fully implicit integration scheme.The numerical results are in good agreement with experimental measurements.It is found that directly using the matrix properties measured at the ply level to characterize the mechanical responses at the constituent level may bring large discrepancies in homogenized stress-strain responses and dominant failure mechanisms.The distribution of fracture plane angles in matrix is predicted,where it is shown to provide novel insight into the microscopic damage initiation and accumulation under transverse tension.

A UNIFIED ENERGY APPROACH TO A CLASS OF MICROMECHANICS MODELS FOR COMPOSITE MATERIALS

Several micromechanics models for the determination of composite moduli are investigated in this paper,including the dilute solution,self-consistent method,generalized self-consistent method,and Mori-Tanaka's method.These mi- cromechanical models have been developed by following quite different approaches and physical interpretations.It is shown that all the micromechanics models share a common ground,the generalized Budiansky's energy-equivalence framework.The dif- ference among the various models is shown to be the way in which the average strain of the inclusion phase is evaluated.As a bonus of this theoretical development,the asymmetry suffered in Mori-Tanaka's method can be circumvented and the applica- bility of the generalized self-consistent method can be extended to materials contain- ing microcracks,multiphase inclusions,non-spherical inclusions,or non-cylindrical inclusions.The relevance to the differential method,double-inclusion model,and Hashin-Shtrikman bounds is also discussed.The application of these micromechanics models to particulate-reinforced composites and microcracked solids is reviewed and some new results are presented.

FRACTURE MECHANISM AND MICROMECHANICAL ANALYSIS OF POLYSYNTHETICALLY TWINNED CRYSTALS OF γTiAl ALLOYS

The fracture behavior and mechanism of PST crystals of a Ti 49%(mole fraction)Al alloy have been studied by using in situ straining and micromechanical calculation. The three dimensional micromechanical model representing the structure of PST crystal has been built, and the stress distribution ahead of the sharp and blunt crack tips either parallel to lamellar interface or perpendicular to the lamellae has been calculated by using finite element method based on linear elasticity of PST crystals. The experimental results show that the fracture behaviors and mechanisms are strongly dependent on the angle of loading axis to the lamellae. The calculation indicates that nucleation and propagation of microcrack along the interfaces are controlled by the normal stress and translamellar microcrack is controlled by shear stress ahead of crack tip.

Evaluation on Self-healing Mechanism and Hydrophobic Performance of Asphalt Modified by Siloxane and Polyurethane

In order to inhibit and remove the thin ice and extend the lifetime of the damaged bridge, the self-healing mechanism and hydrophobic performance of asphalt modified by siloxane and polyurethane (ASP) were studied by dynamic shear rheology (DSR), fluorescence microscope (FM), atomic force microscope (AFM), the fracture-healing-re-fracture test and molecular simulations. The experimental results indicated that the selfhealing capability of ASP increased with increasing heating time and temperature. Furthermore, the addition of siloxane could improve the reaction energy barrier and complex modulus, and it is believed that the self-healing is a viscosity driven process, consisting of two parts namely crack closure and properties recovery. Contact angle of ASP increased with the increasing siloxane content and it deduced that the siloxane could improve the hydrophobic performance of ASP and the ASP molecule model could simulate well the self-healing mechanism and hydrophobic performance of ASP.

QUASI-MICROMECHANICAL CONSTITUTIVE THEORY FOR BRITTLE DAMAGED MATERIALS UNDER TENSION

One of fundamental but difficult problems in damage mechanics isthe formulation of the ef- fective constitutive relation ofmicrocrack-weakened brittle o quasi-brittle materials under complexloading, especially when microcrack interaction is taken intoaccount. The combination of phenomenological and mi- cromechanicaldamage mechanics is a promising approach to construction andapplicable damage model with a firm physical foundation.

Micromechanical Study on Moisture Induced Tensile Strength Reduction of GFRP

The main focus of this paper is to investigate the influence of hygrothermal aging on tensile strength of epoxy resin matrix composites.Firstly,tests of water absorption and moisture induced tensile strength degradation of glass fiber reinforced polymer(GFRP)are conducted.Results show that the moisture absorption behavior of the GFRP follows the Fick’s law,and its tensile strength retention decreases notably in the early hygrothermal aging stage and then gradually approaches a constant.Then,microscale longitudinal and transverse strength prediction models for unidirectional fiber reinforced composites are proposed.They are moisture concentration dependent and reflect the inherent probability of failures of fiber and matrix(or fiber/matrix interface).The moisture diffusing analysis demostrates that the proposed models can predict degradation of tensile strength of epoxy resin matrix composites undergoing different hygrothermal durations.The proposed models are validated by the experiments of hygrothermal residual strength of the GFRP mentioned above.

A macro-mesoscopic constitutive model for porous and cracked rock under true triaxial conditions

The complex mechanical and damage mechanisms of rocks are intricately tied to their diverse mineral compositions and the formation of pores and cracks under external loads.Numerous rock tests reveal a complex interplay between the closure of porous defects and the propagation of induced cracks,presenting challenges in accurately representing their mechanical properties,especially under true triaxial stress conditions.This paper proposes a conceptualization of rock at the mesoscopic level as a two-phase composite,consisting of a bonded medium matrix and frictional medium inclusions.The bonded medium is characterized as a mesoscopic elastic material,encompassing various minerals surrounding porous defects.Its mechanical properties are determined using the mixed multi-inclusion method.Transformation of the bonded medium into the frictional medium occurs through crack extension,with its elastoplastic properties defined by the DruckerePrager yield criterion,accounting for hardening,softening,and extension.MorieTanaka and Eshelby’s equivalent inclusion methods are applied to the bonded and frictional media,respectively.The macroscopic mechanical properties of the rock are derived from these mesoscopic media.Consequently,a True Triaxial Macro-Mesoscopic(TTMM)constitutive model is developed.This model effectively captures the competitive effect and accurately describes the stress-deformation characteristics of granite.Utilizing the TTMM model,the strains resulting from porous defect closure and induced crack extension are differentiated,enabling quantitative determination of the associated damage evolution.

A microscopic approach to brittle creep and time-dependent fracturing of rocks based on stress corrosion model

A brittle creep and time-dependent fracturing process model of rock is established by incorporating the stress corrosion model into discrete element method to analyze the creep behavior and microcrack evolution in brittle rocks at a micro-scale level.Experimental validation of the model is performed,followed by numerical simu-lations to investigate the creep properties and microcrack evolution in rocks under single-stage loading,multi-stage loading,and confining pressure,at various constant stress levels.The results demonstrate that as the stress level increases in single-stage creep simulations,the time-to-failure progressively decreases.The growth of microcracks during uniaxial creep occurs in three stages,with tensile microcracks being predominant and the spatial distribution of microcracks becoming more dispersed at higher stress levels.In multi-stage loadingunloading simulations,microcracks continue to form during the unloading stage,indicating cumulative damage resulting from increased axial stress.Additionally,the creep behaviour of rocks under confining pressure is not solely determined by the magnitude of the confining pressure,but is also influenced by the magnitude of the axial stress.The findings contribute to a better understanding of rock deformation and failure processes under different loading conditions,and they can be valuable for applications in rock mechanics and rock engineering.

Micromechanical modeling of longitudinal tensile behavior and failure mechanism of unidirectional carbon fiber/aluminum composites involving fiber strength dispersion

This paper examines the longitudinal tensile behavior and failure mechanism of a new unidirectional carbon fiber reinforced aluminum composite through experiments and simulations.A Weibull distribution model was established to describe the fiber strength dispersion based on single-fiber tensile tests for carbon fibers extracted from the composite.The constitutive models for the matrix and interface were established based on the uniaxial tensile and single-fiber push-out tests,respectively.Then,a 3D micromechanical numerical model,innovatively considering the fiber strength dispersion by use of the weakest link and Weibull distribution theories,was estab-lished to simulate the progressive failure behavior of the composite under longitudinal tension.Due to the dispersion of fiber strength,the weakest link of the fiber first fractures,and stress concentra-tion occurs in the surrounding fibers,interfaces,and matrix.The maximum stress concentration fac-tor for neighboring fibers varies nonlinearly with the distance from the fractured fiber.Both isolated and clustered fractured fibers are present during the progressive failure process of the composite.The expansion of fractured fiber clusters intensifies stress concentration and material degradation which in turn enlarges the fractured fiber clusters,and their mutual action leads to the final collapse of the composite.