Micromechanical Modeling for Behavior of Cementitious Granular Materials

Crack damage is commonly observed in cementitious granular materials. Previous analytical models based on continuum mechanics have limitations in analyzing localized damages at a microscale level. In this paper, a micromechanics approach is adopted that considers a contract law for the interparticle behavior of two particles connected by a binder. The model is based on the premises that the interparticle binder initially contains microcracks. As a result of external loading, these microcracks propagate and grow. Thus, binders are weakened and fail. Theory of fracture mechanics is employed to model the propagation and growth of the microcracks. The contact law is then incorporated in the analysis for the overall damage behavior of material using a discrete element method. Using this model, the stress-strain behaviors under uniaxial and biaxial conditions were simulated. A reasonable agreement is found between the predictions and experimental results.     

Micromechanical Modeling of a Dump Material

Micromechanical modeling of a fragmented claystone—a difficult waste material produced by open‐pit coal mines in Northwestern Bohemia—is presented in this article. The PFC2D code, which accounts for the discrete nature of geomaterials and represents them as an assembly of unbonded or bonded particles, has been used. First, synthetic claystone was generated and the deformability and strength parameters were calibrated via numerical testing and comparing the results with those of available laboratory and field tests. The pre‐peak, peak, and post‐peak behavior of synthetic claystone was studied, and microscopic indicators of macroscopic behavior were selected and visualized. In order to simulate the dump material, joints were introduced that divided the claystone specimen into fragments. The appropriate microproperties of synthetic dump material were selected by means of a similar numerical testing and calibration procedure. Distribution and redistribution of particle contact forces before, during, and after failure of the dump material specimen were visualized and velocities corresponding to strain localization plotted. According to the study and some previous references, the compressive contact force chain acting in the direction of major principal stress appears as a backbone of microstructure, and compression induced tension as its basic failure mode at particle level.     

Understanding Micromechanical Material Behavior Using Synchrotron X-rays and In Situ Loading

Metallurgical and Materials Transactions A - With the flux of high-energy, deeply penetrating X-rays that a 3rd-generation synchrotron source can provide and the current generation of large fast...     

Micromechanical Formulation of Stress Dilatancy as a Flow Rule in Plasticity of Granular Materials

The paper presents micromechanical formulations of stress dilatancy and their connection to a flow rule in classical elastoplasticity. Dilatancy is inarguably the manifestation of an internal kinematic constraint involving both particle shape and connectivity (texture or fabric) with operative interparticle friction against applied stresses. However, this notion of microstructural dependence is nonexistent in most stress-dilatancy formulations in the literature. We present two different micromechanical approaches that arrive at stress-dilatancy expressions with embedded micromechanical information in the form of a second-order fabric tensor. In connection to stress dilatancy, the underlying nature of the flow rule is next discussed with respect to the dependence of the plastic strain increment vector on the direction of loading (stress increment). It is demonstrated analytically that the flow rule is singular in three-dimensional stress and strain conditions. Finally, the dependencies of dilatancy on fabric are illustrated through various numerical simulations using the micromechanically enriched stress-dilatancy models and a discrete element method.     

Observations of Stresses and Strains in a Granular Material

The use of glass ballotini as a granular material provides the opportunity to simultaneously study internal stress fields and internal fields of deformation as a sample is submitted to boundary perturbations. Digital image correlation makes use of the visible fabric of the material to deduce a field of displacements from one digital photographic image to the next. If the glass granules are immersed in a fluid having the same refractive index, then observation with polarized light exploits the photoelastic properties of the glass to reveal information about the stresses. Again, comparison of digital photographs enables changes in stress conditions from one image to the next to be discovered. Tests performed in a simple loading device which forces rotation of principal axes in parts of the granular mass are presented to demonstrate the unique potential of this dual experimental configuration.     

Micromechanical Modeling of Strain-Hardening Behavior in Dual-Phase Steels

A mathematical model is developed to consider the impact of microstructural parameters, including the volume fraction and the average particle size of martensite, on the flow stress and strain-hardening behavior of dual-phase microstructure. In this regard, the micromechanical approach is applied for partitioning the stress and strain in ferrite and martensite. Martensite carbon content and geometrically necessary dislocations, generated from austenite-to-martensite transformation, and strain accommodation at the ferrite–martensite interface, are involved to modify the partitioned stress of martensite and ferrite, respectively. Having partitioned stress in each phase, the global stress is estimated as the function of steel chemical composition, ferrite grain size, martensite particle size, aspect ratio, and volume fraction. To evaluate the applicability of the proposed model, four dual-phase steels containing 12, 25, 34, and 48% volume fractions of martensite are prepared from the intermediate quenching process, and then after the strain-hardening stages are investigated. Comparing the experimental result and model output reveals that the presented model shows good predictive capabilities to identify strain-hardening stages and estimate the inverse of the strain-hardening exponent.     

Measurement of Particle Dynamics in Rapid Granular Shear Flows

Micromechanics of rapid granular flows is studied in a two-dimensional planar granular Couette flow apparatus. The device is capable of generating particulate flows at different shearing rates and solid fractions. Monosize plastic disks are sheared across an annular test section for several shear rates. The motion of particles is recorded through a high speed digital camera and analyzed by image processing techniques. The average and fluctuation velocity profiles are obtained and granular temperature relations with shear rate are investigated. Average streaming velocity across the shear cell decays slightly faster than exponential, and is rather Gaussian when not too close to the wall. Fluctuation velocities and granular temperature across the shear cell are related to effective shear rate. Interparticle collisions are estimated from the particle trajectories and probability distribution of collision angles obtained from particle collision data. In dense flows, three peaks of collision angles are observed, which signal the onset of triangular structure formulation and cause crystallization. It is found that the distribution of collision angles is anisotropic.     

Normalizing Behavior of Unsaturated Granular Pavement Materials

One of the important components of a flexible pavement structure is granular material layers. Unsaturated granular pavement materials (UGPMs) in these layers influence stresses and strains throughout the pavement structure, and have a large effect on asphalt concrete fatigue and pavement rutting (two of the primary failure mechanisms for flexible pavements). The behavior of UGPMs is dependent on water content, but this effect has been traditionally difficult to quantify using either empirical or mechanistic methods. This paper presents a practical mechanistic framework for determining the behavior of UGPMs within the range of water contents, densities, and stress states likely to be encountered under field conditions. Both soil suction and generated pore pressures are determined and compared to confinement under typical field loading conditions. The framework utilizes a simple soil suction model that has three density-independent parameters, and can be determined using conventional triaxial equipment that is available in many pavement engineering laboratories.     

Behavior of a Fiber-Reinforced Bentonite at Large Shear Displacements

The behavior of a polypropylene fiber-reinforced bentonite was evaluated at large shear displacements by a series of ring shear tests carried out at normal stresses varying between 20 and 400?kPa. Bentonite/polypropylene fiber composites were molded at an initial moisture content of 170%, with fiber lengths of 12 or 24?mm. The fiber thickness was 0.023?mm and the fiber content was either 1.5 or 3% by dry weight. The inclusion of randomly distributed fibers increased the peak shear strength of the bentonite, but the increase in strength deteriorated at large displacements and the residual strengths of both the nonreinforced and fiber-reinforced bentonite were similar. The peak shear strength was found to increase both with increasing fiber length and content. The fibers were exhumed after testing and it was found that the fibers had both extended and broken, with a predominance of broken fibers.     

Microscopic Evaluation of Strain Distribution in Granular Materials during Shear

The evolution of local strains during shear of particles of a granular material is presented in this paper. A cylindrical specimen composed of 6.5-mm spherical plastic particles was loaded under an axisymmetric triaxial loading condition. Computed tomography (CT) was used to acquire three-dimensional images of the specimen at three shearing stages. The high-resolution CT images were used to identify the 3D coordinates of 400 particles. Nine strain components (normal, shear, and rotation), rotation angles, and local dilatancy angles for particle groups were calculated, and their frequency distribution histograms are presented and discussed. It was found that there is no preferred shear direction, and the standard deviation values for shear strain components (εxy, εxz, and εyz) were almost equal for the specific test shearing stage. Shear strains as high as 25.6% were recorded for some particle groups. Furthermore, granular particle groups rotated in the 3D space with almost equal amounts of rotation strains when loaded under axisymmetric triaxial condition. Rotation strain values are very close to the corresponding shear strains. Compared to particle sliding, rotation plays a major role in the shearing resistance of granular materials. The cumulative vertical rotation angles can be as high as 38° and the horizontal rotation angles have values as high as 60°. The statistical distributions of the local dilatancy angle (ψ1) of particle groups were calculated and found to be increasing as shearing continues. The “global” dilatancy angle value is very close to the mean local ψ1 during the first stage of shearing (i.e, when global εz = ?7.3%)     

Dynamic Shear Behavior of a Needle-Punched Geosynthetic Clay Liner

An experimental investigation of the dynamic internal shear behavior of a hydrated needle-punched geosynthetic clay liner is presented. Monotonic and cyclic displacement-controlled shear tests were conducted at a single normal stress to investigate the effects of displacement rate, displacement amplitude, number of cycles, frequency, and motion waveform on material response. Monotonic shear tests indicate that peak shear strength first increased and then decreased with increasing displacement rate. Cyclic shear tests indicate that cyclic response was primarily controlled by displacement amplitude. Excitation frequency and waveform had little effect on cyclic shear behavior or postcyclic static shear strength. Number of cycles ( ≥ 10) also had little effect on postcyclic static shear strength. Shear stress versus shear displacement diagrams displayed hysteresis loops that are broadly similar to those for natural soils with some important differences due to the presence of needle-punched reinforcement. Secant shear stiffness displayed strong reduction with increasing displacement amplitude and degradation with continued cycling. Values of damping ratio were significantly higher than those typical of natural clays at lower shear strain levels. Finally, cyclic tests with increasing displacement amplitude yielded progressively lower postcyclic static peak strengths due to greater levels of reinforcement damage. Postcyclic static residual strengths were unaffected by prior cyclic loading.     

Undrained Shear Strength of Granular Soils with Different Particle Gradations

A series of undrained tests were performed on granular soils consisting of sand and gravel with different particle gradations and different relative densities reconstituted in laboratory. Despite large differences in grading, only a small difference was observed in undrained cyclic shear strength or liquefaction strength defined as the cyclic stress causing 5% double amplitude axial strain for specimens having the same relative density. In a good contrast, undrained monotonic shear strength defined at larger strains after undrained cyclic loading was at least eight times larger for well-graded soils than poorly graded sand despite the same relative density. This indicates that devastating failures with large postliquefaction soil strain are less likely to develop in well-graded granular soils compared to poorly graded sands with the same relative density, although they are almost equally liquefiable. However, if gravelly particles of well-graded materials are crushable such as decomposed granite soils, undrained monotonic strengths are considerably small and almost identical to or lower than that of poorly graded sands.     

Micromechanical Modeling of the Viscoelastic Behavior of Asphalt Mixtures Using the Discrete-Element Method

This paper presents a methodology for analyzing the viscoelastic response of asphalt mixtures using the discrete-element method (DEM). Two unmodified (neat) and seven modified binders were mixed with the same aggregate blend in order to prepare the nine hot mix asphalt (HMA) mixtures used in this study. The HMA microstructure was captured using images of vertically cut sections of specimens. The captured grayscale images were processed into black and white images representing the mastic and the aggregate phases, respectively. These microstructure images were used to represent the DEM model geometry. Rheological data for the nine binders were obtained using the dynamic shear rheometer. These data were used to estimate the parameters of the viscoelastic contact models that define the interaction among the mix constituents. The DEM models were subjected to sinusoidal loads similar to those applied in the simple performance test (SPT). The DEM model predictions compared favorably with the SPT measurements. However, the simulation results tended to overpredict the dynamic modulus, E*, for mixtures made with neat binders and underpredict E* for those that consisted of modified binders. The DEM models gave mix phase angles, ?mix, higher than the experimental measurements.     

Model for Dynamic Shear Modulus and Damping for Granular Soils

This paper presents a simple four-parameter model that can represent the shear modulus factors and damping coefficients for a granular soil subjected to horizontal shear stresses imposed by vertically propagating shear waves. The input parameters are functions of the confining pressure and density and have been derived from a generalized effective stress soil model referred to as MIT-S1. The predicted shear moduli and damping factors are in excellent agreement with high quality resonant column test data on remolded sands and confining pressures ranging from 30 kPa to 1.8 MPa. The proposed model has been implemented in a frequency domain computer code. By simulating the variations in stiffness and damping with confining pressure, the proposed model provides a more realistic simulation of ground amplification that does not filter out high frequency components of the base excitation.     

四连杆曲柄式飞剪剪切力分析

在飞剪剪切过程中,剪切轧件所需的剪切功由传动系统释放的动能和电机做功提供。基于能量守恒定律,建立四连杆曲柄式飞剪机剪切力计算模型,通过理论分析和计算,证明该模型的有效性。借助本文给出的剪切力计算模型,对评估飞剪的工作状态、新产品开发有一定的指导意义。     

Scale Effects of Shallow Foundation Bearing Capacity on Granular Material

Scale effects of shallow foundation bearing capacity on granular materials were investigated to further evaluate the trend of decreasing bearing capacity factor, Nγ, with increasing footing width, B, observed by other researchers. Model-scale square and circular footing tests ranging in width from 0.025 to 0.914?m were performed on two compacted sands at three relative densities. Results of the model-scale footing tests show that the bearing capacity factor, Nγ, is dependent on the absolute width of the footing for both square and circular footings. Although this phenomenon is well known, the current study used a large range of footing sizes tested on well-graded sands to show that the previously reported modifications to the bearing capacity factor, Nγ, using grain-size and reference footing width do not sufficiently account for the scale effect seen in the test results from this study. It also shows that behavior of most model-scale footing tests cannot be directly correlated to the behavior of full-scale tests because of differences in mean stresses experienced beneath footings of varying sizes. The relationship of the initial testing conditions (i.e., void ratio) of the sand beds and mean stress experienced beneath the footing (correlated to footing size) to the critical state line controls footing behavior and, therefore, model-scale tests must be performed at a lower density than a corresponding prototype footing in order to correctly predict behavior. Small footings were shown to have low mean stresses but high Nγ values, which indicates high operative friction angles and may be related to the curvature of the Mohr–Coulomb failure envelope.     

Compression and Shear Behavior of Mudstone Aggregates

In this paper, a staged compression–immersion–direct shear test was conducted on the compacted samples of crushed mudstone aggregates, and its compressive and shear behavior are discussed with attention to cementation effects. Compression behavior of the compacted samples was influenced significantly by the compaction degree as expected. So were the shear behavior and shear strength. Immersion caused an additional compression and a reduction in mobilized shear stress and in the dilatant nature during shear at low applied pressure levels. Moreover, immersion reduced significantly the peak shear strength parameter c with only a little change in ?. The compression lines and critical state lines of the nonimmersed and immersed specimens seem to parallel each other, and the compression line of the nonimmersed and the critical state line of the immersed form the upper and lower bounds, respectively. A gap between the shear stress–void ratio lines of the specimens with and without immersion can be considered to represent a combined effect of cementation retained in a crushed mudstone aggregate itself and an interlocking effect of aggregates.     

High Temperature Micromechanical Behavior of a Pt-Modified Nickel Aluminide Bond-Coating and of Its Interdiffusion Zone with the Superalloy Substrate

Metallurgical and Materials Transactions A - The micromechanical properties of a $$beta $$-(Ni,Pt)Al bond-coating was investigated between $$700,^circ hbox {C}$$ and $$1000,^circ hbox {C}$$...     

Effect of Large Shear Deformations on the Fracture Behavior of a Fully Pearlitic Steel

High-pressure torsion (HPT) has been used for investigating the influence of predeformation on the fracture toughness of a fully pearlitic rail steel. The use of HPT enables one to investigate changes in fracture toughness as a function of predeformation over a wide range of strain while simultaneously studying the influence of the crack plane orientation on the fracture toughness. With increasing prestrain, besides a strong increase in hardness, a pronounced anisotropy in the fracture toughness was found. Both the increase in hardness and the anisotropic fracture behavior can be attributed to the shear deformation process leading to an anisotropic composite structure on the nanometer scale.