A numerical assessment of strain rate effects on the critical state of crushable granular materials

The present study highlights the effects of strain rate on the critical state response of crushable granular materials. A set of drained triaxial tests is simulated using the discrete element method (DEM) to understand the rate effects on the stress-strain and volumetric behaviour of the granular sample. The DEM parameters are obtained by comparing the stress-strain and particle crushing behaviour of in-house experimental analysis on crushable coral sand under a slow strain rate. In DEM, the particles are subjected to varied strain rates under different initial confining pressures and initial densities to capture the rate effects on the macroscopic responses until the critical state. It is seen that crushing increases with increasing confining stress. However, a higher strain rate induces relatively lower crushing and higher strength in terms of both peak stress and residual stress. It is observed that in pressure-volume space, the critical state line alters with the increasing strain rate of the crushable samples, especially at high confining conditions, whereas strain rate effect on critical state seems to be negligible at low confining conditions due to the absence of particle crushing.

Effect of Binder Coatings on the Fracture Behavior of Polymer–Crystal Composite Particles Using the Discrete Element Method

Polymer–crystal composite particles formed by crystals coated with binders are widely used in the fields of medicine, energy, the chemical industry, and civil engineering. Binder content is an important factor in determining the mechanical behavior of composite particles. Therefore, this study aimed to investigate the underlying effect of binder coatings in the fracture micromechanics of polymer–crystal composite particles using the discrete element method (DEM). To achieve this objective, realistic particle and crystal shapes were first obtained and reconstructed based on X-ray micro-computed tomography (μCT) scanning and scanning electron microscope (SEM) images. A series of single particle crushing tests and DEM simulations were conducted on real and reconstructed polymer–crystal composite particles, respectively. Based on the experimental and DEM results, the effect of binder coatings on the crushing strength and crushing patterns of polymer–crystal composite particles was measured. Moreover, the micromechanics of the development and distribution of microcracks was further investigated to reveal the mechanism by which binder coatings affect polymer–crystal composite particles.

DEM Analysis of Single-Particle Crushing Considering the Inhomogeneity of Material Properties

Crushing characteristics of single particles are the basis of granular material simulation with discrete element method (DEM). To improve the universality and precision of crushable DEM model, inhomogeneous stiffness and strength properties are introduced into the bonded particle method, with which the Weibull distribution and size effect of particle strength can be reproduced without deleting elementary balls. The issues of particle strength and carrying capacity under complex contact conditions are investigated in this work by symmetric loading tests, asymmetric loading tests, and ball–ball loading tests. Results of numerical experiments indicate that particle carrying capacity is significantly influenced by coordination numbers, the symmetry of contact points, as well as the relative size of its neighbors. Contact conditions also show impact on single-particle crushing categories and the origin position of inner particle cracks. The existing stress indexes and assumptions of particle crushing criterion are proved to be inappropriate for general loading cases. Both the inherent inhomogeneity and contact conditions of particles should be taken into consideration in the simulation of granular materials.

ON THE SHEAR STIFFNESS DECREASE OF SANDY SOILS DUE TO PARTICLE CRUSHING PROGRESS

Many previous studies have focused on theoretical and experimental evaluations of the crushing of sandy soil particles under high pressure conditions. From the perspective of practical scenarios, the decreased bearing capacity caused by volume shrinkage and the reduced shear strength and stiffness caused by the crushing of sandy soil particles are important aspects. Therefore, this study aims to confirm the decrease in the shear stiffness of sandy soils subjected to various stress levels combined with high principal stresses. Author conducted crushing tests using the high-pressure true tri-axial compression apparatus under the planned stress paths. Tests on isotropic compression without deviatoric stress q and those involving combinations of the mean stress p and deviatoric stress q up to the shear failure line (SFL) on the p-q plane are employed to characterize the particle crushing of sandy soils. The results indicate that the shear stiffness of sandy soils starts to decrease due to particle crushing under a combination of low mean stress p and high deviatoric stress q. Furthermore, experimental formulas regarding the decrease in shear stiffness due to particle crushing were estimated based on the relationships among the mean stress p, deviatoric stress q and deviatoric strain εd.

Effect of Particle Morphology on Stiffness, Strength and Volumetric Behavior of Rounded and Angular Natural Sand

Stress–strain and volume change behavior for clean sands which have distinct particle shape (rounded and angular) with very similar chemical (mineralogical) composition, size, and texture in one-dimensional (1D) compression and drained triaxial compression are presented. The effect of particle morphology on the crushing behavior in one-dimensional loading is explored using laser light diffraction technique which is suitable for particle crushing because of its high resolution and small specimen volume capability. Particle size distribution in both volume/mass and number distributions are considered for improved understanding associated with the process of comminution. Number distributions present a clearer picture of particle crushing. It is argued that particle crushing in granular assemblies initiates in larger particles, rather than in smaller particle. It was found that rounded sand specimens showed greater crushing than angular sand specimens with higher uniformity coefficient. In 1D compression, loose specimens compress approximately 10% more than dense specimens irrespective of particle shape. Densification of angular sand results in improvement in stiffness (approximately 40%) and is comparable to that of loose rounded sand. In general, density has a greater influence on the behavior of granular materials than particle morphology. The effect of particle shape was found to be greater in loose specimens than in dense specimens. The effect of grain shape on critical state friction angle is also quantified.

Experimental Study on Acoustic Characteristics of Particle Breakage Under Combined Shear and Extrusion Load

The jaw crushing loading process is a typical loading process of combined shearing and extrusion. In this paper, by establishing a complete jaw crushing loading process, the sonic test method is used to determine and analyze the particle crushing law to explore acoustic characteristics of particle crushing under the combined action of shear and extrusion. A jaw crushing tester is used to simulate the jaw crushing process of sand aggregate specimens. A rock-soil sound wave detector is used to measure the sound speed, sound amplitude, and sound intensity during the simulated jaw crushing process. It is found that when the jaw angle variation range is 2.6°, the inlet-outlet ratio is 0.332 and the motor speed is 15 r/min, the sound velocity and the sound amplitude curves fluctuate more drastically and the sound intensity is higher. The crushing evaluation of the sand aggregate specimens, which have experienced crushing simulation, shows that when the jaw angle variation range is 3.0°, the inlet-outlet ratio is 0.332 and the motor speed is 33 r/min, higher crushing energy rate and crushing rate are achieved. Through the comparative analysis of each group’s acoustic parameters and crushing evaluations, it is found that both the acoustic parameters and the crushing evaluations reflect the crushing process, but they have similarities and differences. Therefore, to some extent, the acoustic parameters in the crushing process can be regarded as significant indicators for evaluating the crushing effect. This conclusion may be a reference for optimizing working parameters and structural parameters of crushing equipment.

DEM modeling of the one-dimensional compression of sands incorporating a statistical particle fragmentation scheme

This paper presents a novel framework of modeling crushable granular materials under mechanical loadings based on the discrete element method (DEM). The framework is featured with the construction of the one-to-one model in which every particle in a physical experiment has its own numerical twin and allows the modeling of irregular shaped fragments during the continuous breakage process. First, image processing techniques and spherical harmonic (SH) analysis were adopted, respectively, to segment and label particles and to construct a one-to-one model mathematically in DEM. Then, a particle crushing criterion based on the maximum inter-particle contact force was used to predict the crushing events, showing fitting results that agreed very well with a large number of single particle crushing tests. Next, a statistical approach for the generation of particle fragmentation modes of a given type of sand particles based on the principal component analysis (PCA) was proposed. The aim of the PCA was to analyze the statistical trends of the coefficient matrix, which was composed of the SH coefficients of all the particles involved in the analysis. Finally, a successful modeling of a particle crushing event was achieved by replacing the particle, which was judged by the crushing criterion to undergo crushing, with a few sub-particles chosen randomly from a specific fragment template constructed using the micro-computed tomography (micro-CT) data.

Application Ranges of EPB Shield TBM in Weathered Granite Soil: A Laboratory Scale Study

Soil conditioning is a key factor in increasing tunnel face stability and extraction efficiency of excavated soil when excavating tunnels using an earth pressure balance (EPB) shield tunnel boring machine (TBM). Weathered granite soil, which is abundant in the Korean Peninsula (also in Japan, Hong Kong, and Singapore), has different characteristics than sand and clay; it also has particle-crushing characteristics. Conditioning agents were mixed with weathered granite soils of different individual particle-size gradations, and three characteristics (workability, permeability, and compressibility) were evaluated to find an optimal conditioning method. The lower and upper bounds of the water content that are needed for a well-functioning EPB shield TBM were also proposed. Through a trial-and-error experimental analysis, it was confirmed that soil conditioning using foam only was possible when the water content was controlled within the allowable range, that is, between the upper and lower bounds; when water content exceeded the upper bound, soil conditioning with solidification agents was needed along with foam. By taking advantage of the particle-crushing characteristics of the weathered granite soil, it was feasible to adopt the EPB shield TBM even when the soil was extremely coarse and cohesionless by conditioning with polymer slurries along with foam. Finally, the application ranges of EPB shield TBM in weathered granite soil were proposed; the newly proposed ranges are wider and expanded to coarser zones compared with those proposed so far.