Visualization of Crushing Evolution in Granular Materials under Compression Using DEM

Granular materials forming part of natural slopes, embankments, subgrades of foundations, and pavement structures are subjected to both static and dynamic loads during their engineering lives. As a result of these loads, particle crushing may occur. The present study demonstrates that the discrete-element method (DEM) can be used to visualize the evolution of this breakage process. In particular, the evolution of crushing in a simulated granular material subjected to uniaxial compression is presented. Even though DEM does not normally consider particle breakage, it is possible to simulate crushing by replacing one particle that has failed in tension with a combination of many particles of different sizes. The results from the simulation indicate that crushing does not develop uniformly throughout the sample, but rather concentrates in certain regions. These observations agree with experimental results of uniaxial tests conducted on sand. Other results from the simulation satisfactorily agree with experimental results previously reported by other researchers. In this way, by using a simplified failure criterion, DEM can be used to visualize and understand the evolution of granular crushing. This is something that is very difficult to do with laboratory tests alone.     

Time Effects Relate to Crushing in Sand

Based on previously obtained experimental results, a mechanistic picture of time effects in granular materials is presented. Accordingly, time effects are caused by grain crushing, which in turn is time dependent, as indicated by static fatigue of brittle materials. Triaxial compression tests have been performed on Virginia Beach sand at high pressures, where grain crushing is prevalent, to study effects of initial loading strain rates on subsequent amounts of creep and stress relaxation. Grain size distribution curves were determined after each test and the amount of crushing, as characterized by Hardin’s breakage factor, is related to the energy input to the triaxial specimens. A pattern emerges that indicates the importance of crushing for the axial and volumetric strains, while rearrangement and frictional sliding between intact grains play much smaller roles in the stress-strain and volume change behaviors of granular materials at high stresses and shear strains. Because particle crushing is a time-dependent phenomenon described as static fatigue or delayed fracture, the close relation between time effects and crushing in granular materials is established.     

Geomechanical Characterization of the Volcaniclastic Material Involved in the 2002 Landslides at Stromboli

During the 2002 eruption, the NW flank of Stromboli volcano (Southern Italy) experienced large landslides that produced tsunami waves with a maximum runup of 10 m in height. This paper focuses on the geomechanical behavior of the loose volcaniclastic layers, the weakest component of the deposit forming the volcano flank. Tests on the coarse-grained volcaniclastic soil and on the rock material forming the grains were conducted in dry conditions, a feature of the subaerial slope. The rock material, in spite of its high porosity, exhibits relatively high stiffness and strength due to the remarkable continuity of the solid skeleton. Accordingly, the volcaniclastic soil is characterized by high shear strength. During shear tests, however, significant grain crushing occurs, which partly explains the nonlinear shear strength envelope and the strain softening associated with contractive behavior. Grain crushing was also found to affect compressibility and its time dependency, investigated under oedometric conditions.     

Analysis and Implementation of Resilient Modulus Models for Granular Solids

Constitutive equations based upon stress dependent moduli, like K-θ and Uzan-Witczak, are widely used to characterize the resilient response of granular materials for the analysis and design of pavement systems. These constitutive models are motivated by the observation that the granular layers used in pavement structures shake down to (nonlinear) elastic response under construction loads and will, therefore, respond elastically under service loads typically felt by these systems. Due to their simplicity, their great success in organizing the response data from cyclic triaxial tests, and their success relative to competing material models in predicting the behavior observed in the field, these resilient modulus constitutive models have been implemented in many computer programs used by researchers and design engineers. This paper provides an analysis of the nonlinear solution algorithms that have been used in implementing these models in a conventional nonlinear 3D finite-element framework. The analysis shows that these conventional algorithms are destined to fail at higher load levels. The paper offers two competitive methods for global analysis with these models. A comparative study of eight possible implementations of the algorithms described in the paper is made through two simulation examples.     

Impact of Bituminous Subballast on Railroad Track Deformation Considering Atmospheric Actions

High-speed ballasted track design standards usually require the use of sand and gravel layers as subballast to fulfill an accurate protection of the formation layers, not only against traffic loads, but also against the effects of weather. Seasonal changes in soil water content, or suction changes, are responsible for cyclic volumetric strains on railroad trackbed layers, thus on the infrastructure. Being almost completely water-resistant when compared with granular-only layers, bituminous subballast offers a higher protection of the subgrade, consequently improving its behavior along the infrastructure life cycle. This question is investigated through the comparison of the performance of the track formation against atmospheric actions, taking into consideration the unsaturated state of the geomaterials. The method adopted consists of modeling the vertical displacements of both bituminous and granular subballast designs through a finite-element coupled thermo-hydro-mechanical (THM) analysis. The comparison of the two design solutions confirms that the adoption of a bituminous subballast layer might allow important reductions in seasonal vertical displacements.     

Application of Weibull Statistics to the Tensile Strength of Rock Aggregates

Rock aggregates forming part of engineering structures such as embankments, rock fills, pile foundations, and pavements are subjected to static and dynamic loads. As a result of these loads crushing occurs in two different forms: abrasion and particle fragmentation. Particle fragmentation takes place when a particle is subjected to a tensile stress greater than its tensile strength. This tensile strength is a highly variable parameter. The work presented in this study shows that Weibull statistics can be used to characterize the variability in tensile strength of rock aggregates by only using one constant called the Weibull modulus. This parameter can be used not only to describe variability in tensile strength but also to describe the effect of size specimen in tensile strength. The higher the modulus, the lower is the influence of the variability of tensile strength and sample size. More than 390 different specimens coming from a red Biotite Gneiss and a grey Quartzite were tested to validate the use of Weibull statistics. The Weibull modulus was found to be equal to 2.75 and 4.23 for the red Biotite Gneiss and the grey Quartzsite respectively.     

Mechanism of compacting and forming in the course of SHS compaction with a granular shell

By a calculational and experimental method, the mechanism of compacting and forming of flat blanks produced by the method of SHS compaction with the use of a granular shell is investigated. The features of the kinetics of compacting hot solid-liquid products of synthesis are considered. A mathematical model of forming flat blanks in the course of SHS compaction is proposed. The mechanism of deforming and compacting the granular shell is investigated. The effect of size and mechanical properties of the shell on the mechanism of compacting and forming of SHS blanks is evaluated. The technical procedures for improvement of the standard of SHS-compacted flat blanks are presented.     

Compartmentation of the granular layer of the cerebellum

Numerous studies have demonstrated that the cerebellum is highly compartmentalized. In most cases, compartmentation involves the Purkinje cells and the molecular layer, but there is also substantial evidence that the granular layer is subdivided into a large number of highly reproducible modules. We first review the evidence for a modular granular layer. Compartmentation of the granular layer has been revealed both functionally and structurally. First, tactile receptive field mapping has revealed numerous discrete functional modules within the granular layer. The molecular correlates of the receptive fields may be the compartments revealed by histological staining of the cerebellum for several enzymes and antigens. The structural substrate of the receptive fields is the mossy fiber afferent projection map, and anterograde tracing of various mossy fiber projections shows afferent terminals in parasagittal bands within the granular layer that are topographically aligned with the Purkinje cell compartments. Based on this evidence we argue that the cerebellum consists of many hundreds of reproducible structural/functional modules, and that a modular organization is a prerequisite for the efficient parallel processing of information during motor control. The complex organization of the adult granular layer implies elaborate developmental mechanisms. In the second part of the review we consider five developmental models to generate the modular organization of the adult granular layer: 1) the external granular layer is heterogeneous, and its topography translates directly into a modular granular layer; 2) granular layer modules are clones, derived from single external granular layer precursors; 3) modules in the granular layers are a secondary epigenetic response to the compartmentation of the Purkinje cells; 4) modules are secondary to the compartmentation of the afferent terminal fields; 5) modules are sculpted by activity-dependent processes.     

Utilization of waste of Al - B fiber composites for obtaining new powder and sintered materials

Composite powder based on AlB has been obtained by crushing pieces of industrial-waste Al - B fiber composite material at room temperature. The original design of a high precision microhardness tester made it possible to estimate the properties of powder particles both in the near-surface layers and under them. The aluminum fraction in the powder was established to represent a new structurally nonhomogenous material with increased microhardness (1.5 GPa), which grows up to 4 GPa in the near-surface layers. This is caused by formation of defects and stable oxides on external and internal interfaces in the aluminum matrix during crushing. To optimize the process of AlB powder sintering, adhesion on clean and real Al - B and Al/Al interfaces at various temperatures was investigated. By sintering at 910 K, new ceramic materials have been obtained with a density of 2200 kg/m³ and a strength of 90 MPa.     

Study of stresses in hard rolled railcar wheels with a web of different geometry

Results obtained from extensometric and x-ray diffraction studies have shown that standard railcar wheels with a plane-conical web have a high concentration of stresses in the rim region under normal service loads. Fatigue cracks were found to be formed and grow within this region. Wheels with a curvilinear web have advantages over the standard wheels in terms of their stress state. On the average, the total stresses in them are 1.5 times smaller than in standard wheels. As a result, S-shaped wheels have greater fatigue strength and can withstand tests at loads above 100 ton-f without forming cracks. Based on the findings from laboratory and field tests, the new wheels have been certified and recommended for use on new-generation railcars with axial loads of 25 ton-f.     

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.     

Roll Forming Strategies for Welded Tubes

Tubes are used as semi‐finished products as well as final components in almost all areas of the engineering industry. Roll forming of tubes with longitudinally oriented welding lines is one of the most efficient and economic tube production processes. However, numerous roll forming strategies already exist. Each strategy involves a characteristic change of the material properties from the initial slit strip to the final tube. A classification of the different roll forming strategies, which is given in this paper, aims to provide a systematic overview. A finite element analysis of the roll forming process is presented to identify specific forming loads and property changes.     

Predicted Loads in Steel Reinforced Soil Walls Using the AASHTO Simplified Method

The paper investigates the accuracy of the AASHTO simplified method by using load measurements reported in a large database of full-scale instrumented walls for bar mat, welded wire, and steel strip soil reinforced walls. The accuracy of the AASHTO simplified method is quantified by computing the mean and coefficient of variation of the ratio (bias) of measured loads under operational conditions to predicted loads. The paper shows that for steel strip walls, the AASHTO simplified method is reasonably accurate for granular backfill soils with friction angles less than 45°. For bar mat walls, the method is demonstrated to be slightly conservative. The simplified method underpredicts reinforcement loads at shallow overburden depths for steel strip walls with backfill friction angles greater than 45° due to compaction-related effects. It is concluded that these compaction-induced loads near the wall top do not contribute to internal instability due to pullout.     

Time-Dependent Particle Transport through Granular Filters

This paper describes an analytical model of filtration for granular media, based on the mechanics of particle migration under hydraulic loads. A new equation to predict the probability of particle movement through a 3D network model of the filter voids has been developed. Void constriction sizes are determined based on the particle-size distribution and relative density of the filter. An important new development is the differentiation between particles that form part of the filter structure and fine particles that are loose within the filter voids, or coarse particles that are enmeshed in a matrix of fines. The rate of particle erosion and transport is governed by the consideration of mass and momentum conservation. The model describes the time-dependent change of flow rate and base and filter particle-size distribution, porosity, and permeability. The model has application in the design of granular filters for noncohesive uniform, well-, and broadly graded base and filter materials.     

PX1200／180型旋回破碎机腔型改进

通过对PX1200／1800型机械式旋回破碎机破碎腔的尺寸结构进行分析，提出了改进方案并进行了改进，提高了设备的工作效率。     

Scanning electron microscopy of Blastocystis hominis cysts

Scanning electron microscopy of Blastocystis hominis cysts reveals that some cysts have an outer coat, whereas others are naked. If intact, the outer coat forms a fan-like structure around the cyst and its surface is granular. The fragmented outer coat adheres to other cysts and bacteria, forming irregular clumps.     

处理工艺对Fe–Al合金粉末成形性及生坯压溃强度的影响

为了提高Fe–Al合金粉末的压制成形性和生坯压溃强度,分析了氢气还原和真空退火工艺对Fe–Al合金粉末形貌和性能的影响,研究了原始粉末和经过处理后粉末所制管坯的成形率。结果发现,经氢气还原和真空退火处理后,粉末形貌变化不大,粉末性能提高,生坯压溃强度增强,成形性得到改善;尤其是经真空退火粉末的性能得到极大提高,氧的质量分数由0.5%低到0.2%,松装密度由1.82 g·cm?3降低到1.64 g·cm?3,显微硬度由HV 260降低到HV 158,压缩比由63%降低到56%,生坯压溃强度由2.0 MPa提高到2.7 MPa,粉末成形性得到极大改善。在批量化压制长管坯时,对原始粉末预先进行真空退火处理,长管生坯数量成品率由50%提高至100%,产品成本降低。     

某钨选矿厂破碎工艺综合改造实践

针对某矿选厂破碎工艺中存在的磨矿成本过高、设备空耗过大、生产效率低和资源回收率低的问题,通过对原矿卸矿系统、中细破碎系统和矿泥浓缩系统的改造,增加手选作业系统。通过改造,废石选出率超过50%,提高了入选品位,从而减轻选矿厂设备负荷,相应增加了生产能力;采用曲轨卸矿器、空气压缩机和清洗风管,实现了原矿卸矿系统干净、快速、安全;增加了缓冲矿仓和YA1224振动筛,使中细破碎系统均衡满负荷生产;改造了2ZK1836直线振动筛,并增加了螺旋分级机,使矿泥浓缩系统更加环保、节能。     

Endosonographic features of esophageal granular cell tumors

BACKGROUND AND STUDY AIMS: Granular cell tumors of the esophagus are rare tumors. A definite diagnosis is achieved by endoscopic biopsies in only 50% of cases. Endoscopic ultrasonography (EUS) is the best procedure in the evaluation of upper gastrointestinal tract submucosal tumors. The aim of this study was to describe the endosonographic findings of esophageal granular cell tumors. METHODS: From January 1989 to March 1994, 15 patients with 21 granular cell tumors which had negative biopsies were examined by EUS (Olympus GF UM3 or GF UM20,7,5 and 12 MHz). In five cases, the tumor was also studied with a 20 MHz Olympus miniprobe. The final histological diagnoses were obtained by subsequent endoscopic snare resection in 20 cases and surgically in one case. RESULTS: The endosonographic features (with the GF UM3 or GF UM20) of esophageal granular cell tumors were: a) a tumor size of less than 2 cm in 95% of cases; b) an hypoechoic solid pattern in 100% of cases; c) a tumor arising in the inner layers in 95% (second echo-poor layer n=15; third echo-rich layer n=5). In one case, the endosonographic finding was transmural malignant infiltration of the esophageal wall (histologically confirmed). CONCLUSION: When a granular cell tumor of the esophagus is suspected, EUS can show the inner layer location of the tumor and thus contribute to planning the endoscopic resection or follow up. When the tumor also invades the outer layers, EUS can contribute to planning the surgical resection.     

In-Situ Austenite Steel Matrix Composite Reinforced by Granular γ+(Fe,Mn)3C Eutectic

A new in-situ austenite matrix composite reinforced by granular γ+(Fe, Mn)3C binary eutectics (abbreviated to in-situ AMGE) was prepared in as-cast state, in which the modifier, yttrium-based heavy rare earth alloy, was used to influence carbon segregation, manganese segregation and phase formation. The eutectics are formed in the molten pools among austenite dendrites at the later stage of non-equilibrium solidification because the modifier enhances carbon segregation and manganese segregation greatly. Pin-on-disc dry wear tests show that the wear resistance of in-situ AMGE is 1-3 times higher than that of austenite medium manganese steel under low and medium loads, and the loads under which serious wear of in-situ AMGE occurs are much higher than that of austenite medium manganese steel.