Study by X-ray microtomography of the horizontal vibration effects on sand densification

X-ray microtomography experiments were performed in order to evaluate the densification of silica sand submitted to horizontal sinusoidal vibrations carried out at constant frequency (50 Hz) with controlled acceleration and deceleration \(\Gamma \). Packing homogeneity was characterized using relative density distribution through 3D images of the relaxed samples. Information obtained from the images allowed us to evaluate data at grain scale: porosity and pore size distribution, number of contacts per particle, particle shape and size distribution were evaluated and linked to the densification process. Based on the internal analysis of samples, the results confirm and extend the conclusions of previous works regarding the 3-layer densification under vibration and the proposed optimized vibration cycle to get dense and homogeneous samples. They extend them to different initial packings. Additionally, significant correlations are found between density and local particle packing characteristics such as pores size distribution, or the number of contacts per particle.     

Three dimensional fabric evolution of sheared sand

Granular particles undergo translation and rolling when they are sheared. This paper presents a three-dimensional (3D) experimental assessment of fabric evolution of sheared sand at the particle level. F-75 Ottawa sand specimen was tested under an axisymmetric triaxial loading condition. It measured 9.5?mm in diameter and 20?mm in height. The quantitative evaluation was conducted by analyzing 3D high-resolution x-ray synchrotron micro-tomography images of the specimen at eight axial strain levels. The analyses included visualization of particle translation and rotation, and quantification of fabric orientation as shearing continued. Representative individual particles were successfully tracked and visualized to assess the mode of interaction between them. This paper discusses fabric evolution and compares the evolution of particles within and outside the shear band as shearing continues. Changes in particle orientation distributions are presented using fabric histograms and fabric tensor.     

Influence of grinding on the graduation graphitization and densification of coke powder

Petroleum coke powder, made by the delayed coking method at about 500° C, was ground for various times from 15 min to 44 h. Carbon solids were made from the ground coked powder compacts and heat-treated at 1000 to 2800° C without the use of binder materials. The coke particles ground for a considerable time had a spherical appearance and an amorphous structure, and showed non-graphitizability. These ground powders were easily densified and hard carbon solid could be obtained by heat-treatment. However, if the coke powder was pre-heat treated above about 600° C before grinding, no densification occurred and the powder graphitized as well as non-ground ones. The hard carbon solids made from powder ground for 44 h had a bulk density of 1.71 g cm^–3, Shore hardness of 120 and bending strength of about 700 kg cm^–2 at a heat-treatment temperature of 1000° C. These values changed with increasing heat-treatment temperature to a bulk density of 1.93 g cm^–3, Shore hardness of 90 and bending strength of 500 kg cm^–2 at a heat-treatment temperature of 2800° C.     

AIN粉末特性对AIN—BN复合陶瓷致密化的影响

以比表面积分别为4．26和17．4m^2／g两种A1N粉末为原料，添加5％Y2O3作为烧结助剂制备AIN15BN陶瓷（质量分数，％），研究了A1N粉末特性对复合陶瓷致密化过程的影响。结果表明，A1N粉末比表面积对复合陶瓷致密化有重要影响，比表面积高的AIN粉末所制备的复合陶瓷致密化过程主要发生在1500-1650℃，1650℃烧结3h后，复合材料的相对密度达95．6％，继续升高温度，对材料的密度影响不大；而低比表面积的A1N粉末所制备的复合陶瓷的致密化过程主要发生在1650～1850℃，1850℃烧结3h，复合陶瓷的相对密度为86．4％。即高比表面积的AIN粉末有利于获得相对密度高的A1N—BN复合材料。     

AlN粉末特性对AlN-BN复合陶瓷致密化的影响

以比表面积分别为4.26和17.4 m2/g两种AlN粉末为原料,添加5%Y2O3作为烧结助剂制备AlN-15BN陶瓷(质量分数,%),研究了AlN粉末特性对复合陶瓷致密化过程的影响。结果表明,AlN粉末比表面积对复合陶瓷致密化有重要影响,比表面积高的AlN粉末所制备的复合陶瓷致密化过程主要发生在1500~1650℃,1650℃烧结3 h后,复合材料的相对密度达95.6%,继续升高温度,对材料的密度影响不大;而低比表面积的AlN粉末所制备的复合陶瓷的致密化过程主要发生在1650~1850℃,1850℃烧结3 h,复合陶瓷的相对密度为86.4%。即高比表面积的AlN粉末有利于获得相对密度高的AlN-BN复合材料。     

Effect of fabric anisotropy on shear localization in sand during plane strain compression

Summary The paper focuses on the effect of fabric anisotropy on shear localization in cohesionless granular materials. For the numerical simulation, a hypoplastic constitutive model was used. In order to take into account a characteristic length of the micro-structure, the constitutive model was extended to include the second gradient of the Euclidian norm of the deformation rate. The hypoplastic model captures the salient features of granular bodies in a wide range of density and pressure with a single set of parameters. Transversal isotropy is described by the dyadic product of the normal vector of the space orientation of the plane of symmetry. FE-simulations of plane strain compression under constant lateral pressure were carried out with a medium dense specimen for both uniform and stochastic distribution of the initial void ratio. The effect of the direction of the bedding plane and the initial void ratio distribution on the load-deformation behavior was investigated. Moreover, the location, thickness and inclination of the shear zone were also analyzed.     

On the quasi-static granular convective flow and sand densification around pile foundations under cyclic lateral loading

The saturated sand surrounding an offshore pile foundation under quasi-static cyclic lateral load can show the physical phenomena of macromechanical densification and convective granular flow. Based on the results from physical model tests at different geometrical scales, this paper provides a certain quantification of such phenomena and discusses their causes and consequences. The progressive sand densification leads to subsidence of the soil surface and a significant stiffening of the pile behaviour. Conversely, the ratcheting convective motion of two closed cells of soil beneath the pile-head is responsible for an endless grain migration at the soil surface, the inverse grading of the convected material and a direct shear of the sand at the distinct boundary of the revolving soil domain. In this respect, and from a macromechanical perspective considering the soil as a continuum, it appears that the convecting material tends to follow gradient lines of shear stress during its ratcheting motion. Concluding the paper, the practical relevance of these phenomena and their extrapolation to other conditions are briefly discussed.     

Influence of production conditions on the densification of zirconia powders obtained from acetates

Tetragonal yttrium-stabilized zirconia samples were obtained by pyrolysis of amorphous acetates containing both metals, at 950 °C in air. The dilatometric behaviour of samples indicates a dedensification when pyrolysis is carried out with a non-linear heating rate causing an absence of fine open porosity. A destabilization when cooling is observed for samples exhibiting a wide granulometry with agglomerate sizes reaching 100 m. Even when neither one, nor the other phenomenon is noticed, an important aggregation state of the starting powders limits the densification and leads to poor sintered densities.     

化学气相渗积压力对渗积速率和热解炭组织结构的影响(英文)

采用天然气为前躯体在不同压力下使用化学气相渗积法制备炭/炭复合材料。利用甲烷分解热力学与沉积动力学研究了渗积压力对渗积速率和热解炭组织结构的影响。采用偏光显微镜观察热解炭的组织结构。结果表明:随着渗积压力的增加,初始渗积速率增大;但在渗积后期,渗积速率随着渗积压力的增大而降低,导致在高渗积压力下相同时间制备样品的最终密度降低。热解炭组织结构对渗积压力具有很强的依赖性。在低压(1 kPa)下渗积得到的热解炭基体全部为粗糙层结构。在适中的压力(3kPa,5 kPa,10 kPa)下,以炭纤维为圆心由内到外依次得到各向同性和粗糙层热解炭,整个基体以粗糙层为主。在15 kPa下,得到的热解炭组织结构为各向同性和光滑层组织。     

Carbon fabric reinforced polyetherimide composites: Influence of weave of fabric and processing parameters on performance properties and erosive wear

Woven carbon fabric reinforced (55 vol.%) polyetherimide (PEI) composites were fabricated using three types of weaves viz. plain (P), twill (T), and satin-4 H (S) by impregnation technique. Three more similar composites were fabricated with film technique to study the influence of both, weave of fabric and processing technique on the performance properties of the total seven composites including neat PEI. The composites were evaluated for physical and mechanical properties along with erosion wear behavior studied in identical conditions. In almost all properties viz. tensile strength (TS), modulus (TM), elongation to break (e), flexural strength and modulus, interlaminar shear strength (ILSS), etc., film technique proved far inferior to impregnation technique because of improper wetting of fiber strands, as evidenced by SEM studies. CF reinforcement enhanced all the properties of PEI manifold except elongation to break. None of the weaves proved best performer in all the mechanical properties. In case of erosive wear studies, plain weave composite proved slightly better than satin weave composite. Composite with twill weave proved poorest performer. In case of film technique, however, trends were different where plain weave composite proved poorest and satin proved best. Efforts were made to correlate various strength properties with wear resistance W_R. The factor (elongation × toughness) showed fairly good correlation with W_R. SEM studies were conducted to understand wear mechanisms.     

Influence of oxide-based sintering additives on densification and mechanical behavior of tricalcium phosphate (TCP)

In this research, we studied and analyzed the effects of four different oxide-based sintering additives on densification, mechanical behavior, biodegradation and biocompatibility of tricalcium phosphate (TCP) bioceramics. Selective sintering additives were introduced into pure TCP ceramics, in small quantities, through homogeneous mixing, using a mortar and pestle. The consequent powders of different compositions were pressed into cylindrical compacts, uniaxially and sintered at elevated temperatures, 1150°C and 1250°C, separately in a muffle furnace. X-ray powder diffraction technique was used to analyze the phase-purity of TCP after sintering. Hardness of these sintered specimens was evaluated using a Vickers hardness tester. Sintered cylindrical samples were tested under uniaxial compressive loading, as a function of composition to determine their failure strength. Biodegradation studies conducted using simulated body fluid under dynamic environment, revealed that these additives could control the rate of resorption and hardness degradation of TCP ceramics.     

浸渍致密化对碳素复合材料热辐射性能的影响

测定了碳素复合材料的法向光谱发射率和法向总发射率,利用扫描电子显微镜(SEM)进行表面形貌分析,研究了浸渍致密化工艺以及表面形貌与碳素复合材料法向光谱发射率和法向总发射率的变化.结果表明,炭布和短纤维增强碳素复合材料的法向光谱发射率和总发射率均在最终浸渍致密化之后有所增加,在多次致密化后两组样品总发射率间的差距减小,增...     

Experimental analysis of cyclic loading on a cohesionless granular system

The cyclic loading mechanical response of dense granular materials under biaxial compressions was experimentally investigated. Ratcheting occurred and evolved throughout the experiment. The results suggest that permanent displacement primarily occurs during the first cycle; the generation of ratcheting is principally driven by the occurrence of stick–slip events and the changes in porosity. In different cycles, the occurrence of stick–slip events obeys the power law relationship, and the change in porosity tends to approach a homogenous state. Simultaneously, the magnitude of the initial force plays an important role for increasing ratcheting in granular system.     

Influence of phosphate on the transport properties of lead in sand

Temporal moment analysis was used to examine the transport of lead species in sand columns. The influence of sodium phosphate (PO(4(aq))) and hydroxyapatite (HA) on lead transport was also evaluated. Transport properties of lead microparticles (diameter>0.45 μm) were a function of electrophoretic mobility: those particles with electrophoretic mobility less than -1 × 10(-8)m(2)/Vs exhibited significantly lower dimensionless first temporal moment (θ) and second temporal moment (σ(θ)(2)). The forms of lead investigated in this work had a tendency to move in sand over a wide pH range. Although the PO(4(aq)) amendment substantially reduced lead mass recoveries in the sand column effluent, lead microparticles were formed that had a tendency to move rapidly and with minimal dispersion when compared with controls. Treatments with HA provided limited reduction in lead mass recovery and minimal changes in lead transport properties. A colloid stability model was used to predict attachment of lead particles in sand.     

Influence of the shape of fiber cross section on fabric surface characteristics

Fiber cross sections for use in textiles and composites are becoming more and more complex. Shape impacts fiber or filaments properties and therefore the yarn and fabric characteristics. This paper presents the influence of the fiber cross section on the fabric surface characteristics. The material used was polyester staple fibers, of four different shapes: round, scalloped oval, cruciform and hexachannel. All fibers had the same cut length with different fineness. Yarns obtained from these fibers had nominally the same yarn count, torsion value and structure. Plain jersey fabrics were knitted from each of the yarns under identical conditions and then relaxed prior to testing. Friction behavior was evaluated and a roughness-friction criterion developed. An indirect measurement of the real area of contact was obtained in order to provide roughness and friction properties. The influence of fiber cross section on yarn bending rigidity and on the state of the knitted fabric surface was characterized.     

The liquefaction cycle and the role of drainage in liquefaction

In this work the liquefaction cycle is introduced as a framework to describe the coupled processes that take place in fluid-saturated granular media that lead to liquefaction. The modular formulation of liquefaction makes it possible to test the various processes that contribute to liquefaction separately, and to assemble different formulations of the relevant physics into a numerical model for liquefaction. This view on liquefaction is used here to assess the role of drainage in liquefaction. We present a simple scale analysis of the role of drainage. A numerical implementation of the liquefaction cycle shows, however, that the scale analysis is deceptive for the case when strong spatial variations in the permeability inhibit fluid migration. As an illustration the numerical model is used to quantify the imprint of a low-permeability layer on the liquefaction behavior.We greatly appreciate our conversations with Masami Nakagawa and Vaughan Griffiths about this work. The critical and constructive comments of two anonymous reviewers are very much appreciated. The visit of Annemieke van den Beukel to Golden was supported by the Molengraaff fonds, the Schuurman Schimmel van Outeren Stichting, the Stichting Dr. Hendrik Mullers Vaderlandsch Fonds, and the Trajectum Fonds.     

Influence of friction on a harmonically forced rotational oscillator in dry sand

Summary The periodic motion of a body in dry sand is influenced by an unknown resistance in the contact surface. A rotational oscillator is used as measurement device to detect some basic principles. Hypotheses about the pseudo-constitutive laws for the interaction between sand and oscillator allow the calculation of a steady state motion. Experimental investigations confirm the theoretical assumptions and show the dominant influence of friction on the motion.With 9 Figures     

Shape-forming and densification of ceramic superconductors

Processing studies on varied shape-forming and densification of bulk ceramic superconductor, YBa_1·5Ca_0·5Cu₃O_7\t-\gd, are reported in this paper. Polyvinyl butyral-polyethylene glycol-trichloroethylene has been found to be the best binder-plasticizer-solvent system in plastic shape-forming. The effect of initial particle morphology on final densification has been the most sensitive single parameter as compared to compaction pressure and final sintering durations at ∼930°C. 1-2-3 powders of mean particle size ∼ 1·94 μm have yielded sintered densities ∼92% T.D. albeit with lower oxygen intake O_6·7.     

纳米Al2O3化学镀铜粉杂质对烧结致密化的影响

用化学镀铜方法制备了纳米Al2O3化学镀铜复合粉末,并用常规粉末冶金方法对粉末的烧结特性进行了研究.纳米Al2O3化学镀铜复合粉末具有异常的粉末烧结特性和难于烧结的特点.通过对粉末清洗、热重分析(TG)、粉末及烧结试样的场发射扫描电镜(FT-SEM)及能谱(EDS)的观察分析,发现纳米Al2O3化学镀铜粉末表面吸附了化学镀溶液的杂质.这些杂质在粉末高温还原及烧结过程中发生热解,并在铜的表面沉积碳,影响了纳米Al2O3化学镀铜粉的烧结.