Investigation on compressive behavior of Cu-35Ni-15Al alloy at high temperatures

Microstructures and mechanical properties of Cu-35Ni-15Al alloy in cast and porous states were studied by scanning electron microscopy and compression tests. The influence of porosity, deformation temperature and loading rate on mechanical properties of the two kinds of alloys was investigated. The results show that the as cast alloy and porous alloys have almost the same phase constitution: Cu rich phase, Ni rich phase and K intermetallics. The yield strength of porous alloys increases continuously with decreasing porosity, the relationship between porosity and yield stress follows Gibson-Ashby equation. With decreasing deformation temperature, the yield strength of as cast alloy and porous alloy increase. With the increase of loading rate, the yield strength of these alloys shows an increasing trend. After compression, the microstructure of as cast alloy is more uniform, and porous alloys are more prone to have localized deformations.     

Structure reinforcement of porous hydroxyapatite pellets using sodium carbonate as sintering aid: Microstructure,secondary phases and mechanical properties

Porous HAP pellets suitable for loading therapeutic agents were prepared using microcrystalline cellulose (MCC) as pore former and sodium carbonate as sintering aid (SAID). The effect of sintering temperature on the microstructure, mechanical properties and disintegration of pellets prepared at different SAID content was studied. Pellets were characterized by SEM, image analysis, porosimetry and surface area. Secondary phases were identified by PXRD, ATR-FTIR and Raman spectroscopy. Increasing the sintering temperature decreased the diameter, porosity, surface area and friability of the pellets but increased the pore size, tensile strength and disintegration time. The effect of SAID was dependent on sintering temperature. With 5% SAID, a secondary β-tricalcium phosphate (β-TCP) phase was formed, indicated by FTIR peak at 980 cm^?1 and characteristic PXRD reflections, whereas with 10%, a secondary B-type carbonated hydroxyapatite phase (CHA) formed, indicated by FTIR peaks at 878 and 1450 cm^?1, a broad Raman peak in the region 1020 to 1050 cm^?1 and PXRD reflections. Pellets prepared with SAID showed high strength and also porosity. The biphasic HAP/β-TCP pellets exhibited remarkably great strength (4.39 MPa) at the high sintering temperature, while still retaining 43.9% porosity. Relationships were established between the mechanical properties or disintegration time of the porous pellets and the microstructural parameters.     

HEMA-TBA凝胶体系制备多孔氧化铝陶瓷

采用HEMA-TBA凝胶体系制备具有高气孔率, 高强度的多孔氧化铝陶瓷, 研究多孔氧化铝陶瓷的浆料和坯体制备工艺, 并系统研究了分散剂含量和固相体积分数对浆料粘度和悬浮稳定性的影响、干燥和单体含量对生坯性能的影响以及固相体积分数和烧结温度对烧结体微观结构的影响。结果表明: 加入柠檬酸可以使浆料粘度降低, 稳定性提高, 柠檬酸加入量达到2wt%后浆料粘度和稳定性趋于稳定; 固相体积分数的增加会导致浆料粘度和稳定性的增加; 生坯在干燥过程中的收缩比水基体系小很多, 干燥时间也相对更短; 单体含量对生坯强度影响较大, 当单体含量为25wt%时, 生坯强度较高; 通过选择不同的固相体积分数和烧结温度, 可以有效地控制烧结体的微观结构, 气孔率的变化范围在40%~65%, 同时烧结体强度也会随之发生变化, 变化范围在5.7~91.2 MPa。     

Relationship between tensile strength and porosity for foamed metals under equal speed biaxial tension

Abstract

A preliminary experimental investigation has been carried out on an isotropic three dimensional reticulated foamed metal with high porosity under biaxial tensile loading. The approximate relationship between tensile strength of these materials and their porosity has been evaluated under equal-speed biaxial tension loading. The mathematical formula is proved to be in a good agreement with the experimental data for nickel foam.     

Thermal shock resistance of the porous Al2O3/ZrO2 ceramics prepared by gelcasting

Porous Al₂O₃/ZrO₂ ceramics with porosity varying from 6% to 50% were fabricated by gelcasting using polystyrene (PS) as pore-forming agent. The effects of sintering temperature on porosity, strength as well as pore size were investigated. The flexural strength of these porous ceramics at room temperature significantly decreases as the porosity increases. Thermal shock resistance of these ceramics was improved by increasing the porosity. Both the critical difference temperature (ΔT_c) and residual strength of high porosity ceramics were higher than those of low porosity ceramics. These improvements can be attributed to the pores in the specimens which relax the thermal shock stress and arrest the propagation of microcracks effectively, which is confirmed by XRD analysis of specimens which encountered different thermal shock temperature difference.     

高温持续荷载作用下复合纤维对沥青混合料稳定性的影响

为了研究复合纤维沥青混合料在高温持续荷载作用下的稳定性,根据工程中常用纤维和已有研究成果,选用了三种复合纤维,对沥青混合料的变形发展规律及内部空隙结构特征开展试验研究。首先采用加速加载试验,对不掺加纤维和掺加复合纤维的沥青混合料在高温持续荷载作用下的横断面变化和车辙深度进行分析;再利用X-CT扫描技术与VG软件三维重构功能,研究不同纤维沥青混合料的空隙组成及形态特征演变规律。结果表明:高温持续荷载作用下,掺加纤维混合料横断面变形呈现"W"字型,不同纤维在沥青混合料中发挥"加筋"、"增黏"、"吸油"的协同作用,能显著减小沥青混合料的流动变形,且掺加复合纤维III的混合料流动变形最小;掺加不同复合纤维的沥青混合料可采用y=a-b·ln(x+c)进行车辙深度预估。加速加载试验后,掺加三种复合纤维的沥青混合料空隙率最大仅增加7%,分形维数仅降低不到0.5%,表明掺加三种复合纤维基本不改变沥青混合料的空隙结构特征,而且在高温持续荷载作用下,可较好地保持沥青混合料内部空隙的三维形态特征,进而改善沥青混合料的抗变形能力。     

Ice stress-strain curve prediction in uniaxial compression loading in the objective of atmospheric icing risk evaluation

Atmospheric icing is a major cause of damage to electric power networks. During ice storms, “ice shedding off” cables and conductors can lead to major damage to power systems, mostly by mechanical failure. The brittleness of ice might be its most important and critical property possibly leading to hazardous events in urban regions, and involving equipment such as electrical power lines. To avoid or reduce the risks from this phenomenon, it is necessary to understand the mechanical behavior of ice, including ice strain, as well as its maximal strength and residual strength. The behavior of ice depends mainly on the applied stress and strain rate, as well as temperature, salinity, porosity, and particle size. As a fundamental subject on the deformation behavior of ice, the simple case of polycrystalline isotropic ice was chosen in this study. A methodology is developed to determine the stress-strain curve for ice as a function of temperature and several uniaxial compression loading conditions. Mathematical relations are developed for several loading conditions and temperature. The results of this research allow to predict the maximal and residual strength of ice, as functions of strain rate and temperature.     

不同孔隙率红砂岩的动力响应特性试验

采用分离式霍普金森压杆冲击试验系统,对现场采集的试样加工后进行相似速率条件下的冲击试验,得到了冲击荷载作用下一定范围内的不同孔隙率红砂岩的相关动力特性。即在相似冲击速率下,随着红砂岩的孔隙率从5.67%增大到11.86%,其反射波幅值逐渐增大,透射波幅值逐步减小,透射波峰值出现时间随着孔隙率的增大而提前;动力传递率随着孔隙率的增大呈指数下降;动态弹性模量和峰值强度随着孔隙率的增大而明显减小,峰值强度降低率随孔隙率的增加基本呈直线上升;当孔隙率在5.67%⁸.32%区间变化时,峰值强度对应的应变相差不大,但是当孔隙率增为11.86%时,无论是峰值强度还是峰值对应的应变都显著减小。     

Shear and mode II fracture of PUR foams

Polyurethane (PUR) foam materials are widely used as cores in sandwich composites, for packing and cushioning. They are made of interconnected networks of solid struts and cell walls incorporating voids with entrapped gas. The main characteristics of foams are lightweight, high porosity, high crushability, and good energy absorption capacity. Fracture toughness in mixed mode loading is of particular interest because foam cracking weakens the structure’s capacity for carrying loads.Present paper assesses the shear elastic (shear modulus) and mechanical (shear strength) properties of polyurethane foams. Also, three different types of specimens were used to determine mode I and mode II fracture toughness. The shear modulus, shear strength and fracture toughness increases with increasing foam density. Also the effect of loading direction and loading speed is investigated. The authors propose a micromechanical model to estimate fracture toughness based on the tensile strength of the solid material and the topology of the cellular structure.     

The effect of temperature on the hydration of composite cements containing limestone powder and fly ash

The effect of the curing temperature (5, 20 and 40°C) on the degree of hydration, amount of bound water and calcium hydroxide, porosity and the development of mechanical properties was investigated on pastes and mortars prepared with fly ash (FA)?Climestone (L) Portland composite cements. Increasing the curing temperature for ordinary Portland cement (OPC) leads to a more inhomogeneous distribution of hydration products, resulting in an increased coarse porosity and therefore a lower compressive strength after 7?days and longer. In contrast, the FA containing mortars showed higher compressive strength with increasing curing temperature up to 90?days. The reaction of the FA is increased at 40°C and strongly retarded at 5°C. At 20 and 40°C, FA reduces the porosity at later ages. The replacement of 5% of the OPC or FA by L powder did not impair the strength at 5 and 20°C, but lowered strength slightly at 40°C for the FA blended cements. The porosity appears to be the dominating factor regarding the compressive strength, independent of whether part of the OPC is replaced by FA and L powder or not.     

高温处理后闭孔泡沫铝压缩性能及变形机理

目的 探究温度和孔隙率对闭孔泡沫铝材料压缩力学性能和变形机理的影响。方法 将孔隙率为84.3%～87.3%的泡沫铝试件在温度25～700 ℃内进行加热处理,对处理后的试样开展准静态压缩实验。结果 在准静态压缩条件下,闭孔泡沫铝材料在不同温度加热处理后的压缩应力–应变曲线均经历了3个阶段:弹性阶段、塑性平台阶段和密实阶段。孔隙率从87.3%减小到84.3%时,其弹性模量增大了44.4 MPa,屈服强度增大了0.39 MPa,平台应力增大了0.94 MPa。孔隙率为84.3%的泡沫铝,在25 ℃时,其弹性模量为141.4 MPa、屈服强度为4.25 MPa、平台应力为4.75 MPa;当加热温度为500 ℃时,弹性模量减小到了128.0 MPa、屈服强度减小到了4.22 MPa、平台应力减小到了4.51 MPa。结论 泡沫铝的弹性模量、抗压屈服强度和平台应力均随孔隙率的增加而减小;加热温度低于500 ℃以下时,泡沫铝材料力学性能变化很小,但屈服强度和弹性模量均小幅度降低;在压缩载荷下,泡沫铝的变形破坏模式呈现出先从试件铝基体较薄弱部分产生孔壁塑性变形、孔洞坍塌,并逐渐出现断裂压缩带,直至泡沫铝孔洞完全坍塌密实。     

The influence of processing temperature on the structure and properties of mesophase-based polygranular graphites

Two polygranular graphites were prepared by sintering a coal-tar pitch based mesophase and a naphthalene-based mesophase. The influence of temperature on the structural (density, porosity and light texture), mechanical (flexural strength) and electrical (electrical resistivity) properties of the carbons was studied at different stages of carbonization/graphitization (400–2600°C). The results show that the density of the materials increases continuously with temperature, pores mainly appearing below 800°C, during the low-sintering phase and at the initial stages of the solid-sintering process. Above this temperature, porosity decreases due to the densification of the materials. Densification is clearly evidenced by a reduction in interlayer spacing and an increase in crystallite size. Flexural strength and electrical resistivity follow a different trend depending on the temperature range. Thus, flexural strength reaches its maximum value at 1000°C, while electrical resistivity decreases continuously with temperature. Any variation in these properties is mainly related with porosity and crystallographic order. The different composition of the raw materials causes structural, mechanical and electrical changes to occur to different extent in both materials.     

Preparation and properties of a new porous ceramic material used in clean energy field

At high temperature, the oxide redox reactions of ceria can split H₂O and CO₂ to produce H₂ and CO, so porous ceria with high temperature resistance and high specific surface area has an important foreground in clean energy applications. In this work, a reticulated porous ceria ceramic material with interconnected porous structure was prepared by the impregnation technique using organic polyurethane sponges as template. The influences of pretreated sponge, dipping time length, pore size and sintering temperature on the porosity and strength of the porous ceria ceramics were systematically studied. With the increasing sintering temperature, the glass phase occurred and led to an increase in strength, but an decrease in porosity. Eventually, we analyzed the relationships between porosity and strength of these porous materials, aiming to provide theoretical and practical references for its application in clean-energy field.     

Tensile stress-strain behaviour of cementitious composites at high loading rates

The influence of loading rate on the tensile stress-strain behaviour of cementitious composites was studied experimentally. The project was undertaken to obtain an insight into the possible relation between internal structure parameters of composites and their loading-rate sensitivity. Five different types of cementitious composites were applied. Composite structure data were obtained by testing porosity and by quantitative observation of fracture surfaces. Direct tensile tests were performed at four different loading rates within the range 0.001–1000 MPa s^–1. The tensile stress-strain behaviour was significantly influenced by the loading rate and structure parameters of composites. The relative tensile strength increase due to an increase of loading rate was found to be higher for composites with higher total porosity. Recorded stress-strain diagrams obtained at various loading rates are presented and discussed with the aid of continuous damage mechanics.     

Preparation,microstructure and mechanical properties of porous titanium sintered by Ti fibres

Open-cell porous Ti with a porosity ranging from 35 to 84% was successfully manufactured by sintering titanium fibres. The microstructure of the porous titanium was observed by SEM and the compressive mechanical properties were tested. By adjusting the spiral structure of the porous titanium, the pore size can be controlled in a range of 150–600 μm. With the increasing of the porosity, compressive yield strength and modulus decrease as predicated. However, high mechanical properties were still obtained at a medium porosity, e.g. the compressive yield strength and the modulus are as high as 100–200 MPa and 3.5–4.2 GPa, respectively, when the porosity is in the range of 50–70%. It was suggested that the porous titanium be strong enough to resist handing during implantation and in vivo loading. It is expected to be used as biocompatible implant, because their interconnected porous structures permit bone tissues ingrowth and the body fluids transportation.     

Fabrication and characterization of highly porous mullite ceramics

Highly porous mullite ceramics were fabricated by a reaction-bonding technique from a powder mixture of Al₂O₃ and SiC, with graphite particles as the pore-forming agent. The effects of sintering temperature on porosity and strength as well as pore size and surface area were investigated. It has been shown that the strength and pore size increase but the porosity and surface area decrease with the increase in sintering temperature. Due to the formation of a fine-grained microstructure with well-developed necks, an average strength up to 106 MPa was achieved at a porosity of 32.4%. On the other hand, a relatively high surface area of 12.4 m² g⁻¹ was obtained for a 61% porous mullite ceramic, which was observed to have a good thermal-shock resistance to crack propagation.     

Characterization of three- and four-point bending properties of porous metal fiber sintered sheet

A novel porous metal fiber sintered sheet (PMFSS) with high porosity was fabricated by the solid-state sintering method of copper fibers. In this study, both three- and four-point bending setup were established to characterize the bending properties of PMFSS. Similar three stages in the three- and four-point bending fracture process were observed for the PMFSS with 80% porosity sintered at 900 °C for 60 min. Comparing with the three-point bending, it is found that much smaller bending force was obtained in the four-point bending test under the same displacement conditions. Moreover, the porosity and sintering parameters were also varied to investigate the influence on the bending properties of PMFSS. Both three- and four-point bending strength were found to be decreased with increasing porosity ranging from 70% to 90%. Higher sintering temperature produced higher bending strength for the PMFSS sintered in the temperature range of 700–1000 °C. Besides, the extension of holding time also could slightly affect the bending strength.     

BN含量对多孔BN/Si3N4陶瓷结构和性能的影响

以氮化硅(Si₃N₄)为基体, 氮化硼(BN)为添加剂, 叔丁醇为溶剂, 采用凝胶注模成型与无压烧结工艺(温度为1750 ℃、保温时间为1.5 h、流动N₂气氛), 成功制备出具有一定强度和低介电常数的多孔BN/Si₃N₄陶瓷。在浆料中初始固相含量固定为15%体积分数的基础上, 研究了BN含量对多孔Si₃N₄陶瓷材料的气孔率、物相组成及显微结构的影响, 分析了抗弯强度、介电常数与结构之间的关系。结果表明, 通过改变BN含量可制备出气孔率为55.1%~66.2%的多孔Si₃N₄陶瓷; 多孔BN/Si₃N₄复合陶瓷的介电常数随着BN含量的增加而减小, 为3.39~2.25; 抗弯强度随BN含量提高而有所下降, BN质量分数为2.5%时, 抗弯强度最高, 为(74.8±4.25) MPa。     

Effect of Aqueous Film Coating Conditions on Water Removal Efficiency and Physical Properties of Coated Tablet Cores Containing Superdisintegrants

Abstract

Few studies have examined the effect of aqueous film coating process conditions on the physical integrity of the final coated product. Characterization of the aqueous film coating process was previously carried out by selecting water removal efficiency as the response variable to detect and monitor moisture accumulation in the tablet bed [1]. In this study, regression techniques were utilized to obtain the relationship between some physical characteristics of aqueous film coated tablet cores that contained superdisintegrant and several process parameters such as inlet air temperature, spray rate, and pan speed. Tablet response variables measured included residual moisture content, tensile strength and percent porosity. Predicted values of these properties were plotted as a function of the inlet air temperature and the coating solution spray rate. The correlations between the coated tablet response variables and the water removal efficiency of the coating process indicated that coated tablet properties such as residual moisture content, tensile strength, and porosity were linearly correlated with the water removal efficiency of the coating process, which is indicative of the environmental coating conditions present in the coating pan.     

冰冻铸造法制备Al2O3多孔陶瓷

以冰为造孔模板,采用冰冻铸造法制备了Al2O3多孔陶瓷,借助扫描电镜(SEM)观察了孔结构,并研究了冷冻温度、浆体浓度、烧结温度等工艺对多孔陶瓷显微结构、气孔率和抗压强度的影响。结果表明,可获得层状孔结构的Al2O3陶瓷,且孔结构受冷冻温度和浆体浓度的影响较大。当最大气孔率为80%时,抗压强度为16.1MPa;当最小气孔率为32%时,抗压强度为17.4MPa。