Microcrystalline Cellulose: Investigation of Porous Structure of Avicel 102 from Mercury Porosimeter Measurements

To complete study on rheological and mechanical behavior of cellulose, the authors investigated the porous structure of a microcrystalline cellulose: Avicel 102. The research required the raw material itself and two size fractions of the single sample. With them, compacts were obtained. The aim of the work was to evaluate the correlation between particle size, porous structure of powders, and porous network of compacts and the tensile strength of the corresponding compacts. With the method used, an important interparticulate porosity of Avicel 102 was observed, Overall porosity increased when particle size increased on account of the particulate rearrangement. Porous networks of compacts obtained at low pressure presented a smaller overall porosity compared to powders. Higher values were observed when particle size increased, and consequently, the tensile strength of porous network of compacts decreased.     

Mixture law including particle-size effect on fracture toughness of nano- and micro-spherical particle-filled composites

The effects of particle diameter and volume fraction on fracture toughness of nano- and micro-spherical particle-filled composites were investigated. The purpose was to create a mixture law of fracture toughness based on experimental results of spherical silica particle–filled epoxy composites and a theoretical approach. The fracture toughness of composites was found to be tailored independently by exchanging different particle sizes, and elastic and viscoelastic properties were found to be governed by the volume fraction of the particles. In a theoretical analysis, a mixture law of fracture toughness, composed of the elastic moduli, diameter, and volume fraction of particles and the elastic moduli of matrix resins was proposed. Its validity was demonstrated in a comparison with the experimental results.     

Investigations into the mechanical strength anisotropy of Sorbitol Instant compacts made by uniaxial compression

Powder compacts manufactured by the pharmaceutical industry are usually produced by uniaxial compression of powders or granules. This process results in compacts that are anisotropic in their mechanical strength, but this hypothesis has received little attention in the past. In this work, compacts were produced from sorbitol granules using two distinctively different particle size fractions, two compaction speeds and a range of different tablet porosities. The compact tensile strength was assessed by diametral and biaxial compression and by flexural bending. Fracture mechanics, i. e. the critical stress intensity factors in mode I and II loading, and the construction of the fracture envelope were used to investigate failure mechanisms and strength anisotropy. The strength results were also analyzed statistically employing Weibull analysis and analysis of variance. Granule size and compaction speed were identified as major influence factors on tensile strength. The magnitude of the effects found, however, varied between the individual test configurations. Further processing of the Weibull moduli obtained from the different tests confirmed the anisotropy of powder compacts made by uniaxial compression. They also showed that the commonly used diametral compression test to obtain tensile strength values is the least sensitive measure to assess the influence of particle properties on mechanical strength. Biaxial testing was found to be able to detect small changes in crack and flaw structure as a result of small changes in the particle characteristics of sorbitol.     

Microcrystalline Cellulose: Investigation of Porous Structure of Avicel 102 from Mercury Porosimeter Measurements

Abstract

To complete study on rheological and mechanical behavior of cellulose, the authors investigated the porous structure of a microcrystalline cellulose: Avicel 102. The research required the raw material itself and two size fractions of the single sample. With them, compacts were obtained. The aim of the work was to evaluate the correlation between particle size, porous structure of powders, and porous network of compacts and the tensile strength of the corresponding compacts. With the method used, an important interparticulate porosity of Avicel 102 was observed, Overall porosity increased when particle size increased on account of the particulate rearrangement. Porous networks of compacts obtained at low pressure presented a smaller overall porosity compared to powders. Higher values were observed when particle size increased, and consequently, the tensile strength of porous network of compacts decreased.     

Fracture of alumina particulate reinforced aluminium alloy matrix composites

Fracture toughness tests were performed on two aluminium alloy matrices, 2014-0 and 2024-0 reinforced with alumina particulates of different volume fractions and particulate sizes so as to investigate the fracture mechanisms operative in such composites and to determine how microstructural parameters such as volume fraction, particulate size and interparticle spacing affect the fracture toughness. The results indicate that fracture occurred by a locally ductile mechanism. The fracture toughness increased with increasing particle spacing provided that the particle size was less than a limiting value, above which unstable crack growth occurred and the toughness lowered.     

Fracture characteristics of Al/zircon particulate composites

A study has been made of the effects of volume fraction and size of zircon particulates on fracture toughness and micromechanisms of fracture in Al/zircon particulate composites. The composites are prepared by a liquid metallurgy technique using volume fractions of zircon in the range 0·06–0·18 and particulate sizes between 75 and 250 μm. The study was conducted on composites in the cast and the forged conditions. The experimental programme included a particle size distribution study, tensile tests, fracture mechanics tests leading to J_1c and crack tip opening displacement evaluation, fractographic investigations, etc. The process zone size at the crack tip was evaluated from crack tip stresses and strains, and compared with the interparticle spacing and particle diameter in order to understand the micromechanics of cracking. The Al/zircon composites were compared with Al/graphite composites in terms of strength and fracture toughness as a function of volume fraction of the filler phase, and regions of optimum performance were identified.     

Toughening of a brittle thermosetting polymer: Effects of reinforcement particle size and volume fraction

Micron- and nanometer-sized aluminum particles were used as reinforcements to enhance the fracture toughness of a highly-crosslinked, nominally brittle, thermosetting unsaturated polyester resin. Both particle size and particle volume fraction were systematically varied to investigate their effects on the fracture behavior and the fracture toughness. It was observed that, in general, the overall fracture toughness increased monotonically with the volume fraction of aluminum particles, for a given particle size, provided particle dispersion and deagglomeration was maintained. The fracture toughness of the composite was also strongly influenced by the size of the reinforcement particles. Smaller particles led to a greater increase in fracture toughness for a given particle volume fraction. Scanning electron microscopy of the fracture surfaces was employed to establish crack front trapping as the primary extrinsic toughening mechanism. Finally, the effects of particle volume fraction and size on the tensile properties of the polyester-aluminum composite were also investigated. The measured elastic modulus was in accordance with the rule-of-mixtures. Meanwhile, the tensile strength was slightly reduced upon the inclusion of aluminum particles in the polyester matrix.     

Local approach concepts and the microstructures of steels

The paper describes cleavage fracture models that relate local fracture stresses and fracture toughness values to the sizes of brittle initiating particles. In a “quasi-homogeneous” steel, i.e. one possessing a smooth distribution of small particle sizes, a “typically coarse” particle is present in every sample tested and the failure stress or fracture toughness is single valued. When random experimental errors are included, the single valued function becomes a Gaussian distribution. Spatially heterogeneous microstructures produce quite different distributions and these are discussed with respect to extrapolations to low failure probabilities.     

Grain size and porosity dependence of ceramic fracture energy and toughness at 22 °C

A review of the fracture energy and toughness data for dense ceramics at 22 °C shows maxima commonly occurring as a function of grain size. Such maxima are most pronounced for non-cubic materials, where they are often associated with microcracking and R-curve effects, especially in oxides, but often also occur at too fine a grain size for association with microcracking. The maxima are usually much more limited, but frequently definitive, for cubic materials. In a few cases only a decrease with increasing grain size at larger grain size, or no dependence on grain size is found, but the extent to which these reflect lack of sufficient data is uncertain. In porous ceramics fracture toughness and especially fracture energy commonly show less porosity dependence than strength and Young's modulus. In some cases little, or no, decrease, or possibly a temporary increase in fracture energy or toughness are seen with increasing porosity at low or intermediate levels of porosity in contrast to continuous decreases for strength and Young's modulus. It is suggested that such (widely neglected) variations reflect bridging in porous bodies. The above maxima as a function of grain size and reduced decreases with increased porosity are less pronounced for fracture toughness as opposed to fracture energy, since the former reflects effects of the latter and Young's modulus, which usually has no dependence on grain size, but substantial dependence on porosity. In general, tests with cracks closer to the natural flaw size give results more consistent with strength behaviour. Implications of these findings are discussed.     

Effect of inclusions on size of surface flaws in glass-crystal composites

Indentation fracture studies were conducted on three sodium borosilicate glasses containing a dispersed phase of alumina inclusions with different degrees of thermal expansion mismatch between the glass matrices and the alumina. The alumina inclusions were found to cause a significant decrease in the size of the indentation cracks compared to those in the glass. This effect was greatest at the higher values of indentation load, which resulted in cracks of dimensions of sufficient size that their propagation was impeded by the tougher alumina dispersions. The fracture toughness for the composite samples calculated from the indentation data showed a significant increase with increasing crack size. For the smallest cracks in these composites, the value for fracture toughness was well below the value obtained in an earlier study by the single-edge notch-beam technique. The fracture toughness for the larger crack sizes which interacted with the alumina dispersions showed excellent agreement with the notch-beam data. The residual stresses due to the thermal expansion mismatch appeared to lead to a slight increase in the mean crack size regardless of the direction of thermal expansion mismatch.     

The influence of temperature on the mechanical and fracture properties of a 20 vol% ceramic particulate-reinforced aluminium matrix composite

The influence of test temperature on the mechanical and fracture properties of a 20 vol% alumina particulate-reinforced 6061-aluminium matrix composite, in the peak-aged condition, was investigated in the temperature range 25–180 °C. Strength and stiffness were found to decrease but elongation to failure increased with increasing test temperature. However, the fracture toughness was relatively constant over this temperature range. The failure mechanism, the reaction zone around reinforcing particles, the number of debonded particles and void sizes were all significantly influenced by temperature. The role of the matrix/particle interface in the fracture process was also investigated.On leave at the Department of Mechanical Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong.     

Determination of Remnant Residual Stresses in Fracture Toughness Specimens Extracted From Large Components

Abstract: Material fracture toughness data are required to undertake fitness‐for‐service assessments of engineering components containing cracks. Calculations of crack driving force in the component are compared with material fracture toughness values to assess the likelihood of subsequent failure. Experimental measurements of fracture toughness are usually made on small specimens extracted from a larger ‘parent’ component following strict experimental guidelines, formulated to ensure measured toughness values in the fracture specimens are appropriate for use in the full‐size component. Implicit in this procedure is the assumption that the extracted fracture specimens contain no residual stresses, with any residual stresses in the full‐size component being accounted for in the crack driving force calculation. This paper considers a recent conjecture within the structural integrity community that the extracted fracture specimens may themselves contain a residual stress field which may influence measurements of fracture toughness. This could potentially lead to a degree of ‘double accounting’, i.e. the effect of residual stresses may be included in both the material toughness and the crack driving force. This, in turn, could lead to unnecessary conservatism in safety assessments. To explore this conjecture, the results of numerical modelling and neutron diffraction measurements of residual stresses in fracture specimens extracted from two different welded parent components are presented. One of the components is significantly larger than the extracted specimens, with the other being marginally larger than the extracted specimens. Results confirm the intuitive expectation that the residual stresses in specimens extracted from much larger components are negligible, whereas if the dimensions of the extracted specimens are comparable with the larger component then significant residual stresses may remain.     

超音速火焰喷涂WC-17Co涂层微观结构与性能研究

采用超音速火焰喷涂技术喷涂了两种不同WC颗粒尺寸的WC-17Co粉末.对制备的两种涂层的硬度、孔隙率、断裂韧性、结合强度和电化学腐蚀行为进行了测试.结果表明:具有亚微米结构WC颗粒的粉末制备的涂层,在硬度、孔隙率、断裂韧性方面具有一定优势,而含有大颗粒WC相的粉末制备的涂层在结合强度、腐蚀行为方面优势明显,这说明WC颗...     

Thermal Conductivity of Cobalt-Based Catalyst for Fischer–Tropsch Synthesis

In this study, the thermal conductivity of cobalt-based catalyst specimens in the temperature range from 160 ^◦C to 255 ^◦C are measured via a steady-state apparatus. The apparatus and procedures are applied to several specimens of cobalt-based catalyst powder compacts. Specimens with different degrees of porosity are produced by pressing cobalt-based catalyst powder with a particle size of (80 to 360) mesh. The thermal conductivity of cobalt-based catalyst powder compacts is investigated as functions of temperature, specimen density, porosity, and powder size. The results indicate that the thermal conductivity of the catalyst specimens increases linearly with temperature and density and is practically independent of the particle size of the powder in an atmosphere of air, while the porosity dependence of the thermal conductivity is inverse to that of density. In addition, the effects of some measuring factors on the thermal conductivity show that the reliability of the thermal conductivity measurements of cobalt-based catalyst specimens are influenced easily by parallelism, specimen roughness, and moisture content, whereas the specimen thickness and water bath temperature have only a slight effect on the reliability.     

Foaming behavior of Ti6Al4V particle-added aluminum powder compacts

The foaming behavior of 5 wt.% Ti6Al4V (Ti64) particle (30–200 μm)-added Al powder compacts was investigated in order to assess the particle-addition effects on the foaming behavior. Al compacts without particle addition were also prepared with the same method and foamed. The expansions of Ti64 particle-added compacts were measured to be relatively low at small particle sizes and increased with increasing particle size. At highest particle size range (160–200 μm), particle-added compacts showed expansion behavior similar to that of Al compacts without particle addition, but with lower expansion values. Expansions studies on 30–45 μm size Ti64-added compacts with varying weight percentages showed that the expansion behavior of the compacts became very similar to that of Al compact when the particle content was lower than 2 wt.%. However, Ti64 addition reduced the extent of drainage. Ti64 particles and TiAl₃ particles formed during foaming increased the apparent viscosity of the liquid foam and hence reduced the flow of liquid metal from cell walls to plateau borders. The reduced foamability in the compacts with the smaller size Ti64 addition was attributed to the relatively high viscosities, due to the higher cumulative surface area of the particles and higher rate of TiAl₃ formation between liquid Al and Ti64 particles.     

Influence of SiC particles on mechanical properties of Mg based composite

AZ91 magnesium alloy reinforced with different sizes of SiC particulates has been fabricated using powder metallurgy route. Mechanical properties of the specimens have been studied. Yield and ultimate tensile stresses show a decrease with the increase in the size of SiC particulates. The influence of thermal shock between 400°C and 30°C on the mechanical properties was also investigated. The results show a decrease in yield stress and elongation to fracture with the number of thermal shock cycles.     

Effects of sizes of ferrite grains and carbide particles on toughness of notched and precracked specimens of low-alloy steels

Four point bending (4PB) tests of notched specimens and COD tests of precracked specimens were carried out on two steels; one steel was treated into two groups with the same ferrite grain size but different carbide sizes, the other steel with different ferrite grain sizes but similar carbide sizes. The results of the tests show that the toughness measured in notched specimens is mainly determined by the grain sizes, which define the local fracture stress _f; the size of carbide particle plays a minor role. However, on the contrary, in precracked specimens the toughness is sensitive to the carbide sizes, which affect the critical plastic strain _pc for initiating a crack nucleus; the effect of grain size is indistinct. By these inferences the behavioral discrepancy of large grain steel in improvement of crack fracture toughness while reducing the notch toughness is explained.     

TiC-TiB2/Cu复合材料的自蔓延高温合成研究

采用SHS/PHIP工艺制备了TiC-TiB₂/Cu复合材料,通过实验研究了该系列复合材料的微观结构特征和力学性能。结果表明,TiC-TiB₂/Cu复合材料中只有TiC、TiB₂和Cu相存在;随着Cu含量的增加,燃烧温度下降,材料的颗粒尺寸变小;TiC-TiB₂/Cu复合材料的相对密度、抗弯强度和断裂韧性均随Cu含量的增加呈先增后减趋势,当Cu含量为20％时强度最高为580MPa,Cu含量为40％时韧性最高为8.1MPa·m^1/2。     

Development of the Euro fracture toughness dataset

Ten European laboratories have generated the Euro fracture toughness dataset in order to provide an experimental data base sufficiently large to study specimen size and temperature effects on cleavage fracture toughness in the ductile-to-brittle fracture transition regime. The Euro fracture toughness dataset quantifies the fracture behaviour of the quenched and tempered pressure vessel steel DIN 22NiMoCr37. This material is frequently used in nuclear power plants. About 800 fracture toughness tests were performed using compact tension specimens with a size range from 1/2T to 4T.In the lower shelf temperature regime, no significant specimen size effects on cleavage fracture toughness scatter was observed. At higher temperatures, the lower tails of the toughness scatter bands are not significantly effected by the specimen size but with decreasing specimen size the toughness scatter increases due to the fact that the upper part of the scatter band is extended. The presence of a specimen size effect on fracture toughness scatter coincides with the appearance of single cleavage initiation sites at the fracture surface. At the lower shelf temperature both, cleavage initiation sites and size effects are not observed whereas at higher test temperatures both phenomena are present. The specimen size effect trends and the corresponding fracture surface morphology support a weakest-link type cleavage fracture mechanism in the ductile-to-brittle transition regime. A unique correlation between the amount of ductile tearing and cleavage fracture toughness was observed for the steel investigated. This result offers the possibility to determine cleavage fracture toughness from post-test fracture surface examinations.Due to the large number of tests and the wide range of testing conditions, the Euro fracture toughness dataset gives a comprehensive insight into specimen size effects and temperature effects on ductile-to-brittle transition fracture. The Euro fracture toughness dataset includes a large set of raw test data such as load versus load line displacement curves and raw tensile test data for deriving stress-strain curves. The Dataset can be downloaded from the internet via the address ftp://ftp.gkss.de/pub/eurodataset.     

Densification and microstructure development during the sintering of submicrometre magnesium oxide particles prepared by a vapour-phase oxidation process

The densification behaviour and microstructure development of MgO compacts fired from room temperature up to 1700°C at a heating rate of 10°C min^–1 were examined. Starting materials were seven kinds of MgO powder with primary particle sizes ranging from 11–261 nm; these powders were produced by a vapour-phase oxidation process. The original powders contained agglomerates, due to the spontaneous coagulation of primary particles, which ranged in size from 100–500 nm. The MgO compacts densified during firing by three types of sintering: sintering within agglomerates; sintering between agglomerates and grains; and rearrangement of agglomerates and grains. The MgO compact with the lowest primary particle size (11 nm) densified by the first and second types of sintering, but the effects of these two types of sintering decreased when the primary particle size became 44 nm; here the rearrangement of agglomerates and grains primarily contributed to densification of the compact. All three types of densification became less complete with further increases in primary particle size up to 261 nm. The relative densities of the MgO compacts with smaller primary particle sizes (11–44 nm) became 96–98% when the compacts were fired up to 1700°C.