Advanced Materials Research Vols. 41-42

Abstract: Fracture under compression is one of the most commonly studied properties of geomaterials like concrete and rock, in particular since these materials reach their best performance in compression. The fracture process is however rather complex due to the heterogeneous structures of said materials. Over the years fundemental studies of fracture under compression have led to a much improved insight in the details of the fracture process depending on the actual composition of the material. Fracture can be described by means of a 4-stage fracture model, which included as most important aspects pre-peak cracking, which is stable and can be arrested by stiffer and stronger elements in the material structure, and post-peak cracking [1]. The latter macroscopic cracks are basically un-stable and can only be arrested by measures at a structural scale, such as applying confining stress or by using positive geometries. The material structure cannot assist in the arrest of the large energetic cracks other than locally affecting the crack path. In the paper an overview is given of the fracture process in compression. Recently we embarked on studying compressive fracture using a simpler material structure, namely foamed hardened cement paste [2]. Stiff aggregates that are normally included in normal concrete have been left-out; instead a larger than usual quantity of large pores is brought into the material, even up to 80%. Studying fracture processes in this simpler material system ultimately allows for a better understanding of the details of the pre-peak cracking process, which is considered more important than the post-peak process since it defines strength.

Abstract: Ceramic radiator fins were produced by screen-printing copper paste on ceramic substrate, which could replace the traditional technique of direct bestrow copper and meet the requirements of surface mounted technology. This method could be used to manufacture high density, superior thin and micro-sized molectrons. The key processes were screen-printing copper paste, sintering and electroless plating of nickel. The adhesion of copper film onto the ceramic substrate was often reduced after the process of electroless plating of nickel, resulting in the low quality of manufacturing. In this study, we analyzed the ceramic radiator fins which were obtained by the screen-printing copper paste method and using scanning electron microscopy to examine the surface and the cross-sections of copper film and Cu/Ni film. The adhesive properties of copper film during electroless plating was studied. The corrosion resistance of copper film and sintering glass phase on alumina substrate (96%) was also studied in an electroplating bath. The study revealed that the glass phase of acidity of silicon, softening temperature, the interaction conjunction between glass phase and ceramics were important factors.

Abstract: This study investigates the effect of specimen volume on the compressive strength of open cell brittle ceramics. A series of unconfined compression tests were carried out on specimens ranging in volume. The crushing strength and apparent stiffness were measured and these results are correlated with the volume of material stressed. It was found that as the volume of material tested decreased the strength decreased.

Abstract: In order to investigate the size effect of concrete, four sizes of rectangular prisms were tested in uniaxial tension. The cross sections of the prisms were 50x100mm, 100x100mm, 200x100mm and 400x100mm. The concrete was an ordinary one with the compressive strength of 34 MPa and the maximum aggregate size of 20mm. Notches were applied on four side faces and secondary flexure was completely eliminated during the test in order to obtain the exact nominal tensile strength. The size effect was analyzed by four factors, namely tensile strength, fracture energy, critical crack opening displacement and tension softening curves. Clear size dependence was observed in critical crack opening displacement and initial convexity of tension softening curves, and a slight size dependence was observed in tensile strength. On the other hand, size effect was unclear in fracture energy and other part of tension softening curves because of their scatters.

Abstract: In general, cracks in structural members indicate a level of serviceability of the engineering structures. Usually a hairline crack can be detected by visual inspection, crack gauge, fibre optical sensor, or laser sensor. Recently, an attempt to develop a non-destructive tool for health monitoring of prestressed concrete sleepers in railway tracks using the vibration responses has been established at the University of Wollongong, Australia. However, the fundamental understanding of the dynamic effect due to the cracks in sleepers is insufficient. This paper investigates the dynamic effect of the cracks on the vibration signatures of the railway prestressed concrete sleepers. The modal analysis has been used to evaluate the modal changes in vibration characteristics of the prestressed concrete sleepers in the frequency band between 0 and 1,600 Hz. The sample cracks have been initiated by the impulsive forces attributed to the high capacity drop impact testing machine. Comparison of the modal parameters of the intact and cracked sleepers has been highlighted in terms of natural frequencies and modal damping. This understanding will lead to further development of the economical technology to evaluate the structural integrity of railway track and its components.

Abstract: This paper presents a study on the densification and mechanical properties of Al2O3/Ti(C,N) ceramics processed using a pressureless sintering technique. The Al2O3/Ti(C,N) ceramics containing 10, 20, 30 and 40 wt% Ti(C,N) were sintered in the temperatures ranging from 1500 to 1750 °C and in the atmospheres of vacuum, Ar, H2 and N2. It is found that both optimum sintering temperature and Ti(C,N) content exist, where the best densification and mechanical properties are achieved. The Al2O3/Ti(C,N) properties will decrease when either sintering temperature or Ti(C,N) content moves away from their optimum value. The experimental results also demonstrate that among the four atmospheres, Ar gives best results. To improve further the properties of Al2O3/Ti(C,N) composites, Al2O3 and Al2O3/Ti(C,N) powders have used to cover the specimens during sintering, and experiments revealed that covering with Al2O3/Ti(C,N) powder can significantly improve the properties of Al2O3/Ti(C,N) ceramics. Furthermore, the effects of Ti(C,N) content and sintering conditions on densification and mechanical properties are explained in terms of their influences on Al2O3/Ti(C,N) microstructures.

Abstract: A systematic survey of the available data such as elastic constants, density, molar mass, and glass transition temperature of 45 metallic glasses is conducted. It is found that a critical strain controlling the onset of plastic deformation is material-independent. However, the correlation between elastic constants of solid glass and vitrification characteristics of its liquid does not follow a simple linear relation, and a characteristic volume, viz. molar volume, maybe relating to the characteristic size of a shear transformation zone (STZ), should be involved.

Abstract: The effect of purity of amorphous boron raw materials on properties of the hot pressed SiC doped MgB2 superconductor was investigated. MgB2 superconductors with magnetic Jc over 106 A/cm2 and remaining Jc of 105 A/cm2 at 4.2 K and 5 T were fabricated by hot pressing using both high purity (99.00%) and low purity (88.84%) boron powders. XRD analysis shows that purity of the boron powders has little effect on phase component of the MgB2 samples. If the main impurity in amorphous boron is Mg, low purity low cost boron powder is suitable as one of the raw materials for fabricating MgB2. Particle sizes of boron has significant effect on microstructure and properties of MgB2. Smaller boron particle size leads to smaller grain size of MgB2, higher density, higher lattice distortion, and thus higher magnetic Jc.

Abstract: Cracks in railway prestressed concrete sleepers are often caused by the infrequent but high magnitude impact forces. These impact forces are attributed to the wheel or rail abnormalities, e.g. wheel flat, corrugation, dipped joint, etc. The emphasis of this paper is placed on the corresponding fractures of railway prestressed concrete sleepers in track systems under the probabilistic impact loadings. The statistical data of impact loadings in Central Queensland have been obtained through the collaborative project under the research framework of the Australian Cooperative Research Centre for Railway Engineering and Technologies. It is found that for the heavy haul railway network, the track structures, especially the concrete sleepers, may experience the impact loading from 400kN up to 800kN. The corresponding probabilities of such load occurrences are ranging from one in twenty years to one in thousands of years. It is very important for the design and maintenance perspectives to investigate the impact behaviour of concrete sleepers under the impact loadings. This paper presents the impact fractures of railway prestressed concrete sleepers under the single impact loading associated with the probability of occurrence in the soft track environment. Visual interfacial cracks have been evaluated for the crack length and width along the sleepers in relation to the probability of load occurrence.

Abstract: Open-cell Al O ceramic foams with uniform cell structures and dense cell struts were fabricated by centrifugal slip casting using the plant seeds as the templates. The rheological characteristic of Al O slurries with up to 50 vol.% solid content was investigated. The shrinkage matching between the Al O green compact and the plant seeds during drying was studied. The effect of solid contents of slurries on change of green density of cell struts along the height of compacts was analyzed. The drying and sintering schedule of Al O green cakes were studied. The results indicated mass segregation of Al O particles with different sizes during centrifugal process was negligible for slurries with high solid loadings with 50 vol. %. The cell struts of green bodies had high density of 63.8% TD. After sintered at 1500 °C for 2 h, the final products had dense cell struts with high sintered density of 98.9%TD and homogeneous microstructure. The porosity and compressive strength of sintered products was 66.5% and 5.26 MPa, respectively.