Papers by Keyword: Anisotropy

Abstract: This paper presents a new approach for analyzing the microstructure of SiCp-reinforced aluminum matrix composites from digital images. Various samples of aluminum matrix composite were fabricated by hot pressing the powder mixtures with certain volume and size combinations of pure Al and SiC particles. Microstructures of the samples were analyzed by computer-based image processing methods. Since the conventional methods are not suitable for separating phases of such complex microstructures, some new algorithms have been developed for the improved recognition of the particles in the metal matrix composites. One of the most determining attribute of composites structure is anisotropy, but the measurement of this parameter is very difficult and slow. For this reason, the anisotropy of metal matrix composites was investigated by the newly developed algorithm which has an optimized speed.

Abstract: This research is aimed at determining the thermal expansion coefficient, α(T), along three directions, X, Y, and Z, of a graphite sample pressed along the direction of weak interactions (Z), over the temperature range [25-500°C]. The experiment results showed that pressing completely alters the dilatometric behavior of the material, and the shape of the α(T) curve changes significantly from one direction to another. Comparing αX, αY, and αZ, a strong anisotropy in the thermal expansion coefficient is observed. This is in good agreement with results published by other researchers who have shown that the electrical conductivity and thermal conductibility of a similar material exhibits strong anisotropic behavior. Pressing has also led to anisotropy in the basal plane, i.e. between αX and αY which have opposite signs. [Pressing results in a significant decrease in α(T) along the Y axis, which is considerably lower than that reported in the literature. αZ is clearly higher that of non-pressed graphite. The wide divergence among the three coefficients is believed to be attributed to the intensification of the membrane effect.

Abstract: In this paper, mechanical characteristics of KDP crystal anisotropy are analysed theoretically. Vickers indentation experiments are adopted to validate the variation rule of hardness and fracture toughness in different orientation of KDP crystal plane (100), and a model to calculate critical cutting thickness of brittle-ductile transition is proposed for the KDP crystals. The result shows that, on the crystal plane (100), the minimum value of critical cutting thickness of KDP crystal in brittle-ductile transition appears in the direction [110], but the maximum appears in the direction [010]. Finally, the ultra-precision machining of KDP crystal is performed, and the results agree well with the theoretical conclusions. Super-smooth surface with a roughness RMS of 6.6nm is reached as machined in the crystal direction [010], and 11.2nm to the direction [110].

Abstract: This paper presents a multi-spectrum analysis method for the characterization of the surface generation in single-point turning of brittle single crystals. The features on the diamond turned surfaces were extracted and analysed by the power spectrum analysis of the surface roughness profiles measured at a number radial sections of the workpiece. By the analysis of the variation of the spectral patterns in the multi-spectrum plots, the surface roughness and materials effect on surface generation are found to be strongly related to the power spectrum. This provides an important means to explain quantitatively the effect of factors affecting the surface generation in diamond turning brittle crystals.

Abstract: A 3D anisotropic elastoplastic-damage model was presented based on continuum damage mechanics theory. In this model, the tensor decomposition technique is employed. Combined with the plastic yield rule and damage evolution, the stress tensor in incremental format is obtained. The derivate eigenmodes in the proposed model are assumed to be related with the uniaxial behavior of the rock material. Each eigenmode has a corresponding damage variable due to the fact that damage is a function of the magnitude of the eigenstrain. Within an eigenmodes, different damage evolution can be used for tensile and compressive loadings. This model was also developed into finite element code in explicit format, and the code was integrated into the well-known computational environment ABAQUS using the ABAQUS/Explicit Solver. Numerical simulation of an uniaxial compressive test for a rock sample is used to examine the performance of the proposed model, and the progressive failure process of the rock sample is unveiled.

Abstract: Rock is a heterogeneous and anisotropic compound material, containing many shear surfaces, cracks, weak surfaces and faults. Damage and failure in a rock mass can occur through sliding along persistent discontinuities, or fractures. A new micromechanical approach to modeling the mechanical behavior of excavation damaged or disturbed zone (EDZ) of anisotropic rock is presented based on knowledge of the inhomogeneity of rock. In this numerical model, damage is analyzed as a direct consequence of microcracks growth. A study of the effect of elastic and failure anisotropy plus inhomogeneity on the underground excavations reveals that the modes of failure can be significantly influenced by the rock structure on the small and large scales. Fractures that develop progressively around underground excavations can be simulated using a numerical code called RFPA (Realistic Failure Process Analysis). This code incorporates the microscopic inhomogeneity in Young’s modulus and strength characteristic of rock. In the numerical models of a rock mass, values of Young’s modulus and rock strength are realized according to a Weibull distribution in which the distribution parameters represent the level of inhomogeneity of the medium. Another notable feature of this code is that no a priori assumptions need to be made about where and how fracture and failure will occur – cracking can occur spontaneously and can exhibit a variety of mechanisms when certain local stress conditions are met. These unique features have made RFPA capable of simulating the whole fracturing process of initiation, propagation and coalescence of fractures around excavations under a variety of loading conditions. The results of the simulations show that the code can be used not only to produce fracturing patterns similar to those reported in previous studies, but also to predict fracturing patterns under a variety of loading conditions. The numerical model was able to reproduce the associated complex stress patterns and the microseismic emission distribution for a variety of rock structural conditions.

Abstract: The phonon anisotropy property of the GaN wurtzite crystal is studied using angular dependent Raman spectroscopy both theoretically and experimentally. The polarized Raman scattering spectra were recorded from cross-sections of c-axis oriented GaN films as a function of the angle between the incident laser polarization direction and the film normal direction in three different configurations. The Raman intensity of A1(TO) showed a sinusoidal dependence on the rotating angle, as also did the E1(TO) mode, while the E2 mode has a quite different behavior. The theoretical fit takes into account the susceptibility contribution and the phase differential of different vibrating elements.

Abstract: Polymer materials are used in the structural members such as aircraft, ship, automobiles in order to enlarge and lighten the machinery and structures. In such the machinery and structures, the complex destruction phenomena are generating by the deterioration of use conditions of the polymer materials. And the materials are placed under biaxial or multi-axial stress conditions rather than uni-axial stress conditions due to their complicated structures. To understand the basic deformation behaviours and the features in destruction process under multi-axial stress conditions is more important in the reliability and the accident prevention plan. To help overcome this problem, the authors have developed a hydraulic high-speed biaxial loading test device. In this study, we had comparative study for the values of the dimensionless stress intensity factors, F values by the photoelastic and caustics methods to clarify deformation behaviours and characteristics of polymer materials under biaxial stress conditions.

Abstract: An effect of anisotropy on the thermoelectric properties of Bi1Sb3Te6 added with Au alloys prepared by a mechanical alloying process has been studied. The conduction properties including electrical conductivity and thermal conductivity were increased with Au content. The electrical conductivity and the power factor of the perpendicular direction to the pressing direction were larger than those of the parallel direction to the pressing direction. The intensity of (1 1 0) perpendicular plane was larger than that of the parallel plane. It was suggested that the increase of intensity of the (1 1 0) plane would contribute to improve the thermoelectric performance. Although the power factor and thermal conductivity revealed the anisotropic behavior with direction, the Z value showed almost the equal value regardless of direction. From these results, it appeared that the Z value of the Bi1Sb3Te6 added with Au alloy prepared by powder metallurgy process was almost isotropic.

Abstract: We have investigated the exchange bias in CVD grown epitaxial CrO2/Cr2O3 bilayer thin films using hysteresis loops and resonant RF transverse susceptibility. M-H loops indicated an enhanced coercivity without appreciable loop shift and the transverse susceptibility in CrO2/Cr2O3 bilayers revealed features associated with both the ferromagnetic and antiferromagnetic phases. In addition, TS yielded large anisotropy constant (Keff) values depending on the fraction of Cr2O3 present. The large anisotropy fields observed cannot be accounted for by the variable thickness of CrO2 alone and are indicative of possible exchange coupling between CrO2 and Cr2O3 phases that significantly affects the effective magnetic anisotropy.