Papers by Keyword: Void

Abstract: For the B-doped silicon crystals grown with and without N-doping, we measured the temperature dependence of the elastic constant in low-temperature region, to examine whether the N-doping annihilates the elastic softening caused by the gap-states of the isolated single vacancy. We have found that the elastic softening clearly observed for the N-free crystals is not observed for the N-doped ones, suggesting that the gap-states of the vacancies causing the elastic softening are destroyed by the N-doping. This is consistent with the model of Abe [T. Abe, J. Crystal Growth, 327 (2011) 1] in which the nitrogen molecule (N-N pair) occupies the vacancy to destroy its original gap-states. We have further observed that the N-doped silicon, which exhibits no softening in its as-grown state, exhibits the softening after the short-time annealing. This suggests that during the annealing the N-N pair is thermally activated to jump off the lattice site leaving the vacancy.

Abstract: This paper makes a study on the stress state and internal force algorithms of wall beams under the action of vertical load. It also makes a comparative analysis to several typical wall-beam designs at present. Then, by using the finite element, it verifies the accuracy and reliability of internal force algorithm formulas of wall beams in current Code for Design of Masonry Structure, hoping to provide a solid theoretical reference for the selection of wall beam algorithm designs.

Abstract: Void formation in 4H-SiC crystals grown from solution has been investigated by secondary ion mass spectrometry and Raman scattering. It becomes clear that ambient Ar gas is filled in voids and the solvent (Si) partially remains. The result indicates that Ar dissolved in the solvent vaporizes and forms bubbles. The trapped bubbles at the crystal growth front are considered to be incorporated in the growing crystal as voids. We also have developed following methods for suppression of the void formation; (1) dipping seed crystals so that the growth front faces upward, (2) growth under He atmosphere, and (3) the high temperature treatment of the solvent before crystal growth.

Abstract: In all heteroepitaxial systems the interface between substrate and layer is a crucial point. In this work SEM and TEM studies on the interface between silicon substrate and cubic silicon carbide (3C-SiC) layers obtained by chemical vapor deposition (CVD) are presented. A clear connection between process parameters, like the design of substrate cleaning, and the heating ramp, and resulting defect structures at the substrate-layer interface could be found. Whereas the process step of etching in hot hydrogen for oxide removal is crucial for avoiding the generation of closed voids of type 2, the design of the temperature ramp up to growth temperature during carbonization influences the interface roughness. Here a fast ramp helps to obtain a flat interface.

Abstract: Ni/Si multilayer contact structures to 4H-SiC after subsequent annealing steps are investigated with electron microscopy methods. After high temperature annealing step, specific defects in the contact structures are observed. The influence of phase transformations during annealings on the morphology on the contacts is discussed and the explanation of formation mechanism of voids within contact layer is proposed.

Abstract: In this work, crack initiation due to the pre-existence of an initial crack has been predicted according to the criterion of critical stress intensity factor and succeeding crack evolution and propagation are also been performed using molecular dynamic (MD) method in combination with finite element method (FEM). The modified embedded atom method potentials were employed to characterize the interaction among atoms in magnesium alloy in MD simulation. Finite element simulations have been first conducted to provide subsequent MD simulation with boundary conditions constrained at the atoms. The MD simulation shows that atoms around crack arrange disorderly, aggravate rapidly suggesting the onset of crack initiation and eventually results in the failure of alloy specimen. It helps to evaluate the value of critical stress intensity factor for a specific crack configuration, which provides an effective way to determine the stress intensity factor for the specified configuration.

Abstract: AHSS (Advanced High Strength Steels) combine high strength and good ductility. Their outstanding forming and work-hardening behavior predestines these steels for fabrication of strength relevant structural elements and automobile body parts. To characterize a material, not only tensile, but also hole-expansion and bending behavior are important and help predict the stretch-flange-formability. In this study, detailed analyses of the correlation between these three tests and the damage mechanisms during forming have been performed for selected steels. The results show that for AHSS one should differentiate between “local” and “global” failure. Furthermore, not only are certain materials more sensitive to local or global damage, but also various testing methods tend to provoke either local or global damage. Tensile testing provokes global failure whereas hole-expansion tends to induce local failure. A specimen fails during bending with a mixture of local and global modes. These failure modes are strongly attributed to the microstructure. DP-steels yield high elongation during tensile testing and poorer hole-expansion values. High-resolution EBSD has revealed that the microstructure of DP-steels is sensitive to localized damage, which is compensated by work-hardening around damaged regions and thus shifts the loading to un-hardened regions. This makes DP-microstructures well-suited to tensile loading but sensitive to hole-expansion. CP-steels of comparable strength show poorer tensile elongation and higher hole-expansion ratios due to a microstructure which is not sensitive to localized failure (but has limited capacity for work-hardening). The failure mode in TRIP-steels exhibits a similar character as in DP-steels, but only after the martensitic transformation of retained austenite.

Abstract: The size of voids beneath rigid pavement slabs is an important factor which influences the bearing capacity of roads. GPR is an advanced method for detecting voids beneath rigid pavement slabs. Combined with drilling and sampling, equations including parameters of GPR wave amplitude, horizontal size and vertical size of voids are established for analyzing the size of voids. Then, based on the equations, the horizontal size and vertical size of voids can be calculated according to the actual GPR wave amplitude.

Abstract: In recent years, high strength steel plates for building and pipelines have been required to improve ductile fracture properties, assuming ground deformation in earthquake-prone region. The ductile fracture is performed by the result from coalescence of micro-voids followed by the nucleation and growth [1]. Fractured surface morphology reflects the void coalescence process, so it is important to consider the relationship between the fracture surface morphology and the micro-voids formation beneath the fractured surface to consider the ductile fracture properties. The voids nucleate sites are mainly particles such as inclusions or precipitates, and grain boundries. These voids grow and coalesce according to three modes. The first mode is directly coalescence of voids followed by growth [2]. The second one is the coalescence of voids caused by shear deformation followed by internal necking between voids [3]. The third one is the coalescence of voids caused by micro-voids nucleation in shear band between two larger voids [4]. It is expected that these modes influence local elongation property which is one of the indices for ductile fracture property through the formation of fractured surface. In this study, local deformation energy which is measured by load-displacement curve in tensile test is examined by focusing the voids nucleation, growth and coalescence, for high tensile strength plates of TS480-830MPa which is controlled by the microstructure through the cooling rate of heat treatment. The deformation energy is useful to consider the ductile fracture property of steel plates which have a different tensile strength.

Abstract: Using finite element software, the analysis model of semi-rigid base pavement with voids is established. Variation of deflection under load of Falling Weight Deflectometer (FWD) is studied and the reference critical value is obtained to determine whether there are the voids. According to analysis of the deflection basin data under different location, the identification method for the void location of semi-rigid base pavement is established. The field test shows that it is feasible to use this technology in void identification for semi-rigid base pavement.