Influence Of Structural Imperfections Research Articles

Influence of Defects-Induced Stresses on Birefringence in SrTiO3 Single Crystals

Significant applications of SrTiO3 single crystals in electronics require knowledge about the influence of structural imperfections on their optical properties. Birefringence temperature changes were investigated in a few SrTiO3 single crystals in a broad temperature range, from 85 K to 250 K. The birefringence was found to be a non-linear function below the transition Ts at 105 K, and non-linear changes in the optical indicatrix orientation accompanied it. A weak residual birefringence was permanently present a dozen degrees above the phase transition temperature Ts. This is mainly connected with dislocations, which induce local stresses and shift transition points even up to about 200 K. The essential role of imperfections on optical properties was studied in a SrTiO3 24° bi-crystal reduced at 1000 K and under low oxygen pressure. In such an intentionally defected crystal, an increase of non-linearities in Δn(T) dependence was observed below and above the transition point Ts.

Influence of imperfections in the buckling resistance of steel beam-columns under fire

AbstractThis paper presents an investigation on the influence of structural imperfections on the ultimate load capacity of steel welded beam-columns with class 4 cross-section under elevated temperatures. This is done by considering different amplitudes for the global and local (plate) imperfections, and different residual stresses distributions available in the literature. To this purpose, a geometrically and materially non-linear finite element model using Abaqus software has been used to determine the buckling resistance of a steel welded beam-column at elevated temperatures, using the material properties of EN1993-1-2. The imperfection sensitivity of beam-columns is reported: the influences of the amplitudes of the geometric imperfection and the patterns of the residual stress on the load capacity are compared.

The influence of structural imperfections on the LTB strength of I-beams

The structural imperfections, namely, the residual stresses, are generated during most manufacturing processes involving material deformation, heat treatment, machining or processing operations. The intensity and distribution of residual stresses are dependent on the techniques used in the production of steel sections (welding, rolling or cold formed techniques) and their dimensions. An appropriate evaluation of residual stress is of great importance for the structural performance of steel I-beams. Residual stresses can have a significant impact on the stability of structural members. In the case of I-section beam elements, such stresses can impact lateral torsional buckling (LTB) strength, particularly in the inelastic range. For the development of post buckling numerical analyses, the consideration of residual stress distribution models is fundamental for the correct determination of the LTB strength of I-beams. However, there are several models that attempt to represent the distribution and intensity of residual stresses. These models have a different influence on the LTB behavior of I-beams in numerical analysis. Therefore, this paper aims to investigate, through the development of post buckling numerical analyses with the ABAQUS software, the influence of the different residual stress distribution models on the LTB strength of I-beams. Seven different residual stress models were analyzed on simply supported I-beams subject to uniformly distributed loading applied in the shear center and in the upper flange. The results were compared with standard procedures. It was verified considerable divergences in LTB strength depending on the residual stress model applied. In addition, this paper presents a synthesis of the main residual stress distribution models. The analyses showed in this work shall help the choice of the residual stress distribution model on future numerical simulations.

Influence of structural imperfections—twisting and distortion—on the vibrational behavior of carbon nanotubes

Several types of carbon nanotubes in their perfect and imperfect form were simulated, and their vibrational behavior was studied by performing computational examinations with fixed-free boundary conditions. Both computational and analytical results were compared in the case of perfect tubes. Afterward, three kinds of imperfections, that is, twisting angle, z-distortion along the longitudinal axis and xy-distortion along the radial axis, were introduced to the structure of perfect carbon nanotubes, and the natural frequencies of imperfect carbon nanotubes were numerically evaluated and compared with the behavior of the perfect ones. It was concluded that the existence of any type of imperfection in the structure of carbon nanotubes leads to a lower natural frequency and, as a result, lower vibrational stability. However, this trend was more visible for the carbon nanotubes with higher chirality.

Influence of Structural Imperfections and Doping on the Mechanical Properties of Single-Walled Carbon Nanotubes

In This study, finite element models of Single-Walled Carbon Nanotube (SWCNT) in their perfect form were generated. Then, after deriving mechanical properties in agreement with existing literature values, different levels of perturbation and missing atoms in the structure were applied. In another approach, some of the Carbon atoms were replaced with Silicon to produce a Si-doped SWCNT. Mechanical properties of the three kinds of imperfect structures of SWCNT were then compared with the perfect structure results, for both fundamental Carbon Nanotubes, namely Zigzag and Armchair.The results show that with any kind of imperfection, the stiffness of the structure decreases compared to the perfect SWCNT.

Influence of structural imperfections on the photochemistry in durene crystals at very low temperatures

Durene photodehydrogenation in the solid state yields duryl radicals. In doped crystals, the radicals are produced with lambda > 3000 A radiations absorbed by the guests in the high (120 to 300 K) temperature range, and with lambda < 2700 A radiations absorbed by the durene host in the low (2 to 40 K) temperature range. In the latter case, the reaction requires two photons and no substantial activation energy. The fluorescence spectra of duryl radicals produced below 40 K present a multisite structure which is nonreversibly modified by annealing the crystals. The simplified fluorescence spectra exhibited by the annealed samples correspond to radicals located in substitutional sites of the regular durene lattice. From the spectroscopic data obtained at very low temperatures, it is concluded that the durene photodehydrogenation takes place in the structural imperfections of the lattice where the excitation energy can be trapped. These physical defects are different in pure and in doped durene crystals. 7 figures, 2 tables.

Influence of structural imperfections on the graphitization of white cast iron and steel

Influence of structural imperfections on the graphitization of white cast iron and steel