Papers by Keyword: Size Effect

Abstract: Pure Barium titanate (BaTiO₃) ceramics with grain sizes of 0.6, 1.5, and 18 μm on average were prepared by using a two-step sintering method. The wideband dielectric spectra in the range from kHz to THz were measured at 25 and 150 °C to understand the polarization mechanism of fine-grained BaTiO₃ ceramics. It was revealed that the dipole and ionic polarizations decreased with decreasing grain size at 25 and 150 °C when the grain size was smaller than 1 μm. These mechanisms were interpreted as a grain boundary effect.

Abstract: To verify the applicability of a comprehensive constitutive model, which was proposed to analyze size effects in micro plastic deformation, mechanical behaviors of pure copper was adopted as the investigated subject. Unknown parameters were fitted through least square method, and calculated results were compared with experiment data of pure copper as well as those obtained by surface model. Predicted results by the comprehensive model show good agreement with experiment data. Three distinct mechanical domains appear indicating that for pure copper two critical thickness to grain size ratios exist, between which stresses vary rapidly. When concerning the situation of pure copper with only one or several grains across thickness, surface model tends to fail while the comprehensive model performs well, which further verifies the validity and applicability of the comprehensive model.

Abstract: With the ever increasing energy demands, unconventional gas becomes the new force in energy field. However, shale gas horizontal drillings are prone to instability problems because of the brittleness of gas shale. The casing programs of shale gas wells in China and North America demonstrate that the more drilling problems are encountered with the larger borehole sizes. In the thrust faults, based on vutukuri's size effect theory and Mclamore's failure criteria, a new prediction model of collapse pressure in transversely isotropic rock mass is proposed to analyze the critical mud weight of micro-hole in gas shale formations. The trajectory sensitivity analysis is also carried out. The results show that micro-hole drilling technique in thrust faults can significantly reduce the collapse pressure of directional wells by 12% approximately.

Abstract: The natural frequency of cantilever micro-beams is solved analytically on the basis of modified couple stress theory. The governing equations are obtained by a combination of the basic equations of modified couple stress theory and Hamiltons principle. The size effect on natural frequencies of the cantilever micro-beams is analyzed. It is found that the natural frequencies of the cantilever micro-beams predicted by the newly model are size-dependent. The difference between the natural frequencies predicted by the newly established model and classical beam model is assessed.

Abstract: The bending deformation problem of cantilever micro-beams is solved analytically on the basis of strain gradient elasticity theory. The governing equations and boundary conditions are obtained by using Hamiltons principle. The size effect on deflections of the cantilever micro-beams is analyzed. It is found that the deflections of the cantilever micro-beams predicted by the newly model are size-dependent. The difference between the natural frequencies predicted by the newly established model and classical beam model is assessed.

Abstract: The magnetic hysteresis loop is a significant performance of ferromagnetic material. The different-size specimens were fabricated and studied on the basis of magnetomechanical coupling tests. The curve of relationship between model size and magnetization intensity is obtained and hysteretic curve is described in the different conditions. The results indicate that the influencing law is reflected based on size effect of model specimens and performance of magnetic hysteresis and magnetization intensity. The results indicate that the size of Q235 steel model specimens is considerably influential in the magnetic hysteresis loop and magnetization, which accounts for an evident size effect.

Abstract: Modeling study of magnetic and concentration phase transition in ultrathin films of diluted magnet has been carried out under approximation of random interaction between atomic magnetic moments of nearest neighbors and within the framework of Ising model. The dependence of Curie temperature on concentration of magnetic atoms was formed. It is shown that with increasing of thickness of ultrathin film the critical concentration of transition from unordered to ordered magnetic state decreases down to the value equal to the percolation threshold.

Abstract: The influence of the size effect on the features of ADP-KDP-porous glasses composites was studied. The dielectric permittivity results of mixed ADP and KDP embedded into porous glasses with different average pore sizes in a wide temperature range are presented. The data obtained indicate that pore size affects the nature of the dielectric anomaly in the vicinity of phase transition regions.

Abstract: Here, modified analytic embedded atom method (MAEAM) has been utilized to simulate size effect and surface properties of aluminum (Al) nanoparticles. According to the simulation results, we can find that lattice parameter and excess stored energy are size dependent. The simulated excess stored energy ranges from 2.12 to 57.61 kJ/mol, which is in the same order of magnitude with experiment results; surface energy of Al nanoparticles ranges from 0.78 to 1.10 J/m2, which is not invariant but size related. Furthermore, non-uniform lattice distortion has been observed in Al nanoparticles, and mainly concentrates in the first and second shell of surface layers. Theoretical research based on our simulation results provides a novel method to predict excess stored energy of metallic nanoparticles.

Abstract: ndenter size effect on the incipient plasticity of Al (001) surface is studied by using the quasicontinuum simulation method. Two cylindrical indenters with the radii 2.5nm and 40nm are used to penetrate the surface respectively, and in displacement-control in steps of 0.02 nm. Results show that the plasticity under the small indenter is activated by discrete dislocation nucleation events, while the plasticity under the large indenter is dominated by a collective dislocation activity. Contact pressure calculations reveal that reversible incipient plasticity occurs under the small indenter, i.e. the plastically deformed surface can completely recover upon withdrawal of the indenter, while the incipient plasticity under the large indenter seems to be irreversible.