Papers by Author: Dai Ning Fang

Abstract: In this paper, based on the study of effects of various physical mechanisms on the fracture strength of porous ceramic materials, a thermo-damage strength theoretical model applied to each stage of temperature is established. Using the model, the sensitivities of fracture strength to relevant parameters and their variation with temperature are studied in detail. The results show that under low temperature the strength is sensitive to the changes of porosity, pore size and pore shape factor, while under high temperature the effects of porosity, pore size and pore shape factor on the strength compared to the temperature are negligible, yet the strength is very sensitive to the Young’s modulus, thus the Young’s modulus is the dominant of strength under high temperature. This study will provide a theoretical basis and guidance to the design and application of porous ceramic materials.

Abstract: Due to the complexity of service environment of thermal protection system on the aerocraft, the thermal shock resistance (TSR) of ultra-high-temperature ceramics (UHTCs), which are used as thermal protection materials, is no longer the material itself’s. Based on the restrictions of current experiments and the lack of theories, hafnium diboride (HfB2) is used to study the effects of the external constraint conditions and different thermal environment on the TSR of the UHTC in detail. The effects of different initial temperature, different external constraint conditions, and temperature rising rate on the TSR of the UHTCs through numerical simulation are discussed in detail in this study. This study can provide a more intuitively visual understanding of the evolution of the TSR of UHTCs during actual causative conditions.

Abstract: The parameters, such as thermal expansion coefficient and Young's modulus, are considered as a function of temperature and incorporated into ANSYS code to calculate the stress field of UHTCs under high temperature condition in the present work. The stress fields of two kinds of heating cases are calculated and compared. By establishing the relation between the temperature and the mechanical properties of the UHTCs, it is found that the mechanical behavior of UHTCs is strongly affected by the oxide film thickness, initial temperature and the heating rate.

Abstract: A novel method for characterizing a high actuating capability light-weight actuator is described and analyzed. The actuator comprises a truss core and smart face-sheets which result in bending. The deformation of this beam actuator is obtained by changing voltage inside face-sheets made from smart material. Materials selections are discussed for practice, and then optimization is designed to ascertain minimum weight or maximum achievable displacement subject to two types of constrains. Results show the composite trusses have higher stiffness and strength than metal trusses. Comparisons with two corrugated core actuators, the truss-core design has better actuating capability at specified weight.

Abstract: As the size of ferroelectric nanostructures decreases, the fundamental questions of the size dependence of the ferroelectric properties, and of their possible disappearance at a finite critical size, become crucial. In this paper, Landau-Ginzburg-Devonshire(LGD) theory is introduced to study the size-dependent ferroelectric properties of BaTiO3 nanowires. Our results demonstrate that the Curie temperature and mean polarization decrease with the reducing of nanowire diameters. We further show that these size effects are significant only when the nanowire diameters are less than 20nm, Above this, the ferroelectric behaviors of nanowires are almost the same as those of bulk materials and size effect can be neglected.

Abstract: Lattice structures have ranges of thermo-mechanical properties that suggest their implementation in ultralight structures, as well as for impact/blast amelioration systems and heat dissipation media. Considering that proper anisotropy of structure could increase load efficiency, two kinds of 2-D lattice materials designable in specific stiffness and strength of arbitrary direction have been brought forward: variational thickness cell and variational direction cell. The mechanical properties of variational thickness Kagome cell have been analyzed, including effective elastic modulus, yield strength and elastic buckling strength in arbitrary directions. Since the shear buckling of 2-D lattice materials is an important collapse mode especially when relative densities are low, shear buckling strength of various 2-D lattice materials have also been calculated. It is found that compared with the diamond cell, the variational thickness Kagome cell of thickness ratio, m=0.5, possesses the same elastic modulus and yield surface, and higher buckling strength.

Abstract: This paper presents a theoretical model to predict the fracture strength of ultra-high temperature ceramics (UHTCs). According to different mechanisms, the environmental temperature is divided into four ranges. Effects of temperature and oxidation on the fracture strength of UHTCs are investigated in each temperature range. The results show that oxidation plays an important role in enhancing the fracture strength of UHTCs at high temperatures.

Abstract: Thermal shock resistance of Ultra-High Temperature Ceramics is one of the most important parameters in UHTCs characterization since it determines their performance in many applications. In order to reflect practical cases, the temperature-dependent thermal shock resistance parameter of UHTCS was measured since the material parameters of UHTCs are very sensitive to the changes of temperature. The influence of some important thermal environment parameters and the size of the material on the thermal shock resistance and critical temperature difference of rupture of UHTCs at different stages in the thermal-shock process were investigated. The results show that thermal shock behaviour of the UHTCs is strongly affected by the size of the material and the thermal environments parameters, such as the surface heat transfer coefficient, heat transfer condition and initial temperature of the thermal shock.

Abstract: ZrB2/SiC ceramic composites reinforced by nano-SiC whiskers and SiC particles have been prepared by hot-pressing at 1950°C for 1hr under 20 MPa pressure in flow argon atmosphere. Effects of SiC addition on microstructure, mechanical properties and thermal ablation/oxidation behavior of ZrB2/SiC composites were investigated. The results showed that the addition of SiC effectively improved the densification of ZrB2/SiC composites and almost full dense ZrB2/SiC composites were obtained when the amount of SiC increased up to 20 vol%. Flexural strength and fracture toughness of the ZrB2/SiC composites were also enhanced; the maximum strength and toughness reached 600 MPa and 8.81 MPa·m1/2 at SiC additions of 20 vol % and 30 vol%, respectively. The composites possessed good resistance to flame ablation and could keep the whole shape without distinct peeling or cracking after flame ablation by oxyacetylene flame for 3 mins. The more SiC added, the better resistance to flame ablation the composites displayed.

Abstract: In this paper, ZrB2-based ceramics containing up to 15 vol% nano-SiC whiskers were prepared by hot pressing at 1950°C under 20MPa pressure in flow argon. SEM and XRD techniques were used to characterize the sintered compacts. A fine and homogeneous microstructure was observed. The relative density of ZrB2-based ceramic containing 10vol% SiC whiskers reached to 97.7%. The bending strength and fracture toughness of the composite were 550 MPa and 8.08 MPa·m1/2 respectively, while those of the monolithic ZrB2 ceramic (0 vol% SiC whiskers added) were 424 MPa and 4.52 MPa·m1/2 respectively. The grain size of the ZrB2-based ceramics was reduced greatly by the addition of nano-SiC whiskers during the sintering process.