Dai Ning Fang

An Experimental Study on Creep Deformation of PBX with Laser Moiré Interferometry Method

In this paper, the creep deformation of PBX was measured using the moiré interferometry. The experimental results show a different creep process compared with pure high polymer and this phenomenon is preliminary analyzed from damage mechanics.

Mechanical Properties of ZrB2-Based Ceramics Reinforced by Nano-SiC Whiskers

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.

Numerical Simulation for Thermal Shock Resistance of Thermal Protection Materials under Temperature Rising

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.

Thermal Shock Resistance of Ultra-High Temperature Ceramics

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.

A multilayer radome wall structure with passbands having odd times of selected central frequencies

This study focuses on the design of a flat multilayer radome wall structure with passbands having odd times of selected central frequencies. The multilayer structure comprises an odd multiple of five or more layers. And each layer, except for the central layer, is designed as an electromagnetic matching layer at the design frequency, which can be arbitrarily selected. Investigations suggest that the multilayer structure exhibits multiband transmission property, and the central frequency of each passband is an odd multiple of the design frequency. The central-layer thickness, as the results indicate, can be dimensioned in the range from 0.1 to 6 wavelengths to fulfill the requirements of mechanical performances without compromising the transmission capability. Results also show that the multilayer structure at a design frequency of 65u2009GHz turns to be a broadband radome wall structure in the u2009GHz frequency range for both centimeter and millimeter waves applications.

Preparation and Flame Ablation/Oxidation Behavior of ZrB2/SiC Ultra-High Temperature Ceramic Composites

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.

Study on Ballistic Energy Absorption of Laminated and Sandwich Composites

The impact response and energy absorbing characteristics of laminated, foam sandwich and honeycomb sandwich composites under ballistic impact have been studied in this investigation. An improved model is proposed in this paper to predict the ballistic property of the laminated composites. In this model, the material structures related to fiber lamination angles are designed in terms of their anti-impacting energy absorption capability. The ballistic limit speed and energy absorption per unit thickness of the three composites under different conditions are calculated. It is shown that honeycomb sandwich composite has the best ballistic resistance capability and energy absorption property among the three composites.

Research of the Sensitivities of Fracture Strength of Porous Ceramic Materials to the Relevant Parameters

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.

Influence of Electric Fields on the Fracture Behavior of Ferroelectric Ceramics under Combined Electromechanical Loading

In this paper, fracture behavior of ferroelectric ceramics under combined electromechanical loading was investigated using moiré interferometry. It is found that the influence of electric field on fracture toughness is not very larger in the case that the directions of the poling, electric field and crack extension are perpendicular to each other. When the poling direction is parallel to the crack extension direction and both are perpendicular to the electric field direction, the normal strain measured reduced faster than that calculated by FEM with and without electrical loading as the distance away from the crack tip increases. Fracture toughness decreases obviously as the electric-field intensity increases.

Design on Thermal Protection Structure of C/SiC Lattice Composite Materials

A new double-layer lattice structure based on C/SiC composite material is described and being investigated as a means to increase the service temperature of thermal protection structure. The design incorporates a C/SiC double-layer sandwich comprising two pyramidal truss cores. The outer layer of the sandwich structure is the thermal protection layer, which can make the heat redistribute. The internal layer is the insulation layer, which can decrease the temperature of the hot components and increase their reliability. The temperature field of C/SiC lattice thermal protection structures with different geometrical parameters was calculated by the finite element software ANSYS. It is found that the thermal behavior of the double-layer lattice thermal structure is affected by the truss geometry, such as truss length and inclination angle. The thermal protection capacity of C/SiC lattice structure is analyzed and compared with the equivalent solid structure. The results indicated that C/SiC lattice thermal protection structure has lower density and better thermal protection property than the traditional thermal protection structures.