Papers by Author: Kai Duan

Abstract: As a part of a large research program aiming at the cementitious materials containing recycled materials at Central Queensland University – Australia, the current paper presents the preliminary results of a study on the effects of fly ash, which is used to replace cement in concrete, on the concrete compressive strength. For this purpose, systematic experiments have been carried out to investigate the influences of fly ash ratio and age. The compressive strength of concrete specimens with replacement ratios of 15%, 30% and 45%, and aged 7 and 28 days are measured and are compared with those of the concrete specimens without fly ash at the same ages. The results demonstrate that the strength of fly ash containing concrete improves more slowly but more strongly with aging, than their fly ash free counterparts, and an optimum fly ash replacement ratio exists where the maximum compressive strength of fly ash containing concrete can be achieved, and the maximum strength for the specimens aged 28 days and above is higher that of fly ash free concrete. Furthermore, the observation strength behaviours are analysed and discussed in terms of the influences of fly ash on interface reactions and interface bonding strength.

Abstract: Replication, or repeated tests at the same stress amplitude, is used to provide statistical confidence in life data during the development of S-N curves. This paper discusses the effects of replication on the measurement of S-N curves and presents an alternative to traditional replication methods for the determination of S-N curves, particularly for the development of preliminary S-N curves. Using specimens made out of the extruded bars of a magnesium alloy, it is demonstrated that the S-N curve estimated using the data from non-replication tests is almost same as that from replication tests. The advantage of using non-replication fatigue tests is that it uses fewer specimens, in this instance, only half of that required for 50% replication fatigue test, to achieve the same estimation as that of the replication fatigue tests. Another advantage of using non-replication fatigue tests is that it can detect the non-linearity using limited specimens.

Abstract: Fatigue failure of metal components containing notches, cracks and other defects has been a very active research topic for well over seven decades because of its important practical and theoretical implications. Recently, Taylor and his colleagues have re-visited this topic and proposed the Theory of Critical Distance (TCD), which summarizes the early work by Neuber, Peterson and others in a unifying theory and predicts fatigue fracture with the use of a critical distance, L0. In this paper, an experimental and numerical study of the fatigue of notched and unnotched 6061 aluminium alloys is used to verify the TCD and some of the limitations of the TCD are discussed on this basis.

Abstract: YAG nanopowders were synthesized by a co-precipitation method using ammonium hydrocarbonate and ammonia water as the precipitants respectively. The influences of precipitants on chemical compositions, phase transformation and sinterability of the prepared powders, and transmittance of the vacuum-sintered YAG ceramics were studied. The sinterability of powders synthesized using ammonium hydrocarbonate as precipitant is better than that with ammonia water. Pure YAG phase can be obtained by calcining the hydrate precursor at 1200°C, while some impurity phases exist when calcining the carbonate precursor at the same temperature. Transparent YAG ceramics were fabricated by vacuum sintering at 1700°C for 5 h using the YAG nanopowders, and their in-line transmittance is about 60% in the visible light range.

Abstract: Open-cell Al O ceramic foams with uniform cell structures and dense cell struts were fabricated by centrifugal slip casting using the plant seeds as the templates. The rheological characteristic of Al O slurries with up to 50 vol.% solid content was investigated. The shrinkage matching between the Al O green compact and the plant seeds during drying was studied. The effect of solid contents of slurries on change of green density of cell struts along the height of compacts was analyzed. The drying and sintering schedule of Al O green cakes were studied. The results indicated mass segregation of Al O particles with different sizes during centrifugal process was negligible for slurries with high solid loadings with 50 vol. %. The cell struts of green bodies had high density of 63.8% TD. After sintered at 1500 °C for 2 h, the final products had dense cell struts with high sintered density of 98.9%TD and homogeneous microstructure. The porosity and compressive strength of sintered products was 66.5% and 5.26 MPa, respectively.

Abstract: The effect of purity of amorphous boron raw materials on properties of the hot pressed SiC doped MgB2 superconductor was investigated. MgB2 superconductors with magnetic Jc over 106 A/cm2 and remaining Jc of 105 A/cm2 at 4.2 K and 5 T were fabricated by hot pressing using both high purity (99.00%) and low purity (88.84%) boron powders. XRD analysis shows that purity of the boron powders has little effect on phase component of the MgB2 samples. If the main impurity in amorphous boron is Mg, low purity low cost boron powder is suitable as one of the raw materials for fabricating MgB2. Particle sizes of boron has significant effect on microstructure and properties of MgB2. Smaller boron particle size leads to smaller grain size of MgB2, higher density, higher lattice distortion, and thus higher magnetic Jc.

Abstract: This paper presents a study on the densification and mechanical properties of Al2O3/Ti(C,N) ceramics processed using a pressureless sintering technique. The Al2O3/Ti(C,N) ceramics containing 10, 20, 30 and 40 wt% Ti(C,N) were sintered in the temperatures ranging from 1500 to 1750 °C and in the atmospheres of vacuum, Ar, H2 and N2. It is found that both optimum sintering temperature and Ti(C,N) content exist, where the best densification and mechanical properties are achieved. The Al2O3/Ti(C,N) properties will decrease when either sintering temperature or Ti(C,N) content moves away from their optimum value. The experimental results also demonstrate that among the four atmospheres, Ar gives best results. To improve further the properties of Al2O3/Ti(C,N) composites, Al2O3 and Al2O3/Ti(C,N) powders have used to cover the specimens during sintering, and experiments revealed that covering with Al2O3/Ti(C,N) powder can significantly improve the properties of Al2O3/Ti(C,N) ceramics. Furthermore, the effects of Ti(C,N) content and sintering conditions on densification and mechanical properties are explained in terms of their influences on Al2O3/Ti(C,N) microstructures.

Abstract: Artificial neural network (ANN) is an intriguing data processing technique. Over the last decade, it was applied widely in the chemistry field, but there were few applications in the porous NiTi shape memory alloy (SMA). In this paper, 32 sets of samples from thermal explosion experiments were used to build a three-layer BP (back propagation) neural network model. According to the registered BP model, the effect of process parameters including heating rate ( ), green density ( ) and particle size of Ti ( d ) on compressive properties of reacted products including ultimate compressive strength ( v D σ ) and ultimate compressive strain (ε ) was analyzed. The predicted results agree with the actual data within reasonable experimental error, which shows that the BP model is a practically very useful tool in the properties analysis and process parameters design of the porous NiTi SMA prepared by thermal explosion method.

Abstract: In this paper, the extensively-reported “size effect” phenomena in fracture mechanics tests are explained using the boundary effect concept. It is pointed out that the widely-observed size effect in fracture, including the dependence of the fracture energy on ligament, strength and fracture toughness on crack and/or ligament and the strength of geometrically similar specimens on characteristic size, is in fact, due to the boundary influence on the crack tip damage zone. Furthermore, the recently-developed asymptotic model is used to demonstrate that the dependence of strength on crack and ligament lengths as well as on the characteristic size of geometrically similar specimens is a result of the dominance of the distance of the crack tip to specimen boundaries on the specimen failure mode. To verify further the boundary effect concept, the asymptotic model is also applied to two sets of selected experimental data available in the literature, and the implications are discussed.

Abstract: Hydroxyapatite (HA) powder was synthesized by a sol-gel method with Ca(OH)2 and H3PO4 as reactants. The HA granules were then coated with TiH2 powder using a mechanical mixing method. The HA-TiH2 material system produced HA-Ti composites after hot-pressing at 1050°C. The HA-Ti composites are mainly composed of HA and Ti, with small amounts of Ca2P2O7 and Ca3(PO4)2 phases. Fracture toughness and bending strength are 2.4 MPa·m1/2 and 54.3 MPa, respectively for the HA-20vol%Ti composite, higher than those of the pure HA ceramic. The improvement in properties is because of the unique 3D network structure of Ti, which is an ideal reinforcement structure for the weak and brittle HA. According to ISO/TR 7405-1984, hemolysis test was performed to evaluate the blood compatibility of the material. The results show that the hemolysis rate of the HA-20vol%Ti composite is 0.56%. Relative growth rates (RGR) of L-929 cells soaked after 6 days in the HA-20vol%Ti group, pure Ti group, black group and pure Pb group were 132%, 100%, 90% and 6% respectively, while the level of cytotoxicity was grade 0 in HA-Ti composite group. These results imply that the HA-20vol%Ti composite has good biocompatibility and bioactivity.