Papers by Author: Xiao Zhi Hu

Abstract: Layered hydroxyapatite (HA) based ceramic composites consisting of a strong ceramic composite core, 3 mol% yttrium stabilized zirconia (3Y-TZP) with 30 or 40 vol% HA, and a HA-rich porous coating were fabricated using a coating deposition and co-sintering process in the present study. The aim is to develop HA-based bio-ceramic composites that retain the bio-compatibility of HA and the preferred scaffold structure, but have much improved structural properties required for implants. Two different coating techniques, sol-gel derive HA coating and HA solution coating, have been used to deposit the HA coating on the strong HA-containing 3Y-TZP core, which has been pre-sintered at 900°C and has a green-machined surface finish. The upper limit for the final sintering of the layered ceramic composites is set at 1,350°C based on the individual sintering properties of HA and 3Y-TZP. SEM observations have been conducted on the coating surface, sectioned surface of coating and substrate/core to characterize the microstructures of coating and substrate and their interface. Preliminary mechanical tests and XRD tests are also performed to characterise the structural properties at different temperatures.

Abstract: Fatigue crack growth properties of Bisplate 80, a high strength low alloy (HSLA) steel, with extensive weld repair have been studied. Fatigue cracking becomes relevant if extensive abrasive wear damage is repaired through welding because of the change in material properties. In this study, extended compact tension (E-CT) specimens of Bisplate 80 with and without weld repair, and with and without a buffer layer between the weld and parent material are used to evaluate the fatigue crack growth behaviour. Fatigue crack growth rates are closely monitored at the interface regions between the weld, buffer layer and parent metal. Detailed SEM observations are also conducted at those locations. It is expected that the experimental results can provide useful information on the optimum weld repair conditions of HSLA.

Abstract: An energy-based fracture mode has been derived for the mode I crack kinking and branching. The classic -integral has been further explored by a new partial integral path and the analytical solution of the energy release rate for crack kinking and branching from a mode-I crack tip has been established. The crack kinking/branching angle has also been analytically derived. It shows that the Griffith’s theorem and conservation law can be applied to both model I crack extension and model I crack kinking and branching. The branching mechanism for quasi-static mode-I crack has been theoretically investigated. The branching toughness and the K-based criterion for crack branching have been defined. The crack branching phenomena predicted by the present model are in well agreement with the experimental observations reported in the literatures.

Abstract: Contact damage of curved multi-layered systems, consisting of a brittle zirconia (3Y-TZP)/Alumina (Al2O3) coating – prepared by slip casting – on a compliant polymeric substrate from indentation by a hard tungsten carbide sphere is investigated. The essence of this study is to compare the structural performance of layered ceramics, and monolithic ceramic in the brittle coating and compliant substrate of dental crown-like systems. The specimens are loaded at the axis of symmetry. The failure evolution to initiate cracking and final failure patterns in curved multi-layered specimens are compared to those of the single layer specimens of the same thickness and the same study are conducted again with the flat specimens. The onset of fracture is observed in situ using a video camera. It is demonstrated that in all specimens, cone cracking occurs prior to radial cracking, with the latter being defined as the primary mode of catastrophic failure. The multi-layered ZTA (Zirconia Toughened Alumina) specimens prepared by slip casting possessed a distinct final failure pattern in comparison with a single layer specimen. The results of this study provide useful guidelines for building brittle multi-layered coating systems with the functionally graded feature, and complex shapes with geometrical uniformity.

Abstract: The blast-resistant of an aluminum foam sandwich structure under impact has been investigated. Plastic deformation and load distributions in each of the sub-layers during and after impact were calculated by the material point method with the consideration of strain rate effect on deformation of the aluminum foam. The numerical results from the 3D material point method modeling show that the blast-resistant capability of the aluminum foam sandwich structure is excellent, and the energy absorbability of the structure would be underestimated if the strain rate effect on the aluminum foam is ignored.

Abstract: A micro-mechanical model and simulation for the damage behavior of short fiber interleaves (SFIs) were developed based on Mori-Tanaka method and an equivalent approach to interface debonding (Fitoussi etc 1990). The damage evolution and the stress-strain relation of SFIs have been predicted in the cases of interlaminar shear, out-of and in-plane tension, respectively. The simulation indicates that the damage always starts from the interface debonding of fibers perpendicular to load and the matrix cracking in the direction parallel to fibers, and then rapidly spreads to more fibers during loading. The strength and the ultimate strain in out-of-plane tension are much lower than that in interlaminar shear and in-pane tension. The strength and failure probability of interface bonding are the most considerable factors to affect the damage and failure of SFIs. The comparison of the simulation with the interlaminar shear test shows a good agreement.

Abstract: A special test setup was designed and used in compression shear test of unidirectional carbon fiber composites to study the effect of short fiber interleaves (SFIs) on the interlaminar shear behavior. The comparative tests for two kinds of double-notched compression specimens, with and without SFIs, were carried out to determine the interlaminar shear strength and modulus (ILSS and ILSM) and to examine the failure characteristics. To determine ILSM of the specimens with SFIs an inversion analysis method was proposed based on comparing compression displacement of specimens with and without SFIs. The experimental results show that SFIs makes ILSS decreasing due to lower interface strength, and the complex damage process of SFIs leads to a considerable increase of interlaminar shear compliance.

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: A practical method to evaluate the thermal shock stress intensity factors (TS-SIF`s) associated with a flexibly restrained edge-cracked plate (FRECP) is outlined. It is assumed that the edge crack can be represented with a spring couple, and the deformations of the edge-cracked plate are consistent with Euler-Bernoulli deformation theory. It is then demonstrated how a compliance analysis of this simplified representation can be used with a finite element analysis of equivalent crack free plates to evaluate to TS-SIF`s associated with a FRECP. The accuracy of this method is shown to be excellent by comparing it against a direct fracture mechanic finite element analysis. This method is therefore advantageous to a fracture mechanic finite element analysis as it does not require an explicit analysis of a FRECP.

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.