Papers by Author: Chak Yin Tang

Abstract: A micro-meso-macro finite element approach has been developed for simulating the macro-scale damage coupled deformation in a particulate polymer composite (PPC) based structure under tension. A damage model for the PPC structure was developed to define the debonding damage behavior of the structure. The computational results determined in our previous studies by using finite element meso-cell modeling technique were used as the input parameters of the damage model and definition of the constitutive behavior of PPC. A user-defined subroutine VUMAT describing the damage-coupled constitutive behaviour of PPC for defining the material properties of the finite elements for the structure was then built and incorporated into the ABAQUS finite element code. A case example has been given to demonstrate the proposed approach. The macroscale damage process in the simulated component was found to be reasonable.

Abstract: A new kind of Hydroxyapatite (HA) composites was fabricated using a hot-press method in which carbon nanotubes (CNTs) were introduced to the HA ceramic matrix to improve the mechanical properties of the resulting composites. The CNTs were treated using a mixture of nitric acid and sulfuric acid. A homogeneous dispersion of the CNTs in the HA matrix was achieved through dispersing them in the solution of surfactant. The bioactivity of the samples was tested. The microstructures of HA/CNT ceramic composites and the sample surfaces after immersion in SBF were characterized by SEM. The results showed that the introduction of the CNTs into HA ceramic matrix could produce positive strengthening effect without impairing the biological properties of the HA ceramic.

Abstract: The effect of filler shape and volume fraction on the micromechanical damage behavior of particulate-reinforced dental composites was investigated using the finite element method. Threedimensional unit cell models for various filler volumes and shapes, using hydroxyapatite in Bis- GMA as a model system, were used. Young’s modulus and stress concentration factor were calculated. The effects of filler shape on the ease of initiation of strain damage in the matrix and onset of particle-matrix debonding are discussed.

Abstract: In order to obtain more accurate properties after compaction of hydroxyapatite (HAp)/poly-L-lactide (PLLA) composite, high-resolution measurement of mechanical properties method is proposed to determine the properties of each phase separately, leading to information that are valuable for the development of new materials as well as for predictive modeling purposes. The PLLA polymer processing conditions used in hot pressing of the composite strongly influence final mechanical properties of the material in the solid state. Since the aim was to measure PLLA material properties, acceptable findings could only be made using unconstrained, cured in situ nanoindentation tests. A finite element analysis of the in situ indentation experiment was performed to determine required size of plain polymer area, needed for indentation test, which would minimize the particle influence on the matrix elastic behavior.

Abstract: A three-dimensional finite element analysis was conducted to simulate the effects of the varying material parameters on the contraction behaviors of a muscle-tendon complex using an active finite element method. The material behavior of the skeletal muscle was assumed to be orthotropic and the muscle model consists of two parts: the active and the passive parts. An active finite element method was then used for accommodating both the active and passive behaviors of the muscle into the muscle model. In this active-passive muscle model, the active component is governed by an activation level, a time period, a muscle sensitivity parameter and a strain rate. The material property of the passive component was assumed to be viscoelastic and the tendon is assumed to be linear elastic. The effects of activation amplitude and viscoelastic material parameters on the active, passive and total force-length relationship of the cat muscle under isometric contraction were predicted. The predicted results were found to be close to the experimental data reported in the available literature. Hence, the active-passive muscle model was extended to simulate the stress distribution of the cat muscle subject to shortening contraction and different activation amplitude. By varying the magnitude of the material parameters, different muscle behaviors could be generated. The proposed active finite element method lays a good foundation for simulation of human musculoskeletal motion.

Abstract: Wear of ultra-high molecular weight polyethylene (UHMWPE) and its composites is one of the main obstacles that limit the longevity of total joint replacements. Compression molded UHMWPE/quartz composites with organosiloxane as a cross-linking agent for UHMWPE matrix, were tested in nanoindentation and nanowear. The nanomechanical properties of the composite were examined in the light of nanoindentation experiments performed with a diamond tip of nominal radius of curvature of about 150 nm under conditions of various contact loads. Results from nanowear tests show that, in addition to the nanohardness and elastic modulus, the crosslinking procedure has the most pronounced effect on the tribological properties and at 0.5 phr organosiloxane, composites reache their maximum nanowear resistance. These findings are in agreement with the results of conventional mechanical and wear tests performed on these materials.

Abstract: This study descripts processing of biphasic calcium-phosphate (BCP) and poly-L-lactide (PLLA) biocomposite implant material. The composite was obtained by mixing completely dissolved PLLA with granules of high crystalline BCP and was compacted by hot pressing using cylindrical dies at 450 K temperature and 98.1 MPa pressure, for 30 and 60 minutes. Wide-angle Xray structural (WAXS) analyses of BCP, PLLA and BCP/PLLA composite blocks were made followed by calorimetric (DSC) tests in the 320-520 K temperature range. Compression tests revealed that Young’s modulus and compressive strength of the composite increased with extended hot pressing time and were found to be within the bounds of the cortical bone values.