Papers by Author: Qing Li

Abstract: Fabrication of multilayered ceramics signifies an important topic in many advanced applications aerospace and prosthetic dentistry. This paper presents a numerical approach to characterising the transient thermal responses and corresponding thermal residual stresses that are developed in the bi-layered dental ceramic crowns model under a controlled cooling rate from a temperature around its glass transition temperature (typically 550°C) to room temperature (25°C). Finite element method (FEM) is adopted to model the residual stresses in normal or rapid cooling fabrication process. The demonstrative examples take into account the effect of thickness in core veneered all-ceramic restorative prosthesis (specific porcelain bonded to an alumina or zirconia core layer), cooling rates and mismatches in temperature-dependent material properties such as thermal expansion coefficients, specific heat and Young’s modulus. The model of transient ceramic fabrication processing showed significant potential to development of optimal prosthetic devices.

Abstract: Currently, titanium dominates the dental implant materials due to its strength and bio-inerrability. The use of titanium implant had demonstrated considerable surgical success. However, researchers are spontaneously pursuing better materials to achieve better osseointegration in the early stage of implantation. Recently, dental implants based on functionally graded material (FGM) were introduced in pursuit for the goal of enhanced bio-compatibility. The concept for FGM dental implant is that the property would vary in certain pattern to match the biomechanical characteristics required at different regions in the oral bone. However, mating properties do not necessarily guarantee better osseointegration and bone remodelling. There is no existing report available on the long-term effect of FGM dental implant on its hosting bone tissues. This paper aims at exploring this critical problem by using computational bone remodelling technique. The magnitude of bone remodelling due to use of FGM implant is identified over a healing period of four years. Comparisons were made between titanium and various FGM designs, the interesting differences were observed and the optimum FGM design was suggested based on the remodelling results.

Abstract: This paper aims at providing a preliminary understanding in biomechanics with respect to the effect of the particle size of Fully Porous-Coated (FPC) dental implant on osseointegration. 2D multiscale finite element models are created for a typical dental implantation setting. Under a certain mastication force (<200N), a global response is first obtained from a macro-scale model (without considering morphological details on the coated surface), and then it is transferred to micro-scale models (with coated surface morphology details in three different particle sizes). An equivalent strain is analyzed to investigate the effect of particle size of the FPC materials on osseointegration and initiation of bone remodelling. The result reveals that increasing particle sizes has a significant effect on biomechanical and bone remodelling responses.

Abstract: Despite significant success in developing various periodic composites, the challenge remains how to more efficiently design the base cell so that one or more physical properties can be attained. In this paper, the material design problem is formulated in a form of the least square of the difference between the targeted and designed values. By minimizing the objective subject to volume constraints and periodic boundary conditions, an optimal material distribution in base cell can be generated. Different from existing methods, this paper shows how to use the Evolutionary Structural Optimization (ESO) method to design composite material attaining to thermal conductivity defined by the Hashin-Strikman (H-S) bounds. The effectiveness of this method is demonstrated through several 2D examples, agreeing well with commonly known benchmarking microstructures.

Abstract: This study systemically presents an inverse homogenization method in the design of functional gradient materials, which gained substantial attention recently due to their layer-by-layer defined physical properties. Each layer of these materials is unilaterally constructed by periodically extended microstructural elements (namely base cells), whose effective properties can be decided by the homogenization theory in accordance with the material distribution within the base cell. The design objective is to minimize the summation of the least squares of the difference between corresponded entries in target and effective elasticity tensors. The method of moving asymptote drives the minimization of this positive objective function, which forces the effective values approach to the targets as closely as possible. The sensitivity of the effective elasticity tensors with respect to the design variables is derived from the adjoint variable method and it guides the minimization algorithm efficiently. To guarantee the connectivity between adjacent layers, non-design domains occupied by solid materials acting as connective bars are fixed in the design of base cells. Furthermore, nonlinear diffusion technique is introduced to avoid checkerboard patterns and blur boundaries in the microstructures. A series of two-dimensional examples targeted for the elasticity tensors with same extreme Poisson ratios but different densities in each layer are illustrated to highlight the computational material design procedure.

Abstract: This paper provides a preliminary understanding in biomechanics with respect to a fullyporous- coated (FPC) dental implant. A 2D multiscale finite element model is created for a typical dental implantation setting. Under a certain mastication force (<200N), a global response is first obtained from a macro-scale model (without coated surface morphology details), and then it is transferred to a micro-scale model (with coated surface morphology details), which allows determining a local biomechanical field. To facilitate the study in bone remodelling, strain energy density and equivalent strain are analysed respectively. Different porosities of coating are taken into account in this study to investigate the effect of FPC materials on these typical remodelling stimuli. The results evidently reflect the osseointegrative benefits generated from surface coating. The result reveals that increasing in particle sizes has significant effect on biomechanical response.

Abstract: Dental implants have been extensively used in prosthetic dentistry over the last two decades. Clinical experience shows that the healing and osseointegration process can heavily influence the success of the implantation. It is critical to understand the damage extent in different time frames. This paper aims at exploring the mechanical damage of dental implantation over the healing process. The 3D finite element analysis (FEA) models were developed based on computerised tomography (CT) scan technology to investigate the load-induced damage of interfacial osseointegration, as well as cortical and cancellous bone tissues. Unlike the existing linear finite element (FE) stress analysis, this study takes into account the damage accumulation and micro-crack nucleation under a framework of bone/interface remodelling. This study reveals the damage in the surrounding bone tissues and bone-implant interfaces at different stages of the healing process, and consequently premature load tolerances are suggested.

Abstract: Natural human tooth consists of multiple layered quasi-brittle biomaterials, which make dental restorations experience a complex stress state under masticatory contact loading. As such, many restorations are prone to failure and a constant effort is made to improve the mechanical characteristics of the restorative materials. Clinical observations have shown that improved strengths and fracture toughness in ceramic materials do not necessarily lead to an anticipated higher functional longevity of the restoration. While substantial experimental investigations have been carried out to identify the contact induced fracture in such multi-layer material systems, numerical modelling of this event was largely unexplored. This paper presents a new numerical method to account for micro-damage driven fracture in various multi-layered biomaterial structures. In this study, a Rankine constitutive model is adopted and the crack initiation and propagation are automatically implemented in an explicit finite element (FE) framework. The effects of indenter radius, surface curvature and thickness of layered biomaterials on the cracking patterns are investigated. The results show good agreement with the experimental studies in literature.

Abstract: Wear is often of definite influence in the service life of mechanical components and has been recognised as one of the major causes of failure in engineering practice. It is noted that although extensive attention has been paid to phenomenological studies like surface morphology analysis for wear assessment, the physical mechanism of wear particle formation remains unclear. This paper proposes a micro damage and fracture model to simulate the process of wear particle generation. An explicit finite element (FE) formulation is employed to capture the nonlinearities involved. Unlike existing FE analysis (FEA), any initial sub-fractures underlying the wear surface are no longer required. Crack initiation and propagation as well as the corresponding mesh updating are implemented in an automatic fashion associated with the explicit FE framework. The results presented are in good agreement with experimental observation and the reports in existing literature.

Abstract: Ceramics have rapidly emerged as one of the major dental biomaterials in prosthodontics due to exceptional aesthetics and outstanding biocompatibility. However, a challenging aspect remaining is its higher failure rate due to brittleness, which has to a certain extent prevented the ceramics from fully replacing metals in such major dental restorations as multi-unit bridges. This paper aims at simulating the crack initiation and propagation in dental bridge. Unlike the existing studies with prescriptions of initial cracks, the numerical model presented herein will predict the progressive damage in the bridge structure which precedes crack initiation. This will then be followed by automatic crack insertion and subsequent crack growth within a continuum to discrete framework. It is found that the numerical simulation correlates well to the clinical and laboratory observations.