Dental restorative composite (DRC), which normally refers to ceramics filled polymeric based DRC material in modern dental industry, is one of the most important dental materials. It is utilized in making versatile dental applications such as dental fillings, dental implants, dental crowns, dental bridges, and so on. It is light-weighted, low-toxic, biocompatible, translucent, and easy to be tuned in color and, therefore, offers advantages such as biosafety, comfortability and good aesthetics. However, people’s desire of comfortability and good aesthetics limit the choices of materials of DRC. Consequently, the commercially available DRCs are often mechanically fragile, which increases the risk of fracture of DRC under occlusal loading, hinders the application of DRCs and therefore, stimulates efforts to develop fracture-resistant and durable DRC in recent decades.

The conventional method to enhance fracture-resistance of DRC is to increase the content of ceramic fillers. However, when filler content is too high, there could be many drawbacks. How to enhance fracture-resistance of DRC while maintaining a feasible amount of filler content remains a crucial problem in the field of dental materials.

Endeavoring to address above problem, this dissertation aims to enhance fracture-resistance of DRC by conducting bio-inspired design. Inspired by the superior fracture-resistance of natural dentin, a graded DRC (GDRC) was proposed, designed and fabricated and investigated for its fracture resistance and durability by International Organization for Standardization (ISO) standardized experiment. The deformation and fracture mechanism of the bio-inspired graded DRC was investigated by both experiments and numerical simulation.

This work consists of three parts. Firstly, a novel fabrication procedure was invented to construct multi-layered GDRC. By adding different number of ceramic fillers into different layers, graded composition along thickness direction of GDRC was achieved. Indentation experiment was conducted to evaluate mechanical properties of each layer. Experimental results showed that graded mechanical properties along thickness direction of GDRC were achieved. Furthermore, state-of-the-art scanning electron microscope (SEM) and atomic force microscope (AFM) were performed to characterize the microstructure and microscale mechanical responses of GDRC.

Secondly, an ISO standardized three-point bending (3PB) test for polymer-based DRC was conducted to examine macroscopic mechanical behaviors of GDRC under both monotonic and cyclic loading. Compared with the conventional DRC with monotonic composition and mechanical properties along thickness direction and another DRC with alternating composition and mechanical properties along thickness direction, GDRC shows significantly enhanced fracture-resistance while keeping the same filler content as the other two structures.

Thirdly, finite element analysis (FEA) was conducted to study the deformation mechanism of GDRC and the two other DRCs serving as control groups. FEA results showed that the graded structure of GDRC reduced the maximum bending stress, changed its location and relieved the stress concentration in the DRC structure. The FEA model was validated by comparing with the experimental results. By considering both elastic and plastic properties of the DRC, an in depth understanding which cannot be achieved by theoretical solution was obtained. The toughening mechanism of the bioinspired graded multilayer structure can be explained more clearly. Furthermore, the verified material parameters were incorporated to a natural tooth FEA model and illustrated the superior toughening effect of graded structure under occlusal contact forces.

Significances of this work lies in following aspects. First, a novel bioinspired graded multilayer structure was proposed and fabricated. Second, fracture resistance and durability were demonstrated improved by the graded composition and mechanical properties along thickness direction. Third, toughening mechanism of beam-shaped GDRC and bio-inspired design of a tooth filled by GDRC were explored by FEA.