Optimization of Fiber Shapes in Biocomposites

Fibers can be used to improve the mechanical properties of acrylic bone cement. However, debonding of the fibers from the matrix due to the poor fiber/matrix interface is a major failure mechanism for such reinforcements. Optimization of the fiber shape can improve load transfer between the fibers and the matrix, thereby providing improved overall mechanical performance. The goals of this study include: (1) Develop an analytical model to evaluate the effects of fiber end geometry on the pullout load and stress distribution; (2) Determine the optimal fiber morphology for maximum stress transfer in composites using optimization and finite element modeling; (3) Fabricate the fibers with optimal morphology determined by the previous step; (4) Manufacture composites reinforced with the optimized fibers and demonstrate improved mechanical properties experimentally. Analytical solutions were derived to predict the effects of the enlarged end shape on the pullout load and stress distribution. It is shown that the shape of the enlarged end has a significant influence on the stress distribution of the short fiber. A procedure for structural shape optimization of short fibers was developed. The effects of the interfacial bond and fiber orientation were investigated to obtain the optimal fiber shape. The general optimal fiber shape is a variable diameter fiber (VDF). Due to the mechanical interlock, the VDF can both bridge matrix cracks effectively and improve the composite mechanical properties. Ceramic VDFs were successfully fabricated. Static and fatigue tests were carried out on the VDF reinforced composites. Conventional straight fiber (CSF) reinforced bone cement was also tested for comparison purposes. Results demonstrated that both the stiffness and the fatigue life of VDF reinforced bone cement are significantly improved compared with the unreinforced cement. Also, the fatigue life of VDF reinforced bone cement was significantly longer than that of CSF reinforced cement. This study shows the feasibility of a novel fiber (VDFs) technology for reinforced polymers. This fiber family significantly improves the fatigue life of bone cement at a very high level of reliability. VDFs could potentially avoid implant loosening due to the mantle fracture of bone cement and delay the need for revision surgery.