Softness makes strength: Bio-inspired composites reinforced by functionally graded fibers

Inspired by the composite structure of the wood cell wall, we propose in this paper a new class of composites reinforced by axially functionally graded fibers with high stiffness in the middle, degrading to low stiffness at the ends. Analysis based on both analytical shear-lag model and Finite Element model demonstrates that such a functionally graded design can effectively alleviate the shear stress concentration on the fiber-matrix interface. Besides, we find that, by reducing the stiffness of the fiber end, the interfacial debonding is significantly post-poned thus the strength of the composite is improved. A detailed study on the debonding process is conducted and we characterize a distinctive catastrophic propagation mode of the interfacial crack, indicating that the functionally graded fiber enhances the mechanical performance of the composite by raising the threshold of interfacial debonding. Moreover, some of the design parameters governing the fiber modulus profile are dis-cussed with regard to their effects on the mechanical properties of the composite. This work sheds light on achieving high performance composites with relatively poor intrinsic interface properties.

Automated summary

In this paper, a new class of composites reinforced by axially functionally graded fibers is proposed, with high stiffness in the middle and degrading to low stiffness at the ends. The analysis shows that this functionally graded design effectively alleviates shear stress concentration and enhances the mechanical performance of the composite by raising the threshold of interfacial debonding. The study also discusses the effects of design parameters on the mechanical properties of the composite.