The rotation toughening mechanism of barb–barbule joint in the barb delamination of feathers

The barb–barbule structure branching from a feather shaft is a basic unit of a bird feather, and the structure is intricately organized to form the feather vane, which plays an important role in keeping the feather’s integrity during bird flight. In this paper, by coupling nonlinear large deformations of the barb and barbule, an analytical model of delaminating two neighboring barbs on the basis of critical-friction detaching criteria is developed. Considering the rotation and non-rotation of barb–barbule joints in the delamination, a rotatable model (LargeRM) and a non-rotatable model (LargeNRM) are treated to explain the rotation’s contribution to toughening the feather vane. The results show that the predicted interlocking forces of un-detached barbules in the two models were linearly distributed during the delamination. Due to the rotatable barb–barbule joint, the critical detaching force and the elastic strain energy of the two neighboring barbs in the LargeRM are greater than those in the LargeNRM, and this indicates that the rotatable barb–barbule joint could enhance the in-plane delaminating toughness of the feather vane. The present model reveals the nonlinear barb delamination behavior and explains the rotation toughening mechanism of the barb–barbule joint in the barb delamination and further is used to design new bio-inspired interlocking materials, e.g., the feather-inspired Velcro fastener.