Pullout of the Cylindrical Helicoidal Fiber

The multi-layer cylindrical helicoidal fiber structure (MCHFS) exists widely in biological materials such as bone and wood at the microscale. MCHFSs typically function as reinforcing elements to enhance the toughness of materials. In this study, we establish a shear lag-based pullout model of the cylindrical helicoidal fiber (CHF) for investigating interlayer stress transfer and debonding behaviors, with implications regarding the underlying toughening mechanism of MCHFS. Based on the shear lag assumptions, analytical solutions for the stress and displacement fields of the MCHFS during the pullout are derived by considering the CHF as a cylindrically monoclinic material and verified through the 3D finite element simulation. It is found that the helical winding of CHF results in both axial and hoop interlayer shear stresses. Both the helical winding angle and the elastic moduli of the fiber and matrix have significant influences on interlayer stress transfer. This work reveals a new interlayer stress transfer mechanism in the MCHFS existing widely in biological materials.