Fracture of fibrous polymers

The fibrous material obtained by drawing or extrusion exhibits a more than linear increase in the axial elastic modulus with draw ratio, a much slower increase in tensile strength and a decrease in strain to break. The reason for such a behavior is found in the existence of structural defects of fibrous material, which is composed of very long and narrow microfibrillar elements. At their ends the axial connection by taut‐tie molecules is almost completely interrupted. Hence the transfer of tensile stress through such a defect is only possible by lateral shift of the load to adjacent microfibrils. The incompleteness of this transfer, the excess straining of the material in the defect area, and the relatively easy axial displacement of the ends of microfibrils favor the local formation of microcracks which grow and coalesce with close‐by microcracks until a critical size crack is formed and the strained sample fails. The process is favored by a high elastic modulus of microfibrils which is practically identical with that of the drawn or extruded sample. Hence the fibrous polymer fails at lower strain the higher its axial elastic modulus, i.e., the higher the draw ratio.