Inverse notch sensitivity: Cracks can make nonwoven fabrics stronger

The strength of materials is always reduced in the presence of notches and cracks and this phenomenon – known as notch sensitivity – is critical in structural design. Good structural materials (ductile metals, elastomers) tend to be notch insensitive, which was considered to be the optimum behavior. Here, we report that inverse notch insensitivity (where the failure stress of the notched specimen is higher than that of the unnotched counterpart) can be achieved in polypropylene nonwoven fabrics. This behavior is only possible because of the peculiar microstructure of nonwoven fabrics, in which fracture of interfiber bonds provides a source of non-linear deformation and leads to a change in the network topology. The former facilitates crack tip blunting, spreading damage in the ligament, while the re-orientation of the fibers perpendicular to the notch plane strengthens the material and improves the maximum load bearing capability.