Abstract

The determination of the critical puncture force required to pierce a soft material is key to improve surgical technology (e.g. robotic surgery), manufacturing (e.g. food processing) and in-situ material characterization. This seminar discusses a mechanical theory able to calculate the critical depth  and force  required to insert a cylindrical needle with a spherical tip into a hyper elastic material. Needle insertion occurs as a mechanical instability, by which the needle-specimen system snaps between the ‘indentation’ configuration and the ‘penetration’ one, which then becomes energetically favoured. The model considers quasi-static indentation, thus neglects rate-dependent behaviour in the material. It also considers frictionless contact, and the cut material is assumed to be incompressible. Both  and  are functions of the toughness and shear modulus of the cut material, and of the radius of the needle. The scaling relations obtained from the model are then compared against experiments, giving good agreement. To account for frictional and adhesive contact between needle and specimen, the model is extended to correlate interfacial properties with the evolution of the puncture force with penetration depth. The extended model is again compared against experiments to provide validation, and shows that the penetration of tough and soft materials is controlled by friction, while for brittle and stiff materials adhesion prevails. Finally, we analyse the role of volumetric compressibility in puncture, and discover that tough and soft materials develop higher puncture resistance when compressible (lower bulk modulus).