What pressure is the breaking point of human skin?

Experimental Investigation Into the Deep Penetration of Soft


Solids by Sharp and Blunt Punches, With Application to the


Piercing of Skin


Oliver A. Shergold and Norman A. Fleck*


Cambridge University Engineering Dept., Trumpington St., Cambridge, CB2 1PZ, UK


* Author for correspondence


Abstract


An experimental study has been conducted on the penetration of silicone rubbers and human skin in vivo by sharp-tipped and flat-bottomed cylindrical punches. A penetrometer was developed to measure the penetration of human skin in vivo, whilst a conventional screw-driven testing machine was used to penetrate the silicone rubbers. The experiments reveal that the penetration mechanism of a soft solid depends upon the punch tip geometry: a sharp tipped punch penetrates by the formation and wedging open of a mode I planar crack, whilst a flat-bottomed punch penetrates by the growth of a mode II ring crack.


The planar crack advances with the punch, and friction along the flanks of the punch leads to a rising load versus displacement response. In contrast, the flat-bottomed punch penetrates by jerky crack advance and the load on the punch is unsteady. The average penetration pressure on the shank cross-section of a flatbottomed punch exceeds that for a sharp-tipped punch of the same diameter. In addition, the penetration pressure decreases as the diameter of the sharp-tipped punch increases. These findings are in broad agreement with the predictions of Shergold and Fleck (Proceedings of the Royal Society, London, Series A, 2004), who proposed models for the penetration of a soft solid by a sharp-tipped and flat-bottomed punch. Keywords: deep penetration, skin puncture, silicone rubber, fracture mechanics, soft solids














1 INTRODUCTION


The deep penetration of a soft solid by a punch is of widespread technological importance, with applications ranging from the piercing of mammalian skin by a hypodermic needle (or by a liquid jet) in administering an injection, to the failure of rubber seals or tires by the penetration of a foreign body, such as a nail. The dependence of skin perforation upon the mechanical properties of skin, and the shape of the penetrator, is also relevant to the function and evolution of mammalian dentition: the successful predator must have sufficiently strong jaws and sharp teeth to cause skin perforation. And in remote robotic surgery, as well as in training simulators for surgical techniques, it is important to quantify the resistance of tissue to penetration [1].





Deep penetration models


The penetration mechanism observed for soft solids is different from that for strong, ductile solids such as metals, soils and polymers. Deep penetration of strong solids involves radial expansion of material at the penetrator tip [2, 3]. Bishop et al. [2] modelled penetration by the expansion of a cavity in an elasticideally plastic solid, and argued that the penetration pressure is comparable to the cavitation pressure pc, as defined by the pressure to expand the cavity from zero initial radius to a finite final radius. They showed that the cavitation pressure for an expanding spherical cavity is close to that for a cylindrical cavity and so the precise details of the cavity shape are relatively unimportant in the prediction of the penetration pressure. Typically, for metals pc is on the order of 4-5 times the uniaxial yield strength, depending upon the yield strain and the strain hardening rate.





Despite the ubiquitous nature of soft solid penetration, the existing literature provides little insight into the underlying mechanisms of penetration. A limited number of experimental studies indicate that the deep penetration of skin and rubber involves cracking of the soft solid, followed by substantial reversible deformation [4-6]. These studies also suggest that the crack geometry is sensitive to the punch tip geometry and to the material properties of the penetrated solid.





Stevenson and Abmalek [6] showed that a cylindrical, flat-bottomed punch of radius R penetrates natural rubbers by the formation of a mode II ring crack that propagates ahead of the penetrator tip, as shown in Figure 1a. The propagating ring crack forms a column, with an undeformed diameter of 2b and height l , as shown in Figure 1b. In this paper we shall demonstrate that a sharp-tipped punch penetrates silicone rubber and skin by the formation of a planar mode I crack ahead of the tip as shown in Figure 2a. The crack faces are wedged open by the advancing punch (Figure 2b), but on punch removal the planar crack closes (Figure 2c). In this final, relaxed configuration the crack length is 2a.