Experimental investigation into the deep penetration of soft solids by sharp and blunt punches, with application to the piercing of skin.

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, while 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, while 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 flat-bottomed 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 [Proc. R. Soc. London, Ser. A (in press)] who proposed models for the penetration of a soft solid by a sharp-tipped and flat-bottomed punch.