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TECHNICAL PAPERS: Soft Tissue

Experimental Investigation Into the Deep Penetration of Soft Solids by Sharp and Blunt Punches, With Application to the Piercing of Skin

[+] Author and Article Information
Oliver A. Shergold

Department of Engineering, Cambridge University, Trumpington Street, Cambridge, CB2 1PZ, UK

Norman A. Fleck1

Department of Engineering, Cambridge University, Trumpington Street, Cambridge, CB2 1PZ, UK

1

Corresponding author.

J Biomech Eng 127(5), 838-848 (Feb 18, 2005) (11 pages) doi:10.1115/1.1992528 History: Received May 21, 2004; Revised February 18, 2005

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.

Copyright © 2005 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

(a) Steady-state penetration of a soft solid by a flat punch, (b) stress-free configuration after punch removal

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Figure 2

(a) Penetration of a soft solid by a sharp-tipped punch, (b) crack opened to allow punch advance, (c) crack closed after punch removal

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Figure 3

Tensile uniaxial nominal stress versus nominal strain response for two silicone rubbers (44) and human skin (19)

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Figure 4

Sketches of tests used to measure the toughness of a solid, (a) the trouser tear test and (b) the scissor tear test

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Figure 5

(a) Hand-operated instrument for measuring the force required to penetrate the skin in vivo and (b) rubber block penetration test configuration

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Figure 6

Stainless steel punch tips used in the penetration experiments (not to scale) (a) ∅0.3mm hypodermic needle, (b) ∅0.3mm flat-bottomed punch, and (c) ∅2mm sharp tipped needle

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Figure 7

Punch load versus displacement response for the penetration of lower arm skin by a hypodermic needle and penetration of abdomen skin by a flat-bottomed punch

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Figure 8

Typical punch load versus displacement response for the penetration of 10.6mm thick B452 rubber blocks by a sharp-tipped and a flat-bottomed punch. Punch diameter D (mm) indicated.

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Figure 9

Punch load versus displacement response for the penetration of 12.0mm thick Sil8800 rubber blocks by a sharp-tipped and a flat-bottomed punch. Punch diameter D (mm) indicated.

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Figure 10

Penetration of a Sil8800 rubber block by a ∅1mm flat-bottomed punch. Sections are taken at a depth of (a) 0mm, (b) 1.8mm, (c) 9.5mm, and (d) 12.0mm from the front surface of the block.

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Figure 11

Penetration of a B452 rubber block by a ∅1mm flat-bottomed punch. Sections are taken at a depth of (a) 0mm, (b) 4.1mm, (c) 7.1mm, and (d) 10.6mm from the front surface of the block. The column can be seen emerging from the rear surface of the block in picture (d).

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Figure 12

(a) Penetration of the abdomen in vivo by a ∅0.3mm flat-bottomed punch and (b) penetration of the lower arm in vivo by a ∅0.3mm hypodermic needle

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Figure 13

Penetration of a Sil8800 rubber block by a ∅2mm sharp-tipped punch. Sections are taken at a depth of (a) 0mm, (b) 4.5mm, (c) 8.5mm and (d) 12.0mm from the front surface of the block.

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Figure 14

Penetration of a B452 rubber block by a ∅1.0mm sharp-tipped punch. Sections are taken at a depth of (a) 0mm, (b) 3.3mm, (c) 6.5mm and (d) 10.6mm from the front surface of the block.

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Figure 15

Penetration of pig skin in vitro using (a) flat-ended punch, (b) sharp-tipped punch, and (c) hypodermic needle, each of diameter 0.5mm

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Figure 16

Section through a Sil8800 silicone rubber block following penetration by (a) a ∅1.0mm flat-bottomed punch and (b) a ∅1.0mm sharp-tipped punch, and section through a B452 silicone rubber block following penetration by (c) a ∅1.0mm flat-bottomed punch and (d) a ∅1.0mm sharp-tipped punch

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Figure 17

Ring crack diameter 2b versus depth h into silicone rubber block following penetration by a ∅1.0mm flat-bottomed punch

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Figure 18

Crack length 2a versus depth into block following penetration by a sharp-tipped punch, punch diameter D (mm) indicated. Error bars represent maximum and minimum crack lengths from five tests.

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Figure 19

Characteristics of the punch load versus displacement response for the penetration of a soft solid by (a) a sharp-tipped punch and (b) a flat-bottomed punch

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Figure 20

Penetration pressure versus punch diameter for a flat-bottomed and sharp-tipped punch

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Figure 21

pS∕μ versus JIC∕μR for the penetration of a soft solid by a sharp-tipped punch

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