Impression Testing of Self-Healing Polymers

As part of the BIOSANT program (biologically-inspired smart nanotechnology), scientists at NASA-Langley have identified a “self-healing” plastic that spontaneously closes the hole left by the passage of a bullet. To understand and generalize the phenomenon in question, the mechanical properties responsible for this ability are being explored. Low-rate impression testing was chosen to characterize post-yield material properties, and it turned out that materials that heal following ballistic puncture also show up to 80% healing of the low-rate impression. Preliminary results on the effects of temperature and rate of puncture are presented. Introduction After being struck and completely penetrated by a 9 mm bullet, a 6 mm-thick plate of Dupont’s Surlyn shows only a minor dimple (and sometimes a small projection on the exit side.) The hole is said to spontaneously “self-heal” [1]. A 1996 patent [2] specifies Surlyn 8940 for use in shooting-range targets. Although the linear viscoelastic behavior [1] and projectile impact [3] of this and some closely-related materials have been studied, healing itself is essentially instantaneous and the detailed mechanism remains in doubt. Simpler, laboratory-based tests would be of use, not only in studying the mechanism, but also in screening new, experimental materials (which might be designed to heal under different environmental conditions or possess other desirable properties such as resistance to space radiation). The present study explores the use of a moderate-rate impression test [4] as a characterization method. In principle, a single test could give the modulus of elasticity, the strain-hardening exponent, and the strain-rate sensitivity using very small specimens. As will be shown, the results may also be linked more directly to the healing phenomenon.