Chronographs were used to measure initial and final bullet velocity. 223 caliber semiautomatic rifle from a distance of 23 meters at various temperatures. 7.6 cm x 7.6 cm panels of the different materials were prepared and ballistic testing was performed at various temperatures. Mechanical properties were assessed by a Sintech 2W instron according to ASTM D1708 or D638 at crosshead speeds of 5.08 cm/min. The thermal properties of the polymers were characterized by Differential Scanning Calorimetry (DSC) and Dynamic Mechanical Analysis (DMA). The polymers were selected based on chemical structure, tensile strengths, tensile moduli, glass transition temperature, melting temperatures, and impact strength. Table 1 lists the commercially polymers studied here, together with their physical properties. 1,2 The temperature increase produces a localized flow state and the melt elasticity to snap back thus sealing the hole. 1,2 Previous ballistic testing studies revealed that Surlyn materials warmed up to a temperature approx.98 C during projectile puncture (3 C higher than it s melting temperature). Self-healing behavior occurs following puncture, whereby energy must be transferred to the material during impact both elastically and inelastically, thus establishing two requirements for puncture healing to occur: a.) The need for the puncture event to produce a local melt state in the polymer material and b.) The molten material has to have sufficient melt elasticity to snap back and close the hole. Projectile penetration increases the temperature in the vicinity of the impact. Puncture healing in these materials is dependent upon how the combination of a polymer's viscoelastic properties responds to the energy input resulting from the puncture event.
Figure 1 illustrates the puncture healing concept. In this presentation, we report on a suite of polymeric materials possessing this characteristic. These systems have demonstrated healing capability following penetration of fast moving projectiles - velocities that range from 9 mm bullets shot from a gun (approx.330 m/sec) to close to micrometeoroid debris velocities of 4800 m/sec.
The materials being studied are those capable of instantaneous puncture healing, providing a mechanism for mechanical property retention in lightweight structures. This type of material also has the potential to improve damage tolerance in load bearing structures to enhance vehicle health and aircraft durability. In particular, puncture-healing upon impact has the potential to mitigate significant damage caused by high velocity micrometeoroid impacts. Recent Advances in Thermoplastic Puncture-Healing Polymers Self-healing materials provide a route for enhanced damage tolerance in materials for aerospace applications.
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