Rate‐dependent self‐healing behavior of an ethylene‐co‐methacrylic acid ionomer under high‐energy impact conditions

Abstract

In this work, the mechanical and the self-healing behaviors of an ethylene-co-methacrylic acid ionomer were investigated in different testing conditions. The self-healing capability was explored by ballistic impact tests at low-velocity, midvelocity, and hypervelocity bullet speed; different experimental conditions such as sample thickness and bullet diameter were examined; in all impact tests, spherical projectiles were used. These experiments, in particular those at low and midspeed, allowed to define a critical ratio between sample thickness and bullet diameter below which full repair was not observed. After ballistic damage, the healing efficiency was evaluated by applying a pressure gradient through tested samples. Subsequently, morphology analysis of the affected areas was made observing all tested samples by scanning electron microscope. This analysis revealed different characteristic features of the damaged zones affected at different projectile speed. Stress–strain curves in uniaxial tension performed at different temperatures and strain rates revealed yield strength and postyield behavior significantly affected by these two parameters. A rise of temperature during high strain rate tests in the viscoplastic deformation region was also detected. This behavior has a strong influence on the self-repair-ing mechanism exhibited by the studied material during high-energy impact tests

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“…In the latter materials, however, the presence of a reinforcing fabric or foam efficiently adherent to ionomer in some cases reduced the self-healing efficiency. The healing mechanism of ionomers is well discussed in the literature [3][4][5][6][7][8][9]. It is the result of a "welding" effect, where the energy dissipation due to material plastic deformation and projectile friction, leads to material melting; the following viscoelastic recovery and material solidification are able to, at least partially, close and seal the damage.…”

“…The speed of bullets, measured using a laser beam, ranged between 700 and 730 m/s. Hypervelocity tests of ionomer plates were carried out up 4000 m/s with a two-stage Light-Gas Gun; aluminium spheres with 1.5 mm diameter were used as bullets [19,20]. Low-velocity (180 m/s) and mid-velocity (400 m/s) impact tests were performed by shooting steel balls of different sizes using an air gun and a shotgun, respectively [19].…”

“…Thermoplastic materials, such as ionomers based on ethylene-co-methacrylic acid copolymers, partially neutralized with sodium or zinc, have shown self-healing behaviour after ballistic impacts [3][4][5][6][7][8][9]. Blends of ionomer with different polymers, yet with selfmending ability, were also investigated [10][11][12][13][14].…”

“…The self-healing ability of plain ethylene-methacrylic acid copolymer, partially neutralized with sodium ions (EMAANa), and multilayer composites based on the same ionomer have been studied by ballistic puncture tests; different conditions were assessed by varying sample thickness, bullet impact velocity (from 180 m/s up to 4000 m/s), bullet shape and diameter [19].…”

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“…Thermoplastic materials, such as ionomers based on ethylene-co-methacrylic acid copolymers, partially neutralized with sodium or zinc, have shown self-healing behaviour after ballistic impacts [9][10][11][12][13][14]. Blends of ionomers with different polymers were also investigated: blending allows to get mechanical properties tuned over a wide range, yet maintaining self-mending ability [15][16][17].…”