Extreme tensile damage and failure in glassy polymers via dynamic-tensile-extrusion

Dynamic-tensile-extrusion (DTE) is an integrated test technique that allows the study of material deformation at high strain-rates (> 10,000 s -1) and large strains (>1), under hydrostatic tension. This is an important compliment to the more traditional Taylor cylinder impact test, which achieves large strain and high strain-rate deformation, but under hydrostatic compression. Hydrostatic compression is known to suppress many forms of damage in materials. DTE has been previously employed on a number of metal and polymer systems that manifested tensile instabilities. More recently, this technique has explored stable tensile damage in high-density polyethylene (HDPE), which pointed to a pressure-mediated shear damage phenomenon. The current work extends the technique to the behavior of the glassy polymers polymethylmethacrylate (PMMA) and polycarbonate (PC). PMMA was found to undergo unstable brittle fracture at nearly all conditions, and therefore did not yield interpretable experimental results. PC (discussed herein) necked and either failed in a brittle fashion or the neck was arrested prior to failure. In the arrested condition, the neck was seen to become opaque from an apparent accumulation of small-scale damage, and a void nucleated at the centerline. A corkscrew fracture process was observed in PC, though its mechanics are not yet understood. It is worth noting that simulations of pressure-hardening PC indicate that it will not extrude or even neck during DTE without the action of a damage process reducing the flow strength of the material. © The Society for Experimental Mechanics, Inc. 2014.