Understanding the failure behavior of polymers subjected to an ultrahigh strain rate (UHSR) impact is crucial for their applications in any protective shielding. But little is known about how polymers respond to UHSR events at the macroscale, or what effect their chemical makeups and morphology contribute. This dissertation aims to answer these questions by characterizing the responses of polymers subjected to UHSRs, investigating how the polymer molecular architecture and morphologies alter the macroscopic response during UHSRs via hypervelocity impact (HVI), linking the behaviors of UHSR events between the macro- and nano-length scales, and determining the consequences of UHSR impacts on polymer chains. Macroscale UHSR impacts are conducted using a two-stage light gas gun (2SLGG) to induce an HVI. Different molecular weights and thicknesses of polycarbonate were considered. The HVI behavior of polycarbonate is characterized using both real-time and postmortem techniques. The response depends on target thickness and impact velocity (vi). However, negligible difference is observed between the HVI results for the two differing entanglement densities. These contrasts previous conclusions drawn on the nanoscale during UHSR impacts which capture an increase in the energy arrested from the projectile with increasing entanglement density. To link the UHSR phenomena from nano to macroscale, laser-induced projectile impact testing (LIPIT) is conducted on polymethyl methacrylate (PMMA) thin films on the nanoscale in addition to ballistic and 2SLGG impacts at macroscale. Applying Buckingham-Π theorem, scaling relationships for the minimum perforation velocity and the residual velocity across these length scales were developed. It is shown that the ratios between target thickness to projectile radius, between projectile and target density, and the velocity of the compressive stress wave traveling through the target are the governing parameters for the UHSR responses of polymers across theses length scales. The effect UHSRs have on the polymer is investigated via ex-situ analysis by capturing polymer debris using a custom-built debris catcher. Different material-vi combinations are examined. X-ray diffraction and differential scanning calorimetry are used to characterize the HVI debris. Evidence of char was found within the debris. This dissertation advances the knowledge regarding the failure behavior of polymer materials subjected to UHSRs.
| Identifer | oai:union.ndltd.org:MSSTATE/oai:scholarsjunction.msstate.edu:td-7097 |
| Date | 10 May 2024 |
| Creators | Callahan, Kyle Richard |
| Publisher | Scholars Junction |
| Source Sets | Mississippi State University |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | Theses and Dissertations |
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