Features of high rate deformation and shock loading of polymers

Polymers and polymer composites are used extensively in a variety of automotive, aerospace, and protective armor applications. They have several unique responses under high strain rate and shock loading including compression of free or network volume, viscoelastic response, possible crystalline phase transitions, and chemical decomposition. This presentation will highlight examples of these phenomena for several common engineering polymers as an introduction. In particular, timescales associated with viscoelasticity and irreversible decomposition as a function of loading condition will be discussed. There are several applications in which the unique structural control that additive manufacturing (AM) provides can be used to tailor the mechanical properties of polymers under low-to-dynamic (10³-10⁶ s^-1) strain rates. Examples include lightweight structural components in automotive and aerospace applications, blast and shock mitigating materials for soldier, vehicular, and building protection, and other defense applications. However, there have been few examples comparing low strain rate responses, where cell deformation and layer compaction dominate, to high strain rates relevant to these applications. We have shown previously, for example, that shockwave compression of AM structures gives rise to fundamentally different stress localization and compaction mechanisms leading to perturbation of the initially planar wave. Here, we will also discuss a transition of behaviors underlying the responses of AM structures at varying strain rates. We have studied four AM topologies (simple cubic, body-centered cubic, gyroid and primitive), with polyurethane as parent material, from strain rates of < 1 s^-1 to ~ 10³ s^-1. Comparison of the responses from quasi-static to intermediate (Hopkinson-bar) to high strain rates (shock) will be presented, using optical and X-ray imaging and digital image correlation to quantify the differences.