Dynamic Response of Additively-Manufactured Polymer Structures

Additive manufacturing (AM) has created a new paradigm in control of structure-property relationships for a wide variety of material classes and applications. The promise of AM lies in tailoring properties through this exquisite topological design and fabrication, and the possibility of "metamaterial" properties not possible through conventional manufacturing. For example, additive manufacturing (AM) techniques have enabled topological tailoring of polymeric structures at the micrometer scale, producing new classes of materials with exquisite control of structure-property relationships. Relevant to mechanical properties under quasi-static (low strain rate) deformation, AM has produced many examples in which control of deformation mechanics and structural instabilities have led to novel properties, such as high strength-to-weight ratios, tailored thermal management, and auxetic deformation behaviors.<br/>Dynamic loading generally refers to loading at intermediate to high strain rates, including high pressure, high strain rate regimes accessed by shockwave compression from high velocity plate impact, blast wave loading, explosive loading, and fragment impact. While many of the applications involving AM materials exploit novel deformation mechanisms at lower rates, such as designed bending or buckling deformations in the case of ligament structures, many of these mechanisms become "overdriven" at the higher strain rates and pressures found in dynamic experiments. One of the aims of our work is to better understand the transition(s) from controlled structural deformation at lower strain rates to dissipative mechanisms at play under shockwave loading.<br/>Here, we will provide a summary of recent experimental examples of the dynamic responses of polymer-based AM structures. This summary will include examples of optimization of shockwave propagation through control of wave interactions at the micrometer-to-centimeter length scales. For example, we recently demonstrated that unprecedented shockwave dissipation could be achieved in fractal-based AM structures which introduce free surfaces, or interfaces, within a critical spacing (or timescale <i>t </i>~ <i>L/2c_l</i>) that is determined by shock strength, and rarefaction (fan) or release wavespeeds; <i>e.g.</i> the material's sound velocity (<i>c_l</i>) at pressure. The dissipative effect is similar to localization phenomena related to "hot spots" in the shock initiation of explosives, but instead of energy localization leading to reactive burn, rarefaction interactions and material deformation can lead rarefaction interactions and reduction of the shock front. Experimental methods like high velocity optical imaging, surface-based optical digital image correlation (DIC), and time-resolved X-ray phase contrast imaging have been used to capture the features of polymer AM structures under dynamic loading. The structural features in the AM materials can themselves be used as Lagrangian tracers for analysis of material strain, localization, and material flow (material velocities, etc.). This allows for direct, quantitative comparison with simulation.<br/>Investigations of the dynamic responses of AM polymers structures have increased measurably over the last 5 years, due in part to the availability of time-resolved <i>in situ</i> measurements such as X-ray phase contrast imaging, and advancements in AM manufacturing techniques. There are opportunities in this area for quantitative analysis of deformation under a range of strain rates, and improved understanding of the regimes where different structural deformation mechanisms are important. There is also a need for approaches to topological optimization for dynamic contexts, such as blast, shock, and fragment impact.