Monday  |  Pine/Spruce  |  09:20 AM–09:40 AM

#17744, Multimode Shock Loading of Polyurea Elastomers

Due to applied stresses, materials responding to hypervelocity projectiles undergo multiscale deformations. However, state-of-the-art experimental setups can apply a uniaxial state of stress to simplify post-processing analyses, limiting the understanding of material response in realistic loading scenarios. Therefore, this research developed an ultrahigh strain rate loading apparatus using laser-induced shock waves capable of concurrently submitting polymers to combined in-plane and out-of-plane normal and shear stresses to elucidate the intrinsic failure mechanisms while evading time-dependent and plastic deformation processes. To achieve multiaxial shock loading, wave-solids interactions based on elastodynamics were leveraged to convert the initial pressure wave mode into pressure-shear stress waves. Furthermore, Stoneley waves fully engulfed the sample thickness, submitting the polymer film to out-of-plane bending waves. This setup was augmented and synchronized with terahertz time-domain spectroscopy to probe stress-induced conformational changes. Polymers are generally transparent to terahertz waves, marking the suitability of this part of
the electromagnetic spectrum for non-invasive, non-destructive, and in-situ spectroscopic
characterization. This experimental work used polyurea elastomer as a model material system
to represent the general subclass of elastomeric polymers, given its strategic utility in impact-
resistant and shock-tolerant applications. Aromatic polyurea films were deposited on the
hypotenuse of a right-angle prism, where laser-induced pressure waves underwent mode
conversion into pressure-shear-bending stress waves, submitting the polyurea samples to
multiaxial loading. Finite element simulations were used to calculate the stress state within
the sample based on the illumination energy of the high-energy laser. Several failure modes
were observed and visualized using optical and electron microscopies by examining the
surface and cross-sections of the shock-loaded samples. The results of time and frequency
domain analyses of the terahertz signals coincided with the microscopically observed failure
modes while revealing stress-induced conformational changes. The outcomes of this research
report the nanoscale and molecular-scale failure modes of polymers submitted to ultrahigh
strain rate loading. Future research will explore molecular dynamic simulations to associate
the experimental results with specific conformational changes.

George Youssef   San Diego State University
Amritesh Kumar   San Diego State University