Time-resolved spectroscopic characterization of ballistic impact events in polymer and nanocomposite materials

Abstract

A detailed understanding of how materials respond to ballistic shock-loading is critical for the design and development of new protective materials. However, the nonlinear viscoelastic deformation present in polymers and nanocomposites during and immediately following a ballistic impact event is not currently well understood. The dynamic mechanical responses of materials experiencing ballistic shock-loading conditions are quite complex, with large amplitude compressions resulting in strain rates in excess of 106 s-1 and pressures exceeding several GPa. Historically, if one wants to study materials under ballistic shock loading conditions, a gas gun apparatus is necessary to generate appropriate high strain rate events. However, advances in high power ultra-fast laser amplifier systems have opened the possibility of optically generating ballistic shocks which are comparable to a shock wave generated by gas gun apparatus. Time-resolved mechanical property information, such as elastic modulus, bulk modulus, shear modulus, and Poisson's ratio are measured using impulsive stimulated thermal scattering, a laser-based photoacoustic technique.

(cont.) A series of polymeric and polymer based nanocomposite material systems are studied, including multilayered thin films of alternating layers of polymer and hard nanoparticles, un-annealed and fully annealed poly(methyl methacrylate) (PMMA)-polyisoprene block copolymer thin films, and polyhedral oligomeric silsesquioxane (POSS) doped PMMA thin films. The experimental results on these materials clearly demonstrate that this technique is sensitive enough to measure mechanical property differences in samples with only small compositional or structural changes. Since the data can be acquired in real time, or a single shot basis, the measurement is compatible with laser shock loading of the sample. This is demonstrated by preliminary experimental results.