16. Dynamic Fracture-Toughness Testing of a Hierarchically Nano-Structured Solid

The polyurea coating is found very useful in strengthening structures ranging from helmets to concrete structures under impact or blast loading. We believe that the hierarchical architecture of nano and microstructures is the bases of the strengthening mechanism, which provides scale-dependent stress laxation and energy dissipation. Here, a challenge is to characterize the strengthening mechanisms not only in the bulk of the copolymer but also at the coating/substrate interface. To this end, we have found that the tapping-mode images of an atomic-force-microscope (AFM) are ideal markers for digital image correlation (DIC) analysis of nano/micro-scale deformation. The tapping-mode images typically exhibit clustered hierarchical structures of hard and soft domains that can trace multiscale deformation mechanisms. To study the role of the hierarchical deformation mechanisms in dynamic toughening, we have developed a line-image shearing interferometer (L-ISI) for plate impact experiments of dynamic fracture testing. The L-ISI measures the variation of the normal-displacement-gradient over time along a line on the back surface of a pre-cracked specimen loaded by plate impact. The time history of the displacement gradient forms fringes on the streak-camera image, and the fringes are inverted to determine the time history of the crack speed and the dynamic toughness.