Chapter 16 Dynamic Fracture-Toughness Testing of a Hierarchically Nano-Structured Solid Kyung-Suk Kim, Hanxun Jin, Tong Jiao, and Rodney J. Clifton Abstract 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 (LISI) for plate impact experiments of dynamic fracture testing. The L-ISI measures the variation of the normal-displacementgradient 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. Keywords Polyurea · Dynamic toughness · Line-image shearing interferometer · Tapping-mode AFM DIC 16.1 Introduction Recently, a nanophase-segregated elastomeric copolymer, polyurea, has gained significant attention on its nano- and micrometer scale mechanisms of dynamic fracture toughening [1, 2]. Bulk polyurea is typically self-assembled to be clustered in hard isocyanate group and soft diamine group phases at nanometer-length scales, and not homogeneous but segregated. As a consequence, polyurea is known to contain a two-phase microstructure consisting of discrete, rod-shaped, the so-called hard domains which are dispersed within a continuous soft matrix. Figure 16.1a shows bright AFM images of 5–10 nm diameter rods of hard isocyanate group embedded in the soft diamine group matrix. The high-resolution image was obtained with a very sharp AFM tip of 2 nm radius, in a tapping mode. However, when a relatively blunt AFM tip of ~20 nm radius is used with noncontact mode, hierarchical clustering of the hard and soft phases with ~200 nm periodicity is revealed, as shown in Fig. 16.1b. The segregated and hierarchically clustered nanostructures are believed to provide peculiar dynamic deformation mechanisms responsible for ultra-high strength of polyurea under high strain-rate and/or pressure loading. For studying the dynamic response of polyurea, much research on the high-strain-rate response has addressed its deformation under dynamic loading without tackling its resistance to failure associated with failure-initiation and crack-propagation processes. However, the survivability of the structural element that the coating is intended to protect depends on its adhesion to the structural component, and the development of a mechanism-based understanding of failure in elastomers and their interfaces with other materials is critically important. K.-S. Kim( ) · H. Jin Nano and Micromechanics Laboratory, School of Engineering, Brown University, Providence, RI, USA e-mail: kyung-suk_kim@brown.edu; hanxun_jin@brown.edu T. Jiao · R. J. Clifton Plate Impact Facility, Prince Laboratory, School of Engineering, Brown University, Providence, RI, USA e-mail: Tong_Jiao@brown.edu; rodney_clifton@brown.edu © The Society for Experimental Mechanics, Inc. 2021 S. Xia et al. (eds.), Fracture, Fatigue, Failure and Damage Evolution, Volume 3, Conference Proceedings of the Society for Experimental Mechanics Series, https://doi.org/10.1007/978-3-030-60959-7_16 97
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