Monday  |  Salon 11  |  09:20 AM–10:00 AM

#16297, KEY: Role of Nonlinear Viscous Interfacial Slip on Failure of Fibrous Composites under Timescales Ranging from Ballistic Impact to Stress-Rupture over Decades

Many engineering applications involve composite materials consisting of bundles of stiff, ultra-strong, elastic fibers closely packed in a nonlinear, viscoelastic matrix. During failure, the matrix and fiber/matrix interface determine how surviving fibers locally share loads shed from neighbors failing at random flaws. The resulting composites are typically very strong with much lower variability than the component fibers, and exhibit far less creep than the matrix itself, or that reflected in progressive debonding and slip at the fiber/matrix interface. Full understanding of the micromechanical and statistical mechanisms that determine the resulting composite behavior remains elusive, particularly given the wide range of loading timescales in various applications.

At one extreme, the failure mechanism may be stress-rupture under a sustained service load, such as in pressurized composite overwrapped pressure vessels (COPVs). Such applications demand ultra-high reliability, i.e., failure probability of one in a billion (i.e., reliability exceeding ‘nine 9s’), over 10 to 100 years. At the other extreme the application may be the ballistic resistance of body armor (worn by police and soldiers) to bullets or metal fragments impacting, say, at 600 m/s into a 5 mm-thick laminate of UHMWPE fiber in a polyurethane matrix. Survival requires that the projectile be stopped within a few millimeters, and over a few microseconds. A third application, with timescales similar to the first, is a helical bridge cable having several hundred highly drawn, steel wires and with potted sockets (e.g., using zinc as a matrix). Such cables were used in the Arecibo Radio Telescope, which tragically collapsed in spectacular fashion on December 1, 2020.

In all these cases critical to overall performance is the behavior of the matrix and its interface with the fibers (or wires), which is subjected to high local shear loading near fiber discontinuities. The speaker has spent much of his career studying the above composite application areas (COPVs and ballistic impact) where timescales of loading span 12 orders of magnitude. He also worked on an Arecibo Radio Telescope cable in the mid 1980’s (including experimental testing of a removed socket). The speaker will discuss the subtleties of these three applications where theoretical models, supported by experimental observations, are key to explaining various behaviors observed.

Stuart Phoenix   Cornell University