This award supports fundamental research to study how the strength and toughness of semi-crystalline polymers can be enhanced to design lightweight and tear-resistant packaging. Semi-crystalline polymers are widely used in the packaging industry for their easy availability and low price. Yielding and strain hardening are two key characteristics of such polymers. We hypothesize that laminated films that combine materials with differing levels of yield strength and strain hardening will have superior strength and toughness. The research would have direct impacts on the packaging industry in developing high-performance packaging solutions using materials that are already used commercially. The research would also reduce the amount of materials in these packaging systems and therefore their environmental impacts. In addition, other application areas include biological materials, which are often layered and often strain hardening, and metal-plastic composites. Graduate student training and outreach to undergraduate and pre-college students will be integrated with the research.
Multilayered plastics are ubiquitous as packaging materials, yet their large strain behavior is very poorly understood. This lack of knowledge is in sharp contrast of numerous books on small-deformation mechanics and fracture mechanics of composites. That literature gives little or no insight on the effects of severe strain hardening, which appear only at large strain. We hypothesize that bonding yielding layers to strain hardening layers will (1) increase in strength and ultimate strain of laminate films, and (2) improve flaw tolerance and fracture toughness of the laminate films. The research will include fundamental and complementary computational and experimental studies to test these hypotheses. On the computational side we will use custom nonlinear finite element framework with a new constitutive model developed for yielding and strain hardening plastics. On the experimental side, we will work with commercial plastics that range from being elastomeric to highly yield-like. The experiments and computations will be tightly integrated: initial experiments will determine the true stress-true strain behavior of the polymers, which will then be fed to the simulations to make predictions, which will then be tested experimentally.
