Deformation and Fracture of Disordered Solids: Mechanisms Underlying Macroscopic Behavior

Johns Hopkins University, Baltimore MD

Investigators

Abstract

TECHNICAL SUMMARY This award supports theoretical and computational research and education on the deformation and fracture of disordered solids to determine the microscopic mechanisms underlying macroscopic mechanical response. One focus will be the failure modes of amorphous polymers. Here chain connectivity leads to unusual modes of deformation, including craze formation, necking and pronounced strain hardening. A second focus will be general scaling behavior during steady, quasistatic shear or compression of disordered solids. Traditional engineering models of plastic deformation and fracture are based on macroscopic continuum equations with phenomenological constitutive relations for the spatially averaged response. Experiments provide limited information about the microscopic origins of this response. The PI will perform simulations to provide new insight into the connection between macroscopic mechanics and molecular scale interactions and deformation mechanisms. The connectivity of polymer molecules leads to topological entanglements. Most models of the mechanical properties of amorphous polymers assume that these entanglements act like chemical crosslinks. Tracking the motion of entanglements during deformation will provide a detailed test of these models and new information to serve as a foundation for improving them. The effect of polymer structure and molecular friction on entanglements, stress-strain relations and mode of failure will be studied. Studies of entanglements at polymer interfaces will provide microscopic understanding of polymer welds and friction forces between polymers. Studies of sheared systems have played an important role in developing new understanding of non-equilibrium behavior. Simulations will explore scaling behavior in quasistatic shear or compression of disordered systems. Systems evolve through a series of earthquake like events with a power law distribution of sizes. The scaling and spatio-temporal dynamics of individual events, the conditions that nucleate them, and their correlations over long times and distances will be studied. Specific issues will be how inertia and irreversible damage affect non-equilibrium critical phenomena. This project will contribute to the twenty first century workforce by training students in a wide range of interdisciplinary modeling and computational methods. This training will extend beyond the students supported by the grant to students associated with a local IGERT on "Modeling Complex Systems." A new course on multiscale modeling will be developed in coordination with the IGERT. The course will be aimed at science and engineering students from a range of disciplines and course materials will be shared on the web. Outreach efforts will bring research results to a wider audience in partnership with Johns Hopkins based outreach programs for K-12 students, including the Physics Fair, Youth for Astronomy and Engineering, and the Center for Talented Youth. Animations and web-based modules for demonstrating concepts related to the grant will be developed. Recruitment of new students will be done in collaboration with the above IGERT, which is partnering with women's colleges and other universities with a high percentage of students from underrepresented groups. NONTECHNICAL SUMMARY This award supports theoretical and computational research and education to better understand how materials deform in response to applied stress. The research will focus on materials made from long chain-like molecules that are entangled with each other. The PI aims to perform computer simulations of these amorphous polymer materials to understand how they respond to mechanical stress at a molecular level. The research should elucidate how the chains are tangled and the way that the entanglement changes in a precise mathematical sense. This research addresses fundamental processes that impact many technologies. One thrust will examine general aspects of deformation and fracture that may be relevant to the strength and failure of engineered and natural materials on laboratory and tectonic scales. Another will explore specialized behavior in amorphous polymers that affects their function as adhesives, structural materials, and solid lubricants. An improved understanding of the above systems will aid the design and modeling of structural components and the development of tailored materials with improved properties. The research projects will also serve as a testing ground for new computational modeling techniques that couple very different descriptions of materials. These approaches have the potential to improve modeling of a broad range of complex materials behavior beyond the specific projects addressed in this project. This project will contribute to the twenty first century workforce by training students in a wide range of interdisciplinary modeling and computational methods. This training will extend beyond the students supported by the grant to students associated with a local IGERT on "Modeling Complex Systems." A new course on multiscale modeling will be developed in coordination with the IGERT. The course will be aimed at science and engineering students from a range of disciplines and course materials will be shared on the web. Outreach efforts will bring research results to a wider audience in partnership with Johns Hopkins based outreach programs for K-12 students, including the Physics Fair, Youth for Astronomy and Engineering, and the Center for Talented Youth. Animations and web-based modules for demonstrating concepts related to the grant will be developed. Recruitment of new students will be done in collaboration with the above IGERT, which is partnering with women's colleges and other universities with a high percentage of students from underrepresented groups.