Understanding the Mechanics of Crazing through Molecular Dynamics Simulations
Sumit Basu (Indian Institute of Technology), Sudarkodi V (Indian Institute of Technology Kanpur)
From Atomistics to Reality: Spanning Scales in Simulations and Experiments Symposium A
Wed 3:10 - 4:30
CIT 227
Crazing is an important failure mechanism in glassy amorphous polymers. Crazes are crack-like defects whose surfaces are bridged by numerous load bearing fibrils. The fibrils are believed to be drawn out of localized regions rich in micro-voids. With loading, the fibrils elongate (and the craze widens) under almost constant opening stress. Eventually, they break and the craze graduates to a crack. Crazing involves intense localized plasticity prior to failure and hence, controlling craze nucleation, growth and breakdown are potential strategies for increasing the fracture toughness of these materials. Several aspects of the mechanics of a craze are not well understood. These include the origin and size of the initiating micro-voids, the connection between the structure of the underlying entanglement network and craze growth and the final method of craze breakdown. Craze growth seems to be intimately linked to the entanglement density though both scission of chains and disentanglement are believed to play a role in the final breakdown. In the present work, we attempt to answer the above questions through carefully designed Molecular Dynamics (MD) simulations on large macromolecular ensembles. Our results indicate that initial voids of the order of a few tens of nanometers are sufficient to initiate fibrillation. In fact, the size of these voids may be smaller in situations where a large hydrostatic stress is imposed. Local regions of low entanglement density serve as ideal nucleating sites. Craze growth occurs through few ingenuous mechanisms of disentanglement that can operate even with low chain mobility. Local slippage of entanglements leading to depletion of entanglements over some parts of a chain and crowding at others is also possible. Scission of chains does not seem to play a significant role during growth but attains importance close to breakdown. Final failure however, occurs by the disentanglement of short chain segments.