Mechanical Engineering Special Seminar

The Jerzy L. Nowinski Lecture
"Mechanics of High-Rate Deformation of Amorphous Polymers"

Friday, March 24, 2006
106 Center for Composite Materials
10:30 AM

Dr. Mary C.Boyce
Department of Mechanical Engineering
Massachusetts Institute of Technology

The mechanics of polymeric materials under conditions of high rate loading has garnered increasing interest in recent years due to the potential role of polymers in ballistic and blast protection materials and systems. Here, a combined experimental and modeling investigation has been performed to understand the mechanical behavior of amorphous polymers at strain rates ranging from 10 -4 to 10 4 s -1. This range in strain rates was achieved in uniaxial tension and compression tests using a dynamic mechanical analyzer (DMA), a servo-hydraulic testing machine, and an aluminum split-Hopkinson pressure bar. DMA tension tests were used to characterize the viscoelastic behavior of these materials, with focus on the rate-dependent shift of material transition temperatures. Uniaxial compression tests on the servo-hydraulic machine (10 -4 to 1 s -1) and the split-Hopkinson pressure bar (10 3 to 10 4 s -1) were used to characterize the rate-dependent yield and post-yield behavior. The polymers were observed to exhibit increased rate sensitivity of yield under the same strain rate/temperature conditions as the b transition of the viscoelastic behavior. A physically based constitutive model for large strain deformation of thermoplastics was then extended to encompass high-rate conditions. The model accounts for the contributions of different molecular motions which become operational and important in different frequency regimes. The new features enable the model to not only capture the transition in the yield behavior, but also accurately predict the post-yield, large strain behavior over a wide range of temperatures and strain rates. Here we demonstrate the model’s ability to accurately predict the finite strain elastic and inelastic behavior of a number of amorphous polymers, including polycarbonate, poly(methyl methacrylate), poly(vinyl chloride) and plasticized poly(vinyl chloride) over a wide range of strain rates and temperatures. The efficacy of the constitutive model is further demonstrated in finite element simulations of dynamic inhomogeneous deformations. Here, Taylor impact tests on various rods of PC with different initial conditions are conducted and imaged using high speed photography for direct comparison to finite element simulations which utilized the constitutive model. Finally, the ability to preferentially influence these different regimes of molecular mobility using nano-scale fillers is demonstrated where blends of POSS nanoparticles with PC as well as with PVC are found to enable manipulation of the rate dependent large strain deformation behavior.

Contributing Colleagues: Mr. Adam Mulliken, Ms. Sharon Soong, Dr. Sai Sarva, Prof. Bob Cohen

Refreshments will be served