Session: 03-13-02: Testing and Characterization

Paper Number: 162408

162408 - High Throughput Measurement of Plastic Strain Accumulation in Polymers 

We report a high-throughput characterization scheme to determine the plastic strain accumulation in polymers. Permanent strain accumulation is semicrystalline polymers such as polyethylene have been a matter of investigation since the material is being considered for balloon films. Semicrystalline polyethylene does not show significant strain softening present in amorphous polymers but rather a viscous transition from elastic to inelastic and/or plastic deformation. Deformation of the amorphous phase such as interlamellar separation and shear may also be crucial plastic deformation or damage mechanism for semi-crystalline polymers. This allows us to investigate effect of applied stress/strain on residual strain and the hardening behaviour. Under small deformation assumption in a creep/relaxation test, the viscoplastic accumulation mostly happens during the loading only and not the creep, as major part of the strains is recovered due to material viscoelasticity. Tscharnuter et al. reported plastic strain accumulation before yielding. A comparative assessment of residual strain from long recovery experiment and the permanent strain from the Perzyna model could give more insights on whether the unrecovered strain is due to creep, initial loading, or both. In the Perzyna viscoplastic model, the hardening behaviour depends on effective stress. Being an overstress model, it captures the yield behaviour efficiently using an overstress function. Using this approach, local yielding can be modeled from the stress-strain histories obtained through high throughput characterization scheme.

The proposed testing scheme shows how the experimental efforts for time-dependent materials could dramatically be reduced by extracting multiple experimental data sets from a single viscoelasticity test. Inclusion of non-contact strain measurement in the scheme is an asset particularly for ultrathin films, to estimate local residual strain. The methodology is effectively demonstrated on the super-pressure balloon film cut in two different orientations. Residual strains are
measured from 24 hrs recovery test and later compared with the viscoplastic Perzyna model predictions. Subsequently, an yield criteria was also established from the proposed characterization scheme. The yield estimation from experiment and model were in good agreement. Subsequently, the permanent strain from high throughput experiments and the model were
compared.

Presenting Author: Kawai Kwok Purdue University

Presenting Author Biography: Kawai Kwok is an Associate Professor of Aeronautics and Astronautics at Purdue University. He received his Ph.D. and M.S. degrees in Aeronautics from the California Institute of Technology. His research focuses on deployable and multifunctional structures, ultra-thin composite materials, and polymer mechanics. He is a recipient of the NSF CAREER award and the NASA Robert H Goddard Exceptional Achievement for Engineering award.