KINETICS OF STRESS RELAXATION AND CREEP EXEMPLIFIED BY A SAMPLE OF LOW CRYSTALLINITY POLYETHYLENE
时间:2005-04-12
Ke Hong, Ankur Rastogi and Gert Strobl
Physikalisches Institut, Albert-Ludwigs-Universität Freiburg
Hermann-Herder-Str.3, 79104 Freiburg, Germany
Email: hong@physik.uni-freiburg.de
Stress relaxation and creep behavior of a low crystallinity polyethylene was studied with the aim of understanding the kinetics of both processes. The true stress - true strain dependencies were obtained with an INSTRON 4301 tensile testing machine. The material under investigation was a polyethylene vinylacetate copolymer with a crystallinity of 33%.
In stress relaxation experiments the sample was stretched at a constant strain rate to a certain strain. Then, keeping the strain fixed, stress was allowed to relax. Stress relaxation followed a logarithmic time-law until a final value was reached. The total amount of relaxed stress represents the contribution of the viscous stress. A subtraction of the viscous stress from the measured stress yields the true stress-true strain relationship which would be obtained in the limit of zero strain rate. This "quasi-static" stress-strain dependence includes the contribution of the , amorphous network and the stress transferred by the skeleton of crystals.
Creep measurements were carried out by stretching the sample at a constant strain rate to a certain stress, thereafter the stress was kept fixed. The measurements showed that creep also follows a logarithmic time-law. During creep the viscous stress decreases. The loss of viscous stress is compensated by the increase of the stress contributed by the network and skeleton. The kinetics of stress relaxation and creep are related to each other, as can be shown by model considerations.
References:
1. Ke Hong, Ankur Rastogi and Gert Strobl, A model treating tensile deformation of semi- crystalline polymers Ⅰ. Quasi-static stress-strain relationship and viscous stress determined for a sample of polyethylene, submitted to Macromolecules
2. Ke Hong, Ankur Rastogi and Gert Strobl, A model treating tensile deformation of semi- crystalline polymers Ⅱ. Static elastic moduli and creep parameters derived for a sample of polyethylene, submitted to Macromolecules
论文来源:PP’2004 Dail international symposium on Polymer Physics Preprints,june 1-5,2004,Dail,china
