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Stress Relaxation In Metals And Polymers: Theory, Experiment AndComputer Simulations

Published online by Cambridge University Press:  15 February 2011

Witold Brostow ,
Josef Kubát  and
Michael J. Kubát
Witold Brostow
Affiliation:
University of North Texas, Center for Materials Characterization, and Department of Physics, Denton, TX 76203-5308
Josef Kubát
Affiliation:
Chalmers University of Technology, Department of Polymeric Materials, 412–96 Gothenburg, Sweden
Michael J. Kubát
Affiliation:
The Royal Institute of Technology, Department of Polymer Technology, 100–44 Stockholm, Sweden
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Abstract

There exist large amounts of experimental evidence on stress relaxation formetals and their alloys, synthetic and natural polymers, glasses and frozennon-polymeric organic liquids. The results, typically presented as curves a(log t) of relaxation of stress aas a function of logarithmic time t,exhibit common features, apparently independent of the type of Material. Allcurves consist of three regions: initial, nearly horizontal, starting at σ0; central, descending approximately linearly; and final,corresponding to the internal stress σi = σ(>). We discussbriefly the experimental evidence as well as the main features of the cooperative theory which does not involve specificfeatures of different classes of Materials. The bulk of the paper deals withcomputer simulations. Simulation results obtained with the method ofmolecular dynamics are reported for ideal metal lattices, Metal latticeswith defects, and for polymeric systems. In agreement with both experimentsand the cooperative theory, the simulated σ (log t) curves exhibit the samethree regions. In agreement with the theory, the slope of the simulatedcentral part is proportional to the initial effective stress σ0* = σ0 - σi. The time range taken by thecentral part is strongly dependent on the defect concentration: the lowerthe defect concentration, the shorter the range. IMposition in the beginningof a high strain ε destroys largely the resistance of a material todeformation, resulting in low values of the internal stress σo.Since the cooperative theory assumes for particles (atoms, polymer chainsegments) the existence of two states, unrelaxed and relaxed, and has aformal connection to the Bose-Einstein (B-E) distribution, we first simulateB-E systems, recording the formation of relaxed clusters of particles ofdifferent sizes. Differences in cluster sizes predicted from a B-E Model andthose obtained from the simulations are recorded and analyzed. On the jointbasis of experimental, theoretical

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 321: Symposium E – Crystallization and Related Phenomena in Amorphous Materials—Ceramics, Metals, Polymers, and Semiconductors , 1993 , 99
Copyright
Copyright © Materials Research Society 1994

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