There exist large amounts of experimental evidence on stress relaxation for
metals and their alloys, synthetic and natural polymers, glasses and frozen
non-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. All
curves consist of three regions: initial, nearly horizontal, starting at
σ0; central, descending approximately linearly; and final,
corresponding to the internal stress σi = σ(>). We discuss
briefly the experimental evidence as well as the main features of
the cooperative theory which does not involve specific
features of different classes of Materials. The bulk of the paper deals with
computer simulations. Simulation results obtained with the method of
molecular dynamics are reported for ideal metal lattices, Metal lattices
with defects, and for polymeric systems. In agreement with both experiments
and the cooperative theory, the simulated σ (log t) curves exhibit the same
three regions. In agreement with the theory, the slope of the simulated
central part is proportional to the initial effective stress σ0* = σ0 - σi. The time range taken by the
central part is strongly dependent on the defect concentration: the lower
the defect concentration, the shorter the range. IMposition in the beginning
of a high strain ε destroys largely the resistance of a material to
deformation, resulting in low values of the internal stress σo.
Since the cooperative theory assumes for particles (atoms, polymer chain
segments) the existence of two states, unrelaxed and relaxed, and has a
formal connection to the Bose-Einstein (B-E) distribution, we first simulate
B-E systems, recording the formation of relaxed clusters of particles of
different sizes. Differences in cluster sizes predicted from a B-E Model and
those obtained from the simulations are recorded and analyzed. On the joint
basis of experimental, theoretical