  An unusual type of mechanical behavior was registered while studying the
swollen hydrogel compositions "cellulose-polyacrylamide" in the conditions of
multiple cyclic compression tests with the broad variation of the deformation
speed. While increasing the deformation speed the clearly seen inversion of the
positions of compression and decompression parts of the cyclic stress-strain
curves was detected. While carrying out the cyclic compression tests with
relatively low deformation speed (about 100-200 % of the initial sample's
height per minute) the well defined hysteresis of the stress-strain curve can
de seen and in these conditions the decompression part of the curve is situated
inferior the part corresponding to compression. But while increasing the speed
of the deformation the tendency to the progressive approach of the compression
and decompression curves to each other is clearly seen. This effect results in
the full disappearance of the hysteresis at some value of the deformation
speed: the decompression curve coincides with the compression curve. Along with
the further increase of the deformation speed the hysteresis appears again but
the curve corresponding to compression process is situated inferior the curve
corresponding to decompression: the "inversion" of the hysteresis was detected.
The precise character of this process depends upon the stiffness of the
hydrogel under study. Up to date the convincing explanation of this effect can
not been put forward. The authors can only present some hypotheses to explain
this phenomenon.
