The Effect of the First and Second Stress Invariants (Hydrostatic and Deviatoric Stresses) on the Compressibility Function

Abstract

Experimental work was carried out to investigate the separate roles of the hydrostatic and deviatoric components of stress tensor (Using the First and the second invariants and ). The results were expressed in term of stress dependent shear compliance and apparent compressibility function in the time temperature region of the test (up to seconds at  the region of the relaxation).

The Compressibility Function  showed no significant change, when increasing the hydrostatic stress and keeping the deviatoric stress  constant, and by varying  and keeping   constant the change is about 10  for the stress combination of tension-torsion.

The volume was found to increase with time when increasing   and keeping   constant.

In the case of  and  the deviatoric stress  played the major role. All these effects could be rationalized by the idea of the time dependent free volume. If the free - volume increases with time by increasing  this could explain the difference in the effect of and  on  and explain the creep less than recovery.

There is no difference between the compressibility function in creep and recovery (to within the experimental scatter of ( )).

A small increase in B was detected with time, the maximum difference between the lowest and highest  is