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 .