DEGRADATION OF THE DEFORMATION CAPACITY UNDER CREEP DUE TO MULTIAXIAL STATES OF STRESS

A method is proposed, which describes the degradation of the uniform true (rupture) strain phi due to the multiaxial stress states in creep. The method is based on the formulas of Siebel which describe the ratios of the uniform true strains in the 3 principle stress directions for any given states of stress: phi-1/phi-2/phi-3 = (sigma-1 - sigma-m)/(sigma-2-sigma-m)/(sigma-3-sigma-m) sigma-m is the hydrostatic stress. These values for different given stress states are normalized by the principle stress sigma-1ZV for uniaxial tension with the same amount as for the multiaxial stress states. From such ratios [sigma-1-sigma-m/sigma-1ZV] it is possible to find out the real uniform true (rupture) strains phi for the different stress states in taking the uniform true (rupture) strain for uniaxial tension, phi-1BZV, as a basis: [GRAPHICS] The comparison of predicted and tested phi-values proves that the formulas describe the toughness-degradations with the right tendency. For multiaxial states of stress it has to be kept in mind that we have two counterbalancing influences which are on the one hand the reduction of the creep rate due to lower nu. Mises equivalent stresses and on the other hand the reduction of the deformation capacity, described above. A model ist proposed to estimate these influences of the states of stress on the creep rupture times.