Specimen Design for Creep Characterization Under Multiaxial Stress

This paper describes part of an ongoing study to develop a simple tensile test which will maximize the effects of hydrostatic constraint. The test for such purposes is the notched bar test. Two notch geometries are in common use, the ASTM Standard notch, and the Bridgman blunt notch. Both of these tests have shortcomings, which are described in the paper. Alternative geometries, including notches, plane strain holes and slots have been evaluated, using the ratio of hydrostatic stress to Mises stress in a bar made of an elastic, perfectly plastic material. Examples are given of the stress evolution in selected geometries under creep according to a simple Bailey/Norton power law model, and comparison is made with the behavior when a more complex material constitutive law is used, which includes continuum creep damage. The model used in this case is a simplified version of the MPC Omega model, described in API 579 [1]. Since creep calculations involving damage are both computationally intensive and difficult to carry to completion due to numerical convergence problems, approximate methods of predicting specimen behavior under such complex material conditions is being explored. One promising method, based on isochronous stress/strain curves is described and the results compared with detailed predictions using a more accurate constitutive model.