Threshold and flow loci were constructed experimentally at 650°C in the axial-shear stress plane for the cobalt-based alloy Haynes 188. These loci suggest that the stress-dependence of the dissipation potential is on the second deviatoric stress invariant, J2, alone. The functional form of the dissipation is critical to associative potential-based viscoplasticity models because it dictates the direction of the inelastic strain rate vector through normality. The directions of the inelastic strain rate vectors for proportional loading were in generally good agreement with the normality rule. Data suitable for characterizing the material parameters of a viscoplasticity model are presented. Serrated yielding was often present for axial, torsional, and combined axial-torsional loading. By applying proportional and nonproportional load histories at identical equivalent strain rates, J2 was found to generally reduce the data to a single curve.
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