It is possible to define complex moduli with TB,PRONY that gives the same displacement response as frequency-dependent stiffness (TB,ELASTIC) and damping (TB,SDAMP). However, the stress results will differ since viscoelasticity considers the complex moduli in its stress calculations – in the ANSYS Help Viewer, see Section 4.7.3.4 “Stress” in “Mechanical APDL > Material Reference > 4. Nonlinear Material Properties > 4.7 Viscoelasticity > 4.7.3 Harmonic Viscoelasticity” for details. With frequency-dependent stiffness and damping, the elastic modulus is considered to be a real quantity, so there will not be any phase difference between stress and strain.