The modulus of viscoelastic materials varies with excitation frequency.However

during modal analysis of frequency dependent materials

a material evaluation frequency is necessary because stiffness cannot be modified during eigenfrequency procedure.As a result

only those vibration modes are accurate

of which eigenfrequency is close to the material evaluation frequency.In order to obtain vibration modes of solid rocket motor (SRM) using material modulus based on frequency which is the same as the eigenfrequency

an iterative approach was proposed.Results of the iterative technique show that frequency modes obtained from the method are in complete agreement with the eigenfrequency and material evaluation frequency.