AN INTERMEDIATE MODEL METHOD FOR OBTAINING A DISCRETE RELAXATION SPECTRUM FROM CREEP DATA

A convenient method is described for obtaining a discrete stress relax ation spectrum from linear viscoelastic creep data by means of a three -stage process. In stage one, a discrete retardation spectrum is fitte d to the creep data using a least squares procedure, subject to the co nstraint that the discrete spectrum must be a specified order of polyn omial function of the retardation time. In stage two, the resulting ge neralised Voigt model is solved numerically for an imposed step in str ain, to determine the stress relaxation modulus function of time. In s tage three, a discrete relaxation spectrum is fitted to the calculated stress relaxation modulus function. Although three stages are involve d instead of the usual two, the procedure has been found to have certa in practical advantages. These advantages make it suitable for the gen eration of relaxation spectra needed in viscoelastic stress analyses o f solids, for example by the finite element method. In order to illust rate the proposed procedure it is applied to both artificial data and experimental creep data for poly(methyl methacrylate) at 70 degrees C and at the glass transition.