Micromechanical modeling of viscoelastic response of GMT composite

Experimental studies have been performed to obtain creep compliance functio ns of polypropylene (PP) and Glass Mat reinforced Thermoplastics (GMT) with PP matrix. It was found that both GMT and PP in the considered loading reg ion may be considered as linear viscoelastic materials. The obtained viscoe lastic compliance functions were successfully used to describe material beh avior in the stress relaxation test. A micromechanical model based on the c orrespondence principle in the Laplace domain was developed to describe the viscoelastic behavior of GMT. This model considers the GMT composite with a given fiber orientation distribution function as consisting of an infinit e number of unidirectional layers with orientations corresponding to this d istribution function. The viscoelastic properties of the unidirectional lay er are calculated using Hashin's concentric cylinder model that uses the ex perimentally determined viscoelastic properties of PP matrix. The predictio ns for GMT have been compared with experimental data. The model predicts ra ther good initial properties of GMT but it gives slightly less time depende nce than compared to experimental data for both relaxation functions and co mpliance. The cause of the difference (debonding) between matrix and fiber, nonuniform fiber spatial distribution, stress concentrations etc.) is disc ussed.