Non-linear Viscoelastic-Viscoplastic Constitutive Modelling of Creep in Continuous Fibre Glass Mat Thermoplastic Composites

The creep behavior of a continuous fiber random glass mat thermoplastic (GMT) composite has been characterized in this paper. Short term. creep tests consisting of 30 minutes creep followed by recovery were conducted at 14 stress levels over the stress range of 20 to 60 MPa on a single specimen. Statistical analysis showed the material to be linear viscoelastic up-to 20 MPa. However, plastic strains were observed in these tests especially at higher stress levels, which indicated the need for tests on virgin specimens. Long term creep tests over the stress range of 20 to 80 MPa were conducted in increments of 10 MPa to obtain a non-linear viscoelastic viscoplastic constitutive model. Findley's model which is the reduced form of the Schapery non-linear viscoelastic model was found to be sufficient to model the viscoelastic model, while the viscoplastic strains were modeled using the Zapas and Crissman viscoplastic model. A parameter estimation method for such a constitutive model has been developed. The model predictions were found to be in good agreement with the average experimental curves.