Effects of stress levels and fiber orientations on creep behavior of CGF/PP composite and numerical prediction of Burgers model parameters

In this study, the short-term (15 min) tensile creep behaviors of polypropylene (PP) at different temperature and stress levels were examined by DMA creep model test, followed by the characterization on the long-term tensile creep behavior (10 h) continuous glass fiber reinforced polypropylene composites (CGF/PP) at the different stress levels and fiber orientations. The Burgers viscoelastic model was adopted to simulate the materials creep curves and pertinent model parameters associated with the stress levels and fiber orientations were derived. The results show that, with the rise of loaded stress, the creep compliance and steady-state creep rate of PP and unidirectional CGF/PP laminate both increase significantly, and the creep modulus retention rate decrease obviously, demonstrating that the creep behaviors of CGF/PP under low stress levels is dependent on the viscoelastic properties of PP matrix. The tensile-shear coupling effects occur in the loaded angle from 0° to 90° for the off-axis tension with a stress level of 30%, specifically in the 45° where steady-state creep rate and the creep deformation of composites exhibit maximum values. The derived numerical model by means of four element Burgers viscoelastic model to fit the creep curves in different conditions matches well with the experimental data, with a correlation coefficients of 0.99 between them. The numerical clearly illustrated the stress and fiber orientation dependence for the pertinent model parameters. The numerical formula of model parameters was established. The estimated tensile creep curve in the 0° fiber direction at 10 MPa and the estimated off-axis tensile creep curve in the 45° off-axis fiber direction at 30% of stress level are nearly identical with the experimental curve, showing the reliability of the derived numerical model in this paper.