Micromechanics-based thermo-viscoelastic properties prediction of fiber reinforced polymers with graded interphases and slightly weakened interfaces

A three-dimensional (3D) micromechanical study is performed for effective viscoelastic properties prediction of fiber reinforced polymers (FRPs) containing elastic, transversely isotropic/isotropic ellipsoidal fibers surrounded by graded interphases, embedded in an isotropic, viscoelastic matrix. Based on mean-field homogenization theory, an improved double-inclusion model for FRPs considering graded interphase and imperfect interface is established. Applying two-step homogenization technology, elastic-viscoelastic corresponding approach and Time-Temperature Superposition Principle (TTSP), effective thermo-viscoelastic moduli of FRPs are obtained with respect to the grading and geometric parameters. By solving the interface damage tensors numerically, the mathematical errors in the previously used analytical expressions are avoided, and the entire prediction scheme is extended to fillers at any aspect ratio. Comparing the results for composites with 3D randomly dispersed fibers with existing semi-analytical solutions, the accuracy and efficiency of the proposed method is verified. Parameters analyses are conducted, which indicate that the established model and the proposed method should be of great help for predicting mechanical properties of FRPs and designing micro-composites.