A model to describe the cyclic anisotropic mechanical behavior of short fiber-reinforced thermoplastics

Due to the injection molding process, short fiber-reinforced thermoplastic composites show a complex fiber orientation distribution and, as a consequence, an overall anisotropic mechanical behavior. The monotonic and cyclic mechanical behavior of PolyEtherEtherKetone thermoplastic reinforced with 30 wt.% of short carbon fibers was characterized through a series of tests generating various complex loading histories (loading–unloading with creep or recovery steps, cyclic loading with various stress amplitudes) performed at room temperature on samples with various homogeneous and heterogeneous fiber orientation distributions. A three-dimensional model relying on a thermodynamic framework was then developed to represent the anisotropic mechanical behavior of the material, including elastic, viscoelastic, and plastic phenomena. Relevant constitutive laws were defined to describe the phenomena within wide ranges of loading rates and levels, with a limited number of parameters. Elastic anisotropy and plastic anisotropy were naturally described by using a two-step homogenization method and a Hill-like equivalent stress taking into account the fiber orientation distribution. The model was implemented into a finite element code to be able to simulate the response of complex parts with a heterogeneous fiber orientation distribution subjected to a heterogeneous loading. Model parameters were identified by applying a robust and original approach relying on a limited number of relevant experiments. The prediction capability of the model was demonstrated by simulating several types of tests not used for the identification, covering a wide range of monotonic and cyclic, homogeneous and heterogeneous, loading conditions, for various simple and complex fiber orientation distributions. In particular, the model is shown to be able to predict the energy dissipated in the material when subjected to cyclic loading.