Viscoelastic-viscoplastic homogenization of short glass-fiber reinforced polyamide composites (PA66/GF) with progressive interphase and matrix damage: New developments and experimental validation

Abstract In this paper, an original probabilistic micromechanics damage framework involving multi-deformation mechanisms, based on the modified Mori-Tanaka and Transformation Field Analysis (MT-TFA) techniques, is developed to predict monotonic and oligocyclic stress-strain responses in short fiber-reinforced polyamide composites. The proposed model allows simulating actual injection-induced fiber arrangement, which is characterized by arbitrary fractions of randomly oriented fibers distributed in the laminate plane. Furthermore, the modified MT-TFA approach employs a phenomenological model consisting of 4 K-Voigt branches and a viscoplastic branch, formulated under the thermodynamics framework, to describe the rate-dependent viscoelastic-viscoplastic deformation and the ductile damage of the polymer matrix phase. In addition, the Weibull probabilistic density function is utilized to simulate initiation and coalescence of the void-type discrete damage in the vicinity of the fiber/matrix interphase, induced by the fiber/matrix debonding as observed experimentally. The parameters of the developed model are calibrated against the experimental response of glass/polyamide (PA66/GF35) composites via uniaxial loading/unloading tests, by taking into account the actual fiber orientation density function (ODF). The reliability and efficiency of the modified Mori-Tanaka and TFA scheme are assessed vis-a-vis the separate and hold-out experimental data subjected to uniaxial and oligocyclic loading at various loading rates. Progressive matrix and interphase damage are compared in support of the modified MT-TFA technique's capabilities to capture the experimentally observed damage mechanisms. To accurately capture the experimental response, the progressive degradation of the load transfer between the fiber and matrix phases is introduced through a reduction of the active fiber length. The latter is introduced by considering the effect of the interphase void-damage content. The new mean-field formulation provides accurate predictions of the overall response under complex loading paths. It can be combined with other techniques in our future work, such as cycle-jump, towards simulating high-cycle fatigue damage in short-fiber composite structures.