Modelling the influence of the microstructure on the high cycle fatigue crack initiation in short fibre reinforced thermoplastics

In this work, a new criterion to predict fatigue crack initiation in plain Short Fibre Reinforced Thermoplastics is proposed. Several experimental observations in the literature showed that the initiation of a macro crack results from the accumulation of damage in the matrix at the micro-scale. Accordingly, in the proposed criterion the life to crack initiation is predicted through an effective stress acting in the most critical regions in the matrix (close to the fibre ends and lateral surfaces). More precisely, the effective stress is evaluated within a control volume defined as a certain percentage of the matrix volume subjected to the highest stresses. A multi-scale (macro-meso-micro) procedure is adopted for estimating the local stress fields in the polymer. The criterion was proved to successfully predict the influence of the material microstructure (fibre content and orientation distribution) and load orientation.