The understanding of fatigue damage mechanisms of short fiber reinforced thermoplastics are a key issue in order to optimize material processing and propose physically based multiscale fatigue damage models. The presented work aims at a fine description of 3D damage development as observed by synchrotron X-ray microtomography. Damage processes at the micro and mesoscale are fully described in order to extract the elementary damage mechanisms, their sequence and kinetics. The effects of local fiber configuration and orientation are particularly detailed. From observations it is clearly evidenced that cavitation plays a major role in the fatigue damage process as it triggers all elementary damage mechanisms observed at the microscale. It is also shown that a characteristic length appears in the fatigue damage development. This internal length is in the order of magnitude of the spherulite size, suggesting a strong impact of the spherulite size on the fatigue damage development. Finally the effect of local fiber orientation on the micro and meso crack orientation is presented.

Damage mechanisms of reinforced polyamide 6,6 have been studied in 3D through in situ X-ray tomography tensile tests. 3D pictures of the microstructure have been taken during tensile tests to catch damage evolution in the bulk of material. The effects of relative humidity and orientation sampling are particularly investigated in this paper. Main mechanisms have been identified such as fibre failure, debonding, damage at fibre ends and matrix damage (cavitation, fibrillation, damage growth). Qualitative observations reveal that the mechanisms are very sensitive to orientation sampling and relative humidity of the specimen. A specific procedure was developed to propose a quantitative analysis of the results. This analysis shows that identified mechanisms not only have different proportions but also have different kinetics according to relative humidity and orientation sampling of the specimen.