Synchrotron radiation X-ray laminography of internal damage in short-fiber-reinforced polyamide 6 under cyclic tensile loading

Fiber-reinforced plastics are lightweight materials expected to reduce greenhouse gases and the fatigue properties need to be clarified to extend the practical applications. However, the fatigue tests have been limited to the surface or cross-sectional observation of cut specimens. In this study, we conducted non-destructive through-thickness measurements using synchrotron radiation X-ray laminography to investigate the internal fatigue damage behavior in sharply notched carbon fiber (30 wt%)-reinforced polyamide 6 specimens. Under fatigue loading, a large number of voids in the matrix and delaminations at the carbon fiber sides as well as fiber ends were nucleated near the notch tip. Incremental fatigue cycles caused voids coalescence, resulting in the extension of the damaged zone. The unit volume of voids increased locally in severely damaged zone due to coalescence of the voids, while it decreased in adjacent locations because the residual voids were small, which resulted in the larger diversity of unit volume of voids. In the specimen thickness direction, the damage zone was larger near the surfaces in plane-stress condition than inside the specimen in plane-strain condition.