Compressive failure of UD-CFRP containing void defects: In situ SEM microanalysis

Here we present an in situ scanning electron microscopy (SEM) investigation of the compressive failure of unidirectional (UD) carbon fibre reinforced polymer (CFRP) composites with varying pre-existing void content. The experiments were carried out within a dual beam microscope, which couples a SEM with a focused ion beam (FIB), allowing sub-surface investigations of damage. In these tests, the specimen is monitored during the entire loading history, allowing the correlation of microstructural changes and the evolving load-displacement behaviour. Therefore, loading characteristics can be linked directly to failure events. Observations of the sequence of events leading to failure showed direct fibre deflection into a kinked shape eventually followed by fibre fracture. Failure of void-containing CFRP was shown to depend on the void shape as well as the proximity of the void to the kink band. In some cases voids stopped the propagation of kink bands, while in other cases the void caused the kink to deflect in a new direction. The failure structure was observed to change with time, both during hold-load segments as well as after unloading. Through cross-sectional ion beam milling in the unloaded state, the sub-surface damage was observed and shown to be similar to that observed at the surface.