Compression–After–Impact Strength and Surface Morphology in Toughened Composite Materials

Interleaved toughening is a promising approach of improving fracture toughness in composites. Past observations of toughened particles have been conducted using the Optical Microscopy or Scanning Electron Microscopy for localized microstructures at a specific location; however, these methods may not be able to provide a 3–D, birds–eye view of the global particle distribution on a composite interlaminar surface. In this paper, we discovered an interesting optical phenomenon which can be used to observe surface morphology in toughened composites. This novel optical technique is able to reveal the detailed morphological information of toughened particles and individual fibers, including particle distribution uniformity, particle size variation, and fiber misalignment. It was found that particle distribution uniformity is a critical factor in differentiating the high and low Compression–After–Impact (CAI) laminates. High–CAI laminate had uniform and denser particle distribution, whereas low–CAI laminate had very non–uniform particle distribution.