Toughening mechanisms responsible for excellent crack resistance in thermoplastic nanofiber reinforced epoxies through in-situ optical and scanning electron microscopy

Epoxy is a material of choice for demanding applications thanks to its high chemical stability, stiffness, and strength. Yet, its brittle fracture behavior is an important downside for many sectors. Here, we show that the addition of electrospun thermoplastic nanofibers is a viable toughening strategy to design nanofiber reinforced epoxy materials with excellent toughness. Moreover, the use of transparent film-like specimens allowed in-situ imaging during mechanical testing. Optical and scanning electron microscopy, digital image correlation and crack length measurements are used to analyze the toughening mechanisms responsible for high toughening efficiency in detail. The addition of polyamide and polycaprolactone nanofibers resulted in an increased plastic energy uptake up to 100%. In-situ observation of the crack tip showed that the main energy-absorbing mechanism was due to bridging nanofibers. There was a profound decrease in toughening efficiency when nanofibers lacked sufficient adhesion with the matrix only when they were oriented parallel with the crack growth direction. The profound understanding of such underlying mechanisms opens up material design in applications where high toughness is required like adhesives, coatings, and fiber-reinforced composite laminates.