Identification of the effect of nanofiller morphology on interlaminar fracture toughness of hybrid composites

Nanoscale reinforcements have the potential to improve mechanical properties of fiber reinforced composite. Here, effect of nanofiller morphology and dispersion in augmenting mode I fracture toughness of unidirectional carbon fiber reinforced composite materials is studied. The nanofillers used is electrospun carbon nanofibers (CNFs). Unlike most nanofillers which are in particulate form, CNFs exist in both continuous nanofiber mat and particulate forms. This trait allowed us to compare the effect of particulate nanofillers (CNFs dispersed in B-staged epoxy) vs. dry mats on fracture toughness of composites while all other parameters are kept constant. To enhance CNFs-matrix interactions, a novel approach was utilized to functionalize CNFs surface with melamine, so that epoxy functional groups can form strong bonds to matrix. The improvement in mode I initiation fracture toughness with CNF mats was statistically significant, while in B-staged samples, statistical analysis revealed insignificant improvement. In addition, in both CNFs reinforced samples, crack propagation fracture toughness decreased with crack growth and approached that of the composites with no CNFs. The decline was steeper in samples with B-staged CNFs. This behavior was explained by evaluating fracture path via SEM imaging. It was concluded that while CNFs bridge crack tip initially and delay crack initiation, crack deflects towards a lower resistance path by tearing CNFs mat and propagating along resin-rich interface between CNFs and microfibers. These alternative and weaker fracture planes are more readily available in B-staged samples due to poor integration of the B-staged epoxy with the rest of the composite.