Tensile strength and fracture toughness of nanocomposite materials

Are not as high as we expected although very stiff and strong nanotubes or nanofibers (Young’s modulus E~1000GPa) are added into soft polymer matrices like epoxy (E~4GPa).  In our early investigation on the  systematic mechanical property characterizations of nanocomposites (Xu et al., Journal of Composite Materials, 2004--among top 5 in 2005;and top 10 in 2006 of the Most-Frequently-Read Articles in Journal of Composite Materials.) have shown that there was a very small increase (sometimes even decrease) of critical ultimate tensile/bending strengths, and mode-I fracture toughnesses in spite of complete chemical treatments of the interfacial bonding area, and uniform dispersions of nanofibers (click to view a TEM image). Similar experimental results were often reported in recent years. Therefore, mechanics analysis is extremely valuable before we make these “expensive” nanocomposite materials. Our goal is to provide in-depth mechanics insight, and future directions for nanocomposite development. Till now, nanocomposite materials are promising as multi-functional materials, rather than structural materials. Here we mainly focus on two critical parameters for structural materials: tensile strength and fracture toughness. We notice that other mechanical parameters such as compressive strengths and Young’s moduli of nanocomposite materials have slight increase over their matrices.

 A major reason is that very strong nanotube/nanofibers inside nanocomposite materials are not fully loaded due to low efficient interfacial shear load transferring. Since the interfacial shear stress is related to the shear modulus of the matrix, a soft polymeric matrix only offers very limited load transferring from the matrix to the nanotube/nanofiber.  Therefore, future nanocomposite materials for structural applications would require nanoscale reinforcements to carry load directly (aligned discontinuous nanotubes are still not enough). It should be noticed that strong interfacial bonding (such as covalent binding) is a necessary condition, not a sufficient condition in order to increase the failure strength of nanocomposite materials. More importantly, our previous experiments indicate that there is always a large scatter in the strength data of nanocomposites. This phenomenon results from several special features of nanocomposites 1) the high mismatch in the elastic properties of the matrix material and the nanoscale reinforcement and 2) large interfacial bonding area of nanocomposites compared to the same traditional composite materials with the same fiber/particle volume percents.  The first factor will lead to early interfacial debonding between the matrix and the nanotube/nanofiber end, compared to traditional composites with less stiffness mismatch (based on the Dundurs’ parameters often used in interfacial fracture mechanics). The second aspect can be used to explain that initial interfacial defects are easily induced in nanocomposites than traditional composites, and lead to a large scatter in nanocomposite failure strengths.