Fracture of advanced polymer composites with nanofiber reinforced interfaces

An experimental and theoretical study is performed to explore the novel delamination suppression concept based on nanofiber reinforcement of interfaces between plies in advanced polymer composites (Dzenis and Reneker: US patents awarded and pending). Edge delamination was studied on graphite/epoxy composites reinforced by continuous electrospun polymer nanofibers with the diameter around 300 nm. Statistically significant improvements in both edge delamination onset stress and ultimate tensile strength in the composites with nanoreinforced interfaces were demonstrated. A simple FEM model was developed to evaluate the toughening mechanisms. Effects of loading rate and temperature on interlaminar fracture were evaluated using the double cantilever beam (DCB) and end notched flexure (ENF) tests. A thermal activation fracture model was developed for the rate-dependent fracture. Substantial improvements in interlaminar fracture toughness were recorded as a result of nanoreinforcement in the whole range of loading rates. Laminate specimens with nanofiber-reinforced interfaces were developed for dynamic Mode I and II fracture tests using Hopkinson pressure bar (HPB). Crack initiation was captured by crack detection gage. Dynamic stress intensity factors (SIFs) were extracted using a FEA code based on the recorded testing data. Results showed that nanofibers substantially increased the dynamic fracture toughness. SEM fractography was conducted to explore the nanofiber toughening micro and nanomechanisms, which directly related to the specific properties of nanofibers, such as high aspect ratio, high tensile strength, and the nanofiber geometric profiles in composites. Finally, several new SIF formulas for cracks in bimaterial layers were derived suitable for delaminated composite analysis. The results of this dissertation can be used to analyze and further improve the delamination resistance of the novel advanced polymer laminated composites with nanoreinforced interfaces and design and develop new nanofiber-based nanocomposites, including the nanocomposites with superior impact fracture resistance for ballistic and other dynamic applications