A study on fatigue behavior of nanointerleaved woven CFRP

Delamination is the most common failure mode in laminated composite materials and it may cause catastrophic failure in critical engineering structures. One of the ways to prevent this failure is to toughen the matrix against crack initiation and propagation. A powerful method has been proved to be interleaving nanofibrous mat manufactured from thermoplastic polymers between laminate layers [1]: lack of research has been found in literature on the topic of toughening delamination resistance using nano fibers, especially for fatigue behavior. The present work aims to investigate the effect of an interleaved nanofibrous mat on fatigue interlaminar properties of mode I loaded carbon-epoxy composite woven laminates. Double Cantilever Beam (DCB) virgin and nanomodified specimens were fabricated, these latter by interleaving a 40 micron thick mat of Polyamide nanofibers in the mid interlayer, where the crack was artificially induced. Static and fatigue tests were performed in order to determine the delamination resistance curve, and the delamination growth onset [2] and the crack propagation rate vs maximum energy release rate [3] curves respectively. Nine samples per configuration have been tested, and crack paths were investigated by micrograph analysis. Different behaviors between virgin and nanomodified configurations were observed, explaining the different mechanical results. Nanomodified specimens showed a negligible thickness increase (<1%), but presented a noticeable improve in terms of in-plane mechanical properties. In particular, increases of delamination toughness under static load and delamination growth onset under cyclic loads of 150% and 100% respectively were registered. The most important step forward compared to the state of the art was the determination of crack grow ratio laws for the virgin and nanomodified specimens here considered. The results showed that nanomodified specimens decreased the crack grow ratio of about 80%. Micrograph revealed that the presence of nanofibres makes the crack propagating through the nano-reinforced interlayer, resulting in longer crack path and then higher delamination resistance for the nanomodified samples. [1] Saghafi et al. The effect of PVDF nanofibers on mode-I fracture toughness of composite materials. Composites: Part B 72 (2015) 213c216 [2] Sato et al. Intralaminar fatigue crack growth properties of conventional and interlayer toughened CFRP laminate under mode I loading. Composites: Part A 68 (2015) 202-211 [3]Roderick H.M. Composite Materials: Fatigue and Fracture ASTM, 1995