Recent trends in engineering design have involved a shift towards increasingly lighter structures, especially in transportation and primarily due to the rising cost of energy. This shift has placed further emphasis on the importance of fiber-reinforced composites as a lighter alternative to other engineering materials. Glass-fiber reinforced composites, while a low-cost alternative to the higher performance carbon-fiber based composites, are susceptible to fatigue loading in service, precluding their use in many applications. The present work explores the use of carbon nanotubes (CNTs) in conventional glass fiber composites as an additive that can improve the fatigue strength. The research described here is focused on the development, testing and modeling of CNT/glass-fiber hybrid composites, a novel material exhibiting both low cost and improved properties under fatigue loading.

A manufacturing process for small-scale production of these hybrid composites was developed for the purpose of evaluating their relative benefits. The hybrid composites were subjected to uniaxial fatigue and cyclic delamination tests and their performance was compared to that of traditional glass-fiber composites. A solid mechanics-based model was developed as a means for predicting the energetic differences in the formation and propagation of damage in the polymer matrix of the two composite types under cyclic loading. The model was then compared to the experimental results and found to be in relative agreement.

The hybrid composites were found to have fatigue lifetimes in the high-cycle regime of up to two and a half times greater than the traditional composites, with this improvement diminishing slightly at higher loads. Additionally, resistance to both critical and sub-critical delamination crack propagation was improved in the hybrid composites. These improvements have been attributed here to the dissipative mechanisms of CNT fracture and pull-out during matrix cracking, which result in the absorption of strain energy in a way that inhibits damage growth. Additionally, the presence of CNTs is believed to contribute to distributed matrix microcracking of nanoscale dimensions, resulting in a further increase in energy absorbed prior to final failure.

The present study introduces the use of carbon nanotubes as a secondary reinforcement in the polymer matrix of traditional glass-fiber composites as a means for reducing matrix damage and improving fatigue lifetimes. While the results presented here are specific to the current choice of components, the concepts and ideas introduced are general enough to be applicable to other composite systems subjected to fatigue loading.