Composite materials have high in-plane mechanical properties and are lightweight. Thus laminated composites are widely used in engineering applications. However, interlaminar stresses can arise and lead to premature failure at low in-plane stress levels. This constitutes a fundamental weakness in polymeric laminated composite materials. In addition, the full functional capabilities are not utilized in the designs of current composites. In this dissertation, a smart materials strategy is employed to mitigate the weak interlaminar properties of composite materials and to make composites more multi-functional materials.
The first step in our strategy to improve composites is to reinforce the material using a nano-phase material component. A nanoreinforced laminated composite (NRLC) consisting of Multiwalled Carbon Nanotube (MWCNT) arrays that are interspersed between the plies of a fiber reinforced polymeric composite was developed. The MWCNT array and the polymer matrix are arranged within the laminated composite so that the new interfaces yield higher interlaminar properties. The mechanical response of these composite materials under quasi-static loading including interlaminar shear, in-plane tension, and out-of-plane compression was studied and documented. The morphology of the NRLC was characterized using Scanning Electron Microscopy and correlated with its mechanical response. The NRLC material developed achieved substantially stiffer and stronger interlaminar shear properties without significantly compromising the in-plane mechanical properties.
Structural integrity could still be compromised by unpredictable circumstances thus it will be important to continuously monitor composite structures for damage thus providing safety to the users and confidence to the system operator. The approach taken was to develop a built-in damage sensing system for the material. A new integrated and distributed sensing approach based on nanotechnology was developed wherein carbon nanotube arrays or forests were spun into a tough and electrically conductive thread to be used along with conventional fibers in composites. But the nanotube thread has piezoresistive properties and can sense strain and damage. The sensor thread was integrated into composite materials and used for the first time as a sensor to monitor strains and detect damage including delamination. The instrumented composites, named self-sensing composites, were determined to be very sensitive to damage. These materials will help to revolutionize the maintenance of structures, which will now be based on the actual condition not just the length of operation of the structure.
Finally, multi-functionality of composites was investigated in this dissertation. Composite materials should be able to perform multiple functions simultaneously. Overall, the multi-functional composite materials should self-monitor their integrity, respond to their environment in a functional way, and provide multi-functionality. Smart composites can react to their environment by becoming stiffer to react to external loads, changing temperature to prevent icing, or changing their electromagnetic signature. Multi-functionality could also encompass transmitting electrical power or communication through the structural material, providing lightning and static electricity protection, or acting as an antenna. Carbon nanotube materials are ideal to develop smart materials because nanotubes have high strength, toughness, electrical and thermal conductivity, and light weight that cannot be matched by conventional materials. Different smart and multi-functional properties of smart composites based on carbon nanotube ribbon, yarn, and sheet were investigated and shown to be feasible in this dissertation. Overall, this dissertation aims to open up the frontier for smart composites to become the next generation of new material coming onto the aerospace, defense and advanced materials markets.