Hierarchical machining materials and their performance
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Introduction We fly safe airplanes, enjoy the comfort of fast trains, and live in houses powered by wind turbines. These and many other structural applications that employ advanced composite materials are now part of our daily lives. The materials required for these applications must be able to withstand high stresses and serve for decades without failure. Because of increased concerns with air pollution and global warming, additional restrictions on efficiency and environmental impact have to be respected. In the language of engineers, these structural materials must be light, stiff, strong, and durable. Fiber-reinforced polymer composites and, specifically, carbon fiber-reinforced plastic (CFRP) have been shown to outperform other materials in combining all of these properties. This is evidenced by their significant use in aerospace applications;1,2 ∼50% of the structural mass of the newest airplanes is comprised of such advanced composites. These materials take advantage of the high specific stiffness and strength of microscale continuous fibers, and a compliant polymer matrix, both of which are also lightweight. The typical density of CFRP is about 1.6 g/cm3. The microfibers can be layered, woven, braided, knitted, or randomly assembled into a variety of mesolevel morphologies (Figure 1) that allow tailoring of composite performance for different applications.
The vision for the next generation of composite materials is that they will be hierarchically designed down to the nanoscale. The importance of nanoscale features has long been recognized from studies of biological materials, which are all truly multiscale composites.2–4 In addition to using a plethora of different materials, nature also employs structural hierarchy to target multiple properties at the same time, which cannot be otherwise achieved (i.e., both high strength and high toughness). With the discovery of nanomaterials like carbon nanotubes (CNTs) and graphene, it is now possible to take the first steps toward hierarchical designs in man-made materials. Nanoengineering promises structural composites with superior mechanical properties as well as new functionalities. This review focuses on opportunities and challenges for polymer-based hierarchical composites of high stiffness, strength, and toughness enabled by CNTs. Tubular nano-sized carbon filaments were reported more than six decades ago.5,6 It was not until Iijima’s report on CNTs in 19917 that the composites community began intensively exploring CNTs. Interest in CNTs as reinforcement of polymers is due to their high stiffness (1 TPa) and strength (on the order of 50 GPa),8 both of which are superior to those of existing carbon fibers (∼200–800 GPa)—the most advanced fibers available today. Moreover, high failure strain (12%)8 has been reported for multiwall CNTs, promising structural composites with some ductility.
Larissa Gorbatikh, Department of Materials Engineering, KU Leuven, Belgium; [email protected] Brian L. Wardle, Department of Aeronautics and Astronautics, Massachusetts I
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