ABSTRACT
Fiber strengthening has been known for a very long time. Humankind has long utilized natural materials like jute and bamboo and they all have fibrous reinforcement. The focus on understanding the mechanics of reinforcement by fibers is almost 50 years now [1-3]. Several models have been developed to estimate the reinforcement effect of hard particles and high strength fibers. The mechanical behavior of these composites can be described at multiple length scales by different models as summarized in Figure 5.1. At macroscale length (>1 mm), the average property of the material may be considered and material behavior is represented by constitutive laws like Hooke’s law. At micro/meso-length scales (1 µm to 1 mm), the deformation and stress around an inclusion, pore, and crack influence the mechanical behavior, which can be predicted by micromechanical models, dislocation plasticity models, and fracture mechanics. At nanoscale length, the mechanical behavior is influenced by the interaction between the carbon nanotube and the matrix at an atomic level, which can be studied by molecular dynamics. A single CNT of small size (a few microns in length and a diameter less than 100 nm) is expected to cause reinforcement in regions around it at the nanoscale level. It is of basic interest to study whether the strengthening at the nanoscale level is translated into the micro-and macro-level. The applicability of existing micromechanical models and development of new ones for strengthening in CNT composites has received some attention of late [4-7]. Most of the research has been done on polymer-CNT composites, which, in principle, can be applied to metal matrices as well. Carbon nanotubes are different in the sense that strengthening caused by them reaches to very small length scales as compared to larger fibers. This would lead to new mechanisms for strengthening because they can interfere with the deformation mechanisms occurring at nanoscale length. In addition, there are several specific issues with CNT composites, namely clustering, degree of dispersion, alignment, curvature, and single/multi-wall, which influence the mechanical properties and need to be taken into account for bulk fiber reinforced composites. The grain size in the metal matrix composites may also be affected by the presence of CNTs, which will affect the strengthening behavior.
