Specific stiffness

Fiber/composite Elastic modulus (GPa) Tensile strength (GPa) Density (g/cm ) Specific stiffness (MJ/kg) Specific strength (MJ/kg)... 

The very first question that comes to mind when dealing with giant telescopes is the cost-effective feasibility of its optics. Assuming classical materials for the segments blanks, however, there is no need for a very substantial increase in production capacity from existing suppliers provided that the segment size remains below 2-m. Moderately lightweight Silicon Carbide is also considered as a serious and potentially cost-effective candidate, for its superior thermal performance and specific stiffness. 

Figures 9.19 and 9.20 present a survey of the mechanical properties of some (unidirectional) composites, in comparison with some other materials. In Figure 9.19 the values of modulus and strength are plotted as such, while in Figure 9.20 these values have been divided by the specific mass. From Figure 9.20 the enormous advantage of composites with respect to stiffness and strength per unit weight, in comparison to metals, is clearly visible. The modem carbon and aramide composites are superior to those based on glass fibres, for the specific stiffness even by a factor between 4 and 5.

Weight saving over aluminum alloys (high specific stiffness and strength)... 

Figure 1. Specific ultimate tensile strength vs. specific stiffness of current and developmental aerospace structural materials. Data are displayed on a log-log plot in (a), where P signifies PAN-based reinforcements Gr represents graphite fibers 0 0 and 90 0 indicate data collected parallel to and transverse to the fiber direction in uniaxial composites, respectively and Q/I represents quasi-isotropic laminates. The (f represents fiber reinforcements in MMCs. The dashed line in (b) represents the combinations of specific strength and stiffness that are double those of conventional metal alloys.

Discontinuously-reinforced A1 (DRA) provides specific strengths equivalent to the best cross-plied OMCs, and has specific stiffnesses that... 

Unique nanocrystalline microstructures can be produced by controlled crystallization of the fully amorphous product, including nanocrystalline precipitates homogeneously distributed in an amorphous alloy matrix. In some systems, both the strength and ductility increase in this partially crystalline state [24], Other alloys produce nanocrystalline intermetallic or quasicrystalline precipitates, providing a credible path for increasing the specific stiffness. Thus, a significant effort is required to study the kinetics of crystallization, the devitrification pathways and the microstructures and properties that may be produced upon devitrification. The potential for exploration of novel compositions and microstructures in this class of materials is clearly promising. 

Ti-B-X systems alloys are promising for achievement of high specific stiffness. 

Ti-B-Si-X alloys, where X is Al, Zr etc, is a promising system to achieve a good combination of high specific stiffness, specific strength and heat resistance. 

Another characteristic property of polymers, namely their high specific stiffness and strength (which are due to their low density, especially when used in fiber-reinforced composite materials), has led to the use of polymers in transport applications, especially in aerospace industries, where weight saving is of vital importance and materials cost is secondary. However, here again many applications also demand high temperature resistance. 

Compared with aluminium, composites have the capacity to deliver significant weight saving, largely because of improved specific stiffness. For example. Figure 4.1... 

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