Specific strength materials

Fig. 1. Specific strength and Young s modulus of various engineering materials where CF = carbon fiber HM/UHM = high modulus/ultrahigh modulus ...

The introduction of large gas phase volumes into the polymer alters the physical characteristics of the material volume weight, permeability to fluids and gases, and physico-mechanical properties. Moreover, the properties of the polymer matrix itself are changed (owing to orientation effects, supermolecular structure of the polymer in the walls, ribs and tension bars of cells), which drives up the value of specific strength on impact, and results in anisotropy of elasticity. 

Figure 5.57 Design chart of specific modulus versus specific strength (see inset for type of strength determination). Reprinted, hy permission, from M. F. Ashhy, Materials Selection in Mechanical Design, 2nd ed., p. 44. Copyright 1999 hy Michael F. Ashhy.

The use of advanced composites has increased significantly in the last decade. The properties of high-specific strength and stiffness make composites ideal for many aerospace, automotive, and infrastructure applications. Fiber-reinforced composites, which commonly use thermosetting resins such as epoxies as the matrix material, have some inherent deficiencies. These include the need for multistep processing, limited shelf-life, low toughness, sensitivity to moisture, and the inability to reprocess or reform the material [1]. 

As mentioned above, the binder/filler ratio determines the fluidity and hence the process technology for a given syntactic composition. The solid line in Fig. 1 shows how the apparent density depends on the relative microsphere concentration in syntactic compounds, provided there are no air inclusions. The lower limit of apparent density is at the concentration at which the filler is most densily packed at this point the material has its highest specific strength (strength per unit apparent density). Experience has shown that the closest packing of the spheres within the binder is obtained when the material is mixed and cast or molded under vacuum 8 10). 

Table 11 indicates that syntactic plastics are similar in strength properties to monolithic filled systems (glass-reinforced plastics), although their apparent densities are 2-3 times smaller. Hence, syntactic plastics appear to have the highest specific strengths of all known plastic materials. 

Carbon-carbon (C-C) composites with a variety of unique properties can be fabricated by altering the combinations of the type and distribution of filaments and the bonding matrix used. Many engineering applications can be satisfied with a composite material whose density is just 70% that of aluminum and 25% that of steel, but for which the specific strength and stiffness values are four or five times those of steel. To attain the desired properties for such applications requires an understanding of the interrelationship of the fibers and the matrix that holds them. 

Given the modulus and strength values that have been predicted and measured for CNTs, they are potentially an ideal reinforced material for high performance polymer composites with outstanding specific modulus and specific strength with only low concentrations of CNTs (11,39-42). However, one issue of practical importance for nanocomposite is the separation and dispersion of the CNTs within the matrix,... 

Material Tensile strength (MPa) Density (kg dm ) Specific strength (kN mkg ) Breaking length (km)... 

Carbon fiber-epoxy composite materials have been used in advanced engineering structures such as spacecraft, aircraft, automobile transmission shafts and robot structures because of their high specific stiffhess and high specific strength [1, 2]. Since conventional metal alloys are... 

Particleboard utilizes residue materials from other wood processing operations. Sawdust and shavings predominate, although most plants are able to use chips or flakes made from roundwood to meet specific strength requirements in the product. Cost is the major driver, with the proportion of high cost material such as chips or roundwood limited to that necessary to meet product requirements. Particle size distribution and shape have become more important as the effect of these variables on product quality has been recognised, leading to reductions in cost. 

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