Advantages of composite materials

Composites offer outstanding advantages compared to monolithic materials, such as high resistance, high stiffness, long duration under fatigue, low density, and great adaptability to different functions pursued for the structure. Further improvements can also be achieved for corrosion resistance, wear and tear, esthetics, behavior at different temperatures, thermal stabiUty, thermal insulation and conductivity, and sound insulation. 

The fundamental features of the structural performances of composites are the high specific resistance (resistance/density ratio) and high specific stiffness (modulus E/density ratio), and the anisotropic and heterogeneous nature of the material. These last features convey the composite system several degrees of freedom allowing for an optimization of the material configuration. Yet, composites also show some limits that conventional monolithic material do not imply. 

As for the fiber dimensions, composites offer the advantage of high stiffness and high fiber resistance. The usual low breaking strength of fibers is counterbalanced by the energy dissipation of the fiber/matrix interface and by the matrix ductility. The capacity to transfer matrix strengths allows for the development of diffused tensile mechanisms. 

Besides, fibers show a relatively high resistance dispersion. The concentration of local stresses around fibers substantially reduces the cross tensile strength. Conventional materials are more sensitive to their microstructure and to local imperfections impacting the brittle and fragile behavior of material. 

Regarding the macromechanics perspective, assuming the material to be almost uniform, its anisotropy can be exploited as an advantage. The average behavior of the material can be predicted and controlled by knowing the properties of its constituents. Nevertheless, the anisotropic analysis is more complex and more dependent on the measurement procedures, whereas the analysis of conventional materials is easier due to their isotropy and uniformity. 

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The basic nature of composite materials was introduced in Chapter 1. An overall classification scheme was presented, and the mechanical behavior aspects of composite materials that differ from those of conventional materials were described in a qualitative fashion. The book was then restricted to laminated fiber-reinforced composite mafeffals. The basic definitions and how such materials are made were then treated. Finally, the current and potential advantages of composite materials were discussed along with some case histories that clearly reveal how composite materials are used in structures. 

Composite materials are ideal for structural applications where high strength-to-weight and stiffness-to-weight ratios are required. Aircraft and spacecraft are typical weight-sensitive structures in which composite materials are cost-effective. When the full advantages of composite materials are utilized, both aircraft and spacecraft will be designed in a manner much different from the present. 

It is obvious that before the advantage of Composite materials (see Fibre composites - introduction) can be practically realized, they must be fabricated into components. Depending on the particular materials involved (see Fibre composites - matrices and fibres) and the forms required, specially developed processing methods may be necessary. Following is a brief survey of some of the more generally applicable procedures. 

The surface performance under pollution conditions is the comparative advantage of composite materials, and especially silicone rubber, in respect to the cercunic materials, porcelain cuid glass, which were traditioncJly employed for the manufacture of insulators [3,7,8]. In fact, the introduction of sihcone compoimds brought a new era in the field of outdoor ticuismission cuid distribution insulating systems. This change occurs mcunly due to the surface behavior of silicone rubber, cuid especially due to a property known as hydrophobicity [6-8]. 

Not all of the strength and stiffness advantages of fiber-reinforced composite materials can be transformed directly into structural advantages. Prominent among the reasons for this statement is the fact that the joints for members made of composite materials are typically more bulky than those for metal parts. These relative inefficiencies are being studied because they obviously affect the cost trade-offs for application of composite materials. Other limitations will be discussed subsequently. 

Typical S-N (stress versus number of cycles) curves for various metals and composite materials are shown in Figure 6-4 [6-3]. The boron-epoxy composite material curve is much flatter than the aluminum curve as well as being flatter than the curves for any of the metals shown. The susceptibility of composite materials to effects of stress concentrations such as those caused by notches, holes, etc., is much less than for metals. Thus, the initial advantage of higher strength of boron-epoxy... 

The creation of active sites as well as the graft polymerization of monomers may be carried out by using radiation procedures or free-radical initiators. This review is not devoted to the consideration of polymerization mechanisms on the surfaces of porous solids. Such information is presented in a number of excellent reviews [66-68]. However, it is necessary to focus attention on those peculiarities of polymerization that result in the formation of chromatographic sorbents. In spite of numerous publications devoted to problems of composite materials produced by means of polymerization techniques, articles concerning chromatographic sorbents are scarce. As mentioned above, there are two principle processes of sorbent preparation by graft polymerization radiation-induced polymerization or polymerization by radical initiators. We will also pay attention to advantages and deficiencies of the methods. 

This fascinating product will still continue to develop to accommodate new applications, safety, health and environment (SHE) issues, advantages of novel materials like nano-composites, plasma-surface-modified carbon black, development of computer simulation techniques, and finally to develop a cybernetic or thinking tire. 

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.

Foamed composites are lightweight materials reinforced by fibers. Some of the advantages of these materials are excellent strength/weight ratios, workability, excellent corrosion resistance, and design flexibility of molded products. 

Composite piezoelectric transducers made from poled Pb-Ti-Zr (PZT) ceramics and epoxy polymers form an interesting family of materials which highlight the advantages of composite structures in improving coupled properties in soilds for transduction applications A number of different connection patterns have been fabricated with the piezoelectric ceramic in the form of spheres, fibers, layered, or three-dimensional skeletons Adding a polymer phase lowers the density, the dielectric constant, and the mechanical stiffness of the composite, thereby altering electric field and concentrating mechanical stresses on the piezoelectric ceramic phase. 

The model used consisted of a half helmet with three element layers of composite material through the thickness. A half model was used to take advantage of the symmetry present in the problem. The elements used were eight-noded brick elements because these elements can represent the deformation sufficiently. Anisotropic material properties were defined for each element using a user subroutine and impact was designated at the front of the helmet. 

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