Fiber-reinforced composite

Intense nucleation of matrix crystallization on reinforcing fibers leads to transcystaUinity. Recently, transcrystallinity in polymer composites has been reviewed by Quan et al. [51]. TranscrystalUnity is known to occur in several semicrystaUine polymers including iPP, PE, PA, poly (ether ether ketone) (PEEK), and poly(phenylene sulfide) (PPS) in contact with carbon 

At high undercooling a small stress is sufficient to induce the orientation since the ability of molecules to relax decreases with decreasing temperature. A high level of interactions between a fiber and a matrix leads to greater adsorption of polymer chains onto the fiber surface, and these anchored molecules are more susceptible to orientation. 

The role of fiber thermal conductivity was also pointed out [54] the temperature gradient developing during cooling at the interface between fiber and matrix caused by the mismatch of their thermal conductivity facilitated transcrystaUization. 

In addition to the heterogeneous nucleation and shear- or strain-induced nucleation on fiber surfaces, the third reason for the development of transcrystallinity was also indicated [51], related to impurities present in the matrix polymer absorption of the impurities on fiber surfaces can also enhance nucleation and facilitate the transcrystallization. 

Irrespective of the reasons, the strong nucleation on fibers results in structures consisting of columnar entities with transcrystalline morphology. The content of spherulites nucleated in polymer bulk depends on fiber content and nucleation density on fiber surfaces and also inside a polymer matrix. 

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A.K Jain M P Debuisson. Segmentation of X-ray and C-scan Images of Fiber Reinforced Composite Materials. Pattern Recognition, vol 25, N°.3, pp 257-270, 1992... 

Cera.micA.bla.tors, Several types of subliming or melting ceramic ablators have been used or considered for use in dielectric appHcations particularly with quartz or boron nitride [10043-11 -5] fiber reinforcements to form a nonconductive char. Fused siHca is available in both nonporous (optically transparent) and porous (sHp cast) forms. Ford Aerospace manufactures a 3D siHca-fiber-reinforced composite densified with coUoidal siHca (37). The material, designated AS-3DX, demonstrates improved mechanical toughness compared to monolithic ceramics. Other dielectric ceramic composites have been used with performance improvements over monolithic ceramics (see COMPOSITE MATERIALS, CERAMIC MATRIX). 

Particle or discontinuously reinforced MMCs have become important because they are inexpensive compared to continuous fiber-reinforced composites and they have relatively isotropic properties compared to the fiber-reinforced composites. Figures la and b show typical microstmctures of continuous alumina fiber/Mg and siUcon carbide particle/Al composites, respectively. 

Fig. 17. Variety of subcritical damage mechanisms in fiber-reinforced composites, that lead to a highly diffuse damage 2one. (a) Fiber cracking, (b) matrix...

Most recent studies (69) on elevated temperature performance of carbon fiber-based composites show that the oxidation resistance and elevated temperature mechanical properties of carbon fiber reinforced composites are complex and not always direcdy related to the oxidation resistance of the fiber. To some extent, the matrix acts as a protective barrier limiting the diffusion of oxygen to the encased fibers. It is therefore critical to maintain interfacial bonding between the fiber and the matrix, and limit any microcracking that may serve as a diffusion path for oxygen intmsion. Since interfacial performance typically deteriorates with higher modulus carbon fibers it is important to balance fiber oxidative stabiHty with interfacial performance. 

Eiber volume fraction is a quantitative measure of degree of reinforcement of the matrix material in a fiber-reinforced composite. If the volume of a composite material is D and the volume of the fibers is and that of the matrix is then... 

Thermosetting unsaturated polyester resins constitute the most common fiber-reinforced composite matrix today. According to the Committee on Resin Statistics of the Society of Plastics Industry (SPl), 454,000 t of unsaturated polyester were used in fiber-reinforced plastics in 1990. These materials are popular because of thek low price, ease of use, and excellent mechanical and chemical resistance properties. Over 227 t of phenoHc resins were used in fiber-reinforced plastics in 1990 (1 3). PhenoHc resins (qv) are used when thek inherent flame retardance, high temperature resistance, or low cost overcome the problems of processing difficulties and lower mechanical properties. 

Unsaturated polyester resins predominate among fiber-reinforced composite matrices for several reasons. A wide variety of polyesters is available and the composites fabricator must choose the best for a particular appHcation. The choice involves evaluation of fabrication techniques, temperatures at which the resin is to be handled, cure time and temperature desked, and requked cured properties (see Polyesters, unsaturated). 

Ease of cure, easy removal of parts from mold surfaces, and wide availabiHty have made polyesters the first choice for many fiber-reinforced composite molders. Sheet mol ding compound, filament winding, hand lay-up, spray up, and pultmsion are all weU adapted to the use of polyesters. Choosing the best polyester resin and processing technique is often a challenge. The polyester must be a type that is weU adapted to the processing method and must have the final mechanical properties requked by the part appHcation. Table 1 Hsts the deskable properties for a number of fiber-reinforced composite fabrication methods. 

Most processors of fiber-reinforced composites choose a phenol formaldehyde (phenoHc) resin because these resins are inherently fire retardant, are highly heat resistant, and are very low in cost. When exposed to flames they give off very Htde smoke and that smoke is of low immediate toxicity. PhenoHc resins (qv) are often not chosen, however, because the resole types have limited shelf stabiHty, both resole and novolac types release volatiles during their condensation cure, formaldehyde [50-00-0] emissions are possible during both handling and cure, and the polymers formed are brittle compared with other thermosetting resins. 

Resoles can be cured by the addition of base or by heat alone. Their shelf life is thus limited, which is a significant deterrent to their use in fiber-reinforced composites. Resoles are often used in unreinforced appHcations in electronics and high moisture areas. 

Key Words —Nanotubes, mechanical properties, thermal properties, fiber-reinforced composites, stiffness constant, natural resonance. 

The ultimate tensile strength of a uniaxially aligned fiber-reinforced composite is given to reasonable accuracy by the rule of mixtures relation ... 

Also, laminated fiber-reinforced composite materials are obviously both laminated and fibrous composite materials. Thus, any classification system is arbitrary and imperfect. Nevertheless, the system should serve to acquaint the reader with the broad possibilities of composite materials. 

For the remainder of this book, fiber-reinforced composite laminates will be emphasized. The fibers are long and continuous as opposed to whiskers. The concepts developed herein are applicable mainly to fiber-reinforced composite laminates, but are also valid for other laminates and whisker composites with some fairly obvious modifications. That is, fiber-reinforced composite laminates are used as a uniform example throughout this book, but concepts used to analyze their behavior are often applicable to other forms of composite materials. In many Instances, the applicability will be made clear as an example complementary to the principal example of fiber-reinforced composite laminates. 

The basic terminology of fiber-reinforced composite laminates will be introduced in the following paragraphs. For a lamina, the configurations and functions of the constituent materials, fibers and matrix, will be described. The characteristics of the fibers and matrix are then discussed. Finally, a laminate is defined to round out this introduction to the characteristics of fiber-reinforced composite laminates. 

Fiber-reinforced composite materials such as boron-epoxy and graphite-epoxy are usually treated as linear elastic materials because the essentially linear elastic fibers provide the majority of the strength and stiffness. Refinement of that approximation requires consideration of some form of plasticity, viscoelasticity, or both (viscoplasticity). Very little work has been done to implement those models or idealizations of composite material behavior in structural applications. 

Unlike most conventional materials, there is a very close relation between the manufacture of a composite material and its end use. The manufacture of the material is often actually part of the fabrication process for the structural element or even the complete structure. Thus, a complete description of the manufacturing process is not possible nor is it even desirable. The discussion of manufacturing of laminated fiber-reinforced composite materials is restricted in this section to how the fibers and matrix materials are assembled to make a lamina and how, subsequently, laminae are assembled and cured to make a laminate. 

Three principal layup processes for laminated fiber-reinforced composite materials are winding, laying, and molding. The choice of a layup process (as well as a curing process) depends on many factors part size and shape, cost, schedule, familiarity with particular techniques, etc. 

The advent of advanced fiber-reinforced composite materials has been called the biggest technical revolution since the jet engine [1-4], This claim is very striking because the tremendous impact of the jet engine on military aircraft performance is readily apparent. The impact on commercial aviation is even more striking because the airlines stwitched from propeller-driven planes to all-jet fleets within the span of just a few years because of superior performance and lower maintenance costs. 

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. 

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