lecture05 - 10. COMPOSITES 10.1 General Guidelines A. In...

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Unformatted text preview: 10. COMPOSITES 10.1 General Guidelines A. In reinforcing a matrix, stiff and strong fibers are used as reinforcing materials. Rubber—modified polymers and metal/ ceramic composites in which the reinforcing materials are softer than the matrices are very few examples which have low modulus materials embedded in high modulus materials. B. Organic fibers with low temperature resistance shouldonly be used in organic matrix. C. Metallic fibers can be used in ceramic matrix to improve ductility. Using heavy metallic fibers in polymer matrix do not gain weight saving nor improve temperature resistance. D. Ceramic fibers should be used in metal matrix composite in which oxidation and adverse reactivity between metal matrix and carbon and boron fibers can take place. They are not used in polymer matrix. ’ 10.2 Combinations A. Combinations of fiber (discontinuous phase) and matrix (continuous phase) - discontinous olymer/ 01 mer metal ' metal/01 er ‘ ceramic B. Examples and Exceptions ‘ . I ~ a. polymer/polymer — Kevlar/epoxy (bullet-proof vest), liquid crystal polymer/polyethylene (weather balloons) b. polymer/metal —- ARALL (space structure), rubber/stainless steel 0. polymer/ceramic —— phenolic/ glass (Applo spacecraft booster), . polyethylene/concrete and polypropylene/concrete (coast structures, airport runways, road slabs and armor units) d. metal/polymer — steel/rubber (steel belted tires), Boron/epoxy e. metal/metal — bimetals, intermetallic, W/Cu (cryogenically-cooled throust engines for rock), boron/Al, Ag/W and Ag/Mo (contacts used in circuit breakers) f. metal/ceramic — concrete, Al/SiC (aero-propulsion engine) g. ceramic/polymer — glass/epoxy (automotive bumper), glass/ABS (vehicle interior 1 door) » a h. ceramic/metal —— SiC/Al (tubular struts and wing element in rockets), SiC/T i (aircraft engine), Ale3/Al (automotive drive shaft) » i. ceramic/ceramic — SiC/SiC (nozzles in Mirage 2000), SiC/Al—Si (automotive brake ‘ rotor disks), A1203/Al-Si (engine cylinder blocks) 2. WHY USE COMPOSITE MATERIALS? 2.1 Advantages A. Properties a. Polymer matrix composites have high stiffness-to-weight and high strength~to- weight ratios and are excellent for uses in high-performance structures. b. Metal matrix composites have relatively high resistance to thermal impact. c. Ceramic matrix composites have high thermal stability and high corrosion resistance. d. Many composite Materials are designed to improve specific properties such as ductility, corrosion resistance, wear resistance, fatigue resistance, energy absorption capability, thermal expansion, thermal conductivity, etc. B. Fabrication a. Tailorability — Composite structures can be designed to have strong and stiff fibers in the right direction and at right location with the right amount. Combining the structural design with the material fabrication together can help to reduce the amount of scrappage and the fabrication cost. ‘ b. Versatility — Because many types of fiber material, fiber geometry, matrix material, and manufacturing technique are available, the fabrication of composite materials and structures can be very flexible and versatile. ' c. Assembility m A large composite structure can be fabricated from one single process instead of being assembled from many small pieces through secondary joining processes. The assembility is considered to be the primary source of cost reduction in composite fabrication, especially when composite materials and manufacturing processes are expensive. C. Cost — Some composite materials are still more expensive than their metallic counterparts. As the composite technology advances and the production rate increases, their cost are expected to go down. The initial high cost of capital investment is expected to pay off in the long term. 2.2 Disadvantages A. Properties a. Composite materials are inhomogeneous and anisotropic. Formulae used for analyses of conventiOnal metals become invalid. More sophisticated mathematical models for describing the characteristics of composite materials are required. b. Because the fabrication of composite materials involves a random process, it is more difficult to determine the properties of composite materials than to determine the properties of conventional metals. For example, although the Young’s moduli of . composite materials fabricated from different batches are nearly the same, the strengths of the composite materials vary a great deal, depending on the microstructure (including imperfections such as resin rich zones, voids, poor interfaces, etc.) that is highly influenced by small changes in chemical composition, packing geometry and heating history. In other words; whereas Young’s moduli are “microstructurally insensitive”, the strengths are “microstructurally sensitive.” Hence, the properties of composite materials are more statistical than deterministic. B. Fabrication — The manufaturing techniques for composite materials are not identical to those used for conventional metals. New tools and new techniques are required. Frequently, they are more demanding and more sophisticated. C. Cost — High initial cost is due for producing small amount of materials and investing new manufacturing facilities. There is a strong need to identify and to develop low-cost composite materials with high—speed manufacturing techniques. D. Environmental Impact - As the society moves more toward environmentally conscientious, issues such as biodegradability, recyclability, reusability of composite materials are becoming primary concerns in engineering designs. 4. HOW TO STUDY COMPOSITE MATERIALS? 4.1 Fundamental Ideas A. The essence of composite materials technology is the ability to put strong and stiff reinforcement in the right location and in the right orientation with the right amount. For instance, graphite/ epoxy is found to be suitable for high~perforrnance structures since it can be as strong as steel, as stiff as titanium and as light as aluminum. B. It is important for the scientist to understand the nature of the design problem (macroscopic performance) and for the engineer to appreciate the subtleties of the materials (microscopic structure) used in the design of composite materials. C. Composite materials have introduced an extraordinary fluidity to engineering design, in effect forcing the engineers to create a different material for each application to I pursue weight saving and particular properties at a competitive cost. D. Since composite materials are inhomogeneous and anisotropic, difficulties in material analysis and processing may not be avoidable. However, if the characteristics of _ composite materials can be well tailored and the manufacturing techniques can be resolved, significant progress in structural designs can be achieved. E. Although composite materials canbe tailored to be anisotropic and inhomogeneous to meet various structural design requirements, they are not necessarily suitable for structures with complex geometry and/or subjected to complex loading conditions. Also, it is not feasible to use a single composite material tomeet various design needs. 4.2 Scopes of Studying Composite Materials In investigating composite materials, different assumptions may be taken. Depending on the scale of investigation window, i.e. the size of representative volume element (RVE), the following three scopes are usually used: A. Microscopic Study — Based on the viewpoint of inhomogeneity, this scope of study investigates the effects of microstructure and microscopic parameters onthe effective (homogenized) properties of composites. Examples of microstructure and miCroscopic parameters are properties of constituents, reinforcement—matrix interfaces, and packing parameters of the reinforcement, such as volume fraction, distribution, and orientation. The effective properties of composite materials include elastic constants, strength, thermal expansion coefficient, thermal conductivity, etc. B. Macroscopic Study m This scope of study is based on the assumption that composite materials are homogeneous but anisotropic. That is, the microstructure of composite materials is neglected and the composite materials are assumed to have uniform properties. The macroscopic study is very useful in investigating large structures made of composite materials. - C. Mesoscopic Study — Being situated in between the microscopic and macroscopic studies, this scope of study investigates the microscopic performance of composite materials based on a finite number of reinforcing materials, instead of a single reinforcing material used in the microsoopic study and no reinforcing material used in the macroscopic study. 4.4 Strength of Materials Approach A. The elongation (or contraction) 5 of a one—dimensional bar subjected to a uniaxial tension (or compression) force P can be expressed as 5 = 5:: where E, A and l are Young’s modulus, cross-sectional area, and length of the bar, respectively. B. The twisting angle ¢ of a one—dimensional bar subjected to a twisting moment, or torque, T can be expressed as ¢ = ~27— where G and J are shear modulus and polar second moment of area of the bar, respectively. C. The bending curvature K of a one—dimensional beam subjected to a bending M . moment M can be expressed as K : EI— where I 15 the second moment of area of the beam. a. When a conventional metal is to be replaced bya composite material for weight reduction, the first step IS to compare the stiffness—to—weight ratios (—) and strength—to— p a weight ratios («1—) of the two materials. If the composite material has higher ratios than p the metal counterpart, the goal to achieve the weight reduction will be possible. b. When replacing metallic bars and beams with composite counterparts, the axial rigidity EA, the torsional rigidity G] and the bending rigidity E1 of the composite members should be at least equal to those of the metallic counterparts in orderto maintain the same levels of load—carrying capability, i.e. i' (EA)melal : (EA)composite : metal 2 composite = metal = composite = Z where the subscript 1‘ represents for the individual constituents which form the composite materials. It should be noted that these equations do not consider the actual assembly of the individual components through perfect joining condition for the constituents are imposed at the ends of the members. c. When two component beams A and B are put together to form a composite beam C, the bending rigidity of the composite beam is dependent on the bonding/j oining condition of the two component beams. If there is no bonding between A and B, the « bending rigidity of the composite beam is simply the sum of the bending rigidities of the two component beams, i.e. E Cl C = E A1 A + E BI B . However, if there is perfect bonding between A and B, the bending rigidity of the composite beam can be calculated with the use of the equivalent beam approach, i.e. converting the material A into material B (or B into A) and changing the transverse dimension of A. ...
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lecture05 - 10. COMPOSITES 10.1 General Guidelines A. In...

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