3 OBJECTIVE To provide a basic understanding of composite materials and composite types. To understand processing, properties, characterisation and design of composite materials & structures.
4 REFERENCES Krishnan K. Chawla, Composite Materials Science and Engineering, Springer, Matthews, F.L. and R.D. Rawlings, 1999, Composite Materials: Engineering and Science, Woodhead Publishing. Handbook of Composites, American Society of Metals, Derek Hull, Introduction to Composite Materials, Cambridge University Press, 1988.
5 ISSUES TO ADDRESS What is composite? Why are composites used instead of metals/ceramics or polymers? What are the classes and types of composites? What are the typical applications of composite materials?
6 The development of materials over time Composite Materials The materials of pre-history, on the left, all occur naturally; the challenge for the engineers of that era was one of shaping them. The development of thermochemistry and (later) of polymer chemistry enabled manmade materials, shown in the colored zones. Three stone, bronze and iron were of such importance that the era of their dominance is named after them.
7 Classifications of Materials With technological progress, natural materials become insufficient to meet increasing demands on product capabilities and functions. Metals Materials Polymers Ceramics Composites
8 What is composite?
9 Introduction to Composite Materials Historical Perspective Used in ancient Egypt, Americas, and China Straw was used to reinforce bricks Many natural materials are composites Wood, grasses, bones, fingernails, bee hives, bird nests, deer antlers, etc. Composite Materials
10 Introduction to Composite Materials Wood (a natural composite as distinguished from a synthesized composite). This is one of the oldest and the most widely used structural material. It is a composite of strong and flexible cellulose fibers (linear polymer) surrounded and held together by a matrix of lignin and other polymers. Wood has extreme anisotropy because 90 to 95% of all the cells are elongated and vertical (i.e. aligned parallel to the tree trunk). The remaining 5 to 10% of cells are arranged in radial directions, with no cells at all aligned tangentially. Wood is ten times stronger in the axial direction than in the radial or tangential directions. The properties of wood are anisotropic and vary widely among types of wood. A cut-through of a tree trunk Composite Materials
11 Introduction to Composite Materials A composite material is a macroscopic, physical combination of two or more materials in which one material usually provides reinforcement. In most composites one material is continuous and is termed the matrix, while the second, usually discontinuous phase, is termed the reinforcement, in some cases filler is applied. FUNCTION: Dispersed (reinforcing) phase... The second phase (or phases) is imbedded in the matrix in a continuous or discontinuous form. Dispersed phase is usually stronger than the matrix, therefore it is sometimes called reinforcing phase. Can be one of the three basic materials or an element such as carbon or boron
12 Introduction to Composite Materials Matrix material serves several functions in the composite Provides the bulk form of the part or product Holds the imbedded phase in place Shares the load with the secondary phase Protect the reinforcements from surface damage due to abrasion or chemical effect Bonding strength between reinforcement and matrix is important
13 Introduction to Composite Materials Matrix... The continuous phase, the primary phase Purpose is to: transfer stress to other phases protect phases from environment Matrix considerations... End use temperature Toughness Cosmetic ıssues Flame retardant Processing method Adhesion requirements
14 Introduction to Composite Materials Composite Materials Combination of 2 or more materials Each of the materials must exist more than 5% Presence of interphase The properties shown by the composite materials are differed from the initial materials Can be produced by various processing techniques Composite materials- a new emerging class of materials to overcome a current limits of monolithic of conventional materials
15 Introduction to Composite Materials Composites are not new materials. Perhaps the first important engineering structural composite was the Biblical straw-reinforced, sun-dried mud brick adobe. Laminated structures such as bows have been used since prehistoric times. In the early 1900s doped fabric was employed in early aircraft surfaces. Reinforced phenolics were developed in the 1930s and glass-reinforced plastics in the 1940s. More recently, emphasis turned to reinforcements, with graphitic and boronbased fibers developed in the 1960s. High-performance aramids, such as Kevlar, were developed in the 1970s. This and the previous decade have seen new developments in both fiber and matrix with lightweight aerospace MMCs and high-temperature CMCs showing major advances. Composite Materials
16 COMPOSITE MATERIALS Composite materials have been utilized to solve technological problems for a long time but only in the 1960s did these materials start capturing the attention of industries with the introduction of polymeric-based composites. The primary barrier to the use of composite materials is their high initial costs in some cases, as compared to traditional materials. Regardless of how effective the material will be over its life cycle, industry considers high upfront costs, particularly when the life-cycle cost is relatively uncertain. In general, the cost of processing composites is high, especially in the hand lay-up process. Here, raw material costs represent a small fraction of the total cost of a finished product. There is already evidence of work moving to Asia, Mexico, and Korea for the cases where labor costs are a significant portion of the total product costs. The recycling of composite materials presents a problem when penetrating a highvolume market such as the automotive industry, where volume production is in the millions of parts per year. With the new goverment regulations and environmental awareness, the use of composites has become a concern and poses a big challenge for recycling.
17 Driving Force for Composites Common driving forces for the use of composite materials include the ability to save weight, increase mechanical properties, reduce the number of elements in a component, obtain a unique combination of properties, and to increase shaping freedom. Increasingly, composites are being used for the above while also achieving a reduction in part cost. Many of these driving forces, together with the manufacturing cycle, often offset the higher raw material costs of the composite constituents to produce a commercially viable end product. The criteria on which composite materials are selected for a particular application are naturally dependent on the industrial sector for which they are intended. Composite Materials
18 Driving Force for Composites For example, aerospace has traditionally been driven by performance, where longer cycle times and increased scrap levels were tolerated, whereas high volume applications, typified by the automotive industry, require rapid and highly automated techniques but where the full potential of composites in terms of mechanical properties is seldom reached. Composite materials selection criteria as a functional of industrial sector
19 Why are composites used instead of metals/ceramics or polymers?
20 Introduction to Composite Materials CERAMICS Hard, corrosion and wear resistant BUT BRITTLE METALS Soft, conductive, high fracture toughnes, ductile HEAVY, low temperature use GLASSES Non-crystalline solid, hard, brittle, vulnerable to stress concentration POLYMERS Easy to shape Low modulus, low temperature use If two heads are better than one, could two materials be better than one? - COMPOSITES
21 Introduction to Composite Materials A comparison of the properties of ceramics, metals, and polymers If two heads are better than one, could two materials be better than one? - COMPOSITES Composite Materials
22 Design Objectives Performance: Strength, Temperature Manufacturing Techniques Life Cycle Considerations Cost
24 COMPOSITE MATERIALS Composite materials are combinations of materials put together to achieve particular function.
25 COMPOSITE MATERIALS ADVANTAGES OF COMPOSITE MATERIALS Unique combination of properties For example, tungsten wire is very stiff (405 GPa) but very dense (19.3 Mgm -3 ). A combination of graphite fibre in epoxy resin is nearly as stiff (306 GPa) but with a density of only 1.5 Mgm -3. The carbon fibre itself is much stiffer than the tungsten; values up to 1000GPa at a density of 2.6 M gm 3 are possible.
26 COMPOSITE MATERIALS ADVANTAGES OF COMPOSITE MATERIALS Properties can be controlled in a wide range If only a small number of simple materials are available the variation of a given property among those materials is digitised to the values the individual materials may show. Figure given below shows the variation of coefficient of expansion with volume fraction for a composite consisting of aluminium containing silicon carbide particles. It is seen that matching the coefficient of thermal expansion with that of other materials is easily obtained. The similar nondigitisation of properties is important in, for example, acoustic wave devices and in many other matching situations, e.g., in prosthetic devices.
27 COMPOSITE MATERIALS ADVANTAGES OF COMPOSITE MATERIALS Properties can be controlled in a wide range Variation of coefficient of thermal expansion with volume fraction of SiC particles in aluminium. Ordinate values x 10-6 K -1.
28 COMPOSITE MATERIALS ADVANTAGES OF COMPOSITE MATERIALS Composites can sometimes attain a value of a given physical property not attainable by either of the two components alone. Thermal conductivity of various materials are given below and it is quite clear there that composites can attain a lower thermal conductivity than that of others. This may be regarded as a rather special example since the combination is with air or vacuum as one of the components. However, it indicates the generally important message, which stimulates the imagination, that often empty space can be an important component of the properties of a solid.
29 Advantages of Composites
30 Advantages of Composites Low weight, high specific properties (many natural, and biological materials are composites) Use of extremely high property (strength and modulus) constituents Design flexibility: The rule-of-mixtures - an additional design degree of freedom Synergistic effects: Role of the interface, of heterogeneity / anisotropy / hierarchy Anisotropy: property directionality Heterogeneity: chemical variability
31 Limitations of Composites Some of the associated disadvantages of advanced composites are as follows: High cost of raw materials and fabrication. Composites are more brittle than wrought metals and thus are more easily damaged. Transverse properties may be weak. Matrix is weak, therefore, low toughness. Reuse and disposal may be difficult. Difficult to attach
32 Limitations of Composites Properties of many important composites are anisotropic - the properties differ depending on the direction in which they are measured this may be an advantage or a disadvantage Many of the polymer-based composites are subject to attack by chemicals or solvents, just as the polymers themselves are susceptible to attack Composite materials are generally expensive Manufacturing methods for shaping composite materials are often slow and costly
33 Limitations of Composites Repair introduces new problems, for the following reasons: Materials require refrigerated transport and storage and have limited shelf life. Hot curing is necessary in many cases requiring special tooling. Hot or cold curing takes time. Analysis is difficult. Matrix is subject to environmental degradation.
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