Introduction to Composite Materials Introduction to Composite Materials

Chapter 1

Introduction to Composite Materials

A composite material can be defined sheets of continuous fibers in different orienta- as a combination of two or more materials that tions to obtain the desired strength and stiffness results in better properties than those of the indi- properties with fiber volumes as high as 60 to vidual components used alone. In contrast to 70 percent. Fibers produce high-strength com- metallic alloys, each material retains its separate posites because of their small diameter; they con- chemical, physical, and mechanical properties. tain far fewer defects (normally surface defects) The two constituents are a reinforcement and a compared to the material produced in bulk. As a matrix. The main advantages of composite ma- general rule, the smaller the diameter of the fiber, terials are their high strength and stiffness, com- the higher its strength, but often the cost increases bined with low density, when compared with as the diameter becomes smaller. In addition, bulk materials, allowing for a weight reduction smaller-diameter high-strength fibers have greater in the finished part. flexibility and are more amenable to fabrication The reinforcing phase provides the strength processes such as weaving or forming over radii. and stiffness. In most cases, the reinforcement is Typical fibers include glass, aramid, and carbon, harder, stronger, and stiffer than the matrix. The which may be continuous or discontinuous. reinforcement is usually a fiber or a particulate. The continuous phase is the matrix, which is a Particulate composites have dimensions that are polymer, metal, or ceramic. Polymers have low approximately equal in all directions. They may strength and stiffness, metals have intermediate be spherical, platelets, or any other regular or ir- strength and stiffness but high ductility, and ce- regular geometry. Particulate composites tend to ramics have high strength and stiffness but are be much weaker and less stiff than continuous- brittle. The matrix (continuous phase) performs fiber composites, but they are usually much less several critical functions, including maintaining expensive. Particulate reinforced composites usu- the fibers in the proper orientation and spacing ally contain less reinforcement (up to 40 to 50 and protecting them from abrasion and the envi- volume percent) due to processing difficulties ronment. In polymer and metal matrix compos- and brittleness. ites that form a strong bond between the fiber A fiber has a length that is much greater than and the matrix, the matrix transmits loads from its diameter. The length-to-diameter (l/d) ratio is the matrix to the fibers through shear loading at known as the aspect ratio and can vary greatly. the interface. In ceramic matrix composites, the Continuous fibers have long aspect ratios, while objective is often to increase the toughness rather discontinuous fibers have short aspect ratios. than the strength and stiffness; therefore, a low Continuous-fiber composites normally have a interfacial strength bond is desirable. preferred orientation, while discontinuous fibers The type and quantity of the reinforcement generally have a random orientation. Examples determine the final properties. Figure 1.2 shows of continuous reinforcements include unidirec- that the highest strength and modulus are ob- tional, woven cloth, and helical winding (Fig. tained with continuous-fiber composites. There is 1.1a), while examples of discontinuous rein- a practical limit of about 70 volume percent reinforcements are chopped fibers and random mat forcement that can be added to form a composite. (Fig. 1.1b). Continuous-fiber composites are At higher percentages, there is too little matrix to often made into laminates by stacking single support the fibers effectively. The theoretical 2 / Structural Composite Materials

Fig. 1.1 Typical reinforcement types strength of discontinuous-fiber composites can a low-viscosity resin that reacts and cures during approach that of continuous-fiber composites processing, forming an intractable solid. A ther- if their aspect ratios are great enough and they moplastic is a high-viscosity resin that is pro- are aligned, but it is difficult in practice to main- cessed by heating it above its melting tempera- tain good alignment with discontinuous fibers. ture. Because a thermoset resin sets up and cures Discontinuous-fiber composites are normally during processing, it cannot be reprocessed by somewhat random in alignment, which dramati- reheating. By comparison, a thermoplastic can cally reduces their strength and modulus. How- be reheated above its melting temperature for ad- ever, discontinuous-fiber composites are gen ditional processing. There are processes for both erally much less costly than continuous-fiber classes of resins that are more amenable to dis- composites. Therefore, continuous-fiber com- continuous fibers and others that are more ame- posites are used where higher strength and stiff- nable to continuous fibers. In general, because ness are required (but at a higher cost), and metal and ceramic matrix composites require discontinuous-fiber composites are used where very high temperatures and sometimes high pres- cost is the main driver and strength and stiffness sures for processing, they are normally much are less important. more expensive than polymer matrix composites. Both the reinforcement type and the matrix af- However, they have much better thermal stabil- fect processing. The major processing routes for ity, a requirement in applications where the com- polymer matrix composites are shown in Fig. 1.3. posite is exposed to high temperatures. Two types of polymer matrices are shown: ther- This book will deal with both continuous and mosets and thermoplastics. A thermoset starts as discontinuous polymer, metal, and ceramic matrix Chapter 1: Introduction to Composite Materials / 3

Fig. 1.2 Influence of reinforcement type and quantity on composite performance

Fig. 1.3 Major polymer matrix composite fabrication processes 4 / Structural Composite Materials

composites, with an emphasis on continuous- material is anisotropic (for example, the compos- fiber, high-performance polymer composites. ite ply shown in Fig. 1.5), it has properties that vary with direction within the material. In this example, the moduli are different in each direction (E0° ≠ E45° ≠ E90°). While the modulus of 1.1 Isotropic, Anisotropic, and elasticity is used in the example, the same depen- Orthotropic Materials dence on direction can occur for other material properties, such as ultimate strength, Poisson’s Materials can be classified as either isotropic ratio, and thermal expansion coefficient. or anisotropic. Isotropic materials have the same Bulk materials, such as metals and polymers, material properties in all directions, and normal are normally treated as isotropic materials, while loads create only normal strains. By compari- composites are treated as anisotropic. However, son, anisotropic materials have different mate- even bulk materials such as metals can become rial properties in all directions at a point in the anisotropic––for example, if they are highly cold body. There are no material planes of symmetry, worked to produce grain alignment in a certain and normal loads create both normal strains and direction. shear strains. A material is isotropic if the prop- Consider the unidirectional fiber-reinforced erties are independent of direction within the composite ply (also known as a lamina) shown material. in Fig. 1.6. The coordinate system used to de- For example, consider the element of an iso- scribe the ply is labeled the 1-2-3 axes. In this tropic material shown in Fig. 1.4. If the material case, the 1-axis is defined to be parallel to the is loaded along its 0°, 45°, and 90° directions, fibers (0°), the 2-axis is defined to lie within the the modulus of elasticity (E) is the same in each plane of the plate and is perpendicular to the fi- direction (E0° = E45° = E90°). However, if the bers (90°), and the 3-axis is defined to be normal

Fig. 1.4 Element of isotropic material under stress Chapter 1: Introduction to Composite Materials / 5

Fig. 1.5 Element of composite ply material under stress

Fig. 1.6 Ply angle definition 6 / Structural Composite Materials

to the plane of the plate. The 1-2-3 coordinate Consider the unidirectional composite shown system is referred to as the principal material in the upper portion of Fig. 1.7, where the unidi- coordinate system. If the plate is loaded parallel rectional fibers are oriented at an angle of 45 de- to the fibers (one- or zero-degree direction), the grees with respect to the x-axis. In the small, modulus of elasticity E11 approaches that of the isolated square element from the gage region, be- fibers. If the plate is loaded perpendicular to cause the element is initially square (in this ex- the fibers in the two- or 90-degree direction, the ample), the fibers are parallel to diagonal AD of modulus E22 is much lower, approaching that of the element. In contrast, fibers are perpendicular the relatively less stiff matrix. Since E11 >> E22 to diagonal BC. This implies that the element is and the modulus varies with direction within the stiffer along diagonal AD than along diagonal material, the material is anisotropic. BC. When a tensile stress is applied, the square Composites are a subclass of anisotropic mate- element deforms. Because the stiffness is higher rials that are classified as orthotropic. Ortho- along diagonal AD than along diagonal BC, the tropic materials have properties that are different length of diagonal AD is not increased as much in three mutually perpendicular directions. They as that of diagonal BC. Therefore, the initially have three mutually perpendicular axes of sym- square element deforms into the shape of a par- metry, and a load applied parallel to these axes allelogram. Because the element has been dis- produces only normal strains. However, loads torted into a parallelogram, a shear strain gxy is that are not applied parallel to these axes produce induced as a result of coupling between the axial both normal and shear strains. Therefore, ortho- strains exx and eyy. tropic mechanical properties are a function of If the fibers are aligned parallel to the direc- orientation. tion of applied stress, as in the lower portion of

Fig. 1.7 Shear coupling in a 45° ply. Source: Ref 1 Chapter 1: Introduction to Composite Materials / 7

Fig. 1.7, the coupling between exx and eyy does ites are normally laminated materials (Fig. 1.8) not occur. In this case, the application of a ten- in which the individual layers, plies, or laminae sile stress produces elongation in the x-direction are oriented in directions that will enhance the and contraction in the y-direction, and the dis- strength in the primary load direction. Unidirec- torted element remains rectangular. Therefore, tional (0°) laminae are extremely strong and stiff the coupling effects exhibited by composites occur in the 0° direction. However, they are very weak only if stress and strain are referenced to a non– in the 90° direction because the load must be car- principal material coordinate system. Thus, when ried by the much weaker polymeric matrix. the fibers are aligned parallel (0°) or perpendic- While a high-strength fiber can have a tensile ular (90°) to the direction of applied stress, the strength of 500 ksi (3500 MPa) or more, a typical lamina is known as a specially orthotropic lam- polymeric matrix normally has a tensile strength ina (θ = 0° or 90°). A lamina that is not aligned of only 5 to 10 ksi (35 to 70 MPa) (Fig. 1.9). The parallel or perpendicular to the direction of ap- longitudinal tension and compression loads are plied stress is called a general orthotropic lam- carried by the fibers, while the matrix distributes ina (θ ≠ 0° or 90°). the loads between the fibers in tension and stabi- lizes the fibers and prevents them from buckling in compression. The matrix is also the primary 1.2 Laminates load carrier for interlaminar shear (i.e., shear between the layers) and transverse (90°) tension. When there is a single ply or a lay-up in which The relative roles of the fiber and the matrix in all of the layers or plies are stacked in the same detemining mechanical properties are summa- orientation, the lay-up is called a lamina. When rized in Table 1.1. the plies are stacked at various angles, the lay-up Because the fiber orientation directly impacts is called a laminate. Continuous-fiber compos- mechanical properties, it seems logical to orient

Fig. 1.8 Lamina and laminate lay-ups 8 / Structural Composite Materials

Fig. 1.9 Comparison of tensile properties of fiber, matrix, and composite

Table 1.1 effect of fiber and matrix on 1.3 Fundamental Property Relationships mechanical properties Dominating composite constituent When a unidirectional continuous-fiber lam- Mechanical property Fiber Matrix ina or laminate (Fig. 1.11) is loaded in a di Unidirectional rection parallel to its fibers (0° or 11-direction), 0º tension √ … 0º compression √ √ the longitudinal modulus E11 can be estimated Shear … √ from its constituent properties by using what is 90º tension … √ known as the rule of mixtures: Laminate … Tension √ E E V E V (Eq 1.1) Compression √ √ 11 = f f + m m In-plane shear √ √ … Interlaminar shear √ where Ef is the fiber modulus, Vf is the fiber volume percentage, Em is the matrix modulus, and Vm is the matrix volume percentage. The longitudinal tensile strength s11 also can as many of the layers as possible in the main be estimated by the rule of mixtures: load-carrying direction. While this approach may work for some structures, it is usually nec- s11 = sVf + smVm (Eq 1.2) essary to balance the load-carrying capability in a number of different directions, such as the where sf and sm are the ultimate fiber and ma- 0°, +45°, -45°, and 90° directions. Figure 1.10 trix strengths, respectively. Because the proper- shows a photomicrograph of a cross-plied con- ties of the fiber dominate for all practical vol- tinuous carbon fiber/epoxy laminate. A balanced ume percentages, the values of the matrix can laminate having equal numbers of plies in the often be ignored; therefore: 0°, +45°, –45°, and 90° degrees directions is E E V (Eq 1.3) called a quasi-isotropic laminate, because it car- 11 ≈ f f ries equal loads in all four directions. s11 ≈ sVf (Eq 1.4) Chapter 1: Introduction to Composite Materials / 9

Fig. 1.10 Cross section of a cross-plied carbon/epoxy laminate

Fig. 1.11 Unidirectional continuous-fiber lamina or laminate 10 / Structural Composite Materials

Figure 1.12 shows the dominant role of the fi- n12 = nfVf + nmVm (Eq 1.6) bers in determining strength and stiffness. When 1/G12 = Vf /Gf + Vm/Gm (Eq 1.7) loads are parallel to the fibers (0°), the ply is much stronger and stiffer than when loads are These expressions are somewhat less useful transverse (90°) to the fiber direction. There is a than the previous ones, because the values for dramatic decrease in strength and stiffness re- Poisson’s ratio (nf) and the shear modulus (Gf) sulting from only a few degrees of misalignment of the fibers are usually not readily available. off of 0°. Physical properties, such as density (r), can When the lamina shown in Fig. 1.11 is loaded also be expressed using rule of mixture relations: in the transverse (90° or 22-direction), the fibers and the matrix function in series, with both car- r = r V + r V (Eq 1.8) rying the same load. The transverse modulus of 12 f f m m elasticity E22 is given as: While these micromechanics equations are useful for a first estimation of lamina properties 1/E22 = Vf /Ef + Vm/Em (Eq 1.5) when no data are available, they generally do not yield sufficiently accurate values for design pur- Figure 1.13 shows the variation of modulus as poses. For design purposes, basic lamina and a function of fiber volume percentage. When the laminate properties should be determined using fiber percentage is zero, the modulus is essen- actual mechanical property testing. tially the modulus of the polymer, which increases up to 100 percent (where it is the modulus of the fiber). At all other fiber volumes, the 1.4 Composites versus Metallics E22 or 90° modulus is lower than the E11 or zero degrees modulus, because it is dependent on the As previously discussed, the physical character- much weaker matrix. istics of composites and metals are significantly Other rule of mixture expressions for lamina different. Table 1.2 compares some properties of properties include those for the Poisson’s ratio composites and metals. Because composites are n12 and for the shear modulus G12: highly anisotropic, their in-plane strength and

Fig. 1.12 Influence of ply angle on strength and modulus