10 Structural Composite Materials Figure 1.12 shows the dominant role of the fi- V12=Vilt+VmVm (Eq1.6) bers in determining strength and stiffness.When l/G12='/Gr+'/Gm (Eq1.7) 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 (v)and the shear modulus (G) sulting from only a few degrees of misalignment of the fibers are usually not readily available. off of0° Physical properties,such as density (p),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- P12=PiVt+PmVm (Eq1.8) rying the same load.The transverse modulus of elasticity E22 is given as: While these micromechanics equations are useful for a first estimation of lamina properties 1/E22=VE+'/Em (Eq1.5) 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 in- creases up to 100 percent (where it is the modu- lus of the fiber).At all other fiber volumes,the 1.4 Composites versus Metallics E22 or 90 modulus is lower than the Eu 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 Vi2 and for the shear modulus G2: highly anisotropic,their in-plane strength and 1.0 1.0 0.8 sninpow 0.8 0.6 0.6 0.4 0.4 0.2 wnwixew/sninpoW leulpny6uoT 0.2 0 0 0 30 60 90 30 60 90 Ply Angle(0) Ply Angle(0) Fig.1.12 Influence of ply angle on strength and modulus
10 / Structural Composite Materials Figure 1.12 shows the dominant role of the fibers in determining strength and stiffness. When loads are parallel to the fibers (0°), the ply is much stronger and stiffer than when loads are transverse (90°) to the fiber direction. There is a dramatic decrease in strength and stiffness resulting from only a few degrees of misalignment off of 0°. When the lamina shown in Fig. 1.11 is loaded in the transverse (90° or 22-direction), the fibers and the matrix function in series, with both carrying the same load. The transverse modulus of elasticity E22 is given as: 1/E22 = Vf /Ef + Vm/Em (Eq 1.5) Figure 1.13 shows the variation of modulus as a function of fiber volume percentage. When the fiber percentage is zero, the modulus is essentially 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 E22 or 90° modulus is lower than the E11 or zero degrees modulus, because it is dependent on the much weaker matrix. Other rule of mixture expressions for lamina properties include those for the Poisson’s ratio n12 and for the shear modulus G12: n12 = nf Vf + nmVm (Eq 1.6) 1/G12 = Vf /Gf + Vm/Gm (Eq 1.7) These expressions are somewhat less useful than the previous ones, because the values for Poisson’s ratio (nf ) and the shear modulus (Gf ) of the fibers are usually not readily available. Physical properties, such as density (r), can also be expressed using rule of mixture relations: r12 = rf Vf + rmVm (Eq 1.8) While these micromechanics equations are useful for a first estimation of lamina properties when no data are available, they generally do not yield sufficiently accurate values for design purposes. For design purposes, basic lamina and laminate properties should be determined using actual mechanical property testing. 1.4 Composites versus Metallics As previously discussed, the physical characteristics of composites and metals are significantly different. Table 1.2 compares some properties of composites and metals. Because composites are highly anisotropic, their in-plane strength and Fig. 1.12 Influence of ply angle on strength and modulus
Chapter 1:Introduction to Composite Materials /11 E Ez V Fig.1.13 Variation of composite modulus of a unidirectional lamina as a function of fiber volume fraction Table 1.2 Composites versus metals Metals typically have reasonable ductility,con- comparison tinuing to elongate or compress considerably Condition Comparative behavior relative to metals when they reach a certain load(through yielding) Load-strain relationship More linear strain to failure without picking up more load and without fail- Notch sensitivity ure.Two important benefits of this ductile yield- Static Greater sensitivity Fatigue Less sensitivity ing are that(1)it provides for local load relief by Transverse properties Weaker distributing excess load to an adjacent material Mechanical property Higher variability or structure;therefore,ductile metals have a great Fatigue strength Higher capacity to provide relief from stress concentra- Sensitivity to hydrothermal Greater tions when statically loaded;and(2)it provides environment Sensitivity to corrosion Much less great energy-absorbing capability(indicated by Damage growth mechanism In-plane delamination instead of the area under a stress-strain curve).As a result, through thickness cracks when impacted,a metal structure typically de- Source:Ref2 forms but does not actually fracture.In contrast. composites are relatively brittle.Figure 1.15 shows a comparison oftypical tensile stress-strain curves for two materials.The brittleness of the stiffness are usually high and directionally vari- composite is reflected in its poor ability to toler- able,depending on the orientation of the rein- ate stress concentrations,as shown in Fig.1.16. forcing fibers.Properties that do not benefit from The characteristically brittle composite material this reinforcement (at least for polymer matrix has poor ability to resist impact damage without composites)are comparatively low in strength extensive internal matrix fracturing. and stiffness-for example,the through-the- The response of damaged composites to cyclic thickness tensile strength where the relatively loading is also significantly different from that of weak matrix is loaded rather than the high- metals.The ability of composites to withstand strength fibers.Figure 1.14 shows the low cyclic loading is far superior to that of metals,in through-the-thickness strength of a typical com- contrast to the poor composite static strength posite laminate compared with aluminum. when it has damage or defects.Figure 1.17
Chapter 1: Introduction to Composite Materials / 11 stiffness are usually high and directionally variable, depending on the orientation of the reinforcing fibers. Properties that do not benefit from this reinforcement (at least for polymer matrix composites) are comparatively low in strength and stiffness—for example, the through-thethickness tensile strength where the relatively weak matrix is loaded rather than the highstrength fibers. Figure 1.14 shows the low through-the-thickness strength of a typical composite laminate compared with aluminum. Metals typically have reasonable ductility, continuing to elongate or compress considerably when they reach a certain load (through yielding) without picking up more load and without failure. Two important benefits of this ductile yielding are that (1) it provides for local load relief by distributing excess load to an adjacent material or structure; therefore, ductile metals have a great capacity to provide relief from stress concentrations when statically loaded; and (2) it provides great energy-absorbing capability (indicated by the area under a stress-strain curve). As a result, when impacted, a metal structure typically deforms but does not actually fracture. In contrast, composites are relatively brittle. Figure 1.15 shows a comparison of typical tensile stress-strain curves for two materials. The brittleness of the composite is reflected in its poor ability to tolerate stress concentrations, as shown in Fig. 1.16. The characteristically brittle composite material has poor ability to resist impact damage without extensive internal matrix fracturing. The response of damaged composites to cyclic loading is also significantly different from that of metals. The ability of composites to withstand cyclic loading is far superior to that of metals, in contrast to the poor composite static strength when it has damage or defects. Figure 1.17 Table 1.2 Composites versus metals comparison Condition Comparative behavior relative to metals Load-strain relationship More linear strain to failure Notch sensitivity Static Fatigue Greater sensitivity Less sensitivity Transverse properties Weaker Mechanical property variability Higher Fatigue strength Higher Sensitivity to hydrothermal environment Greater Sensitivity to corrosion Much less Damage growth mechanism In-plane delamination instead of through thickness cracks Source: Ref 2 Fig. 1.13 Variation of composite modulus of a unidirectional 0° lamina as a function of fiber volume fraction
12 Structural Composite Materials 100 80 60 40 20 0 2024-T3 7075-T6 Carbon/Epoxy Aluminum Aluminum(0°，t45°，90)s Sheet Sheet Fig1.14 Comparisonofthrou-te-thickess tensile strenghofa composite laminate with auminum alloy sheet.Source:Ref3 80 7075-T6 60 Aluminum Sheet 90 T-300/5208 Carbon/Epoxy (0°，±45°，90)5 20 0 0 2 4 6 8 Tensile Strain(%) Fig.1.15 Comparison of typical stress-strain curves for a composite laminate and aluminum alloy sheet.Source:Ref3
12 / Structural Composite Materials Fig. 1.14 Comparison of through-the-thickness tensile strength of a composite laminate with aluminum alloy sheet. Source: Ref 3 Fig. 1.15 Comparison of typical stress-strain curves for a composite laminate and aluminum alloy sheet. Source: Ref 3
Chapter 1:Introduction to Composite Materials /13 100 (%)y16uans 80 7075-T6 60 Aluminum 40 T-300/SP286 Carbon/Epoxy 20 (0°，±45°，903 0 0 25 50 75 100 Hole Out (% Fig.1.16 Compared with aluminum alloy sheet,a composite laminate has poor tolerance of stress concentration because of its brittle nature.Source:Ref 3 R=1.0;Kt=3.0 Carbon/Epoxy (0°，±45°，90)5 1.0 0.8 7075-T6 0.6 Aluminum wnwxew 0.4 0.2 0 102 103 104 105 106 107 Cycles to Failure Fig.1.17 Comparative notched fatigue strength of composite laminate and aluminum alloy sheet.Source:Ref 3
Chapter 1: Introduction to Composite Materials / 13 Fig. 1.16 Compared with aluminum alloy sheet, a composite laminate has poor tolerance of stress concentration because of its brittle nature. Source: Ref 3 Fig. 1.17 Comparative notched fatigue strength of composite laminate and aluminum alloy sheet. Source: Ref 3
14 Structural Composite Materials shows a comparison of the normalized notched The specific strength(strength/density)and specimen fatigue response of a common 7075- specific modulus (modulus/density)of high- T6 aluminum aircraft metal and a carbon/epoxy strength fibers (especially carbon)are higher laminate.The fatigue strength of the composite than those of other comparable aerospace metal- is much higher relative to its static or residual lic alloys(Fig.1.18).This translates into greater strength.The static or residual strength require- weight savings resulting in improved perfor- ment for structures is typically much higher than mance,greater payloads,longer range,and fuel the fatigue requirement.Therefore,because the savings.Figure 1.19 compares the overall struc- fatigue threshold of composites is a high percent- tural efficiency of carbon/epoxy,Ti-6Al-4V,and age of their static or damaged residual strength, 7075-T6 aluminum. they are usually not fatigue critical.In metal The chief engineer of aircraft structures for structures,fatigue is typically a critical design the U.S.Navy once told the author that he liked consideration. composites because "they don't rot [corrode] and they don't get tired [fatigue]."Corrosion of aluminum alloys is a major cost and a con- 1.5 Advantages and Disadvantages of stant maintenance problem for both commer- Composite Materials cial and military aircraft.The corrosion resis- tance of composites can result in major savings The advantages of composites are many,in- in supportability costs.Carbon fiber composites cluding lighter weight,the ability to tailor the lay- cause galvanic corrosion of aluminum if the fi- up for optimum strength and stiffness,improved bers are placed in direct contact with the metal fatigue life,corrosion resistance,and,with good surface,but bonding a glass fabric electrical design practice,reduced assembly costs due to insulation layer on all interfaces that contact fewer detail parts and fasteners. aluminum eliminates this problem.The fatigue 2.0 High-Strength 1.5 Carbon Epoxy Aramid /Epoxy 是 1.0 Titanium (Ti-6Al-4V) Steel(260 Ksi) E-Glass/Epoxy* Aluminum(7075-T6) 0.5 0 0 50 100 150 200 250 300 Specific Modulus(106 in.) *[±45°，0°901s Fig.1.18 Comparison of specific strength and modulus of high-strength composites and some aerospace alloys
14 / Structural Composite Materials shows a comparison of the normalized notched specimen fatigue response of a common 7075- T6 aluminum aircraft metal and a carbon/epoxy laminate. The fatigue strength of the composite is much higher relative to its static or residual strength. The static or residual strength requirement for structures is typically much higher than the fatigue requirement. Therefore, because the fatigue threshold of composites is a high percentage of their static or damaged residual strength, they are usually not fatigue critical. In metal structures, fatigue is typically a critical design consideration. 1.5 Advantages and Disadvantages of Composite Materials The advantages of composites are many, including lighter weight, the ability to tailor the layup for optimum strength and stiffness, improved fatigue life, corrosion resistance, and, with good design practice, reduced assembly costs due to fewer detail parts and fasteners. The specific strength (strength/density) and specific modulus (modulus/density) of highstrength fibers (especially carbon) are higher than those of other comparable aerospace metallic alloys (Fig. 1.18). This translates into greater weight savings resulting in improved performance, greater payloads, longer range, and fuel savings. Figure 1.19 compares the overall structural efficiency of carbon/epoxy, Ti-6Al-4V, and 7075-T6 aluminum. The chief engineer of aircraft structures for the U.S. Navy once told the author that he liked composites because “they don’t rot [corrode] and they don’t get tired [fatigue].” Corrosion of aluminum alloys is a major cost and a constant maintenance problem for both commercial and military aircraft. The corrosion resistance of composites can result in major savings in supportability costs. Carbon fiber composites cause galvanic corrosion of aluminum if the fibers are placed in direct contact with the metal surface, but bonding a glass fabric electrical insulation layer on all interfaces that contact aluminum eliminates this problem. The fatigue Fig. 1.18 Comparison of specific strength and modulus of high-strength composites and some aerospace alloys