200 150 2 100 5 Central hole dia. 1.[0l4s 50 =10mm 2.[02/452ls Specimen width 3.[0/452s =50mm 4.[452/02ls 0 5.[4514s 0.01 0.02 0.03 0.04 0.05 Strain FIGURE 4.13 Effect of stacking sequence on the tensile properties of woven fabric laminates with a central hole.(Adapted from Naik,N.K.,Shembekar,P.S.,and Verma,M.K.,J.Compos.Mater.,24,838,1990.) directions,which is smaller for the 181 fabric style than for the 143 fabric style. Tensile properties of fabric-reinforced laminates can also be controlled by chang- ing the lamination pattern(see,e.g.,parallel lamination vs.cross lamination of the laminates with 143 fabric style in Table 4.4)and stacking sequence(Figure 4.13). 4.1.1.6 Sheet-Molding Compounds Figure 4.14 shows the typical tensile stress-strain diagram for a random fiber SMC(SMC-R)composite containing randomly oriented chopped fibers in a CaCO3-filled polyester matrix.The knee in this diagram corresponds to the development of craze marks in the specimen [9].At higher loads,the density of craze marks increases until failure occurs by tensile cracking in the matrix and fiber pullout.Both tensile strength and tensile modulus increase with fiber volume fraction.The stress at the knee is nearly independent of fiber volume fractions >20%.Except for very flexible matrices (with high elongations at failure),the strain at the knee is nearly equal to the matrix failure strain.In general,SMC-R composites exhibit isotropic properties in the plane of the laminate;however,they are capable of exhibiting large scatter in strength values from specimen to specimen within a batch or between batches.The variation in strength can be attributed to the manufacturing process for SMC-R composites. They are compression-molded instead of the carefully controlled hand layup technique used for many continuous fiber laminates.A discussion of process- induced defects in compression-molded composites is presented in Chapter 5. 2007 by Taylor Francis Group,LLC
directions, which is smaller for the 181 fabric style than for the 143 fabric style. Tensile properties of fabric-reinforced laminates can also be controlled by changing the lamination pattern (see, e.g., parallel lamination vs. crosslamination of the laminates with 143 fabric style in Table 4.4) and stacking sequence (Figure 4.13). 4.1.1.6 Sheet-Molding Compounds Figure 4.14 shows the typical tensile stress–strain diagram for a random fiber SMC (SMC-R) composite containing randomly oriented chopped fibers in a CaCO3-filled polyester matrix. The knee in this diagram corresponds to the development of craze marks in the specimen [9]. At higher loads, the density of craze marks increases until failure occurs by tensile cracking in the matrix and fiber pullout. Both tensile strength and tensile modulus increase with fiber volume fraction. The stress at the knee is nearly independent of fiber volume fractions >20%. Except for very flexible matrices (with high elongations at failure), the strain at the knee is nearly equal to the matrix failure strain. In general, SMC-R composites exhibit isotropic properties in the plane of the laminate; however, they are capable of exhibiting large scatter in strength values from specimen to specimen within a batch or between batches. The variation in strength can be attributed to the manufacturing process for SMC-R composites. They are compression-molded instead of the carefully controlled hand layup technique used for many continuous fiber laminates. A discussion of processinduced defects in compression-molded composites is presented in Chapter 5. 1 2 3 4 5 1. [0]4S 2. [02/452]S 3. [0/45]2S 4. [452/02]S 5. [45]4S Central hole dia. = 10 mm Specimen width = 50 mm 200 150 100 Stress (MPa) Strain 50 0 0 0.01 0.02 0.03 0.04 0.05 FIGURE 4.13 Effect of stacking sequence on the tensile properties of woven fabric laminates with a central hole. (Adapted from Naik, N.K., Shembekar, P.S., and Verma, M.K., J. Compos. Mater., 24, 838, 1990.) 2007 by Taylor & Francis Group, LLC
1400 203 Glass fiber 300 43.5 200 Knee 29.0 /point Tensile strength 100 14.5 Matrix 0 0 0 0.5 1.0 1.52.0 Strain(%) FIGURE 4.14 Tensile stress-strain diagram of an SMC-R laminate.(After Watanabe,T. and Yasuda,M.,Composites,13,54,1982.) Tensile stress-strain diagrams for SMC composites containing both continuous and randomly oriented fibers(SMC-CR and XMC)are shown in Figure 4.15.As in the case of SMC-R composites,these stress-strain diagrams are also bilinear.Unlike SMC-R composites,the tensile strength and modulus of SMC-CR and XMC composites depend strongly on the fiber orientation angle of continuous fibers relative to the tensile loading axis.Although the longitudinal tensile strength and modulus of SMC-CR and XMC are consi- derably higher than those of SMC-R containing equivalent fiber volume fractions,they decrease rapidly to low values as the fiber orientation angle is increased(Figure 4.16).The macroscopic failure mode varies from fiber failure and longitudinal splitting at =0 to matrix tensile cracking at 0 =90.For other orientation angles,a combination of fiber-matrix interfacial shear fail- ure and matrix tensile cracking is observed. 4.1.1.7 Interply Hybrid Laminates Interply hybrid laminates are made of separate layers of low-elongation(LE) fibers,such as high-modulus carbon fibers,and high-elongation (HE)fibers, such as E-glass or Kevlar 49,both in a common matrix.When tested in tension, the interply hybrid laminate exhibits a much higher ultimate strain at failure than the LE fiber composites (Figure 4.17).The strain at which the LE fibers 2007 by Taylor&Francis Group.LLC
Tensile stress–strain diagrams for SMC composites containing both continuous and randomly oriented fibers (SMC-CR and XMC) are shown in Figure 4.15. As in the case of SMC-R composites, these stress–strain diagrams are also bilinear. Unlike SMC-R composites, the tensile strength and modulus of SMC-CR and XMC composites depend strongly on the fiber orientation angle of continuous fibers relative to the tensile loading axis. Although the longitudinal tensile strength and modulus of SMC-CR and XMC are considerably higher than those of SMC-R containing equivalent fiber volume fractions, they decrease rapidly to low values as the fiber orientation angle is increased (Figure 4.16). The macroscopic failure mode varies from fiber failure and longitudinal splitting at u ¼ 08 to matrix tensile cracking at u ¼ 908. For other orientation angles, a combination of fiber–matrix interfacial shear failure and matrix tensile cracking is observed. 4.1.1.7 Interply Hybrid Laminates Interply hybrid laminates are made of separate layers of low-elongation (LE) fibers, such as high-modulus carbon fibers, and high-elongation (HE) fibers, such as E-glass or Kevlar 49, both in a common matrix. When tested in tension, the interply hybrid laminate exhibits a much higher ultimate strain at failure than the LE fiber composites (Figure 4.17). The strain at which the LE fibers 1400 300 203 43.5 29.0 14.5 0 200 100 0 0 0.5 1.0 Matrix Knee point Glass fiber Tensile strength Strain (%) Tensile stress (MPa) Tensile stress (103 psi) 1.5 2.0 FIGURE 4.14 Tensile stress–strain diagram of an SMC-R laminate. (After Watanabe, T. and Yasuda, M., Composites, 13, 54, 1982.) 2007 by Taylor & Francis Group, LLC
17.4 GPa 300 21.4 43.5 GPa -X Longitudinal 豆 200 29.0 ssels 100 12.4 GPa 14.5 Transverse 4.32 GPa 0 0 0.5 1.0 1.5 2.0 2.5 Strain (% FIGURE 4.15 Tensile stress-strain diagrams for an SMC-C20R30 laminate in the longitudinal (0)and transverse(90)directions.(After Riegner,D.A.and Sanders,B.A., A characterization study of automotive continuous and random glass fiber composites, Proceedings National Technical Conference,Society of Plastics Engineers,November 1979.) 700 101.5 600 87.0 500 XMC 3 72.5 豆 400 58.0 SMC-C20R30 300 43.5 SMC-R65 200 SMC-R50 29.0 0 100 14.5 SMC-R25 2030 40506070 90 0 0 10 80 Fiber orientation angle(degrees) FIGURE 4.16 Variation of tensile strength of various SMC laminates with fiber orien- tation angle.(After Riegner,D.A.and Sanders,B.A.,A characterization study of automotive continuous and random glass fiber composites,Proceedings National Tech- nical Conference,Society of Plastics Engineers,November 1979.) 2007 by Taylor Francis Group,LLC
300 43.5 29.0 14.5 200 100 Tensile stress (MPa) Tensile stress (103 psi) 0 0 0.5 1.0 Strain (%) 21.4 GPa 17.4 GPa Longitudinal 12.4 GPa Transverse 4.32 GPa 1.5 2.0 2.5 0 FIGURE 4.15 Tensile stress–strain diagrams for an SMC-C20R30 laminate in the longitudinal (08) and transverse (908) directions. (After Riegner, D.A. and Sanders, B.A., A characterization study of automotive continuous and random glass fiber composites, Proceedings National Technical Conference, Society of Plastics Engineers, November 1979.) XMC 3 SMC-C20R30 SMC-R65 SMC-R50 Fiber orientation angle (degrees) SMC-R25 700 600 500 400 Tensile strength (MPa) 300 200 100 0 0 10 20 30 40 50 60 70 80 90 101.5 87.0 72.5 58.0 Tensile strength (103 psi) 43.5 29.0 14.5 0 FIGURE 4.16 Variation of tensile strength of various SMC laminates with fiber orientation angle. (After Riegner, D.A. and Sanders, B.A., A characterization study of automotive continuous and random glass fiber composites, Proceedings National Technical Conference, Society of Plastics Engineers, November 1979.) 2007 by Taylor & Francis Group, LLC
3 0 0 Strain(%) FIGURE 4.17 Tensile stress-strain diagram for a GY-70 carbon/S glass-epoxy interply hybrid laminate.(After Aveston,J.and Kelly,A.,Phil.Trans.R.Soc.Lond., A,294,519,1980) in the hybrid begin to fail is either greater than or equal to the ultimate tensile strain of the LE fibers.Furthermore,instead of failing catastrophically,the LE fibers now fail in a controlled manner,giving rise to a step or smooth inflection in the tensile stress-strain diagram.During this period,multiple cracks are observed in the LE fiber layers [10]. The ultimate strength of interply hybrid laminates is lower than the tensile strengths of either the LE or the HE fiber composites(Figure 4.18).Note that 200 40 150 Modulus 30 Strength 100 20 )sninpow 50 10 0 0 0 20 40 60 80 100 Relative carbon fiber content(vol%) FIGURE 4.18 Variations of tensile strength and modulus of a carbon/glass-epoxy interply hybrid laminate with carbon fiber content.(After Kalnin,L.E.,Composite Materials:Testing and Design (Second Conference),ASTM STP,497,551,1972.) 2007 by Taylor&Francis Group.LLC
in the hybrid begin to fail is either greater than or equal to the ultimate tensile strain of the LE fibers. Furthermore, instead of failing catastrophically, the LE fibers now fail in a controlled manner, giving rise to a step or smooth inflection in the tensile stress–strain diagram. During this period, multiple cracks are observed in the LE fiber layers [10]. The ultimate strength of interply hybrid laminates is lower than the tensile strengths of either the LE or the HE fiber composites (Figure 4.18). Note that 4 3 2 1 0 0 1 Strain (%) Tensile load (lbs) 2 3 FIGURE 4.17 Tensile stress–strain diagram for a GY-70 carbon=S glass–epoxy interply hybrid laminate. (After Aveston, J. and Kelly, A., Phil. Trans. R. Soc. Lond., A, 294, 519, 1980.) 200 40 30 20 10 0 150 100 50 Strength Modulus 0 0 20 40 Relative carbon fiber content (vol%) Tensile strength (ksi) Tensile modulus (Msi) 60 80 100 FIGURE 4.18 Variations of tensile strength and modulus of a carbon=glass–epoxy interply hybrid laminate with carbon fiber content. (After Kalnin, L.E., Composite Materials: Testing and Design (Second Conference), ASTM STP, 497, 551, 1972.) 2007 by Taylor & Francis Group, LLC
the ultimate strain of interply hybrid laminates is also lower than that of the HE fiber composite.The tensile modulus of the interply hybrid laminate falls between the tensile modulus values of the LE and HE fiber composites.Thus,in comparison to the LE fiber composite,the advantage of an interply hybrid laminate subjected to tensile loading is the enhanced strain-to-failure.However, this enhancement of strain,referred to as the hybrid effect,is achieved at the sacrifice of tensile strength and tensile modulus. The strength variation of hybrid laminates as a function of LE fiber content was explained by Manders and Bader [11].Their explanation is demonstrated in Figure 4.19,where points A and D represent the tensile strengths of an all-HE fiber composite and an all-LE fiber composite,respectively.If each type of fiber is assumed to have its unique failure strain,the first failure event in the interply hybrid composite will occur when the average tensile strain in it exceeds the failure strain of the LE fibers.The line BD represents the stress in the interply hybrid composite at which failure of the LE fibers occurs.The line AE represents the stress in the interply hybrid composite assuming that the LE fibers carry no load.Thus,if the LE fiber content is less than ve,the ultimate tensile strength of the interply hybrid laminate is controlled by the HE fibers.Even though the LE fibers have failed at stress levels given by the line BC,the HE fibers continue to sustain increasing load up to the level given by the line AC.For LE fiber contents greater than ve,the HE fibers fail almost immediately after the failure of the LE fibers.Thus,the line ACD represents the tensile strength of the interply hybrid laminate.For comparison,the rule of mixture prediction,given by the line AD,is also shown in Figure 4.19. Rule of mixture C B ◇ 0 Ve 0.5 1.0 Volume fraction of low-elongation(LE)fibers FIGURE 4.19 Model for tensile strength variation in interply hybrid laminates. 2007 by Taylor Francis Group,LLC
the ultimate strain of interply hybrid laminates is also lower than that of the HE fiber composite. The tensile modulus of the interply hybrid laminate falls between the tensile modulus values of the LE and HE fiber composites. Thus, in comparison to the LE fiber composite, the advantage of an interply hybrid laminate subjected to tensile loading is the enhanced strain-to-failure. However, this enhancement of strain, referred to as the hybrid effect, is achieved at the sacrifice of tensile strength and tensile modulus. The strength variation of hybrid laminates as a function of LE fiber content was explained by Manders and Bader [11]. Their explanation is demonstrated in Figure 4.19, where points A and D represent the tensile strengths of an all-HE fiber composite and an all-LE fiber composite, respectively. If each type of fiber is assumed to have its unique failure strain, the first failure event in the interply hybrid composite will occur when the average tensile strain in it exceeds the failure strain of the LE fibers. The line BD represents the stress in the interply hybrid composite at which failure of the LE fibers occurs. The line AE represents the stress in the interply hybrid composite assuming that the LE fibers carry no load. Thus, if the LE fiber content is less than vc, the ultimate tensile strength of the interply hybrid laminate is controlled by the HE fibers. Even though the LE fibers have failed at stress levels given by the line BC, the HE fibers continue to sustain increasing load up to the level given by the line AC. For LE fiber contents greater than vc, the HE fibers fail almost immediately after the failure of the LE fibers. Thus, the line ACD represents the tensile strength of the interply hybrid laminate. For comparison, the rule of mixture prediction, given by the line AD, is also shown in Figure 4.19. A B 0 0.5 vc Volume fraction of low-elongation (LE) fibers 1.0 Tensile strength C Rule of mixture D E FIGURE 4.19 Model for tensile strength variation in interply hybrid laminates. 2007 by Taylor & Francis Group, LLC