2000 Carbon/Epoxy,[0]12 1600 Ef=128 GPa Xf=1880 MPa ■1.55% 1200 800 L 400 0.4 0.8 1.2 1.6 Strain,% FIGURE 8.6 Flexural stress-strain response of a [0]12 carbon/epoxy test specimen. 2000 Carbon/PEEK(APC-2),[0s 1600 Ef=115 GPa X=1632 MPo 5f=14% g120 800 400 0 0.2 0.40,6 0,8 1.0 Strain,% FIGURE 8.7 Flexural stress-strain response of a [O]carbon/PEEK test specimen For the case where the three-point flexure specimen is not strain-gaged, the flexural modulus may be determined from a plot of load,P,vs.center deflection,δ，as L3△P E=4wh3△8 (8.4) This relation,however,assumes that shear deformation is negligible.For [90 beams(fibers perpendicular to the beam axis),shear deformation is generally insignificant and Equation(8.4)should be accurate.On the other hand,the results of Zweben [9]for [O]n beams(fibers parallel to the beam axis)shown in Figure 8.8 illustrate that for certain unidirectional composites,large support span-to-thickness ratios,L/h,are required to minimize the influence of shear deformation and thus produce an accurate flexural modulus. ©2003 by CRC Press LLCFor the case where the three-point flexure specimen is not strain-gaged, the flexural modulus may be determined from a plot of load, P, vs. center deflection, δ, as (8.4) This relation, however, assumes that shear deformation is negligible. For [90]n beams (fibers perpendicular to the beam axis), shear deformation is generally insignificant and Equation (8.4) should be accurate. On the other hand, the results of Zweben [9] for [0]n beams (fibers parallel to the beam axis) shown in Figure 8.8 illustrate that for certain unidirectional composites, large support span-to-thickness ratios, L/h, are required to minimize the influence of shear deformation and thus produce an accurate flexural modulus. FIGURE 8.6 Flexural stress–strain response of a [0]12 carbon/epoxy test specimen. FIGURE 8.7 Flexural stress–strain response of a [0]16 carbon/PEEK test specimen. Ef L wh P = 3 3 4 ∆ ∆δ TX001_ch08_Frame Page 128 Saturday, September 21, 2002 4:59 AM © 2003 by CRC Press LLC