19.4 Application to Glass-Epoxy Systems Tsai and Sun [15, 16] previously determined the strain rate dependence of an S2/8552 (glass fiber) lamina. S2 glass fibers are considerably less brittle than carbon fibers. The results are plotted with the NU failure envelope in Fig. 19.13, and the properties are listed in Table 19.2. Interestingly, the rate dependence based on the shear strength (F6) is similar to the previous values determined for lamina utilizing the 8552 system (although, the value for E2 appears to be inaccurate). This indicates that for the range of fiber stiffness and fiber volume fractions considered, the strain rate dependence of the matrix-dominated material properties is dependent solely on the matrix constituent. Significantly, the analysis indicates that once the strain rate dependence is determined for a given fiber/matrix combination, in this case IM7/8552, that same rate dependence holds when the matrix is used with other similar fiber types. Thiruppukuzhi and Sun [17] obtained static and intermediate strain rate tensile data for the S2/8553-40 lamina system. The NU Theory envelope is presented in Fig. 19.14. The rate dependence was determined to be mf ¼0.06, which was similar to the other epoxy systems. The master failure envelope is provided in Fig. 19.15. An overall comparison of the evaluated lamina rate dependencies is provided in Table 19.3. The individual envelope fits appear to be quite good for the experimental data. Thus, the NU Theory is found to be useful for predicting the strain rate dependent failure of a given lamina for the domain of stiff fibers combined with comparatively ductile matrices. For a given matrix, it was determined that the fiber type was insignificant to the material strain rate dependence, which indicates that considerable testing time may be saved in characterizing material performance across 0 20 40 60 80 100 120 140 160 -200 -150 -100 -50 0 , t6 (MPa) Shear Stress Normal Stress, s2 (MPa) Master Failure Envelope for AS4/3501-6 NU Shear Quasi 0.001 /s 0.1 /s 250 /s 400 /s Fig. 19.12 Master NU Failure Envelope for AS4/ 3501-6; data from [12] 0 20 40 60 80 100 120 140 -200 -150 -100 -50 0 , t6 Shear Stress (MPa) Normal Stress, s2 (MPa) Master Failure Envelope for S2/8552 NU Shear Quasi 0.01 /s 1 /s 600 /s F6 mf = 0.065 Fig. 19.13 Master NU Failure Envelope for S2/8552; data from [15, 16] 194 J.D. Schaefer et al.
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