206 J.D. Schaefer and I.M. Daniel Fig. 25.10 Comparison of static angle-ply yield envelopes for IM7/8552 Fig. 25.11 Angle-ply yield envelopes at static, intermediate and dynamic strain rates for IM7/8552 0 20 40 60 80 100 120 -200 -150 -100 -50 0 50 100 Shear Stress, τ6(MPa) Normal Stress, σ2 (MPa) NU Quasi-static NU Intermediate NU Dynamic Quasi-static Intermediate Dynamic NU -F2c y F 2t y F6 y lamina and angle-ply data may be combined into a single yield envelope, as shown in Fig. 25.13. Thus, from a single set of lamina tests the strain-rate-dependent yield behavior of angle-ply laminates may be predicted with enhanced accuracy using a mitigated testing framework. 25.7 Conclusion This investigation provides a direct comparison by which to evaluate the Northwestern Yield Criteria for lamina and the validity of its application to composite laminates. The results indicate that the failure-mode-based Northwestern Failure Theory may be recast as a set of yield criteria to effectively predict the yielding of both lamina and angle-ply laminates.
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