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ADVANCED MECHANICS OF COMPOSITE MATERIALS Episode 13

Tham khảo tài liệu 'advanced mechanics of composite materials episode 13', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Chapter 7. Environmental special loading and manufacturing effects 407 Fig. 7.36. Fatigue diagrams for a unidirectional carbon-epoxy composite loaded along the fibers with various stress ratios. Fabric composites are more susceptible to cyclic loading than materials reinforced with straight fibers. This fact is illustrated in Fig. 7.37 showing the experimental results of Schulte et al. 1987 . The foregoing discussion deals with high-cycle fatigue. The initial interval 1 N 103 corresponding to so-called low-cycle fatigue is usually studied separately because the slope of the approximation in Eq. 7.69 can be different for high stresses. A typical fatigue diagram for this case is shown in Fig. 7.38 Tamuzh and Protasov 1986 . Fig. 7.37. Tensile fatigue diagrams for a cross-ply 1 and fabric 2 carbon-epoxy composites. 408 Advanced mechanics of composite materials Fig. 7.38. Low-cycle fatigue diagram for unidirectional aramid-epoxy composite loaded along the fibers with R 0.1. Fatigue has also some effect on the stiffness of composite materials. This can be seen in Fig. 7.39 demonstrating a reduction in the elastic modulus for a glass-fabric-epoxyphenolic composite under low-cycle loading Tamuzh and Protasov 1986 . This effect should be accounted for in the application of composites to the design of structural members such as automobile leaf-springs that being subjected to cyclic loading are designed under stiffness constraints. Stiffness degradation can be used as an indication of material damage to predict fatigue failure. The most sensitive characteristic of the stiffness change is the tangent modulus Et specified by the second equation in Eqs. 1.8 . The dependence of Et on the number of cycles N normalized to the number of cycles that cause material fatigue fracture under the preassigned stress is presented in Fig. 7.40 corresponding to a 45 angle-ply carbon-epoxy laminate studied by Murakami et al. 1991 . 7.3.4. Impact loading Thin-walled composite laminates .