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Handbook of Materials for Product Design Part 7

Tham khảo tài liệu 'handbook of materials for product design part 7', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Composite Materials and Processes 5.31 5.3.3 Balance All laminates should be balanced to achieve in-plane orthotropic behavior. To achieve balance for every layer centered at some positive angle 0 there must exist an identical layer oriented at -0 with the same thickness and material properties. If the laminate contains only 0 and or 90 layers it satisfies the requirements for balance. Laminates may be midplane symmetric but not balanced and vice versa. Figure 5.11e is symmetric and balanced whereas Fig. 5.11g is balanced but unsymmetric. 5.4 Quasi-isotropic Laminate. The goal of composite design is to achieve the lightest most efficient structure by aligning most of the fibers in the direction of the load. Many times there is a need however to produce a composite that has some isotropic properties similar to metal because of multiple or undefined load paths or for a more conservative design. A quasi-isotropic laminate layup accomplishes this for the x and y planes only the z or through-the-laminate thickness plane is quite different and lower. Most laminates produced for aircraft applications have been with few exceptions quasi-isotropic. One exception was the X-29 Fig. 5.3 . As designers become more confident and have access to a greater database with fiber-based structures more applications will evolve. For a quasi-isotropic QI laminate the following are requirements It must have three layers or more. Individual layers must have identical stiffness matrices and thicknesses The layers must be oriented at equal angles. For example if the total number of layers is n the angle between two adjacent layers should be 360 n. If a laminate is constructed from identical sets of three or more layers each the condition on orientation must be satisfied by the layers in each set for example 0 60 s or 0 45 90 s Ref. 11 p. 199 . Table 5.1412 shows mechanical values for several composite laminates with the high-strength fiber of Table 5.4 and a typical resin system. The first .