Laminate Design Rules

    Although the final stacking sequence will be determined through analysis, the following general rules provide a starting point. They also cover some issues that most analyses will miss. Of course, there are always exceptions to each of the rules.

    • Fibers should be aligned in the direction of principal loads or stresses; for example, axially for a beam and circumferentially for a pressure vessel.

    • Unidirectional laminates are somewhat fragile — some off-axis plies are necessary to hold the laminate together.

    • Angle plies carry shear loads; +/-45° plies are the most effective.

    • Shells, even under pure axial loads, buckle in both the axial and circumferential directions. Hoop fibers are needed for stability.

    • Constant angles are difficult to maintain, especially on doubly-curved surfaces. Account for variations in the analysis, and try to use a limited number of angles.

    • Angle differences between plies should be minimized — [0°/45°/90°/-45°] rather than [0°/90°/45°/-45°] — to minimize interlaminar shear stresses caused by shear coupling.

    • Nonsymmetric laminates hold their shape only at cure temperature. They will warp (for open sections) or develop significant residual stresses (for closed sections) at other temperatures. Symmetric laminates avoid this problem.

    • Quasi-isotropic plies are isotropic in in-plane stiffness only. Strength and bending stiffness are not isotropic.

    • Quasi-isotropic, intermediate modulus carbon laminates have roughly the same stiffness as aluminum but about two-thirds the density.

    Accurate material properties are not always available for composites. Generating a complete set of composite performance data is extremely expensive. Analysis of composites requires four elastic constants and five strength constants, determined using five different tests. Therefore, materials are often selected based on whether or not property data is easily obtained. Most composites suppliers have considerable data available for designing with the materials that they offer, and can provide design guides and assistance, although some additional testing may be needed.