The goal when designing composite structures is making them tolerable to failure during its service life. Laminate design in composite materials are an important phase during the development of the laminate to ensure it will withstand the loads or mantain the stiffness that has been design to.
When designing there are different options to take account
- Selection the materials that are going to be used: fiber and resin.
- Selection of the fiber orientation and the lamina stacking sequence.
- Selection the number of plies needed in each orientation.
Laminate design for strength
In case that the stress of a structure is known and remains the same during the service operations, the recommended lamina orientation may be selected in the following way.
Using the standard Mohr’s circle technique, define the principal normal loads and the principal directions. Select a [0/90] cross-ply configuration with the layers 0º aligned in the direction of the maximum principal load 1 and the 90º layers aligned in the direction of the minimum principal load 2. Taking to the account of the angle X the final laminate configuration would be [X/(X+90)].
In case the structures varies in direction or the loads are unknown a common approach in laminate design is to make it quasi-isotropic: [0/±45/90]s. The next phase is to reduce the selection of plies for each angle and define the total thickness of the laminate. For this, the design charts or carpet plots are used when defining the ply ratios.
Laminate design for stiffness
Antoher type of laminate design in composite materials is designing for stiffness. The stiffness of a component is how it deflects when applying loads. The properties of a material that affects directly to stiffness are: axial, bending and torsional stiffness.
Laminate stacking sequence plays an important role in the stiffness of a laminate because depending on the elements of the ABD matrix, a laminate would have better resistance to a type of external loads or another. For example, in the case of symmetric laminate, its stiffness against the in-plane loads is related ti the elements in the [A] matrix and for its stiffness against bending, buckling and torsional loads is related to the elements in the [D] matrix. To modify the [A] and [D] matrices it can be vary the fiber type. Volume fraction, fiber orientation angles, lamina thicknesses and the number of layers of each orientation.
For cross-ply stacking sequence (0/90), the [D] matrix for all symmetric laminates contains nonzero D16 and D26. For bending deflection, buckling loads and vibrational frequencies of symmetric laminates are not available. A few closed-form solution are available in the literature for unsymmetric laminates with a nonzero [B] matrix. However, the coupling effect of this matrix becomes small when the laminate contains more than six to eight layers.
