A composite laminate is a laminate that contains more than one layer, or ply, of reinforcement materials.
This pile of materials is called a laminate stack.
In a composite laminate, structural properties, such as strength, stiffness, and dimensional stability depend on the stacking sequence of the plies.
Understanding where the stresses and direction of the loads are is key to estimating the number of plies and the orientation of plies needed in the laminate stack.
What loads is the area to be laminated subject to? Knowing where the loads are gives you an idea of the type and weight of reinforcement material you need, and helps you visualize how to orient the fibers, that is, parallel to the loads, for maximum strength.
This knowledge, in turn, helps you cut and then position the layers of the laminate stack for strength, stiffness, and stability.
For example, if you’re using a plain weave fabric with warp and fill fibers running respectively at 0° and 90°, you’re getting strength in two directions. But if you need strength in a different direction, you would place the next layer at an angle to the first, typically + or – 45°.
Another example, of using +/-45°, would be to use a 0° unidirectional fabric and a 45° unidirectional fabric and orient the plies in a balanced stack as +45°, -45°, 0°, -45°, 0°, +45°. This example is symmetrical and balanced. In a symmetrical stack, the plies above the center or mid-line of a laminate are a mirror image of the plies below.
In a balanced stack, there are equal numbers of + and – angle plies.
Here are some examples of how symmetry and balance affect composite laminates:
- Symmetrical/Balanced: Laminate remains flat, with consistent stress on the midline. Example +45,-45,0,-45,+45
- Nonsymmetrical/Balanced: Causes curvature of the laminate. Example 0,+45,90,0,-45,90
- Symmetrical/Nonbalanced: Causes the laminate to twist. Example -45,0,-45
- Nonsymmetrical/Nonbalanced: Causes the laminate to twist and curve. Example 90,+45,0,90,+45,0
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