Ply Optimization: Staggering

In the previous article, we introduced a lot of parameters that are used to optimize the laminate to be manufactured. “Optimizing” a laminate can refer to reducing the waste produced during the manufacturing process or decreasing the chance that defects in the finished part may occur. Now we will look at the so-called staggering, why it is needed and what exactly it is supposed to do.

Background: To accurately calculate the mechanical parameters of CFRP laminates - like the moduli, strength, the coefficients of thermal expansion, and stress responses of the material - the Classical Lamination Theory (CLT) is used. The CLT yields accurate results only when laminates are designed “symmetric and balanced” and “quasi-isotropic”. This means that the order of the plies of the laminate is of symmetric structure and that the most common angles are covered. Usually, these are 0°, 45°, 90°, 135°. Note, however, that a -45° ply is equivalent to 135°. Quasi-isotropic laminates lead to the most predictable parts and are therefore easier to design with.

Figure 1: Two symmetrically designed, quasi-isotropic laminates. The left laminate uses two central plies (135°). The right laminate only uses one central ply. The “vertical offset” describes the offset in “up”-direction, which is perpendicular to the layup table surface. The lateral offset describes the sideward offset of the ply. It is given as a fraction of a tow-width. Ply.5 of the left laminate is therefore shifted by 1/8’’ inch (0.5 * ¼’’). Ply.4 of the right laminate is shifted by (0.33 * ¼’’).

Using the same orientation multiple times can lead to defects in the laminate. Layup tools usually have a (albeit very narrow) gap between their tows by design to prevent them from sticking to each other’s sides during layup. If these gaps are not covered from above, certain regions between the tows may never be directly covered by prepreg material, which leads to air being enclosed inside the laminate. These air pockets are considered weak spots in a laminate and must be avoided at all costs. This is where the staggering comes into play. Plies are shifted sideward relative to the other plies of the same orientation. This leads to the effect that the gaps are immediately covered by the matrix of the following ply. The matrix will melt and expand during the consolidation and the autoclave process to fully fill these gaps. The staggering is usually given as a fraction of the tow width, meaning that a lateral offset of 0.5 is equal to 0.5 times the used material width. The following illustrations visualize the effect of the staggering on the coverage of the boundary.

Figure 2: Lateral offsets of 0 and 0.5. The plies are shifted by exactly half a tow width, which also means that the gaps of one ply are directly above/below the center line of the tows of the other ply.

Figure 3: Lateral offsets of 0, 0.33 and 0.66. The plies are shifted by a third and two thirds of the tow width. This means that their gaps are not directly above each other but rather spread apart and distributed evenly.

A good analogy is a brick wall:

Figure 4: In this figure you can see a sketch of two walls. The wall on the left may look tidier, but it lacks stability. Since the lighter and weaker mortar is stacked, the wall’s strength is greatly reduced in this position. In an even distribution of these weak spots, the walls average stability is much higher.

In this final visualization you can see how the tows of staggered plies are placed. This visualization is taken from the CAESA® Composites TapeStation, which we use to program and emulate laminate manufacturing processes.

Figure 5: A 3D visualization of 3 staggered 45° plies using values of 0, 0.33 and 0.66 tow widths. It is clear that the gaps between the tows are well distributed along the direction of the laminate thickness.

Staggering is one of the parameters needed to optimize the properties and the manufacturing of CFRP parts. The next article will be about another parameter, the so-called “boundary coverage type”. We hope your interest has been sparked and we hope to see you again soon!

Until then, stay safe and stay tuned.