Metals are isotropic materials. Composites are anisotropic materials. Composite parts cannot be treated like metallic parts.

Cutting holes in a CFRP (Carbon Fiber Reinforced Polymer) structure can indeed affect its mechanical properties, especially stiffness and strength. You're right in your intuition that the fibers in CFRP play a significant role in determining its mechanical behavior, so cutting through them can reduce its structural integrity if not done correctly. However, it's not necessarily a bad idea to cut CFRP for your design; it just requires careful consideration of the fiber orientation and post-processing steps.

Here are some key factors and recommendations to consider:

1. Fiber Orientation and Load Path

• Importance of Fiber Alignment: The stiffness and strength of CFRP depend largely on the alignment of the fibers relative to the loads the part experiences. When you cut into CFRP, you sever some of the continuous fibers, which disrupts the load transfer through the fibers.
• Optimized Fiber Layup: When cutting holes (or making cutouts for airflow as in your case), it's important to ensure that the fibers in the remaining material are aligned with the directions of the primary loads (radial or circumferential forces in your case). If possible, you should design the layup of the CFRP laminate to account for the holes in advance. For instance, a quasi-isotropic layup with fibers at 0°, 90°, and ±45° could help maintain more balanced stiffness around the holes.
• Effect on Load-Carrying Capacity: Removing material for airflow could lead to localized stress concentrations near the edges of the holes, weakening the structure. In metal structures, rigidity often drops in proportion to the removed material, but in CFRP, the effect can be more significant due to fiber discontinuities.

2. Cutout Shape and Size

• Rounded Shapes: Sharp edges or corners in the cutout design could lead to stress risers in CFRP, which can cause delamination or crack initiation. Rounded cutouts, like the "rounded triangles" you mentioned, are a good idea because they help to reduce stress concentrations. The transition between the spokes and the disc should be smooth to distribute stress more evenly.
• Size of Cutouts: Larger cutouts could weaken the structure more, but if the spoke widths and remaining material around the holes are sufficient, the design can still be strong. It's important to maintain a balance between cutout size and the remaining material's stiffness.

3. Post-Processing to Prevent Fraying or Delamination

• Edge Finishing: After cutting, it's crucial to finish the edges properly to prevent fiber fraying and delamination. You can sand and seal the edges with epoxy or a similar resin to ensure the fibers are well-bonded and protected. This step is critical, as unsealed fibers can lead to crack propagation or environmental degradation (e.g., moisture ingress).
• Precision Cutting: CFRP should be cut using tools that minimize fiber pull-out, such as waterjet or laser cutting. These methods provide clean cuts that are less likely to damage the fiber structure compared to mechanical cutting methods.

4. Lamination or Reinforcement

• Reinforce Around Cutouts: If your design allows, you can reinforce the areas around the cutouts by adding extra layers of carbon fiber or using different weaves in those regions. This reinforcement can help maintain stiffness and strength despite the material removal.
• Adhesive Bonding: Another option is to bond a secondary structure (e.g., a CFRP ring) around the holes or along the spokes to improve stiffness without adding much mass.

5. Effect on Thermal Expansion

• One of the benefits of CFRP is its very low coefficient of thermal expansion (CTE), which is likely why you chose it. Cutting out material will not significantly affect the CTE properties unless you remove so much material that the remaining structure behaves differently due to changes in the overall stiffness.