Reference: Abbott, Richard. Analysis and Design of Composite and Metallic Flight Vehicle Structures 3 Edition, 2019

Unlike metallic materials such as steel and aluminum, the performance of composites is heavily influenced by the manufacturing process and subsequent quality assurance. Below are examples of factors that affect composite mechanical properties:

4.1.5.1. Material Storage

Composites, particularly pre-impregnated fabric (pre-preg), are sensitive to storage temperature, humidity, and time. Often pre-pregs are supplied with a shelf life based on storage temperature. Quality assurance personnel are responsible for tracking the cumulative ‘out-of-freezer’ time. Using material outside of the expiry period voids any warranty unless substantiated by test.

4.1.5.2. Material Preparation

Following removal from the freezer, pre-preg must be fully thawed and moisture removed prior to lamination. Failure to do so will result in moisture ingress and reduction in strength.

4.1.5.3. Layup Conditions

Composites are sensitive to contamination and environmental effects during the layup process. Work must be done in a clean room at set limits of temperature and humidity. These limits are set by the specific resin used.

4.1.5.4. Vacuum Bagging / De-bulk / Autoclave

Laminate strength depends on proper consolidation of the plies. Vacuum bag leaks or insufficient de-bulk frequency are likely to create pockets of entrapped air and excessive variance in laminate thickness, strength and durability.

4.1.5.5. Curing

Resins require specific curing schedules that include temperature ramp-up rate (degrees per minute), sustained time at temperature, followed by a cool down rate. Specifics are determined by the particular resin used. Falling outside of these limits may not only decrease strength but also increase brittleness in the matrix.

4.1.5.6. De-molding

Depending on the part geometry and mold design certain components require heavy persuasion to de-mold (i.e. mallet and wedges). If care is not taken, this process can introduce cracks into the matrix that are difficult to detect by the naked eye.

4.1.5.7. Trimming

When parts are trimmed manually using hand tools, care must be taken to ensure smooth edges (no frayed fibers) and avoidance of sharp stress concentrations.

4.1.5.8. Secondary Bonding

Adhesive joints are highly sensitive to contamination and surface roughness. For this reason, peel ply (consumable fiber used as facing ply on bonding surface) should be removed only immediately prior to bonding. If peel ply is not present, the surface must first be wiped with alcohol, sanded with low grit paper, followed by another alcohol wipe immediately prior to bonding. Both methods must be qualified by test.

4.1.5.9. Post Curing

Similar to curing, this step is sensitive to temperature ramp-up, time at temperature, and ramp-down. Usually, follower coupons (test sample coupons of a set laminate schedule) are placed in the same oven cycle as primary structures. These coupons are then tested to substantiate the particular post-cure cycle. Thermocouples are often placed on the thickest laminate locations to digitally log time vs. temperature. This data is also useful for determining when the structure has heat-soaked to a uniform temperature and begins the timer for the sustained temperature portion of the post-cure.