Determination of Significant Composite Processing Factors by Designed Experiment (MSFC Center Director’s Discretionary Fund Final Report, Project No. 95–23)

To determine composite material properties’ effects from processing variables, a 3 factorial designed experiment with two replicates was conducted. The factors were cure method (oven versus autoclave), layup (hand versus tape-laying machine), and thickness (8 versus 52 ply). Four material systems were tested: AS4/3501–6, IM7/8551–7, IM7/F655 bismaleimide (BMI), and shear tests on IM7/F584. Material properties were G₁₂, v₁₂, E_1C, and E_2C. Since the samples were necessarily nonstandard, strengths, though recorded, cannot be considered valid. Void content was also compared. Autoclave curing helped material properties for the low modulus fiber material but showed little benefit for higher stiffness fibers. The number of plies was very important for epoxy composites but not for the BMI. E₁ was generally unaffected by any factor. Particularly high void content did correlate to reduced properties. Autoclave curing reduced void content over oven curing but a moderate amount of voids, <1 percent void content, did not correlate with material properties. Oven cures and hand layups can produce high-quality parts. Part thickness of epoxy composites is important, though cure optimization may improve performance. Significant variations can be caused by processing and it is important that test coupons always reflect the layup and processes of the final part.