The following information was obtained from the ASM Handbook, Volume 21-Composites and has been summarized for your convenience. Composite materials are used because of their superior strength and lightness, both important factors in rocketry, and thus an understanding of how to use them is necessary for optimizing structural and aerodynamic performance.

INTRODUCTION TO COMPOSITES

Composite: a macroscopic combination of two or more distinct materials, having a recognizable interface between them

• Optimized to achieve a particular balance of properties for a given range of applications
• Materials that contain a continuous matrix constituent that binds together and provides form to an array of a stronger, stiffer reinforcement constituent --> resulting composite material has a balance of structural properties that is superior to either constituent material alone
• Typically have fiber or particle phase that is stiffer and stronger than the continuous matrix phase
• Good thermal and electrical conductivity, coefficient of thermal expansion (CTE) that is less than the matrix and/or good wear resistance

Levels of Classification 

1. Matrix constituent

• OMC: organic-matrix composites
• PMC: polymer-matrix composite
• Carbon-matric composites (carbon-carbon composites)
• Typically formed from PMCs by including extra steps of carbonizing and densifying the original polymer matrix
• MMC: metal-matrix composites (other type still in research phase: IMC- intermetallic-matrix composites)
• CMC: ceramic-matrix composites
    
  2. Reinforcement form

• Particulate reinforcements: all dimensions roughly equal (reinforced by spheres, rods, flakes, and many other shapes of roughly equal axes)
• Whisker reinforcements: aspect ratio typically between ~20-100

Above two are classified as "discontinuous" - reinforcing phase is discontinuous for the lower volume fractions typically used in MMCs. "Filled" systems are materials (usually polymers) that contain particles that extend rather than reinforce the material. They are not generally considered to be particulate composites because they are included for the purpose of cost reduction, not reinforcement. In some cases, though, the filler also reinforces the matrix material.

• Continuous fiber laminated composites: contains reinforcements having lengths much greater than their cross-sectional dimensions
• Composites have both continuous and discontinuous phases
• Continuous fiber-reinforced: length of fiber is such that further increase in length doesn't increase elastic modulus or strength of composite
• Most continuous fiber (or continuous filament) composites contain fibers comparable in length to overall dimensions of composite part
• Each layer, or "ply," has specific fiber orientation direction. Layers can be stacked --> entire laminate has highly tailorable overall properties

In laminated composite, neither of the phases (discont/cont) are truly continuous in 3D. Many applications require isotropy in a plane, which is achieved by controlling fiber orientation within a laminated composite

• Woven composites
• Braided fiber architecture
• Knitted fiber architecture: knit fiber bundles or "tows" to create interlocking fibers that often have orientations slightly or fully orthogonal to primary structural plane (in order to have properties in the "out-of-plane" dimension)

In order to provide a useful increase in properties, there generally must be a substantial volume fraction (~10% or more) of the reinforcement

General Use Considerations

Fiber-reinforced composites: strength and stiffness can be controlled by specifying fiber orientation

• Highest levels of properties achieved when all fibers aligned along primary loading direction within the composite
• But this also causes lowest specific properties for loads perpendicular to fiber direction --> anisotropic properties (change depending on fiber direction) must be considered
• Solution: choose laminate architecture such that isotropy within a plane is achieved (often done with OMCs)

Dealing with low properties in off-axis condition:

1. Use axially reinforced material in components with largely axial loading, so that off-axis stresses on composite are minimal and material is used most efficiently
2. Cross-ply reinforcement so that some fraction of fibers are aligned along off-axis loads

• For example, orient laminating plies at equal numbers of 0°, 90°, +45°, and -45° plies (there are other stacking sequences that will result in in-plane isotropy)

Factors to Consider:

• Strength
• Stiffness
• Damage tolerance
• Fatigue resistance
• Environmental resistance
• Thermal conductivity
• Electrical conductivity
• Wear resistance
• CTE (want low)
• Chemical inertness